shiners780

Is the weight of springs and torsion bars considered sprung weight or unsprung weight? Primarily in reference to a 944's suspension (I am not sure if the suspension geometry of other models would result in different answers.)

My initial thinking is that the weight of the front springs would be considered partially sprung weight and partially unsprung weight. And that the whole weight of the rear torsion bars would be considered sprung weight (consider a car in stock form without rear coilovers installed, just torsion bars w/ shocks.)

I figure the racing crowd would know the answer.

shiners780

Anyone?

SundayDriver

You generally include 1/2 of the weight of the spring. I assume you do the same for a torsion bar.

JackOlsen

From Fred Puhn's book 'How to Make Your Car Handle:

"Since the torsion bars are moving with the body, they are close to 100% sprung weight. Coils have a good portion of their mass moving with the wheel movement, so they are (very roughly) half unsprung weight. Since you want as little unsprung weight as possible, torsion bars are better."

Larry Herman

Yeah, that may be true, but for the track I'll trade 33 mm rear torsion bars for 1600 lb springs any day. Plus racing springs only weight several lbs, so it really isn't an issue.

JackOlsen

I have coils on my own car.

SundayDriver

I have read that, but I am not sure I agree about torsion bars. It does not matter where the weight is hung so much as whether it is on the car side or tire side of the suspension that controls it. Seems to me that a torsion bar is part of each, like a spring. I could well be wrong, but that is just how it appears to me.

M758

Remember however that the movement on t-bar is very small. In fact it is just a minor twist around 33mm diameter. I think that is reason they can be considred "sprung". Now while many 911 or 944 guys po-pu t-bars and prefer coils it is due to limied availibily and the design for our cars. Yes 33 mm t-bar vs 1600lbs spring the choice is clear. However the Ferrari F1 car has used t-bars in a least 1 championship winning chassis in the past 5 years. Not sure about the current design, but that tells me that t-bars can have nice advantage in purpose built cars.

shiners780

With a conventional spring, the physical weight of the spring can be moved up or down relative to the ground in two different ways:

1. Lifting the tire/wheel/bearings/hub etc. (the unsprung weight mass) will compress the spring moving it's center of mass upward.

2. Pushing down on the car body/chassis etc. (the sprung weight mass), will compress the spring moving it's center of mass downward.

In either of these two situations, the mass (or weight) of the spring is being moved relative to the ground. This means both the sprung and unsprung mass of the car has a controlling factor of the spring's mass.

The torsion bar, on the other hand, can only have it's mass controlled by the movement of the car, not the suspension. The suspension movement rotates the bar, but that does not change the bar's position relative to the ground. This means that the torsion bar's controlling factor (regarding mass movement, not rotational movement) is the sprung mass of the car...making the torsion bar part of the sprung mass.

I don't know, I could very well be wrong here as well. I'm just takin' the brain out for a little exercise walk...

M758

Jim agree with your logic.

SundayDriver

I think you should look at a simple spring - mass - damper model. The unsprung weight is the mass. The mass and spring constant yield a natural frequency which you have to damp with the shocks.

Any mass that move (liner or rotary) is part of that unsprung mass. So if you take a torsion bar and fix one end, then twist and release the other, you have oscillation, hence there is some unsprung mass. It is not a factor of whether the bar moves with the chassis or not (though that certainly has implications on the suspension tuning). Hence some part of the torsion bar is unsprung as it oscillates and must be controlled with the shocks.

Same is true for springs, whether they are conventioanlly mounted or inboard. Everything I have seen says you include 1/2 of the spring and shock weight in unsprung mass and I think you would do the same for the torsion bar.

However, with a torsion bar, the equivalent unsprung mass would, I think, have to be 1/2 the rotational mass (polar moment).

mein51

I'm gonna agree with Mark on this one. You do have to rotate the mass of the torsion bar when the suspension moves just as you would have mass displacement with the spring's compression. Is this negligible? Probably as negligible as the weight difference between a torsion bar and coil spring. I could see the advantages to both for different situations - like the F1 car scenario talked about earlier for a lower CG. Either way, I only reindexed my torsion bars once and that will be the last time

SundayDriver

I missed this comment earlier - good observation. The biggest issue with coil springs is that when you compress them, you introduce both a bending and rotational force. This bends the shaft of the shock. One race engineer told me that they have measured as much as 30% of the total damping from the friction of the shock seal due to shock shaft bending - this is a very bad thing as you can not tune properly. Hyperco makes a perch that helps this issue, but they are prone to failure and short life.

You still have the twisting which you want to eliminate. FYI - I run a large torrington bearing at one end of each spring to keep the spring from sticking and reduces the twisting force and therefore the bending.

A torsion bar eliminates this issue - this is a very big deal in something like an F1 car. Also the torsion bar has advantages in packaging and will help to slightly lower the CG.

Geo

Mark, I have heard this as well from my buddy who writes the software for Roehrig Engineering's shock dynos (they also have a damper division). For high end racing, this effect is not inconsiderable.

Regarding the advantages/disadvantages of coils vs. bars, remember that coils are bloody easy to change in the garage vs bars. If I were not limited by the rules I would run coilovers in a heartbeat over the bars. Think changing weather. When my car is done (what, 2015? ), when I show up at the track, the spring set-up I have on the car will stay on the car, at least for any given day. With coils, if it starts to rain, switching to softer springs and adjusting ride height is infinity easier than bars.

I suppose you could spend a couple weekends in the shop and working out marks on the bars and various bits to line then up to switch set-ups and maybe I'll do that at some point, but coils are still much easier. That alone makes them the better choice IMHO.

shiners780

Mark, let me pose this question: (Anyone else feel free to discuss as well)

The sprung weight of a car is 2000 pounds.
The unsprung weight of the car is 100 pounds.

The torsion bars, not yet installed or included in either category above, weigh 10 pounds total. We need to install the torsion bars, then put their weight into a category listed above.

Install the torsion bars and lower the car to the ground. How much total weight are the front springs and rear torsion bars supporting?