Nowa

Hi all,

I recently did something I regret (I think) I bought a set of new 18" alloys (for my 05 WRX) and I had a choice of having them fitted with 215/35 or 225/40 tyres (same price). I choose the latter without even thinking about the acceleration drop. I still have my original 17" alloys with my original 215/45 Bridgestone tyres, quite heavily worn so soon to be fitted with new winter tyres and I "think I can feel" the drop in acceleration with my new wheels

Are there any math geniuses out there who can tell me how much of a drop in acceleration I got with these new wheels? Is the acceleration directly proportional to the circumference, so is it just a matter of calculating the circumference of the two wheels, then divide the difference by the smaller original wheel to get a % in drop?

Many thanks in advance!

g7prs

iTrader: (5)

The circumference should remain the same i think, as you go up a wheel size you come down in tyre profile. Is it not to do with the greater weight of the bigger alloys?

scoobiewrx555

You are not going to feel a 1.9% difference in tyre diameter from an acceleration point of view. The difference in 0-60 will be miniscule.

What you will have though is less of an error between your speedo indicator and your true speed. Normally with the 215/45/17's when your true speed is 70mph your speedo will be reading c.75mph. The difference in tyre diameter just means your speedo will likely be reading c.74mph instead now.

Weight difference won't be a lot either. Your 17" WRX wheels are actually quite heavy. It's likely your new 18" wheels will either weight about the same or may even be a tad lighter as is often the case with decent aftermarket wheels.

Nowa

Thanks for your reply, well, you could very well be right, the new wheels are heavier, but they are bigger as well, at least higher, I'd say about an inch...

Nowa

Thanks for the explanation! Yes, I did suspect that my speedo would be slightly off with the new wheels, and unfortunately, the new wheels are heavier... Or perhaps I'm imagining things at least they felt a bit heavier when I changed them... They are called "Inovit Redline"

Aaron1978

iTrader: (12)

this is one of the most useful things i've come across on snet so saved it.

Car Bibles : The Wheel and Tire Bible Page 2 of 2

Also the inovit redline wheels where known to be heavy, the early sets anyway.

Aaron

carl heath

you didnt actually read that lot did you ? lol

Potiriadis

Have to say,

I swapped the standard WRX rims on my 07 for 18" Inovit Redlines, and I was convinced they were a shade lighter. Not as light as PPF7's would have been, but then you pays your money.....

BlueBugEye

iTrader: (12)

If you've gained an inch in height it sounds to me like you have the wrong tyres fitted - when I swapped my standard 17" WRX wheels for 18" PFF7s not only were the wheels considerably lighter but they were virtually the same rolling radis and therefore height etc.

rob200970

I fitted 18" Inovit Redlines to my 2006 WRX and have not noticed any difference in Performance apart from better handling round the Nurburgring. I fitted 225/40/18 Goodyear F1 Assymetrics to them, and the total rolling diameter is the same (give or take a couple of mm). The wheels themselves are obviously going to be an inch bigger as you have gone from 17" to 18", and as long as you have gone down from 45's to 40's, there should be no obvious difference. Stick with them, they look great!!

RareRims.co.uk

If there are two wheels, both the same weight, one 17" and one 18", then you will still have a performance decrease
This is due to where the weight is located, having a greater effect.

The heaviest part of the wheel is the 'barrel' of the wheel, this being 1/2" further out will mean that the leverage effect is greater, obviously the tyre weight is also concentrated further out from the centre of the wheel.

In terms of performance decrease, the only thing we have tried that gave a tangible result was Dixon Cheng and his Integra type R

He swapped 15" Slipstreams for 16" wheels of another make, kept the same make and model of tyre, and couldnt get within 3/4 of a second of his previous lap time.

Now this is on a relatively torqueless Integra, so you shouldnt notice the difference to the same degree on a subaru.

In terms of wheel WEIGHT and difference, its said that every 1kg of wheel weight, is equatable to 10kg of in car weight.

So lose 5kg off the wheels and its like not having a passenger.

dunx

iTrader: (3)

Technically it's the inertia of the wheel/tyre assembly that affects acceleration....

As stated you can feel a difference even if ( like mine, 17" vs 19" ), the overall weight is the same.

The grip is in a different league, but the acceleration is blunted just enough to feel through the "bum-dyno".

dunx

eggy790

iTrader: (20)

How true is this?

dunx

iTrader: (3)

In terms of work done fighting the inertia of a larger heavier wheel, it's spot on...

However, losing 5 kg is quite easy if you are trying. The problem is that you may "need" a certain tyre width and thus the wheels width is almost fixed.

You really only need to do some maths.

215/35/17 vs 225/45/17 vs 235/35/19

907 vs 832 vs 809 wheel revs per mile

-9% vs 0% vs +3%

dunx

P.S. I can feel the 3% difference my "show" wheels make...

P.P.S. The other option is a possible next choice