Thoughts on Cobalt Front Suspension
01-28-2010, 02:59 PM
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Thoughts on Cobalt Front Suspension
Powell Raceshop, Jan 2010
A good handling car needs attention to suspension – springs, bars and joints, weight, and the wheel tire package.
Production cars you are pretty much locked in to what you have, because it is quite expensive to make changes to another type of suspension. Even weight reduction - both overall mass and mass reduction at one end or the other, while important, is hard to achieve. Most times, the major weight savings are most easily achieved at the light end of the car. That means in the case of the Cobalt the hardest weight to lose is at the front, which is where it matters the most.
Also for a good handling car, aerodynamics (down force versus drag) comes into play. In the extreme, down force is so important that mechanical grip through suspension becomes secondary. That is not the case with a production car.
Finally , Wheels and Tires: the type and weight of these items are very important. In fact, of all the things that can be done to a car, tires are probably the most significant change that can be made, easily and for little effort and cost.
We need to look at what the car is – in the case of the Cobalt TC , the last of the Delta platform performance iterations and an excellent handling car – and understand the goals.
At all times, if the car is to be driven on the street, remember that the overall spring rates chosen may be ideal for a pure track car on a hot dry race track, but not so good overall for the road in many different traction conditions.
The last and the best! GM describe the FE5 Turbocharged cars: 08 /09/ 10: 25 mm front stab bar, 24 mm rear stab bar, higher rate front and rear springs, re valved shocks and struts, front knuckles with shorter steering arm (steering rack is the same) resulting in quicker steering.
Cobalt's strut-type front suspension is compact yet provides long wheel travel, with 90 mm of compression travel and 87 mm of rebound, which is substantially more than most competitors and enhances overall ride and handling. Monotube rear shocks are used for a more precise road feel. L-shaped front control arms contain a forward pivot bushing, which transmits most of the cornering force, and a larger, high-damped composite rear elastomeric bushing that allows rearward movement to minimize harshness from road surface impacts. The front control arms are made from steel on the FE1 suspension and are aluminum on the FE3 and FE5 suspensions.
The direct-acting front stabilizer bar connects to each strut for precise steering response and improved cornering agility with decreased body roll. All front suspension components are mounted to a new, rigid cradle assembly comprised of four large hydroformed steel tubes and two side members that are integrally welded to front and rear cross members. The cradle is mounted to the Cobalt's space frame at four widely spaced points.
Front suspension issues: The geometry of the strut is such that as the strut rotates when the driver turns the steering wheel, the front sway bar link gets some high loads applied through it at an angle . Stock sway bar drop links wear out quickly and are not adjustable
The front spring rate for the stock TC is pretty good, but for serious track driving the spring rate could usefully be increased.
Friction is the enemy of suspension design and with Macpherson strut suspension, excessive suspension joint compliance combined with friction is not a good thing.
What do most folks want? In the Cobalt market, “drop” or a lowered car is the first thing considered. Oftentimes, there is no consideration given to the negative effects of lowering a production car.
Lets start with what we got and how it is this way:
Production cars are designed around certain well defined parameters. It starts with a marketing assessment – what does the customer want?- and sometimes that assessment does not suit a small group of drivers/ enthusiast buyers .
Some 300,000 Cobalt’s have been sold annually.
Most buyers want good fuel economy, safe cars meeting 5 star crash ratings, (whatever that may mean) a peppy feel, smooth, comfortable ride and generous interior space and comfort. And a good sounding audio system. The result is a pretty bland, soft riding automatic transmission equipped car painted black, white or silver...
The manufacturer has some Federal laws to consider there are engineering requirements for safety crash protection standards, as well as fuel economy considerations that we hardly think about.
For example: Engineers must provide sufficient clearance from tire to fender to allow installation of tire chains; enough ride height to satisfy bumper crash regulations; side impact and roll over protection; load path engineering to divert the power train mass down and underneath the cabin in the event of a front collision; fuel system design to prevent fires in the case of a collision; noise emissions both interior and exterior, approaching/ passing/leaving, for induction and exhaust.
This means, practically speaking that aftermarket dropped suspensions, revised motor mounts, revised fuel system plumbing; revised body kits, free flow exhaust systems are not possible. These are all common changes made by enthusiasts to their cars, but if they were produced by the manufacturer at the get go, the cars would FAIL FEDERAL LAWS requiring cars to pass NHTSA/EPA safety/environmental criteria before sale.
Okay. Before we go forward with Delta suspension modifications, its time to review an excellent synopsis by Mike Kojima on handling, from the Nissan Sentra enthusiast forum, because you need to understand why you are going to change your car – more than just “tuck those wheels, make some noise, go fast.”
http://www.se-r.net/car_info/suspens...#Understanding Vehicle Dynamics
• Friction circle: This is basically a vehicles performance envelope. It's expressed in lateral G’s, accelerating and braking G’s. When graphed, the friction circle looks like an egg with the X axis lateral G’s and the Y access braking and accelerating G’s.
