Does tyre pressures effect rolling road figures?
04 December 2003, 10:14 AM
#4
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In theory it makes a difference. In practice it tends not to make much if any difference to power unless the difference in pressures is drastic.
Rich
[Edited by RICH WILD - 12/4/2003 10:14:34 AM]
Rich
[Edited by RICH WILD - 12/4/2003 10:14:34 AM]
04 December 2003, 11:18 AM
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Isn't it that in theory it shouldn't make a difference, but in reality it does make a small difference? 
The calculated flywheel figure from a dyno run should not show any difference due to tyre pressure; any gains via less rolling resistance during the power run will also affect the coast down (when drivetrain losses are calculated). At the wheel figures should show the difference.
In reality, higher tyre pressures also results in slighter higher calculated flywheel power. Don't know why - maybe same reason as you can spin the wheels easier in reverse?
Richard
The calculated flywheel figure from a dyno run should not show any difference due to tyre pressure; any gains via less rolling resistance during the power run will also affect the coast down (when drivetrain losses are calculated). At the wheel figures should show the difference.
In reality, higher tyre pressures also results in slighter higher calculated flywheel power. Don't know why - maybe same reason as you can spin the wheels easier in reverse?
Richard
05 December 2003, 12:46 PM
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Huge difference. Think of a baloon with a flat piece of glass squashed onto it. The more air, the less surface area needs to be in contact to create the neccesary pressure to resist each surface.
F=PxA
So the more pressure, the less contact area and hence also less drag created.
However, it isnt so simple, too much pressure will cause the rear to become too bouncy, and will cause lost of optimal traction req'd to take off. Hence a good balance needs to be achieved, and is different in every car. Depending on the acceleration of the car, I would say during maximum acceleration, up to 75% of the weight of the car could be transferred to the rear wheels.
Hence by umping enough pressure to compensate for that short burst of movement may seem idealic, but does nothing for balance and poise later when cornering etc.
Hope that helps answer a small part of your question, and hopefully also give you a clue that the dynamic nature of the engineering is very difficult to simulate, let alone explain in theory.
[Edited by prana - 12/6/2003 12:07:57 AM]
F=PxA
So the more pressure, the less contact area and hence also less drag created.
However, it isnt so simple, too much pressure will cause the rear to become too bouncy, and will cause lost of optimal traction req'd to take off. Hence a good balance needs to be achieved, and is different in every car. Depending on the acceleration of the car, I would say during maximum acceleration, up to 75% of the weight of the car could be transferred to the rear wheels.
Hence by umping enough pressure to compensate for that short burst of movement may seem idealic, but does nothing for balance and poise later when cornering etc.
Hope that helps answer a small part of your question, and hopefully also give you a clue that the dynamic nature of the engineering is very difficult to simulate, let alone explain in theory.
[Edited by prana - 12/6/2003 12:07:57 AM]
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