Handling Adjustments

[Bart Carter]

Any changes made to a car's suspension and braking systems must be done with care and
understanding.  If you have any doubts, you should contact a knowledgeable and 
experienced mechanic.  Suspension packages are available.  Be sure they do what you want if
you go that direction.

Changes can affect steady-state cornering force, vehicle transitions, straight line stability,
braking, high speed handling and any combination.

Try to change only one thing at a time.  If more than one thing is changed simultaneously, you can't
attribute direct results.  Try to follow a series of changes that will result in your desired goal.
Test your changes in a safe place.  Make sure that if you exceed your car's limits, the
result will not be dangerous.  Keep records of your test results.  The absolute best way to
evaluate your suspension is using tire temperatures.

The following is a list of changes and their results that are generally accepted by
knowledgeable suspension tuners.  Please note that it is best to increase the traction of
the end of the car that has the least traction without losing traction from the other end.
My ideal handling is an ever-so-slight understeer at the front that lets you point the car with 
the throttle. (RWD) Remember that at some point a change will achieve the opposite result of 
what you want.  You should keep a written record of your changes and their results.

Change                Reduce Understeer/     Reduce Oversteer/
                      Increase Oversteer     Increase Understeer
Tires---
Front pressure        Increase (2.5 psi)     Reduce (2.5 psi)
Rear pressure         Reduce (2.5 psi)       Increase (2.5 psi)
Front section width   Increase               Reduce
Rear section width    Reduce                 Increase
Front aspect ratio    Lower aspect ratio     Higher aspect ratio
Rear aspect ratio     Higher aspect ratio    Lower aspect ratio
Front tread depth     Reduce                 Increase
Rear tread depth      Increase               Reduce

Wheels---
Front wheel width     Wider                  Narrower
Rear wheel width      Narrower               Wider
Front wheel weight    Lighter                Heavier
Rear wheel weight     Heavier                Lighter

Alignment---
Front wheel camber    More negative          More positive
Front wheel caster    More positive          More negative
Front wheel toe       Toward toe-out         Toward toe-in
Rear wheel camber     More positive          More negative
Rear wheel toe        Toward toe-out         Toward toe-in

Anti-sway bar---
Front                 Soften                 Stiffen
Rear                  Stiffen                Soften

Spring rates---
Front                 Soften                 Stiffen
Rear                  Stiffen                Soften

Shocks---
Front                 Soften                 Stiffen
Rear                  Stiffen                Soften

Suspension Bushings---
Front                 Soften                 Stiffen
Rear                  Stiffen                Soften

Brake Proportioning---
Front                 Reduce pressure        Increase pressure
Rear                  Increase pressure      Reduce pressure

Weight Distribution---
Front                 Reduce                 Increase
Rear                  Increase               Reduce

Aerodynamic Spoilers---
Front                 Increase downforce     Reduce downforce
Rear                  Reduce downforce       Increase downforce

[tfung]

Thanks Bart!
very helpful!

[greg]

Thanks for putting this together, Bart - great stuff!

[Bart Carter]

There is a companion post called "Tuning with tire temperatures."

It will give you the information to make your handling adjustments.

[Bart Carter]

I am going to chronicle adjustments to my car as I make them so others can see some real results. My goal is leave the suspension as close to stock as possible, but improve the handling. The M5 handles very well, except for some understeer. So my first goal is to increase the traction up front, but not at the expense of the rear.

The first thing to do was to take some baseline measurments, then change things one at a time.

My starting baseline is completely stock. I have the Dunlap 8080 tires. As the car understeers stock, I wanted to measure tire temperatures to give me some direction for my next modification.

I started out with 42 psi front and 40 psi rear (cold) tire pressures. I then went to a local autocross and took tire temperature readings coming off of a hot lap.

The temps for the front tires read hot to cold from the outside of the treads to the inside. 110 - 95 - 85. This is a good indication that more negative camber would be beneficial. The rear tires read even accross the treads, indicating that they were fine.

With the tire temps so uniform, the tire pressures used seem to be correct. Please note that an autocross is not as good as a skid pad! Your tires do not get up to road racing temperatures. But it was a good measurement to get my first baseline measurement.

