Rolling resistance: Tyre diameter vs width...

[CommodAnt]

Hi guys,
I'm looking at filling up the wheel arches on my ride to not only make it look better, but to fill the void surrounding the wheel which has got to be hurting fuel economy. I would ideally like to maintain the same level of traction from the vehicle (it still needs to stop and handle well), but overcome these aforementioned issues.

It is generally accepted that a larger diameter wheel is good for fuel economy due to lower rotating speeds within the drivetrain and wheel bearings leading to less friction, but bad for acceleration due to the associated gearing changes. I am assuming that due to the larger rims having a larger contact patch with the road that this would increase rolling resistance from the tire itself?

If this is the case, then why not fit larger diameter wheels (+2 in) with a narrower profile (-1/2 in) to get a comparable contact patch with the road, and decrease all other friction processes internal to the vehicle (assuming I'd already done the calculations - which I haven't).

Am I missing something here or does this seem like common sense that has been overlooked? Please discuss.

[theholycow]

Larger diameter tires have less rolling resistance, according to some studies I read a couple days ago, as well as according to my own knowledge of how tires work.

I thought that contact patch was almost entirely dependent on air pressure and nothing else, but I do have some doubts now. Still, I continue to advocate taller and wider tires for their effect on gearing and rolling resistance. OTOH, they weigh more and could have more aerodynamic drag -- though it sounds like you're looking to fill your wheelwell more completely to reduce aerodynamic drag.

Based on the new information I read, a taller wider tire will have lower RR not because of change in contact patch, but because of lower load on the tire. Put simply, a standard modern automotive tubless radial tire's load capacity is determined by the volume and pressure of air it holds, and a less loaded tire has less RR. A taller wider tire will be less loaded than a shorter narrower tire at the same weight.

I've got two threads about this linked in my sig, one about tire width and the other about pressure. In my research the other day, I posted a crapload of links and analyzations to one of those threads, I think the pressure one.

I'd say the big issues with taller tires are:
- Additional ride height for the car may be aerodynamically bad
- The effect on gearing has to be decided for each car and driver. If you think taller gearing will help you, good.

[Rower4VT]

Contact patch is mostly dependent on tire pressure and vehicle weight. Doesn't sound like it could be completely true...but it is. There are websites out there that explain the math of it. Like a bicycle, taller, narrower, higher-pressure tires provide for lower rolling resistance.

Check out Continental Vanco tires. I think I'm getting a set of 195/70R15 to replace my 205/60R15. The Vanco tires are built for small delivery trucks and heavier vans. They handle higher pressure, have a stiff sidewall (better handling), and the tread is designed for lower rolling resistance. Some of the Vanco models I believe are rated at 65-70psi. Might give a bit rougher ride, but I bet you coast a lot further than you could before.

[theholycow]

Originally Posted by Rower4VT

Contact patch is mostly dependent on tire pressure and vehicle weight. Doesn't sound like it could be completely true...but it is. There are websites out there that explain the math of it. Like a bicycle, taller, narrower, higher-pressure tires provide for lower rolling resistance.

I've been saying that for years.

Unfortunately, it may be untrue. In the last week there have been experiments and measurements by one gassavers.org user that may show that it's wrong, and I found some reference material that says it's wrong too. If it is wrong, I can't say I understand why, but it is what it is.

Take a look at the links in my sig about tire pressure and tire width. It's all in those.

[CommodAnt]

Originally Posted by Rower4VT

Contact patch is mostly dependent on tire pressure and vehicle weight. Doesn't sound like it could be completely true...but it is. There are websites out there that explain the math of it. Like a bicycle, taller, narrower, higher-pressure tires provide for lower rolling resistance.

Check out Continental Vanco tires. I think I'm getting a set of 195/70R15 to replace my 205/60R15. The Vanco tires are built for small delivery trucks and heavier vans. They handle higher pressure, have a stiff sidewall (better handling), and the tread is designed for lower rolling resistance. Some of the Vanco models I believe are rated at 65-70psi. Might give a bit rougher ride, but I bet you coast a lot further than you could before.

I'm wondering if the stiffened sidewall = more weight. I read that shaving 1kg of each wheel was the equivalent to shaving 10kg off the entire car as far as accelleration is concerned.... I suppose that also applies to decelleration, so it might not be such a bad thing. 70psi will be one hell of a rough ride.

[Rower4VT]

The one's I'm looking at are the Vanco2's. They're actually a pound lighter than what I have now...dispite the fact that the sidewall is 1/2 inch taller. Max psi is 51, so I actually probably won't exceed 55psi on these tires if I get them. In terms of a taller+narrower tire to replace a 205/60r15, I don't have many choices. There are more options for 205/65's and 195/65's but they won't give me the additional "overdrive" I'm looking to get. 205/70r15 might be an option also, but I'm thinking there would be significant rubbing....the only way to know for sure would we to test them out, and I don't have the time or money to do so...unless I get luck on craigslist or something.