Determining Optimum Tire Size

Wider tires obviously provide more contact patch, but yield less energy per unit area - so they will run cooler - which could reduce available traction. They also weigh more.

So the question is - how does one determine the optimum tire size for a given weight of car (like the GT40? Obviously HP factors in, but I'm more concerned about braking and cornering performance.

I've seen GT40's with front tires ranging from 195mm to 245mm (and there is probably someone running 255's in front). Cars like the Mazda MX5 (which weigh about the same as a GT40) only use 205's up front. Cars in the 3,000+ lbs range seem to use 235-245s (GT3, M3, A4, etc).

Is there any real advantage to running fronts wider than ~205 on a 2,400 lb car?

And of course, how does one determine the optimum rear size for a given HP (say 400 HP) and weight distribution?

I should state that "optimum" in this case means track performance on a "typical" track (and yes, I realize the optimum may vary slightly from track to track).

Thanks!
Mark

I think that if you look at any given class of racecar, F1 to FF you will find that performance is limited by, engine power, downforce, weight, and grip (TIRES).

So in all race cases if not restricted I think you will find race engineers will want more, more, more and more.

Take a minute to consider what the modern Indy car would look like without any one of these variables limited. Not to mention all of them. They would quickly become 800 pound racecars with 2000hp, 20,000 pounds of downforce, and enough tire to hold it on the track. Lap speeds would climb above 250MPH and reach the point that the car could not be driven by humans.

Since GT40's are really only limited by what will fit under the bodywork and not by the rulebook then truely more is better, to a point.

Practical limits are about as you have stated 8-9 inchs of tread width at about 26 inch in height on front and about 12-14 inches wide by 27 inches tall at the rear.

Thats a 245-40-17 and a 335-35-17 front to rear more or less.

I have 225-40-17 and 285-35-17 on mine by the way with about 370ish HP and a open 3.44 diff. After about 1/2 way thru 2nd gear I can just about mat it if I "roll" it on. I am using dunlop SP8000's. This is a fairly middle of the road Z rated tire.

Top of the line DOT rated slicks at these widths would be plenty of grip at this power level.

I also think that really wide tires, > 12", on the rear should only be done after fully considering the suspension geometery. You can get to the point where the rear has so much more grip than can be ballanced with the front, in the middle of a corner, that limiting rear grip is the only way to ballance the car. You guys who are tracking their cars chime in here.

Also I don't think modern builders are designing for wider tires when they lay out the suspension design. I might be wrong about this but if I am I am sure we will hear from them.

If you are asking what will work best performance wise then about 8 X 251/2 fronts and 12 X 26 rears is about right. Modern race slicks at these widths will develop more than enough possible grip. The trick is to get it from them with a proper setup.

How to do that is a MUCH larger, and more important, question.

Modern street tires have a very wide temperature range and thus the wider the better keeping the proper balance front to back. Slicks however have a much narrower temp band and the width/heating could be a consideration.

Optimum tire size depends partly on what size wheel you intend to use. I'm leaning towards using 15" but my problem has been the lack of decent speed rated tire sizes available for a 15 inch wheel. I'd like to run 235/50/15 front and 295/50/15 rear but haven't had much luck with finding anything decent.

I think that I read a thread that JP started (or someone else) to get Avon to make some decent tire sizes for 15 inch wheels but don't know if anything happened?

Chris

Good point Chris - side wall height would factor into it. Personally my plan is to run 17" wheels for best performance since there is more available and this should reduce unsprung weight as well. I'd like to have a set of 15" wheels as well - but this would be more for show and performance would not be as critical.

Howard - I didn't even think about how wheel width would impact suspension geometry. I agree 8 or 9 inches of tread up front and maybe 12 inches in the rear is probably about right for a GT40, given what can be fit reasonably and the capabilities of the rubber and the car itself.

So let me re-phrase my question. If you were designing a ~2,400 lb ultimate race car from the ground up, how would you determine the best tire size? Assume you had a tire company that would make any size you requested. Clearly 24" wide front tires would place a lot of rubber on the pavement, but would probably not stop any faster than a narrower tire, and would be a pain to turn. So what determines this optimum point?

I find it interesting that sub 2000 lb cars seem to run fronts around 175mm, 2300-2500 lb cars seem to use 205-225mm, and 3000-3500 lb cars seem to use 235-245mm. Why didn't the sub 2000 lb guys use 235mm rubber? It seems they would have done so if there was more grip to gain - so the wider tire must not buy them anything - but why? Is it (lack of) heating? Ratio of unsprung to sprung mass? Something else?

