Optimum wheel diameter for competition use.

[TheJDMan]

Another observation I made while watching the CTS autocross was that very tall wheels 20" and bigger don't seem to be used on any of the really serious competition autocross cars. Yes, there were a number of late model Camaros, Mustangs and one SSR with 20" diameter wheels but all the older PT style cars seemed to be running 18" wheels. Even the 2015 Z/28 comes equipped with 19" dia wheels. That would seem to indicate that there is no performance advantage in running 20" or larger wheels. I also see that many sports car road racing series like CanAm and even Indycar seem to be running 18" dia wheels. Now I realize the the sanctioning bodies in CanAm and Indy probably regulate the wheel dia but that is not the case in autocross. I know of no rule in autocross that regulates what wheel diameter you can or cannot run. So from a purely competition performance standpoint what does everyone consider to be the optimum wheel diameter for autocross?

[bryant]

the bigger the wheel the heavier it is. i suspect that is part of the equation. beyond that not sure what diameter is optimum.

[srh3trinity]

I remember a few years back where they went through this with Bad Penny. He decided 17's were the sweet spot, but tire selection and brake size pushed him back to 18's. That is what I see now. 18 is about as small as you can run with big brakes and there are more competition tires available in that size compared to a 17 or a 19.

[Tomswheels]

One advantage is simple, on my 2013 Mustang, 18 inch wheels with 275/35/18 offered quite a bit lower roll center than taller 19 or 20s...

[Craig510]

It all depends on the class. CP has a weight penalty for larger than 16". Goodguys and optima have a 200tw rule. 315/30R18s are the widest Falkens, Dunlops max out at 285, BFG makes a 335, but they have not been (readily) available this season. As far as I know, Nitto is the only brand making wider than a 275 in a 17, but a 255 Falken has about the same grip, so if you want wider rubber you have to go 18.

[parsonsj]

Reposted:

Wheels are far heavier than tires -- so you can discount the tire part, and worry only about wheels (and the wheel/tire combination tends to have the same diameter, so again, you can discount the tire). The physics about wheel weights is called moment of inertia, or MOI.

The basic equation for MOI for a cylinder rotating around a center axis is I=M*R(squared). M = mass, R is the radius. So while weight matters, radius (diameter) matters a whole lot more.

Most of a wheel's mass is in in the hoop, so the approximation of a wheel as a cylinder is appropriate.

Using your 18" vs 20" example:

MOI of 18" wheel (let's say it weighs 25 lbs): = 25 lbs * 9(squared) = 2025 lbs square inches.
MOI of 20" wheel (let's say it weighs 28 lbs): = 28 * 10(squared) = 2800 lbs square inches.

It weighs 10% more, but its MOI is 38% more. In other words, the 20" wheels require 38% more energy to accelerate/decelerate than do the 18" wheels.

A rule of thumb from Mazda Racing is that each 1" of wheel diameter effectively adds 50 lbs of weight to your vehicle. Again, using the 18 vs 20 wheel example, that's 2" * 4 wheels * 50 lbs = 400 lbs. If you put 20" wheels on your car, it will "feel" as if it weighs 400 lbs more than if you used 18" wheels.

[parsonsj]

Bigger wheels hurt performance -- all the time, in all forms of acceleration (positive, negative, and change of direction).

Bigger brakes help performance, but only for negative acceleration, and only really in cases where brake fade becomes an issue. They hurt performance for positive acceleration and change of direction.

So the dynamic is to figure out the best compromise. For low speed autocross (< 60 mph), 15 or 16 inch wheels are probably best, since a 40-80s autocross run isn't going to cause brake fade. On road courses, bigger wheels to allow 13 to 14 inch brakes are probably best, so 17 or 18 inch wheels are needed to package the brakes.

Of course, that all assumes you can get good tires in all of those sizes... which you can't. So most of us who spend a lot of time on the track end up at 18" wheels. There are lots of tire choices, and lots of brake choices available in that size.

[dontlifttoshift]

Fixt.


One advantage is simple, on my 2013 Mustang, 18 inch wheels with 275/35/18 offered quite a bit lower center of gravity than taller 19 or 20s...

[JustJohn]

It's nit-picking, but I would have used the hollow cylinder approximation to at least take out hub diameter. Assuming a 4" hub and I=1/2m (Ri^2 +Ro^2) you would get 4250 and 5825. Higher values but 27% more relative work. Agreed that even shrinking tire sidewalls won't be enough to make up the difference.

