robschwieb

Hey guys, I could use a bit of help here. I'm a big dummy and forgot to take some loaded torsion bar measurements.

I'm in the process of prepping for an LS swap. This last weekend I dropped my NA trans, pulled the 5th gear, and put it into my 9U turbo trans. I'm waiting on a paper gasket to close up and install the new trans.

While there is plenty of room with the trans out I'd like to install my new 29mm torsion bars. The problem is that I didn't take the measurements needed before putting it up on jack stands.

I did, however, measure my ride height front and rear. With the 250lb front springs & spacers, the stock torsion bars, and 17s ride height is spot on where I want it. On flat ground, a bubble level on the door sill is dead center. It's a tad high in the front but the few extra pounds from the LS1 will take care of that.

Anyway, as I see it I have two options.

Option 1:
Put car back on ground and add weights to rear to simulate trans being installed and take my measurements. (Prefer not to go this route)

Option 2:
Since ride height doesn't need adjusted but the spring rate is increasing by a known amount, can I just use the spreadsheet calculator that is floating around the internet for our cars? It has default values for a 23.5mm bar setup and that's what I have. I can't see any reason those values would differ all that much from mine. From there I could just adjust my "H" measurement (wheel hub to banana bar) by the amount the calculator specifies to keep my current ride height with the larger 29mm bar. It gave me a value of -1.71in.





Any input would be appreciated!

harveyf

No one else is responding so I'll jump in. First, the math on this is pretty simple but you are missing a few key measurements. That said, I think you're being optimistic to think you can nail it right out of the box. I find it pretty straightforward to pick a setting, any setting really, with the weight off the rear and engage the torsion bar splines just a fraction. Like a 1/4" or so. Carefully lower the car and see where you are at. Now you can come back off it, disengage the torsion bar, and move the hub in the desired direction. Use a bottle jack to hold it in position. At this point, you'll have to rotate the torsion bar until you find the sweet spot where it will just slide in. Again, just enough to engage. Lower the car. Measure. Repeat as necessary. The trick is to not drive the torsion bar home until you're sure you've got it, as they are a beast to withdraw once fully inserted. Of course, do the above on both sides at the same time, so as to keep things nice and balanced.

If you are still changing the weight in the front, I wouldn't fully commit on the rear until you've finished in the front. That just means leaving the torsion bar partially engaged until the very end. You wouldn't want to drive it this way but it fine just sitting static in the shop.

I have welded a big washer to the end of my torsion bars, to give me a way to pull them when fully engaged.

KevinGross

I went through this process many years ago when installing stiffer torsion bars in my own car. If I recall, I did fairly simple math to account for the old and new wheel rates, given that I was also installing the 968 M030 rear shocks with mild springs (100 lb/in at the spring). I assume this is what your spreadsheet does, and for me it worked out well in terms of figuring out the required difference between the original unloaded spring plate angle and the new unloaded spring plate angle. You'll still want to corner balance the car afterward, of course.

mikehayes

I used this exact sheet when I upgraded my torsion bars earlier this year. It's actually pretty good. I calculated the angle, then cut out a triangle with that angle and taped it to my banana arm, and rotated my trailing arm from being parallel with the bottom line of the triangle to being parallel with the top. I installed everything, torqued it, lowered the car, and realized that I nailed the ride height, but one side of the car was about .5 inches higher than the other. No helping it; it's a guess and check process no matter how much math you do. It's nearly impossible to know they're perfectly matched side to side.