Larger diameter rear wheel - Why?

[mrsupra]

There is a recent trend in sports car/supercar design that I do not understand - why are the rear wheels a larger diameter than the front? I'm not asking about width (obviously rear wheels are wider to fit wider tires), I'm asking about diameter. It seems that every modern sports car - from the Corvette and Evora to the LaFerrari and 918 Spyder - has rear wheels that are of a larger diameter than the fronts. This makes no sense to me. What is the functional purpose for this, if any? It isn't because of the brakes, because the front brakes are usually larger. As far as I can tell, all it does is increase unsprung weight. Is there any functional reason to have larger diameter rear wheels, or is it just a styling thing?

Thanks.

[giantfan10]

There is a recent trend in sports car/supercar design that I do not understand - why are the rear wheels a larger diameter than the front? I'm not asking about width (obviously rear wheels are wider to fit wider tires), I'm asking about diameter. It seems that every modern sports car - from the Corvette and Evora to the LaFerrari and 918 Spyder - has rear wheels that are of a larger diameter than the fronts. This makes no sense to me. What is the functional purpose for this, if any? It isn't because of the brakes, because the front brakes are usually larger. As far as I can tell, all it does is increase unsprung weight. Is there any functional reason to have larger diameter rear wheels, or is it just a styling thing?

Thanks.

maybe its because the larger the drive wheel the more distance the car travels per each revolution of said wheel versus a wheel with a smaller diameter

[mrsupra]

There is a recent trend in sports car/supercar design that I do not understand - why are the rear wheels a larger diameter than the front? I'm not asking about width (obviously rear wheels are wider to fit wider tires), I'm asking about diameter. It seems that every modern sports car - from the Corvette and Evora to the LaFerrari and 918 Spyder - has rear wheels that are of a larger diameter than the fronts. This makes no sense to me. What is the functional purpose for this, if any? It isn't because of the brakes, because the front brakes are usually larger. As far as I can tell, all it does is increase unsprung weight. Is there any functional reason to have larger diameter rear wheels, or is it just a styling thing?

Thanks.

maybe its because the larger the drive wheel the more distance the car travels per each revolution of said wheel versus a wheel with a smaller diameter

That could just be done with different gear ratios, though. Also some AWD cars (911 turbo, for example) have larger diameter rear wheels, so that can't be it.

[Tim.Wright]

Possibly to balance the wear/durability of the front and rear tyres on high powered rwd cars.

[ian_s]

it'll be 100% down to styling and have no sound technical reason behind it.

[Tim.Wright]

That's also a large possibility.

[Facts Only]

Interesting question. I wonder if from a styling and dynamics point of view they wanted a larger diameter rear tyre but didnt want a larger sidewall than the front so increased the rear wheel size as well.

[Jolle]

I can see different reasons why to have different sizes front and rear. Esthetics is one, large diameter would mean a larger contact spot on the tarmac and smaller wheels would mean less weight for faster steering.

so sizing wheels, from diameter, how wide and hight of the sidewall should be all tuned for its particular function. It's almost silly that with most cars the diameter is the same front an rear.

[J.A.W.]

I can see different reasons why to have different sizes front and rear. Esthetics is one, large diameter would mean a larger contact spot on the tarmac and smaller wheels would mean less weight for faster steering.

so sizing wheels, from diameter, how wide and hight of the sidewall should be all tuned for its particular function. It's almost silly that with most cars the diameter is the same front an rear.

"Its almost silly that with most cars..."

If by "most cars" - you mean ordinary every-day road vehicles..
.. then it is surely -a matter of being rational & practical.. that all 4 road wheels ( + spare) are the same/interchangeable..

Of course for mega-pricey supercars, such tedious conventions are - per se - irrelevant..

[bl4zar_]

Couldn't it be that since most of the performance cars have rear traction, rear tyres are affected by higher deformation of the sidewall, and so a lager wheel diameter (-> smaller tyre sidewall) is appreciated?

[DaveKillens]

There is a relationship between diameter and contact patch. For a given width, if you increase the diameter, you increase the contact patch.

Thus, if the regulations limit the rear tire width but you require more contact patch, then you would increase diameter if allowed.

Usually cars are also limited in total width. If instead of increasing the diameter, you would then make the tire wider, and that would have a possible impact on room for the suspension geometry. You make the tire wider by 2 cm, then you would probably have to make the suspension arms 2 cm shorter or attempt to relocate the pick-up points closer to the center of the car.

[Jolle]

I can see different reasons why to have different sizes front and rear. Esthetics is one, large diameter would mean a larger contact spot on the tarmac and smaller wheels would mean less weight for faster steering.

so sizing wheels, from diameter, how wide and hight of the sidewall should be all tuned for its particular function. It's almost silly that with most cars the diameter is the same front an rear.

"Its almost silly that with most cars..."

If by "most cars" - you mean ordinary every-day road vehicles..
.. then it is surely -a matter of being rational & practical.. that all 4 road wheels ( + spare) are the same/interchangeable..

Of course for mega-pricey supercars, such tedious conventions are - per se - irrelevant..

Dare to think outside the box a bit. In a normal boring four door fwd saloon, the rear tires do almost nothing. They could be of a harder compound, quite narrow and with a bigger diameter for the least resistance. Many new cars haven't got a spare anyway, and with nice hard tires, they could endure the lifetime of the car itself.

[J.A.W.]

Dare to think outside the box a bit. In a normal boring four door fwd saloon, the rear tires do almost nothing. They could be of a harder compound, quite narrow and with a bigger diameter for the least resistance. Many new cars haven't got a spare anyway, and with nice hard tires, they could endure the lifetime of the car itself.

"Dare to think outside the box a bit"?

Yeah, J..
I suggest you try that line on the makers of those boring econo-boxes who have to build them to comply with gov't mandated
road rules which specify such commonality between the road wheels on 'safety' grounds..

[Tim.Wright]

In a normal boring four door fwd saloon, the rear tires do almost nothing. They could be of a harder compound, quite narrow and with a bigger diameter for the least resistance. Many new cars haven't got a spare anyway, and with nice hard tires, they could endure the lifetime of the car itself.

Rear grip is what gives you stability. I.e. stops you from losing control. It also does the job of straightening the car up on corner exit as the steering is returned to zero. All that comes from the rear tyres so it's not correct to say they do almost nothing.

[George-Jung]

Could it also be that a larger diameter rear wheel than the front.. results in slightly less revalutions compared to the front.. hence cancelling out temp. difference between front and rear.. due to RWD?