Coping with understeer predicamen

[GreenV8s]

number 46 said:

You seem to be saying that you can have 6 quaters of grip !!! What!!

That's vector arithmetic for you, 1 + 1 doesn't always equal 2. I did say it was unintuitive!

[TVR Bob]

GreenV8s said:

number 46 said: 'In particular, gentle acceleration tends to encourage understeer, while engine breaking or hard acceleration both tend to encourage oversteer' Interesting- How does it do that then !!!!!! I'm afraid I don't know a short answer other than 'it just does, OK?'. The long answer is that it's due to the rather subtle interaction between two things that happen when you accelerate or brake. The first effect is that torque applied to the wheels uses grip in the direction of travel and leaves less grip available for lateral acceleration. Because of this effect, if you accelerate or use engine braking in a rwd car you will increase the slip angle on the rear wheels and encourage oversteer. This is a non-linear effect normally referred to as the circle of friction. What's unintuitive is that small amounts of torque have very little effect on the amount of grip left for lateral acceleration, in fact even if you use three quarters of the available grip for acceleration/braking you still have roughly three quarters left for lateral acceleration. This can seem quite strange unless you're use to dealing with vectors. The second effect is caused by weight transfer between the front and rear axles. Acceleration transfers weight off the front wheels onto the rear. For a given lateral acceleration this increases the slip angle at the front and reduces it at the rear - it encourages understeer. This is a linear effect - twice the acceleration causes twice the weight transfer. Notice that this tends to counteract the first effect. As you accelerate there is one effect causing understeer and one causing oversteer. So what happens? The answer is quite interesting (well I think so anyway). Under gentle acceleration the second effect dominates and you get understeer. Under hard acceleration the first effect dominates and you get oversteer. Of course the terms 'gentle' and 'hard' are relative to the amount of grip and the center of gravity height and wheel base of the car. Finally think what happens when you apply engine braking. In this case both effects increase slip angle at the rear wheels and encourage oversteer. Lifting off mid-bend is a good way to end up in the scenery if you are in a powerful rwd car. Hope this makes sense, it's rather hard to explain it without pictures!

That's what I (sort of) said in my previous post!

[GreenV8s]

TVR Bob said:

That's what I (sort of) said in my previous post!

Yes, I think you are a quicker typist than I am!

[HarryW]

Making it a bit clearer Peter .
Interstingly (to me anyway ) did a track training day a few years ago when the sharp lift off whilst understeering was taugh to promote oversteer,seemtoremeberthe first couple of times the oversteer coming in a bit too quick . Conversely there was a raod going AJP Tuscan (grant) there too , who with the speed required to get it to understeer for the given radius corner couldn't get it to understeer in the first instance .
Very satisfying to get right though. However I haven't practiced it enough to apply the principle,so rely on the wait for the front end to regain grip before getting back on it. Have done the car park bit in the snow before now to practice the wind the steering off whilst applying the power,agian very satisfying and at very safe 'snow' speeds.

Harry