C&D: Effects of Upsized Wheels

[Kelvari]

Found something interesting while surfing the web tonight. Apparently Car & Driver did a test using different sized wheels on VW Golf (15" - 19") and posted their results.

Quick summary is that smaller wheels are better for performance and fuel economy, while larger wheels are better for braking and cornering.

Full article here

 

[Zenith]

It's really hard to make big claims about something like that from a few tests...

Larger wheels and tires have the effect of gearing the car up. This slows acceleration and lowers the rotation rate of the wheels relative to ground speed. This can increase or decrease your car's mileage depending on what speeds you're driving at. If I run 3000 rpm at 70mph then increase the size of the wheels and tires, my car will rev lower at the same speed.

Increasing rotational inertia (Not weight, not mass, inertia) also increases the amount of effort the engine will have to put out in order to accelerate the car and the amount of effort the brakes will have to put out in order to decelerate the car. Usually having bigger wheels and tires does this, but not always.

Changing the the diameter of the wheel tire combo also makes your speedometer inaccurate. My car's speedo is 10% low.

 

[Anna+Reece]

Hmm. Interesting. Though I do know, in the drifting scene, it is common for people with low power cars to run smaller wheels, because it is easier to break traction with smaller wheels.

 

[MrWednesday]

Hmm. Interesting. Though I do know, in the drifting scene, it is common for people with low power cars to run smaller wheels, because it is easier to break traction with smaller wheels.

Smaller wheels = shorter effective gearing (greater effective gear ratio)

The force the wheels deliver to the road is equal to the torque at the wheels divided by the wheel radius. At the same torque, a smaller wheel will direct more force to the pavement.

 

[niky]

Note... this was a test of wheel size... not tire size. They kept all as close as possible to overall stock diameter.

You can see this in their chart, where they show speedometer error, which was the same for most of the tires except the biggest one.

Not really news. There have been countless similar tests before. Bigger wheels, more inertia, slower car, worse economy. Smaller wheels, the opposite, but less cornering grip... although the overall difference in braking is tiny... a spread of less than ten feet. That can easily fall within test-to-test variability.

What would be fun is if they examined how much pressure you'd have to pump into the smaller sizes to match the lateral skidpad grip on the larger sizes... though that would definitely negatively affect braking.

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I doubt the W or V rating had anything to do with the results. It's the carcass that determines the speed rating, not the compound. You can get cheapo tires rated as V or W. And Yokohama offers Advan racing tires in small sizes that are only "H" rated. I wouldn't read as much into that difference as C&D did.

 

[Keef]

It gets slightly more complicated than that when you factor in that not all wheels weigh the same. Some, like the ASAs from the test (or similar ASAs because that design isn't available for Golfs anymore), weigh 23 pounds a piece in 17x8, whereas my RPF1s weigh about 15 while being an inch wider with a lower offset and therefore more massive hub.

And then of course you have to account for the weight of the tire itself which is both less of an issue because a high performance tire's benefits are well worth its weight, but also more of an issue because the tire's weight is around the perimeter of the rotating mass which amplifies its effect. They used all-season tires in the test and performance or not they're still relatively thin with weak sidewalls. The construction of my NT01s or similar high-end summer tires is considerably more stout - my tires in 235/40-17 weigh about 23 pounds a piece, nearly twice as heavy as the 205/60-15 all-seasons they replaced despite way smaller sidewalls which is the strongest part of the tire.

And if you want to get really complicated we can talk about how the rigidity of the wheel under load effects the car's overall handling performance. Option magazine out of Japan tested this recently by modifying an identical set of wheels to shave weight. Arguably not nearly as effective as engineering a light wheel from scratch but nevertheless an interesting read. I'm always impressed by Japan's attention to detail not seen elsewhere in the aftermarket community.

 

[dudejo]

One thing I'd be interested in knowing is what provides more inertia between the drivetrain and the road wheels.