Donor Gearboxes

**el chupacabra** · 26th January 2023, 17:59

I just modelled the whole crank assembly up and it was a little over 300mm wide but then fusion crashed before I could screenshot it and share the layout. I’m thinking both cranks spinning in the same direction so I can space them apart more and make the engine slightly shorter.

**husaberg** · 26th January 2023, 19:39

https://www.trademe.co.nz/a/motors/m...6?bof=6EikuEfh

this is interesting no idea what a tiger 800 is it a twin?

the guy that did the bsa vtwin a the norton rotary at EMU engineering i am prety sure used one of these he will have a CNC map for it.
https://www.trademe.co.nz/a/motors/m...5?bof=6EikuEfh.

The triumph ratios are not actually bad, better than the firestorm.

**jato** · 26th January 2023, 22:20

https://www.gearingcommander.com/index.htm I don't want to hijack the design or anything but i looked up a gsf 600 gearbox and the gears are spaced way too far apart for a track bike- first is only 36% of top so there will be big drops in rpm heading down the back straight

**el chupacabra** · 27th January 2023, 06:46

Hmmmmmm. I haven’t bought it yet, maybe it’s back to hunting. I preferably want something common so if I decide to make another it’s not too hard to find parts. Maybe an sv650 will pop up in my price range.

The triumph looks good, but I’ve never seen one before and don’t want it to be a mission to find parts, the VTR gearbox would be good if it had the clutch and cases.

**F5 Dave** · 27th January 2023, 06:50

Originally Posted by husaberg

https://www.trademe.co.nz/a/motors/m...6?bof=6EikuEfh

this is interesting no idea what a tiger 800 is a twin?

the guy that did the bsa vtwin a the norton rotary at EMU engineering i am prety sure used one of these he will have a CNC map for it.
https://www.trademe.co.nz/a/motors/m...5?bof=6EikuEfh.

The triumph ratios are not actually bad, better than the firestorm.

800 is a triple. I'd not be looking at bikes with broad spread power and road orientation as a shopping ground.

That site posted looks like a great resource.

**jato** · 27th January 2023, 09:03

Rainy day so i had a good look at Gearing Commander - some of the earlier suzuki 400 (impulse?) look real good but probably hard to find more than one. late model YZ125's have TZ like ratios but not so sure how a 125 gearbox would handle two 85 's firing in pairs ... in theory if you speed up the primary ratio say 35 - 40 % then the torque peaks would be no more than a 125 single. some (early) yamaha gears have been tested at rockwell 64 or greater and would shatter with a determined mid race shift... it'd be interesting to find a late model yz gearbox and do a hardness test on them. otherwise the gsxr600's have reasonable ratio's - first around 45% of top and bullet proof . maybe 40 or 50 % bigger/heavier/stronger than needed but common and easy.

**el chupacabra** · 27th January 2023, 11:51

The more I look the more it looks like the only suitable gearbox is something from an mx bike. But I’d be limited to 5 speeds. Unless I use a 125cc box and maybe a bigger clutch, but that could be a hassle.
Most nz tracks would only require a top speed of ball park 240kmh.

I do recall reading the rsw250 uses the gearbox from the rsw125 and even the rsw500 used the same gearbox. Maybe a 125cc mx gearbox might work. . .

I could hang out for a gsxr600, but I’ve given up on Sv and other tourer gearboxes as the ratios look horrible.

I could find another rz350 gearbox and use a banshee first gear but they are getting a bit expensive and I don’t really want to pull my motor apart to measure the cases.

So many options

**F5 Dave** · 27th January 2023, 15:31

Shipping is a bit ouchy these days but pretty good deal I'd think.

https://www.ebay.com/itm/27518064213...Bk9SR8jYqZS-YQ

Maybe same seller has a clutch.
Hmm, he does, combine shipping and not too bad.

**husaberg** · 27th January 2023, 15:48

Originally Posted by F5 Dave

800 is a triple. I'd not be looking at bikes with broad spread power and road orientation as a shopping ground.

That site posted looks like a great resource.

hi

you might be surprised by the Triumphs ratios, i was.
RZ350 is
2.571
1.777
1.318
1.083
0.961
0.889 or 1.68 spread

TZ which is of course crazy close
1.933
1.421
1.125
0.962
0.870
0.815 or 1.118 spread

Trumpy is a pretty reasonable
2.21
1.86
1.50
1.28
1.13
1.04 or 1.17 spread

These are far closer then a 85Mxer
which a is a range of about 2.5 to .885. for an old Cr80
these are so much closer then a MB50 its not even funny like 3.0-0.885

these are the YZ85.
Gear Ratio Â– 1st Gear: 27/11 (2.454)
Gear Ratio Â– 2nd Gear: 32/17 (1.882)
Gear Ratio Â– 3rd Gear: 26/17 (1.529)
Gear Ratio Â– 4th Gear: 22/17 (1.294)
Gear Ratio Â– 5th Gear: 26/23 (1.130)
Gear Ratio Â– 6th Gear: 25/25 (1.000)

the old pommy shitter turing Triumphs is a closer ration box than a std YZ85

i would also wager there is a few other ratios that fit the Triumph from other models as they were all about commonality plus god save the queen etc they won ww2 with triumph motors powering the Aux electrics on Lancaster bombers

**jato** · 27th January 2023, 15:55

or a bit closer and cheaper? https://www.ebay.com/itm/19080688011...Bk9SR4S5_ZS-YQ
so many to choose from - do 600's have back torque limiters? (i've never worked on one) for a recent project i used an early nineties gsxr 750 clutch - very simple and light - i think the 600's have a lot of interchangeable parts with a thousand other models.

**Grumph** · 27th January 2023, 16:03

I'd go 600 size box. It's sized for about the amount of torque you'll get. Much lighter than the Triumph boxes too.
In my experience the main criteria isn't the overall spread, it's the top 2 or 3 gears. The ability to pull through the gaps 4-5, 5-6 is paramount.
That's why the TZ box is so close in the top 3 - 4 ratios. No torque to pull big gaps.

When we went from a 12% gap 4 - 5 in the Aermacchis to an 8% gap it meant the things would keep accelerating in top. Std box it simply died at top end.

**jato** · 27th January 2023, 16:17

this clutch looks appealing - its got the little 34T gear that slides onto splines on the back of it - good for driving a little oilpump that can circulate oil to the shafts and your cast-in spray bar above the gears - a 230 crf pump and gear will mesh spot on , or just about every suzuki fourstroke pump. the primary gear most likely will be module 2.25 or maybe 2.5 so easy to get cutters to match for your layshaft... https://www.ebay.com/itm/14483570970...Bk9SR5Tnjpa-YQ

**jato** · 27th January 2023, 16:24

Agree regarding the last few gears - bit of a headwind and shifting up with a drop in revs to the bottom of the powerband and you are a sitting duck

**el chupacabra** · 27th January 2023, 16:50

Bike busters in nz have a 1996 gsxr600 box for $275, I’m going to see if they have the clutch and any other parts. Although I don’t think I’ll be able to work out the shaft centres accurately without the cases.

I’ll see what the triumph dude can do the lot for.