•
• Understeer: This is when, at the limit of vehicle traction, the front of the car slides first before the rear. Race car drivers call this "push". This is the way that many cars come set up to behave from the factory as it is the most predictable for average drivers. The crash mode for understeer is that when the limit of adhesion is exceeded, the car will plow straight ahead off the road nose first. This is not the fast way to have your car set up but if you are a dork mode driver. When the car understeers you should regain control if you let off the gas, unless of course you run out of road first. That is what air bags are for. Even my uncoordinated evil twin sister could get that right, maybe. It is not efficient for extracting maximum lateral G’s because the car will dynamically use the front tires excessively for turning, overloading them while the rear tires basically just hold the back of the car up. Front wheel drive cars like ours tend to exhibit understeer as the final terminal mode of balance.
•
• Oversteer: This is when, at the limit of vehicle traction, the rear of the car slides first before the front. Race car drivers call this "loose". The rest of us call this "spinning out", "spinning a shitty", "doing a brodie" or even crashing. The final crash mode of oversteer is backwards, tail first into the woods or in the worst case spinning round and round with the driver as a helpless passenger. Since the infamous days of Ralph Nader and the Corvair, most auto manufactures avoid oversteer like the plague. Oversteer is difficult for a dork to handle because recovery requires judicious use of countersteering and throttle feathering to control; fine motor skills that only some of us can deal with. Although oversteer looks neat and macho it is really a slow way to drive except in pro-rally on the dirt which I don’t know too much about. Oversteer is slow on the pavement because hanging the tail out bleeds off a great deal of speed going through a corner. Conserving the momentum is the fast way around as turn.
• Neutral: This is the fast way around a turn where all four wheels slide evenly. Since the total friction circle traction of each tire is being used, all the available grip that the tires have is being put to the ground. Racers call this "drifting". This not to be mistaken for the idiotic Japanese Option Magazine video stuff which makes a mockery of proper driving technique. Neutral is the fast way around a corner most of the time. Neutral is also the hardest handling mode to achieve for the suspension tuner.
• Polar Moment of Inertia: Or PMI as we will refer to it, is a description of how a cars mass is distributed along the length of the vehicle. A car with a high PMI is like a rear engine, rear drive car like a Porsche 911 or a front engine, front wheel drive car like our beloved SE-R, same thing only the poles are different, so to speak. A car with a low PMI would be a mid engine car like a Boxster. Low PMI cars have most of their mass about the middle, high PMI cars have the mass at one end or another. Low PMI cars are the easiest to get a neutral balance out of due to the balanced, centralized mass. High PMI cars like to oversteer, in the case of the 911 or understeer like our cars. To get a feel for this phenomena, hold a bowling ball in one hand and rotate it back and forth by twisting your wrist. Now get a set of dumbbells of the same weight, grab the middle of the bar and do the same thing. Bet the bowling ball wants to rotate easier right? Guess what type of car will be easier to get neutral!
• Slip Angle: This is the wonderful thing that allows us to tune our cars suspensions despite the design limitations caused by the PMI. Proper manipulation of slip angle is the great equalizer and is what suspension tuning is all about. Slip angle is the difference in which a cars wheels are pointed vs the angle that the tires contact patch is placed on the road. The main thing that affects slip angle is the manipulation of the individual load placed on each wheel while cornering. This is the key for suspension tuning. A front wheel drive car has most of the weight on the front wheels. So the front wheels run at higher slip angles and develop understeer. Conversely the same for a rear wheel drive, rear engine car developing oversteer. That is also a reason why a mid engine car with equally loaded tires will be more or less neutral. Slip angles, weight distribution and PMI are the main factors in how a vehicle will handle.
Because our cars are front heavy, front tire overloaded, front wheel drive cars, does that mean that we are condemned to econobox hell for driving fun? Heck no! By design we can not change the basic layout of our cars to significantly change the PMI or weight distribution but we can sure tweek the slip angles of the tires to achieve world class handling out of our killer econo transportation units.
The easy way to tweek the slip angles are with anti-sway bars and springs. Shock absorbers, going against what people think that they do, are not really for changing the handling balance. Shocks mostly act as spring dampers and affect understeer/oversteer balance mostly only in transient (which is big word for a change from straight line travel to turning) maneuvers like initial turn-in and zig zaging around slalom cones.
Changing to heavier springs changes the slip angle differential by resisting the cars tendency to roll on the end of the car that they are installed on. The resistance of the heavier spring to compression causes more weight to be transferred to the outside wheel of the end of the car that they are installed on as the car tries to lean over in a corner. This causes that wheel to proportionally run at a higher slip angle than it normally would. If you put heavier than stock springs in the rear of your SE-R while not changing the spring rate of the front, the car would tend to understeer less.
