Physics of car understeer and oversteer

[user079622]

I am interested in physics behind oversteer and understeer for car in turn, how position of c.g. affects car drive dynamics.

I will assume that aerodynamics forces are 50%/50% on each shaft.

If I move c.g. toward back, friction force at back wheels increase but centrifugal force increase as well, so how I can anaylze if I want to know will car oversteer or understeer?

 

[berkeman]

https://en.wikipedia.org/wiki/Understeer_and_oversteer

 

[sophiecentaur]

I am interested in physics behind oversteer and understeer for car in turn,

You really need to read around this topic rather than rely on just one or two suggested sources. It's an engineering topic and the terminology and the explanations can often not totally appear to agree amongst sources - so you have to read round.

There is a quantity in car steering that's referred to as slip angle this is basically the difference between the direction that the tyre is pointing relative to the direction the car is travelling. The tyre doesn't so much 'slip' when turning. It is the distortion of the rubber at the forward and rear parts of the contact patch with the ground. There is some actual slippage between the tyre surfaces in the times when it is settling into contact and releasing contact (like the action of a clutch) but there is also significant distortion between those phases. The tyre sort of 'walks' over the ground. The greater the lateral force, the greater the slip angle. Whether a car oversteers or understeers depends on the difference between slip angles of the front wheels and the rear wheels. Greater slip at the front requires more turning of the steering wheel. That's understeer and is a fairly stable way of cornering. Greater slip at the rear wheels requires careful use of the steering wheel or the rear wheels can slip sideways. That can require immediate correction of the steering wheel to avoid doing a 180. The relative slip will, as you suggest, depend on the position of the centre of mass of the car in a fairly obvious way (but not too obvious; more reading round is needed because friction between tyre and road is not a linear business).

Once you've grasped a bit about slip angle it starts to make some sense.

 

[user079622]

@sophiecentaur

Why all sports car have c.g. in the middle(or close to middle), has this something with my topic?
Porsche 911 has engine behind rear axle, is hard to stop momentum of heavy engine once when oversteer happen?

 

[sophiecentaur]

@sophiecentaur

Why all sports car have c.g. in the middle(or close to middle), has this something with my topic?
Porsche 911 has engine behind rear axle, is hard to stop momentum of heavy engine once when oversteer happen?

There's never a simple answer to questions about cars. Rear engine and rear wheel drive avoids the need for a transmission shaft. A transmission shaft would mean the driver would need to sit higher (not good). Race cars have different sized wheels front and back and very little mass at the front. Good for steering response, I suppose. Front engine can allow for more load and passengers and getting at the engine for repair (old cars particularly). So many factors to balance.

BUT. You won't learn about car physics (or cars or Physics, btw) by questions and answers. You can't rely on your questions being the most relevant and there are a lot of blind alleys. Google (and other search engines) is the way to go and also books on race car design. My main source is an ancient book by Colin Chapman of Lotus cars (iirc) called The Sports Car. Mostly based on the Jaguar XK 120 and as old as I am. AAAHHHH!

 

[user079622]

@sophiecentaur

This is my man! Do you know who he is?

 

[sophiecentaur]

@user079622
Nah, I'm no petrolhead. He looks a bit more modern than my guy though. Does Google help you for info?

 

[user079622]

@sophiecentaur

If you are from UK, you must know who is Sir Adrian Newey!
Yes it helps, but I thought that this is not complex subject...That I only need include centrifugal force and friction force...

 

[sophiecentaur]

but I thought that this is not complex subject...That I only need include centrifugal force and friction force...

If you read about the way tyres behave on roads you will soon find that it is a very "complex subject".Also, the suspension is a massive contributor to cornering ability and has been responsible for many accidents on bends because the tyres have to be in full contact with the road as much as possible.

Also, as I said before, I am not a petrolhead and have never watched F1 racing (even on TV). I know names of a few drivers and also that horrible little man Bernie Eccleston. 'Nuff said.