Rear Torsion Bar Reindexing Question

moneymanager · 10-02-2009, 10:18 AM

Car is a 74. I'm swapping out torsion bars for bigger ones. Will use Wil Ferch's calculator to determine desired new trailing arm angle.
If I set the new angle, can I mount a wheel and check the ride height with the tire on the ground WITHOUT reconnecting the shock? Sure might make it easier. Thanks,

aston@ultrasw.c · aston@ultrasw.c's Garage

In my experience no.

In fact you might need go for a drive to get everything to settle.

btw the calculator works great!!!

janz · 10-02-2009, 10:40 AM

If you have gas shocks it will make a difference. When I did mine the calculator did not help very much. Even after I thought I had finally got it right the car settled a bit more after sitting for a few days. Took me about a dozen tries (PITA) but finally got it perfect for what I was looking for. You might get lucky like others I've read about here and hit it right but I had to spend a bunch of time and work to get mine perfect. Hope you consider changing the bushings on the spring plate as the old ones take a shape that makes it hard to work around, I went with the neatrix rubber and really made it a bit easier to get to my final height. You should roll the car a bit or better yet take it for a drive after each adjustment, once you get the rears right then you need to do a corner balance using the front adjustments. Also start off with the front adjusted evenly so when you do the corner balance it wont throw your rear ride height off to much. This is one of the worst jobs I had done on the car and now I know why they charge so much to get it right, very very time consuming to get it right. But well worth it for ride, braking and cornering performance, what a difference it made. Good luck and have patience.

moneymanager · 10-02-2009, 10:43 AM

Janz...read your thread about the constant slippage. Sounds like a nightmare. But you finally did get there? And it was replacing the torsion bars that did it?
Aston, did you get the setting right on the first try?
Thanks,

janz · 10-02-2009, 11:03 AM

MM I think it was a combination of the old bushings and the old torsion bars that kept wanting to go back to their worn in position. Took a while to get there but its done and corner balance is only 40 lbs off one way with car empty and 40 lbs off the other way with me sitting in it. But ride heights are perfect for what I was looking for and wow does it corner and brake nice now and the ride is pretty good for a 911. Folks that go for a ride with me are amazed how well it handles and how well it rides. Really enjoying the ride now that the hard parts are over with. 142 with hands off wheel = perfect straight line. Did I mention that each time I changed it a new alignment was needed. The shop I used actually felt sorry for me and only charged me $85. for the last alignment and they did a great job and even mentioned how there was full adjustment now instead of "its almost to the end and there is no adjustment left". Loving my 911 now.

aston@ultrasw.c · aston@ultrasw.c's Garage

one side yes, the other side no, my mistake.

taking notes as you work helps as well

also if you are serious about driving i.e. track, you will want to "corner balance" and forget fine tuning ride height.

moneymanager · 10-02-2009, 11:25 AM

Janz, glad you got squared away. I've done this job but last time I was only trying to do the neatrix bearing replacement, so I was only trying to get back to the stock ride height...which I'm coming to find is easier than changing the angle to fit a new torsion bar.
Aston...that's good news. Maybe I'll luck out and get close on the first try. This is mostly a track car so has been and will be re-cornerbalanced.

boyt911sc · boyt911sc's Garage

jht,

Does your '74 have an adjustable springplates? If it does, it will save you a lot of work for +/- adjustment of about 1/2 inch. The calculator by WF is very helpful to get close to the desired height. And at times, a second adjustment might be needed.

Tony

moneymanager · 10-02-2009, 07:49 PM

Thanks Tony, I'm hoping for a one shot solution but resigned to maybe doing it twice. We'll see! Unfortunately I don't have adjustable spring plates. I keep telling myself this is the last time I'm going to need to do this.

Gordo2 · Gordo2's Garage

For what it's worth,

I found Will's calculator to be great. My first shot using the #'s ended up a little lower than what I wanted, the second adjustment was much faster/easier since I knew what was doing. I have adjustable spring plates - the shop that did my corner balance was able to balance out without having to re-index, and actually told me the car was pretty close when I brought it in.

Good luck, Gordo

911st · 10-02-2009, 08:59 PM

I suspect the calculator is geared for stock rear camber.