Antisway bars work in much the same way. Antisway bars are torsion bars attached to the cars chassis and are linked to the right and left control arms. Antisway bars offer resistance to independent side to side wheel movement. This is how these bars limit sway in the turns and hence their name. While limiting sway, the antisway bars also cause weight transfer to the outside wheels. By altering the diameter of the antisway bars or installing them where there were none before adds yet another chassis tuning element. If you were to increase the size of the rear antisway bar on an SE-R you would be increasing the amount of weight transfer to the outside rear wheel, thus causing it to run a bigger slip angle. This would give you more oversteer.
Tire pressure also can affect the slip angle. Higher pressures reduce the slip angle and lower pressures increase it. A great deal of suspension tuning can be done for free by adjusting the tires pressure.
Alignment also has a great deal of effect on a vehicles handling balance. Caster and camber affect how a tires contact patch is positioned on the ground by compensating for a tires tendency to flex and lift the inside tread while cornering, By helping keep the tread flat, these settings can increase or decrease the available friction circle traction on an end of a car thus affecting balance. Toe in or out can affect balance also by changing how a vehicle turns in.
So now lets get back to the Cobalt.
Let me suggest three important suspension tuning activities:
• Low rider looks
• Drag strip
• Road race/ performance driving
Low rider looks.
Have at it! Remember the limitations: if the car is too low, the tires will hit the inner upper fender , rubbing through the plastic fender liner which will last 5 second. The struts will bottom on the external and internal bump stops, the axles will run out of their intended alignment range and the car will bounce horriblly when the springs coil bind or the bump stops collapse.
Best solution: air ride. Expensive. Effective.
Drag Strip.
Here the car needs to be adjusted in trim height to move weight forward. Raise the back, drop the front. Not too much at the front– see low rider looks – the axles wont like it.
Not only raise the back, but limit rear suspension travel. Drag bags go inside the rear springs and are remotely inflatable, cost under a hundred bucks from Summit Racing. A good buy.
For the rest: Drag racers need to pay attention to the suspension geometry at the front to make sure the control arms stay in place during launch. The strut rebound control if adjustable should be set to control how quickly the shock lets the springs return to normal ride height. The front end will lift in the launch. That’s why the rear normally squats so drag bags limit that.
The control arm bushings, especially the inner rear trailing arm bushing needs to be made less compliant. But the need to retain the ability to flex in and out as the wheels go up and down. Poly control arm bushings cannot flex in and out as the arm goes up and down without destroying the bushing. Do not use poly in this application it is dangerous. And disconnect the front bar. It can self steer the car under acceleration.
The suspension arms need to be reinforced if steel or replaced with FE5 alloy arms. On a cost benefit basis it is hard to justify fabricating new lower control arms. If this was done, the suspension geometry should be reworked. I don’t know how; that’s a whole other engineering study. Ballard the guy with the world’s fastest Drag race FWD Cobalt has probably done something so ask him.... finally the axles need to run parallel to the trans output shaft, but that is a motor mount discussion.
Road Race.
Here we go. Keywords: Friction. Control arm location. Damping.
The first is not a simple task. The friction in a Macpherson strut can come from the spring hat bearing, the lower ball joint, the leading bushing which is by design a stiff rubber joint that flexes and a trailing bushing that moves and yields a lot by design and finally the drop links that connect the sway bar to the strut.
What you can do to improve the front suspension for friction? The first is to make sure the strut is assembled correctly, the top spring hat bearing is in good condition and well lubricated with the top spring hat correctly positioned. If it is not, it will cause the spring to be loaded on an angle which creates binding in the strut assembly and worst case can make the car pull to one side or the other down the road and not self center correctly.
While talking of the top spring hat #7 in the drawing below, make sure the tab is on the inside so the spring is aligned correctly centered on the strut shaft; grease the bearing carefully #8 this is a ball bearing in a plastic case and can be fractured through improper assembly so take care. The celasto bump rubber #4 should be attached to the bottom of the spring hat #7, using glue....
The next thing to look at it are the front lower control arm bushings. There is a trade-off. The best low friction suspension set up transfers a lot of road noise and impact harshness from the road to the suspension and the cabin. If you don’t mind that, then go ahead with this:
Replace the leading inner bushing with a delrin/tool steel shaft bushing assembly, and the rear inner with a spherical joint.
The stock ball joint is fine. The OEM drop links for the sway bar are not, and the very best on the market are powergridinc.com links, fully adjustable low friction spherical joints with weatherproof boots. All of these aftermarket suspension parts are available from OTTP .
