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Discussion Starter · #1 ·
From time to time it's come to my attention that many BMW owners do not really know their suspension - that is to say, how components interact, how they affect a vehicle's handling, etc. This can lead to confusion when it comes to determining what to spend you hard-earned $$$ on, and can also lead to detrimental choices at the track. Several people have asked me recently what terms like "bump" and "rebound" actually mean, so I figured there are probably more that can benefit from a brief crash course in suspension terminology and cause-and-effect.

General terms
Axle: Whether a old truck solid axle or a modern independently sprung suspension, an "axle" is the centerline of a car's front or rear wheels and their related components.

Spring: A piece of flexible metal that holds up the car. They come in many varieties, but the only ones you'll come into contact with on a modern BMW are coil springs. They are a coiled round bar of steel, engineered to resist a specific amount of force, measured often in pounds of force needed to compress the spring for each inch of travel. IE: 600lb springs require 600lbs of force to compress 1", or 1200lbs to compress 2".

Coilover: A shock or strut that has a coil spring located cocentrically around it, most often with an adjustable spring perch used to change ride height.

Shock, dampener: The shocks we're concerned with are hydraulic. They use a liquid and valves to dampen the actions of the spring to which they are attached. Without dampening a spring that is compressed and released will expand and contract in a violent manner. The job of the shock is to control how fast the spring expands after compression is stopped, and to modify how quickly the spring compresses in the first place.

Strut: The same as a shock, but mounts in a way that it also provides longitudinal location and stiffness for part of the suspension, in lieu of control arms.

Anti-sway bar, anti-roll bar, sway bar, roll bar: These are all the same thing. They are a bar of tubular metal that connects the two ends of the front and/or rear axle. They are a torsion spring, that is to say they resist twisting motion. Because they are anchored at the left and right of the vehicle, and to the frame in between, they receive input from both sides. As one side compresses, whether by a bump in the road or by body roll, it twists the sway bar. The sway bar resists this twisting motion, and thereby reduces the effect of the bump or body roll. Because of the way they mount to the vehicle's frame, sway bars can rotate freely if both sides of their axle contact a bump at the same time. This means that you can have better ride comfort and the same degree of roll resistance by stiffening a sway bar than by simply stiffening the related coil springs.

Strut bar: This is a bar, solid or tubular, that connects the top of the shocks or struts in the front and/or rear of a car. It does not move. It is effectively a frame reinforcement, specifically reinforcing one of the weakest points in most cars. It stiffens the frame of the car, increasing its torsional rigidity. This is a very good thing! The frame of your car IS for all intents and purposes a giant, very stiff spring. When you hit a bump or turn the wheel the frame of your vehicle bends a little. When it bends, it stores energy. When the load goes away, that energy is released and will affect, ultimately, your tires' adhesion. The stiffer your car's frame, the less it will be deflected by outside forces, and the more accurately your car's suspension and tires will work.

Tires: The black things you drive on. These ARE springs, in addition to the rest of their complex properties. The important thing to remember is that they are undampened springs, controlled only by tire pressure. The higher the pressure, the higher the effective spring rate. High means more resistance to the tire deflecting during a lateral load (cornering), but also means less deflection to vertical loads (bumps).

Understeer, push: A condition where the vehicle wants to go straighter than your steering input.

Oversteer, loose: A condition where the vehicle wants to turn more than your steering input.


Adjustment points
Bump: This controls the stiffness of a shock relating to compression. Sounds just like a spring, and it pretty much is. The difference being that while a spring's stiffness always has the job of holding up your car, the shock's bump setting only acts on temporary forces likes bumps and roll. Adding more bump will act very much like stiffening the spring on the same corner.

