Anyone know how rotating mass effects a chassis dyno

markislive78 · 01-18-2005, 02:46 PM

TitanTalk,

Anyone know how much 20'' wheels will affect a chassis dyno? I dyno'ed 289.2 rwhp @ the dyno a while back with 20's. I'm just curious of how engine rotating and reciprocating masses, IE... Flywheel, Trans gears and shafts, driveshaft, rear gears, axles, wheels and tires effect drive train loss. My 20's weighed in @ 86lbs per wheel and tire vs. 75lbs on stock 18's. That's a diff of 11lbs...Anyone knows how muck that hurts?

I have a 2wd which comes with an ol heavy/rusty steel drive shaft. Wouldn't it seem logical to use one of those nice light aluminum drive shafts that come on 4wd's or Titans w/Off-road packages to help combat the extra rotating weight created by my 20's?

Any responses or ideas would be greatly appreciated....

juma · 01-18-2005, 03:38 PM

a rule of thumb in racing is 1 pound rotating equals 10 pounds of weight. so each of those wheels is like and extra hundred pounds to haul. drive shaft, about the same. the aluminum one on the 4WD won't work on the 2WD but you can get a aluminum or fiber one on the aftermarket and it should make quite a difference. looks like we might have clearance problems with that brace about half way up the drive shaft.

juma

markislive78 · 01-18-2005, 05:41 PM

Quote:

Originally Posted by juma

a rule of thumb in racing is 1 pound rotating equals 10 pounds of weight. so each of those wheels is like and extra hundred pounds to haul. drive shaft, about the same. the aluminum one on the 4WD won't work on the 2WD but you can get a aluminum or fiber one on the aftermarket and it should make quite a difference. looks like we might have clearance problems with that brace about half way up the drive shaft.

juma

Juma,

Thanks for the reply...So rotational mass would only include drive wheels(rear tires) and not the front ones because they are not part of the drive train right? So it's like i'm carrying around another 200 lbs with 20's right?

VShortt · 01-18-2005, 05:47 PM

According to everything you guys have said, it should be more like 220 pounds.

juma · 01-18-2005, 06:15 PM

Quote:

Originally Posted by markislive78

Juma,

Thanks for the reply...So rotational mass would only include drive wheels(rear tires) and not the front ones because they are not right? So it's like i'm carrying around another 200 lbs with 20's right?

no, you include all wheels so its more like 400 pounds. I think on a dyno, you'd have to add that weight to your vehicle weight to get a realistic number to use to calculate torque and hp. at the track, its evident. search on driveshaft and you may find some links that have more info on that. juma

juma · 01-18-2005, 06:18 PM

Quote:

Originally Posted by VShortt

According to everything you guys have said, it should be more like 220 pounds.

yeah, but this isn't exactly scientific... 200-220, 400-440 is a close enough estimate, probably. juma

markislive78 · 01-18-2005, 07:02 PM

I just don't see how the 20's in the front could impact drive train loss... when the are not attached to the drive train

GETSOMZ28 · 01-18-2005, 07:02 PM

The effect on performance isnt just the extra weight, but that weight is further out from the center of the wheel too (rotational inertia). So you lose performance on two fronts when you go with a bigger heavier wheel.

GETSOMZ28 · 01-18-2005, 07:08 PM

Quote:

Originally Posted by markislive78

I just don't see how the 20's in the front could impact drive train loss... when the are not attached to the drive train

They dont on a dyno, but they do on the street (gotta move all four tires)!

bradyb · 01-19-2005, 09:09 PM

GETSOM is right, the larger wheels take more power to turn and push so they rob horsepower and lower your dyno numbers. The blings blings are all for show and not for go.

Boone-Titan · 01-19-2005, 10:03 PM

The further away from the axis of rotation the more net loss each Lb equals. To put it simply, it eats more energy (HP) when starting to rotate and thru the full range of acceleration. I doubt very seriously if the driveshaft would effect things much. The steel and the aluminum driveshafts are both hollow. The steel one is smaller in diameter. The Alum one although lighter is spinning its mass at a larger radius. I bet its a wash for the driveshaft. The other thing effected by larger wheels is torque. Think of when your trying to break a bolt loose. The longer the breaker bar the more torque you can apply to the bolt. Now reverse this and imagine your trying to turn the bolt while the breaker is still on the bolt head. Which lenght of breaker bar would be easiest to turn? The shorter one and hence the same applies to the smaller wheel.