Transmission 700-r4 - Drive shaft

[Phil's 1970]

I am replacing the turbo 350 with a 700-r4 in my 1970 c/10 longbed truck. I know the drive shaft will need to be cut 3-inches. (Question) Can I have a one piece drive shaft made for it or do I just need to have the original one cut. The truck has been lowered about 2 1/2 inches. Thanks

[Longhorn Man]

I would just cut the front one, a 2 piece is a better design

[raycow]

I must respectfully disagree with Longhorn Man. The one-piece shaft is less trouble overall.

Apart from the center bearing itself, the main issue is U-joint angles, which must be carefully matched to avoid vibration. There is no problem as long as you keep the truck completely stock, as the angles are correct from the factory. However, if you do a transmission swap or change the ride height, the U-joint angles change too. This is easy to fix on a one-piece because you have only two U-joints, but with three joints you will be pulling your hair out.

If you need convincing, log on to any 58-64 Chevy passenger car board. These cars have two-piece shafts and their vibration problems are legendary when they are modified. Btw, I also have a 60 Belair, so believe me when I say I have been there.

Ray

[cdowns]

try findinding a driveshaft shop tthat is foolish enough to make a one piece shaft for a longbed truck // GOOD LUCK

[raycow]

You can run a one-piece shaft at least up to 6 feet long - and I don't need to have a driveshaft shop make one for me because I can find them in salvage yards - which means that an OEM was wrong enough to make a 6 foot shaft.

Ray

[Longhorn Man]

i never said trouble free, I said better.
I also disagree with the one piece being better, and the comparison to an impalla is apples to oranges.
a shaft that is over 6 feet long needs to be made out of thin walled material, and they need to increase the diameter of the tube for the shaft. Most places (and OEMs) used alluminum for this.
If you look under a 67 - 72 truck, there's not a lot of room for a huge shaft through the hole where the carrier bearing goes. On top of that, you have to remember that the rear axle will go up and down, AND, it will move a bit left and right due to the panhard bar set up. if it had upper control arms, then the rear would not move left and right. So you need room all around the shaft for this kind of movement. In stock form, an unloaded truck doesn't squat much unloaded, but as soon as you put 500 pounds in the bed, it will start to squat and more so when in motion. Also, again, with that squat is some left and right movement. You can see the shaft will be rubbing on the crossmember.
Now, imagine the shaft being bolted to that crosmember with a carrier bearing. That shaft is going nowhere. The up and down and side to side movement is all done on the U joint in the middle of the driveshaft.
As long as no C notch has been done to the frame (up to a 4 or 5 inch drop) then the ppl at GM built the driveshaft and rear axle built the truck to be driven at those angles and height. GM tested the trucks severly overloaded. 2500 pounds or more in the bed of a 1/2 ton truck, becouse they know people are stupid enough to do that. At these loads, the rear suspention is fully compressed, and the axle as almost on the frame.
Now lets talk about a mojor drop on a truck... like air bags with a C notch cut on the frame. 7 inches or so of a drop. If you deflate the rear bags with a one piece shaft, the one piece shaft would now be angled up from the rear of the trans to the diff, and the shaft would get bent on the crossmemner, and possibly even hit the floor of the cab if you were to remove the crossmember.
Now, put a carrier bearing there, and the front shaft stays in the same place, and the middle U joint makes all the angle.
Look at the U joint/driveline angles on a jacked up 4X4, U joints can take some seriouse angles... more than you'll need in the examples I've given, which cover most 2 wheel drive aplications for the majority of the trucks we are talking about here in 67 - 72 world (2 wheel drive)
Now, lets talk about another reason why a 2 piece is better, angles. If you need to have an extreme angle on the drive line... a single shaft may not be able to make it work. Off the top of my head I can't remember the angle a typical 'run of the mill' U joint can handle. Just for sh!ts and giggles, lets say 15 degrees. (picking that one for easy math that almost any of us can do in our head also) So lets say this driveline needs to be at a 40 degree angle. A one piece shaft would only be able to handle a 30 degree andgle, but, if you had a 2 piece shaft, then you could easily make that extra 10 degrees. You could even reduce the wear and stress on the U joints by making them 13.3 degrees each.
Now, there are in fact drawbacks to a 2 piece shaft, but it is only cost, and quality.
You do in fact want high quality pieces, not craptastic auto zone made in china garbage. On top of the added cost of quality pieces, you'll be paying added labor for the extra pieces and extra balancing. You'll also want the whole assembly installed properly. Make sure nothing is in a direct line being that U joints need some angle in them in order to live a long life.

