Posted: Mon Aug 25, 2008 11:01 am Post subject: gearbox play
I've been trying this summer to get my primary drive tensioned properly, and have run into some difficulty. I've been running a belt drive for a couple years with no trouble. Last fall I noticed my final chain getting tight, so I would adjust things, and it would be okay for a while before tightening up. So then I bought a left-side gearbox adjuster and put that in thinking it would solve the problem, but it didn't. Now I'm wondering if the problem might be that the hole for the top gearbox mounting bolt is worn. That would explain why it would start out loose enough, but get tighter, even though the gearbox adjusters are keeping the bolt locked in place. So I'm curious if anyone else has experienced this, and can anything be done to fix it? Thanks for any info.
Joined: 10 Nov 2006 Posts: 416 Location: west cumbria
Posted: Tue Aug 26, 2008 8:54 am Post subject:
I usually find the final drive pulls the gearbox back and tightens the primary.
Non-the-less, try, with the top bolt just nipped, over tightening the primary chain then slacken it off to the correct tension and lastly fully tighten the top bolt. Now set the rear chain. All the backlash will be taken out of the hole preventing the gearbox moving forward.
If you are using both transmission adjusters to set the belt tension, then I don't see how the top bolt could move. The bottom mount possibly.
Is there a loose enough fit in the adjuster banjos that allows the transmission to twist in the frame? Does the belt run true? Is the belt too tight? It will get tighter as primary gets hot.
I understand the concept of bushing the mounts to get rid of slop but have trouble understanding how the dual tensioners could work. It doesn't make a lot of sense to me that you can twist the transmission when it is sandwiched between 2 plates and then bolted at 2 points. Especially the bottom mount which is not slotted and even more so if you've bushed the holes to remove slop. I would think the whole assembly would try to pull itself straight when you tighten everything up, and if you did manage to hold it skewed with the adjusters there would not be complete contact between the faces of the gearbox and mounting plates.
I think maybe the dual tensioners are working fine, and the top bolt isn't moving. But if the hole is a little oversize, then the gearbox could still move some front to back. To get my primary belt adjusted, I took a ride with it loose. People always say make sure your primary isn't too tight or you'll frag your gearbox bearings, so I try to err on the side of looser. But there's such a thing as too loose, and you can tell when a belt drive is too loose, because it will flap like crazy, or even skip. So I rode, tightened it up, repeated until I got it just tight enough that it didn't flap. Then I tightened up all the adjusters and the mounting bolts. But after my next long ride, I check the belt and it's pretty tight again. So I loosen it up a tiny bit, and of course, it starts flapping. Pretty frustrating. I think I'm going to tighten it up to where it doesn't flap, and call it good enough. If it seems too tight after that, I'll just leave it and hope for the best.
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