Tire and rim weights

[BabyDiesel]

It became very apparent to me today that my coefficient of rolling resistance (Crr) has increased since I went with the new wheels Even with running at higher air pressures than normal, coasting distance is not as it was, which is making my P&G less effective. The tires are also running a lot warmer since the switch. So back the MX-3 wheels I go...

[nimblemotors]

So did I understand that right, you put a normal 5in wide tire on a 4in space saver spare rim?

[CapriRacer]

Originally Posted by BabyDiesel

It became very apparent to me today that my coefficient of rolling resistance (Crr) has increased since I went with the new wheels Even with running at higher air pressures than normal, coasting distance is not as it was, which is making my P&G less effective. The tires are also running a lot warmer since the switch. So back the MX-3 wheels I go...

Did you change both tires and wheels? If so, then what tires did you change from and to?

Be aware that when you get new tires, you will take a fuel economy hit from the increased RR of the tires - as in, all other things being equal, new tires have larger RR than worn tires.

[BabyDiesel]

Originally Posted by nimblemotors

So did I understand that right, you put a normal 5in wide tire on a 4in space saver spare rim?

You are correct nimblemotors.

Originally Posted by CapriRacer

Did you change both tires and wheels? If so, then what tires did you change from and to?

Be aware that when you get new tires, you will take a fuel economy hit from the increased RR of the tires - as in, all other things being equal, new tires have larger RR than worn tires.

I changed only the wheels, CapriRacer. The tires were swapped from the MX-3s to the space savers. They had close to 15,000 miles when I swapped, so RR should not had changed.

With the 14x5.5 rims, the tires barely got warm after driving 40 miles. With the 14x4 rims, the tread gets actually hot in that same drive, driven the exact same way. The increase in heat is what is cuing me in on an increase in RR.

What is your expert opinion on the matter? I did not expect the tire's heat to increase as it has. The only thing I can think of is a combination of increased hysteresis and the tread being affected by the pulled in sidewall.

[niky]

If the wheels are wider than the tread, there's a bit more stretch in the sidewall... which stiffens them. At least that's the theory, and there are racers who use that to get more performance out of the same tire.

Jaguar, for the XFR-S (the "S" signifying that this is the "Sporting" R-model... ), widened the wheels about an inch without changing the tire model used, as part of the handling upgrade package.

I guess you can figure out if it's that or the rubbing that's causing the extra heat by pumping the tires right up and observing the extra heat goes away.

[CapriRacer]

Originally Posted by BabyDiesel

......... With the 14x5.5 rims, the tires barely got warm after driving 40 miles. With the 14x4 rims, the tread gets actually got in that same drive, driven the exact same way. The increase in heat is what is cueing me in on an increase in RR.

What is your expert opinion on the matter? I did not expect the fire's hear to increase as it has. The only thing I can think of is a combination of increased hysteresis and the tread being affected by the pulled in sidewall.

First, the stock tire needs a 5" to 6 1/2" wheel and you are using 4" I suspect the tread is overly arching and the over all movement of the tread through the footprint has increased = more RR.

[nimblemotors]

I wonder if the narrow rims bulge the tire too much?
I need some narrow 15in tires to put on the 4in wide rims, no cheap option it seems. How long will the spare tires themselves work?

[RPM]

Tire manufacturers specify a recommended rim width range for each tire size. A 185/70 R14 tire typically requires a minimum rim width of 5". In this case, 4" wheels are a liability and something I personally wouldn't consider. I can image how poor the handling must be. It's interesting that the coasting distance decreased. Thanks for sharing that info!

[aerohead]

Originally Posted by Miller88

I like the look! Can you get more pictures farther away?

Also - are donut wheels safe to drive on permanently? Aren't they a lighter gauge of steel?

I think the wheels are as robust as a standard wheel.The space-saver tires a speed limited to 50-mph though,and about 50-miles range as they are gummy and wear quickly.
CAR and DRIVER put a set of four on a Corvette and skidpad tested it,returning almost as much cornering G's as the wide,high-performance OEM tires.

[BabyDiesel]

I am going back to the 14x5.5 MX-3 wheels. I can't stand the hit in fuel economy that I have taken! I had a 48 mpg tank before I swapped and was keeping my average in the 47's easily, I even had multiple 50+ mpg trips. Since then my fuel economy has tanked. This tank is sitting at 45.2 right now, and it is a struggle to get a 50 mpg trip. I will say an easy 2 mpg hit, which hurts when you are so close to breaking that mythical 50 mark

Using a calculation of increasing 10% Crr equals a 1.1% reduction in fuel economy, it appears that I have gained a minimum of 40% Crr from swapping the wheels! I find that hard to believe... 3 pounds of extra wheel weight wouldn't be this detrimental though, and that is the only other thing that changed.

I am