Effects of wheel/tire weight

[derekf]

I am not an engineer. I've never taken any engineering classes, or physics classes, or anything like that -- so if the answer is obvious then please be gentle.

With that said, what real effects do the weight of wheels/tires cause?

Giving it rational thought, one would assume that the added weight would resist any changes in velocity, be it acceleration or braking.

However, when I went to the ultra-heavy 17" Cragar Soft 8s on my El Camino compared to the old stamped steel 15", I felt (seat-of-pants only) a difference - but it wasn't the expected difference: the car felt much faster from a dead stop. The old tires would spin some at launch, the new ones did not. Some of that should probably be blamed on the difference in tires (going from 255/50-15 to 275/35-17) but I didn't buy high-end soft compound tires.

So -- assuming that I'm right, and higher-weight wheels/tires just resist changes in velocity, has anyone used increased wheel weight as a tuning tool? I mean - if you've got issues with wheelspin, would increased wheel weight help dampen it?

How much difference does wheel weight make in the quarter -- if going from 15 to 40 pounds per wheel adds two seconds to ET, then obviously it sucks as a tuning aid? What about on a chassis dyno - do wheel weights make a difference at all (common sense says 'yes', but is sometimes wrong).

If heavier = more resistance to speed changes, once you're at cruising speed, would a heavier wheel increase gas mileage?

[parsonsj]

Wheels (and brake rotors for that matter) are flywheels. They store energy, and the faster they spin, the more energy they store. Since they don't provide their own, they must take energy from the vehicle itself, taking acceleration energy away from the vehicle.

So they resist acceleration, both positive and negative, resulting in reduced performance in both directions. There's no way around it ... you have to pay the energy costs.

Lighter is better, but the more important factor is Moment of Inertia, which changes based on the square (I = kmr squared) of the radius. If two wheels have the same weight, but one has a radius 1" (15" to 17") bigger, the MOI of the bigger wheel will be nearly 30% higher. MOI is a continuum (MOI can be calculated at any place along the radius, to get the total MOI of a rotating disk requires integration), but since most wheels concentrate the bulk of their weight along the outside (the hoops that hold the tire), MOI calculated at the outside edge is a decent approximation.

These same physical laws show that a heavy solid disk will accelerate down an incline due to gravity faster than an equal diameter (and lighter) hoop. Try wrapping your mind around that!

I can't explain your seat-of-the-pants experience with the lead-weight Cragars (confirmation bias?), but I'd be skeptical without actual physical evidence. Those bigger, heavier Cragars are taking energy from acceleration and storing it as potential energy. Then your brakes have to step up and absorb that energy (by converting it to heat) to bring your car to a stop.

And no, a heavier wheel won't increase gas mileage. It would be at best negligible in its effects. You'd think that cruising means that you aren't accelerating, but that's not quite true, unless you are on a perfectly level surface. Otherwise, to maintain constant speed, you need to accelerate to go up and down hills. It is true that the potential energy stored in your wheels going down a hill can be used to help the car accelerate up a hill, but most of us don't drive that way. So the heavier wheels will probably eat at your gas mileage, but not at a rate that you'd be able to measure. The one beneficial effect would be that it will be easier to maintain a constant speed on level ground.

As far as how much a bigger, heavier wheel will hurt ET, that's beyond my capabilities. It requires integration, and so many simplifying assumptions that the best way would be scientific experiments (run the car, change the wheels, run the car, change back, run the car, etc.).

jp

[Twentyover]

OK, don’t beat me up too bad because I read some of David Vizard’s stuff. Afterall, my name is Greg, and I race small British sportscars (stated like I’m in AA)

Vizard published (think it PHR) a number of years ag0, the difference in acceleration using a lightened flywheel on a 302 Ford. I don’t remember if he gave the actual difference in moment of inertia of the 2 flywheels used; but in the story a lihter flywheel achieved an acceleration gain equivalent to something like 30 additional hp in 1st gear, and something like 20 additional hp in 2nd gear, 12 hp in third, 4 hp in 4th. These apparent increases in hp were due to the engine not needing to consume hp to accelerate (angular) a heavier flywheel.

How does this apply to your situation? Not really sure, except I’ve got to be lieve lighter is better. Always better. As to how it applies to a chassis dyno, think you need to bring a sedond set of wheels to the rollers to find out

[Norm Peterson]

I mean - if you've got issues with wheelspin, would increased wheel weight help dampen it?

How much difference does wheel weight make in the quarter -- if going from 15 to 40 pounds per wheel adds two seconds to ET, then obviously it sucks as a tuning aid?

The first question begs the question of whether wheel weight/MOI would completely suppress it or not. If it does not, the increased MOI will then make the wheelspin harder to rein in to regain traction (the additional wheel MOI then represents greater rotational momentum working against you).

A fair estimate can be made if you know lots of things about the car beyond just the wheel weight. Mostly, the differences are small, assuming no wheelspin. My own ballpark estimate for a 400 HP 3500 lb car is about 6' and less than 0.050 second difference in a 3rd gear 75 - 105 mph sprint for the above 15# vs 40# wheel weights and all else held constant (a little less than a 1% acceleration difference, according to a comparative acceleration spreadsheet). That's enough to see, and easily more than enough to lose a race at the dragstrip by, but it's not enough to feel without some sort of external reference.


Norm

[KwikGT]

Derek,

Both JP and Norm are %100 correct but I believe your question was "how much can wheel size/weight effect my quarter mile time?"

In drag racing terms the answer is "it can make a huge difference".