So my first adjustment was really adding more tire pressure to the front than I was using at the time. I already knew the M5 understeered, so from past experience I knew that adding more pressure up front would be a good move.

Next stop: wider rims and more camber.

[johann]

Very interesting Bart!

Would a harder sway bar rear reduce understeer by reducing grep rear or does it help the front by reducing weight transfer to the front outer wheel?

Cheers,
/Johan

[Bart Carter]

Originally posted by johann
Would a harder sway bar rear reduce understeer by reducing grep rear or does it help the front by reducing weight transfer to the front outer wheel?

Johan, actually both happen with a stiffer REAR bar.

It reduces weight transfer to the front AND increases weight transfer to the rear. Thus the front sticks better and the rear becomes looser.

My goal is to get the best grip on the front without increasing the stiffness of the rear bar. I think I can do that with some simple but effective modifications.

If one was to really get into adjusting for track conditions, you can have a rear bar made that can be adjusted for different tracks or surfaces. Center would be normally neutral. Then, depending on the surface or speed of any given track, you can add a little understeer or oversteer with the rear bar adjustments.

[johann]

Thanks Bart!

So, Ideally adjustable camber is a better way to balance between oversteer and understeer?

The rear bar for my M3 that I am waiting for is adjustable, hopefully it's not too hard in the softest setting sp it will always oversteer.

Cheers,
/Johan

[Bart Carter]

Originally posted by johann
So, Ideally adjustable camber is a better way to balance between oversteer and understeer?

In general, assuming that you are not putting in too much negative camber, dialing in more front negative camber will take out understeer. This is assuming that the tire is working mostly the outside part of the tire due to the suspension, not due to tire roll or tire size. You can increase tire pressure or increase tire/rim width for this.

Remember that everything is a compromise.

For the street, you would not want to go beyond, say, 1.5 degrees negative camber. Tire wear and other considerations may compel you to go to sway bars, springs, etc. If you feel you need stiffer springs and shocks for more control at high speed handling, then do this first before adjusting camber.

But before you go down your own path, first check with tuners that have success with dialing out understeer on an M3. Then, before you make any decisions, I HIGHLY RECOMMEND you check your tire temps on a skidpad. (see tire temp tuning post). This will give you a lot of answers that you can actually measure on your own setup. I never would have made one change on my car without doing this first.

To get rid of understeer and not change the other characteristics of your car, you can put wider wheels, tires on the front. Be aware of wheel offset when doing this.

Remember, you are looking to increase front traction without decreasing rear traction.

[Bart Carter]

My next modification to reduce understeer was to add wider rims and tires on the front and add some more negative camber.

First, I bought another set of stock rear (9.5 inch) rims and 275 tires. The tires came on the rims, Bridgestone S03s.

To make the rims/tires work on the front properly, I added a set of 3 mm spacers. These did two things: they gave the tires another 3 mm from the struts so they wouldn't rub and gave the wheels about the same offset as the stock fronts so they would track properly.

Important, the 3 mm spacer still allowed the rims to be hubcentric. Longer than stock wheel bolts also are needed.

I also added a set of KMAC adjustable camber plates. They are supposed to give you a 2 degree adjustability, but I only was able to gain a little over 1/2 degree change. My camber changed to 1.1 degree negative and my caster to 7 degrees positive. I am going to call KMAC and ask them about the limitations.

When running the driver's school the next two days, the difference was amazing. Almost all the understeer was gone. I could bring around the rear with trailing throttle oversteer. With a cooldown lap, there was no opportunity to get any tire temps to see just how they were working.

I moved my Dunlaps from the rear to the front and put the S03s on the rear. In retrospect I wish I would have changed the S03s to the front on the second day for a comparison. I feel that a little more negative camber on the front, or better tires on the front would have given me a more neutral handling car. I left the tire pressures at 42/40 cold for the track.

I feel at this point the changes I made are sufficient for anyone driving on the street. I am going to "fiddle" a little more though. Next step, skidpad with pyrometer testing.

[Bart Carter]

BTW, I want to thank all the members who have done these modifications before me, and their feedback, so I could make such informed decisions about what fitments would work without trial and error.

And, especially to Dick Roberts, who pioneered the 3mm spacer solution.