1999-2009 [Subscribe to Daily Digest] |
1. Is there any agreement between engineers as to what constitutes neutral steering? Is there some objective standard, say
mathematical, whereby someone can clearly say above or below this certain number it is understeer and above or below this
certain number it is oversteer? I ask this question because what is oversteer to you might be understeer to Coolknight.
There is an absolute agreement on the definition of over and understeer. There are mathamatical formulas that define different components of a vehicle's "understeer coeficcient". When you sum the appropriate terms you get a number. If it equals zero, the car is neutral. If it is negative it is oversteer, if it is positive it is understeer. The first order term of the equation is "(Static weight at front/cornering stiffness front)-(Static weight at rear/cornering stiffness of the rear)" The two static weight terms basically correspond to the F/R weight distribution. The cornering stiffness term corresponds to the tires, their inflation pressure, and the instantaneous load on them. In the end it is a measure of the balance of lateral force generated by the front and rear tires. Increasing cornering stiffness will increase the lateral force available for a given normal force. Everything GM designs is understeer at it's limits with quite a bit of room before it approaches oversteer.
2. Can Firestone really prove that going from understeer to oversteer really is as dangerous as this opinion suggests?
Yes. Oversteer can be usefull for a skilled driver however it requires counter intuitive inputs from the driver. At thime you need to steer in the opposite direction than your intended path. Also assuming no damping any vehicle that is oversteer has a critical speed associated with it at which the car becomes unstable. (infintesmal input produces infinite output) Oversteer is generally not a good thing to give the masses.
3. I have always read that understeer is bad but Firestone suggsts that it is not. When is it good and when is it bad?
Car magazines and skilled drivers don't like _huge_ amounts of understeer. They tend to prefer vehicles that are very close to neutral steer in most conditions but can be coarsed into over or under steer through application of the throttle or other inputs. Most passenger cars have tons of understeer to keep the masses safe but it does not make for a very fun driving experience.
4. How would you characterize the 9-5's understeer/oversteer gradient?
The 9-5 is definately understeer. (more so than I like but not enourmously so.) With adjustments in tire pressure you can change the cornering stiffness of the tires and therefore the under/over steer characteristics. When doing this I advise a few things. Start with the manufacturer's advised tire pressures front and rear. From there increase front tire pressure. Do not increase the front tire pressure to one that is greater than the max printed on the tire sidewall. Drive the car hard and see when it feels good to you. Be careful when doing this since you may have chagend the way your car handles and may be oversteer which can be dangerous if you are not expecting it. My base 9-5 was fairly well balanced with the front tires at 39 to 40 psi and the rears at 30 to 32. I forgot the numbers exactly. My Aero is running 44 psi up front and 38 psi in the rear and is still understeer. (less so than stock) The ride is not great aat these settings but I am fearful of rim damage and the higher pressures decrease the likelyhood of that.
When playing with tire pressures there are a couple of things to be careful of. As the tire pressure goes up the energy absorbed by the tire decreases in a pothole event (less chance of rim damage and more load going through the suspension) The ride will also be degraded with increased tire pressure. By the way, the reason that I advise on increasing the front tire pressure and not decreasing the rear is to increase your overall lateral force capability. Otherwise have fun playing and be careful...
-Joe
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