Effect of weight of wheel+tire

[H4MM3R]

Originally Posted by elhigh

Yes.

The 205 is of course slightly wider, but it's only an extra centimeter. If you're mostly driving at cruising speeds, not a lot of stop-and-go, then there probably won't be much difference between the two.

extra centimeter (X4) for each tire adds a lot to the "frontal area". Plus the added weight. Also more contact with the road increasing rolling Resistance. Even though it seems small when you look at only one tire, the total out come is different. You will see a drop FE guaranteed.

[s2man]

I went from 195's to 185 LRR's. While that single centimeter on the nominal width is a only 5% reduction, the actual tread width of the new tires is 19% narrower. 5.25" vs 6.5"

I also got rid of the "big 15's" and put on 14" rims. I wish I had before & after weights.

[aerohead]

The big boys refer to polar-moment-of inertia,which has to do with the mass of the wheel/tire combination,and where the mass is with respect to the center of rotation.They place the unit in a three-wire rotating pendulum,twist the thing up,and then time the frequency of "swing",to quantify inertial effects. Acting like four flywheels,its true that during "transient" episodes,when the car is accelerating or de-accelerating,either the engine or brakes must overcome the effects of the momentum due to the inertia of the wheel/tire combo.And I agree with whoever says,that,once up to a constant cruise speed,the mass of the units has little consequence to mpg,all else being equal.Composite wheels demonstrate the lowest mass,even against magnesium,have already received D.O.T.approval in the U.S.,and appeared on the market in the 1980s on a Chryslerodge Shelby something or other.I tried to locate some for my exhibit and struck out.They seem to have bombed in the market,I don't know why.There are alot of things I don't know about!

[james]

Yes like everyone said, the light weight wheels affect mpg more when there is more accel and decel. In city driving, there is a lot of this, so it would make a more sizable difference. The VX, which is designed with fe in mind, has light weight wheels and small tires, and since everything else on the car is basic and simple (except the engine), I figure the difference must be significant enough to warrant the expensive wheels. honda could have made the car more aero, but they didn't do much. who knows why. I think that the light wheels are a good match for the tall gearing. It makes accelerating from low speeds with tall gearing easier. I notice that the car is really easy to stop at low speeds...this is partly just because of the light weight of the car, but probably also the light wheels.

[boxchain]

I've heard that for every pound of unsprung weight you can remove (wheels, tires, etc) its like removing ~3 lbs of weight from the vehicle. But you know what people say.

It makes some sense though. Yes, you can coast further with a steel wheel since you have a higher polar moment of inertia. But you paid for that when you fought it while putting in power to accelerate, and since both in and out have frictional losses and other inefficiencies, the lighter the wheel the better. QED.

In an engine you have a flywheel; same concept but in this case you need it to smooth out the harsh peaks that come from piston compression and the resulting explosions.

IOW: go with the lighter wheels esp if the D is the same. Think about getting some cheap alloys.