miatabusa gets the shaft lead

Project Miatabusa Gets the Shaft!

by Dave Coleman

The last time any of you heard anything but excuses about Project Miatabusa was January. I almost can't believe that myself. The excuses? Well, we were busy... I was busy winning a LeMons champoinship and then enjoying the spoils of victory. Tim (who supplies all the actual talent for this endeavour) was busy putting a CBR1000 engine in the Angry Hamster. And Alex, who's really good at talking the rest of us into doing silly things like this, was busy trying to talk us back into finishing it. 

It finally worked.

Hayabusa gearbox

To re-cap just a bit, the idea was to take the Hayabusa's relatively fragile transmission (it'll pull a car around for a while, but before long things start breaking from the substantial extra load) and replace it with a shaft that holds a Miata flywheel. That's pretty simple to say, but actually doing it... 

miatabusa cad drawing

To turn the idea into reality, Tim designed a complex, 4-piece Hayabusa-to-Miata adaptor. If you never saw these before, you REALLY need to go read the first few parts of this project to get up to speed.

Miatabusa hayabusa flywheel shaft

And there it is, just as easily as we said it, Tim made it. Ok, not quite as easily...

To understand just what you're looking at, we've got to walk through how everything works. With the original shaft, on the left, the giant gear is driven directly off the crank. That gear carries the Hayabusa's multiplate wet clutch, and when the clutch is disengaged, the gear spins freely on the shaft. That detail will be important later. The small gear next to the giant one is the drive gear for the oil pump. Next up is a bearing, then 1st gear, 5th gear, 4th gear, 3rd gear, 6th gear, 2nd gear, and finally another bearing.

Look closely at the new Miatabusa shaft and you can see we moved the big bearing to the other side of the gear. This puts it halfway between the gear and the flywheel, two things that really should be well supported.

The oil pump drive gear is actually a separate part from the main drive gear. It sits on a loose spline and is normally held in place by the bearing. Tim threaded this portion of the shaft so the gear could be held in place by a threaded collar.

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Der Bruce
Der Brucelink
Thursday, September 08, 2011 8:14 PM
I can't remember where but I thought you said you were going to run a gear reduction somewhere so that the tranny isn't running 11k with the motor? I'm WAY looking forward to video, with a Yoshimura exhaust maybe!?
Thursday, September 08, 2011 9:05 PM
The gear reduction comes from the big gear and custom shaft that they've bolted the flywheel to. It's why they didn't bolt the flywheel directly to the crankshaft like on any normal engine. The engine will see 11k RPM however the tranny will see about half that? I can't remember what the ratio was...
Alex Vendler
Alex Vendlerlink
Thursday, September 08, 2011 9:06 PM
.6 to 1
Dave Coleman
Dave Colemanlink
Friday, September 09, 2011 4:20 AM
No need for memory, guys, the past is just a click away:
Friday, September 09, 2011 5:12 AM
Glad to see Miatabusa is back! Have you made any calculations on road speed vs engine speed? There seems to be a lot of gearing going on.

And did you mention a turbo? I don't remember that being in the plans. Guess I need to read the articles again since it has been 9 months from the last update. Damn you're lazy Dave ;) (kidding of course)
Friday, September 09, 2011 5:13 AM
I'd like to see the shop that you guys are building this car in. The parts you guys are turning out are very impressive! I can't wait to see the finished product...might be worth selling my 240 and building my own Miatabusa once the parts go on sale. They are still going to be sold right?
Friday, September 09, 2011 5:41 AM
This project gives me faith in humanity
OMG Its Weasel
OMG Its Weasellink
Friday, September 09, 2011 6:24 AM
i just crapped myself.
Friday, September 09, 2011 6:54 AM
very nice I was on the verge of getting impatient over here
Friday, September 09, 2011 7:20 AM
Very nice!

I bet many of the custom fastners you had to make can be sourced through OEM motorcycle parts. Many bikes/ATV's use similar style parts in their clutch, middle and final drive units for most of their shaft drive models. Clever interpretation of part numbers will often reveal their dimensions/specs.
Now ya know...

Too bad the OEM 6-speed sequential tranny has shown to be too fragile. What are the weak points you have found? Many companies do MC tranny upgrades too. maybe next time?
Dave Coleman
Dave Colemanlink
Friday, September 09, 2011 7:32 AM
The gears themselves are quite strong, but the dogs wear very quickly, as does the rest of the shift mechanism. The lack of a reverse gear is also a real bummer.

The total gear ratio spread, from 1st to 6th, is only 2.5:1 on the Hayabusa. Its 3.85:1 on a Miata gearbox.