Rebound: This controls how quickly the spring descends after compression. This is a VERY important setting when tracking a car. Without enough rebound a spring will hop and skip along the ground, making it hard to put down power, braking or cornering force. If you have too much rebound, the spring will not be allowed to expand quickly enough to return to its proper ride height before the next input. Too much of this will lead to "jacking down", which is a term for when a vehicle lowers itself until it is riding on it's bump stops (hard rubber, used as a last-ditch safety feature to keep tires from rubbing on very hard bumps). You can use rebound to control roll. By increasing rebound you decrease the velocity of the spring's expansion. If the spring cannot expand, it cannot push that side of the vehicle up. So the vehicle must either not roll as much, or lift its inside tire. Obviously you don't want to set so much rebound as to end up with the latter.

Sway bar settings: Increasing a sway bar's stiffness will reduce roll, but also increases the sprung tension on that axle. Granted, most of the increase in tension is apparent only when there is body roll - but even statically there is an increase. This means you must modify your rebound settings when you change your sway bar settings (just as you must whenever you add spring tension).

Camber: This is a tire's difference from vertical when looked at from head-on. + means the bottom of the tire is in more than the top. - means the bottom is out more than the top. Remember, this setting is only accurate when the car is NOT moving. As the wheel moves with the suspension, the camber will change depending on the geometry of the suspension. What is important is observing how the tire wears, and by taking tire temperature readings on the outside, center and inside to make sure that all of the tire is being used when it is at full load.

Castor: This is the difference between a wheel's centerline and the axis of suspension travel on that corner when viewed from the side. This is rarely adjusted on a street car. The more forward the wheel's centerline relative to the suspension's axis, the better the car will soak up bumps. And the more it will nosedive when you hit the brakes. The reverse is also true.

Toe: This is the difference between the direction a tire is pointing and straight ahead, when looking down from above. + means the tire points in. - means the tire points out. Positive toe makes a car more stable. This is useful at higher speeds and for general steering comfort. Negative toe makes a car more darty. This is useful to initiate quick directional changes, but can also reduce your total level of grip with excessive tire scrub (more on that later).


Options
Reduce understeer:
-Enter turns slower! Then power out.
-Reduce front tire pressure, and/or increase rear tire pressure.
-Reduce front spring tension, and/or increase rear spring tension. This can be done by swapping springs, or by changing sway bar settings/diameters.
-Reduce front bump, and/or increase rear bump.
-Reduce rear camber, and/or increase front camber.

Reduce oversteer:
-Do the opposite of the above "reduce understeer" options.

Reduce roll:
-Increase tire pressure all around.
-Increase bump settings.
-Increase rebound settings.
-Swap springs for stiffer ones.
-Increase sway bar settings/diameters.

Increasing traction for applying power:
-Reduce rear tire pressure.
-Reduce rear spring tension.
-Reduce rear bump.
-Increase rear rebound.
-Reduce rear sway bar settings/diameters.

Increase braking traction
-Reduce tire pressures.
-Increase front spring tension.
-Increase front bump.
-Reduce rear rebound.
-Reduce all sway bar settings/diameters.
 

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Excellent Post! Thanks for taking the time to do this


Brian
 

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All this and you forgot to talk about track and offset? :)
 
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Sticky!
 

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These "Reduce Understeer / Reduce Oversteer" recommendations are flat-out wrong regarding tire pressure.

Edit - I went to Tire Rack for this to confirm what I have always thought:

Guide To High Performance Handling

Adjustments Decrease Understeer Decrease Oversteer
Front Tire Pressure Higher Lower
Rear Tire Pressure Lower Higher
 

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Discussion Starter · #8 ·
All this and you forgot to talk about track and offset? :)
-Trying to keep it simple. Add if you want.

These "Reduce Understeer / Reduce Oversteer" recommendations are flat-out wrong regarding tire pressure.

Edit - I went to Tire Rack for this to confirm what I have always thought:

Guide To High Performance Handling

Adjustments Decrease Understeer Decrease Oversteer
Front Tire Pressure Higher Lower
Rear Tire Pressure Lower Higher
-Nope. Although it depends on where you start out with regards to tire pressure. If your tires are way under-inflated to begin with, then adding air will certainly help.

