The effects of wheel diameter

[TeoRacing]

Hello!
I would like to start a discussion about wheel diameter and its effect on handling for a saloon road or racing car.
I would like to ask why would it be beneficial to run with wheels of larger diameter. And i mean the whole diameter, incuding the height of the tires when viewed from the side.
Don't put into consideration the fact that a smaller wheel will be lighter or that the sides of a larger diameter tire will be harder.
Let's assume that all the variables between a 15" and a 17" rim wheel will be the same. The only difference will be the whole diameter of the wheel.
I have heard from someone that the shape of the footprint of a larger diameter tire is quite different from the the footprint of a 15" wheel. And this is beneficial to its handling, giving us the "right" to tell that we must use a large diameter wheel if we want to make better times in a circuit.
I am looking forward for your thoughts! Thanks!

[AU N EGL]

Can we also add wheel and tire wt to this discussion too?

[TeoRacing]

Originally Posted by AU N EGL

Can we also add wheel and tire wt to this discussion too?

I am afraid, no!
What we must look at, is the effect of wheel diameter to the handing of the car and only this!
We must assume that tire width, unsprung mass (via a lighter rim for the 17" case), tire shoulder hardness are all identical.

[Goran Malmberg]

There ar numbers of things to take in to conciderations and I can not say that one diameter is better than another. However, if we want to use the
tire diameter to serve a certain condition it might be of interest to use a
larger or smaller diameter tire.

If we take an outer diameter that is larger we will get a longer footprint.
A longer-not so wide footprint will loose more area for slippangle than a wider shorter footprint.

Goran Malmberg

[TeoRacing]

Originally Posted by Goran Malmberg

If we take an outer diameter that is larger we will get a longer footprint.
A longer-not so wide footprint will loose more area for slippangle than a wider shorter footprint.

Goran Malmberg

What you actually mean by "more area for slip angle" ?
What i know of this is that with a small diameter wheel the footprint is more wide than that of the larger diameter wheel (so we agree).
With the 15" wheel the car will be more agile but with the 17" the car will be more stable in fast turns.
Also for the 15" to take a given turn, the tire must be "deformed" more than the larger diameter tire, so with the 15" tire and for the same turn the tire will have to produce a larger slip angle in order to pass the turn with the same speed.

Anyone on this?
Also, is there evidence that with a larger diameter tire, the surface of the footprint will be larger?
The shape of the footprint will certainly be different but what about the surface?

[AU N EGL]

Larger diameter tires also gives you a taller gear vs a smaller diameter tire

Originally Posted by TeoRacing

Also, is there evidence that with a larger diameter tire, the surface of the footprint will be larger?
The shape of the footprint will certainly be different but what about the surface?

YES Wider tires are Better.

"For the same vertical load and internal pressure, a tire with a wider tread has a shorter, wider contact patch then a narrower tire. The area of both contact patches is the same if the internal pressure and load are the same. Lateral tread deformation is showed that deformation builds up along the length of the contact patch until the restoring forces of the tread and carcass excees tread grup and sliding begines.

Shorter contact patch at the same slip angle begins to slip at roughly the same distance from the leading edge as with a longer contact patch. But shorter contact patches has more of its length stuck to the road then a the longer, narrower contact patch: and therefore a larger portion of its overall area is gripping. A larger portion of contact-patch area gripping means more total grip. So for the same load and same slip angle, a wider contact patch generates more grip then a narrow contact patch."

Source: "The Racing and High-Performance Tire" by Paul Haney

[TeoRacing]

Wider is certainly better than a narrow one, but my question was about 2 tires with identical width but different diameter!

[Al Weyman]

Well you can fit bigger brakes and they look soooo cooool! :-)

Seriously though it is an intersting topic, one reason I am planning ging up to 18" from 16" on my IROC is simple, tyre choice, there are no wide Toyo 888's (the control tyre) in the 16" range unfortunately as I doubt I would bother especially as we have a rule that a standard wheel must still be able to fit over the brakes and calipers and spin freely.

[Goran Malmberg]

Originally Posted by TeoRacing

Wider is certainly better than a narrow one, but my question was about 2 tires with identical width but different diameter!

If we just have a larger diameter tire of the same width, everything else beeing equal, (without specifying what I mean by equal), we will probaley
have a larger footprint of the same width. By that we will rise the µ number of friction for better grip. There will be a small number of "sliploss", but we will still gain grip. Then we have braking and acceleration where slip is not hurt by longer footprint, resulting in better grip.
Goran Malmberg

[johnny yuma]

TeoRacing are you accepting the fact that the larger the diameter the larger the footprint [given equal widths of tyre on road],or are you saying the contact patch in your theoretical comparison is kept equal,and is there any merit in the WHEEL ITSELF either being 17 inch OR 15 INCH?

