Geoff Weeks

It is why I cringe when I hear someone say they are taking an old truck and adding O/D aux and an O/D main to gain road speed. The reason they aren't spitting driveshafts left and right, is the shorter the shaft the higher the critical speed, but it can't be "assumed" that just because the added O/D shortened the shaft you are not in critical territory. As my problem seams to indicate, totally stock might even be a problem. I have the factory manuals and they spell out the size and length of the shafts the truck came with, plug those into Spicer's calculator and one can see there is a problem!

Best is I understand, it is a speed at which the shaft starts to get a standing wave set up in it. If you run at that speed for long, the shaft will start to "whip" and deform. Eventually it will fail catastrophically. Depending on speed it can fail in a very short time period.

As someone who was a mechanic. I can look at something and say "that isn't strong enough" but can't tell why and when it would fail. Not an engineer. However I was always surprised by the skinny little driveshaft on a truck that have 21500 GVW. When you add in the O/D in the main transmission, I just knew I wouldn't be re tubing the driveshaft back to stock size. Both my K-7's have the stock drivetrain. When I started researching it I found something interesting. There are many "charts" on the internet listing critical speed for drivelines, most aimed at racers. (afterall, most would think the OEM did their homework). When I found Spicer's calculator on the net, it show much more conservative values for max driveline speed. I think the difference is between "all day everyday" usage and racing. There was also something listed that I hadn't heard of "half true critical speed" which was safe to pass through, but to be avoided as cruise speed as a 1/2 harmonic can be troublesome. So I am ending up with a much larger shaft (but aprox the same length). The U joints on my shaft now were obsolete by the end of the K's run (late 40's) and although mine are still ok, it is not possible to find replacements. If a new shaft is going in, new yokes for available joints only makes sense.

The air vent for the centrifugal fan discharge is a dead give a way that it was powering a generator. I can't think of any other application that would have that.

Then not the same guy, It would take some doing to pry him from out west to NH I think. Last time (many years ago) I stopped in for a visit at his place in Idaho, he was just building a nice shop, so don't think he'd be keen on moving.

Most likely, lives in Alaska/ and Idaho or did, now mostly in Idaho I think. Used to post under Tundra on JOT

Jim, we need to start an "antisocial" media! Glad to see you are still here and posting. Still have the Marmon, but don't know for how much longer.

Looks like a generator engine. 1) painted Onan colors, 2) the air passage on the side of the "bellhousing" for exhausting cooling air for the generator.

My question is how long has it been known? I was chasing a vibration problem on my 1942 IHC. 2 piece driveshaft (jack shaft and drive shaft) and found the rear had a lot of run out. Looked way too small for the size truck (2 1/2" tube) and too long for that sized tube. BUT that is what the factory manual calls out for that wheel base. I ran the numbers through Spicer's calculator and sure enough it's operating range included running at critical speed for the shaft! Long story shortened a bit, I am getting a new shaft made that is 3 1/2" shaft, and converting to modern joints. The front jackshaft is short enough not to run into critical speed or even 1/2 true critical. My other almost identical truck with a longer wheel base has a longer shaft that flares from 2 1/2" at the joints to 3 1/2" in the middle. That is also what is called for in the manual and also runs well into critical speed according to calculations. I will have to add a 2nd jackshaft to keep the driveshaft short enough to keep away from critical speed. So, did they not know about this "back in the day"?

It is looking like we may have too. I am trying to keep my "off brand" stuff off this board. but it has been what? over two months? I am beginning to think it is DOA without hope of coming back to life.

I am going through JOT withdrawal.

It has been right around 2 months, has any progress been made?