velohead66

Just wondering what effect "rolling circumference" has on acceleration.

All things being equal, if I want to accelerate more quickly (say at a trackday), would it be preferable to upgrade to tyres/wheels giving a larger or smaller rolling circumference.

And how does this work, in terms of physics.

All comments appreciated, thx.

scoobian

smaller circumference = lower gearing = quicker acceleration but lower top speed

smaller circumference requires lower profile tyres or smaller wheel diameters or combinations of

John 37

Not always as you can also spend more time gear changing. As an example, if you fit smaller tyres, the 0 - 62 time gets worse as you need an extra gear change. Whether this is an issue dependes on the track you're on. Now wer're talking about tailoring gears to tracks and that's a big subject.
I probably wouldn't bother unless you've got a spare set of wheels.

John

scoobian

this might be handy too
tyre/wheel calc

dunx

iTrader: (3)

I had this idea, 225/35/17 tyres on std STI rims is 7% lower gearing, a cheap +19 bhp over a std. car.
But scale this up to my 400 bhp, and it "adds" 28 bhp, almost enough to feel through the "seat of the pants" dyno.

HTH

dunx

P.S. I ran my classic on 195/45/15's for a while, and it really shifted, and drifted nicely too !

John 37

[quote=dunx;8538627]I had this idea, 225/35/17 tyres on std STI rims is 7% lower gearing, a cheap +19 bhp over a std. car.
But scale this up to my 400 bhp, and it "adds" 28 bhp, almost enough to feel through the "seat of the pants" dyno.

HTH

dunx
quote]

I'm afraid not. Power is a measure of how fast something can be done. By reducing the wheel diameter, you increase the tractive force but reduce the speed. The end result is the same power. That's not surprising since it's the same engine.
For example, a car doesn't give more power in 1st than in 4th.

John

dunx

iTrader: (3)

K.I.S.S. !

for a given mass in a gravitational field, it's easier to call it weight, it's an analogy of the effect of reducing the rolling radius on the cars performance.

Your car accelerates quicker in second gear than in fourth. Doesn't alter the actual power....

dunx

velohead66

So are we getting into the perinial of power vs torque.
What's the phrase, power sells cars, torque wins races.


The car is MY98 with 205/50/16 at the moment.
I want a 4x semi-slicks on a new wheel.

I was tempted by.....
215/40/17 1x wheel = 6.6kg 1x tyre = 9.7kg rolling circ 1897mm

but then I heard smaller rolling circ = faster acceleration.
so maybe a better choice is....
215/45/16 1x wheel 6.2kg 1x tyre = 9.5kg est rolling circ 1885mm

Given trade offs like, flex on tyre wall, weight, acceleration due to circumference, what is best choice.

I suppose gearing (in terms of gear changing) is unimportant to me, at this stage, but interesting to hear it is ultimately relevant.
Top speed is also unimportant.
But acceleration, and speed through corners are the crux - although would these be a trade off too ?

scoobian

in reality, the rate of acceleration seems to be what you're interested in - and this will improve/increase by reducing the rolling circumference
the downside would be that - as previously pointed out - you may reduce 2nd gear such that it interferes with your 0-60 time......if thats an issue, then be careful about your gearing, if 0-060 ISNT that important, but speed thru the gears IS, then its a worthwhile change.

dunx

iTrader: (3)

TBH I don't think you'd really notice a difference of only 2%

dunx

velohead66

Not interested in 0-60 what so ever.

I guess it wont make too much difference, but as long as all enhancements/ mods are complimentary, (eg weight loss of other components other than wheels) then there "may" be a small improvement, overall.

At least I know that particular mod (ie smaller rolling circ) wont be be slowing me down !!