The Stovebolt Site Forums The Shop Area Engine Shop Gearvendors overdrive

I just bought a Gearvendors overdrive unit for my '51 3/4T with the 4.57 ratio 4 speed transmission. I'm wondering if I should remove the center bearing between the two drive shafts and have a longer drive shaft made from the Gearvendors unit to the rear axle, or keep the center bearing and have a really short front shaft.

You would be better going with the single shaft. The forward section of the 2 peice shaft on that year is an enclosed type, and won't be compatible with the GV unit.

The real answer is can one long shaft withstand the speed you will be turning it?

http://www.stovebolt.com/techtips/driveline_setup.html
This Tech Tip has links to Spicer Driveshaft manuals, and they can help you figure the critical speed for the long shaft you are considering, and then you will see if a long shaft will work, or if you need a two piece shaft still.
The manuals will also help you double check the angles to be sure the driveshafts are installed and setup right for minimal vibration and long life.

A driveshaft can only turn so fast before it is no longer stiff enough to resist turning into a banana and leaving the truck.
Note: you want the driveshaft to stay in the truck
Contributing factors to critical speed other than speed and length, are material, wall thickness, and diameter.

For your figuring, you most likely have Spicer 1350 series U-joints in your original shaft.

I think (a rough guess) you should keep a two piece driveshaft, even if the first one is only about a foot long. If you want to use factory parts use 1 ton driveshaft and carrier bearing, having them shortened as needed.
New shafts will probably be easier if you don't already have what you need, and you can adapt a new modern carrier bearing.

Grigg

Another benefit to having a carrier bearing is it reduces the load on the tail bearing in the new and quite expensive Gear Vendors unit.
With a short shaft on the back of the GV unit it only has to support half of that short shaft, and if it is out of balance at least it is a short one...

Grigg

A rule of thumb for driveshafts is 55" ctr/ctr of joint for a gas engine with a 3" tube and .083 wall which nearly all are. Diesels can go to 72" with a 4" tube and 2500 RPM. Set up the OD unit so that the driveshfts going in and coming out of it are the same, approx 2-3 degrees but measure the original angle first so that you maintain it.

How long is the Gear Vendors unit? If it is a foot or longer in length the resulting driveshaft would be shorter than the driveshaft used in a 1/2-ton '55 1st with open drive.

I'm not sure of the length of the Gear Vendors OD as installed behind an SM420, but given the comparison above I bet you would be fine with a single piece drive shaft as long as it was of similar diameter to that used in the 1/2-ton.

That's what I was thinking, considering that there's only a shade over 9" difference between 1/2 ton and 3/4 ton in wheelbase. Either way, the original TT forward part won't work with the GV Overdrive.

Reasonable logic..
But don't forget that the shaft will be spinning about 25% faster now that he has OD, and this could be above the critical speed for that size and length driveshaft.
Do the figuring as explained in the manuals in the driveshaft Tech Tip, and or call a driveshaft shop and ask if they can make a shaft to do what you want, and survive.

Grigg

Grigg knows about driveshafts. ;}