Unsprung & Rotating Weight


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Everyone knows the importance or advantage of building race vehicles light weight, but does anybody know of any actual tests that have been conducted to derive some sort of formula or get some hard data to support that? Here's what I mean. Coming from an asphalt circle track background, I was taught that removing 1 pound of unsprung weight (spindle, brake caliper, etc.) was the same as removing 4 pounds of sprung weight. And that removing 1 pound of rotating weight (crankshaft, flywheel, etc.) was like removing 10 pounds of non-rotating weight. And that when you combined the two, removing 1 pound of unsprung rotating weight (tire, wheel, brake rotor, etc.) was the same as removing 40 pounds of sprung non-rotating weight.

However, I have never been able to find any hard data or formulas that support these theories. It's easy to find stuff that says lighter is better, but that's about it. I'm also curious if everyone thinks this is a real important issue with off-road and desert racing, as much as it is to the asphalt circle track guys. Cause to them it's extremely important. Heck, we used to gun drill all our wheel studs for goodness sake. (I always thought that was like splitting hairs, but everybody else ended up doing it too)...

Also, with all this in mind, are there any Trophy Trucks or Class 1's that run the carbon/carbon or carbon fiber rotors? or inboard brake systems like on Chapman's Lotus cars? Or is all that too expensive compared to it's benefit even for a TT? Someone said in a different post that with the new 39" tires there was a lot of gyroscopic force when you are spinning a 100 lb. tire and wheel at 130 mph. WOW, that seems unbelievably heavy. Is that really necessary to have them that big? If you could cut 10 pounds off per wheel that's like (based on the above formula) taking 1600 lbs off the truck!!


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Unsprung weight is important off road. The trucks are pretty much on par with round-y-round cars if you consider the ratios. A 6000 pound truck with 600 pounds of junk going up and down in the rear is not a bad ratio. This is about 10% of the vehicle weight for the axle or 5% per corner. IRS can be much less.

Rotating weight is more related to vehicle acceleration and deceleration. This is not a huge factor off road. What most don’t realize is how much rotating mass, from the engine out, affects driveline stress and failure. Like the saying "It's not the fall that hurts, it’s the sudden stop;" high rotating speed of driveline components creates a lot of potential energy.

Carbon/carbon brakes are few to none off road because of the expense and abuse. There are some titanium rotors and brake hats out there. The Ti hats are more to keep some of the heat from getting to the hubs than saving weight. There have been some inboard mounted disc brakes on IRS cars.

A 39” tire is worth it as long as the vehicle is big enough and heavy enough. A good example of this is driving over potholes. Every inch of increase of tire diameter makes a very big difference in reaction; same thing with bumps. Try hitting a Mexican style pothole in a VW Rabbit some time.


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RE: "the bigger the tire, the smaller the hole. "

That's true but the real issue has more to do with running over rocks at higher speeds without getting flats. The 39" tires have a larger volume of air to allow the tire to deform more to suck up the rocks as well as give a larger foot print for traction while running 27 pounds of air pressure. Another misconception about the 39" tires is that they weigh more than the 37's. That's not true. The 39's don't require a liner and they are actually about 7 lbs lighter when mounted.

As far as weight on a TT goes it's not that big of an issue. Ask some of the most winningest drivers out there, Ashley, Herbst, etc. they'll all tell you that light would be nice like in asphalt racing but it's just a pipe dream and doesn't work in off road. Some of the lightest TT's running have severe issues with reliability and end up DNFing a lot. Also, take note that the heaviest trophy truck in the entire fleet of SCORE vehicles won the TT points championship last year and is on it's way to another championship this year after winning the last two races overall.