I am considering replacing the OEM wheels on my HCHII with Motegi Traklite wheels (10.4 lbs. forged aluminum) Has anybody else had experience with changing rims to reduce unsprung weight? Any comments on the impact on FE in the area of reducing weight vs aero-advantage?

Matt in Moore Tafetta White 2008 HCHII

I believe the OEM wheels are fairly light, also, there is an aero part to this equation. Keep that in mind.

Before you get too far... simply work out the percentage of weight you are saving... on a vehicle that weighs about what... 2000lbs.... If you are only saving 10lbs, I'm afraid its not going to make a lot of difference. If you are going to pull weight off all over and drop it by 100 or so, then maybe.

From the calculations I've found elsewhere, each pound of unsprung weight is equivalent to 4 pounds of sprung weight. The reduction with these wheels is about 32 lbs (8lbsx4) over the OEM rims. This equates to about 128 lbs sprung weight reduction. Rotational inertia is reduced by about 22-25%. My question is still:how much aerodynamic effect do the "pie plate" wheels have versus spoked wheels? I haven't found any data on aero testing.

Obviously the flat wheels have better aerodynamic characteristics at higher speed, but is this really a factor for my car since I rarely get even as high as 60 miles per hour? The rotational inertia is EVERYTHING when it comes to accelerating. The more unspung roational mass you have to get moving the less fuel efficient it is. So where is the breakpoint? At what speed does the aero effects become more efficient than a spoked wheel? AND, since my typical commute is 60% freeway (50-55MPH, 25% state highway (40MPH), 15% city street (25MPH), is the aero really effective?

Matt in Moore Tafetta White 2008 HCHII

Whatever unsprung weight saving you achieve will be negated by the high performance tires your new wheels will require. I am assuming you are increasing to 17"+? If you can find 15" Motegis at a much lower weight. Your FE will increase.

However $2000 (wheels) can buy a lot of gas.

nf

P.S. I don't think the aerodynamics of the wheels amounts to anything measurable.

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The Motegi TrakLite wheels I am looking at are 15x7, and would be fitted with Michelin Pilot Exalto AS. The wheels currently sell for about $189 from tirerack.com.
I have no desire to increase the diameter. That adds drastically to the rotational inertia and requires a major increase in power needed to accelerate.

Matt in Moore Tafetta White 2008 HCHII

I suspect there is some reason for the shape of the rim due to limiting drag. There has to be some measurable difference, I just don't know how much. In racing (at least in F1), teams go through a lot of trouble creating center caps for the rims to limit the amount of drag the inside of the rims generate. So there has to be something there.

Back to the original poster:

Also, the unsprung weight once up to speed helps you maintain your speed. The more weight the more inertia you have working for you. Granted the energy required to get a heavy wheel up to speed may cost you more than the payoff.

What is the rim size anyway? Is it 17"? What is the weight of a low profile 17" tire that has the same circumference as the oem? I suspect it is going to be a heavier tire and wider. Both which are now working against you in the FE department.

Go ahead and change the wheels if you like the look or style of the new wheels better than the OEM wheels. But don't look for a significant increase in FE without some trade-off. And, the trade-off is probably either safety or reliability, or both. If it was that easy to increase FE by modifying the product, and without sacrificing safety or reliability, the good engineers at Toyota should have done it already.

Of course, they have missed some really obvious improvements. I can easily imagine reprogramming the cruise control, as one example. If they just program it to slow down a little for each up-hill, and speed up a little on the down-hill, they could provide improved FE to average drivers, with no change in driver behavior. (Perhaps the cruise control should alter its behavior whenever the ECO button is engaged.)

Do it! Look forward to the pics.

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