Sidebar:
To make the front suspension assembly capable of camber adjustment, GM recommend ovalizing the top strut to knuckle hole. If you want to get fancy you can buy an aftermarket eccentric bolt for this application but it is not really needed. As you lower the car, the camber curve of the suspension gets steeper, so you get most of the camber you need just by lowering the car. Only in auto x or serious road racing will you need as much as 3 degrees or so of negative camber. SCCA and GRAND AMERICAN preparation rules limit front camber to – 3 degrees.
Springs and shocks.
Most aftermarket springs lower the car, but also rely on coil binding three or more coils. Most of the aftermarket lowering springs are also a softer spring rate than stock. Some manufacturers make high quality springs for Cobalt- Eibach, H&R, Pedders. Some others are less than good. Make your choice wisely. I use Pedders springs.
DO NOT cut the bump stops when installing lowering springs for road racing. The generic information for the Eibach springs recommends this, but in fact it is not required. If the bump stop is cut the spring ends up as the first limit of suspension travel (or the shock piston rod bottoms in the housing) which is not good.
Most aftermarket shocks are not very good. As it happens, the current 2010 Cobalt TC strut and shock assemblies are well priced, have very good damping control and are not adjustable. That’s okay. Simple is good. The capability of adjusting shocks leads to poor adjustments and the may not be equally adjusted around the car...
Coil Overs.
Putting the spring around the shock with an adjustable pedestal is popular. Most race cars use coil overs. They also use Hypercoil springs which are typically linear in rate and very good quality, available in many diameters and lengths with ranges from 75 lbs to 2500 lbs! Coil overs provide the ability to adjust ride height, and the shock itself is generally adjustable; a nice overall package.
THE REAR:
Note that at the rear suspension, Cobalt has insufficient wheel/tire clearance to use coil overs, so the existing spring has to be replaced with an adjustable spring perch and replacement spring, and a conventional shock is used.
THE FRONT:
At the front, there are two main ways to go: modify the existing strut housing to take an adjustable spring perch, or replace the entire strut housing. You get nothing for nothing. Unless you are prepared to spend about 5,000 dollars the typical aftermarket coil over set up may not satisfy your needs for durability and performance. Pure coil overs need a modified top strut plate.
This picture is of a KW front coil over that uses the stock upper mount and the stock strut bearing.
In this picture the coil over is shown with the top mount removed. The little blade sticking out of the top of the shaft is the adjuster. This is a KW coil over with a progressively wound spring.
This is the GM proving grounds in Milford Michigan. Back when GM had money a few years ago they built this road course. Note the white “U” shaped turn. Called the “toilet bowl” it is an absurdly 30 ++ degree banked concrete corner. It is here that a Cobalt test car broke a knuckle and lead to the competition knuckle for racing use.
The Turbo LNF cars were intended to get a really trick new front suspension knuckle but that got chopped in the middle of its development cycle by the bean counters. Instead a revised LNF knuckle with a shorter steering arm and revised spindle height came about.
This picture shows (closest to the front) the LNF knuckle , the G6 derived competition knuckle and the LSJ knuckle. The middle one is the strongest looking casting with a slightly shorter steering arm, the LNF has the shortest steering arm. These knuckles are lined up, so its easy to reference the edge of the steel bench and see how much shorter the steering arms became. The steering ratio went to 14.8:1 from 16.5:1 for LSJ. The steering rack and pinion is are the same for all cars.
This picture shows the first installation of an Ohlin’s 3 way adjustable coil over shock with a remote canister in a Cobalt built in our shops in 2004. The shock is anchored to a Ground Control upper camber plate. The Ground control plate is for a VW and is very well built and inexpensive. In racing the ground control plates did yield slightly in extreme conditions , but all we do is replace them at regular intervals and yell at the race drivers to stay the heck off the speed bumps and the concrete apex berms. Then its all good. The strut tower is re-drilled for the 4 bolts plate, and the strut tower is opened up to move the top of the coil over as far back as possible to improve the castor angle. Because the tower is welded and tied into the firewall there is not a lot of room to go far rearward.
This is a rear ohlins shock in the oem position. The picture below is of the rear coil spring adjustment plate we machined to put in the stock spring seat at the rear with a Hypercoil rear spring. Grand Am spring rates for Cobalt are in the 4-600 lb range at the front, and 6-900 lbs at the rear.
A good handling car needs attention to suspension – springs, bars and joints, weight, and the wheel tire package.
Production cars you are pretty much locked in to what you have, because it is quite expensive to make changes to another type of suspension. Even weight reduction - both overall mass and mass reduction at one end or the other, while important, is hard to achieve. Most times, the major weight savings are most easily achieved at the light end of the car. That means in the case of the Cobalt the hardest weight to lose is at the front, which is where it matters the most.
Also for a good handling car, aerodynamics (down force versus drag) comes into play. In the extreme, down force is so important that mechanical grip through suspension becomes secondary. That is not the case with a production car.
