Setting rear ride heights - MT 647 installation blog update

[bevans6]

So the next to last step in my sisyphean transmission install is making sure the driveshaft doesn't bind on the axle when the suspension is on the bump stops.  If it did, it would probably tear the tail off the transmission housing.  I bought new bump stops, ripped the old ones off by shearing off the 1/2" bolts that screw them onto the frame extension, cleaned everything up, and jacked the suspension up until the driveshaft was just this close to the axle, around 1/8" or a bit less.  I measured the distance between the axle platform and the bump stop mounting flange, right around 1 1/2".  Next, I made up some shims that were about 1.200" thick, screwed the new bump stops to them, and jacked the bus up with them held in place until they were fully compressed and bearing the entire weight of the bus.  They were 1.25" thick at that point (they started out at 1.8" thick, compared to the manual diagram which showed them at 2" thick).  So I needed a shim indeed.  I decided to make the shims .950" thick giving me around a half inch of leeway between the installed, fully compressed number and the driveshaft is in trouble number.  Oddly, the location on the driveshaft that comes the closest to the axle is about exactly half way between the two U-joints, so 1" of movement on the axle results in 1/2" of movement on the driveshaft at that point, which was handy once I figured that out.

So I decided to take it a step further.  The ride height in the manual was 5 9/16" flange to flange, or 3 9/16" flange to uncompressed bump stop.  I decided that to retain ride comfort and not be hitting the bump stops on bumps, I would raise the rear ride height somewhat.  First thing was to jack up the rear axle so when I aired up the bus it would rise up to ride height off the blocks, then I aired up the bus.  Using a 15 cfm shop compressor and an 80 gallon tank, it took about 15 minutes to fully air up the bus (based on when the suspension stopped taking air, the front suspension took almost twice as long as the rear suspension).   So with the bus up on it's airbags and stable, I measured the ride height distance between the two flanges - book sez 5 9/16, drivers side was 6 1/4, passenger side was 6 1/2".  I've read of biasing the rear ride height to accomodate road camber, maybe that is what some previous guy did here.  Also looked at the adjustment range on the ride height valves - they were adjusted to their max height (without bending stuff).  Seems that someone had already raised the rear ride height by 3/4 to 1", which is what I had planned to do anyway, so I left it.  I'm going to let it set overnight and see what changes, but for now I think that all I have to do is actually install my new bump stops with their custom machined shims, which means drilling out and re-tapping the 1/2" NC threaded holes, and drilling and tapping three 1/4" holes on each side to hold the shims in place, and I should finally be down to the final tasks.

Final tasks are to install the rest of the bolts that hold the driveshaft flange together and torque them to spec, torque the U-joint strap bolts on the transmission end U-joint to 125 ft lbs (going to be a PITA since I haven't been able to figure a way to get a torque wrench on them), and grease the driveshaft.  In order to even have a hope of doing these things the driveshaft has to be able to turn, which means I need to be able to release the parking brake, which is the other reason I aired up the bus.  With that done I finally get to start the engine (finding out if my flex plate/ring gear/starter motor modifications and engineering actually worked), run the engine long enough to start finalizing the fluid levels in the transmission/engine/cooling system, and look for leaks.  That step passed, I will see if the transmission turns the rear axle forward and backwards, put the wheels back on and go for a drive.  I figure three more days, tops...   

Brian

[chessie4905]

On your spacers, you did allow for deteoriation of the bump stop rubber over time? They can squash down to 1/2 inch or less.

[bevans6]

I did, that is what the extra 1/2" of shim is for.  I need 1.5" for safety, which includes about 1/4" of "I ain't going there" before the driveshaft actually touches the axle, I have 1.25" in the compressed rubber bump stop, I added .95" for a total of 2.2" fully compressed including shim, which actually gives me .7" of "extra" shim.  I think that makes sense and I added it up right.  I think checking the bump rubbers for deterioration is going to be part of the annual check up, mind you, and I have a couple of spares...

Brian

[chessie4905]

You could also consider polyurethane bump stops. They hold their shape and condition much longer. Will probably require some searching to find a comparable replacement. Also a smaller diameter thicker driveshaft tube might give more room also if needed down the road.

[Tom Y]

Brian, I put lock tite on the flange bolts. After driving my bus from Az to Pa when I bought it and finding them all lose. Lucky I made it home.  Tom

[bevans6]

I hear you. I use AN spec or SAE gr 8 self locking nuts and bolts on the flange, I really don't want them to come loose either!

Brian