Project Miatabusa Part 17: EVO II is Finally Here!

,

Now, the only problem with leaving the clutch basket in place is that there isn’t room for it. Between the power takeoff gear and the flywheel, we need room for a short spur for the Miata flywheel to pilot on, a Miata rear main seal, and a bearing to support the flywheel. The stock clutch basket takes up most of this room.

This solution was relatively easy. With our clutch being a solid plug, we don’t need it to be nearly as tall as the stock clutch stack. We simply made the plug as thick as all the outer clutch discs if you stacked them up by themselves (this gives us the same total contact area with the basket that the stock clutch had). Then we cut the basket down to the size of our new plug.

Slide everything together and the solid clutch adaptor locks the shaft to the sprung hub.

Now things start getting really fancy. Our original design used two bearings, one at either end of the shaft. At the front of the engine was the tiny needle bearing used by the factory, and at the other end, by the flywheel, was a double-row ball bearing we mounted in the wedge-plate adaptor that's bolted to the stock Hayabusa clutch cover face. The factory Hayabusa bearing that used to live right next to the power takeoff gear was eliminated.

We had limited ourselves to two bearings on principle, not wanting to over constrain the design. A straight shaft running between two bearings can always stay aligned with both bearings, but running between three bearings requires all three bearings to be perfectly aligned. If we had two bearings in the Hayabusa case and one in our wedge plate, there was a chance we’d be forcing the shaft to bend, ever so slightly, to hit all three bearings. Add in the variables brought on by thermal expansion, and limiting ourselves to two bearings seemed the wise choice at the time.

Miatabusa EVO I used this double-row ball bearing on the outboard side of the gear.

With the sprung hub and our cut-down clutch basket taking up the real estate where our bearing used to live, we would have to push that bearing farther toward the flywheel. Not only was it now the wrong shape for the available space, having the gear several inches from the nearest bearing didn't seem like a good idea. If the shaft flexed at all, it could open up the gear lash and make things worse. Since Dr. Ferdinand Hayabusa had put a bearing next to the gear when he designed the thing in the first place, we decided this time to bow to his wisdom and use that same bearing, even if it meant using a 3-bearing shaft.

The EVO II shaft uses the two factory bearings, plus a third by the flywheel.

The next problem was that that third bearing was gonna be a doozy. Our EVO I shaft was made of two pieces with an elaborate splined interface that was really hard to put together and even harder to take apart. The wisdom of experience has shown us the virtue of a simpler one-piece shaft that makes serviceability dramatically better, but leads us down the path to a monster bearing.