Why don't car manufacturers....

[shinn497]

make cars designed with smaller diameter wheels? In addition, why don't they make the speed sensor able to be reset so that you can change overall wheel diameter without messing with the speedometer.

I was looking at the FR-HS concept car 17411 and the one thing that popped out at me was just how massive those wheels air. The thing is this is a hybrid car so it makes me wonder if there are other solutions. In particular why not use smaller diameter with a smaller side wall. You can keep the low profile stiffness, get more suspension travel, and keep the handling feel while getting improved acceleration and fuel economy.

The only advantage I see to larger diameter wheels is the increased contact patch. (as a physicist I know this shouldn't be true but it is) .

Is there something I'm missing. I'm in my mid 20's and I think almost any wheel over 16 inches looks positively stupid (In a jeremy clarkson accent). Then again, used to own a car with 18s that was always cracking wheels and losing tires to potholes (yay california)...call me cray.

Anyway, sorry to rant, but I'm wondering if there really is a reason for this outside of aesthetics/trends/herrafrush.

[Jeff Kleiner]

One practical reason is to allow fitting larger brakes within the wheel but the reality is that in many cases it is simply fashion.

Remember that tire diameter is what plays into speed sensors, filling wheelwells, etc. A 15" wheel/tire combination can have the same overall diameter as an 18" one depending upon the tire's aspect ratio. Similarly wheel diameter does not dictate contact patch; section width, aspect ratio and to a lesser extent wheel width do.

Jeff

[Rich]

One practical reason is to allow fitting larger brakes within the wheel but the reality is that in many cases it is simply fashion.

Remember that tire diameter is what plays into speed sensors, filling wheelwells, etc. A 15" wheel/tire combination can have the same overall diameter as an 18" one depending upon the tire's aspect ratio. Similarly wheel diameter does not dictate contact patch; section width, aspect ratio and to a lesser extent wheel width do.

Jeff

Perfect answer ^^

[shinn497]

Shazbot I think I was talking out my *** about the contact patch vs diameter. Physically speaking, in an ideal world, it should be based soley on the pressure and mass (of the car) ONLY. Wheel width relates moresoe to the material used. I.E. softer compound == less sidewall stiffness == thicker width.

I remember seeing some experimental data that demonstrated pretty clearly that that wheels behaved very non-ideal and that diameter and width did indeed lead to a larger contact patch. Furthermore, they also calculated that the contact patch size was smaller than that required to lift the car up. This made sense considered that the sidewalls would also contribute.

I was looking into brake/sizes and your right. Many super cars have big brakes because cooling and brake fad are an issue and larger brakes can dissipate heat better. But this is only due to the fact that many super cars are surprisingly heavy 3000+ pounds. The ones that are lighter, tend to go REALLY fast and need the better cooling anyway.

However, if you look at really really light cars (elise, atom, etc. etc. ) they dont require such large brakes and wheels. The elise's maximum wheel size is 16.

Wouldn't it be great if someone made a sub 2000 pound car that could have a power to weight ratio that rivaled a super car.? Then it could use just stock car brakes and be fine. Hrmmm hrmmm.

Anyway certainly for an obtainable car like the FT-HS or brz such large wheels really aren't a necessity.....

[PhyrraM]

I think it is mostly styling, or more specifically proportions.

Cars are getting huge. Wheels would look tiny in the traditional sizes. The showcar crowd always pushes boundaries, but when the 'big wheel thing' started taking off, the OEMs followed because they likely saw the problem coming. When the Magnum/Charger/300 came out I remember thinking "Holy crap, that car must be huge to make 20" wheels look good".

Don't F1 cars still use 15" (or close) wheels?

[CHOTIS BILL]

For some reason I am thinking F1 use 13” wheels. Probable wrong though.

Bill Lomenick

[G_Hodgkins]

It all has to do with styling and proportions. It looks odd to have tiny wheels

[shinn497]

Yeah f1 cars are 13 inches in wheel diameter. The tire diameter is twice that. I think this makes perfect sense. However there could also be regulations.

[shinn497]

On a side note I live in socal and I see a bunch of SUVs with 22" rims.

Clarkson was right.

[Mike N]

Large diameter wheels are very much a market (style) driven thing. Yes I understand fitting big(ger) brakes but it has gone way beyond that.. Reducing rotational inertia, unsprung mass etc would all help performance and economy so it would seem that the real technical advancement would be to go smaller and not bigger. Don't tell me it can't be done, the 13" wheels on a F1 car prove that wrong. I'm surprised that the Prius actually has 15" wheels and not 13's or even 12's.

[CHOTIS BILL]

I did a quick Google search for F1 wheel size and the only link that worked seems to say that F1 uses 13” and Champ cars use 15” wheels.

Why Formula 1 (13inch) and Champ Car (15inch) use different wheel size?

Bill Lomenick

[VD2021]

Another reason is the longer circumference. A full rotation allows the vehicle to travel further.

[tirod]

Overall diameter and gearing have limits. What we are seeing is fashion. There may have been some benefits with lower profile tires and grip, but the offsetting advantages are already mentioned - short sidewalls are much stiffer and rim damage is now common.

Off road racers don't use short sidewall tires, they can't take the punishment of 85mph impacts on broken rock. Same for rockcrawlers, at 2 psi, the tire needs to wrap and grip slick rock. Short sidewalls simply don't.

Car and Driver did a track test of tires, rating each incremental increase in rim diameter with the same spec tire and measuring the differences. In 1/4 times, the larger rims got slower as the diameter increased. In ride, it got harsher and the transients sharper to the point of being abrupt, on the track there was improvement in grip with an incremental reduction in overall lap time. Had rotor size increased, times would have fallen further.

Overall, just increasing rim size nets little gain without corresponding changes in gearing, damping, spring rate, and brake improvements, but will get you the negatives of higher costs and more frequent damage. All to the makers benefit in profits.

What we have are lemmings driven by the fantasy of "cool looking rims" and the complete lack of education about what they are really doing - installing a low profile dry summer tire for use 24/7/365. It's all about looks, not real performance on bad roads in horrible weather. What we are suffering is just like '50s tail fins, the '60s with air shocks, or today's diesel commuter trucks. Fashion, not reality.

Goes to "Observe the masses and do the opposite." I'm picking up an AWD '99 Forester today. If all the tires do their fair share, then being forced to use less capable tires and rims is minimized.

Nothing to do with any future kit plans, not at all.