Different sized wheels and Gearing question

[alolympic]

Hi Guys,
Got a question which should have a pretty simple answer but I just can't be sure myself.

Let's presume that a car has a wheel and tyre combo with a different rolling circumference front to rear e.g. larger at rear than front.

If so, which wheels will determine the gearing?
Does it depend on which wheels are driven?

Reason is, I am looking to downsize my front wheels but not sure if this will effect the gearing on my RWD car.
Anyone help?

[GORDON STREETER]

The driven wheels determine the gearing.
I hope you haven't got ABS !

[grantp]

Originally Posted by alolympic

Reason is, I am looking to downsize my front wheels but not sure if this will effect the gearing on my RWD car.
Anyone help?

Apart from the gearing and Gordon's observation about ABS there are a few other issues that might come up especially if you are talking about a road car and a significant change in wheel size.

[GORDON STREETER]

I wasn't going to go into all the other things like caster angle on a Sunday Grant

[grantp]

Originally Posted by GORDON STREETER

I wasn't going to go into all the other things like caster angle the day after a big party.

EFA.

Quite right.

I was thinking more along the lines of simple things like ground clearance, brake sizes and the potential for totally screwing the handling. Of course it is possible that they won't matter much for the intended purpose. But if gearing is an issue for some reason is is just possible that other things might be as well.

[alolympic]

Cool, thanks Guys, glad to get an answer.
No, there is no ABS, the car is a kit car from 1962.
No real original spec to upset, the wheel change is being driven from testing and trial and error.
Brakes will clear, I am only thinking of going from a 16" wheel to a 15". I will try and keep the rolling circumference within 5% of the rears anyway.
Thanks guys

[grantp]

Originally Posted by alolympic

Cool, thanks Guys, glad to get an answer.
No, there is no ABS, the car is a kit car from 1962.
No real original spec to upset, the wheel change is being driven from testing and trial and error.
Brakes will clear, I am only thinking of going from a 16" wheel to a 15". I will try and keep the rolling circumference within 5% of the rears anyway.
Thanks guys

Is this a Rochdale Olympic by any chance?

I seem to recall that there are a couple of people around here who may have useful experience.

[alolympic]

Originally Posted by grantp

Is this a Rochdale Olympic by any chance?

I seem to recall that there are a couple of people around here who may have useful experience.

Yep, although, in reality, it is a Rochdale Olympic body, not much original left.
Just looking to lighten the steering and reduce unsprung weight on the front.