Lighter wheels help my car runs faster in the 1/4?

[SlowCreamPuff]

Made 3 runs yesterday.
[email protected] (2.087 60')
[email protected] (1.996 60')
[email protected] (2.197 60')

My previous best was a 12.0@118. The only change to the car since then was Fikse FM/5 wheels.

There are a few things that I can think of. Cooler temperature at the track, I didn't let the engine cool down yesterday, and more miles on the car. Mine is a 2001 GTS w/ K&N, smooth tubes, Corsa 3", and almost 40,000 miles.

I am sure it's a combination of all these factors, but I am happy.

What is your experience? I am surprised the car ran so well given the engine was pretty hot when I made all these runs! All the runs are with 335 Michelin Pilot Sport at street tire pressure and no power-shifts. My previous best 60' was 1.8xx. I got a little too greedy on the last pass yesterday, hoping to get into the 11's and spun too much.

 

[AlvaSpeed]

What are the weights of the new wheels?
Old vs New.

 

[SlowCreamPuff]

I wonder if anyone knows the exact weight difference?

My estimate is 2 to 5 lbs saving on each wheel.

 

[Martin D]

Hey Slow....

Yes, if you think about it what you are doing is decreasing rotational mass. My car got faster when I changed Rotors....I was 8 lbs per rotor lighter. It made a difference.

On down the line: I know Ben mentioned he saved 8 pounds per wheel over stock when he swithced. Also, if you go for the Unitrax titanium half shafts, you can save an additional 2 pounds per wheel! How about a carbon fiber drive shaft??

congrats on your runs!

Regards,

 

[Snake Bitten]

Loose the cats...

 

[joe117]

You weren't "decreasing reciprocating mass", it was the rotating mass. We were talking about this in another post.

My question is this, since the tread circumference of the wheel is moving, rotationally, at the same speed as the car, then would you simply be accelerating that outer tread mass to the same speed that you accelerate any other mass in the car?
If that is true, why would a reduction of 10 pounds of tire weight be better than 10 pounds anywhere else in the car?

Perhaps a lighter unsprung weight might cause less hop and thus spend more time in road contact

What am I missing?

 

[Tom F&L GoR]

Joe, it's the rotational speed of the wheel. Imagine spinning a plastic plate on your finger, then try spinning a steel plate (same size) on your finger. It'll take more effort to get the steel plate to the same rotating speed. Admittedly it doesn't affect how fast you can walk with either plate spinning on your finger as much, but every bit helps.

Given a wheel only rotates at about 500 RPM (I didn't do the math yet) and folks see a benefit, think about flywheels that rotate at 5000 RPM, and you can see why a lighter flywheel would also make a difference.

a little technical background on lightened rotating parts

 

[Newport Viper]

good read

 

[joe117]

Yes, that is a good read.
First, the wheel weight question. The answer still depends on the weight savings from the wheel swap in the first post. Look's like the tech article was saying that the outer edge of the rotating mass was the most important so changing the wheel alone might not be worth much. The .2 sec improvement seems to be within that ball park though.

Second, I think the article answers the question I've had about a lightened flywheel on a Viper. In speaking of flywheel weight reduction on large low rev engines compared to high rev engines, David Baker says,

"it's not worth spending much money lightening the flywheel of a 7 litre Chevy engine revving to under 5,000 and geared for 60 mph in first as the vehicle will be very insensitive to the reduction in weight."

That sounds a little like a Viper, not exactly but close.