Slip angles - Rob Fleming
Would someone be kind enough to explain how slip angles, tyre pressures and the driven axle affect the handling of a car?

Thanks

Rob
Re: Slip angles - mike harvey
Rob,
Without the aid of diagrams, the slip angle is a vector of the lateral and londitudalal forces between the tyre and the road. In a straight line, all of the forces are longditudanal, but when cornering, lateral forces begin to apply. If these forces added together become greater than the ability of the tyre to grip the road, the tyre will skid. Tyres have best adhesion to the road surface, curiously, when they are slipping around 10-20% ( It varies with the compound, construction etc.) Hence the slip angle, the point of maximum grip, is not truely reflective of friction, and forces can exceed the theoretical maximum of 1 G. The ackerman steering principle, which is supposed to ensure that both steering wheels turn at the correct angle for the radius of their respective arcs is not used on modern cars because the designers plan for an amount of drift on cornering.
Hope this helps, it makes sense to me because I've written it!
Cannot help with tyre pressures other than for the car to handle well,the tread needs to remain flat, in contact the road surface. When cornering, the tyre sidewall deflects and rolls the tyre over, lifting the inner edge away from the road surface. To some extent camber takes care of this, but if the pressures are too low, the sidewalls will distort and reduce the 'footprint'. Normal pressures are a compromise, and you may need to adjust them to your preference. You may pay the penalty of increased wear rate though.

regards, Mike
Re: Slip angles - Stuart B
To add to Mike's excellent description above, I think tyre pressures affect the handling only in so far as by adjustments you can affect the degree of grip at either end so balancing the car out, or maybe deliberately unbalancing it if that is what you want to do.

As for effects of driven axle, to keep it simple the tyres have only so much grip and the more you ask them to do, ie in terms of applying force to the road surface by cornering, braking or accelerating, then the more chance there is of the adhesion being lost.

Therefore with a fwd car the front axle has to drive, steer and most of the braking, so this type is likely to understeer first when the limit is reached. Very often the back wheels do not do a lot more than stop the back end from dragging along the road, an exaggeration I know but I think it's fair comment.

So with rear drive the task of all the tyres is more balanced and the car will either naturally oversteer or understeer dependant upon the suspension set up, unless a deliberate attempt is made to unbalance it into oversteer by using excess power or upping the tyre pressures, playing about with the brake balance whatever.

Four wheel drive in my experience, which I admit is limited, I find reacts inherently like a fwd car but more balanced if thats any help. Plus dependant upon the power split front to rear can be made to give power oversteer, but you are usually going a bit quick when that happens.

My personal preference for quick driving is for a rwd car which is set up to give slight understeer, then if you do go sideways it is an inherently more stable situation than a determined oversteerer.

As Mike said I wrote it so I know what I mean hope you do too.
 

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