Polar moments of inertia... 2

[BioMax]

This is more of a "fire starter" I think, but I would really like to get some outside oppinions.

On any on-road car you would want low polar moments of inertia, for fairly obvious reasons. BUT on off-road desert vehicles "they" are leaning to high PMI. The theory is solid, in that low PMI allow the vehicle to change directions quickly. Where in the dirt, that isn't a very desireable trait.

My question is two fold:

1. What are your opinions on this theory?

2. If an overly complicated rear suspension, using heavy bell cranks or some other sort of leverage system, is used would it add to the "dynamic" PMI or is unsprung weight not part of the equation?

I do realize that any moving suspension part attached to the stationary frame is only partially considered unsprung.

As I have stated before I agree with the theory, but I have seen trucks built, with what seem to be way too much unsprung weight, in order to apease the dirt gods...

 

[GregLocock]

"On any on-road car you would want low polar moments of inertia, for fairly obvious reasons"

Show me the money. This smells like dogma, not an engineering argument.

"on off-road desert vehicles "they" are leaning to high PMI. The theory is solid, in that low PMI allow the vehicle to change directions quickly. Where in the dirt, that isn't a very desireable trait."

"1. What are your opinions on this theory?"

Doesn't seem unreasonable. Our road car has a high PMI, it is delightful to drive on gravel roads - the initial yaw rate is reasonably slow, so it is easy to catch with your right foot. 70 mph tail out in gravel is ten times as much fun, and ten times safer, than 60 mph in a hairpin in a Lotus Esprit on tarmac.

"2. If an overly complicated rear suspension, using heavy bell cranks or some other sort of leverage system, is used would it add to the "dynamic" PMI or is unsprung weight not part of the equation?"

Unsprung weight is most defintely part of the equation, I'd be tempted to lump the whole lot in to the PMI. If you think about it the angular momentum is what is important, and the unsprung mass is fully coupled to the body, in yaw.

Best way to investigate the effect of PMI initially is to use the bicycle model, tests I'd use would be step steer, pulse steer, and swept sine. The first two can be analysed by eye, the latter really needs to be looked at in the frequency domain.

I'm not going to tell you the answers, but you will learn a lot from studying those responses.

However in my experience you have to make huge changes in PMI to make a significant difference in these tests, and without adding ballast you may find making huge changes too difficult.

Also bear in mind that low cg height is probably far more significant than any fancy-pants theories.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[BioMax]

I can agree with it being much more fun in the dirt than on the street...

What about over bumps?

If the vehicle is upset vertically the suspension would be forced to absorb the movement and then the unsprung weight would be folowing the bump (as best it can) rather than being used as polar weight. Right?

If low PMI is dogma then why do most of the "super cars" run mid-engine config?

 

[GregLocock]

"If the vehicle is upset vertically the suspension would be forced to absorb the movement and then the unsprung weight would be folowing the bump (as best it can) rather than being used as polar weight. Right?"

Sorry I was only thinking about the yaw axis. Yes, in pitch the unsprung mass is decoupled from the body's PMI while you are on the ground.

"If low PMI is dogma then why do most of the "super cars" run mid-engine config?"

Well, that's a good question - but the cynic in me says dogma and fashion.



Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[Fabrico]

They "leaned" to high PMI for that very reason over 20 years ago and never gave up. Mid-engines come and go and also win big races, but they have not taken over.

As I recall, Formula 1's have around 5% unsprung weight per corner. That's 10% per axle. A Trophy Truck has about 600-700 pounds unsprung in the rear. But the weight of those trucks average over 6,000 pounds. The unsprung is a shocking number but it's still right around 10%.

The old Baldwin TT is about the only one left with a rear arm system that operates similar to the old quarter-elliptic setups. It works but desite it's trick appearance and complexity, it has never proven to be superior to a good 4-link with coil-overs or gas-spring shocks.

CG would seem critical, but for scudding across the desert it is often ignored by the big boys. The 70-90 gallon fuel cell is more often than not, mounted above and to the rear of the rear axle. Then the 2 spare tires are placed above and to the rear of the cell. Some even put full sized spare drive shaft complete with U-joints, on top of the roof! Though some appear to be a big engineering blunder, they work very well and win some big races. These are serious vehicles costing over half a million dollars now.

 

[BioMax]

I cannot deny that from the outside looking in not many of the trucks look like they should work and yet they work exceptionally well. I just look at some ot the rear suspension systems "they" are using. Those with what I believe to be called, mesanine arms. On top of that 4"x.250 wall axle housings, with a 12" ring and pinion and all of the trussing used. It seems that some of it is for the sake of "hey look what I can do." I think the Riviera truck is one of a dying breed of straight foward design and engineering, while still holding it's own. I know that Pflueger is having a fully independent TT build and the rumors are it works really well. Way back in the day there was an independent truck built that (as rumor has it) was forced to return to straight axle rear suspension because it worked too well.

I just wanted to get some outside oppinion and I appreciate it all...

Fabrico: The pic you posed looks a lot like a Robby Gordon truck.

 

[Fabrico]

Good call, it is a Robby Gordon truck. It's just a good example and not one I work on. The Mike Smith built SCORE Class 1, built as a truck with a buggy body (a "Truggy", for the Herbst's) has a 14 inch ring gear in the rear end. That's not 14 bolt, but an honest to goodness 14 inch gear. T/F dragsters use a 12" gear. Typical axles are the same as T/F at around 2.75", both being gun-drilled. I don't think they are spending this kind of money just for piece of mind.

I used to think the same way but you have to consider what wins races year after year. But that's not to say, at all, that there cannot be anything better. I think there is plenty.

 

[BioMax]

I'm not going to tell any of the TT or class 1 guys that they're over kill. I know with the new trend heading towards 39" tires, the weight that is carried around (sprung and unsprung) is tremendous. Especially with the horsepower that the engine builders are capable of, there is that much more to keep from breaking.

What I guess I am asking is, why build an elaborate rear suspension (mesanine arms, or what have you) and add to the unsprung weight when there is room to stay simple and less weight? It seem that all that is added is high PMI and cool points.

I don't mean to keep "beating a dead horse" but there has to be some method to the madness. These are top notch builders doing this. Am I missing something?

 

[Fabrico]

Though certainly popular in the past and still a few in use, I'm not aware of any current builders using the so-called mezzanine arm setup for competitive unlimited racing such as Trophy Truck. These were popular from about 1990 to perhaps 1998 or so in the unlimited classes, and were pre-dated by the quarter-elliptic spring setups which mounted in basically the same place. Builders like Kudela (Baldwin TT), and Nelson (Larry Ragland TT "Arnold") built some good working machines. Because they work and are proven tough, they do still show up on limited class vehicles (see picture) and pre-runners.

While the setup provides an exceptionally "clean" 4-link environment, it does not provide advantageous movement ratio of the shock absorbers and springs.

As far as the appearence of high unsprung weight, it's not so much the weight itself, but the ratio of sprung vs unsprung that is important. A poor ratio's affect on ride quality and perhaps even stability is arguable, but there is no doubt it is negative to traction.

In addition to PMI and cool points, there has been significant increases to average speed. If keeping it simple means staying with smaller wheels and other components, that just makes the bumps bigger.

 

[BioMax]

Okay, that does make a little more sense. Thank you!


Fabrico: On a different site... there is a guy saying that there is someone out there who is running a genuine reverse 4-link on an a truck, that even finished the 1000. Have you seen the truck or even heard of it?

 

[Fabrico]

All that I have seen are up front; got one on my Grand Cherokee.