Shafted! How a tiny part can make a huge fknBANG under your car
SPOILER ALERT - if you haven't seen the episode where Marty dyno-tunes his RX-7 (click HERE) or the LATEST EPISODE (CLICK HERE) featuring the result of that, then go watch both those episodes before diving into the smelly mess which is the mop-up of this week's action. We will say this: it is the first time an MCM car has failed on the dyno!
Marty's RX-7 had a good spin on the Haltech chassis dyno after all its recent engine work, but the fun was cut short by a failure from under the car. While the 12A rotary had managed to keep all of its Doritos inside its elliptical housings, the drivetrain wasn't quite so lucky.
This can be a good and bad thing, as it saves you an engine rebuild... however grenading drivetrain components often have a way of causing a lot of damage due to them spinning under the floor. When one end of that chain breaks, it can flail freely into the car, breaking the back off the transmission or snout off the diff and beating up the floor of the car.
In Marty's case it appears a universal joint let go. These four-sided pieces are how the tailshaft (also known as a "propellor" or "prop" shaft) is connected to the transmission and the diff. The tailshaft needs to move up and down as the diff moves up and down with the suspension, and the universal joints (also known as "unis") ensures the driveline angles don't bind up.
These unis use needle roller bearings under a steel cap to spin at high velocity with the drivetrain. They're fitted to the tailshaft's yoke using a pair of metal caps which bolt down, joining the whole lot in spinny matrimony, and on old cars were considered a maintenance item that you had to re-grease.
This isn't possible on newer cars as they're not designed to be pulled apart and have the bearings re-greased. In fact, on some later-model cars the transmission output flange is connected to the driveshaft via a rubber coupler, not a uni. These rubber dampers wear over time and vibrations from them will make you feel like you're riding inside a spinning washing machine as it falls off a mountain.
Checking for excess movement in the universal joints is important to prevent failures down the track. As Marty found out, sometimes those failures mean spending a bunch of quality time with the mop when you sploosh fluid all over Tuning Fork's dyno...
On modified cars, the other thing to inspect is pinion angle.
Universal joints have to run at a slight angle, which is where you sometimes hear of "pinion angle" when it comes to live-axle diffs. This is how much the diff faces up or down, and is normally between 1-3 degrees - diffs are set-up so as the car accelerates and the suspension squats, the pinion ends up at the primo angle.
The key to keeping your unis happy is to have the pinion yoke and trans yoke parallel, even if the driveshaft is running on an angle. While it is theoretically perfect to have everything run in a dead straigth line, you need the car's suspension to move through its arc, and that affects the pinion angle.
Lowering your live-axle car can affect these angles and, over time, lead to excessive wear. And this is why it can be a good idea to stick an angle finder on the pinion and transmission output to double-check angles before doing too many kilometres on a modified car you've bought.