Donor Gearboxes

[el chupacabra]

Slowly but surely making progress.

Still a lot to chisel away before patterns are made.

[jato]

looking good! a friend is asking if 125 cranks and cylinders could be shoehorned into the case...

[el chupacabra]

Thanks. An Athena 112cc kit should bolt in with no additional work. Good for Hp somewhere in the low 30s making the bike 450cc. I am unsure about a 125, the cylinder could be made to work but the 54.5 vs 47.8 mm stroke might be harder to accommodate without a redesign of the lay shaft gear size.

[el chupacabra]

A couple of hours at the PC and a few wines later, we have some more progress with some small hurdles to overcome, namely how thin the case is by the RHS crank wheels (about 2.0mm).
There also isn't too much meat for studs in between the cranks. The Lower engine mount is a bit ugly too.

[el chupacabra]

This is with the cassette plate in position.

[jato]

More good progress! should we send a couple of casks of wine and wait for the result? re case strength in recent times i've had quite a few (mostly KTM) engines arrive at the workshop that have cracked or broken in use - anaemic gusseting and thin sections trying to contain lots of power and merciless gearshifts... some of those ktm sections were slightly under 2 mm thick. compressive loads maybe ok but in areas in tension more strength is needed. the bottom mount is crying out for something alright.

[el chupacabra]

I'm sure twice the wine consumption would half the development time haha. I've beefed up the areas as I was worried about being able to flow aluminium through such a thin area when casting as 2.5mm will be close to the limit and I don't want to push it. I could always add material during the casting and remove it with and end mill during machining to make sure the pour is successful.

I have also started laying out all of the studs and everything is pretty straight forward apart from the upper cylinder where the case studs get close to the cylinder studs. I say stud but it will likely be recoiled for a cap head. I'm wondering if a slot where i have highlighted in the last picture would be a good solution to get access to a bolt with a spanner and not having the cap head in the transfer port guide. . . hmmm, things to think about.

[jato]

For the upper cylinders' rear case bolts would an option be to have 4 studs coming into the gearcase cavity (visulising 4 back-spotfaced pads in there with loctited flange nuts so nothing goes visiting the gearmesh) another (not ideal but practical) option could be to cant your proposed capscrews forward 10 degrees or so to give clearance to the cylinder studs.

[el chupacabra]

I could make a sub plate that holds the studs for the cylinder, or I could make a large blanking plug for the case stud that is threaded and has the cylinder stud in it centrally. Or thread the plug so the cylinders can be secured with cap heads for the top bolts.

I’ll investigate tomorrow before I carry on. I want to be able to take the cranks out without removing the gearbox. I also don’t want to make general assembly too difficult, it might just work how I have it designed. I plan on using O rings instead of silicone to seal the case halves.

[husaberg]

betcha that single crank suggestions looking mighty attractive now
Yamaha changed the crankcases pretty much every couple of years v angles intakes etc trying to get the best compromise between flow and compactness.
they also had to run special offset carbs and ugly intake rubbers

Normal Honda do stuff not the easy way just as a form of corporate masturbation, but not with the NSR's.
the Honda has a sight loss to slim the engine but tiny compared to the YZR.

4 of the 6 petals only
What i do notice though with yours having the cranks in all the same direction this should pick up soe power compared to contra rotating cranks that have one side fighting the intake flow


Kawaski

RD200 47mm stroke

[el chupacabra]

In a perfect world i would make a single crank with flying webs or twin cranks geared together. But one of my key design constraints is to be able to use off the shelf available spares. (The 20mm shafts on the YZ85 crank would definitely be too weak for 120Hp)

Im all for changing the design if something gets pointed out that I’ve done that could be improved. But here’s the design philosophy I want to follow:
Must have available fast moving spare parts (YZ and Triumph)
The less custom parts inside the crankcase the better
Must be rideable by a half decent club level racer (600cc level power not 1000cc levels)
Must be deigned for longevity and ease of servicing
Power to weight ratio of 1:1 would be nice

I could always make my own crank wheels for the clutch side and beef them up. Hmmmmm but then the off the shelf cranks won’t drop in and I’d still need at least 3 custom gears manufactured.

[el chupacabra]

I almost rolled it right back so like Honda the rear cylinder studs would also be the case studs. But that would involve re modelling the intake ports and changing the V angle to 80 degrees. So I came up with this not so elegant solution, but not so difficult to implement.

[husaberg]

In a perfect world i would make a single crank with flying webs or twin cranks geared together. But one of my key design constraints is to be able to use off the shelf available spares. (The 20mm shafts on the YZ85 crank would definitely be too weak for 120Hp)

Im all for changing the design if something gets pointed out that I’ve done that could be improved. But here’s the design philosophy I want to follow:
Must have available fast moving spare parts (YZ and Triumph)
The less custom parts inside the crankcase the better
Must be rideable by a half decent club level racer (600cc level power not 1000cc levels)
Must be deigned for longevity and ease of servicing
Power to weight ratio of 1:1 would be nice

I could always make my own crank wheels for the clutch side and beef them up. Hmmmmm but then the off the shelf cranks won’t drop in and I’d still need at least 3 custom gears manufactured.

i was being cheeky not trying to show disrespect to your design
i get it but i strongly disagree 20mm would not be enough Aprila rsw125 was that size for years only the very last rsa was 22mm crankpin
The Barton engines were also 20mm pins on three cylinder 480's or so. at over 100 plus hp.
or are you meaning the wheel shafts?
if thats the case pretty sure cr and ktms were beefier 22 i think?
you are right off the shelf is great. RD200 would not be that easy to find that''s for certain .

[el chupacabra]

I was meaning the wheel shaft part of the crank the primary gear is keyed to. On my banshee the bearing ID is 25mm, on the YZ it is 20mm. I was thinking a 20mm shaft with two Gears, the coupling and primary drive would be a weak link. (Picture to help show what I mean).

it’s easy enough to make a crank wheel on the lathe with a longer shaft to run a support bearing but that’s a lot of work. Use the original magneto side wheel and maybe bore one of the magneto wheels to make 20mm and key it to the 20mm primary and join the cranks so to have 2 instead of 4

Also, don’t worry about causing any offence, I welcome the criticism or other ways of doing things better. I appreciate all the pictures as I use them as a visual guide to make sure I’m on the right track.

[husaberg]

I was meaning the wheel shaft part of the crank the primary gear is keyed to. On my banshee the bearing ID is 25mm, on the YZ it is 20mm. I was thinking a 20mm shaft with two Gears, the coupling and primary drive would be a weak link. (Picture to help show what I mean).

itÂ’s easy enough to make a crank wheel on the lathe with a longer shaft to run a support bearing but thatÂ’s a lot of work. Use the original magneto side wheel and maybe bore one of the magneto wheels to make 20mm and key it to the 20mm primary and join the cranks so to have 2 instead of 4

Also, donÂ’t worry about causing any offence, I welcome the criticism or other ways of doing things better. I appreciate all the pictures as I use them as a visual guide to make sure IÂ’m on the right track.

Early KtM85 is 22mm shafts both sides.
Late ie post 13 is 25 and 22mm they have a very nice cylindrical roller on the 25mm side NJ 205
later ones are 20mm crankpin earlier are weirdly 22 pin.

The Honda CR80/85 IS 22MM mainshafts either side.

all the rest YZ RM and KX are all 20x52x15. bearings.

the other thing with KTM cranks is they have a keyed rather ten splined primary drive gear.

https://www.mitaka.co.uk/CATALOGUE/MX_CRANKS.htm