Ideal wheel diameter?

[petevb]

To avoid the thread hijack we had going I figured we should continue this in it's own thread. From: How to get more heat in your tires

With complete freedom, what would be the ideal wheel diameter/ sidewall height for a club race car? 18", 16", smaller?

Modern racers (LeMans, etc) have standardized on 18s when the rules let them do so (as opposed to say F1, where they can't). This makes me think that an 18" wheel has an advantage on a heavier car with modern suspension.
Does the heavy car need the large contact patch and small sidewall that the 18" wheel offers? Or are they primarily packaging to clear big (endurance) brakes? Modern cars also have a friendly camber curve to keep those larger tires flat to the road...

Lightweight earlier cars like many of our club racers, on the other hand, probably don't need the contact patch as badly (witness the thread about getting tires hot enough) or the bigger brakes as badly, and they often don't have ideal camber curves. Which means they can get away with smaller wheels and/ or taller sidewalls up to a point?

So is there a weight vs tire diameter rule of thumb? 3000 lbs=~18", 2500=~17", 2000=~16"? Or are we just packaging the smallest wheel that fits over the minimum brake we need?

Maybe the more "advanced" the car is (read better camber curves, more downforce, stickier tires) the larger diameter you'd want? Gearing comes into play as well, but of course, but since you can get a shorter gearset I'm not sure it matters for this discussion...

What's fastest, or is this not simple enough to have a rule of thumb answer?

[304065]

Here's how I think about it, in order of priority

Rules limitations (width)
What will fit under fenders (also a function of rules limitations)
Cross-sectional width and contact patch size for a particular tire
Availability of competition tires (also a function of rules limitations)
Availability of wheels of correct offset to achieve desired track
Wheel weight
Cost of tires

For example, I started off on 225/55/15 on 15x6 Fuchs. 225s handle kind of weird on sixes although they will fit. Tires were fairly cheap and the combination of the very light fuchs with a reduced polar moment compared to a larger wheel made this a good combo.

Then I went to Boxster sevens with spacers for correct offset and 16s. These are heavier, more polar moment, not as responsive as the Fuchs, but tires are the same price.

I'm going back to a set of custom-offset 15s, these will fit over my "S" brakes and small vented undrilled rotors. Big brakes are not just not allowed by the rules, they are heavy, both in terms of unsprung weight and also rotational mass.

"Don't brake. It slows you down."

[Bill Verburg]

It's not a simple question w/ a simple answer. Everything that you use on a car is a compromise and they all need to work together. Some focus on a single aspect, such as weight, to the exclusion of other equally significant factors. That is a mistake.

There are many advantages from using a bigger wheel, not all are applicable to all cars. The pluses are the ability to accommodate bigger brake rotors, an advantageous contact patch shape(for motor sport) and to provide shorter stiffer sidewalls.

disadvantages are weight(usually), cost, relatively poor ride and loaded tire radii.

strictly for motorsport use
A 2000# car doesn't generally need the bigger rotors, a 3000# car does. a 2.7 lt wt doesn't need more than 15" wheels a 964RS does

all benfit from the shorter wider contact patch provided by lower profile tires that run at lower slip angles generating more grip and less heat.

loaded radius is intimately tied into trans gearing most stock transmissions can benefit from lower gearing but a custom geared motorsport trans doesn't.

Of course it's best to use lighter components but the cost of doing so can be more than monetary.

[byndbad914]

Originally Posted by petevb

Or are we just packaging the smallest wheel that fits over the minimum brake we need?

This gets my vote along with verifying tire availability for what you want. What I mean is that if I went with 15x10s and bigger on my car, they were all 25" diam or larger for stock cars basically. I used 930 Turbo brakes on my car so I could go with 15s BTW. That said, I went with 16s cuz I could get 10 and 11" width tires in 23.5 up to 25.5 on the rear. I can easily swap tire diams for easy "gear ratio" changes. The point is the car is lower to the ground with smaller diam tires, the smallest diam wheel that fits the brakes (assuming constant width) is the lightest unsprung weight which is critical, and the small tire is lighter too. Additionally I intend to try bias ply slicks next swap which should be lighter yet as they don't have the extra construction weight.

Problem is I can only get the bias-ply rears in 23.5" diam, not the 25.5 that I prefer. However, the local track I basically run on exclusively is changing over the Winter here and I suspect I may want the smaller diam tire to get an extra shift in a couple spots now v. riding out the rpm band in one gear with the 25s (I expect to run out of the single gear in two spots now). Sorta O/T other than making the point of how critical tire size selection can be in your choice of wheel.

