Setting car height 15” rims

[Sailkane]

Current wheel setup is 8 ( custom ) and 9.5 x 15”.
Running 225/45 Front (23” diameter)
245/40 Rear (22.8” diameter)
22&29 tbs
Spring plate calculator says@ race height 19.67” angle.

Rough fender measurements ?
F24.5 R 24? Does this seem to low?

Also do you run any rake in your cars?

This is an Autocross car.

[Bill Verburg]

Originally Posted by Sailkane

Current wheel setup is 8 ( custom ) and 9.5 x 15”.
Running 225/45 Front (23” diameter)
245/40 Rear (22.8” diameter)
22&29 tbs
Spring plate calculator says@ race height 19.67” angle.

Rough fender measurements ?
F24.5 R 24? Does this seem to low?

Also do you run any rake in your cars?

This is an Autocross car.

You've left out some pertinent variables
ET of the wheels? w/ your set up the closer the front is to ET27 and the closer the rear is to ET19 the more flexibility you will have

camber? Hoosiers like camber and the more camber the more room for the tire neaar the lips

looks like Hoosier tires?

w/ the right setup the chassis can go lower( can and should are not the same thing), I run 225/45 and 255/40 x17 on 8 & 9.5 at very close to the height you spec

assuming the wheel ET's are where they need to be the main issue is bump steer from the front geometry

you definitely want steering rack spacers and o/s tie rod ends, max on the spacers and the o/s depends on A-arm angle and camber, there is a limit here as well when the ball joint starts to run into the wheel

The front A-arm should be no more than horizontal and ideally should point down as it leaves the chassis, this ensures minimum bump steer and best possible instant center heights which means best possible roll center height

[Sailkane]

Ok…but as with your car your rear tire diameter is 25”.
If you dropped to a 15” rim with a 22.8” diameter do you need to raise your car body height to achieve the same #’s? Or do you just subtract the 2.2” 25” diameter - 22.8” diameter) from the new rear height and set it at 21.8” rear height.

[Bill Verburg]

Originally Posted by Sailkane

Ok…but as with your car your rear tire diameter is 25”.
If you dropped to a 15” rim with a 22.8” diameter do you need to raise your car body height to achieve the same #’s? Or do you just subtract the 2.2” 25” diameter - 22.8” diameter) from the new rear height and set it at 21.8” rear height.

Back when my car was new I ram 7 & 8 x15 205/50 & 225/50 tires, tire OD's 22.8 & 23"

I'd really need to do some digging for the fender heights, I had all that in a notebook that i haven't seen in a few decades. What I do remember is that the top of my shoes barely fit under the front rubbber chin spoiler. So probably ~ 1" lower than now

the limiting factor is lowering w/ the right wheels and tires is suspension geometry, particularly in front

A sure eyeball indicator of geometry at least in front is the angle of the A-arm

This is very bad


This is lowered from stock ~108mm and still good, a rack spacer can correct this when you get to ~ 140+ you really want the o/s tie rod ends





stock compared to race height


vorspur is toe in, the change in this value w/ suspension travel is bump steer, ideally a vertical line, biggest possible (+) linear slope is next best, lowering the car w/o compensation moves the static value higher, upper red horizontal line, rackspacer and o/s tie rod ends move it back down

the bigger t-bars will greatly limit travel and thus limit bump steer, but the roll center still needs to be kept as close as possible to the mass center line, the greater the down angle of the a-arm and the lower the static camber the higher the roll centers will be

[HarryD]

More important that ride height us corner balances. My car was lowered and corner balanced by a very good race shop. One side is 1/2” higher than other. Corner balances are dead on.

Anyone who has driven my car in the track is impressed about its handling.

[regnihe]

Different than the OP questions, but related. Race application. Rule change has allowed us to remove weight from the car. (87, 911). Started by taking about 100 lbs. of weight plates our of the passenger floor. Removed another 30 lbs from various places, mostly from the front. We checked corner balance and were very close to optimal.

Question. Is there a diminishing return were removing "low" weight, like weight plates begins to loose effect because the CG is rising?

Thanks.

[Bill Verburg]

Originally Posted by regnihe

Different than the OP questions, but related. Race application. Rule change has allowed us to remove weight from the car. (87, 911). Started by taking about 100 lbs. of weight plates our of the passenger floor. Removed another 30 lbs from various places, mostly from the front. We checked corner balance and were very close to optimal.

Question. Is there a diminishing return were removing "low" weight, like weight plates begins to loose effect because the CG is rising?

Thanks.

The dynamic effect of weight removal other than on straight line accel/decel depends on the length of the lever arm and the mass removed

the lever arm being the distance from the center of Mass, which is roughly midway across the transverse torsion bar tube

from the extreme end front bumper to the CoM is ~2707mm
from the extreme end of the rear bumper is ~1584mm

from the roof to the CoM ~964mm

The moving mass to or from floors, being close to the CoM, has little effect on yaw or pitch dynamics. It's primary effect being on linear +/- acceleration.

[Bill Verburg]

Don't know if this will help the OP but back in the day thsis was the way the factory did it


Also a summary of various 911s over the years

[Sailkane]

As always Bill you have everything I was looking for .
Thanks for sharing!

[Travis Neff]

Bill,

I always love your data sharing. I do have a question on the data you have for the 74 3.0 RS & 84 SC/RS

143 front
14 rear (should this be -14 rather than +14)

Seems odd that the rear is +2mm and the front is -35mm

Also a summary of various 911s over the years
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