With the number of threads out about suspensions and tire swaps, I thought it might be handy to review some of the basics on how a car'* chassis actually works, so as to better understand what some of the changes will actually do when the rubber meets the road. I'm not looking to write a book on chassis dynamics, I just want some of the new folks to start thinking of what the suspension actually does.

Using the independent suspension of the Bonneville as an example, each wheel has to do a few things for us to go merrily bouncing on down the road.

First: All four wheel have to support to weight of the car. But they all don't support the same amount of weight...the car is much heavier up front than it is in back, so the front wheels take more of the load when the car is sitting still. That'* the cars balance...typically it is about 60% on the front wheels, 40% on the rear. You can actually measure this if you have a scale that reads high enough (shipping scales) by raising the car up on blocks heigh enough to slip a scale under one wheel at a time. If you do it right, the sum of each wheel equals the total weight of the car.

Now while our suspension is called "Independent" it really isn't. We have sway bars on the front, and some of us have sway bars on the rear too. These transfer some of the weight from the wheels on one side to the wheels on the other side to try to level the car out. They don't do much when the car is sitting still. They do come into play when the car turns (more on that in a little bit.)

The next thing the suspension has to do is absorb bumps in the road. We have springs on all four wheels along with shock absorbers. The springs suck up the sudden changes in weight at each wheel when it hits a bump and moves up and down. It also does this when the car turns..(more on this in a little bit) The shocks slow down the speed the springs can compress...they keep the car from going "Boing!" like a rubber ball...that is really all they do. By compressing nitrogen and oil inside their guts, they dampen out the up and down motion of the wheel and transfer that energy to the body of the car. The fellows with air suspensions have the same thing, but with the added twist that the car will use a built in air compressor to change the dapening rates inside the shocks on it'* own. Note: The tire itself is part of the shock absorbing system. It flexes and deforms under load to soak up some of the road defects before the springs and shocks ever see it.

Ok, that'* how the car'* suspension works sitting still...now lets give it a shove and roll it down the road. That adds acceleration and braking loads to the mix. Braking being the more important of the two.

Now, think back to what happened the last time you stomped on the brakes to stop the car. Did you see the nose of the car dip? This is Weight Transfer in action...and it'* a good thing. Look at the driver side of the car...both wheels on that side spin counter-clockwise as the car is moving forward.

When you stomp on the brakes, the front brake calipers squeeze down on the rotors and try to stop the tire from turning. Ok, pretty straight forward. But what you might not have realized is that all the energy being sucked up by the brakes has to go somewhere. Some of it gets turned into heat. The rest ends up as a mechanical force that tries to spin the car in the same direction as the wheels. (The car tries to flip over on it'* roof !)

Now, obviously it can't do it...the car is too chunky for that to happen. But it does succeed in lifting the back end of the car up. So under braking, a weight transfer takes place. The front wheels have more of the cars weight on them..way above the normal 60/40 distribution when the car is sitting still. If you have really kickin front brakes and super sticky tires, you might even be able to lift the rear tires off the ground and have 100% of the cars weight on the two front wheels. This weight transfer is actually a good thing, since the amount of force the tires can generate is based on the friction they have between the rubber and the road. A big part of that force is the pressure pressing the two together...i.e. the amount of weight the tire is supporting!

So, us folks with drum brakes on the back tires really are not at any performance dissadvantage over folks with disk brakes on all four corners when it comes to overall stopping power on dry pavement...the harder the front brakes can be worked, the less the rear brakes matter.

Ok, that'* braking in a nutshell. Acceleration is even simpler...it'* braking in reverse. You stomp on the gas and the front wheels try to spin faster counter-clockwise. And just like under braking, the car tries to spin as well...only this time in the opposite direction (clockwise when looking at the driver'* side of the car.) This is why the nose of the car rises up in the air under hard acceleration...a weight transfer is taking place towards the rear wheels as the front wheels try to flip the car on it'* back. This is actually a limiting factor on how hard a front wheel drive car can accelerate before the wheels break traction and start spinning in a front-wheel burnout.

So, time for another cup of coffee at this...back in a few minutes to cover turning forces. <bell rings..class dissmissed >

Thanks, Curt. Your post should help a few people.

Alot of members here may also benefit from understanding that a PARTIAL FE1/FE2 swap can imbalance the car, and that there'* alot of pieces to the handling puzzle.