I would NOT run 18s+ unless I had a car I determined needed 14" rotors and six-piston calipers, which mine certainly doesn't. 17s only if the tire sizes that worked best fell there.

edit - also don't forget I went with the smaller brakes cuz they were perfect, less weight by being simple alum calipers with 12" rotors. Smallest brakes that work = least unsprung weight. I have a tank of a V8 914 (but that was for safety with all the cage) but I really focus on trying to keep unsprung weight down.

edit2 - need to keep my thoughts better organized - another key to going with smallest tire/wheel combo is the lower inertia means the car accelerates thru its rpm band faster and tightens the gears up relative to each other to get more of a "close ratio" gear selection without dumping money into a trans.

[byndbad914]

here is a pic of the car... you can see the front tire diam of 23" allows me to run essentially stock front geometry (not aggressively lowered so that the strut bottoms out or the lower control arm is level at static v. angled down like you want it to be). There is just over 3" of clearance at the front and I can go lower (2.5") if the track wasn't so rough I run at. You can see the 23" diam tire almost completely (no shrouding at the top by the fender) it is so small.

The rears here are 25.5" so I have some rake but can run a gear "longer" so I don't have to mess with upshift/downshift within 200ft of a turn. If I go to 23.5" diam the car levels out. That was a big driver because I wanted wide-ass tires and 10s/12s fit on the car.

[petevb]

I find the lotus Elise/ Exige an interesting example. 2000 lbs- very similar to our racers. Street cars use 16" front wheels (you could likely clear the 288mm diameter front disks with a 15" wheel) and 17" rear wheels (clearly more than needed for brake clearance).

The Exige Cup, a ~250 hp track version of the same car upgrades the brakes to 308mm diameter, but keeps the 16"/ 17" wheels.

The 400 hp 3.0L 2005 "Sport Exige" used larger 344mm/ 330mm diameter brakes to handle the extra power despite its lighter 1900 lb weight. Rim size increased to 17" front, 18" rear.

It looks to me like, when it doesn't need to worry about caliper size, lotus likes 16" front (with tall 50 series sidewalls) for the front. I guess, like our cars, there isn't much weight over the front to deflect the sidewalls. The rears a slightly lower profile 45 series tire, perhaps because of the extra weight.
I might have expected the basic "track" version (cup) to use lower profile tires, but they choose not to.
The full-on "race" version gets both big calipers and bigger wheels, though not, interestingly, 18s on the front.

I don't know, but I feel like a wide-tired 930 probably shouldn't use 15" wheels, despite the fact that they would fit...

[byndbad914]

you sorta should remember that current styling plays a big roll in car design. The current trend right now is to run a 1" larger wheel on the rear (an old street rodder thing that somehow has really taken off in recent years). So I would bet the larger rear is strictly styling, since at the largest the rear brakes should only be as big as the front (no reason to put larger diam rotors or more caliper on the rear!).

If that car had 16" rears with same sidewall height tire (so overall 1" shorter wheel/tire combo) it would accelerate better and stop better at that wheel.

[petevb]

Originally Posted by byndbad914

If that car had 16" rears with same sidewall height tire (so overall 1" shorter wheel/tire combo) it would accelerate better and stop better at that wheel.

Hmm- that seems tricky to predict. The contact patch would be smaller as you describe. Less area, less friction... certainly it would stop better with the larger wheel if it was locked up. Seems tough to predict weather the extra traction would balance the inertia.

I might buy your "styling" argument for the street car, but the fact that they keep the larger rear wheel on the race car makes me think there may be more to it. Certainly the rear is taking ~50% more load than the front- this might prefer a taller wheel...

[RaceProEngineer]

Originally Posted by byndbad914

you sorta should remember that current styling plays a big roll in car design. The current trend right now is to run a 1" larger wheel on the rear (an old street rodder thing that somehow has really taken off in recent years). So I would bet the larger rear is strictly styling . . . . . If that car had 16" rears with same sidewall height tire (so overall 1" shorter wheel/tire combo) it would accelerate better and stop better . . . . .

Bingo!

We have been racing, off and on, since the early 1970s, and never before has the after-market (in other circles, called "tuner") suppliers had such powerful influence on racing. Indeed, it has always been the other way around!

This is simply a LoPresti Guess: If Colin Chapman were still around, cars that Pete is citing as examples, along with their "bigger is better" accessories, would not be from the Lotus stables.

