19 replies to this topic

[DEXISWOLF]

A mini specialist has told me that a 3.44 final drive on a 998 with 13s would be near undrivable and to get something like a 2.9 but that would be city e stunned slug territory wouldn't it and I already have the 3.1 ratio so my question is would it work wih 13 inch wheels :)

[jaydee]

Does he know that 13" on a mini are low-profile?
If he knows, tell him that the old 997 cooper had something like a 3.6 diff on 10" wheels....

[Cooperman]

The tyre size is unimportant in calculating the gearing, what is important is the rolling radius.
To compare the rolling radius of a wheel/tyre combination do the following:
take the tyre width (e.g 165) and multiply that by 1/100th of the aspect ratio (e.g. 1/100th of 60). Divide the result by 25.4 and add the radius of the wheel.
So for a 165/60 x 12 wheel/tyre, the rollong radius would be:
165 x 0.6 = 99 mm. Divide by 25.4 = 3.9". Add the wheel radius which is 6" anbd the rolling radius is 9.9".
Now do that for a 175/50 x 13 and you get 175 x 0.5 = 87.5 mm. Divide by 25.4 = 3.44" + 6.5" = 9.94" rolling radius.
So the 12" and 13" are almost the same.
A 3.1 FDR will be OK, but a lower FDR, such as a 3.44, would be better for a 998. The original 997 and 998 Cooper had a 3.76 FDR to improve acceleration.

Edited by Cooperman, 01 August 2012 - 11:44 PM.

[DEXISWOLF]

Thanks guys I already bought the final drive but he had me worrying he did have a mk 1 and I pointed out that that would have the 3.4 or lower but he just said well that's on tens

[JVA10L]

The tyre size is unimportant in calculating the gearing, what is important is the rolling radius.
To compare the rolling radius of a wheel/tyre combination do the following:
take the tyre width (e.g 165) and multiply that by 1/100th of the aspect ratio (e.g. 1/100th of 60). Divide the result by 25.4 and add the radius of the wheel.
So for a 165/60 x 12 wheel/tyre, the rollong radius would be:
165 x 0.6 = 99 mm. Divide by 25.4 = 3.9". Add the wheel radius which is 6" anbd the rolling radius is 9.9".
Now do that for a 175/50 x 13 and you get 175 x 0.5 = 87.5 mm. Divide by 25.4 = 3.44" + 6.5" = 9.94" rolling radius.

That will NOT give you the rolling radius, the only way you can measure this is by marking the tire and the ground. Roll the car forward several times, mark the floor again and measure how far between the marks and divide by the number of revolutions you the wheel turned.............

What you are measuring there is the rolling CIRCUMFERENCE. This can be calculated from Coopermans rolling radius as follows:- Diameter x Pi (in other words 2r x 3.1412).

[cooperrodeo]

Dilligaf is correct.

[Ethel]

Surely they're both valid ways of comparing wheels & tyres. I take Dilligaf's point, but you'd also have to consider tyre pressure and differing centrifugal effects for different tyres.

All "standard" Mini wheels & tyres are similar enough to make much less difference to gearing than final drive swaps.
3.44 is the best all rounder final drive ratio, you certainly wouldn't want anything taller on a 998, unless you're more interested in fuel economy.

[mike.]

The final drive in my 998 city e was a killer. 2nd gear was needed for steepish hills and was slow accelerating.

Now have a 3.44 in my 1275 and that seems much better suited to a mini, 1275 or otherwise.

[Ethel]

It's also why it's irrelevant if you are trying to choose between components you don't yet own. You are right, if you want to know accurately compare engine revs to distance travelled. You are measuring the rolling circumference however.

[tiger99]

Centrifugal effects have been mentioned, but are not actually included in either method of measurement. However, I would suggest that error will be fairly small. I would also suggest that, as long as you are consistent, either method of measurement will be useful for comparison purposes.

If you want the true rolling radius at speed, including centrifugal effects, you probably want a device to count drive shaft or wheel revolutions and an accurate GPS, or just use the tyre manufacturers data, where available. A bit of Googling might find it.

[Cooperman]

I've used my method for comparison for over 50 years in many different types of car for competition. My gearing usually works out OK, so it is an excellent guide. It can be expensive to have to buy lots of different tyres and wheels, then fit them and measure them to get the gearing correct. Remember, the overall gearing can easily alter by 2% to 3% due simply to tyre wear for a given size or for different tyre pressures for different conditions. For example, a 165/70 x 10 will give a different overall gearing when pumped up to around 40 psi for a test on a race track compared to when set to 30 psi for a slippy and slightly loose surface.
What the calculation as I described gives is a fair comparison.

[JVA10L]

I now understand why Cooperman ignored the contradiction to his very clear and accurate post on how to calculate rolling radius.

[Guess-Works.com]

IMO they are not inaccuracies, just rather a method of negating variable errors...

My ratio calculator http://www.guess-wor.../Tech/ratio.htm uses a similar mathematical formula, which over time has proven to be one of the most useful resources available for both competition and road users in determining gear ratios, final drive and wheel sizes, as well as solutions to the inevitable speedo inaccuracies associated.

[Guess-Works.com]

No I'm afraid is isn't...

I think you're confusing accuracy with variability...

When measuring the rolling distance of a wheel, as you do, you are only measuring it for that wheel at that tyre pressure on that car, on that day at that altitude at that temperature, and the list of VARIABLES goes on...

And I would put money on the fact that if you then replaced that wheel with another wheel off the same car and did the measurement again you would get an ever-so-slightly different result.

This is why using a mathematical calculation gives more reliable and comparable results, ok may not be 100% true to real life, but it's near as damn compared to drawing chalk lines on a wheel and the ground and using a tape measure.

[Ethel]

I don't think there is an argument, everyone is making valid, but different, points.

You're right about the definition and significance of "rolling". Cooperman was also right in his advice of how to calculate the effect of different wheels 'n tyres on gearing.

Anyone reading will have benefited from both posts