Pressure and traction are a bell curve. Too much or too little both reduce traction, but in different ways. To reduce understeer, change your rear tire pressure to decrease from maximum traction and/or change the front pressure to increase from maximum traction. This usually is done by reducing front pressure and increasing rear pressure, or just increasing rear pressure. Doing the reverse will sharpen your turn-in (less tire flex), but will in fact increase understeer once you get near the limit of the car's total traction.
 

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These "Reduce Understeer / Reduce Oversteer" recommendations are flat-out wrong regarding tire pressure.

Edit - I went to Tire Rack for this to confirm what I have always thought:

Guide To High Performance Handling

Adjustments Decrease Understeer Decrease Oversteer
Front Tire Pressure Higher Lower
Rear Tire Pressure Lower Higher
That's what I always thought, too, but the bell curve explanation makes sense. Typically rental cars have underinflated tires, so if I keep the car for more than a day, I will usually go add air to them, usually more to the front than the rear, since most rentals heavily understeer and overload the front tires. But it seems the higher pressure/more understeer works on the M5.
 

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I also was confused by the advise to increase REAR tire pressure to reduce understeer. Most of my autocross experience was in smaller cars like a Datsun 510 and a Miata, and for those we typically dropped rear pressure or raised the front pressure to reduce understeer.

When I road raced a Formula Ford, the tire pressures were set to get even temperature across the tire without a dramatically lower or higher temp in the center compared to the edges. Then, we controlled oversteer/understeer with spring rates (coarse) or sway bar settings (fine).

You really need to know if you're near the optimum point of the bell curve before you decide which way to change pressure (or try it both ways and see what the results are).
 

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Discussion Starter · #13 ·
I also was confused by the advise to increase REAR tire pressure to reduce understeer. Most of my autocross experience was in smaller cars like a Datsun 510 and a Miata, and for those we typically dropped rear pressure or raised the front pressure to reduce understeer.

When I road raced a Formula Ford, the tire pressures were set to get even temperature across the tire without a dramatically lower or higher temp in the center compared to the edges. Then, we controlled oversteer/understeer with spring rates (coarse) or sway bar settings (fine).

You really need to know if you're near the optimum point of the bell curve before you decide which way to change pressure (or try it both ways and see what the results are).
-I'm entertaining a theory that your argument might be true in autocross in a way that does not apply to track driving. That is to say that higher pressure would work better in a short autocross run as the tires don't get up to temperature the way they do in multiple laps of track driving.

I know that in my experience (which involves almost no autocrossing), once you get the tire pressures set for generally good traction, reducing understeer is simply a matter of slightly increasing stiffness in the rear of the car. This can be done by increasing sway bar stiffness, spring rate, bump, and/or tire pressure. Or you can augment an increase in rear stiffness with softening stiffness at the front. But you definitely want to increase the stiffness bias to the rear to reduce understeer.
 

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I love posts like this.

I don't know jack sh*t about the inner workings and mechanics of cars, and I really want to learn.

Stuff like this makes it easy to understand and is a great resource.

Thanks.
 

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Pressure changes have varying effects. However, in general you increase pressure to gain grip on a given end of a car "up to a point." The issue here is that you are balancing contact patch deformation under load versus sidewall spring rate. Increasing pressure increases sidewall spring rate which reduces the transient amplitude of the sidewall on turn-in. Hence increasing rear pressures for example can give more stick in the rear as it reduces the transient deformation of the sidewall on turn-in.

For steady state grip, for a given setup there is really only one optimum pressure -- the one that results in the best management of the contact patch at maximum load. However, rarely is the contact patch at steady state conditions.

You only gain grip by lowering pressure if you are already at a pressure level that results in sub-optimal contact patch management.
 

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Discussion Starter · #18 ·
Ok, i thought castor was a bean.
Caster, is what we are interested in.
cheers
-Yeah, I made two typos. Castor should have been caster, and dampeners should have been dampers.
 

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Excellent thread!!!! :applause:
 
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