Project Miatabusa Part 17: EVO II is Finally Here!

Project Miatabusa #17

Miatabusa EVO II is Finally Here!

by Dave Coleman

If you’re one of the 6 people in the world who isn’t familiar with this car, step back, go HERE, and read every word of the drama (or comedy, or tragedy, we won’t really know until the end) that is Project Miatabusa. For the rest of you who have been waiting to hear what we did about the sackfull of hammers trying to beat their way through the bottom of our car, wait no more:

Step 1: Redesign everything

First, the root of the death rattle problem was our initial decision to remove the sprung hub that normally lives in the Hayabusa’s clutch basket. This led to a horrific gear backlash rattle caused by a difference of opinion between the crankshaft and the flywheel. The crankshaft thought it would be really groovy to accelerate and decelerate with every piston stroke, while the flywheel figured it would be just swell to spin around smoothly and continuously. Usually in these cases, when they’re bolted directly to each other, the two parties find some compromise, but when you stick a gear in between, the gear gets the worst of the argument.

The gear with the red arrow is the root off all our challenges.

The obvious solution is to put the Hayabusa’s sprung clutch basket back in, (since it seems to have been designed to prevent exactly this rattle), and drive the flywheel through it. When the crank wants to accelerate, it can compress the springs in the hub instead of forcing the flywheel to speed up, and when it wants to decelerate a few milliseconds later, the springs can extend again, letting the flywheel keep going at its steady pace. If we’re lucky, this harmonious new living situation will keep the peace in our crankcase.

Solidly connecting the gear to the shaft didn't work, but using the factory needle bearing wouldn't fly either.

The first redesign this requires is letting the power takeoff gear float on the flywheel shaft instead of being brazed directly to it like it was on Miatabusa EVO I. This will let the gear shuffle back and forth with the crank, while the shaft rolls steady with the flywheel. Back when the flywheel shaft was just the stock Hayabusa’s transmission input shaft, the power takeoff gear rode on a big needle bearing for just this reason, so the obvious thing to do is just put the stock parts back in there.

Not so fast. In the stock application, the gear would also intermittently spin on the shaft whenever the squidmonkey resting his balls on the airbox grabbed a handful of clutch. In our application, that needle bearing will never spin, it will just rock back and forth as the springs negotiate the differences between the crank and flywheel. Needle bearings really don’t like this kind of job, since some spots on the bearing race will see lots of action while others will see none. This ultimately leads to failure modes that would dump tiny bits of hardened steel into our crankcase.

So instead, we replaced that fancy needle bearing with a bronze bushing which is much better suited to this kind of cyclical, short-travel business.

Yeah, so we took our bushing to the mall for a Glamour Shots appointment, you got a problem with that?

Now that the stock sprung hub and clutch basket could move on the shaft, we had to figure out how to connect the sprung side of that hub to the flywheel. The stock Hayabusa clutch is a standard multi-plate affair, with half the plates splined to the ID of the clutch basket, and half splined to the shaft. So our path was pretty obvious. Simply make a big, thick, clutch plug that splines to both sides at once, making it act like a perfectly solid clutch stack.

Two problems down, eleventy billion to go.