As it was, I needed to re-adjust the belt tension and replace the lower belt guard (using some lock-tight blue on the replacement screw). The belt was not visibly damaged, the pulley had minimal damage -- something like two stone nicks. The belt tension, however, was pulled from the 6mm specified slack to around 80mm! I think I'm also going to get some of the aftermarket stainless mesh grills to keep stones out of the radiator cowl, and pay more attention to rattles.
On further investigation, I found there are some engineering shortcomings in that lower belt guard. It was designed such that one little screw could loosen from vibration and the entire guard will drop off its mounts. There are no sheet metal tabs against the hex flats, back-up cotter pin, clip, safety wire, or even a second screw through a closed slot.
If the belt guard did drop off, it would pass between the belt and pulley with an inverted-U channel. That would virtually guarantee that the guard would get trapped and pinched between the pulley and belt. This would, at the very least, either lock up the rear wheel with damage to the $99 guard, the $239 belt, and the $99 pulley (plus labor) or snap the belt and damage the pulley. Naturally, the possible catastrophic result is death or serious injury to the rider along with totaling the $17K+ bike.
It seems to me that a good Design FMEA should have caught this likely scenario from one little screw in the suspension/driveline loosening, and the risk could have been mitigated with secondary locks or travel limits.
Additionally, a redesign of the lower guard to wrap as a U channel from the outside of the belt would have allowed the guard to drop off. Or, alternatively, the channel riveted seam from metal to plastic could be shifted to have the top of the channel as plastic that may shear off without locking the rear wheel. In that alternative, it would need to be tested to make sure the wheel would not lock.
This entry was submitted by David T. Humphrey and edited by Rob Spiegel.
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William K. - I agree with your comments, "redundant supports and safety wires on the support bolts would be the best way."
For the record, this belt guard mount has been trouble-free, since I attached the guard screws with lock-tite blue AND the back mount also with safety wire.
The screws are small enough diameter that I am VERY careful on torque. The front screw is a button head, so I was not able to cross-drill the head for safety wire (or I would). I didn't replace that button head with a SHCS, because I didn't want a form that could rub or catch on the belt in any condition.
I also monitor this component (and the top belt guard) very closely now that I am more aware of their vunerability.
Thanks for the information about the problem of stuff getting caught in the belt. That is one of the things that those touting belt drive never mention. I have not had a rock get caught up in a chain, although I did have a chain come off one time and wrap between the big sprocket and the spokes. I came to a very squirrly stop with the rear wheel locked, fortunately I was just starting up when it happened. I learned how critical chain tension is, and a bit about chain paths. That bike got a back-side chain idler and always had correct belt tension from there on for the next 20 years.
But still it seems that there should be a way to avoid that kind of hazard with the belt guard.Safety wires on the bolt is a good approach, but redundant supports and safety wires on the support bolts would be the best way.
William K. "The fundamental design flaw was putting anything inside the belt loop. If there really was a need for the guard it would be covering the loop so as to prevent anything from getting caught in the drive pulley or getting chewed by the belt teeth. When a correctly implemented belt guard falls onto the belt it will make a lot of noise and wear the belt, but never result in a loss of control."
I agree with you in principle; however, the one other design consideration that I have become aware of is that rocks or sticks can be deflected by the rear tire and land in that belt loop causing a lock-up or damage to the belt and rear pulley. This guard was designed to reduce this possibility without having the guard itself creating other problems of making frequent belt inspection impossible without partial disassembly. Design is indeed a trade-off of sometimes conflicting considerations.
I still feel that it is important to have better retention of this guard to avoid potentially lethal consequences. Mine is now safety wired.
You are certainly correct about the "social stigma" associated with drive shafts.
Any manufacturer would love to have that kind of loyalty to their product.
I suppose that it would be possible to come up with a rugged enough type of U-joint, but it would probably be heavier and I know that it would cost more, and besides that it would make field changing the driven end ratio awhole lot more complex, at best.
Thanks, William. Sounds like we're not going to see any dirt bikes with drive shafts soon. I would imagine there is also a social stigma against drive shafts among some bike groups.
Probably a bike without a chain could be safer, but there are other things to be considered. Some motorcycles wind up being used in rather harsh terrain and in a very abusive (to the bike) manner. The result is that a bike with a drive shaft would be damaged to the point of being disabled and needing shop repair in a fairly short time, since the universal joint is a relatively fragile part, and does not generally like large changes in angle. Toothed belts are much better in that area, and are often used, but a damaged belt requires replacement, since they are not generally repairable. Chains and sprockets do wear more than belts and pulleys, but chains are field repairable, and also replacable with less effort than a normal belt replacement requires.
So the application requires consideration. In addition, a belt will chew up a leg almost as much as a chain will, if one contacts the side of either one. At least that is what I have read, but not experienced.
It is very exciting to have a rear wheel lock up. It happened to me one time just coming foo the line, when "the engine broke".
The fundamental design flaw was putting anything inside the belt loop. If there really was a need for the guard it would be covering the loop so as to prevent anything from getting caught in the drive pulley or getting chewed by the belt teeth. When a correctly implemented belt guard falls onto the belt it will make a lot of noise and wear the belt, but never result in a loss of control.
The bike that I built had an idler that held the chainin line with the engine-end sprocket at the same time that it kept the chain adjusted correctly. It was similar in shape to a pulley, except that the edges did not taper very much. The result was that as the chain would stretch, which they certainly do, the slack was easy to adjust out, and when the limits of the adjuster were reached the chain was due to be replaced anyway. PLus, it looked good.
Of course, there are those who claim that "real bikes have chains", but that would be part of a different discussion.
Harley, Triumph and other potato-potato/chug-chug bikes should be using these proven, decades old technologies, but they're not. Until then, keep your wrenches handy, and tighten everything biweekly.
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