Bulletproof Gearbox "Upgrades"

bushmechanic · Jul 19, 2014

my engine and Tyre choice have a habit of destroying rear hubs, rear spokes and ripping the gearbox in half. (105mm bore and bolted paddles)

Ive had 6 gearbox failures In Total, about half of those with the 100mm bore and knobbies but none in the last 150 hrs with a bolted paddle, 75rwhp and a few tricks.

first time it broke was at 160hrs as built from the factory, i screwed up and landed a big ******* flat jump with a bit too much throttle on then on landing the G force on the lever shifted into 3rd .. bang. this happened a second time after a tiny stick flicked up off the front wheel and hit the shift lever

so I looked into the shift star and detent arm. ours is different to almost every other bike. the spring is softer and the shift star is very pointy without the traditional double curve.

that will make more sense when I add more pics tomorrow

tapped a 5mm thread into this hole for a 5mm CSK bolt and used a 2010 KTM 450SX detent arm spring. approx doubles the force on the arm

reprofiling the shift star more in line with other bikes

more pics to come plus circlip upgrades and hardness testing/retempering of the 3rd and 4th sliders

bushmechanic · Jul 23, 2014

shift star profile differences

this shows the basic difference, other bikes have a double profile on the star

when you shift gears your input does not actually slide the gears into place, what you do is put the detent arm over the top of one of the points.

the actual shift is done by the detent arm spring so when you have a double profile it can wait for the "right" moment to slide

the berg star just slams it straight in... it is faster.

bushmechanic · Jul 23, 2014

circlip upgrades

another issue is the circlips being too pissweak

these are the 08 clips from my bike after the first failure.

the 08 clips are much tougher than the pre 08 clips

tip from KTM talk via craigy was both are crap compared to the kawasaki clips (92033172)

there is an opportunity to use one of these locking stop discs I made on the output shaft, can only install one of them.

I still have one left from back in 2008 to go in my 628

bushmechanic · Jul 23, 2014

re tempering gears

so with the shift star and detent arm fixed (in 2008-9) there were no more failures from the bike shifting itself into gear at the wrong time and the upgraded clips have not been a problem.

but about this time I put the 105mm piston in and started running bolted paddle tyres.

now the gears would just rip in half under load, I sent one box away to be shot peened crack tested hardness tested and cryogenically treated. seemed good but the hard facing flaked off 5th .. it was an old gear, maybe was due to go anyway.

the last mod was just take a standard box (no cryo) and re-temper one of the gears (4th slider) in an oven for 1 hr at 250 deg C

with all those mods Ive had a good run of 150 hrs with the big engine and the paddle tyre although i mostly run in 5th gear

I picked out a set of gears today for the 628 build http://husaberg.org/mechanical/17998-bushmechanic-mongrel-628-trailbike-2.html#post150233

and re tempered some of them in the same way, will be changing gears more often so did 3rd and 4th sliders and the gears that engage into their teeth , 5th and 6th.

they tested 63-65 HRC before and 58 HRC after. polished one side before they went in .. the flash is deceiving, the color we are chasing is dark purple.

steve · Jul 23, 2014

Interesting stuff as always

bushmechanic · Jul 23, 2014

thanks Steve

yes it is interesting, pretty funny that in the end it is a very simple and cheap solution to the main issue of gear strength, one hour in an oven LOL

Im still pretty impressed with my old husaberg this part of the berg engine is the only OEM bit that i have really had significant trouble with, all the other failures were from aftermarket stuff.

spanner · Jul 24, 2014

Just picked up 6th gear set (the ones that are subject to pitting) from the shot peener`s... the shot just bounced off

He said they are harder than gears he normally deals with.

I think you are on the right track by baking them in the oven

Possibly they made them that hard because some of the gears run a needle roller on the ID.
How to keep the inside ID cool while cooking the OD

...

Cheers Bushie for another Great thread.

bushmechanic · Jul 24, 2014

interesting spanner

Dave Hopkins often mentions breaking early husky boxes while racing he had them pulled back from 58 to 55 or even 53 If i recall correctly, under 52 was too soft

looks like ours start over 60 .... closer to 65

perhaps to keep the centre of the gear cool you could mount the gear on a solid aluminium shaft and have the shaft go through the oven wall and into a bucket of water

bushmechanic · Jul 26, 2014

testing today the new spring on the detent arm pulls the arm down (toward the centre of the cases) quite a lot

since there is a lot of slop in the arm pivot on this bike it can just about fall off the thin shift star

I just filed the pivot spacer down .. tightened it up.

Bulletproof Gearbox "Upgrades"

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