The Effects of Rotational Inertia on Automotive Acceleration

i got lost halfway down but some great info. rotational mass vs static mass

 

ARRG not math with letters

 

I'm a little bummed after reading some of that. I'd always heard that trimming 1lb on rotational mass was equivalent to cutting 5lbs of sprung weight. I guess it's double the savings at best.

 

There is only one Professor on here, and you aren't it....I know you can't see it STEVE....rotate on this!....LOL......math....MB, rolling starts, now math...I'm headed outside for some fun in the sun...geezzzz....asa I get the **** cleaned off my shoes....OH, I might add this just for the hell of it...this isn't for you Stevey...when I head over to the track for the test and tune nights, you line up with whomever is in the other lane...it could be a stock car, street car, pro stock car, as you really don't know...they run everyone thru as fast as they can...and in fact, farm trucks...got my *** cleaned by one of those hopped up diesels one night...900HP/1000FtLbs of torque...it's all in fun...really doesn't matter as long as you are having fun...some of those kids can drive, some can't, some can shift, some can't, some can launch, some can't....I do it for fun, as I loved it as a kid, now, I learn from the kids...but, the old guy win's a lot of them just being consistant on every run...it's for fun...stay off the streets racing...you can play, but I wouldn't be racing...you just might lose it and hit a curb...or someone else....have fun...top down time...

 

Doc, is everything alright? You see a little out of sorts today. For the record, I keep it on the track but as with most of us, youthful rotating masses made me loose myself once in a while. But not often.

Les

 

les im pretty sure doc is fine. just a little pms!!

 

Excellent article, and it was fun to go back to my Calc-based Physics days for a moment. I'm lying there, actually...

To me, the article was a bit wordy in places it shouldn't have been. The example of the flywheel "theoretically" making a 100hp car do a ten second 1/4 mile was simply extra fluff that distracted from the whole lot, and that scenario would only exist on paper.

Particularly with the part on braking, I think he focused too much on the part about lighter rotors making you stop better. They don't, and I'm not aware of any racer that has purchased lighter brakes with the intention of stopping harder from weight savings alone. Where the bigger, lighter brakes DO come in handy is the heat dissipation, and the more pistons you use to clamp down on the rotor with, the bigger hat you need for said braking (also to simply fit the pistons). Interesting points he made about the lugs being negligible, as I have always thought this and I've seen vendors selling "ultralight" titanium lug bolts for use with lightweight wheels... Now I know it's $500 down the drain.

Let us also remember that the wheel isn't the ONLY thing spinning on the hub during acceleration... So if you save 5 pounds per corner on a new wheel and tire setup, and ~7 pounds on each corner in rotor hat weight, then you've effectively shed 96 pounds of static weight from the car, using these numbers. 96 pounds may only mean an improvement of .1 second in the 1/4, assuming all other factors remain constant, but it's the other 99% of the time your machine is in action that the biggest gains will be realized.

While the number doesn't seem huge, think about this: each and every time your suspension cycles (as in, EVERY bump that you encounter), it has to move the attached weight back and forth on an arc. It is at the limits of the arc that the greatest effect from the weight is felt... pick up a fifteen pound barbell and hold it straight out from your body. Now, rotate your torso back and forth as fast as possible, trying your best not to move your arm. Rotate from the 10:00 position to the 2:00 position 50 times. Try this same experiment again with a 6 pound barbell, and note the results.

Your car doesn't have muscles that fatigue, but mass is mass and this all plays an effect on shocks, bushings, tie rods, ball joints, etc... a machine that has to work harder to achieve the same results will be slower than one that uses lighter components.

Les can attest to the fact that having lighter components is FAR more than simply going in a straight line the fastest... it's about endurance, agility, and the ability to do the cycles thousands of times with as little stress on the machine as possible.

