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HOW TO CALCULATE YOUR SHAFTS FOR SMOOTH SAILING
HOW TO CALCULATE YOUR SHAFTS FOR SMOOTH SAILING
When you’re elbows deep into your engine transplant there are a million things to consider and make sure you get right. Turbo placement, water line routes, steering-shaft clearance, and then there is shopping for all those bolt-ons. But, as fun as it is to get carried away with choosing go-fast parts and chasing big power numbers, there’s a practical aspect to driveline swaps that is often overlooked — one that can poorly affect the way your finished build drives, or, in the worst case, cause all manner of damage from extreme vibrations.
It’s to do with your driveshaft — but we’re not just talking balancing, although that is often the first scapegoat. Get the universal-joint angles wrong, and you could be running in circles trying to correct a 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 referring to 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 stress, 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? What we mean is, the quicker an inert object moves, the greater the forces acting on it. Slowly rotate your driveshaft while crawling along at 20kph, and everything will be hunky dory. Whip it up to 7500rpm 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. These are set by two or three things, depending on whether you have a single or two-piece shaft: your engine/gearbox angle; your diff-pinion angle; and, if two piece, your centre-bearing height (in relation to the aforementioned two items).
Thankfully, there are some helpful companies out there that make this process of measuring these operating angles not as painful as you might think. Tremec, for example, has a free app for smartphones called ‘Toolbox’, which, as well as calculating speed, rpm, and tyre size, will measure your angles. However, while the app is easy to use, it’s only useful for those of us with a rearwheel-drive car with single-piece driveshaft. The process can be done in minutes and gives you a pass or fail. If you have a twopiece shaft, the easiest way to crunch your numbers is using the torsional-analysis calculator provided at spicerparts.com. But the Tremec app will come in handy if you don’t want to splash out on a digital protractor, as you can use the app (and your smartphone) to measure the angles that you’ll need to input into the Spicers calculator. As a case study, we will measure up Master Mechanics’ Mazda B2000 from the cover of Issue No. 259.