Some things you state here are very confusing, such as "People have had similar issues on all sorts of bikes...". What issue are speaking of? Premature gearbox wear, spline shaft wear, or catastrophic failure?
All of the above. My comment in particular though was related to spline wear. The most catastrophic type of gearbox failure in terms of parts cost is the destruction/chipping of the drive teeth, destruction/chipping of the dog teeth (in a dog engagement gearbox) or destruction/chipping/wearing of the shaft splines. Other failures are munched or worn selector forks, stuffed bearings and so on, but those aren't the types of wear I’m speaking of.
"You just get to make up your own mind" - while that's easy to say, what criteria will we use to make up our own mind? Because Woody's made one we should decide if we should buy it? I'm just not getting this.
It’s got nothing to do with Woody’s, they just build nice wheels. My point was this - I’m not trying to convince you to buy one (or not).
"...a cushhub reduces gearbox wear..." How does it do this? What kind of wear? What exactly does a cushhub eliminate that causes this wear? It's a general consensus from whom?
All a cush hub does is reduce ‘shock loads’ (so to speak). To a negligible degree it also reduces the total energy transfer (some energy is wasted, principally as heat during the compression of the rubber and as friction between the rubber and the housing). What it really does though is increase the time period over which the energy is transferred by adding more elastic (or close enough to it) deformation to the system. Increasing the timeframe means a reduction in the peak stress applied across the contact surfaces in the drivetrain during the transfer of the energy. This in turn reduces the risk of chipping, pitting, spalling or elastically deforming any of the metallic engagement surfaces within the entire drivetrain as the reduced peak stress (the peak stress being reduced by the compression of the rubber as the total energy is transferred over a longer time frame) minimise the strain in the material at the contact interfaces. You then need to understand some basic contact mechanics and the Hertzian and Non-Hertzian models to realise why this can reduce wear at the engagement surfaces...but that’s beyond what I’m willing to write (plus I would need to dredge it up from the deep recesses of my mind where it hopefully still lurks).
Nevertheless, the consensus I referred to though was that of the people that have done much more riding under harsher conditions than myself and reported less wear and tear on their drivetrain after installing a cush hub (and even better, slipper clutches).
A cush hub also makes for a smoother ride, less ‘unpleasant’ drivetrain lash when changing gears and general reduction in NVH.
I thought the knobby tire had a similar effect on drive train shock as a cush hub. And the slipper clutch. It is to cushion crankshaft pulses, right?
You are correct that knobbies will reduce the peak stress on the drivetrain since less friction can be generated between the tyre and the track/road surface so there is less force and total energy being applied to the system in the first place. Whether it’s enough of a reduction and whether it’s a consistent reduction on all road surfaces is another story though. This is why cush hubs and slipper clutches are important in supermoto as the road tyres in use on high traction race tracks will definitely generate more force than knobbies on a loose surface ever could.
There is no direction in this system as far as a cush hub is concerned, there is no upstream/downstream difference. This is because the engagement surfaces are symmetric on both sides (and hence in both directions) in this drivetrain. The overall drivetrain extends from the top of the piston to the tyres contact patch....engine pulses, tyre compression lock, it doesn't matter, it's all just manifested as tension and force in the drivetrain just on different sides of symmetric shapes. These forces will always act to speed up the engine and slow the bike, or, slow the engine and speed up the bike in an equal and opposite manner (in a lossless ideal drivetrain anyway). The job of the drivetrain is to link the energy states of these two end entities so that they share a similar energy; i.e. the engines energy is being applied to the wheel and vice versa when decelerating. A slipper clutch or cush hub acts to limit the peak load and rate of transfer within that drivetrain between the two systems.
Why don't dirt bike trans wear out prematurely from running them so hard on a track, jumps, winding it out as it hits the ground, etc.
They do. They wear, break and need rebuilding and parts replacement from doing all of those things. This is the prime reason that landing a jump with the throttle open is bad juju.
What about shaft drive? What do they have for a cushion?
I have no personal experience with them but I reckon you would find that they have synchromesh boxes, slipper clutches in the road models and variant of a cush drive using rubber (or something similar) dampers in the drivetrain.
I can't equate the spring washer wear to a failure of the bike's drive train. I would seem more to me that it was a simple engineering decision that just wasn't up to the application. They just weren't strong enough. How does that mean the system isn't up to the task?
You might be right, it could have been poorly chosen washers. However, when I looked at the damage to my clutch basket and the washers behind the springs it was indicative of high peak stress loads (i.e. significant and non-uniform deformation) rather than failure due to cyclic loading or repetitive friction (smoother and more uniform wear on the surfaces involved). The only way to transfer high peak loads through a spring is to compress it completely and cause it to bind (unless you choose absolutely the wrong spring coefficient for the application). What I saw indicated that there was enough energy to have caused the springs to completely compress, bind and then buckle into the edges of the clutch basket and in the process this places unequal loading on the face of the spring washers and snaps them down the middle. This is my basket, I have no idea what other peoples looked like, I’m just assuming it’s a similar failure mechanism – I could be wrong there.
They also directly contradict SimpleOne claim that the gearboxes are lighter on a dirtbike, they actually say they are 'beefed up' and that wear is a non-issue running them on the pavement without a cush hub.
Maybe in their experience but that is not mine. The gearbox in my TE630 is miniscule compared to that in my ZX-6R - there is a huge reduction in the size of the mating surfaces on both the drive teeth and the dog engagement teeth of the Husky; and both are 600cc. My ZX is factory equipped with both a slipper clutch and cush hub but I would say that is also about reducing undesirable/unsafe handling traits such as rear wheel hop induced by compression lock during high speed downshifts as it is to do with drivetrain longevity and general reliability. I can't say what the differences were in splines.
From what I can find browsing the interwebs is road bikes come with a cush hub to make them smoother. Lots of other bikes without them with lots of miles with no premature wear or failure.
Yep this is another reason, makes a bike feel much more refined with reduced noise, vibration and harmonics.
BTW, one sumpermoto racer thought the cush hub was to ease transitions from decel to accel in cornering. Proper clutching and shifting makes WAY more difference on gearbox life than whether you have a cush hub or not.
Yes. As well all know, just because its on the internet doesn't mean it's valid.
Can't say for spline shaft wear.
None of us can say empirically. Well, not unless you or some other benefactor wants to fund my destructive testing regime until I achieve a reasonable p-value to make the results significant.
Anyway for me, doing the rebuild at just under 15,000km's is a major pain in the arse, and I will spend a bit of time and money now in the hope of preventing such a short term failure occurring again. I really hope it was just a crap batch of original countershafts, but I'm now sufficiently paranoid to spend a bit more to try and mitigate the factors involved as much as possible. Personally I think the causes of spline wear there are rusting and repetitive shock loading, so this partially mitigates one of the contributing factors in my view.