How Big Will Wheels Get?

"MtrRider Just Wants Doritos" (MtrRider) 09/17/2013 at 00:33 • Filed to: None

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The Tesla Model S pictured above has 21" wheels, which would have been huge on a car ten years ago. They don't look out of place on the car either, like many older car with big "donk" wheels. Today, more cars are getting 20" wheels and bigger from the factory. How long will they continue to grow? How big will they be in 15 years?

DasWauto > MtrRider Just Wants Doritos 09/17/2013 at 00:38

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As long as beltlines continue to go higher, wheels will get bigger to compensate (to reduce the slab sided look and to keep the wheels from looking dinky on a huge car).

Meanwhile, I will stick to looking for 15s for my e30 and my mazda3 will stay on 16s (probably, 17 at most, if I decide to bother).

GhostZ > MtrRider Just Wants Doritos 09/17/2013 at 00:42

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Bigger wheels (when done properly, Donks excluded) are almost universally better for cornering and daily driving if you can make more power for the same MPG. I'd say for cutting edge modern cars, about 33 ft/lbs per inch is a good measurement for the reasonable upper limit for wheels. This is provided that the transmission is geared normally.

15" wheels work great on a 200ft/lb car, 18" wheels work great on a 330 ft/lb car, and 21" wheels work great on a 400ft/lb car. Of course, you have to take into account how much weight you are moving.

Beyond those sizes, you're just killing your torque at the wheels without any gain in contact patch, comfort, or efficiency.

PRBot II > MtrRider Just Wants Doritos 09/17/2013 at 00:59

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15 years? We might see dubs on a Corolla. As for theoretical long term stuff... if in the year 3333 pop culture says 1,000" wheels are hot and there's a viable option for making a profit, then... ?

JasonStern911 > GhostZ 09/17/2013 at 01:28

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Please elaborate. It would seem like wider tires would be more beneficial as power increases, but given the same contact patch, a larger diameter rim would only detriment performance unless the purpose was to fit larger brake rotors, as otherwise there is more unsprung mass reducing grip when traveling over an uneven road. I know WRC cars used to run 16" rims on the gravel stages and 18" rims (with larger diameter brake rotors) on the tarmac stages. Given the lack of limitations on rim sizes at the time, it seems like they would have run 20"+ rims on tarmac if there were benefits beyond aesthetics.

Collin > MtrRider Just Wants Doritos 09/17/2013 at 01:34

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In sedans, I don't think we're going to see much of a move past 22 inches.

Then again, the 17s on my E39 look plenty large, thank you very much.

Matt Brown > MtrRider Just Wants Doritos 09/17/2013 at 02:26

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I read somewhere that it started leveling off a couple years ago. I know when you go with bigger wheels you transmit exponentially larger loads into the suspension, so there is some engineering justification to limit the size.

GhostZ > JasonStern911 09/17/2013 at 02:46

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Here's a simple explanation, some of which you may have heard before, but I'll try to be comprehensive:

Road grip is primarily a factor of contact patch, which is the area that the tire makes contact with the road. This is related to surface area on the outside of the tire. Wider tires have a wider contact patch and larger diameter tires have a longer contact patch. However, MPG can be significantly affected by rolling resistance, which is both affected by the weight of the tire, which is related to the volume of the tire.

If you do the formulas, increasing the width of a tire with the same radius and width by 50% will increase its surface area by 50%, but increasing that same tire's radius instead by the same amount (50%) will increase the surface area 360%.

Whereas volume is the opposite. Increasing the radius of that tire by 50% will make its volume 50% larger, wheras making the width 50% longer will make the volume 225% longer.

Since the volume of a tire is related to its weight, increasing the radius will drastically increase the contact patch area (360% for a 50% increase) while only increasing the total volume by less than increasing the width will. Volume isn't directly related to weight (since the inside of the tire is obviously hollow).

