Advantages of using bigger Wheels

[Un0Turb0-WP]

Are there any advantages of using bigger wheels on a race car?
I was hoping to use 15" wheels but they wont fit over the brake calipers/discs
So i was thinking of using 17" wheels as they are more common then 16" wheels here...

Just wondering as i've seen some touring cars using 18" even 19" wheels..

Thx!

[greenamex2]

There is a theorhetical advantage, asides component clearance, of using larger diameter wheel, particularly if used with lower profile tyres.

The main benefit would be tyre choice, 16's are pretty hopeless.

The reason touring cars etc have 18 and 19's is due to brake clearance and the desire to appeal to mass markets.

F1 cars seem quite happy on 15's (may even by 13's)!

[Al Weyman]

You also get a larger footprint with the larger diameter tyre/wheel. F1 cars would go even quicker if the rules did not stipulate 13 inch wheels which is why sports cars use 20 inch wheels etc. as I understand it.

[greenamex2]

On the downside, the wheel tyre combination will weigh more.

The 'larger footprint' argument is not a guarantee and is dependant on the outside diameter increasing. Even then there are a number of arguments that say the footprint will just change in shape, not area.

I believe Al's previous Escort Mk2 bloke example could be quoted here!

[Al Weyman]

Very true!

[graham bahr]

not only do bigger wheels tend to add weight, i've got some 8x15's and 8x17's same pattern compomotives and the 17's are nearly twice the weight, the extra diameter adds more to the flywheel effect which is bad news, they also tend to be bad news from an areodynamic point of view, in that they tend to jack the ride height of the car up, and without radical suspension mods, your never get the ride height right down without seriously compromising geometry

[Al Weyman]

On a similiar subject I was admiring someones massive AP discs the otherday and he actually told me that he recons he has gone too big and they would be better a little smaller for the same weight and gyroscopic effects you are describing.

[retro_msport]

I like the Escorts to be on 15's, its a fair compremise between brake diameter and ride height, with a 21.5" tyre they can be run on the chassis rails at full squat (40-50mm static ride height) with 17" wheels serious mods have to be done at the front, to get the tca's at an acceptable angle, to the point of totally scraping the front suspension and starting again if the minimum ride height is to be maintained.
And weight is an issue.. A 13" split rim is seriously light, and the tyre is lighter too, going up to a 17" is adding a fair chunk to the unsprung weight.
I think if the benifits outweight the losses then its a positive thing, so ask yourself do i need that big a brake disc, will i gain in tyre footprint or am i going to have to drop profile so the tyre reamain at the same diameter, is the larger wheel going to add to the frontal area, is it going to mess my gearing up... and a whole load of other questions

[greenamex2]

On the brake front we work on the principle that you fit the smallest brakes that will just allow you to lock the wheels.

Obviously as I run on genuine road tyres this means I have quite light brakes.

[graham bahr]

agreed, as far as i can see with brakes size is'nt the main issue but managing rather managing the heat, and getting the right materials and fluid.

if you can keep a brake cool enough to keep it working and have suffient pad area and effort to lock the it, then fitting bigger discs will do nothing.

when were braking from high speed the caliper must be exerting 100's of lbft braking effort, so if we make the disc 1 inch bigger we gain a mere 0.5 inch more leverage, now whats another 0.5 inch leverage when we have 100's of pounds foot leverage?

[dtype38]

Originally Posted by graham bahr

when were braking from high speed the caliper must be exerting 100's of lbft braking effort, so if we make the disc 1 inch bigger we gain a mere 0.5 inch more leverage, now whats another 0.5 inch leverage when we have 100's of pounds foot leverage?

If you have a 12" disc and you make it 1" bigger, then the same brake pad force will give you 13/12 (roughly) times the braking torque, ie 8% or so more. That's not that insignificant

[graham bahr]

Originally Posted by dtype38

If you have a 12" disc and you make it 1" bigger, then the same brake pad force will give you 13/12 (roughly) times the braking torque, ie 8% or so more. That's not that insignificant

er no, your get half that, dont forget that you might have a 12 inch disc, but your only using 6 inches of leverage, as the disc spins around its centre and your clamping one side only

[dtype38]

Last time I looked 1" extra on 12" was the same percentage increase as 1/2" extra on 6"


Although I bet I know which one my wife would prefer

[graham bahr]

ok not going to get deep in maths its not my strong point unless its calculating compressions and engine sizes,

what i will add is that i have recently had been in dialog with a major brake manufacturer who catagorically stated that unless you massively increase disc size it has very little effect on brake performance, and that the two key issues are pad surface area and material, everything else is secondary

[Al Weyman]

If Graham is wrong then why do they put diddy little disc brakes on racing push bikes as surely the old system brake caliper on the outer rim would be more efficient.

[dtype38]

OK, so to get back to the point of the thread... Advantages of bigger wheels. I think we've decided that its a compromise between the tyre and the rim. The tyre needs to be as large as possible to get a decent length of contact patch without getting so large that the weight and speed have gyroscopic effects that detract from the handling. Within that outer tyre size, the rim should then be as big as possible to a) minimise the tyre wall height to give maximum stability to the tread, and b) give maximum space within for the brake discs so that they can easily be kept cool (not so that they can be as big as possible). Beyond that we get into the specifics of the rules of individual race series and what is permitted. Is that fair?

[graham bahr]

spot on my man

[thatorangething]

Spot on indeed. Hi Graham, still owe you a meal out, haven't forgotten! Don't forget Graham's original posting, which touched on a very significant point- that if you put a bigger overall diameter tyre on any vehicle, then you destroy it's geometry and the c of g is raised too, so unless you plan to relocate every suspension mounting higher in the bodyshell to make up for it, thereby restoring wishbone angles etc, expect an inferior car. Also, traction is a factor, so you have to decide how you want your car to drive. I had a very interesting chat with one of the guys at compomotive a while ago, and he described how they made a HUGE difference to a drag racing fiesta by taking the 15s off and fitting 13s - the extra sidewall height and flex just gave it so much extra bite.

[retro_msport]

Originally Posted by thatorangething

I had a very interesting chat with one of the guys at compomotive a while ago, and he described how they made a HUGE difference to a drag racing fiesta by taking the 15s off and fitting 13s - the extra sidewall height and flex just gave it so much extra bite.

That will be the wrinkle effect of a drag slick, the bigger the sidewall the more wrinkle and the 12" x tyre width contact patch becomes a possibilty.....
But dont go round corners

[Al Weyman]

The idea though is to fit a much lower profile tyre say a 35 with your 18" rims then the overall diameter will remain the same as if you were using 15 or 16 inch stock wheels. Also the opposite to the drag wrincle effect would occur, i.e. stiffer sidewalls enhancing high speed cornering stability, this surely must be a fact or why would the LeMans cars fit 20 inch plus rims as even they must have a limit to the size brake they need and I would have though many would have been inboard aka F1 so not an issue.

[graham bahr]

Originally Posted by Al Weyman

this surely must be a fact or why would the LeMans cars fit 20 inch

i agree to a point, but then again LeMans cars are designed around such big wheels particulary with regards to suspension, geometry etc, and massive budgets, so these huge wheels probably weigh less than the 13x6 minilites on my RS