Springs, ELC, Struts, Sways, tires, etc.

One other misconception: Lower profile tires are not the answer when looking for better cornering. They are USUALLY worse. Sidewalls NEED to flex to keep the contact patch in contact with the road surface during lateral G situations.

Ok, class back in session. time to start looking at how the suspension and wheels move.

The rear wheels are pretty easy to understand...they just move up and down in a straignt line relative to the front and rear of the car and a slight arc compared to the centerline of the car. On an older Bonneville with rear wheel drive, the rear suspension is a tied together with the axle. That means the two wheels on the rear have to work together. On newer Bonnevilles with front wheel drive the rear wheels are independent. They can move up and down to follow weight changes on their own. (Unless a sway bar is connecting them together.)

When I say "weight changes" I really mean the amount of load on each wheel at any given moment...it'* the combination of the cars mass being acted on by gravity, forward acceleration, and latteral acceleration (caused by making the car turn!)

When a car turns, the car'* mass resists turning...it always wants to go in a straight line it was traveling in. So, when the front wheels turn and start forcing the mass of the car to change direction, forces in the opposite direction are generated. We see this as "body roll"

I'm going to digress a little and cover an important concept in Physics...Center of Mass!. There is a single point in the car that if you could attach a cable to, you could lift the entire car up off the ground and have it hang there, nice and level like it was sitting on it'* wheels. For the Bonneville (and most other cars) the center of mass is right around the height of the front seat, just a little to the right of the driver'* right hip. Pretty convienent eh? Good place to park the coffee cup on the morning drive

The point is that the center of mass for the car is the point that the car always wants to rotate around. Problem is that the forces that make the car move are focused down at the contact patches of the four tires..where the tires touch the earth...well below the car'* center of mass. This leads to some funky motions when the car moves and turns. "Body Roll" is one of the manifistations of the turning forces being down at pavement height and the center of mass being up higher...the car wants to lean to the outside of the turns!

The sway bar(*) try to counteract this rather uncomfortable feeling motion. Swaybars are steel tubes that loosely connect the wheels on one side of the car to the other. When a weight transfer happens in a turn, the force on the wheels on the outside of the turn increases...the body rolls to the outside of the turn. At the same time, the weight on the tires on the inside of the turn decreases. The car is trying to flip over again..this time on it'* side. Now, the shape, size, and placement of the swaybars resists the weight transfer...as the wheel on one side tries to move upward, the swaybar torques up and transfers some of that force back to the other side of the car, keeping the suspension somewhat level.

What Will was harping at (and I tend to agree with) is that all the components we have been talking about are part of a "system". Somebody at GM took a lot of time figuring out how they should all work together to produce the disired results. That'* not to say that swapping components around is a bad thing. I just emphasize that if you start re-engineering parts of the system without thinking of all the other bits in the system...you might come to discover the end result is a lot less than you were hoping for! It might downright suck...at worst case you might end up with a car that exhibits some really nasty and dangerous handling qualities. (I do this from time to time on Roadracing Motorcycles...and I've occassionally got to field test my leathers too, as evidenced by the black burn scars on them from the big slides.)

Ok, with swaybars and weight supporting bits of suspension aside, time to start making the car turn corners.

For starters, pull the front wheel off any Bonneville and you'll see some bits that allow the wheel to move up and down and turn side-to-side. On mine, I have a McPhearson strut and lower control arm arrangement. The strut is a fancy way of combining the upper suspension mount, coil spring, and shock all in one easy to replace package. I will not go into the advantages and disadvantages over other suspension setups..they are what they are.

What is important to note is the geometry of the pieces and how they connect! For example, the front wheels DO NOT move up and down in a STRAIGHT LINE !!!.. They move up and down through an ARC ! This is the first improtant part of suspension geometry in action. As the body rolls to one side in a turn, the suspension moves up and down and the arc tries to keep the tire perpendicular to the road. Cool term: This is known as "Camber"

Another thing to note is that the top of the suspension is tipped a little towards the rear of the car. WTF? This is the concept of "Caster" in action. If you drew an imaginary line through the center of the steering knuckle connections (the parts that allows the wheel to turn left and right) down to the pavement, you will notice it doesn't quite match up with the center of where the tire touches the road. It is always a little in front of the tire'* contact patch. This is important to the car'* stability and steering. Just like a shopping cart'* wheels (also known as casters) the tire naturally wants to turn itself back to a point that follows the direction the car is moving. Neat trick eh? You let go of the steering wheel on a car in a turn, it spins around till the car is traveling in a straight line again! This is important to how well you can control the car. Were the geometry of the front end changed so that the magic line through the steering knuckle to the road is BEHIND the contact point of the tire and the road, you would have a little problem..the car would always be trying to turn on it'* own...you would have to constantly correct it at the steering wheel to keep the car going straight.