I did some more math and assumed a 1 inch thick rim, and ignored the weight of the inside of the rim (since that will probably be about the same no matter how wide the wheel is) as diameter increases, volume of that metal part (and thus, weight) increases by the same amount, but as width increases, that volume increases by a little bit higher amount. The difference is only about 2%, but as that rim gets thicker, the difference increases too. a 2" rim has a 5% higher weight for the same surface area, which can mean a lot in terms of handling. (20lbs vs 21lbs for the same contact patch)

The math isn't perfect, but you get the idea. More diameter means more grip without a huge increase in rolling weight. This is great for modern cars. The problem is that more diameter also means less torque at the wheels, which can slow you down, Or, if your car is too light and spinning wheels, drastically improve your off-the-line grip . It also raises the top speed and allows bigger breaks.

WRC cars probably run 16% rims on gravel to counteract the increased diameter of having thicker-sidewall dirt tires, that, and to increase the torque since tire slip doesn't burn away rubber on dirt as much as it does on gravel.

So why don't WRC cars run 20" rims on tarmac? They don't need the tall gearing. It would slow them down because on an 18" rim, they aren't spinning tires anyway. Widening the tires is difficult, so they increase it as far as they can go to add grip, but without reducing torque too much. Since they aren't going 200+mph also, they can run the same gearing for both gravel and tarmac (so no differential swapping needs to take place) by just switching tires. That's my take on it, but someone who is more informed will probably chime in.

There's also the issue of tire deformation, heating, and the vector of the contact patch (making sure that the force the tires counteract is in the same direction as the contact patch) that really make it more complicated than I can explain, but I hope this sheds some light on it.

Tentacle, Dutchman, drives French > DasWauto 09/17/2013 at 03:38

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This!

Or rather, what you actually refer to is "proportions". Cars largely follow the Rules of Thirds in their design/proportions. Also called The Golden Cut, or Golden Ratio, in the arts world. The ratio is about 1:1,6

See http://en.wikipedia.org/wiki/Golden_ra…

Cars get bigger, so do wheels. Untill engineering gets too tricky. Larger wheels means more unsprung weight and more rotational inertia. Weight can be reduced, but only to a point, beyond which it will be too expensive/economically feasible.

Party-vi > GhostZ 09/17/2013 at 05:28

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Er, larger wheels translate to smaller sidewalls which translates to awful ride quality. Imagine how great new cars would ride if they still had 18" or 17" wheels as standard options.

GhostZ > Party-vi 09/17/2013 at 05:37

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I was talking about larger wheels with the same size sidewalls. Although, improving tire technology has let us reduce the sidewalls without causing the tire to stiffen up (and thus ride worse because more bumps are translated to the suspension and chassis). That being said, good tire technology on large sideways is still the best way to go.

Bob Loblaw Made Me Make a Phoney Phone Call to Edward Rooney > GhostZ 09/17/2013 at 09:29

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Larger wheels with same size sidewalls? Where's that happening?

Turbineguy: Nom de Zoom > MtrRider Just Wants Doritos 09/17/2013 at 10:05

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I had this discussion back in Jan with the head engineer on the C7 at a GM media event in NYC. This is a constant battle between the design guys and the engineers. Design guys want bigger wheels for proportion and visual excitement, while the engineer types see them as greater unsprung weight and more NVH issues to deal with. I'm surprised no punches have been thrown yet.

feather-throttle-not-hair > MtrRider Just Wants Doritos 09/17/2013 at 13:54

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You can buy people movers with stock dubs. Don't think this is going to stop any time soon.

JasonStern911 > GhostZ 09/17/2013 at 23:15

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Thanks for the write-up.

Who knew these guys were the pioneers of maximum traction? :)

jeebus > MtrRider Just Wants Doritos 09/18/2013 at 14:00

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Model T's rocked 21s too. So I guess nothing has really changed.

Boss2452stolemylunchmoney > GhostZ 09/18/2013 at 22:56

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The big thing that you're missing in terms of larger diameter is inertia. Inertia increases to the 4th power as radius increases while area only increases to the 2nd power. Therefore, larger wheels severely and negatively impact acceleration both in acceleration and in braking. Larger wheels will roll better and provide marginally better traction, especially over irregular surfaces, but wider wheels corner better.