new rims and fuel economy

[guzzla]

I was ahving a chin wag with some mates about tires etc. We all run 35in radials but one also has a set of bias creepys. He went on to say that when he runs them on his cruiser - as opposed to his 35in radial MTRs his fuel gauge just about falls out of the dash at the speed the extra fuel is used.

I know that my 35in radial claws with ROH rims @ 40psi weigh 42kgs each though im unsure how much extra the creepys weigh.

I had been thinking of swapping to alloys just to tidy the old girl up a bit but (currently looking at the mickey thompson DC1 rims) its got me interested in any possible gains/savings by just switching to alloy rims. Im sure there is a gain of some sort as the overall turning weight is reduced not to mention the effort saved under braking.

I know there is a million ways to take weight out of the cars but Id like to focus on this particular item.

Any decision I make to switch to alloys isnt going to be based on proposed fuel savings. Its more to do with jazzing the old girl up a bit.

Id be interesed to hear your thoughts / experiences

Regards,
Nathan

[Guy]

good quality alloys will be strong and considerably lighter than a steel rim.
The fuel use is more to do with rolling resistance of the bias construction (as I understand it)

[-Scott-]

Technically speaking:

Wheels have a "moment of inertia" (can't remember the exact term) associated with rotation about the axis. The higher this number, the more energy required to turn it.

Typically, heavier wheels will have a higher moment of inertia, and use more energy (fuel) to turn them. Whether the difference between steel and alloys would be noticeable on a large 4wd, I have NFI.

Of course, you can think of the wheel/tyre combination the same way, which probably contributes to the creepy's higher fuel economy. But how much is mass/moment of inertia and how much is to do with other tyre characteristics I don't know.

[danaz]

If your trying to save weight rotating weight is the best place to save it. Dont know if alloys would make a huge difference but they would make a difference.

And ye bias plys have a lot more rolling resistance than radials.

[turps]

good quality alloys will be strong and considerably lighter than a steel rim.
The fuel use is more to do with rolling resistance of the bias construction (as I understand it)

Have you actually checked the wieght difference between alloys and steel rims?
I have held a Walker Evans beadlock and these are quite heavy. And I would say equivalent to the weight of a steel beadlock. So would assume a normal alloy wouldnt be to far off the weight of a std steel rim.

[Highway-Star]

Thats one of the reasons modern cars run low profile tyres, these spindly alloyed alluminium mags weight a huge amount less than their steel counterparts and also less than the rubber tyre that would be there if the tyre was a standard profile.

Removale of weight further out on the wheel (closer to the tread as opposed to the hub), has a greater effect on reducing the moment of inertia, its logical. So with a 4WD where you are running 35" tyres with 15 or 16" rims, the effect of going to alloys is going to be allot less helpfull.

With my Zuk, the effect of alloys is slightly noticable, when I ran standard 26" tyres on it, it got consistantly about 0.5km/L more than Dads stocker of the same model, and mine accelerated better. However it is possible that something else in the car could cause this (or the difference in our driving styles). Now I have got 27" tyres, the effect of the small extra moment of inertia and slightly higher gearing is enough to knock me back to the same fuel economy as Dads stocker with 26's and steels.

The best benefit of alloys in my opinion, is when you get a flat tyre. When I had a blowout about 2 years ago, my tyre was easy to remove, picked it up as easy as pie, but my spare is a steel, and it is allot heavier! Lining it up on the studs is more anoying beacuse its heavier and harder to move precisly.

Also I think alluminium has a better heat transfer coefficient, so it will cool your brakes a little bit quicker. Don't quote me on it though.

[Slunnie]

good quality alloys will be strong and considerably lighter than a steel rim.
The fuel use is more to do with rolling resistance of the bias construction (as I understand it)

I reckon this is what its all about, significantly more than tyre/rim weights and inertia. I've found even within radials that the tread pattern has a significant impact on the rolling resistance and open pattern tyres will cost easily at the bowser, definately enough to notice. The bias construction also suffers significantly more deformation when rolling than any radial, especially if you look at the crown profile and realise it needs to mash as flat as the road. Likewise this costs fuel.

WRT weight, it'd interesting to know the weight difference between the MTR and an equiv creepy.

[KiwiBacon]

Under acceleration and braking, the worst case is the weight on your wheels is worth double the dead weight on your truck.

So if you can save 40kg from your wheels, the same performance could be gained from dropping up to 80kg from the rest of the truck.
Don't expect earth shattering results. It's a place where weight is a good consideration (esp when buying new wheels and tyres), but it's not worth spending thousands extra over.