There can be a break even point with tires if you assume weightless wheels. Probably see that when we get magnetic bearings. Given a 25" outside tire diameter weighing 28 lbs on an 18 inch wheel, you could achieve roughly the same inertial mass on a 20 inch wheel with a tire that weighed 25.8 lbs.


Reposted:

Wheels are far heavier than tires -- so you can discount the tire part, and worry only about wheels (and the wheel/tire combination tends to have the same diameter, so again, you can discount the tire). The physics about wheel weights is called moment of inertia, or MOI.

The basic equation for MOI for a cylinder rotating around a center axis is I=M*R(squared). M = mass, R is the radius. So while weight matters, radius (diameter) matters a whole lot more.

Most of a wheel's mass is in in the hoop, so the approximation of a wheel as a cylinder is appropriate.

Using your 18" vs 20" example:

MOI of 18" wheel (let's say it weighs 25 lbs): = 25 lbs * 9(squared) = 2025 lbs square inches.
MOI of 20" wheel (let's say it weighs 28 lbs): = 28 * 10(squared) = 2800 lbs square inches.

It weighs 10% more, but its MOI is 38% more. In other words, the 20" wheels require 38% more energy to accelerate/decelerate than do the 18" wheels.

A rule of thumb from Mazda Racing is that each 1" of wheel diameter effectively adds 50 lbs of weight to your vehicle. Again, using the 18 vs 20 wheel example, that's 2" * 4 wheels * 50 lbs = 400 lbs. If you put 20" wheels on your car, it will "feel" as if it weighs 400 lbs more than if you used 18" wheels.

[parsonsj]

There can be a break even point with tires if you assume weightless wheels. Probably see that when we get magnetic bearings. I'm not familiar with "magnetic bearings". Do you mind explaining a bit?

[JustJohn]

Sure:
112402112403
Probably 50 years from a consumer application. Really solves the wheel weight problem though.

[parsonsj]

Does the electromagnet also provide the rotational force on the rotor?

[LS7 Z/28]

Reposted:

Wheels are far heavier than tires -- so you can discount the tire part, and worry only about wheels.

I don't know 1/10 of what you do, but based off of the last set of wheels and tires I bought I didn't think this was the case. As odd as it seems I think that these overly heavy street tires like BFG rivals for example in 335/30ZR18 weigh close to, or equal to what a decent lightweight three piece 18X12 wheel weighs. Maybe I'm crazy. I wish I would have scaled them before mounting them, but these "performance" street radials are heavy as hell and modern lightweight wheels are remarkably light for their load rating. Maybe I'm way off but I saw this posted before on a different thread and it made me think about it and question this statement.

[parsonsj]

but I saw this posted before on a different thread and it made me think about it and question this statement.Let me try again: tire diameters are going to be about the same, so the only difference between a 25" tire for a 20" wheel and 25" tire for an 18" wheel is the difference in sidewall height. The weight difference there is pretty small, and can be ignored, especially since the driving factor for the tire's MOI is its diameter, which is the same in both cases. So we can discount the tire, and concentrate on the wheel.

Hope that's clearer.

[JustJohn]

Does the electromagnet also provide the rotational force on the rotor?

It can but then you are really just walking away from a combustion engine altogether. Probably the best example is Japan's mag lev trains, which are in essence big linear bearings oriented to also provide motive force.

I really wasn't trying to be a jerk or derail the original OP's discussion, just make the point that the negative impacts of large diameter wheel's increasing MOI penalty can be minimized with careful design and material selection when you have to compromise for other factors like large brakes.

Most CAD packages will calculate MOI with a few button presses and I'm surprised to never have seen the influence of optimizing this part of the design for competition level wheels.

[parsonsj]

I'm surprised to never have seen the influence of optimizing this part of the design for competition level wheels.I think most motorsports sanctioning bodies drive this based on tire manufacturers' participation. For example, Formula1 uses 13" wheels, and has for a considerable time, I believe. There is some movement to an 18" wheel, but that's only to make the cars more "relatable" to fans, and that is being driven by Pirelli (the sponsoring tire manufacturer for F1). Given the current cars' capability to generate more than 5g braking force (where tears from the drivers spatter the inside of their helmets), there is no need for more brake on those cars. The 18" movement is being driven by Pirelli's marketing.

