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Re: Steering problems with Aero rims on SPG... Posted by Peter [Email]  (#2804)  [Profile/Gallery]
(more from Peter) on Thu, 27 Dec 2001 22:43:14
In Reply to: Steering problems with Aero rims on SPG..., Jon   [Profile/Gallery]
, Thu, 27 Dec 2001 16:15:13
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What type of tyres (size and brand model) are you running, also what are your pressures set at? What you are experiencing is a camber effect from the road working against the alignment torque of the tyre and the sidewalls. If the vehicle alignment is in specs I do not recommend changing either the toe settings or the camber settings as under different conditions when the car is driving correctly you can experience accelerated tyre wear. Most high performance tyres that have a solid centre rib will have this tendency more then a luxury type tyre. Most people seem to ignore the tyre pressures as a way of resolving handling problems or performance gains. Welcome to the world of give and take. Make sure that the load carrying capacity of the previous tyre at the recommended tyre PSI is the same as the new PLUS sized tyre. I.E. if the old tyre was 800 lb. @32 PSI (SAAB) then the new one should be 800 lbs. @ whatever that tyres PSI is at that pressure. DATA spec books have these calculations. If these are correct on your vehicle you may want to start playing with the tyre pressures. Keep in mind that stiffnes typically peaks at about 20% more than the design value for a pressure 70% more than design. An increase in pressure more than likely will have a reduced effect on aligning stiffness because it creates a shorter contact patch and increases camber stiffness, were a low psi will make for a more comfortable ride but a loss of tyre life and handling are its negatives.
Because the tyre size (width) has been increased, it is normal to for cornering (road ruts) aligning stiffness to increase. This is because the tyre has to ride a inclinating out of the rut. The effect of camber angle on maximum force is dependant the on crown shape. A round tyre (185/65/15) develops a maximum lateral force with a large negative camber, with the optimal slip angle being very small. The larger width of the tyre (205/50/16) creates a flatter crowned tyre, which works best in the 0-4 degree negative camber range. Some tyres being run at negative camber angles compensate for slip angle distortions to help keep a flatter crown on the road.. The tolerance for camber seems to increase with an increase in normal force. This is more then likely due to several contributing factors, increase in pressure, sidewall stiffness, contact pressure.
When dealing with road deviations one must try to find a compromise. Have you made any other alterations to your car besides a wheel/tyre size, increase in track? Are you getting more pull from the solid rear axle tracking the ruts or from the front end. I believe that you can contact your local tyre dealer and find out who ther regional tyre rep is. The tyre rep will be able to supply you with a basic (cheat) sheet, that they give to the local amatuer racers, they sell DOT race tyres to.
I hope that you are able to rectify your problem.
peter
posted by 24.27.70....
, Thu, 27 Dec 2001 16:15:13, Thu, 27 Dec 2001 22:43:14 <-- Viewing This Message
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