MAGK944

I am going 50/50, half is sprung and half is unsprung. Reasoning: anything that acts as the "spring" is by nature attached to the sprung AND unsprung mass of the car.

Torsion bar delete on two of my cars, 600# springs, no issues. As others have said that bottom mount needs to be flush and don't use excessive spring rates back there (i.e: greater than 750#)

Chris White

[QUOTE=MAGK944;6870120]I am going 50/50, half is sprung and half is unsprung. Reasoning: anything that acts as the "spring" is by nature attached to the sprung AND unsprung mass of the car.
[QUOTE]

Only half a beer for you!

The torsion bar is 100% sprung since none of its mass is displaced by suspension movement.

aben8057

Is there any easy way to have them not engaged without having to drop the whole rear?

rlm328

+1 Chris,

Sprung is anything that is hinged. Torsion bars are directly attached to the frame.

And yes it is safe to convert.

rlm328

Delete them and go with coil overs.

alxdgr8

I think that is what he wants to do, but he wants to remove or "deactivate" them without dropping the rear carrier. As far as I know, there isn't a way to do it. It would be nice if it was setup like 911's where there is a whole in the rocker to access them.

SamGrant951

You can drop 1 side at a time to remove them without removing the whole assembly, there is enough clearance.

Van

If you can get the car road worthy by Sept 16, I can get you into Watkins Glen (it's actually my event)...

Dare

Hmmm, that's not how I see it. The torsion bar's job is to twist as the trailing arm swings through its range. One end of the bar is locked to the frame. However the other end rotates with the trailing arm. Although it is very small, the end of the bar that moves with the trailing arm does have mass, and this mass contributes to the overall mass of the trailing arm assembly. Thus I think MAGK944 is right in that part of the mass of the torsion bar is properly counted as the unsprung weight (i.e. mass) of the rear suspension.

And with that said...

JohnKoaWood

[QUOTE=Chris White;6870167][QUOTE=MAGK944;6870120]I am going 50/50, half is sprung and half is unsprung. Reasoning: anything that acts as the "spring" is by nature attached to the sprung AND unsprung mass of the car.
But what if the car is on a treadmill...

Chris White

Nope, the torsion bar does not 'move' with the suspension movement, it rotates – but the mass that actually rotates is so incredibly small that’s it is for all intensive purposes meaningless (we are talking grams…)
To be technically correct you would have to count the center of the bar as sprung and the outer edge (so approx 12mm from center as a ratio of the full length of the trailing arm (lets approximate at 1 meter) – that means that on the furthest outside edge of the Torsion bar the mass that is unsprung is 1000mm/12mm * .5 (because the center does not move) or .41% - because the inner end of the torsion bar is fix it is completely sprung so the percentage of the torsion bar weight that is unsprung is .2%

So you are 0.2% correct….(and I am 99.8% correct!)

vt951

Hmmm, I'll discuss with Dan next time I see him. That would be awesome. Road trip!

EDIT: It's about a 9 hr drive from Cincy to the Glen. That would be a long way to go for a shake down.

Van

9 hours is a long way... But it should be nice camping!

Dare, think of the torsion bar thing this way: picture the lid on a trunk. When you open it, you have to apply a force to overcome the weight of the lid (and the friction of the hinges... but let's take that part out of the equation for now).

If you set a 20 lb rock on the lid, you now have to lift that along with the lid. If the rock is farther away from the hinge, the lid is harder to open. If the rock is closer to the hinge, it's easier to open. (Leverage)

Now, take the rock off, and replace the pin of the hinge with a 20 lb uranium pin. The opening the lid will feel like it did originally - the 20 lbs of extra weight that the trunk now has will not effect the feel of the lid.

And now, let's put some resistance back into the equation (friction from the hinge). If our new 20 lb pin has a lot of friction, or can act as another source of resistance (a torsion bar spring, perhaps?), it can make the lid feel heavier, although it is not actually contributing any weight to the lid.

Van

P.S. and, Chris, I think you're wrong about 0.2% being sprung because of the diameter of the torsion bar... because the torsion bar also has that diameter on the other side of the pivot point acting as a counter balance.

If you take the spring (torsion bar or otherwise) off the rear suspension and support the body of the car, you can put a scale under the wheel and measure that you have, let's say, 95 lbs lbs of unsprung weight. If you had a control arm that had a bar going in the other direction from the pivot point, and you put a counter weight on there, you would actually reduce your unsprung weight because the scale would read less.

(However, this would introduce a host of other problems - like inertia of the suspension and overall weight of the vehicle.)

(As I'm thinking about this - is this the rational for inboard shock absorbers? To keep all of those components 100% sprung?)

Dare

Exactly. This is why your trunk lid with the uranium pin doesn't feel heavier when you hold it still against the pull of gravity. However the rotating portion of the trunk lid, which includes the pin, now has more mass, and therefore a greater moment of inertia. Thus it will take a greater amount of energy to set the lid in motion relative to a lid with an aluminum pin.

Now back to the trailing arm. You're right in that the mass of the torsion bar does not contribute to the unsprung static weight of the suspension (i.e. the amount of force exerted by gravity on the trailing arm assembly as measured at the wheel axle). However it does contribute to the unsprung mass of the trailing arm assembly, thereby affecting the forces required to move the assembly up and down. This in turn affects the suspension frequency of that corner of the vehicle.

Totally agree. But this isn't really about suspension, is it? It's about beer!