Quote:
Originally Posted by 6 Grrrs
I don't remember the length, but it is a one piece driveshaft. The original was one piece as well. Shortbed regular cabs typically do not have a carrier bearing/2 pc driveshaft (at least in the '88-98 2wds).
Short story...I was about 300-400 miles into the 2007 Power Tour (Kalamazoo MI), when I decided I couldn't handle the drivetrain vibrations anymore. Monday Morning at 7am I was at the local driveshaft shop to have them check my driveshaft for balance. I had just installed 4.56 gears (from 3.73s) in the 14bolt with a new Eaton diff, new bearings, etc. I finished it the day before we left and it felt fine the little bit I drove it around town, but I never got up to highway speeds. I also had pulled the driveshaft and had it shortened/rebalanced (only about 1/2" shorter) because ever since I did the T56 swap, the driveshaft was a little long and was always a pain in the a$$ to get out (pickups have the fuel tank inside the framerail and my 3" dual exhaust is on the other side - needless to say there really isn't much room to remove the driveshaft). Whne the straps were unbolted, you had to rotate the yoke & drivewshaft to find the 'sweet spot' to get it out.
So the guy who built the shaft in 1999 or 2000 shortened it and 'rebalanced' it. I installed it and was on my way. So back to Kalamazoo...when I pulled the driveshaft, they set it up in the lathe. It had .040" runout!!! It also had no balance weights on it (which I never noticed), AND the front yoke had spread approx 1/8", so the front ujoint was moving all over the place. Basically the shop in Kalamazoo would not repair it. I spent about $350 for a new custom built shaft with about .003" runout, and balanced to within the desired amount (I dont remember exactly). My old shaft was 4" in diameter, this new one was 3" (Cromoly I think). When I questoined the guy building the shaft (owner of the shop) about the smaller diameter, he told me it is the same basic shaft he builds for 1000 hp drag cars, and he would stand behind it for the life of the truck.
Well we were on our way and the vibration had all but stopped for the most part. I have since made some changes to my rear springs, and I believe I affected my pinion angle. The truck launches great (1.78 60 ft on ET Street Radialls - used to be 2.20s, with a best of 1.97). I have Cal Tracs on it too. With the CTs there is some pinion adjustment, but very little, and I believe only down.
My truck is lowered 5" in the front & 7" in the rear, which is pretty low for a C1500. There is a pretty severe vibration around 70mph, and get worse as I go faster. I would imagine the driveshaft is spinning pretty fast with the 27.2" tire and the 4.56's. With the 3.73's I never had any vibration issues.
I guess it wasn't the short story I thought it was  |
Tod,
Everything you describe point to our common problem. When you installed the 4.56 gears the speed of the driveshaft increased, given the same vehicle speed. So before the gear swap, the vibration might have started at a higher speed, above the normal highway cruising speed. Doing some calculations, with the 3.73 rear end the driveshaft was spinning 3450 RPM, given a 27.2 inch tire, at 75mph. With the 4.56 gear the driveshaft was now spinning 4200 RPM, all other things being equal. With the 3.73 gears, 4200 driveshaft RPM was 95 mph, so you never felt the vibration. On top of that, you were running a very heavy 4" steel driveshaft.
So when the guy made the new shaft out of 3" cromoly, it was significantly lighter. Frank from the DSS believes that driveshaft weight is critical for smooth operation. He has come to this conclusion based on his high speed testing with the NASCAR teams that run a 4" driveshaft. Despite the fact that they use a .063" wall thickness tube, they are still quite heavy, and can become unstable at the very high speeds that they experience.
CF has a very high stiffness to weight ratio, which makes it idea for driveshafts.
Andrew