Finally , Wheels and Tires: the type and weight of these items are very important. In fact, of all the things that can be done to a car, tires are probably the most significant change that can be made, easily and for little effort and cost.
We need to look at what the car is – in the case of the Cobalt TC , the last of the Delta platform performance iterations and an excellent handling car – and understand the goals.
At all times, if the car is to be driven on the street, remember that the overall spring rates chosen may be ideal for a pure track car on a hot dry race track, but not so good overall for the road in many different traction conditions.
The last and the best! GM describe the FE5 Turbocharged cars: 08 /09/ 10: 25 mm front stab bar, 24 mm rear stab bar, higher rate front and rear springs, re valved shocks and struts, front knuckles with shorter steering arm (steering rack is the same) resulting in quicker steering.
Cobalt's strut-type front suspension is compact yet provides long wheel travel, with 90 mm of compression travel and 87 mm of rebound, which is substantially more than most competitors and enhances overall ride and handling. Monotube rear shocks are used for a more precise road feel. L-shaped front control arms contain a forward pivot bushing, which transmits most of the cornering force, and a larger, high-damped composite rear elastomeric bushing that allows rearward movement to minimize harshness from road surface impacts. The front control arms are made from steel on the FE1 suspension and are aluminum on the FE3 and FE5 suspensions.
The direct-acting front stabilizer bar connects to each strut for precise steering response and improved cornering agility with decreased body roll. All front suspension components are mounted to a new, rigid cradle assembly comprised of four large hydroformed steel tubes and two side members that are integrally welded to front and rear cross members. The cradle is mounted to the Cobalt's space frame at four widely spaced points.
Front suspension issues: The geometry of the strut is such that as the strut rotates when the driver turns the steering wheel, the front sway bar link gets some high loads applied through it at an angle . Stock sway bar drop links wear out quickly and are not adjustable
The front spring rate for the stock TC is pretty good, but for serious track driving the spring rate could usefully be increased.
Friction is the enemy of suspension design and with Macpherson strut suspension, excessive suspension joint compliance combined with friction is not a good thing.
What do most folks want? In the Cobalt market, “drop” or a lowered car is the first thing considered. Oftentimes, there is no consideration given to the negative effects of lowering a production car.
Lets start with what we got and how it is this way:
Production cars are designed around certain well defined parameters. It starts with a marketing assessment – what does the customer want?- and sometimes that assessment does not suit a small group of drivers/ enthusiast buyers .
Some 300,000 Cobalt’s have been sold annually.
Most buyers want good fuel economy, safe cars meeting 5 star crash ratings, (whatever that may mean) a peppy feel, smooth, comfortable ride and generous interior space and comfort. And a good sounding audio system. The result is a pretty bland, soft riding automatic transmission equipped car painted black, white or silver...
The manufacturer has some Federal laws to consider there are engineering requirements for safety crash protection standards, as well as fuel economy considerations that we hardly think about.
For example: Engineers must provide sufficient clearance from tire to fender to allow installation of tire chains; enough ride height to satisfy bumper crash regulations; side impact and roll over protection; load path engineering to divert the power train mass down and underneath the cabin in the event of a front collision; fuel system design to prevent fires in the case of a collision; noise emissions both interior and exterior, approaching/ passing/leaving, for induction and exhaust.
This means, practically speaking that aftermarket dropped suspensions, revised motor mounts, revised fuel system plumbing; revised body kits, free flow exhaust systems are not possible. These are all common changes made by enthusiasts to their cars, but if they were produced by the manufacturer at the get go, the cars would FAIL FEDERAL LAWS requiring cars to pass NHTSA/EPA safety/environmental criteria before sale.
Okay. Before we go forward with Delta suspension modifications, its time to review an excellent synopsis by Mike Kojima on handling, from the Nissan Sentra enthusiast forum, because you need to understand why you are going to change your car – more than just “tuck those wheels, make some noise, go fast.”
http://www.se-r.net/car_info/suspens...#Understanding Vehicle Dynamics
• Friction circle: This is basically a vehicles performance envelope. It's expressed in lateral G’s, accelerating and braking G’s. When graphed, the friction circle looks like an egg with the X axis lateral G’s and the Y access braking and accelerating G’s.
•
• Understeer: This is when, at the limit of vehicle traction, the front of the car slides first before the rear. Race car drivers call this "push". This is the way that many cars come set up to behave from the factory as it is the most predictable for average drivers. The crash mode for understeer is that when the limit of adhesion is exceeded, the car will plow straight ahead off the road nose first. This is not the fast way to have your car set up but if you are a dork mode driver. When the car understeers you should regain control if you let off the gas, unless of course you run out of road first. That is what air bags are for. Even my uncoordinated evil twin sister could get that right, maybe. It is not efficient for extracting maximum lateral G’s because the car will dynamically use the front tires excessively for turning, overloading them while the rear tires basically just hold the back of the car up. Front wheel drive cars like ours tend to exhibit understeer as the final terminal mode of balance.