As a pure track machine, the narrow spread and lack of reverse might be fine. If you want to drive to the track, not so much... Having a flywheel and big clutch are also nice when getting a car to start moving.
Friday, September 09, 2011 7:41 AM
I was looking at the reduction gear, couldn't you guys machine one of those? We used to machine our own sprockets in SAE and our mill is pretty cheap (we can't do the complex 3-dimensional cut needed to make the custom clutch cover). Are the teeth too small to cut on a mill? We would program a cut and then repeat umpteen times to create a finished sprocket.
Friday, September 09, 2011 8:18 AM
Awesome build Dave, Tim, and Alex. This project almost makes the Miata not Ghey..... almost. But, I absolutely love the badassery of those custom billet parts!!
Friday, September 09, 2011 10:07 AM
F-N-R box would cure the lack of reverse, but would need one helluva chain or some sort of ring-gear conversion. But, the 'Busa trans dies often enough in 1000lb dune buggies, I can't imagine it would be happy in a 2000lb car.
Dave Coleman
Dave Colemanlink
Friday, September 09, 2011 10:27 AM
Bingo! The poor track record in dune buggies was a big motivator for using a proper automotive transmission.
Friday, September 09, 2011 2:51 PM
@ 8695Beaters: Gear tooth profiles are fairly complex even on straight teeth, especially if you want them to be strong and quiet. It's possible to machine a gear like that (companies do it all the time), but if there's nothing wrong with the stock one then it saves a lot of machining or hobbing, heat treating, and surface hardening. A chain sprocket for a lightweight, low powered FSAE car is a completely different animal than a reduction gear capable of putting down some serious mileage. The load transfer is completely different also; a chain has a tendency to distribute load among multiple sprocket teeth, whereas a gear applies all of the load over a very small (and therefore highly loaded) area.
It was a great question though, and I'm sorry for answering out of turn.
Friday, September 09, 2011 4:15 PM
That answers my question and then some...Thanks!
Friday, September 09, 2011 5:54 PM
Ahh, I see. The dogs/forks/etc prob get a severe beating with no lift/clutchless shifting all the time as well as the loss of the cush drive that was on the motorcycle clutch and wheel hub.

A buddy had a Busa-Buggy and drove it like an animal. Actually ripped the shifter off the floor twice! (Did the same thing to his M3...) It is so much easier to mad-dog motorcyce engines in cars/karts than in the original bikes they came in (a little more consequence in the motorcycle application....)

As for re-gearing, I doubt the gear on the crank could actually be modified to a smaller size and I also doubt that the secondary/clutch gear would fit in the cases if much bigger. It would be interesting tho...
Friday, September 09, 2011 6:01 PM
Remember there are other places that ratio changes can take place, like the Miata transmission and the rear diff. Nothing is without compromise of course, but there are a few ways to go about it in a RWD car.
Friday, September 09, 2011 6:21 PM
Q: Why didn't you find a way to also keep the original dual bearing behind the clutch gear? I worry that your fabricated side cover/bearing holder will see load that it was not designed for. The original cover with it's 2 dowels and aprox 10 6mm bolts was in no way intended to carry any load what-so-ever but simply be a cover. Now that will be carrying virtually all of the load in shear while the main case isn't helping out at all. You can see that the OEM design has the load path directly from the right side main journal directly into the OEM double bearing with a virtually straight path. All this can be seen in the top pic on page 3. Not at all a criticism, just some thoughts from a guy who has spent his entire life in the motorcycle industry. ;-)
Friday, September 09, 2011 6:28 PM
"Remember there are other places that ratio changes can take place"
Of course, but I thought we were addressing the motor.
Hey Guys
Hey Guyslink
Friday, September 09, 2011 7:22 PM
Exocet Miatabusa? Come on Dave you know you wanna. Plus, the exocet has a US distributor now, exomotive.
Eric Hsu
Eric Hsulink
Saturday, September 10, 2011 6:13 AM
Dave: at 11krpm, you might consider a steel or aluminum flywheel. There's a reason why SFI doesn't allow cast flywheels in drag racing...
Saturday, September 10, 2011 6:29 AM
@EH, don't forget the primary reduction of .6(?) which will keep the flywheel under 7k when the motor is at 11k
Dusty Duster
Dusty Dusterlink
Saturday, September 10, 2011 2:54 PM
At least once a week while visiting this site I wondered what became of the evil Chimera that is Miatabusa.
Dave Coleman
Dave Colemanlink
Monday, September 12, 2011 10:58 AM

You make a very good point about the bearing loads all being carried by part of the case only intended to carry a cover. The same is true of a lot of other loads, like torque reaction loads, the weight of the drivetrain itself, and the doubling of that weight when the car compresses going down the corkscrew.

Because of all that, we've already been working hard to make sure those loads are distributed elsewhere. We have a brace connecting the bellhousing adaptor to on the engine mounts on the cylinder head, the adaptor also bolts to the starter cover flange on the nose of the block, and finally, the engine mounts will incorporate some more bracing connecting the bottom of the bellhousing adaptor to factory Suzuki engine mounting bosses.

We have considered running the original Suzuki bearing in addition to the one we're running now, but we're concerned about over-constraining the shaft and causing unanticipated loads as it deals with changing operating temperatures.
Tuesday, September 13, 2011 4:18 PM
"over-constraining the shaft " leads to blindness???

The gears will also be trying to push themselves away from each other, adding even more loads to items not originally intended to handle these loads. Sounds like you have thought of all this already as I suspected.

GL with the build, when do you plan to put it on track?
Dan Barnes
Dan Barneslink
Tuesday, September 13, 2011 4:54 PM
@ mxpop - All E36 and E46, and likely similar cars before and after, will eventually let go of the shift knob and allow you to throw it into the back seat after enough fast 1-2 shifts. I grabbed some (really old) glue from the model airplane box that's working pretty well. But that's just the knob, not the shifter coming off the floor.
Thursday, September 15, 2011 5:29 AM
Thanks for the update... Can't wait to see some video of it on the track. I'd love to see the tach of both the engine and flywheel.
Thursday, September 15, 2011 6:11 AM
DB, thanx for the explanation but I would rather stick to my version of the story so I can point/laugh at my buddy for being a hack... ;-)
Wednesday, September 21, 2011 5:03 AM
I really really really really REALLY hope you guys produce and sell the parts for this swap. I'll be one of your first customers!!
Dave Coleman
Dave Colemanlink
Monday, September 26, 2011 10:06 PM
Ok Pyro3113, we're gonna hold you to that!
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