Acceleration differences...

[Alfamania]

I was just wondering of your opinion on this:

If a car has 14 inch wheels from factory and by me fitting 17 inch wheels with the closest similarity of circumference would it accelerate just as fast as it did with the factory wheels? I think not....What do you guys think? 

[Cool Jesus]

Ok, testing out our knowledge on the science of physics then.
Thinking logically, if the rolling circumference is unchanged then there should be no difference. However there is a change in the profile size of the tyre. Large profiles give more flex to the wheel's dynamics, so cornering, bounce and shock absorption to just name a couple and gave better or nicer ride comfort. I'm picturing photos of drag racers, with their huge tyres and you can see the rear wheels folding up on the side walls when they take off to give the running gear some flex with the torque being used. Certainly not saying the 14" wheels are doing this, but they would be flexing a little. Well, in my mind the lower profile does away with these attributes and hence you would find changes (for better or worse) in cornering, bounce and shock, and even acceleration perhaps. You may even find wheel weight to be lighter, meant less weight for the engine to initially turn.

So, a smaller (or even equal) circumference would give faster acceleration but require more engine rpms to maintain speed. Large circumference would require greater engine rpm to equal the acceleration but less overall rpm to maintain speed down the track.

But in real terms the differences may be irrelevant as to what you can feel as the driver. If anything, with similar rolling circumference you may find the engine not work as hard as it did with the 14" as I'm sure you'd be loosing a couple of kilos at each corner with current larger alloys.

[extraball]

http://www.caranddriver.com/features/effects-of-upsized-wheels-and-tires-tested

[Duk]

The heaviest part of a wheel is the rim itself. The part of the wheel that has the tyre beads against it and forms the metal side of the air container.
The further this part of the wheel is from the axle line, the greater the polar moment of inertia the wheel has. So all things being equal, over all diameter and over all mass, the 17" rim will have more 'flywheel effect (it'll take more effort to accelerate it at the same rate and decelerate it at the same rate)' than the 14" rim. Even tho the 14" wheel will have more and so heavier, side wall, the effect of the actual beaded area of the rim will be greater over such a large (1.5") distance.

And drag tyres distort massively so that there is more rubber in contact with the track to provide more grip/traction.

Case in point about the effects of polar moment of inertia and how its effects can be calculated.
In this case how the mass of an object, when it's at the front of a car, can effect the car's turn in and understeer characteristics.
Ages ago I looked at moving the (V6) engine in my 75 back 1". Obviously it was a lot of work to do such a thing, but I was lucky enough to have access to Fred Phun's 'How To Make Your Car Handle'. In that book it shows you how to calculate such changes.
Long story short, by moving the engine back 1 inch, I would have reduced the engines polar moment inertia effect on the car by about 5%.
I also calculated how much of an effect removing the battery from the front of the car would have. That calculated to being 3 times the effect of moving the engine back 1". Now obviously the battery weighs a lot less than the engine, but because of its distance from the longitudinal (front/rear) centre of gravity of the car, it has heaps of polar moment inertia effect on the car. The impact absorbing type bumpers have even more effect again.

[colcol]

And when General Motors went from a straight 6 in the VL to the V-6 in the VN the centre of gravity was lowered and the weight mass was moved back more to the centre where you want it.
Look at a Commodore road car and then a V-8 Touring car and everything is placed towards the centre of the car and lower than standard, such as the wiper motor goes from the scuttle to the floor, it all helps, Colin.