•
• Oversteer: This is when, at the limit of vehicle traction, the rear of the car slides first before the front. Race car drivers call this "loose". The rest of us call this "spinning out", "spinning a shitty", "doing a brodie" or even crashing. The final crash mode of oversteer is backwards, tail first into the woods or in the worst case spinning round and round with the driver as a helpless passenger. Since the infamous days of Ralph Nader and the Corvair, most auto manufactures avoid oversteer like the plague. Oversteer is difficult for a dork to handle because recovery requires judicious use of countersteering and throttle feathering to control; fine motor skills that only some of us can deal with. Although oversteer looks neat and macho it is really a slow way to drive except in pro-rally on the dirt which I don’t know too much about. Oversteer is slow on the pavement because hanging the tail out bleeds off a great deal of speed going through a corner. Conserving the momentum is the fast way around as turn.
• Neutral: This is the fast way around a turn where all four wheels slide evenly. Since the total friction circle traction of each tire is being used, all the available grip that the tires have is being put to the ground. Racers call this "drifting". This not to be mistaken for the idiotic Japanese Option Magazine video stuff which makes a mockery of proper driving technique. Neutral is the fast way around a corner most of the time. Neutral is also the hardest handling mode to achieve for the suspension tuner.
• Polar Moment of Inertia: Or PMI as we will refer to it, is a description of how a cars mass is distributed along the length of the vehicle. A car with a high PMI is like a rear engine, rear drive car like a Porsche 911 or a front engine, front wheel drive car like our beloved SE-R, same thing only the poles are different, so to speak. A car with a low PMI would be a mid engine car like a Boxster. Low PMI cars have most of their mass about the middle, high PMI cars have the mass at one end or another. Low PMI cars are the easiest to get a neutral balance out of due to the balanced, centralized mass. High PMI cars like to oversteer, in the case of the 911 or understeer like our cars. To get a feel for this phenomena, hold a bowling ball in one hand and rotate it back and forth by twisting your wrist. Now get a set of dumbbells of the same weight, grab the middle of the bar and do the same thing. Bet the bowling ball wants to rotate easier right? Guess what type of car will be easier to get neutral!
• Slip Angle: This is the wonderful thing that allows us to tune our cars suspensions despite the design limitations caused by the PMI. Proper manipulation of slip angle is the great equalizer and is what suspension tuning is all about. Slip angle is the difference in which a cars wheels are pointed vs the angle that the tires contact patch is placed on the road. The main thing that affects slip angle is the manipulation of the individual load placed on each wheel while cornering. This is the key for suspension tuning. A front wheel drive car has most of the weight on the front wheels. So the front wheels run at higher slip angles and develop understeer. Conversely the same for a rear wheel drive, rear engine car developing oversteer. That is also a reason why a mid engine car with equally loaded tires will be more or less neutral. Slip angles, weight distribution and PMI are the main factors in how a vehicle will handle.
Because our cars are front heavy, front tire overloaded, front wheel drive cars, does that mean that we are condemned to econobox hell for driving fun? Heck no! By design we can not change the basic layout of our cars to significantly change the PMI or weight distribution but we can sure tweek the slip angles of the tires to achieve world class handling out of our killer econo transportation units.
The easy way to tweek the slip angles are with anti-sway bars and springs. Shock absorbers, going against what people think that they do, are not really for changing the handling balance. Shocks mostly act as spring dampers and affect understeer/oversteer balance mostly only in transient (which is big word for a change from straight line travel to turning) maneuvers like initial turn-in and zig zaging around slalom cones.
Changing to heavier springs changes the slip angle differential by resisting the cars tendency to roll on the end of the car that they are installed on. The resistance of the heavier spring to compression causes more weight to be transferred to the outside wheel of the end of the car that they are installed on as the car tries to lean over in a corner. This causes that wheel to proportionally run at a higher slip angle than it normally would. If you put heavier than stock springs in the rear of your SE-R while not changing the spring rate of the front, the car would tend to understeer less.
Antisway bars work in much the same way. Antisway bars are torsion bars attached to the cars chassis and are linked to the right and left control arms. Antisway bars offer resistance to independent side to side wheel movement. This is how these bars limit sway in the turns and hence their name. While limiting sway, the antisway bars also cause weight transfer to the outside wheels. By altering the diameter of the antisway bars or installing them where there were none before adds yet another chassis tuning element. If you were to increase the size of the rear antisway bar on an SE-R you would be increasing the amount of weight transfer to the outside rear wheel, thus causing it to run a bigger slip angle. This would give you more oversteer.
Tire pressure also can affect the slip angle. Higher pressures reduce the slip angle and lower pressures increase it. A great deal of suspension tuning can be done for free by adjusting the tires pressure.
