Understeer and Oversteer

December 12, 2006 at 12:02 am Leave a comment

Most cars have understeer built into them, because it is inherently safer. The car when driven into a corner too fast, will tend to become heavy on the steering, run wide at the front, generate excessive tyre scrub, and thus warn the driver to “back off”, with all these signals being transmitted through the steering.

An oversteering car will tend to have the rear run wide, and the driving technique required to correct it is quite unique. Extreme examples of understeer, are high powered front wheel drive vehicles aka SAAB Viggen “goes like a bullet, handles like a lead tipped arrow”, the reviews said. The old Vee –Dub, and the infamous Chevrolet Corvair. as good examples of oversteerers. Both had a pronounced weight bias to the rear. The Chevvy Corvair resulted in a book called “Unsafe at any Speed” by Ralph Nader, which lambasted the American auto industry for manufacturing such a vehicle.

Vee-Dub pressures were set at up to 30 p.s.i. on the rears, and as low as 17 p.s.i. on the fronts, to help curb the excesses of oversteer. In other words, adjusting tyre pressures can affect the steering characteristics of your car.

So if it’s so desirable, why would you want to change an understeering car. Some drivers prefer that the car be “balanced”, and might even go to the extreme of buying a mid engined car such as a (later model) Toyota MR2, or choose a Alfa Romeo with 50/50 weight distribution, front to back. Using reverse logic to the Vee-Dub, you can make an adjustment to your tyre pressures to reduce the understeer on your family sedan, or small F.W.D. An extra 4 p.s.i. (30 kPa) in the front tyres, and lower 4 p.s.i. in the rear tyres, will change the steering characteristics of your car. Of course, the usual disclaimers apply. You must always have enough air in the tyres to carry the load, so don’t do it when the car is fully laden for your vacation.

Other performance vehicle makers will tailor the handling by having a wider tyre on the rear, maybe even a lower profile, so that the rear will always out corner the front tyres at the same slip angle, and won’t overtake the front of the car in a spin, which is very embarrassing, even dangerous for the driver. Yet they only provide one spare wheel!

Entry filed under: Balance & Alignment. Tags: , , , .

Mag Wheels Reading your Tyre’s Message

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