SIDE ISSUE WHY ARE FORMULA 1 ON 13 INCH ? aero? weight?

[Al Weyman]

F1's are governed by the rules.

Another thing that comes into the occassion and is especially true on short 10 lappers is a taller tyre wall (smaller diameter wheel, taller profile tyre) will reach operating temrature quicker and also prehaps get too hot towards the end of the session and start going off. I know when guys used to run in ModProds with Mk1 and 2 Escorts some choose to stay with the 13" rims as they found going up to 15" no advantage, one bloke fitted 17" rims and actually went slower!

[AU N EGL]

Al has hit the button here too. Many races are dictated by the rules as to what size wheels and tires you can run.

[Al Weyman]

I hasten to add in the above examples we were on Falken road rubber but the guy with the 17" wheels and low profiles I recon never got them up to operating temprature in the 10 lap sprints which probably contributed to the lack of performance (over a grand down the pan!).

[Adam43]

Well the argument that there is a larger footprint is clear (see attached file too).

However it is difficult to make all other things equal! Different wheels have different weights. Often it is difficult to get tyres from the same manufacturer that allow the other parameters to remain constant.

For me, the larger footprint may not be worth the change in gearing; rarely am I at a circuit that requires a higher top speed. And as others above have said there is the issue of tyre temperatures, although this can, to a certain extent, be compensated for by pressures.

I ran 18in instead of 16in for a little while, but was eventually convinced that they were slower. Although I suspect this was down to weight, they were heavy wheels, and this didn't overcome the advantages discussed above and also an increase in width.

[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. I'd be surprised if you significantly increased the overall wheel diameter. For example, I'm about to have a go with some 18"x8" wide rims instead of my current 15"x7" rims. Much bigger you might think, but the set of 235-40R18s I'll be fitting are actually slightly smaller in overall diameter than the 225-60R15s I'm currently on. So much bigger wheel, slightly wider tyre, but slightly shorter contact patch (although very marginal for both) Where I'm hoping to gain is in much better control of the contact patch.

Anyway I think the question is about overall diameter which is an interesting point. I've looked at a lot of wheel/tyre combinations recently because I've been trying to match up the overall diameter of my current tyres which is about 655mm. I've actually found it quite difficult to find virtually any tyre over 650mm diameter for any rim size. I think the Toyo 245-40R18 is just over and maybe the Dunlop D02 in that size, and I think Hosier do some, but thats about it. Is there some law of physics that says manufactures don't want to do circuit racing tyres any bigger? Centrifugal force problems perhaps?

[AU N EGL]

Originally Posted by dtype38

IIs there some law of physics that says manufactures don't want to do circuit racing tyres any bigger? Centrifugal force problems perhaps?

There is but the author of this thread did not want to get into wheel size and tire wts,

[TeoRacing]

First of all i would like to thank everyone who participated in this discussion!

One question for everyone:
Let's say we have a 15" diameter wheel and a 17" diameter wheel. When i am talking about diameter i mean the whole diameter of the rim and tire, not just the diameter of the rim.
It is certain that these two tires will produce a differently shaped footprint.
It is also quite certain that the whole surface of the 17" footprint will be larger.
Let's assume that in practise the surfaces of the two footprints are exactly the same.
The shape of the 15" wheel's footprint will be short and wide while the shape of the 17" wheel's footprint will be long and narrow.
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!!!

Thanks in advance!!!

[Al Weyman]

If the overall diameter of the tyre is the same on both surely the footprint will be identical. As I see it the difference is in the stiffness of the tyre wall in the 17 inch and 15 inch wheels. The more flex in the taller tyre I believe means the tyre warms up quicker in race conditions (may be too much meaning the tyres inflate maybe 10 or 15lbs during a sprint race) also there is a different 'feel' with the stiffer lower profile covers. I would also imagine the response is marginally quicker on the low profile jobs. Also I would have thought the lower profile tyres would be more stable during the course of a race with less fluctuation of tyre pressures. I also maybe entirely wrong of course :-).

Also to be taken into concideration would be the spring effect of the taller profile tyres the lower ones feeling much more harsh and stiff.

