Lighter Weight Wheels --- Worth It?

[Formula413]

Originally Posted by Christ

PS drive wheels are obviously more important than non-drive wheels...

How so?

[Christ]

Simple answer - If you can only afford to reduce the rotating mass of two wheels, make them the drive wheels. Obviously, the drive wheels are the ones that would benefit the most from reduction in rotating mass, since it would essentially be a reduction of weight that the engine has to turn during acceleration, and overall weight as a bonus, for cruising. Less work for the engine == better FE overall.

I thought I said something to this effect earlier?

[Formula413]

But all four wheels need to turn for the car to move, and the engine provides the energy for that. I don't see how it matters where the axle(s) is/are.

[Christ]

Would your engine benefit from a lighter flywheel? Less mass to turn..

Yes, the engine presents the power to turn all four wheels, but it only directly drives two of them, in most cases.

For purposes of acceleration, if you have two heavy wheels and two light wheels, the light ones should be on the drive axle, as this will prevent less necessity of force to turn the wheels.

Since it requires less torque to turn a lighter wheel, it will be dually beneficial to your engine to have light wheels on the drive axle.

Dual benefit:
1. Engine uses less power to accelerate rotating mass

2. The wheels will weigh less overall, therefore there is an overall weight reduction in the vehicle, thus making the engine work less to keep moving the vehicle.

As you can see, the first benefit speaks more to acceleration, the second benefit speaks more to keeping that motion.

If this doesn't do it for you, you'll have to google it or something... this is the best explanation I can think of.

[MechEngVT]

Christ,

Formula is right. For normal driving and/or hypermiling, all four wheels need to spin for a vehicle to move. Therefore the engine sees the load of all rotational inertia not just that rotational inertia that is mechanically connected to the driveline.

In drag racing it may be beneficial to increase the rotational inertia of the wheel/tire and absolutely minimize the weight of the non-drive tires. Higher drive tire mass/inertia reduces the tendency to generate wheelspin when massive torque is applied quickly and the non-drive tires are only there to support vehicle weight. If racers find benefit for lightening non-drive tires I fail to see why a "normal" driver could not achieve similar benefit.

Unless you argue that the engine will accelerate the drive wheels at a faster rate to a higher speed there is no benefit, obvious or not, for lightening the drive wheels over the non-drive wheels. If you do in fact argue this case then we're talking more about racing and less about normal driving. Even under full-race conditions few DOT approved tires can sustain drive slip greater than about 12-15% without breaking loose so the drive wheels cannot exceed non-drive wheel speed by much and not for long.

[Christ]

Wow.. you've taken that so far out of proportion it's not even funny.

I'll chance one more reply to it.

First off, the original case was that you had only 2 light weight wheels, the other two were stock.

Secondly, the engine does not SEE the inertia required to spin the non-drive tires.. it only sees them as dead weight. The fact that they roll means nothing.

The engine is directly driving (via transmission) the drive axle, therefore, weight reduction on this plane means that the engine benefits twice, once from less power necessary to accelerate, and again from less power necessary due to overall weight reduction to MAINTAIN that movement.

We're not talking about racing here, we're talking about real world application. Get your head out of the books and THINK about it.

Once again, if you lighten the rotating mass of an engine, it does not have to work as hard to accelerate, therefore, since the wheels on the drive axle are directly connected, and PART OF THE DRIVELINE, lightening them means that the engine does not have to provide as much work to acceleration.

If you lighten the vehicle as a whole, it will achieve the same effect, but not nearly on as large of a scale, considering that every 100lbs of power train mass lost translates to a higher net efficiency increase when compared to equal VEHICLE weightloss.

What part of this had anything to do with racing?

Facts are easy to ignore when you're reading from a book, such as the fact that lower required inertia gives to more efficiency due to less work.

If it takes less torque to initially turn and accelerate the wheel, such would be the case with the engine's driven axle, obviously the engine can put less work into the same achievement.

Basically, what you're trying to say is that putting the lighter wheels on the non-drive axle would achieve the same result. I laugh.

I want you to actually try this. I've done the math, I've pointed out all the necessary test points, and I've actually done the test. I have 8 sets of wheels for my car.. I've tried them all, in almost every conceivable combination... believe it or not, I wasn't just talking out my ass when I typed:

Originally Posted by Christ

Simple answer - If you can only afford to reduce the rotating mass of two wheels, make them the drive wheels. Obviously, the drive wheels are the ones that would benefit the most from reduction in rotating mass, since it would essentially be a reduction of weight that the engine has to turn during acceleration, and overall weight as a bonus, for cruising. Less work for the engine == better FE overall.

[MechEngVT]

Christ, if you're so confident that your opinion (and it is just that) has been formed by data, then provide that data and your experimental methods/conditions for us to review and pick apart. I contend that there is no physical basis for your assertion since what vehicle system a component belongs to has no effect on its dynamic behavior and both drive and non-drive wheels exert equal resistance to motion when accelerating equal masses of equal shapes through equal changes in rotational speed.

Until you provide your data and they can be independently repeated/verified, you are talking out of your ass.

[Christ]

Originally Posted by MechEngVT

Christ, if you're so confident that your opinion (and it is just that) has been formed by data, then provide that data and your experimental methods/conditions for us to review and pick apart. I contend that there is no physical basis for your assertion since what vehicle system a component belongs to has no effect on its dynamic behavior and both drive and non-drive wheels exert equal resistance to motion when accelerating equal masses of equal shapes through equal changes in rotational speed.

Until you provide your data and they can be independently repeated/verified, you are talking out of your ass.

Ok, for the sake of argument, and because I don't really feel the need to prove that which is obvious to someone who is only going to complain and find a way to ninny his/her way out of being wrong anyway, I concede, and you, sir/ma'am, are correct.

It is quite unfortunate that you'd rather take your "knowledge" over practical application.

[almightybmw]

The flywheel example says it all. What happens when a drag car lifts the front tires? Does it care about moving them now? just unsprung weight now. That's extreme.

How about this: I have many bicycles. I'll find the heaviest wheel and tire combo I can, and put it on the rear. Find the lightest and put it on the front. Then swap weights. I'll have more than one person ride the bike, to get a better poll basis. I'll have a cadence meter set to record across the test track to use as a standard deviation.

Would that be good enough for the disbelievers?

[guudasitgets]

Unless you leave the heavy tire/wheel combination off the car, sitting on the ground, then they are part of the car and they have to be rotated, doesn't matter where they are, and to the engine it doesn't either. Unsprung weight and suspension dynamics are a whole different thread. Handling would change in cornering. We did this in testing at Road Atlanta on a Formula Ford, two of the tires were mounted on our rain tire steel wheels and we swapped them front to back. There was no diffrence in acceleration, time-to-RPM down the straights were identical. Theres your data; case closed.