The rolling resistance of your tyres is the biggest wheel factor in fuel economy, others have mentioned that already. Tread design and carcass construction are the big variables there.

Those 18" wheels that rice burners use are slowing their cars down. A smaller wheel and taller profile tyre has less weight and lower polar moment of inertia.
However, lower profile tyres have less rolling resistance (possibly due to less sidewall deformation) which has fuel economy benefits on the highway.

[cruiser60series]

moment of inertia =
(mass at a point) x (distance to that point from center)SQUARED

doubling the distance will result in 4 times as much inertia.

My physics lecturer showed me an example of two wheels. One was solid disc and the other was just a rim - both had the same mass and diameter.

He spun them up to the same speed and put them on the carpet. The ring went alot further than the disc as it had much more inertia. Because all the mass was concentrated to the edge of the shape. It would also of consume alot more energy to reach the same rotational speed.

Just mentioning this to say that a priority in economical wheel design would be reducing the heavy components at a greater distance from the center.

[KiwiBacon]

Just mentioning this to say that a priority in economical wheel design would be reducing the heavy components at a greater distance from the center.

Heavy wheels work like a flywheel. They store more energy.
They'll only cost your fuel economy if you are in a situation where you're constantly using the brakes to slow down (burning that stored energy to heat).
In a rolling situation they will not change fuel consumption at all. Energy that goes into accelerating them is returned on coast down.

[cruiser60series]

They'll only cost your fuel economy if you are in a situation where you're constantly using the brakes to slow down (burning that stored energy to heat).

Usually what happens when we drive, who gets up to speed in their driveway and rolls the rest of the way to work.

Also in these situations everything is frictional and losses are everywhere and additional energy is constantly required to keep things rolling.

What your saying is like having a 2 tonne trailer behind your car to go further (as moment of inertia is the rotational equivalent to mass). Sure once you get up to speed you coast further and have greater stored energy. But you still need to consume the energy to get it there and since there are losses and energy is not conserved you don't get it all back.

[Guy]

good quality alloys will be strong and considerably lighter than a steel rim.
The fuel use is more to do with rolling resistance of the bias construction (as I understand it)

Have you actually checked the wieght difference between alloys and steel rims?
I have held a Walker Evans beadlock and these are quite heavy. And I would say equivalent to the weight of a steel beadlock. So would assume a normal alloy wouldn't be to far off the weight of a std steel rim.

Not even close an allied beadlock (cold forged alloy) only weighs about 14Kg and that is a very robust wheel (over 2 inch thick centre section) a steel wheel of comparable strength would weigh considerably more.

[KiwiBacon]

What your saying is like having a 2 tonne trailer behind your car to go further (as moment of inertia is the rotational equivalent to mass). Sure once you get up to speed you coast further and have greater stored energy. But you still need to consume the energy to get it there and since there are losses and energy is not conserved you don't get it all back.

That's a rather retarded analogy. The difference between 20kg of rotating weight and a 2 ton trailer is roughly 1960kg.

A rotating wheel is almost 100% efficient at storing energy. It gives back almost all of the energy you put into it.
How you get that energy out is up to you (brakes, wind resistance, tyre rolling resistance etc). But the weight (or inertia) itself does not consume energy.

[cruiser60series]

But the weight (or inertia) itself does not consume energy.

Are you saying lugging around more weight doesn't consume more energy? Or bigger wheels need less energy to turn than smaller ones?

It gives back almost all of the energy you put into it.
How you get that energy out is up to you (brakes, wind resistance, tyre rolling resistance etc)

If you spin a wheel and put brakes on it how is that storing the energy at 100% efficiency, the energy has been dissipated. You must put more energy in to keep going.

That's a rather retarded analogy. The difference between 20kg of rotating weight and a 2 ton trailer is roughly 1960kg.

I wasn't being quantitative just qualitative.

[KiwiBacon]

Are you saying lugging around more weight doesn't consume more energy? Or bigger wheels need less energy to turn than smaller ones?

Weight in motion stores energy. How you chose to dissipate that energy is up to you.

If you spin a wheel and put brakes on it how is that storing the energy at 100% efficiency, the energy has been dissipated. You must put more energy in to keep going.

Using brakes to burn momentum to heat does not affect the efficiency of the energy store.

That's a rather retarded analogy. The difference between 20kg of rotating weight and a 2 ton trailer is roughly 1960kg.

I wasn't being quantitative just qualitative.

[guzzla]

so in the attempt to get a little bit back on topic who had swapped to allow rims and noticed a difference. likewise who has both steel and alloy rims and has the technology to provide weights for both.

other than that All of what has been said makes good sense and logic.