The effects of wheel diameter

[Al Weyman]

Funny enough that jag size will also fit my car and there are some real bargain sets around att he moment as long as you are not after anything wider than 8.5 inches and as I said previously Graham's post is also why I will probably step up a size, basically tyre availability.

[graham bahr]

Yeh I know, but I've long stopped letting looks bother me too much.

yeah i think most of us start out like that, i rememebr long ago not wanting to remove lights, trim etc from my car to avoid spoiling the look, but gradually the bits came off as it didnt alter the look much unless you were up really close and the performance gains were deemed worthwhile, and as a few bit go or the look changes its easier to carry on

as for wheel size i definatly have more grip by a long way on 17s over 15's but its impossible so say how much is down to the diameter, i cant do a valid back to back test as i simply cannot get 15inch rubber in a similar width and compound.

my 15 wheels were 8 wide and my 17's are 9.5 the wheel tyre combos weigh the same due to a swap to lighter split rims.

i dare say the brakes run cooler in the bigger rims or i have the option to run bigger brakes still, well in the dry anyway i dont have enough 17's for wet use, so the brake size wont be getting an upgrade

[Adam43]

Originally Posted by dtype38

I'm not sure that last bit is to do with this question Adam. I suspect that when you went to 18" instead of 16" you're talking rim size and on the larger size you went to a lower profile tyre.

You're right, it isn't, the diameter was slightly bigger on the 18, but not significantly. It is my only practical example and also shows how difficult it is to answer this theoretical question with practical experience (all other things being equal). I dare say no one has ever managed to acheive in practice what is being suggested here.

With the constraints on the question, there may not be much to say

Which shape will give a benefit to car's handling and why?
!!!Let's assume that the only variable is the shape of the footprint!!!

A larger size footprint will give more grip (generally), although this may not necessarily have a positive effect on the handling.
Will the change be the same front and rear?

[Dex]

the "bigger tyre=bigger contact patch" argument is seriously flawed guys.

A tyre is a circular shape - so in theory the contact patch would be an infinitely thin line going across the tyre. We know that the contact patch is formed by the deformation of the tyre surface due to the weight of the car.

So if we assume a fixed tyre pressure, then the resistance to deformation is caused by two things - the air pressure inside the tyre and the sidewall stiffness of the tyre.

if we had a tyre with no sidewall strength at all then the contact patch would simply be a function of the corner weight divided by the tyre pressure.
But it's actually "corner weight/tyre pressure - sidewall strength"

So let's consider a big fat wide low profile tyre, and a skinny high profile one. Both running the same pressure, fitted to the same car. Which has the greater sidewall resistance - generally the low profile tyre - as there's a lots less rubber to deform. So the Higher profile, skinny tyre has the least resistance to deformation of the tyre overall - and thus the Biggest contact patch.

NB - See drag cars - big wrinkle wall sidewalls allow massive deformation, giving a nice big, long contact patch to give better grip off the line.

So the skinny high profile tyre has a larger contact patch (for a given TP) than the low profile - but it's the wrong shape for a circuit car - it's the right shape for accelerating and braking, but it will be awful for cornering, and the big sidewall is likely to cause all sorts of issues with tyre heat, handling, *feel* etc.

Remember folks, with contact patches (as with so much else in life) size isn't everything - the shape can be far more crucial.

[Adam43]

Hence the word "generally". I don't think any one is being absolute. I'm not sure seriously flawed is the correct emphasis, although the secondary effects you mention are all valid.

As you point out the theoretical situation never exists and the weight of the car remains constant.

Does this original problem assumes same width?

The balance between lateral and longitudual force on the patch is interesting, especially when you say it is good for acceleration. At low speed, where the grip is key, the larger diameter provides a worse gearing, but the grip may help? Probably a little help.

[Goran Malmberg]

Originally Posted by Dex

the "bigger tyre=bigger contact patch" argument is seriously flawed guys.

