Project Vehicross Part 8: Pinion Angle 101
It’s been awhile since we’ve done anything with our funky Isuzu project. There’s a simple reason for that: I was planning to replace it with a Mazdaspeed 3 (in 2016) and have a fun, practical, and incognito hatchback to replace the fun, tough, and weird Isuzu. Due to a series of unfortunate events (some relating to a lazy previous owner, and some relating to unfortunate shortcomings in Mazda’s quality), we decided to offload our 3 and keep the Isuzu. That meant dealing with some of the issues we’d been ignoring for months while we fussed with the Mazda.
The biggest problem we’ve been ignoring is our driveshaft. See, back when we lifted our VX with an OME Lift Kit we mentioned that our driveshaft started making odd noises. After doing some poking around, both under the truck, and through the brains of some truck people, we realized the problem was due to our driveline angle. In a rear or four wheel drive application, the driveshaft must cover some vertical distance to meet with the pinion gear in the axle. When Isuzu designed our suspension, they set the driveshaft at a certain length and a certain angle to prevent undesirable harmonics. These harmonics will cause unwanted noise and, if severe enough, can even cause failures in the driveshaft, rear end, or the transmission or transfer case.
When a live axle rises and falls throughout its motion range, it does not do so linearly- it actually moves in an arc. At factory ride height, the driveline angles cancel each other out, but when you lift the suspension, the pinion angle increases and the difference causes a vibration, which one can hear inside the passenger compartment. Determining which angle is the issue is fairly straightforward. If the vibrations come under acceleration, the transfer case/transmission angle is too steep. If the noises come under deceleration, the pinion angle is too steep. Our noises always came under coasting situations (side note, if you hear noise under both conditions, but only at certain speeds, you have a driveshaft imbalance).
It should be noted that the front end of the VehiCross does not have these problems. Not only does the front driveshaft use CVs, but because the front end is independently sprung, the front diff does not move when we lift the front. The front axles are put under more strain as they operate at more of an angle, but since they are not causing us problems, we are only working on the rear end.
Fixing the mismatched driveshaft angles can be done with some adjustable suspension links. Independent4X supplies off-road parts for a number of Japanese imports and has a very nice Isuzu section. I4X is one of the few places you can get aftermarket Isuzu parts to build your Trooper, VehiCross, or Amigo into a Jeep shaming rig. They make their own heavy duty turnbuckle style lower links for just such an emergency, and they also offer options of lower links, upper link, or all three in a kit. We decided to start with the lower links only and see if that did the job.