That is why guys put light weight, skinnies on the front of their drag cars.

It all goes back to that rotating mass, an object that is at rest wants to stay at rest... stuff we learned back in Science class.

Like Norm said, the difference may not "feel" extremely noticable but when you are drag racing every bit counts.

I know that the drag guys say for every 100lbs you add or subtract from an average car roughly translates to .10 of a second on/off your ET. That could be 3 car lengths difference at the end of the quarter!

I'm not sure if that translates the same to weight added/removed from the rotating mass or not but the bottom line is this. The less rotating mass the faster the car.

[KwikGT]

Also, as far as the chassis dyno goes, it will effect your horse power output at the wheels.

If you took two identical cars but one had heavier wheels, the one with lighter wheels will register more HP on the dyno.

Again, how much depends greatly on the car and amount of weight added.

Like JP said, even the rotors make a difference. Many people don't know that a lot of the reason that performance rotors are cross drilled is for weight reduction as well as cooling properties.

[derekf]

Thanks, y'all. My guess as to why the Cragars made it feel faster would be that the tires were a little better, plus the added weight was just enough to keep the tires from spinning -- so I was faster, since I had traction.

[jaybee]

"I'm not sure if that translates the same to weight added/removed from the rotating mass or not but the bottom line is this. The less rotating mass the faster the car." Actually rotating mass should have a larger effect on acceleration than other mass because it has to accelerate both in rotation and along the path of the vehicle.

[KwikGT]

I thought so too, but I wasn't sure how much.

[TonyL]

There was a really good article in Eurotuner about this very science. There was a difference But it was so small only a machine (a dyno) could really tell.

[Norm Peterson]

Actually rotating mass should have a larger effect on acceleration than other mass because it has to accelerate both in rotation and along the path of the vehicle.

It does, and it depends on the MOI and the angular acceleration (which depends on where in the powertrain it is and also on the total gearing effects if it's 'upstream' of the differential gears).

Norm

[KwikGT]

Norm, you are far too intelligent for this site! Half of us thought you just spoke Latin! WTF do you do for a living??

[David Pozzi]

Not only is wheel weight a consideration but MOI is more connected to the weight of the outer rim section. Spun outer rims are much lighter than cast outers and have a much lower MOI, even though the wheel centers may weigh more.
Herb Adams showed a calculation for lighter flywheel weight on a road course. I haven't seen it done for wheel weight but the wheel's MOI must be known to calculate it.

A simple coast down comparison would be a good way to compare and more reliable than an acceleration comparison.
David

[BillyShope]

When performance calculations are made, rotating inertias are converted to "equivalent weights." In other words, the rotating inertia of, say, the flywheel is equivalent to so many extra pounds thrown in the trunk. Again, for a flywheel, the equivalent mass is equal to the flywheel inertia (mass-length^4) multiplied by the square of the ratio of overall gearing to rear tire radius. For a flywheel, the equivalent weight is significant; for a road wheel, it isn't.

And, for a road wheel or flywheel, the rotating inertia varies with the fourth power of the radius, assuming all dimensions are scaled proportionally.
http://home.earthlink.net/~whshope
over 140,000 page views

[406 Q-ship]

^^^What they said......

I have a thought to why the car appears to have better traction (beyond the wider contact patch of the 275s) that would confirm the MOI reasoning. If the original rim and tire combination could spin the tires but still had a reasonable amount of grip, the new heavier wheels are using up that power that used to spin and to accelerate the wheel and vehicle (don't forget the car got heavier too). If you were to get the new heavier wheels to spin harder and surpass their friction with the surface, now the MOI and flywheel effect will take over and the spinning wheels will be harder to recover.

[JEFFTATE]

So ...
I need a lightweight flywheel ???
Hmmmmm...

[406 Q-ship]

So ...
I need a lightweight flywheel ???
Hmmmmm...

Depends on the weight of the vehicle, usually heavier cars like heavy flywheels.

[parsonsj]

So I need a lightweight flywheel ???

I doubt it, especially for a street car. If you want to be annoyed at every traffic light, put a really light flywheel in your car. Since the flywheel has less inertia, you need to supply more energy via more throttle opening as you release the clutch. It changes the whole feel of the car, and gets old fast.

Once you're moving, then subsequent gear changes aren't affected as much... it's moving from a dead stop (and god help you if you are putting a car on a trailer or going up a hill) where you really notice it. I'm on my third flywheel on my car... and it weighs 32 lbs.

jp

[Norm Peterson]

So ...
I need a lightweight flywheel ???
Hmmmmm...

You only need a light flywheel if its advantages are sufficiently important to you such that you can live with its disadvantages (considering the entire range of anticipated car usage). Even then, it's not necessarily a "yes or no" question; there is such a thing as "too light", and there's your own adaptability to consider.

I know that I could live with less than the current 40 lbs combined clutch and flywheel weight in my EFI 350 powered 3500 lb car, and could probably do with less than whatever is OE in my '08 Mustang. Then again, I don't drag-race, do burnouts, or launch all that hard from stop signs and traffic lights.


Norm

[Randy67]

So ...
I need a lightweight flywheel ???
Hmmmmm...

If you want to see what a light flywheel is like, come see my car over in Loganville. It has a 10 lb steel flywheel with a total flywheel & clutch weight of 24 lbs. Thankfully I have a 4.06 first gear in the Richmond which helps quite a bit. If I had a taller gear like a Muncie, no way would I want this light of a flywheel. I agree with Norm, flywheel weight needs to match the intended usage.