Bill's point about shorter, less compliant sidewalls is certainly valid, but only applies to street tires and SOME R-DOT radials. With very few exceptions, the height of the sidewall on a slick is proportional to its diameter - shorter tire = shorter sidewall. But, paradoxically, the taller slick does not always have the larger contact patch!

Pete's correlation of car weight :: wheel diameter - 3000 lbs=~18", 2500=~17", 2000=~16" is likewise interesting, and we do see much of that at the track.
However, when one approaches the question from a performance perspective, doesn't it just make sense that the already heavy 3000 pound car would benefit from lighter UNsprung weight? Or perhaps the philosophy is, "The car is already so ponderous, what does a few more pounds matter, even if they are outside the chassis?"

And for Driver Ed or Track Day events, it really doesn't matter. But for competitive racing, well, John, Tim, et al, that's another story.

Ed

[byndbad914]

Originally Posted by petevb

Hmm- that seems tricky to predict. The contact patch would be smaller as you describe. Less area, less friction... certainly it would stop better with the larger wheel if it was locked up. Seems tough to predict weather the extra traction would balance the inertia.

So here is my thinking before the phone call, followed by what the pros at Hoosier said.

My thinking - all tires are "theoretically" a circle. Ground is a flat plate. Circles on a flat plate make a line contact, so in theory the longer the line the more "contact", NOT the bigger the diam. So as you can see with my car, I went smallest tire I could with widest I could fit cuz from an engineering standpoint that made sense to me. NOW, we all know the tire "squishes" to more than a line contact... that is going to be very dependent on tire mfr as to how much it squishes down, but that said, I couldn't imagine a larger diameter squishing out more than maybe a few % larger contact patch.

So I called Hoosier a minute ago - he said that they have a special setup to measure contact patch and the larger diam tire (say 16" to 18") in the same width squishes out maybe a few thousands wider contact patch. He said he would run the smallest diameter tire in the widest possible width - the sidewall will be short and sturdy and the contact patch is governed more by width of wheel than diameter of tire, which makes complete sense to me (hence my wheel purchase shown above).

So, if you have a 10" width max, the very slight loss in a few thousands of contact patch would not drive me to go to 18" wheels v. 16" wheels. I would take the 16" wheel hands down any day assuming the tire I could buy had identical sidewall heights (therefore the 16" wheel is 2" smaller diam and the tire is 2" smaller diam). You would have the same sidewall rigidity on a much lighter combo with nearly identical contact patch.

[byndbad914]

Originally Posted by petevb

I might buy your "styling" argument for the street car, but the fact that they keep the larger rear wheel on the race car makes me think there may be more to it. Certainly the rear is taking ~50% more load than the front- this might prefer a taller wheel...

Sorry shoulda done one post but forgot this...

Corvette pretty much started this whole craze with the C5 having a 1" larger diam wheel and I clearly recall that it was purely a styling thing. Additionally, the Vette isn't rear heavy. And larger diam tires aren't always better load rated - that is sidewall height dependent and the taller the sidewall the lower the load rating. So if you intend to run a 26" diam tire regardless of wheel, then the larger wheel will give a shorter, stiffer sidewall and better performance.

However, Corvette designed by look and trend, not pure performance when they did that. At the time, the larger wheels had been coming out and all the "high end" car builders jumped back onto the old-school 14" front 15" rear except with 16" and 17", then 17" and 18" and now 20" front and 22-24" rear for the "Hot Wheels" look that is so incredibly popular right now. Look at any car winning the Ridler Award in the last few years and every one of them have monster wheels with larger rears than front. That is all about look with those cars and street rods in general, absolutely nothing to do with performance. Because the cars are designed that way, the race cars are simply following the factory by putting same diam but wider wheels with race tires because, at minimum, the car would look funny with smaller rear tires than the fenderwell was shaped too.

I worked with race teams for years and TRUST me, it is amazing how little thought can go into what they are doing v. following what the factory did cuz "they pay those engineers for a reason", but understand that design drives car sales more than engineering going back to the Arkus Duntov era of Corvette for instance.

IMO the big reason for bigger wheels is how much brake you need to have in the car. For 24hr races, the larger brakes will work better to keep heat generation down, so I understand why they run 14"+ diam rotors in serious race cars, but for my piddle-dick run some 20-30 minute sessions race car, that would wasted weight and $$. Also consider NASCAR still forces 15" wheels and those guys don't seem to have too much trouble with brakes over the course of 500 laps at speeds ranging 150-200mph and I have seen those rotors GLOW cherry red heading into the turns at night.