Front/rear track width effects

[rpmag]

I have accumulated some basic data on sporting car suspension. All are RWD with front or mid-engines.
Approximately 70% have a wider rear track and the remainder wider or equal front track. Switch to a racecar list and the percentage is reversed to 70% wider front track.
This racecar % could be accounted for by the need to accomodate rear rim width's, but is there another reason why there is such a predominance of wider rear track sporting road cars?

 

[hemipanter]

I did checked mu tire spring rate up using the cornering weight scales. You might allready know some numbers but here is mine anyway. 40 kg/mm, at 1,5 bar. Dimension 15 wheels x 13 wide x 24,5 diameter Avon slicks.
Goran Malmberg

 

[BillyShope]

That's reasonable. I usually use 1500 pounds/inch (about 30 kg/mm) in my calculations, but, for my purposes, I could probably be just as happy using your number. Unless you're a working engineer, tire data is not that easy to come by.

 

[GregLocock]

That's very stiff, almost twice what a typical production tire is at that pressure. Rule of thumb: pressure is about 80% of the spring rate at 2 bar, the rest is sidewalls.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[hemipanter]

I checked the roll stiffness out using my ½ scale model. The A-arm is parallel and equal length in order to lessen Wr changes. The chassis has a bearing at ground level to create a fixed Rc so that a force applied only create a rotating moment to the chassis.

With a Tw of 600 mm we will now apply a force to the chassis at Cgh distance. The chassis roll get 9,5 dgr. Using the same force but a Tw of 800 mm the chassis will now roll 7,0 dgr.

As the suspension system here is a parallelogram the Wr in both the 600 and 800 cases remains the same. During heave there will therefore be no difference in deflection.

Goran Malmberg

 

[rpmag]

an update for those who may be interested. Going through my database of front/rear track differences in sportscars a new fact has come to the fore. When I compared F/R track in cars released in the last 4 years the ratio has changed from that previously noted. It is now 3:1 wider at the front.
Wider at front:
Noble m15 89mm wider at front
Ferrari F430 53mm wider at the front
Lamb Gallardo 30mm wider at the front
Porsche Cayman 42mm wider at the rear