Rear Torsion Bar Measurement and Adjustment

by George Lyle

I don't recommend you try to correct sag by cranking in a "compensating" amount of twist [of the spring plate/torsion bar]. The relationship between arm position and body position isn't linear. I tried it and ended up too high!

This is the method I use for adjustment. I use a dime-store protractor with a piece of thread through its center hole and a nut to hold the thread vertical. It reads to 1/2 degree, which is less than the tolerance for the arm position.

I get the floor of the bus level front-to-rear (saves calculations) and then use the protractor to measure the angle of the plate. I hold the flat side of the protractor against the bottom of the spring plate and push the spring plate up a bit to take the play out of the splines. The difference between the protractor reading and 90 is the spring plate angle. Note that the angle is different for different models of bus, so be sure to use the right specs. I get mine from Bentley. If you turn the inside spline one tooth and the outside spline one tooth in the other direction you will get a difference of 2/3 degree, which is fine enough to get the arm into spec.

Don't be tempted to crank in a little extra twist. Handling, CV life, and tire life will suffer.

Be aware that if your loaf is properly adjusted, you _won't_ be able to see the horizon through the back window. This kind of surprised me, but that's the way it is!

If you work carefully and use the correct numbers you have a good chance of doing it only once!

Remember to douse all the fasteners in Liquid Wrench the day before you do this!

There are 44 splines on the inner end and moving one spline changes the plate angle 8.18 degrees.

There are 40 splines on the outer end and moving one spline changes the plate angle 9.00 degrees.

Neither of these is the "fine tuning". The two used _together_ is the way that you adjust the angle. If you turn one end one spline and the other end one spline in the opposite direction you will get an angle change of 0.82 degrees, which is the tolerance allowed for the spring plate angle. Thus, there is always one setting that will get the torsion arm into the correct angular range. You measure the current angle of the spring plate, find the difference between that angle and the desired angle, divide the difference by 0.83, round to the nearest integer, turn the torsion bar in the inner spline by that number of notches _opposite_ the desired direction, and move the spring plate on the outer spline by that number of notches _in_ the desired direction. Check the measurement and you're done!

If you try to adjust the angle by just turning the outer spline, you will end up with the rear end way too high, like against the upper stop. The bus will handle very strangely, will have horrible rear tire wear, will ride very roughly as the suspension tops out, and will damage the CVs and boots.

A simple $0.50 protractor works great. It's simple, accurate to 0.5 degree, and you can get one just about anywhere. You can set the bars right the first time.

A list member suggests:

> There are four bolts that hold 
> the torsion housing covers on. old-timer clued me in on this. 
> On the actual resetting of the splines on the outer end of the torsion 
> bar with the splines on the inside of the spring plate, you can line up 
> the bottom of the springplate (is a straight edge) to intersect the 
> bottom front bolt hole for the spring plate covers.

This procedure works great, if you drive a bug! It's a dune-buggy folk trick for setting the springs a bit stiffer on a Baja bug. Unfortunately, buses have different settings than the bug, and different models of buses (deluxe, campmobile, etc.) have different spring plate setting angles.

I've had no problem with doing it by following the procedure in the Bentley manual. The only deviation I make is to level the bus front-to-rear before I begin, rather than adding and subracting the deck angle as Bentley recommends.

If it's so easy to do it right, why do it any other way?

The easiest way to check your bus is to park it on a level surface, open the sliding door, put a carpenter's level on the door sill, put a driver in the driver's seat, and read the level. If it is close to level your suspension is fine. (I'm assuming that you have equal-sized tires all around, of course!) Buses normally sit a bit nose-high without a driver in the driver's seat.

> I'd check my 73 for you, but I imagine my rear end would be comparable
> to yours (ie. sagging a BUNCH).  Do you get tired of not being able to
> see cars behind you in your rear-view mirror?  Sounds like its time to

Be very careful using the rear-view as an indication. In a properly-adjusted bus you _cannot_ see the horizon in the inside rear-view mirrors. The mirror is positioned higher than the upper edge of the rear window. I assumed that my bus was sacked out because I couldn't see the horizon and wasted quite a bit of time. If you adjust the bars too high you will end up with strange handling, rapid tire wear, and CV joint/boot damage.

I'd strongly suggest that you stick with the standard spring plate settings. There is always the temptation to "crank in a bit more", but, in my experience, it never works well. Be sure to hold the spring plate up while measuring to take slack out of the splines.

In order to correctly set the torsion bars, you _must_ adjust both ends of the bar. The tolerance is only 50 minutes of arc (5/6 degree), and the only way to have the plate fall within this tolerance is to use the differing number of splines make the fine adjustment. If you use only the outer splines, you get an adjustment of 8.2 degrees per spline!

This job isn't hard to do, but it does take some thought and planning in order to have it come out right.

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