[dtype38]

Originally Posted by TeoRacing

Let's say we have a 15" diameter wheel and a 17" diameter wheel. When i am talking about diameter i mean the whole diameter of the rim and tire, not just the diameter of the rim.

Al there is assuming that you're using the same size rim on both of those tyres. His answer would be different if he had assumed the same tyre profile! The point is, that I don't think your question as put has an answer which has any meaning in the real world. It's like asking if your engine would be better or worse with bigger carbs (all else being equal). The answer is that it depends if your existing ones are too small or not! In the same way, the effect on the handling of a car of putting bigger diameter tyres on depends on so many other variables that to answer in isolation would be virtually meaningless.

Originally Posted by TeoRacing

It is also quite certain that the whole surface of the 17" footprint will be larger.
Let's assume that in practise the surfaces of the two footprints are exactly the same.

Err.. I'm not sure you're making sense there. If its certainly larger, why would you want to assume its the same? And are you still using your inital assumption that the two tyres are the same width? Perhaps if you try and get clear in your own mind what it is you're asking and maybe presented your own thoughts on this subject, then we might be of more help.

[TeoRacing]

My question is as simple as that:
What's the effect of the shape of tire footpring on handling and why?
Which is better, the long and narrow one (large overall diameter) or the short and wide one (smaller diameter wheel)?

[AU N EGL]

Originally Posted by TeoRacing

My question is as simple as that:
What's the effect of the shape of tire footpring on handling and why?
Which is better, the long and narrow one (large overall diameter) or the short and wide one (smaller diameter wheel)?

That is what I answered in post #6

Wider is always better for more grip.

[Goran Malmberg]

I wrote a shortcut version of a study made in Norway of different tire size.
". The weight of the wheel affect unsprung weight. But what we are talking now, is rotational inertia, which is something different. It is NOT enough to put the wheel on a scale. We must take in to account WHERE the weight is located on the wheel. This is the same phenomenon as a mid-engined car compares to a nose and rear heavy car.

An experiment shows the following.
The wheel has a linear acceleration down the board. It also has a linear acceleration around its own axle. Both require energy. The energy to rotate the wheels is given by Kr=(I * w^2) / 2.
The s”straght line” acceleration is Kv = (m * v^2) / 2,

We have a 10,89 feet long board raised 5 feet (26,377 gdr) in one end, and the other end resting on the floor.
A downhill “road” situation. We have tree wheels.
A, 8x17 with 235/40 tires weigh 44p,
B, 15 inch with 205/60 tires weight 35 p, and
C, 14 inch with 195/60 tires weight 32p.
We will take each wheel, hold it on top of the board, and let it roll down by gravity to the floor. Measure the time with a photocell. A tire with higher rotational inertia will then take MOORE energy to start to turn around. That energy loss will make it take a longer time to roll down the board. Here is the numbers.
A 1,61 sec. Moment of mass 1,272 Virtual mass 75p.
B 1,59 sec. Moment of mass 0,946 Virtual mass 57p.
C 1,56 sec. Moment of mass 0,704 Virtual mass 51p.



A car that weights 2200 pound (1000kg) that does 0 to 60 mph at 10 sec will loose 3 tenth of a sec, using the 17 incher used in this example!!!
A 10 sec quartermile car will loose about 25 horses!"



Goran Malmberg

[Al Weyman]

Yes and another possible effect of the larger wheel moving the mass of the weight of the tyre wheel combination also effects the braking as the enertia in the larger 'flywheel' (if you look at the wheel and tyre like that) will require more force to slow it down. I don't know much on the science or calculations on all of this but I have tried things over the years, when I raced a Mini it definitely felt a lot better on 10" rubber than 13". Why I don't know but it did. Oh and Dtype you must not put 18" wheels on your car thats sacralige. My IROC was fitted with 16inch standard and 18 won't look wrong but on a Dtype??????

[graham bahr]

heres an effect with a difference, i found by going to 17 wheels could run ex touring car rubber, grippy and cheap as chips! (almost)

[dtype38]

Originally Posted by Al Weyman

Oh and Dtype you must not put 18" wheels on your car thats sacralige. My IROC was fitted with 16inch standard and 18 won't look wrong but on a Dtype??????

Yeh I know, but I've long stopped letting looks bother me too much. If I did I wouldn't have stuck a great big roll over hoop out of the top of the bodywork! Anyway, the original D-types had 16" or 17" wheels on, so the 18s aren't any more wrong than the 15s I've got on at the mo.... except maybe that they're 10 spokes off an XJ saloon