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Old 01-14-2010, 01:08 PM   #1
Grider Pirate OP
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Airhead 5 Speed gearbox questions

I have 3 5 speed gear boxes. All different. The first is from an '82 R80GS, and it has a kickstarter. Next is the '88 from my RS, and finally the box on my '95 PD.
The first question is:
Is the 'weakness' of the kickstarter primarily that it runs un-bushed in the rear cover? I had the cover off, and could fairly easily have turned some bronze bushing stock and machined the cover to remedy this problem, if that indeed is the weak point. I didn't, because it was needed in the RS till I get that trans sent off to, and back from, Anton.

Second question:
The gear on the INPUT shaft that is driven by the coupler looks ugly. It looks very much like the picture on Joerg's site. Is this common? Sorry, no pictures, I'm at 'work'.

The '88 R100RS trans will be shipped to Anton as soon as I get off my ass and box it. It has never shifted as smoothly as the '95, especially from 4th to 5th, it's stuck in second gear, and it needs a circlip, so off it goes.
The '95 is almost Japanese in it's shifting.

A REALLY general question is: Why oh why doesn't a well set up airhead transmission last forever? Duane Ausherman talks about the fact that they don't, and offers up that running synthetic will help, but dang! I have had manual transmissions from low end Japanese econo-box cars last 400,000+ miles with no maintenance aside from changing out the gearlube every couple years.
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Grider Pirate screwed with this post 01-14-2010 at 01:15 PM
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Old 01-14-2010, 03:26 PM   #2
Wirespokes
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That is one of the weaknesses - no bushing in the back cover, but another one is the lack of heat treating of the gears.

Not sure about your second question - depends how gnarly it is.

I know what you mean about car and truck transmissions lasting hundreds of thousands of miles - why not these? They're definitely beefy enough!

One of the issues (and maybe the main one) is the shifting dogs; there will always be some friction between the dog and it's mate before sinking home. That grinding creates metal particles that then deteriorate the bearings. Cars and trucks use brass synchros instead that don't have that problem.

And then there's the large output shaft bearing that's only oiled via the channel hanging from the roof of the box - a splash system - and takes the most force in the box (especially in 5th). This is one reason to take it easy if the bike hasn't been ridden for a while - to get that bearing oiled well before subjecting it to heavy forces.

The alternator rotors tend to be the weakest link in the electrical system and the transmission in the mechanical end of things. Ways to prolong its life and make things easier on it is to: shift without grinding (relaxed easy shifts), change oil frequently, use synthetic, and take it easy in first (short shift), and use fifth only as an overdrive and don't subject it to much stress.
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Old 01-14-2010, 05:26 PM   #3
Andy-Gadget
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Quote:
Originally Posted by Grider Pirate
I have 3 5 speed gear boxes. All different. The first is from an '82 R80GS, and it has a kickstarter. Next is the '88 from my RS, and finally the box on my '95 PD.
The first question is:
Is the 'weakness' of the kickstarter primarily that it runs un-bushed in the rear cover? I had the cover off, and could fairly easily have turned some bronze bushing stock and machined the cover to remedy this problem, if that indeed is the weak point. I didn't, because it was needed in the RS till I get that trans sent off to, and back from, Anton.
My understanding of the weakness isn't the shaft in the back cover, but the gear mesh on the input shaft.
The real problem is that the idler gear on the kick start, needed so the engine turns the right way, engages fully with the gear on the back of the input shaft, this isn't good for gears as they need clearance between the teeth to work correctly, and the gear flanks suffer great wear from this lack of clearance.
The fix would be to design a stop that kept the gears out of full mesh, best of luck, I only have one kick start box in use, and it is on my R65LS, and so is less "stressed" than any of yours

Quote:
Originally Posted by Grider Pirate
Second question:
The gear on the INPUT shaft that is driven by the coupler looks ugly. It looks very much like the picture on Joerg's site. Is this common? Sorry, no pictures, I'm at 'work'.
Lack of clearance, see above.

Quote:
Originally Posted by Grider Pirate
The '88 R100RS trans will be shipped to Anton as soon as I get off my ass and box it. It has never shifted as smoothly as the '95, especially from 4th to 5th, it's stuck in second gear, and it needs a circlip, so off it goes.
The '95 is almost Japanese in it's shifting.

A REALLY general question is: Why oh why doesn't a well set up airhead transmission last forever? Duane Ausherman talks about the fact that they don't, and offers up that running synthetic will help, but dang! I have had manual transmissions from low end Japanese econo-box cars last 400,000+ miles with no maintenance aside from changing out the gearlube every couple years.
I will probably cause a fight with this statement, but BMW gearboxes aren't very well engineered, but come from the factory very well built.
Guzzi gearboxes on the other hand are well engineered, but come from the factory not very well built.
To iluminate the differences, look at how the output shaft is located on the two boxes, the BMW output shaft is supported at either end by shallow groove ball bearings, shimmed on the outside of the bearings, this shimming has to be a compromise between the cold and hot shaft/case relationship.
The Guzzi box has a double row angular contact bearing at the back (drive end), with the shaft locked inside it with a nut, and the bearing locked in the case with a collar, can't go anywhere, the other end of the shaft is a parallel roller bearing that allows any difference in expansion to occur with no stress on the box.

The boxer gearbox age weakness is the front (non-drive end) bearing, which is the fifth gear end, and so the stressed end over time.
If a box has sat for a long period, the drive end bearing is most likly to give grief due to its position relative to the breather, and the moisture problems that this can cause.
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Old 01-15-2010, 05:44 AM   #4
AntonLargiader
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I've never seen the rear cover be a problem. Overuse WILL tear up the kick gear, though:



However, sounds like you're talking about the input gear. Depending on the year, they get broken/worn ears or worn teeth (which usually means all three helicals are damaged). I just replaced a set of them on one of my own transmissions.

IMO a well set up Airhead transmission DOES last a long time. Maybe not forever, but 80~100k miles if you keep the oil clean.
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Old 01-15-2010, 09:36 AM   #5
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D'oh!

Thanks Anton. I also just realized my comparison to an automotive transmission is unfair, since most are 'straight through' (input locked to output) in top gear. That would be a pretty tough trick to pull off on a shaft drive bike!
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