Front End Unsprung Weight
Has anyone calculated the front end unsprung weight?
I'm looking for control arms, brake, knuckle minus wheel/tire since that varies car to car.
I am trying to figure out my suspension frequency so I can pick a good torsion bar.
For my own learning, would the entire control arm be unsprung on a torsion bar system? Usually there is a percentage of the control arm that is considered unsprung because the load is applied part way down the length of the control arm (like a coil spring).
Thank you!
The control arms will still be 50% unsprung mass. The reason for this has nothing to do with the location of the spring. One end of the control arms is bolted to the spindle (unsprung mass), the other end is bolted to the frame (sprung mass). So generally, 50% of the mass of the control arms is counted as unsprung mass.
Now, if you want to get
really technical, you need to find the CG of the control arms, because that would actually determine how much is counted. From the CG to the suspension is unsprung, from the CG to the frame is sprung. It probably won't be a 50% split, but generally that's close enough for what most people are doing.
The shocks are also typically counted as 50% unsprung mass. Again, if you want to get technical there's also a motion ratio involved, that does depend on where they're attached to the control arm (they aren't traveling the full suspension travel distance).
The torsion bars are completely
sprung mass. They are supported at the frame on both ends. One end in the cross member, the other in the frame mounted end of the LCA. The torsion bar doesn't travel with the suspension, and even the LCA hex end is frame supported. That's one of the advantages of the torsion bar design. For a coilover or coil spring, again it's 50% of the mass of the spring multiplied by whatever the motion ratio is for where it's mounted. But that doesn't matter in this case.
Don't forget the strut rods, they're also 50% unsprung. And that should be pretty close to 50% as they're pretty uniform. :D
Here's some of the weights. I don't have the weights for the spindles or brakes, although, that also depends on which brakes you're running, they aren't all the same (67-72 disks, 73+ disks, 9" drums, 10" drums). I have some extra 73+ disk spindles and stuff laying around so I can get those weights tomorrow if you'd like.
Stock pair of LCA's w/tabs, bushings and pins:.......21.2 lbs
Stock pair of UCA's (73-76) with ball joints:.......................9.8 lbs
Stock pair of strut rods 73-76 w/bushings:.............4.8 lbs
Stock pair of tie rods & ends:...............................4.6 lbs (these will also count 50%)
Now, that's all measured with a bathroom scale taking the average of 3 measurements, so, some variation would be expected.
And finally, while the frequency might be relevant at the design level, it doesn't matter nearly as much as the suspension travel on the street. If you lower your car any from stock, you have to increase the wheel rate to keep the suspension from bottoming out. The stock torsion bars bottom out right from the factory, these cars were substantially undersprung. Personally, I'd figure out the ride height you want first, then figure out how much suspension travel you have left, and work from there.
I lowered my Duster about 2" from stock. That also sets the control arms roughly level with the ground, so you get the best camber gain curves from the suspension. But I run 1.12" torsion bars (300 lb/in), and had to use much shorter lower bump stops and taller upper bump stops to re-center the suspension travel range around my new ride height.
