Effects of wheel/tire weight

[justasquid]

Holy resurrection batman!

The best visual aid I have ever seen to explain it is this. Take a hammer, hold your hand as close as you can next to the head of the hammer and rotate your wrist. It will spin very easy. Then place your hand farther away from the head of the hammer and try to rotate it. The farther you move your have away, the harder it is to spin. No change in mass, but a huge change in the amount of power it takes to rotate it. This also applies to stopping it as well.

So, for example, if you have a wheel that is 30 lbs, but most of its weight is near the center, it will spin easier than a 20lb wheel that has most of its weight on the out edge. So its not just about total weight, but also where the majority of the mass is located.

Then multiply that by 4, and you see how easily a larger dia wheel can affect things.

[Norm Peterson]

The torque equasion refers to how much force you can apply due to your current leverage (a wrench on a nut, and the length of the wrench is linearly important to how much force you can put on the nut), Don't confuse this leverage as the same concept as loss of torque due to mass being on the outer edge.. torque is spinning that edge (the edge is not what is generating torque).... it's a very different equasion when you are talking leverage radius for a force applied to spin an Axis point (what the engine is doing), VS something being spun around an Axis at a radius causing a loss in Intertia.

Torque and rotational inertia are inseparable, and work in both directions. The formula . . .

[Torque] = [rotational Inertia] x [Rotational Acceleration]

does not make any distinction between accelerating the rotational inertia or decelerating it.

Loss of engine torque that goes into spooling up the flywheel rotationally when you're accelerating car is the condition you have once the clutch is engaged.

But it also works the other way when you start from a full stop, when flywheel inertia at some rpm is converted back to a torque as the flywheel gives up some rpm as the engine rpm and transmission input shaft rpm are made to match. Giving up some rpm = rotational deceleration, which shows up (briefly) as a separate source of torque that adds to whatever the engine is putting out. Here's where a "too-light" flywheel shows up as lacking - it isn't giving you as much "extra" torque for quite as long as you may want because it simply doesn't have it to give (you may end up launching from a higher rpm and putting up with greater clutch slip or wheelspin to crutch this deficiency). This gets into rotational momentum, or rotational inertia x rotational speed.


Norm

[John Wright]

Weight of the wheel has more effect than just the flywheel effect...it effects the shocks and springs, and the suspension's ability to control the movements of those parts as well.

[SSChevelle]

A heavy weight wheel also increases Unsprung weight, ie. It reduces the suspensions ability to react as quickly to uneven road conditions.

[67CamNKC]

When we were running our alcohol funny car and dragster, we ran into this. Some competitors used beadlocks on their rear wheels, some used liner locks. A liner lock is an inner tube that when inflated pinched the bead of the tire to the rim. The liner locks weight 10 lbs a over the bead locks. We actually always ran the liner locks because it gave us a better tuning window and we didn't low power tire shake or spin near as much. Some actually used as a tuning tool on worse racing surfaces. Some friends of ours found a tenth (in a 5 sec quarter) difference between the two if/when he could stay in it down the track......

[sik68]

I did this a while back for my personal use, comparing 17's to 18's with a LOT of simplifying assumptions. Feel free to check for errors:


The left column is for the 17" setup, and the right column is for the 18" setup. Note the subscripts on the variables.


And Using those same wheels during acceleration to 60mph:


This shows me that light weight wheels / tires are effective from a cumulative point of view of a longer distance event, like a road race. Power saved during acceleration, deceleration, and turning all adds up!

I'll see if I can post a similar calculation I did for flywheels also.

[syborg tt]

Ask any road or Mountain Biker the importance of a lite weight wheelset. It can make or break a ride.

This shows me that light weight wheels / tires are effective from a cumulative point of view of a longer distance event, like a road race. Power saved during acceleration, deceleration, and turning all adds up!

I'll see if I can post a similar calculation I did for flywheels also.

[TnBlkC230WZ]

Let's not forget the effects on ride and handling. The shocks and other suspension conponents have to control the movment of the extra weight.

[ace_xp2]

I did this a while back for my personal use, comparing 17's to 18's with a LOT of simplifying assumptions. Feel free to check for errors:

Not sure if your actual intent was to use two tires of the same diameter. But you listed 275/40r17 and 275/40r18, which means your Dtire should be an inch smaller or larger for whichever applies, likely larger for the 18 at 26.7.

A W.A.G. here, but for those running without power steering, Isn't there a gyroscopic effect here as well? In other words, wouldn't a heavier assembly, especially further out from center, make it harder to turn?

[Taman]

How old were your old tires. They may have been brick hard. Did the new ones have a softer compound? You also went wider, so you should get better traction. I bet if you put on the same tire on the 15 as you have on the 17, both new. The 15's wouldn't feel so much different.

[Norm Peterson]

I did this a while back for my personal use, comparing 17's to 18's with a LOT of simplifying assumptions. Feel free to check for errors:

We're pretty close on the distribution of wheel weight between the hub and the barrel (I end up using about 67% for a 17 x 9, 68% for 18 x 9.5 . . . those numbers are handy because I just ran a spreadsheet simulation using those sizes).

You might try breaking the tire MOI down into sidewall vs tread; I think that will providde slightly better results when the tire profiles and/or sidewall heights are significantly different between the tires being compared. Maybe it won't make any huge difference in the bottom-line vehicle acceleration, but at least you won't be having to write quite as many "weasel-words" qualifying the answer.


Norm