Mark

Mark.. I don't really think that the weight of a given car has as much to do with tire selection as grip and loading requirements. Lets look at a couple of examples. First a 1000 pound open wheel car with no areo aides. Nearly zero down force. Also it has a very low drag number as far as the chassis/body is concerned therefor the tires that would be put on our first example will induce most of the drag. Lastly we have only 100 hp to work with. In this example you can see that very quickly going wider that lets say 6 inches at both ends will begin to reduce total performance because of the limmited ability of our 100 Hp to overcome the increased drag of wider tires. I would think that the fastest way to lower lap times on this example would to add a modest set of wings and gain some downforce. Something like we see used on F2000 FF's.

On the second example we still have the 1000 pound chassis but we also have 1000 hp. Still no wings, ground effects, sealed bottoms or sucker blowers. On this second example we will need some tire width even if we gain drag doing so. We will cover the tires with bodywork like a CANAM car anyway. Whatever drag we do gain we have a LOT of HP to overcome it with. But again after a resonable set of tires are put on the car the very next thing to do is add DOWNFORCE and lots of it. 20 TO 30 TIMES THE WEIGHT OF OUR CAR SHOULD DO.

With this kind of down force the tires really are not providing the cornering force in as much as they are simply withstanding the loading being applied to them . Very stiff sidewalls and the ability to shed heat are going to be more important than the marginal increases in grip provided by sticky pole tires. We are getting huge downloading from the areo aids. Many times more total performance than can be acheived with wider tires alone.

Next were going to take about a 100 of our hp and drive a great big blower and suck all the air out from under the car like Lauda's Alfa F1 car did nearly 30 years ago.

Then we are going fence in the bottom of the car to seal the vacume we have created. After that we will add a computer and a hyd. active suspension system to keep the ride height exactly where we want it all the time.

And if this isn't fast enough for you we'll add another 1000Hp. Oh shit we're back to the undriveable racecar again.

Simply put its just money. More money. Less rules. Faster racecar.

Maybe we shouldn't put tires on our unlimited racecar after all. How about we just put a friging rocket on it and fly it real low. Thust vectoring ought to get it around the corners...........

See what you did. Hell I don't know what kind of tires to put on the damn thing. I put on what I could afford on mine. I am sure you will too in the end.

In the end effectiveness of a given size tire is more important than width anyway.

Hi Mark

You're right; there is definitely an ideal weight per square inch for a car's tyres. Of course, there are losses through friction for wider tyres too, and the 'balance point' will be different for each tyre manufacturer. Your tyre manufacturer of choice should have all the data for working out ideal front tyre width, if you can tease it out of them and then work out how to interpret it.

Has anyone out there done back to back testing with tyre widths for a GT-40? Some of the racers? Even if you use slicks, the width to ultimate grip would have some relevance to road tyres.

Without data I would guess:

Front tyre: 205 to 225. Maybe 235 with a really, really sticky road tyre. I remember the 1000BHP Testarossa based Koenig Competition used 205s on the front, and it was much heavier than 2400lbs, so this 'narrow' width must be competent even if it doesn't look as cool.

Rear Tyre: if you have 300+ RWHP, I would use the widest you can get!

Sorry, Mark. All I've really done is give you back the information you started with! Maybe start with a 215 on the front and take it from there? Let us know your testing results ....

Mark!
Depending on car use we could select different size tires.
For street use it might be comfortable and economically to go "realistic" in width. But for ultimate cornering capacity, or the most grip in general, the wider the better. Wider tires run cooler, so softer rubber is to be used. Between the thumb and fingers we can use tire width accordingly to weight distribution of the car. We must also play around with inflation for even tread temperature. And a wide tire on the street may run higher coold pressure since they will not see the same rise in temperature as used on the track. When tire gets wider and softer blend, the grip is increased and therby increase the suspension load calling for heavier springs, which in turne rise the tire temperature even more. So one must keep an eye on thigs to get right. It is not as simple as mounting wider tires and see if the car corners better. Allignment may also be altered.

I am using Avon slicks by now and they last say 2000 miles.
Not very economical.So for strict street driving a narrower tire say 11" rear and 8" front best quality sports car street tires should do.
Goran Malmberg