So why not make the car perfectly nuetral steering by adjusting the caster to zero (the centerlines of the steering being exactly where the contact patches of the tire are?) Well, surprise! GM thought of that for you. Remember back in discussing the effects of hard braking on the suspension...the car tries to flip as the back end lifts up? The suspension compresses and the car squats down..reducing the actual caster on the front wheels. So, the car has a positive caster and is stable with self centering steering when it is just cruising down the road...But is also has a nuetral to slightly positive caster when the car'* suspension is fully compressed by braking and hard cornering. In other words, the geometry remains on the safe side of nuetral steering at all times! (See, them engineers are not so dumb after all )

The last little geometry problem is called "Toe-In"... the car is natually "pidgeon toed" and this is done intentionally. The front of the tires are closer together than the rear of the tire..this is done by adjusting the tie rods that connect the steering knuckles to the steering rack that move the wheels when you turn the steering wheels. The toe-in means the tires are trying to roll towards each other when traveling in a straight line (it causes a little more wear, but we live with it.) Toe in comes into play when the car turns.. the outside wheels have to turn in a bigger circle that the inside wheels. Nothing we can do about that, it the way the world is. The Toe-in helps stabilize the wheels in turns, so they roll in the right direction when the suspension get'* through compressing from all the turning forces acting on them.

Ok, time for more coffee while y'all chew on that.

I was out looking at my car and came up with an example of how you all can better visualize camber and caster on your own cars...

Straighten up the front wheels on your car and take a look at the two front tires from the front. If the car is sitting on level ground, you'll see the tires are just about perpendicular to the ground. They may even appear to be tipped in at the tops. The car would be travelling in a straight line with a level suspension and the contact patch on each tire is just about in the center of the tread...nice even load distribution.

Now, turn the steering wheels all the way to the left, like you were in a hard left turn.

Go look at the front tires again...looks a little odd doesn't it? The contact patch has shifted towards the outer bead on the driver'* side tire and onto the inner bead on the passenger'* side tire...like the tires tipped to the left at the same time they turned to the left?

What you are seeing is caster and camber in action. The geometry of the front suspension and steering are tipping the tires in the direction the turn.

What you don't see is the suspension compressing on the passenger side and lifting on the driver'* side (The car is just sitting still...it isn't generating or responding to side loads that an actual left turn would create)

Now, go have a friend sit down in the passenger seat of the car while you watch the suspension and tires...the entra weight on the passenger side of the car simulates some of the weight transfer you would see do to cornering forces. Watch the tires..the contact patches move back towards the center of the tire'* tread as the extra force is added to the side of the car that would be on the outside of the turn!

Pretty cool eh? The geometry of the front suspension is designed to try and maximize the contact patch in a turn based on the tires and rims GM expects on the car when it left the factory! I keep saying these engineering folks aren't all that dumb.

Some of you may have seen the Autocrosser'* trick of running the camber way up on the front tires...so the car is sittng with the tires tipped in at the top. What they are trying to do is create the bigger contact patch when the car is running VERY stiff sidewalls in a tight turn...in other words, doing exactly what the stock suspension does with stock tires. Of course, when the car is running straight, the inside endge of the tire is getting hard use..wearing it out in a hurry...that'* a tradeoff some of them are willing to pay for in the persuit of higher cornering forces. The smarter folks start putting adjustable coil-overs and adjustable upper & lower A-Arms to dial the suspension in and match the actual tire & rim combination to the car'* weight distribution.

man this is great cant wait for the next session

I agree with Willwren, but that does not mean that higher profiles work better either. Like all things, its all about balance. I think the 60'* used in our cars strike a nice balance between a stiffer sidewall for better steering response, while still having enough flexibility to keep the tires on the road and not beat us up too badly over bumps. Probably why GM chose them! Please keep in mind that a 75 series, for instance, CAN grip just as well as a 60 or a 50. The difference is that the much higher sidewall starts deflecting so much as the cornering forces build that steering starts getting very sloppy. The only plus to this is that you can usually feel that very clearly, so you have a very good idea what is happening at the contact patch, compared to a low profile which gives no warning and loses traction RIGHT NOW!

