Rotational Mass Calculations -- How much of a difference?

[MannyC]

I posted this in one of the other areas, but it got almost no reads. So here goes:

Guys,

As some of you may have read, I sheared a wheel off my car at Button Willow this past weekend. I was running 96 wheels on street tires. I will be getting some new track wheels and running stock tires for a whole and was told that the new 19" Purners are about 18 pounds lighter than the 96 wheels are, per wheel. This would be 72 pounds in savings, not calculating in the rotational mass equation (is that 4X the #?) If so, that would be somewhere about 300 pounds of savings, right?

My question is, how much of a difference will I notice on the track with these wheels compared to the stock wheels? Details would be great. Is it like having a 300 pound person sitting in your car with you? Does the car slide more in turns or is it more of a slower acceleration issue or more?

Thanks

 

[Paolo Castellano]

Superbar, call John Purner. He will give you the scoop. 888-577-0144.

 

[Miles B]

The rotational mass savings mainly benefits your rear wheels. It means your engine doesn't have to spin up as huge a pair of flywheels every time you accelerate.

Give me a couple of hours til I finish my PS pump and I will try and do a few calculations to show you the effect.

What you really need to know for rotational savings, though, is the weight distribution. If the rims are still just as heavy, and all the weight comes off the hub area, there is very little benefit.

 

[Makara]

The rotational mass savings mainly benefits your rear wheels. It means your engine doesn't have to spin up as huge a pair of flywheels every time you accelerate.

Forgive me if I'm missing something, but since the front wheels have to accelerate too as the car speeds up, wouldn't their weight impede acceleration as much as the rears?

 

[Miles B]

Ugh. See this is what you get when you type something when trying to figure out why your power steering is busted.

Yep, you're perfectly right of course; I had the dyno on my mind as I was looking at one earlier.

 

[Miles B]

OK I started calculating it, and was looking for some wheel weight examples, and found a spreadsheet a guy made with it all done already. You can put in the numbers for your wheels, and the Viper, and see the effects.

http://www.siscom.net/~welter/racing/rotational.xls

My numbers showed about a 20 ftlb difference to the road, with my estimation on the wheel and tyre dimensions. This is *aside* from the benefits of less mass to accelerate in a straight line.

 

[cgmaster]

Lighter is better any way you look at it. Removing weight from anything or just moving weight closer to the center of a rotating mass helps alot. Second it also helps on the suspension. The shocks work less since there is less weight to move. If you remove 300 pounds of sprung weight you should see a huge improvement in lap times and power. This will also help braking but not as much. Also watch the weight of your tires there can be a difference in tire weights from different builders.

 

[DChan415]

Well remember the 19's will probably alter your gearing ratio. My car felt slower after I put on the 19" HREs.

 

[MannyC]

Shouldn't it be the opposite effect? Bigger lighter wheels = more speed? One full turn of a 19" wheel = more distance than one full turn of a 17" wheel. Right? Does it not work this way?

 

[Viperzilla]

I think he meant that it didn't feel as quick. The 19" probably add to top speed. I'm not a gear-freak, mechanic, or engineer, so correct me if I'm wrong.