NVR2L8

Hey guys,

In my continuos search for understanding my Z06 and how to reduce weight and improve performance, I ran across an interesting article on rotating mass, and I thought I would share.

Although the article is directed a little bit more to drag racing, the physics of rotating mass is the same.

http://www.w8ji.com/rotating_mass_acceleration.htm

RaleighSS

Good link ... makes me wish i was a ME rather then a EE lol .. good stuff after taking time to read it second time.

sperkins

Did I really just spend 10 minutes reading an article that is trying to convince me that lighter wheels, lighter tires, lighter rotors, lighter driveshaft, lighter flywheel, lighter clutch and lighter a crankshaft won't make your car faster?!

69427

Being an engineer, I'm already familiar with the topics he discussed, but I do have to salute the writer for making a topic (which is frequently boring when delivered by college professors) reasonably interesting and easy to follow.

Everett Ogilvie

You did not waste your time! One of the most important things to take away from the article is where the weight is concentrated away from the center line. As he said and as we all know, a light tire makes a huge difference (because of how far out the majority of the weight is). A wheel makes more difference than a driveshaft, but not as much as a tire. A rotor makes more difference than a driveshaft, but not as much as a wheel - it is all RELATIVE. Work on the "lowest hanging fruit" items first - those that make the most difference. It does not mean don't work on the other items later if you can afford to.

The other important thing the article mentioned is the RATE of accelerating or decelerating the object in question - he even gave an example of a road course car, implying that it speeds up and slows down all those parts at a great rate and very often, so some things matter more on a road race car than a drag car.

Overall I thought it was a pretty good article (glad I am a ME!).

One more thought - he touched on unpsrung weight - on a drag car it is not as important as a road course car where the suspension action is constant and can go on for extended periods of time, lap after lap. Take the rotor example - making your rotor smaller and/or lighter may not have as significant inertial effects as making your tire smaller/lighter, but it also impacts unsprung weight which is very important for the action of the suspension. Most of the things we do to our cars have multiple effects.

69427

Re-read it. He tried to explain (unsuccessfully to some apparently) that not all weight reductions are as "economically or acceleration efficient" as other weight reductions. F=mA still applies.

drivinhard

"I would never bother changing from steel to an aluminum driveshaft in my car, because my car takes 11 seconds to go 1/4 mile. The car weighs 3000 pounds, and this means I might save 20 pounds of weight and 1/2 horsepower lost to spinning that weight over the length of the track. $400 is not a good investment at all for 1/2 horsepower over the length of the track, or the extra 1/2 horsepower applied for 11 seconds I have to extract at the end and convert back to heat with my brakes."

Never underestimate the cumulative effects of the relentless pursuit of small gains.

NVR2L8

I am sorry that I am not as smart as you, but I was not familiar with the fact that the ratio of rotating mass is not just a simple calculation. I have been told by several people that a multiple of 2-3:1 was a good rule of thumb.

I now know that the result of the rotating mass weight is variable based on numerous items, mainly the larger the diameter of the item (like wheels / tires) is a much higher ratio than a smaller diameter item (driveshaft, etc.).

Even more variables include the speed of the rotation, the acceleration and deceleration of the rotation, and the unsprung condition of the car during the rotation.

I hope several of us "mortals" will benefit from the article. I am sorry that you did not.

sperkins

You guys...
Maybe some of you aren't to the point of squeaking out tenths, but some of us are.

Here's the deal. You take two equal cars. You drive one and I modify the other with all the crap I just listed and I'll beat you every time. EVERY TIME.

69427

The author is not saying that a lighter component (such as an appropriately sized driveshaft) will not increase performance. He is saying that if funds are limited (as in the $400 mentioned) it is not an efficient use of money. The $400 is better spent on items that are heavier, have a higher moment of inertia, spin faster, etc.

An example: I drool over the titanium bolts I see advertised in the Speedway Motors catalog. I'd love to replace a lot of steel bolts with those. However, I don't have unlimited funds, and I need to spend my cash where I get the biggest bang for my buck.

RX-Ben

I noticed a perceptible difference out of slow turns going from a 7.25" QM clutch (and flexplate) to a 25lb RPS (9"?). I want that time back. It was so light that I pushed a P0300 code every time I started the car and couldn't do a crank speed sensor calibration.
4lbs off the prop shaft/couplings made no difference and I didn't expect any.
I did not notice a diff w/lighter rear discs, didn't expect any accel diffs (just for susp).
None of this should be news.
The QM is not exactly $ efficient, since you will need a trailer and winch. 4.5" QM is ideal.
I am not sure how light the lightweight LS1 cranks are (any offer >10lb loss?) - might be a decent loss in MOI there if the weight comes off the bobs. Maybe not so great for the bearings.

AU N EGL

A few Years ago I talked a Speed World GT engine builder in Concord NC.

you can make engines very very light, and squeak out every little 1/10 of a HP possible. BUT then the life of the engine drops dramatically. at that time the SWGT 346s were lasting 3 maybe 4 race weekends.