Auto Racing: Under/Oversteer, wheel drive vehicles, front wheel drive vehicles

Question
Hello,

I have a general question that I hope you could answer thoroughly. I used to consider myself quite a car geek. However, I was reading around the other day when I found this great web page which said that if you want to reduce understeer you need to move the center of gravity to the rear, and if you want to reduce oversteer you need to move the CoG to the front. I used to think that the more weight you have on the front wheels leads to more front grip and the same for the rear wheels. You know when a pickup gets its rear wheels stuck in mud, you get people to sit in the back to get more grip. Horizontal friction is proportional to vertical loading. Of course, if there is "excessive" frontal weight, the nose would simply refuse to turn due to inertia and one would get massive understeer.

Isn't lift off oversteer in front wheel drive vehicles based on this fact? The weight is transferred to the front wheels, thus increasing front grip while the rear wheels lose grip and slide outwards.

What made me wonder more was that I was watching this video about drifting and the guy was talking about the mid-engined MR2. He said the car has a very light nose so there is basically no understeer (exactly what the web page was claiming)! Instead, since the engine is in the back, it was a great oversteering car. That's what the guy said. So I thought it must be a case of HOW MUCH weight is there (front:rear). The Porsche 911 for example has most of the weight at the rear. I haven't driven one but according to what I believed was right, it should understeer a lot. According to what I have recently read, it should have no understeer at all.

I would really appreciate it if you could give me some insight into this topic.

Thank you in advance.

Regards,
Hamza

Answer
Hello Hamza:

You answered your own question in the first paragraph, INERTIA - -  if there's too much inertia on one end of the car, it will certainly lose grip and start to slide.  

In racing cars , we try desperately to get a car balanced and 'neutral' , and the article you read about moving the CG was an attempt to simplify a VERY complicated subject.

The Porsche 911 you mention is a rear engine car and the weight distribution is about 45% 65% with a rear weight bias.  However, the rear tires are larger, and under heavy braking, the car becomes more neutral, so it STOPS faster.  Then in the corners, the larger traction 'patch' of the rear tires evens out the handling .  Porsche has spent decades and probably mega millions arriving at this level, and yet they still have intense competition to deal with.

You are right that basically, the heavy end has more grip , but when you factor in the roll angle, roll moment, and polar moment of inertia, the subject gets much more difficult to understand. I usually try to measure one factor by itself, do what I can to improve it, and then move to another. I eventually end up with a better perfoming car, but understand, I am not an engineer. I work a lot from the seat of my pants.

What happens in a race car is that as one enters a corner, the car is under braking so there is a maximum weight transfer to the front. The extra weight becomes part of the inertia and overloads the gripping capacity of the tires. The car understeers. We then work to reduce the pitching action and keep some of the weight in the rear, so braking is more effective, and steering works better.   Then when we add throttle, weight transfer moves back to the rear.  Wheel spin will cause considerable overster, called power oversteer. Now if the car tends to roll heavily, then the inside tires are unloaded and not gripping, if the back is lighter than the front we will now have power oversteeer, AND roll oversteer.

I frankly thing that front wheel drive is an abomination and have done no work with them in terms of handling .  I know that all other things being equal, I can beat the pants off one in a race with my 35 year-old rear drive Datsun.  That is partly because with the FWD, they have to turn in much later and can't add throttle as early as I can.  Also, if they suffer wheelspin, they lose steering, where I can use it to steer.

I wish I could offer better answers to your question, but basically grip is a function of the vertical loading as you said. Controlling understeer and oversteer in a racing car is a much more complicated subject.

Good luck

Dan Liddy
Sarasota, Florida USA