On my site I show some testing to see what different tire pressure and width
course to footprint size. In the case I use Avon racing type slick tires. There are also a chart based on the test.
However, we can not change tire size (diameter) on a racing car without altering the ride height. If a groundforce car this may course major uppsetting of the underside aero. If adjusting rideheight by coilovers then, A-arm geometry will change. So, tire diameter must be calculated with when designing the car. Also, to take advatage of a good A-arm geometry, wheel diameter is pretty dependent on steering angle.
Therfore we can not select tire diameter independentley from other factors.
But of course, it is interesting to discuss tire dynamics for itself.
Goran Malmberg

[JohnD]

Can't contribute to this learned discussion.
Just wish that the bankers who run 23" rims on thier Range Rovers would read it.
John

[dtype38]

Originally Posted by JohnD

Just wish that the bankers who run 23" rims on thier Range Rovers would read it.
John

Went to a motorshow a couple of years ago and saw an X5 with 22" rims with rubber bands on, black tinted glass, massive stereo, and...... lowered down to about 5-6" ground clearance. I just stood and laughed out loud. The two large coloured gentlemen didn't look pleased, particularly when I asked if it was an M5 "Scenic"

[trikesrule]

This a no brainer also. The shoulder hardness ie construction are identical. So the larger wheel will be in front. I actually did this very thing this season in speedway. My driver pleaded with me please put the larger diameter wheels/tyres back on. Said the 17' were better on the limit suggesting better control of the contact patch. The stop watch agrees. Don't forget I'm on clay guys. Might be a little different on the black stuff...........trikes

[Char1ie]

So the advantages and disadvantages are:

Smaller wheel - lighter, wider tire wall means tires heat up quicker

Bigger wheel - heavier, narrower tire wall means tires heat up slowly. And you can get loads of tires in big sizes.

If I'm using Toyo Proxes and I have a choice of 15s or 16s I think I'll stick with the lighter 15s with cheaper tires.


Charlie

[Notso Swift]

Dare I introduce aerodynamics
One of my old cars went up 1" in width contact patch), car went slower, basically car didn't need the extra grip. The OD was within a couple of mm and the gearing actually got better with the wider tyre. Because the OD was basically the same the contact patch was the same length
... and I bought 6 custom made rims for this result! FWIW the bigger rims combination was arginally lighter, because of the better wheels.
Oh and another thing, you generally run lower profile tyres with a little less pressure (hot)

[trikesrule]

Tyres...mmm Never seen a 'cheap' tyre perform better than a more expensive item. Had some comments from a Targa competitor last year that run Toyo Proxies. Was all good. Notsoswift we went to a lower profile tyre on the clay and had to go up in psi to keep the tyre on the bead and to better control the tyre from rolling under. Mate of mine works at Bridgestone (Hobart) here down under. He said they tell the customers to run more psi...........trikes

[dtype38]

We need to make sure we're comparing apples with apples here. I have some friends run in a race series which has Toyo Proxies in T1R version (fast road) as the control tyre and they mostly dislike them. I've heard good things though about the Toyo Proxies in 888 version (road legal track tyre) so I'll be giving them a go in low profile form. Be interesting to see how they compare to my deep profile Yoko 032Rs.

[ian.stewart]

I am using 888Rs, and for what the tire is used for in my case the grip is superb [Drags and Sprinting] cant comment too much about fast cornering yet, but on testing the felt really good
Going back to the topic, Profile must surly have an effect on footprint caused by sidewall deflection, I use 225/50/15s, when I drag the car I run the tires at about 14psi this seems about the optimum without causing the centre of the tread to colapse, but allows the sidewall deflection to give a longer footprint, I have tried low profile 17s on the back of similar compound width and rolling dia but no matter what we did to the pressures the tires constantly gave a slower 0.3 and 0.4 sec 60ft time

[ian.stewart]

Originally Posted by ian.stewart

I am using 888Rs, and for what the tire is used for in my case the grip is superb [Drags and Sprinting] cant comment too much about fast cornering yet, but on testing the felt really good
Going back to the topic, Profile must surly have an effect on footprint caused by sidewall deflection, I use 225/50/15s, when I drag the car I run the tires at about 14psi this seems about the optimum without causing the centre of the tread to colapse, but allows the sidewall deflection to give a longer footprint, I have tried low profile 17s on the back of similar compound width and rolling dia but no matter what we did to the pressures the tires constantly gave a slower 0.3 and 0.4 sec 60ft time

Further to the sidewall deflection giving a larger footprint, of coures I understand these are specific use tires but have a look here, in some cases 7500hp, yes 7 thousand 5 hundred horses and being put to the concrete,
http://www.youtube.com/watch?v=9O0b90G8Yhg

Ian

[dtype38]

Originally Posted by ian.stewart

Going back to the topic, Profile must surly have an effect on footprint caused by sidewall deflection,

