BAD VIBES — DRIVESHAFT ANGLES RESOLVED
DRIVELINE VIBRATIONS ARE DAMN ANNOYING, AND CAN SERIOUSLY WRECK STUFF, TOO. HERE’S A WAY TO CHECK YOUR UNI JOINT ANGLES, AND ENSURE EVERYTHING IS SWEET
e love a good factory restoration, but you’re probably reading this magazine for the same reasons that we’re producing it — the love of modifying cars. Most of the cars we feature have been modified to an extent, some more than others, and the majority ended up receiving an engine and drivetrain upgrade along the way. But, as fun as it is to get carried away with choosing go-fast parts and playing the numbers game, there’s a practical aspect to driveline swaps that is often overlooked — one that can poorly affect the way that your finished build drives.
It’s to do with your driveshaft, but we’re not just talking balancing. Get the universal-joint angles wrong, and you could be running in circles trying to correct the driveline vibration that just won’t go away. Who would have thought that those science and maths lessons in school would come in handy …? Yes, we’re talking about physics and geometry, but don’t worry if you weren’t paying attention in school, because it doesn’t have to be hard. Maybe a bit confusing, but not all too difficult.
Spin something, and you’ll subject it to the principle of inertia, defined in Isaac Newton’s first law. Don’t worry, we’re not going to get all complex here — ‘inertia’ describes the resistance of an object to a change in its state of motion. Huh? The quicker an inert object moves, the greater the forces on it. Slowly rotate your driveshaft while crawling along at 20kph, and everything will be hunky-dory. Whip it up to 4500rpm while doing a big old burnout, and the forces acting on it are going to be a bit more inclined to try to break it free, especially if there is something out of whack — balance, or the universal-joint operating-angles tolerance. Of course, those are two extremes. In the real world, which is where your car will spend 99-per-cent of the time, any wayward forces are most likely to manifest themselves as a driveline vibration. This is where things can get confusing, because the first suspect is almost always a poorly balanced driveshaft — even when it might not be the case.
It’s generally accepted that in a front-engine rear-wheel-drive vehicle, the diff-pinion angle should be pretty close to the angle of the engine and transmission. But, when it comes to vehicles with a two-piece shaft, things get a whole lot more confusing.