Geoff Weeks

what is the rim width? I am wondering how well they would fit on the narrow (er) rim. 9.00's normally go on a 6.5 or 7" rim. As I said my rims are stamped with the max tire size My truck with disk wheels has 6.5" 6 bolt wheels, the spoke are only 6", which is why they are stamped with the 8.25 tire size.

Mine are still in their work clothes. what size rims do you have on yours? I've got 6.5 (edit 6 inch on the 8.25, 6.5 with 900's) with 8.25x 20 rubber and they are stamped max tire size 8.25. The one with 9.00 had disk wheels.

That is a valid point, they did put carrier bearings (both trucks have them) . They may have done "seat of the pants" engineering, however. If it didn't vibrate too bad it was good to go, now we know how and when the harmonics will kick in. I am itching to see how it runs out when the new shaft get here.

My guess is they didn't have a clue. The reason being that the truck I'm working on with the small tires I believe it came with (7.50x 20) would put driveshaft speeds at or near critical at around 50-55. With 8.25x 20 tires the max speed is around 60 MPH with the engine at 3000 rpm, max recommended speed. Even if you figure 45 MPH as max back in the day, it would be in dangerous territory, with the smaller rubber. My other K-7 sits on 9.00x 20's and that would put the max speed near 70, something it cannot reach. empty weight is 10,500 lbs 269 CID isn't going to move it that fast.That one I can see why it wasn't a concern 3000 rpm x O/D ratio of .823 puts the driveshaft at around 3650 rpm. Because the spec's for tire size are so far removed from driveshaft spec's I don't think they would have tied them in some way ("your can't order that wheelbase, with that tires size") I think they just were not well versed on the problems associated with shafting at that point. I could be wrong, but that is my take on it.

My Marmon with 2spd rears sets up a vibration up against the gov in low range top gear, I haven't plugged in the numbers but I suspect that puts the shaft into or near critical. It has a Eaton O/D 15 speed, I never run it there, when I was climbing a hill and nearing where it vibrated, I would upshift the rears and down the main and carry on, so shaft speed is decreased. If I get bored I'll have to measure and plug in the numbers, but I bet that is what it is, feels like it anyway. Only shows in top gear, so any engine speed in lower gears doesn't vibrate, has to be driveshaft speed related. I never ran the truck up against the gov in high range on the rear axles.

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?