Good point, so to add a bit of complication, do we need to consider footprint shape/size in different parts of the lap (assuming circuit racing here). What I mean is that I'm pretty sure that the footprint actually changes shape and size depending on how the tyre is being loaded. For example, during braking the fronts are rather squished onto the tarmac (bigger footprint), and similarly for the rears in acceleration. Presumably for best straight line top speed we want as little rubber on the tarmac as possible within the bounds of getting enough power down and keeping steerage. Cornering, however, I guess we want as much rubber attached as possible? If so then the relative width of the rims compared to the tyre also has a significant influence. A relatively tall tyre with a narrow rim will tend to "roll under" and lose footprint in cornering, but will be much better at "squashing" into the tarmac for braking and acceleration, whereas a low profile on a relatively wide rim will be better at cornering, but less good for acceleration?? errr....

[Goran Malmberg]

Discussion like this is rather confusing. I understand that it might be of interest discuss wheeldiameter and if larger is better than smaller, but in the real life, this is not how it is. The wheeldiameter must be taken in to concideration when constructing the racecar, and can not be seen as a separate phenomenon. For now I am building a Corvette race car and is using quite large wheels, 650mm front and 710 mm rear diameter tires. The reason for doing so is rather complicated, and there is a number of things that make me go this size. And once the car is designed for this type of tires, it is impossible to select another size since it will upset the handling of the car,
either the groundforce or A-arm geometry will suffer in such case. On many road driven-race cars this is not an issue, we can just change ther ride height after the tires, but on a highley engeneered race car this is not possible, so wheel diameter must be looked at in the right perspective.
Goran Malmberg

[Al Weyman]

So I have the option of 18" wheels and can then use a 255 x 35 x 18" Toyo 888 or save my money and use my standard 16" rims but the largest 888 (control tyre) is 225x45x16 (smaller than the standard fitted 245x50x16) how much will I benifit from the extra £600 outlay for the 18"?

[Goran Malmberg]

Originally Posted by Al Weyman

So I have the option of 18" wheels and can then use a 255 x 35 x 18" Toyo 888 or save my money and use my standard 16" rims but the largest 888 (control tyre) is 225x45x16 (smaller than the standard fitted 245x50x16) how much will I benifit from the extra £600 outlay for the 18"?

I guess the 255 is the best, but cant tell by how much. The tire act as a second spring so another sidewall height changes damper and spring setting of the car. Lower profile tires is more sensitive to camber compensative A-arm geometry, so you might run in to either cornering or braking-acelerating problems. Another problem in my own case was that the spindles, both front and rear, is in the middle of the wheel so the wheel diameter is needed to keep steering angle reasonable. The car has little cambercompensation since it has a lot ground force together with low cgh, which helps using lower profile tires.
So, the picture is complicated to tell exatley what will happen. But I think low profile is the way to go if the rest of the car is properley set up for it.
But cost... cant tel if worthwhile.
Goran Malmberg

[Notso Swift]

Originally Posted by Al Weyman

So I have the option of 18" wheels and can then use a 255 x 35 x 18" Toyo 888 or save my money and use my standard 16" rims but the largest 888 (control tyre) is 225x45x16 (smaller than the standard fitted 245x50x16) how much will I benifit from the extra £600 outlay for the 18"?

If you are running on a budget (aren't we all) then consider how much difference fresh tyres will make... taken over a life cycle you may find that you are better off with the smaller one because your "average" lap time is better, even if the peak is better with the other tyre.
I can fit a big 235 x 17" on my car but the price is more than double the 205 x 15" I run on

[Al Weyman]

What car do you run Notso? My car is marginally overgeared on some circuits and the smaller diameter tyres may help ther so thats another concideration.

[Notso Swift]

A very, VERY different beast to yours, Turbo 4WD with a restrictor. 1150 kg race weight and 300 hp

[AU N EGL]

Wheel Diameter is not the only factor to consider for a car. Al's Camaro and my Corvette would not do well on smaller tires. Wider tires have more corning grip and better hard braking into fast corners.

So it is not only the diameter of the tire/ wheel combo, but the tire width based on the cars size and hp/tq produced should be considered too.

[trikesrule]

Targa competitor runs 888'. Interestingly he changed from Yoko's. Targa person runs a Pug GTi...........trikes

[Notso Swift]

AU N EGL,, Depends on the circumstance some classes have rim width limits, so a 255 would be worse on an 8" or even a 9" wheel than a 225.