I set my rear at -2.5 deg of camber and came in about .5" low to.

Each deg of neg camber is soothing like 6-8mm difference in height.

A solid reason to set alignment before corner balancing if you do not have a race shop that can to them together. My shop could not.

Thus, ride height first, alignment second, corner balance last.

I believe if you are going to set is at a race setting add one deg (apx .3" higher at the fender) to his number to get closer to the correct height.

Otherwise, if you have adjustable spring plates, set the at bottom third of there range so you can bring it back up if needed.

Still, it is a great tool.

boyt911sc · boyt911sc's Garage

Quote:

Originally Posted by moneymanager

Thanks Tony, I'm hoping for a one shot solution but resigned to maybe doing it twice. We'll see! Unfortunately I don't have adjustable spring plates. I keep telling myself this is the last time I'm going to need to do this.

MM,

When I first started tinkering with bigger TB's from 19/24 mm (front/rear) SC stock, first upgrage was 19/26 mm. It did not last long. Next was 21/27, a few months later 21/28. As of today I have 22/29 on the car and my son wants to try the 23/30 TB's we bought last spring at Hershey. So you could see that I got some practice replacing and re-indexing TB's. These were solid TB's and haven't tried yet the 'hollow ones' which I have no intention (???). They are too expensive!!! Wait, let me check with Bret F. Just kidding. Have fun doing the project.

Tony

moneymanager · 10-03-2009, 06:05 AM

Tony, you ARE on the slippery slope. Since I drive this thing on the street some of the time, I'm
hoping that this is my last stop! I began with 19/23 and am hoping I'll be happy enough with 21/26...Don't need to replace all my crowns and fillings right now. BTW, do you reconnect the shock each time you lower the car to the ground to check ride height?
911ST, thanks for the tip. I do have or will have again a couple of degrees of negative camber. So if I proceed per Wil's chart and he suggests 22-23°, I should probably use 23-24°?? If I've got it right, this may save an hour of pleasurable labor!

911st · 10-03-2009, 08:55 AM

It should help based on my experience. However, assumptions are built into his model. One being the front to rear weight distribution. If your car dose not approach the norm, or say my 85 has a higher percentage of weight on the rear, this could effect our relative out comes.

I would add adjustable spring plates if you are thinking of corner balancing. Or at a minimum, adjustable sway-bar links. At least one has to be adjustable or if you are trying to get your corner balance right there is a chance that you will end up with some preload when you reconnect the sway bars.

Another note. Rubber bushings are part of the spring rate. However, they zero out at rest after some time. Thus, one could put a lot of work into setting up a car with rubbers and find later it has drooped in height.

Thus, it is still a craP shoot.

If not going to corner balance I like the tripod method that Chuck at Elephant racing came up with. Suspend the car from the jack point on one side with no wheels on that side and measure the side on the ground to the torsion bar ends.

Wil Ferch · 07:57 PM

Just saw this.....

If you have the Pocket size tech spec books that were published over the years from the 70's through the 80's .....you will see that the spring plate original angle might be X for a given car...but that the angle is 1 degree shallower if the factory car was delivered with Bilstein gas pressure shocks....yes they will otherwise yield a ride height anywhere from 3/8" to 3/4" higher compared to hydraulic-only shocks that don't tend to "lift".

You also have to remember these cars are not riding on needle bearings or Chuck Moreland's Polybronze bushings. Over the years the rubber bushes take a set and/or are oblong to a degree.... and some of the cars show some variation a bit beyond what the calculator predicts. But in most cases with cars that have tight rubber bushes in good condition...I'm told the calculator works fairly well and dispenses with the "I had to do it 5 times" syndrome we were forced to endure before.

moneymanager · 10-08-2009, 01:15 AM

Amen. Bottom line, it helped me a lot. I got much closer on the first try than I ever would have just taking a wild guess. One side was right on, the other needed a second iteration, but only one.

JeremyD · JeremyD's Garage

I just did this - the I phone app makes it easy - and I was within a 1/2 inch of where I wanted to be. Easy to adjust out with the adjustable plates

Spring Plates Torsion Bars and I Phone