Alignment also has a great deal of effect on a vehicles handling balance. Caster and camber affect how a tires contact patch is positioned on the ground by compensating for a tires tendency to flex and lift the inside tread while cornering, By helping keep the tread flat, these settings can increase or decrease the available friction circle traction on an end of a car thus affecting balance. Toe in or out can affect balance also by changing how a vehicle turns in.
So now lets get back to the Cobalt.
Let me suggest three important suspension tuning activities:
• Low rider looks
• Drag strip
• Road race/ performance driving
Low rider looks.
Have at it! Remember the limitations: if the car is too low, the tires will hit the inner upper fender , rubbing through the plastic fender liner which will last 5 second. The struts will bottom on the external and internal bump stops, the axles will run out of their intended alignment range and the car will bounce horriblly when the springs coil bind or the bump stops collapse.
Best solution: air ride. Expensive. Effective.
Drag Strip.
Here the car needs to be adjusted in trim height to move weight forward. Raise the back, drop the front. Not too much at the front– see low rider looks – the axles wont like it.
Not only raise the back, but limit rear suspension travel. Drag bags go inside the rear springs and are remotely inflatable, cost under a hundred bucks from Summit Racing. A good buy.
For the rest: Drag racers need to pay attention to the suspension geometry at the front to make sure the control arms stay in place during launch. The strut rebound control if adjustable should be set to control how quickly the shock lets the springs return to normal ride height. The front end will lift in the launch. That’s why the rear normally squats so drag bags limit that.
The control arm bushings, especially the inner rear trailing arm bushing needs to be made less compliant. But the need to retain the ability to flex in and out as the wheels go up and down. Poly control arm bushings cannot flex in and out as the arm goes up and down without destroying the bushing. Do not use poly in this application it is dangerous. And disconnect the front bar. It can self steer the car under acceleration.
The suspension arms need to be reinforced if steel or replaced with FE5 alloy arms. On a cost benefit basis it is hard to justify fabricating new lower control arms. If this was done, the suspension geometry should be reworked. I don’t know how; that’s a whole other engineering study. Ballard the guy with the world’s fastest Drag race FWD Cobalt has probably done something so ask him.... finally the axles need to run parallel to the trans output shaft, but that is a motor mount discussion.
Road Race.
Here we go. Keywords: Friction. Control arm location. Damping.
The first is not a simple task. The friction in a Macpherson strut can come from the spring hat bearing, the lower ball joint, the leading bushing which is by design a stiff rubber joint that flexes and a trailing bushing that moves and yields a lot by design and finally the drop links that connect the sway bar to the strut.
What you can do to improve the front suspension for friction? The first is to make sure the strut is assembled correctly, the top spring hat bearing is in good condition and well lubricated with the top spring hat correctly positioned. If it is not, it will cause the spring to be loaded on an angle which creates binding in the strut assembly and worst case can make the car pull to one side or the other down the road and not self center correctly.
While talking of the top spring hat #7 in the drawing below, make sure the tab is on the inside so the spring is aligned correctly centered on the strut shaft; grease the bearing carefully #8 this is a ball bearing in a plastic case and can be fractured through improper assembly so take care. The celasto bump rubber #4 should be attached to the bottom of the spring hat #7, using glue....
The next thing to look at it are the front lower control arm bushings. There is a trade-off. The best low friction suspension set up transfers a lot of road noise and impact harshness from the road to the suspension and the cabin. If you don’t mind that, then go ahead with this:
Replace the leading inner bushing with a delrin/tool steel shaft bushing assembly, and the rear inner with a spherical joint.
The stock ball joint is fine. The OEM drop links for the sway bar are not, and the very best on the market are powergridinc.com links, fully adjustable low friction spherical joints with weatherproof boots. All of these aftermarket suspension parts are available from OTTP .
Sidebar:
To make the front suspension assembly capable of camber adjustment, GM recommend ovalizing the top strut to knuckle hole. If you want to get fancy you can buy an aftermarket eccentric bolt for this application but it is not really needed. As you lower the car, the camber curve of the suspension gets steeper, so you get most of the camber you need just by lowering the car. Only in auto x or serious road racing will you need as much as 3 degrees or so of negative camber. SCCA and GRAND AMERICAN preparation rules limit front camber to – 3 degrees.
Springs and shocks.
Most aftermarket springs lower the car, but also rely on coil binding three or more coils. Most of the aftermarket lowering springs are also a softer spring rate than stock. Some manufacturers make high quality springs for Cobalt- Eibach, H&R, Pedders. Some others are less than good. Make your choice wisely. I use Pedders springs.
DO NOT cut the bump stops when installing lowering springs for road racing. The generic information for the Eibach springs recommends this, but in fact it is not required. If the bump stop is cut the spring ends up as the first limit of suspension travel (or the shock piston rod bottoms in the housing) which is not good.
