Kinematics on historic rally car

Non-engine, eg: aerodynamics, gearboxes, brakes, suspension
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HSEXPERT
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Kinematics on historic rally car

Post by HSEXPERT »

Hi Guy

As you know better than me, kinematics is a very important issue discussion nowadays, also on what rally historic cars is concerned.
We can see i.e., a big difference on tyre wear on 2 rally cars exacly the same, same power, same suspension/brakes, but with different weight distribution, and most important with a different position engine/gearbox and rear axle or even the tank/spare wheel (lower on car).
The thing I need to know better is what's more important, on a RWD: to put the tank and spare wheel mostly on top of the rear axle, or put them on rear end boot, but lowering them on chassis (as we can't do both...)?

Thank you,
Paulo
Guy Croft
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Post by Guy Croft »

Hi Paulo!

Good question. I think we may be talking primarily about the ‹Å“Polar Moment of Inertia¢ž¢ (PMI) here - one characteristic of handling that dictates the ease with which a rally car will change direction.

The polar moments that control the behaviour of the car when changing direction are a function of the mass of vehicle forward and rear of the car¢ž¢s centre of gravity and their distance from it, ie: the longer the car and the greater the imbalance of F-R weight distribution the worse the response to turn-in. Think of a Lancia Stratos if you want a prime example of low PMI.

Conversely a car with a high polar moment has high directional stability (ie: it resists change of direction).

The more weight you hang outboard of the axles at front/rear the greater the 'pendulum' effect too, same thing you get when the weight distribution on a car trailer is wrong, the trailer tries to swing the back end of the towing vehicle.

So the answer is - if you want quick turn-in - to put as much weight as possible near the middle of the car, front nad rear, and in the case of your particular question, I guess, over the axle not to the rear of it. Yes, the height of the installed components will affect the behaviour in roll of the vehicle - but - I figure that if the suspension is sufficiently stiff the latter will be insignificant compared with the performance gain from reduction of PMI.

I hope that helps,

GC
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