Curt, your overview of how suspensions work is certainly one of the clearest and easiest to understand I have ever read. Wish I had been able to show that to the kids I hired when I was in the parts business. Would have saved a lot of time and grief!

Glad you guys are diggin this.

On tires and aspect ratios. Yep, you are 100% correct. The trend in the industry has been away from Bias Ply tires towards Radials, and from large sidewalls towards narrow, thick sidewalls. But, the trend in suspensions has been along the same lines...the automotive engineers are designing the suspensions around the tire technology available at a reasonable cost at the time.

So, just because a gumband wrapped around a 24" rim will tuck under the fenders and fit an older car does not imply that it will work correctly. The converse is also true, a 75 series tire on a 14" rim might fit a modern sports car...but it isn't going to work as well as the tire and rims the car was designed for.

Is there some lattitude for experimentation and taste..of course. If you don't stray too far away from the OEM, you are probably going to be successfull. Get too far away from the OEM and it is you that will have to play the role of auto engineer to get the rest of the suspension working around your choice of rubber and rims.

Speaking of which, I also wanted to touch on one of my own personal pet-peeves regarding racing equipment on street cars: Tires and Brakes.

Tire compound is important to both the life and how much traction it can generate. I've seen autocross, drag tires, and roadracing tires on street cars. (Usually the ricer crowd, but there'* at least one Grand Prix running around Daytona on roadracing tires.) I can tell you it ain't cool. Here'* why: Street tires are fully vulcanized when they leave the factory. They operate in a temperature range that is marked on the sidewall. This operating temp is carefully set so the tire delivers the maximum adhesion to the pavement when it is up to that operating temp. If it doesn't make it to that temp, then it doesn't stick like it is supposed to. On a street car, that temp range is pretty low..you would probably only be able to overheat your street tires if you tried to drive the speed limit across Deal'* Gap. The OEM tires would scream and howl in the process, but they would Stick!

Sporting tires are not fully vulcanized when they leave the factory in most cases. These tires are supposed to be heat cycled a few times on the race track before they reach their maximum adhesion capability. They also operate at a much higher temp than a street tire. (Gimme your street tires for a few hot laps around the road course at Daytona and I'll give you back the shredded carcass when we are done ) Run a race tire on the street and it will never fully cook and it will stick to the road like it was coated in butter. Race tires and Street tires are completely seperate animals and need to be treated accordingly.

When in doubt, look back at the original equipment tires on your car...remember, GM doesn't make tires, the engineers designed the suspension around the tires that were available for the car when it was produced.

Brake pads work the same way. They work best when they are in the correct operating temp range. Those mondo brakes on SCCA road cars need a lot of heat to work effectively and it'* a heck of a lot hotter than your Bonneville will ever see. (Watching the Rolex 24 from the infield at night gives you a whole new prospective on it...when they hammer the brakes and dive into the first left-hander of the infield, the front rotors are hot enough that they GLOW!.) So unless you really are going to take it to the track, leave them on the shelf...it'* another example of how bigger isn't always better. Same goes for cold air ductwork and massive rotors. They may fit, but unless your driving style and use of the car warrant them, they may not haul your car down any quicker than the stock rotors, calipers, and pads.

Ok, personal rant is over...on to some more suspension bits:

Strut Tower Braces: Does your car have one? No? If you plan to do some aggressive corner carving, then go get yourself one. They do work and here'* why:

Remember back when we were talking about Caster and Camber? Well, one of the key points to controlling the steering and making it consistant was knowing where the tires contact patch was relative to the imaginary line drawn through the steering knuckle. Well, one end of that line is the lower ball joint, the other end is the pressed steel strut towers in your fenderwells. They are pretty impressive chunks of metal, but they can flex under the compression loads. Throw the car into a hard turn and the strut tower on the outside of the turn is going to flex inward..changing the suspension geometry on that side of the car.

By connecting the tops of the towers together with a steel bar, the top of the shock tower on the outside has something to push against (tower on the oposite side.) It isn't a perfect solution..an X brace connecting the towers to each other and the lower control arm mounts would be ideal, but the little problem of the engine being in the way precludes that. That makes connecting the tops about as good as it gets without re-engineering the front end of the car.

