tire width vs. drag (Cd)

[3-Wheeler]

Originally Posted by winkosmosis

...So that's a typo? Wider tires really do decrease rolling resistance? I knew it...

Actually, saying a wider tire alone decreases rolling resistance is half the story.

Any tire shape that increases the contact patch with the road decreases rolling resistance.

The main reason why?

The larger contact patch allows the tire carcass to flex less, which in turn takes less energy to roll.

Pumping a tire to a higher pressure accomplishes the same thing, but not as greatly as going to a larger contact area.

Why?

Because as you pump up the tire, what happens to the contact patch?

Right, it gets even smaller, which then in turn causes the carcass to be stressed over a smaller area.

Instead, think ATV tires or similar and you get the idea on a better approach.

However there are side effects:

1) A big balloon tire can easily upset the handling of the vehicle
2) Offers more frontal area to the wind, with an increase in drag
3) In extreme cases can raise the vehicle CG and affect handling
4) More unsprung mass, which lowers ability to respond to bumps
5) More gas to accelerate the mass of the larger tires
6) Harder on brakes when slowing down
7) Less mechanical advantage of disks/drums, making pedal pressure go up

Jim

[MetroMPG]

Originally Posted by CapriRacer

I got it from the "Tires and Passenger Vehicle Fuel Economy" Report

Thanks for that. And for posting your web site - a great resource.

I learned something new in this thread.

[donee]

Hi All,

What am I missing? The experience in the Prius comunity is that the bigger section tires have worse mileage. Dramatically worse. For example, people who have put on 17's (225 section?) have a hard time toping 40 mpg as reported on Prius chat, in 2nd Gen Prius. Wheras people with the 185R15's easily top 50 mpg. Both the tires have about about the same diameter.

Also, a constant load rating tires that is bigger in diameter will have a smaller section (similar enclosed air volume). One would expect the larger diameter tire to have less rolling resistance due the smaller flexing.

[Frank Lee]

Like most everything there's likely a continuum.

[MetroMPG]

I don't think this thread is meant to suggest that wider is automatically better.

I would assume that "wider = lower Crr" is true when you compare tires with otherwise identical design & materials.

If you switch tire design, then it's apples & oranges. Maybe that's what happened with the Prius people.

EG. I've shown that my 155/80/13 tires roll significantly farther than my 175/70/13 tires. But (A) they're different brands, and (B) very different tread designs. (I can't speak about (C) the materials in them).

[orange4boy]

Thread resurrection.

Been looking into this again. Part of the reason tire width makes such a big difference in the real world is that wheels have such a bad CD that a small change in their frontal area has a big impact on the total CD of a car.

Tires and wheels count for around 50% of the drag in a streamlined car like the Prius but are only a small part of the frontal area. Seemingly small changes in wheels should make a proportionally larger difference in total drag.

The air under a car flows outward from the centre and hits the tires at a yaw angle. Because of this all four tires are, for practical purposes, part of the frontal area. If we assume that's the case this may then double the frontal area change from the usual assumption when estimating changes in FE from wider tires. (I didn't do this in the following calculation)

At highway speeds these changes would far outweigh changes in RR.

This would seem to back up the assertions of many Prius owners who have seen a big reduction in their MPG from a increase in tire width.

Plugging some numbers into the calculator also backed up my thoughts on the subject.

I got about a 4% reduction in mpg from wider tires using the following:

Data from http://www.energy.ca.gov/transportat...%20Testing.pdf(thanks to Capriracer for finding that.)
In that study the CRR dropped .002 going from a 175 to a 235 which closely matches the change in width in the Hucho study Darin posted.
CD + .026 (Hucho)
Frontal area + .4 square feet. (calculated)

Results at 60 mph Narrow tire: 45.67 mpg wide: 43.74 mpg: ~4% difference
@ 75 mph 5% difference
At 40 mph it's a wash.

There are no hard numbers showing a strong correlation between tire width and RR. There does seem to be a slight reduction in CRR for wider tires of the same dia. but there would need to be a huge reduction for it to wash out the hard CD numbers we have from Hucho.

That's my 2 cents based on the limited data and using the calculator.

[fjasper]

When going to a larger diameter _wheel_, weight increases rapidly. An aftermarket alloy 14" wheel weighs 14.6lbs, but the 17" version of the same wheel weighs 22.5lbs. An extra 32 pounds of unsprung rotating weight. If the same width tire is used, the tire may be slightly lighter (less sidewall), but nowhere near enough to make up for the increased wheel weight.

Big wheels are for styling, unless you need room for big brakes. Performance tests have shown smaller wheels generally perform better, with the possible exception of slight improvement in braking (possibly due to better brake cooling).

[Vekke]

What goes to prius there are good points shown, but I say few more:

- Prius aerodynamics and add on parts is designed to work best on that car which gives that 0.25 cd. So if you put wider tires air dont go smoothly pass the tires as they are optimised. That about one inch more wider tires dont seem much but if look that at you 3D computer screen you immediately see that who has designed something this much wrong when designing tire spats etc.

- Larger wheels and tires weight more, that has huge effect on city driving and in P&G "mode"
- rims might have less aero look. Prius original rims are optimised for aero vs looks to prius. Aftermarket rims only for looks.
- low profile tires are not best for rolling resistance

New Michelin Tires Designed For EV's and Hybrids

Michelin thinks 175/70 R10 would be best bang for buck when it comes to fuel economy tire. So 175 tire can have enough load ratings vs low rolling resistance characterics. Sad that they are not yet availebly :/.

[basjoos]

I've often wondered what the rolling resistance of the skinny antique car tires were. The wooden/steel banded horse-drawn wagon wheels have a very low rolling resistance, which they have to be, considering their limited output equine power source. The wheel/tire technology of the early cars was derived from the earlier horse-drawn wheel/tire technology and so would be expected to also have low rolling resistance to make the most of the limited power output of the early automotive engines. A Ford Model A weighs about 2200 lbs and runs on 3" wide tires mounted on a 19" wheel. These wheels and tires are still in productuon. It would be pricy, but it would be interesting to see what kind of mileage and handling/braking a set of these would produce if mounted on a modern car.

[Joenavy85]

Originally Posted by fjasper

Big wheels are for styling, unless you need room for big brakes.

and big brakes are (relatively speaking) not needed in the ecomodder world, since we try to minimize the usage of them