[brad_man_72]

ujoints, even really good spicer ujoints are designed to operate at no more than 3*, any more than that and you greatly decrease ujoint life and increase odds of a vibration and a shudder on takeoff.
I've built several 3" diameter 1 piece driveshafts for longbed 67-72 chevy trucks w no issues. None that were c-notched.
I've built many diveshafts made of steel up to 4" in diameter.
there are many situations where a 2 piece driveshaft is a better solution, most longbed 67-72 chevy trucks a one piece shaft will perform better than a 2 piece shaft.

2 piece shafts get a bad reputation because most people that work on them don't understand how sensitive they are and that someting as simple as changing 1 ujoint properly can upset the ballance enough to destroy a carrier bearing. Then throw in the people who don't mark anything, get the center yoke out of time and put it back together 180* from how it was ballanced.

[Longhorn Man]

a 3 inch shaft in a LWB? Were these trucks mildly dropped, or ever used as trucks? It's been quite a while since I've been under a coil truck, but I coulda sworn that would be too large in diameter.
The other problem with trying to get someone to build a shaft like that is many of the shops that will do it, shouldn't. We have a place here in town that'll do it, but they won't stand behind the work they do (and for good reason).
Gearing and intended use, along with if the truck has overdrive or not are also important things to consider when a real long one piece shaft is being made.

[Phil's 1970]

Wow, thanks for all the information. I am still confused. I called the local driveshaft shop to see what they had to say. He stated that he has built them both ways for customers but he would not tell me which one was best, he would do it either way.
This is a stock 350 250hp. Mostly will be used for cruising but may be used to pull a trailer if needed. Thanks.

[brad_man_72]

sorry we build them out of 3.5 diameter tubing, the truck I'm most familiar with that we built a one piece for is lowered about 5" in the rear. I've seen him put a load in the back, and I've seen him go over 80mph.

true alot of people doing driveshaft work have no business pushing the limits of driveshaft design.

for those who don't understand how lenth effects the diameter tuing needed, imagine a 6ft pole 1" thick. Just the effets of gravity will cause it to bow in the middle. Now take that shaft and spin it 4000rpm it will turn into a jumprope really fast. I'm sure their is a driveshaft diameter calculator online somehere.

[Phil's 1970]

I have read alot of good information on this and also talked to some local folks. After thinking about this all weekend I have decided to keep the two piece and have it cut. Thanks so much.

[brad_man_72]

have it cut, welded, and ballanced as an assembly by a reputable driveline shop.

good luck

[Phil's 1970]

Here I go again, Drive shaft shop man called today and stated that all my u joints and yoke was shot. He stated he could build a one piece (3 1/2 inch shaft) cheaper than cutting and rebuilding the two piece. This truck has a 2 1/2 inch drop. Questions: How much of the C-notch will I have to cut out? Will I be all right just having the one piece shaft cut 3 inches shorter than the orginial two piece or does it change the length not having the center mount? Anyone know what the length would be for the one piece? I do not have the engine and transmission back in yet. Merry Christmas.

[raycow]

Assuming that the center U-joint runs at an angle, I would expect the one-piece shaft to be slightly shorter than the two-piece, but I don't know how much shorter.

Ray