Most aftermarket shocks are not very good. As it happens, the current 2010 Cobalt TC strut and shock assemblies are well priced, have very good damping control and are not adjustable. That’s okay. Simple is good. The capability of adjusting shocks leads to poor adjustments and the may not be equally adjusted around the car...
Coil Overs.
Putting the spring around the shock with an adjustable pedestal is popular. Most race cars use coil overs. They also use Hypercoil springs which are typically linear in rate and very good quality, available in many diameters and lengths with ranges from 75 lbs to 2500 lbs! Coil overs provide the ability to adjust ride height, and the shock itself is generally adjustable; a nice overall package.
THE REAR:
Note that at the rear suspension, Cobalt has insufficient wheel/tire clearance to use coil overs, so the existing spring has to be replaced with an adjustable spring perch and replacement spring, and a conventional shock is used.
THE FRONT:
At the front, there are two main ways to go: modify the existing strut housing to take an adjustable spring perch, or replace the entire strut housing. You get nothing for nothing. Unless you are prepared to spend about 5,000 dollars the typical aftermarket coil over set up may not satisfy your needs for durability and performance. Pure coil overs need a modified top strut plate.
This picture is of a KW front coil over that uses the stock upper mount and the stock strut bearing.
In this picture the coil over is shown with the top mount removed. The little blade sticking out of the top of the shaft is the adjuster. This is a KW coil over with a progressively wound spring.
This is the GM proving grounds in Milford Michigan. Back when GM had money a few years ago they built this road course. Note the white “U” shaped turn. Called the “toilet bowl” it is an absurdly 30 ++ degree banked concrete corner. It is here that a Cobalt test car broke a knuckle and lead to the competition knuckle for racing use.
The Turbo LNF cars were intended to get a really trick new front suspension knuckle but that got chopped in the middle of its development cycle by the bean counters. Instead a revised LNF knuckle with a shorter steering arm and revised spindle height came about.
This picture shows (closest to the front) the LNF knuckle , the G6 derived competition knuckle and the LSJ knuckle. The middle one is the strongest looking casting with a slightly shorter steering arm, the LNF has the shortest steering arm. These knuckles are lined up, so its easy to reference the edge of the steel bench and see how much shorter the steering arms became. The steering ratio went to 14.8:1 from 16.5:1 for LSJ. The steering rack and pinion is are the same for all cars.
This picture shows the first installation of an Ohlin’s 3 way adjustable coil over shock with a remote canister in a Cobalt built in our shops in 2004. The shock is anchored to a Ground Control upper camber plate. The Ground control plate is for a VW and is very well built and inexpensive. In racing the ground control plates did yield slightly in extreme conditions , but all we do is replace them at regular intervals and yell at the race drivers to stay the heck off the speed bumps and the concrete apex berms. Then its all good. The strut tower is re-drilled for the 4 bolts plate, and the strut tower is opened up to move the top of the coil over as far back as possible to improve the castor angle. Because the tower is welded and tied into the firewall there is not a lot of room to go far rearward.
This is a rear ohlins shock in the oem position. The picture below is of the rear coil spring adjustment plate we machined to put in the stock spring seat at the rear with a Hypercoil rear spring. Grand Am spring rates for Cobalt are in the 4-600 lb range at the front, and 6-900 lbs at the rear.
Last edited by qwikredline; 01-28-2010 at 07:01 PM.
01-28-2010, 03:57 PM
#3
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01-28-2010, 05:17 PM
#13
Learned something new! I always thought the steering rack changed from LSJ to LNF, now I know it is the knuckle that controls the ratio. Thanks!
edit: any chance maybe the pics could be re-hosted on ss.net for the photobucket impaired, or in case the files are moved/deleted some time in the future?
edit: any chance maybe the pics could be re-hosted on ss.net for the photobucket impaired, or in case the files are moved/deleted some time in the future?
01-28-2010, 06:02 PM
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John, we all know you don't know what you are doing and TTR will just come in and blow your knowledge outta the water (all the while not copying your parts). 
You know I am kidding! I love your knowledge, and enjoy you sharing it with us! Keep up the good work
You know I am kidding! I love your knowledge, and enjoy you sharing it with us! Keep up the good work
04-09-2010, 11:25 AM
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I'm kind of concerned about running the hardcore rear bar with race camber up front, 2.5* maybe? will figure that out with more testing? Anyone running a similar setup?
04-09-2010, 11:55 PM
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my concern is to much oversteer. I love a loose setup, just worried it may snap out quicker than I can catch it. Autocross it would be great but it's the track days and soon to be hill climbs that worry me.
04-10-2010, 09:53 AM
#24
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run the 1.0 bar your ride balance and handling will be perfect... run the 1.25 it will be fine, just pay attention in the rain...dont drive crazy
