View Full Version : Drive Shaft Conversion

05-24-2019, 02:34 PM
OK, now I need some advice.

The ’48 has the weirdest drive shaft arrangement WO ever put together. It is a two piece shaft with a center carrier bearing. The u-joints, as I have griped about a lot, are ball-and-trunnion joints; basically hogged out of premium grade non-obtainium in today’s market. (See the attached picture).

To keep grit and crud out of those precious joints, Willys used a flex boot. When we were putting things back together, we found rubber boots for the u-joints, but they were Jeepster boots. I’ve gone through two sets of them – so far – and I’m ready for a third.

So, I went over to Fort Wayne Clutch and Driveline today to talk about boots. Steve and Chris poked around under the truck for a bit and declared that finding a boot to fit the u-joints was going to be a real fishing expedition. Then came the “Oh By The Way”; there is enough play in the joints that they really should be replaced.

At the least, I would have close to $250 in just replacing the boots – and who knows how much more to find replacement joints. Plan B was a bit simpler – make a new one piece drive shaft with a slip spline and flange and use modern joints that can be serviced and don’t need boots; guaranteed not to rip, rust, bust, turn to dust or smell bad! Everything new for about $350. It would be a 62” shaft, 3 ˝ inch steel tube.

That would also get rid of the carrier bearing that transmits all the noise and vibration up into the frame and cab. Now, this sounds almost too good to be true – I’m suspicious.

What questions do I need to ask about this?

05-24-2019, 03:42 PM
Keep the original for a conversation piece. Build a new one piece for your jaunts to town.

05-24-2019, 04:33 PM
LarrBeard, if I had a dollar for all the carrier bearing style driveshafts I received tech calls for, to put together a one piece. Just a few things have to be considered.

1) The critical speed of the driveshaft as a one piece.

In your case a well balanced 62" Center of U-Joint to Center of U-Joint at installed position, 3 1/2" diameter .095 wall one piece slip between center driveshaft, has a critical speed calculated at 3400 rpm. Not enough. Critical speed is a bending moment. Like a jump rope whipping out in the middle. At approach to critical speed the shaft can and will vibrate, hum, shake and spontaneously dissemble (I could tell a NASCAR tale here). Increasing the diameter to a 4 inch tube bumps you up to 3900. A 5 inch Aluminum tube moves you to near 5000 rpm. That's perfect. You never want to run near critical speed. It beats the hell out of everything. If the driveshaft fractures, it will happen in the middle. The flailing parts are still attached to things that are spinning. It is not good. I have seen semi truck frame rails cut in half. I have seen 5" Aluminum shafts for about $450. Money well spent for what you are trying to accomplish. http://www.driveshaftspecialist.com/Aluminum%20html/TruckAluminum.html

2) The drive line angles must be equal to each other.

This is not to bad usually. Basically the center-line of the output shaft MUST be as close to parallel to the center-line of the pinon as you can get them. This is accomplished with wedges in the spring pads and moving the cross-member/motor mount up and down. The tough part is the shaft needs to be as close as possible to in-line in the "PLAN" view. Or as if you are looking straight down from above. My guess is the output shaft and the pinion are not in line.

Since you have a trailer, can you wait till Toledo? I can crawl under it and we can check how "tolerable" it will be. What I believe are some of the most brilliant Drive line Engineers are here in Toledo. I am positive we can figure out a good conversion if you want to go that route.

The stock 3 piece shaft solves a lot of issues. By adding a third joint the angles are less sensitive so to speak (there is more division). You now have two short shafts and that eliminates the critical speed issues. I don't think they transmit noise, unless the carrier bearing itself is bad. There should be some isolation between the bearing and the frame. A piece of rubber usually or the whole bearing is mounted in rubber. If not, an isolation device is a good idea.

I like the large diameter one piece shafts. My 77 Chevy Silverado is converted from a two piece. My 2016 GMC came from the factory with a 5" Aluminum one piece.

I don't know Larry... I am torn on yours. A big one piece is how it would have to be done. They look good but, very modern in a "Olden Day Coach". How much to fix up the old one?

One other thing. There is nothing wrong with going to a modern Cardan style 2 piece shaft assembly. Just a custom mount for the center bearing is all you would need.