Lets talk unsprung weight

[Ralph LoGrasso]

I'm curious as to what kind of effect unsprung weight will have on a given car in controlled circumstances. Just how detrimental to handling / acceleration / decelaration will the addition of unsprung weight be (ie, larger wheels / tires). Are there any calculations that can be performed to see what kind of effect you're looking at? ie: 1 added pound of unsprung weight will decrease acceleration to given speed (handling / braking) by X seconds (g's, feet) all else remaining the same.

Besides the calculations, how detrimental would the addition of say 10 lbs of unsprung weight over stock be at each corner (again larger wheels / tires). Can the problem be solved with stiffer springs and larger sway bars, (even sticker tires in terms of handling)?



Thoughts?

[dgumoe]

ralph, im by no means an expert in suspension, i do however know that unsprung weight sucks. thats why irs is a nice feature. it reduces unprung weight because the differential is bolted to the frame, which allows you to run a lighter spring rate because there's less crap bouncing on the suspension. you can run a stiffer spring but obviously ride quality suffers. i think the latest PHR has a writeup on this sort of thing

[baz67]

Ralph,

Think this way. Take ten lbs and hold it in your hand with an outstreched arm. Now take your arm and move it up and down. Now imagine that same weight at your shoulder area moving up and down. Does that help you see how that is not good.

Now for how it affect braking and acceleration. The added mass of a larger rim acts like a flywheel. It will require more work to get it moving, more power, and conversly more braking to slow it down. So you get a double wammy with added unsprung on the wheel.

You wheel is basicly a gyroscope. The more mass, the more energy it takes to move it. That could equel slower steering responce.

[wickedmotorhead]

I'll add my simple explanation of weight transfer.

The weight of the chassis and all the parts on it is sprung weight. Tires, wheels, and typically a fraction of parts connected to the frame are unsprung weight.

Some figures I found:
Tires, Wheels, Uprights: 100% Unsprung weight
control arms and axle shafts: 50% Unsprung of an average of the two ends
Shock and spring: 36% unsprung because the lower end is moving 72% of the unsprung weight's motion (ex. mounted to the lower control arm)

Now take a ratio of sprung to unsprung for a crude measurement. One auther suggests that a ratio of 5 is good and 2 is very poor. It does not get much more simple than that, but that doesn't get into any of the physics. Typically a high ratio will stick better on rough roads and ride better since the car body isn't shocked by large rotating masses bouncing of the road, but instead the body's inertia plants those lightweight Kinesis wheels to the pavement and makes them scream for mercy. Its more of a short and not so technical answer but hopefully gives you a better idea. Quite simple if you add weight its going to take more power to move it. The key is getting the correct weight transfer. One simple equation of the many more to give you a general idea.

Fore/aft Weight transfer=[(car weight(lbs)*acceleration(g)/(g))*(CG Height)]/wheelbase

Thats how many lbs will be shifted from the front to the rear for a given acceleration rate. Thats just straightline stuff. Then you get into lateral weight transfer.

Lateral weight transfer= [[***** weight(lbs)*(speed (MPH))^2}/{14.97x(Radius of turn(ft))]*[CG height]]/Track width

This is assuming the rotation of the CG due to roll is negligible due to small roll angle and low CG

And for fun the maximum speed you can take that corner without tipping over=2.74*SQRT[(Radius of turn*Track)/(CG Height)]

These are only the tip of the iceberg. Then you get into roll couples and centrifugal forces on roll centers finding CGs of the unsprung weight and calculating front and rear weight transfer due to unsprung weight. Hope these give you a way to see the rough mechnics in a quick fashion. I suggest getting a good vehicle dynamics book to further discuss like Millikens "Race Car Vehicle Dynamics"

Shane

[DeepBlue68]

I think someone mentioned it above, but I just wanted to reiterate: don't forget to think about the increased rotational inertia of a heavier wheel. Besides the effects that increased wheel/tire weight will have on the car's acceleration/deceleration/turning, it will be even harder for the engine to get that extra wheel weight turning. It's almost like keeping the same wheels and knocking a few horsepower off the engine.

[Ralph LoGrasso]

Thanks for all the responses, guys. There's some great tech in here. I'm going to have to come back to this thread later and read it again to absorb it all. Shane, I'll check out that book next time I'm at the book store. Sounds like a good read. I guess the moral of this story is the lighter the wheel (less unsprung weight)--the better.


I really feel bad for the guys with stock brakes and 26+" rims on their suv/trucks.

Thanks again guys,

[Norm Peterson]

You won't find RCVD at your local bookstore. But you will find it at the SAE.org online store. It's a bit pricy at ~$90 or so, and I think there's also a companion workbook option. A word of warning - you may not get a lot out of RCVD unless you've either formally studied engineering or done fairly extensive reading of the softcover chassis & suspension books with some independent thought.

As far as the effect of wheels & tires and acceleration is concerned, this can be calculated without a whole lot of effort from mechanical engineering text equations.

Torque = [rotational mass moment of inertia] * [rotational acceleration].

There's enough known stuff to get from net flywheel torque to acceleration in mph/sec or whatever units you're using, and you can re-arrange the terms and express it in terms of effective mass added to the car weight for the overall effects. For a wheel/tire, mass moment of inertia is a function of the weights and the radii^2 (actually, it's an integration problem, but there are enough common formulas such that you shouldn't need to go that deep into the math). A decent little spreadsheet problem, or part of a larger one that tackles overall vehicle acceleration.

On suspension movements and sprung:unsprung, the heavier the unsprung mass is relative to the sprung mass, the more it acts to move the sprung mass around as it tries to follow the road contour. Don't forget that the spring pushes on both the axle/suspension and the chassis.

Norm

[Ralph LoGrasso]

Norm,

Thanks for the response. I think I'll hold off on that book then, since I have no engineering experience. I'm glad to hear I don't have to get into integration and such, I cringe at the terms. I think I'll try to set up a spread sheet problem when I get some free time. It will be nice to know the effects.

Thanks again,

[Norm Peterson]

If you'd like, I could review any such spreadsheet.

Norm

[Ralph LoGrasso]

Thanks for the offer, Norm. I replied to your PM.

[zbugger]

...Lets talk unsprung weight...

Oh please..... I have a big enough headache already....