Ok, the only things I've got left to offer thoughts on are bushings and such. Anybody else want to offer up some words of wisdom on chassis stuff?

Really looking forward to this continuing! AWESOME information.

My thoughts on Bushings....

Ok, this has come up a few times, usually in regards to swaybar end links, but it aplies more to contol arms and the swaybar mounts.

First, the control arm bushings. Look at your stock suspension. Note that the lower ball joint that plants the steering knuckles are connected firmly to the lower control arm. The lower control arm in turn is "loosely" connected to the rest of the chassis. The connection is the rubber bushings. Ok, GM chose to do this for dampening vibration from the road at the expense of suspension accuracy. Those bushings let the control arm move up and down as the wheel follows the road and changes in suspension loads.

The down side to this is that the rubber control arm bushings can also deflect a little in and out along with side to side as cornering loads get transfered to the chassis of the car. The engineers took that into account when designing the entire system...the amount of deflection makes small changes in caster, camber, and toe-in...but generally this just makes the car feel a little loose in the corners, you don't see really gross suspension geometry changes until you push the car really hard in the corners.

One thing we can do is change the material in the bushings. We have a lot to choose from. In terms of relative stiffness, Poly bushings would plant the control arm a little stiffer, Graphite would be a little stiffer still, and solid metal wil be the stiffest of all.

Race cars use the last approach...solid metal (usually aluminum) bushings or fabricated LCA'* with spherical end links connecting them to the chassis. This gives the highest degree of accuracy to the geometry of the suspension..it doesn't deflect at all. It also transfers every vibration to the chassis of the car. Rather unpleasant for a street car....just try keeping the coffee in the cup going down the road in a solid suspension. It is also a high maintenance item. Those joints have to be lubricated and check for wear constantly. Most roadrace cars are going to fully adjustable suspensions on all four corners..coil over shocks adjustable for preload, rebound, and dampening mounted to fully adjustable upper and lower control arms. High dollar and high maintenance in one package...but if you want a car that handles like a go-cart on every track you plan to run, that'* the price you pay.

The "right up the middle" of these two extremes are Poly and Graphite bushings. (PST and other sell these). By going to them, you increase the accuracy of the suspension a little when subjecting the car to hard corners. But you also increase the amount of vibration being transmitted to the chassis. Spring rates are changed a little as well, since that available mush in the rubber bushings was included in the overall design of the suspension as a system...it'* not much of a change, but there just the same.

Along the same lines are the bushings that support the sway bars. As I said earlier, the sway bars "loosely" couple the left and right sides of the suspension together...transfering loads back and forth to attempt to level the car out in the corners. The swaybars are hollow steel tubes, heat treated to make them like springs. The shape and diameter of the swaybars account for most of their function, but how they are mounted can change the way they transfer loads as well.

The swaybars are connected with 10 rubber bushings (four on each end link, two along the middle) How much these bushings can compress and deflect determines the amount of load that can be transfered through the swaybar. Put a stiffer material in place of the rubber and the swaybar does a better job transferring loads.

Now, a lot of us have put poly bushings on the end links. This is a relatively harmless change. The real action on the swaybar happens in the middle, where the steel tube is twisted by the loads at either end. That makes the bushings that connect the swaybar to the chassis of the car the most important two in the whole system. The tighter those hold the swaybar in place, the more focused the energy is on twisting the bar and transfering the load from end to end.

For a little fun experiment, bend a piece of coat hanger wire in the shape of the swaybar. Now, grab the ends and give them a little twist in opposite directions like the car'* suspension does in a turn. Watch the middle...it moves!! Now, anchor the middle of the wire like the two chassis bushings do to the swaybar. Do the twist experiment again...see how much more efficient the twisting becomes? The tighter the middle of the swaybar is anchored in the middle, the more load it can transfer and resist the twisting motion.

Give that some thought when buying your bushings...figure on replacing the two body mount bushings before worrying about the end links. Better yet, start thinking about a heavier swaybar as part of the entire suspension upgrade and keeping the rubber bushings (for noise dampening. ) if the stiffer suspension and harsh ride quality fits your driving style. It all works together: Swaybar stiffness + Spring Rates + Shock Absorber dampening rates + Bushing rigidity + Tire & Wheel dimensions. All in the context of the basic geometry and rigidity of the chassis they are hooked up to.