tire width vs. drag (Cd)

[CapriRacer]

Originally Posted by basjoos

.........A Ford Model A weighs about 2200 lbs and runs on 3" wide tires mounted on a 19" wheel. ...........

According to Tire Guides, Model T's came with 21" wheels (tire size 4.40-21) and those carried over into the Model A until 1930 when the wheels became 19" (tire size 4.75-19).

[Frank Lee]

There were T's with both tire sizes, though they weren't as different in size as the numbers indicate. 1909-1924 model T's all had 30x3 1/2 clincher tires on 23'' rims in the rear, some were sold into 1926 with clinchers. Front tires were all 30 x 3 clinchers on 24'' rims in USA up until 1919, when demountable rims became an option, then all tires were 30 x 3 1/2.

In 1925 21 x 4.40 "balloon" tires were introduced as an option. They had a new system for measuring size - the outer diameter was about the same as the old 30'' clinchers, but they were named after the rim diameter instead, since that diameter now was more rigid on the tires with steel wíres in the bead, more like modern tires. Balloon tires gave more mileage and were more comfortable (clinchers needs 55-60 psi air pressure while 21 x 4.40 tires needs 27 psi). In 1926 21'' steel wire wheels came as an more and more popular option. They were standard on closed cars at the end of production in 1927.

-a "T" forum

[Varn]

I got my vw in the fall It needed new tires so I went for 155/80x13 instead of the stock 175/70x13, it had 215/60x13 on it by a prior owner.

My new tires are taller than the last ones on it. The new tires are winter tires rather that "all (3) season" tires. I figure that narrow would be good for aero and for winter traction.

I am going to run them all year. I figure that the winter tread will cost some mileage but not getting stuck is worth it. Hoping to see better mileage once the weather gets warmer. I am still running some humongous mud flaps behind them. Going to take them off when I get a chance.

Thanks for sharing some real data on the cd.

[Varn]

I always figured that the size of the tire has nothing to do with tire patch size. It in my mind is a function of tire pressure. You have 500 pounds on a tire and there is 50 psi in the tire. you will have 10 sq inches of contact patch. a wide tire will have a wider patch and a narrow tire will have a longer patch but both have 10 sq inches.

Originally Posted by 3-Wheeler

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

[Frank Lee]

Originally Posted by Varn

I always figured that the size of the tire has nothing to do with tire patch size. It in my mind is a function of tire pressure. You have 500 pounds on a tire and there is 50 psi in the tire. you will have 10 sq inches of contact patch. a wide tire will have a wider patch and a narrow tire will have a longer patch but both have 10 sq inches.

Urban legend- it's not so. Yes contact patch area will vary with psi but not with that math.

[Varn]

I wasn't aware that gas laws of charles and boyle don't apply to car tires.

Originally Posted by Frank Lee

Urban legend- it's not so. Yes contact patch area will vary with psi but not with that math.

[Frank Lee]

Well... they don't.

Go measure it. I did.

[zonker]

Prior to reading capri racers conclusion that wider and taller tires will improve FE, I was planning on mounting a set of 15x4 alloy rims with 165/80-15 VW Beetle tires in the efforts of getting the ultimate FE tire/wheel combination for my 'stang.

hmm... i think i have an opportunity to do an ABA test of the effects of larger tires on my mustang ragtop.

I have another mustang with the same exact 14" polycast wheels as whats on the white rag now, except the tires mounted on it are 215/70-14 radials instead of the 195/75-14 tires i am using now.

I'll weigh the both wheel/tire combos and then do my 92.6 mile FE run back to back and see if the 215's will yield better FE numbers.

[CapriRacer]

Originally Posted by zonker

Prior to reading capri racers conclusion that wider and taller tires will improve FE, I was planning on mounting a set of 15x4 alloy rims with 165/80-15 VW Beetle tires in the efforts of getting the ultimate FE tire/wheel combination for my 'stang.

hmm... i think i have an opportunity to do an ABA test of the effects of larger tires on my mustang ragtop.

I have another mustang with the same exact 14" polycast wheels as whats on the white rag now, except the tires mounted on it are 215/70-14 radials instead of the 195/75-14 tires i am using now.

I'll weigh the both wheel/tire combos and then do my 92.6 mile FE run back to back and see if the 215's will yield better FE numbers.

Word of caution. A-B-A testing means everything else remains the same.

One of the things that clouds this issue is the differences between tires.

http://www.energy.ca.gov/transportat...%20Testing.pdf


This says there can be as much as a 60% difference between tires of the same size. The effect that is important - the difference in F/E caused by tire width alone - is pretty small - 0.1%. This means any test has got to eliminate differences in tires or the effect will be unobservable.

Zonker, if you are going to do this test you need to make sure you have otherwise identical tires - same make, model, and state of wear (New would be good).

And because OE tires are NOT usually identical even within a line of tires, it might be good to post what tires you are considering using BEFORE you conduct the test. I can steer you away from potentially different tires.

It would be bad if you spent all the time and effort to conduct the test only to have the results invalidated because of a poor selection of tires.

[arcosine]

I am averaging slightly over 40 mpg this winter and spring in my Saturn (sc1) with narrower but taller tires, 165/80R15 vs 185/65R15. I have just switched the fronts to 205/70R15, the biggest tire that will fit without rubbing on the struts. Unfortunately the front skirts are not working, as the tires have cut the bungee cords and the right side ripped off, so there is some drag penalty. I will report the mileage after the next fill up, in about 3 to 4 weeks. Coast down tests have not been definitive yet, but it seems a bit slower. This may be due to the skirts though. At 62 mph (100 kph) on the speedometer the car is actually at 70 mph on the GPS and the 2 liter engine is at 2200 RPM. I have to make sure I don't speed. The car is a real cruiser.