-Tom Wood's Driveshafts-

We try out his 1350 constant velocity slip-yoke shaft

It's almost impossible to get away from any single modification without others being required for the first to work. Here's a good example.

The blue project suburban got treated to a set of 4.88s to raise rpms for better all around performance. While we got exactly what we wanted in terms of engine performance, with the faster turning driveshaft came new vibrations. Even since the truck was initially lifted, vibrations were present, but not obnoxious. Years have passed, and various trail damage has made for a less-than-appealing highway experience. New gears only compounded the problem.

So we turned out sights to the only real solution - a new shaft. Local shops have produced excellent products for us in the past, and though we would have no problems returning with future business, we wanted to try something new. Having run a dual u-joint shaft originally, the idea of a constant velocity (CV) shaft has always been interesting. The promise is lower vibrations, longer joint life, and the inherent bragging rights that come with such a slick piece of gear.

Tom Wood has been making custom driveshafts for some years now, and is well established in the four-wheel-drive market. He sponsors a number of Calrocs and NARCCA competitors, and many of his shaft have withstood the ultimate tests of off-road abuse.

What interested us was the CV shaft he offered that allowed us to retain the slip-yoke on our stock NP208 transfer case. It comes in standard form with 1/2 ton (1310) u-joints and a 3" diameter shaft. While that may be plenty for blazers and trucks that don't tow heavy loads, there are some of us out there that simply require [read: want] more. After talking with Tom, we settled on 1 ton (1350) joints and a 3.5" diameter tube. We measured for the length that would be required, and ordered one.

Upon installation, we found two problems. First, the shaft didn't seem to have enough compression, and we now had more vibrations than we did with the old shaft. We called Tom again, and he gave us a list of things to go over to make sure that this shaft would work properly.

• First we double checked our measurement for compression. We were supposed to have one full inch of compression, and we had exactly half that.

• Next, we measure our exact tire size unloaded at normal pressure, 40 psi in our case. Our worn 35" BFGs came out to just shy of 34". Tom was concerned that with too small a tire, and a numerically high ring and pinion that the shaft was turning too fast, and approaching critical speed.

• Finally, we purchased a protractor, and measured the pinion & driveshaft angles. For a CV shaft, these two angles should be within two or three degrees of each other. Our pinion was at 2.5°, and the driveshaft at 11° - nice.

Compare the measurements between the driveshaft angle, and the pinion angle.	A big dial is handy for when it's getting late, and the eyes are getting blurry.

We took this information back to Tom, and another phone conversation yielded a new plan. Tom was commited to building us a shaft that worked right, and we were right there with him, so: 1) we would return the shaft for upgrades, 2) it would be shortened the appropriate length, 3) the tube diameter would be upgraded again from 3.5" to 4", 4) he would send some shims to point the pinion upward.

6° steel shims were included in the kit, along with a pair of center pins.	New u-joint straps are inexpensive, and ensure that we have the proper clamping force on the u-joint caps.

We got the shaft back, well packaged as usual, and got right to installing the new shims, but not before checking the compression of the shaft as installed. It was perfect. The procedure for installing the shims is about the same as installing lift springs. Carefully support the receiver hitch, unload the suspension and seperate the springs from the axle. The shims actually bolt into place with a new center pin, so be sure you have the spring pack clamped together before loosening your current center pin. Place the shim in place so that it points the pinion in the appropriate direction. We wanted to point the pinion upward, so the shim was placed with the skinny end forward, and the fat end rearward. Tighten the center pin, and cut off the excess with a sawzall or die grinder. Situate the axle back into place, hook up the u-bolts, and follow the tightening procedure outlined in the u-bolt literature. Now is a good time to replace any corroded u-bolts and u-bolt hardware. One of ours actually sheared on disassembly, and so a new set was ordered.

Clamp that spring pack together before loosening the center pin.	Shims installed; ignore the broken u-bolt. The truck sat a short time while new parts were ordered.	One bolt was particularly tight during removal, and after only a turn or two - POP!	This is the first time it has happened to us, but apparently it's not that uncommon.

So how does the shaft work - exactly as advertised. Bearing in mind that a lifted vehicle does sacrifice some road manners when compared to the stock condition, this is as close as we've been since the suspension and tires went on in early 2000. There is a tiny hum at about 45 mph, but that quickly dissipates as vehicle speed passes 55 mph.

Spline engagement was something that we were initially concerned about, but there is a solid 5.5" of slip yoke penetration, so we feel confident that this shaft is up to par. In fact, not only does this shaft perform up to factory expectations, when weighing the overall beef of a 4" diameter steel tube and 1 ton u-joints into the equation, this shaft is far and away the best version of a bolt-on heavy duty driveline.

That's juicy.	1 ton beef.	Spine engagement is good, and so far, vibes are almost non-existent.	One more.

The upgrade, while expensive, has great value to trucks turning lots of gear and big tires. Smaller, lighter trucks would probably be just fine with a 1310 series half ton joint setup, and would likely enjoy the great cost savings over this big guy CV. This will be the last driveshaft we buy for this truck. Lifts may change, and with that so would the tube length; u-joints will come and go, but this is CV shaft what we want.

As for the old dual u-joint driveshaft, look for a tech article in the future where we will re-tube it with something thickwall (schedule 80 maybe), and build a driveshaft dedicated to the trail.

Now, referencing the opening paragraph, we commented on how "no upgrade stands alone." Every mod requires some work somewhere else so that it will function properly. That said, one of the next tech articles will show how to calculate speedometer error, evaluate options for correction, and apply some of that to the project suburban since after the 4.88s, it now has a 27% error in the speedometer reading.