Interesting also about the comment near the bottom regarding low-profile tires... since the entirety of the tire's weight is concentrated at the end of the rolling stock's radii, it's easy to see why a narrower drag radial on a smaller diameter wheel nets better straight-line acceleration... couple that with the issues of sidewall flex, wheel weight of a 19" wheel versus a 16", etc... it all comes to play nicely together. Once at speed (refer back to his flywheel equations), it matters less and that's why a road course car benefits more from a low-profile, wide tire since there is more tread available for lateral forces (and the sidewall it's attached to doesn't flex nearly as much as the sidewall on a wheel and tire combo with same OD but smaller rim and higher aspect ratio tire).

On that last point, it's one of the reasons why the N/A cars make horrible drag racers, but excellent corner carvers: that big 19" wheel is a torque robber and a bear to get rolling off the line, hence the bogging effect on any launch < 4,000 RPM... But once rolling, we can hang with many of the big boys, because the car simply bites the road in the corners with the minimal sidewall flex. As you decrease the wheel's diameter, you improve the 1/4 mile capabilities but steal from the other hand on the handling abilities. Since the N/A is already at a huge horsepower deficit, it's why many owners tend to modify their cars to tackle the corners, not the strip. We build off the platform we already have a head-start on.

A good read indeed.

 

after reading this what i SQUIRREL!!! were was i?

 

Pop a yellow jacket from the truck stop and go back and read it, you'll agree with me LOLOLOL

BTW we gotta get your car off the stands and start wrenching again, I was bored as hell all day.

 

soon soon. i think i have the rear calipers sold as well as my mesh screen for the slk. should be enough to cover all the work rob is doing to my 55 setup. email from svo today. tracking number still isnt working but i have some info and will wait till fri. hope rob gets the calipers into the booth next week. powdercoat has been a nightmare to get off!

 

Good I'm going through withdrawals. I have a laundry list of things to check/swap/tighten before the dragon.... Only two more months!!

 

Post Mechanical Stress.....

 

I kinda skimmed the article. Nothing really new for me. Weight savings means less to move up and down, accelerate or decelerate and makes a difference. How much difference is the debate. The closer the weight to the center of rotation the less of a problem. If you want a real world example to think about go back to the mid 70's when it was all about fuel milage. The cars that did the best had 13" wheels and tiny tires (yes they were light with tiny engines too). Less friction, weight, etc. For racing what matters depends on the car. How heavy it is, how much power, how fast the track. For light lower power cars it's often better to have less tire for aero and weight reasons. Heavy higher power cars need all the rubber they can get even if it's heavy. The taller the tire (weight farther away from center) the more important the weight of the tire itself. I've used my stock rims with both light and heavy tires. I've used my Forgestar's with both medium and heavy tires. For me what's important is cornering grip level. The unsprung weight makes more of a difference in how the car handles than how fast it goes but it does make a difference

As far as big lightweight brakes. They stop better but mostly because it takes less pedal pressure to achieve the same braking force which, for me, makes it easier to threshold brake. I can say from experience that being close to loosing traction under braking will stop shorter than engaging ABS. The most important braking component is the tires and next is drivers ability to modulate the pedal. Actually the fastest stops are ever so slightly slipping. The larger components of a BBK have greater capacity to absorb and radiate heat. It keeps the fluid and wheel bearings cooler so braking is more consistent and reliable. With stock brakes I was constantly bleeding them and with the BBK I'm not obligated to bleed them between heats. I always bleed them before an event however.

Unsprung weight makes a difference, just not enough to easily be felt by the buttaccelarometer or buttdecelarometer. I'm not disappointed in the lightweight parts I've purchased but the price of them is out of proportion with the gains. By far the greatest increase in speed at the track was from wider wheels and tires. (corner grip) More rubber means faster corner speeds, lower stopping distances and traction for acceleration.

I could go on but I'm confusing myself.

Les

 

Very interesting! But SRT's are autos, and light wheels are too expensive I do shave a pound off the tire weight every trip to the track!! LOL

 

I like your thinking better...lol