MetroMPG.com mailbag: what about lightweight rims?
 

Got another one this week:

Lightweight rims are a definite efficiency booster.

They're particularly good for city driving - or any situation where speed is changing frequently (pulse & glide, anyone?). All the "high performance" cars - at both ends of the performance spectrum - have 'em. Ferarris to Civic VX's.

Honda Canada just added the rims from its hybrid model as standard equipment on its non-hybrid 5-speed model (along with LRR tires, underbody panels and a deck lid spoiler) to lower the fuel consumption enough to earn an efficiency rebate from the government. (And now the non-hybrid looks identical to the hybrid).

Weight reduction in general is a favourite ecomodding tactic, but pound for pound, it's even more effective to take off rotating mass than static mass. I've read in various places that taking off 1 lb of rotating mass is the same as removing anywhere from 7 to 16 lbs elsewhere (there's the Internet for ya). [EDIT: this bit of fuzzy Internet fluff is corrected below.]

The problem for me is my car has an uncommon bolt pattern.

Whatever the Honda pattern is, it's not the Geo's! Ask me how I know.

When I was shopping for my first Firefly, I had a set of VX rims sitting in the garage waiting. Unfortunately, the closest I got to putting them on the car was in Photoshop....

I even took the VX rims in to my machinist friend (the guy who made the motor/tranny coupler for the ForkenSwift) to see if he would drill out a new set of holes in the VX rims that would fit the Firefly's stud pattern (that's why on the rear wheel in the above pic, you can see I Photoshopped the 8-hole pattern, to see what it'd look like).

But he said, nuh-uh.

Honda (or the rim supplier) optimized these rims so much for weight that they scooped out material on the back side in between the original stud holes.

So buddy said: "If I were in my 20's and this was for MY OWN car, I might try it. But I'm older and wiser now, and I've seen stuff break. I just don't think there's enough meat in the hub area of these rims to machine out your new holes without compromising them."

Got another reply from the (shy) asker of the original question:

True.

But going to a 12 inch rim would mean using a smaller diameter tire as well. And that would effectively reduce my gearing (making the engine spin faster at a given road speed). I expect that would reduce my fuel economy more than any savings I'd gain from the lighter rim/tire combo.

Also, my car has quite good low rolling resistance tires. If I could find a set of lighter rims that would fit, I'd want to keep using these tires.

I believe this is primarily a concern for auto handling. From the POV of efficiency, weight is weight is weight. Dropping 10-15lbs of my vehicle weigh via lighter wheels is equivalent to dropping 10-15lbs off my ass. :thumbup:

Thank you for questioning this. It forced me to do a bit more digging for a credible source on the subject --- and put this bit of fluff to rest: "taking off 1 lb of rotating mass is the same as removing anywhere from 7 to 16 lbs elsewhere"

First - agreed on the handling benefits of unsprung weight.

On the issue of rotating mass...

In fact, you'd have to drop 20-30 lbs off your ass to equal it. The relationship is more like 1:2.

Here's a credible source giving an overview of the 1:2 relationship, in response to this question:

And a perhaps less credible source adding to the above... with formulae!

It's not always 1:2, it depends on the distributions of the rotational mass. :)

That's true. Both sources mention that. But it's more than 1:1!

Based on what the almighty wikiality says, the second quote is incorrect since the distribution of weight on a car wheel isn't nearly as pronounced as on a bicycle, aka the ideal case of a mass at some distance from the center. Car wheels tend to be pretty beefy IME, and weigh more than or about the same as a tire, and since the disk or drum is rotating too, there's even more mass close to the center. This implies the moment of inertia is likely significantly less than .5mr^2 IMO. Offhand I would guess that a tire wheel combo has maybe ~25% more KE than a static body moving at the same speed.

This does bring up an interesting point though. If you've looked at tire specs, you may have noticed that wider tires weigh significantly more than their skinnier counterparts. Since all this weight is near the edge, I would guess that the choice of tire can influence acceleration/deceleration as much as or more than lighter wheels could.

That being said, the only fuel efficiency property that depends on mass in an ideal situation, Rolling resistance=mg(Crr), depends on mass alone AFAIK. While larger wheels/tires may hurt acceleration, braking, and handling, I don't see how they do anything to influence the energy needed to move a vehicle down the road.

I haven't looked at tire specs, but what you say seems reasonable.

It also makes sense that wheel/tire mass makes little difference at constant speed.

But I'd still get the lighter wheels if I could. The majority of my fuel savings comes from the P&G technique = repeatedly accelerating (and at a fairly brisk rate). In those circumstances, or for driving that's primarily urban, it makes sense to reduce wheel/tire mass. For highway cruising or DWL, not so much.

About the question of RR being solely a mass issue, I wonder how much aerodynamic lift a typical car generates at highway speeds... :) (Just kidding.)

I don't think it makes any difference at any speed unless the driver can't account for the increase in KE via having larger coasting distances. I mean, less mass is good for normal city traffic in that marginally less energy is lost when everyone gets on the brakes from some speed. But if we stay off the brakes, having more KE at some given speed isn't an issue at all since it'll just take us slightly farther as we coast down. IMO, lighter wheels just aren't worth the cash to those looking to get better mileage compared to just about anything else.