In Sprint Cup, that's a 15" wheel, and is also driven by NASCAR which is being driven by Goodyear. Some of the reason for keeping the wheels and tires the same is to reduce team costs.

Anyway, the point is that most motorsports regulate wheels and tires, and so that is probably stifling innovation with wheel sizes. Within the size constraints, the F1 teams do use advanced materials and state of the art design to reduce weight and aerodynamic drag.

[alcino]

For me I look at it from the tire budget side. As wheel sizes get taller and wider so does the tire's prices that fit those wheels. This can be applied to other tire types but since this is pro-touring I am assuming no restrictions on wheels and 200TW tires. Well, not many manufactures make these "Extreme Performance Summer" tires and sizes are really limited, especially the wider sizes. Also all the serious autoxers are buying 2 sets of tires(slicks or 200tw) a year to stay competitive. Not because of wear but because of the loss of grip from heat cycling. I got by with one set last year(full season autox and 11k driving) but after the half I was getting noticeably slower to the point where I was 2-3 seconds off in a 60 second course(this is a lot!). My friend who does track days(HPDE) 5-7 times a year gets 3 sets a year. As you can see a tire budget can quickly surpass an expensive set of custom wheels in a short time. So this year I'm trying to budget for a second set before the 3/4 mark. Helping this budget was staying with a smaller(but big) 17"x10" wheel. All the 200TW brands make the 255/40/17 size(plus a few 275s) giving me plenty of options and they run roughly $180-200 a tire. These same width tires in 18, 19, 20" jump an extra $60-130 each. Plus if they are wider (300+ mm) you could be looking at $400+ a tire(ouch michelin PS2!).

To be competitive you must have quality tires period! The wider and fresher, the better...but it comes back to the old adage that "speed costs, how fast do you want to go?" Well I want to go as fast as 255/40/17 twice a year and my skills will let me. If budget doesn't matter then get the widest tire you can find and fit in car, then get the wheels that fit them.

[parsonsj]

If budget doesn't matter then get the widest tire you can find and fit in car, then get the wheels that fit them.I agree. The bigger the footprint the better. I'd only add that you should use your front tires as the limit, and keep the backs within 40mm of that size. A rear tire that's a lot bigger than the front will cause significant understeer (push) and that's no fun on the AutoX.

[Cyclone03]

I still a noobe here and to autocross. I'm running NITTO NT555 300TW tires. I'm planning to use them up this year while I learn to drive.

I'm running an 18X8 wheel all around on my 68 Mustang. Currently I'm running 225/45 18 I need the height to keep the headers off the ground, I will most likely change the wheels when I go to a 200TW tire. Now truth is if I could get a good 200tw 15 inch tire I would change my front brakes so I could use my friends Mini Lites ,but that aint going to happen. While playing around with different wheels I've found I can fit a 19X9 on the front! because the 19" wheel will go over the upper control arm ball joint.
Now in an autocross is a wider front tire generally faster. I have more testing to do but I'm thinking a 235/40 is pretty much my limit with an 18x8 5"BS and the same with a 17x8 (may need 1/8-1/4" spacer),but a 19X9 may be doable with a 245. Will I loose more than I gain due to centripetal force with the 19 ?
Is wider always better?

[CarlC]

As a safety note, not all aftermarket wheel manufacturers are created equal. "Engineering" is often someone performing some CAD work to make a pretty design with zero thought of the dynamics of the application. Some of the designs are scary, and some have failed under hard use.

During testing with the Camaro years ago 17's were the sweet spot but the 18's had not yet been fully embraced by the aftermarket. The 18's on the car now are far and away the best package @ 18 x 9.5 275/35 on all corners. Since the tire is everything, we chose the tire that best suited what the car would be subjected to (One Lap) then picked a rim, which dictated brake size limits, which led to suspension tuning.......

[TonyHuntimer]

There's some great information here! 19s and 20s are just too big for the project I'm working on. This thread just adds extra focus on keeping it locked in 18s.

Now if I could find a strong (and good looking) magnesium wheel in an 18" diameter that, it would be awesome! It will have to be 11" wide for 315s all the way around.

:)
Tony Huntimer
TestCarDatabase.com