Chain will NOT stay on

#1
Hello Everyone. I bought this old bike a few months ago, I haven't been able to ride it a whole lot. The problem I am having is that the chain keeps coming off the rear sprocket. Nothing I do seems to rectify this issue. I started with replacing the chain, since the other one broke. This did not do anything, as it came off again shortly after. Then I switched the sprocket on the rear. This also did not do anything. I sat on the bike and adjusted the chain tension, aswell as check the rear wheel alignment to no avail. I have no idea what to do at this point, this bike has been a nightmare so far.
 

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mustangfrank

Well-Known Member
#2
Hello Everyone. I bought this old bike a few months ago, I haven't been able to ride it a whole lot. The problem I am having is that the chain keeps coming off the rear sprocket. Nothing I do seems to rectify this issue. I started with replacing the chain, since the other one broke. This did not do anything, as it came off again shortly after. Then I switched the sprocket on the rear. This also did not do anything. I sat on the bike and adjusted the chain tension, aswell as check the rear wheel alignment to no avail. I have no idea what to do at this point, this bike has been a nightmare so far.
Two things to check: Alignment of the rear sprocket to the clutch sprocket, I usually check with the chain off and a cheapo magnetic mount laser cube I have aligning/eyeing it from the rear sprocket to the front, laser beam on top of the teeth tips. Trick is figuring out a mount for the laser...I get by with a bent up spring clamp, clamped to the rear of the rear sprocket. Just a little bit of engine "twist" in the mounting holes or rear axle adjustment with a long chain setup like yours can be the culprit. This method doesn't verify the rear wheel is perfectly "straight" in the frame but it at least checks the chain path is straight. I normally have the rear wheel centered before cinching down the engine bolts after checking with this method.

The other is to put the bike on a stand with the rear wheel elevated and roll the rear wheel slowly by hand watching for the chain to slack and tighten, checking for an out of round sprocket or hub mount, sometimes they're just out of round on the mounting bolts. I put a piece of masking tape on the frame down tube where the chain passes closely to see the chain movement easier. Trick is finding a spot on the top of the chain near the middle to watch.

Best I can put my methods into words, do a search if these don't help, the topic has come up recently and sort of often.
 
#3
You have a swingarm suspension and no jackshaft, that is the main problem. In that situation, the chain tension will vary drastically with suspension travel. You need a jackshaft positioned as close to the swingarm pivot as possible. Try to position the jackshaft so you can draw a straight line through the jackshaft, swingarm pivot and rear axle when the swingarm is at mid-travel. To see what is happening with your current setup, check your chain tension with the suspension compressed, extended, and at mid-travel.

Easier, but probably not as good, solution: remove that fixed roller and install a spring-loaded chain tensioner that has plenty of travel to compensate for the variable chain slack.

Here is a good, but pricey, example.
https://www.monstercraftsman.com/universal-bolt-on-mini-bike-tensioner-black-wheel/
https://www.monstercraftsman.com/universal-bolt-on-mini-bike-tensioner-green-wheel/
 
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mustangfrank

Well-Known Member
#4
You have a swingarm suspension and no jackshaft, that is the main problem. In that situation, the chain tension will vary drastically with suspension travel. You need a jackshaft positioned as close to the swingarm pivot as possible. Try to position the jackshaft so you can draw a straight line through the jackshaft, swingarm pivot and rear axle when the swingarm is at mid-travel. To see what is happening with your current setup, check your chain tension with the suspension compressed, extended, and at mid-travel.

Easier, but probably not as good, solution: remove that fixed roller and install a spring-loaded chain tensioner that has plenty of travel to compensate for the variable chain slack.

Here is a good, but pricey, example.
https://www.monstercraftsman.com/universal-bolt-on-mini-bike-tensioner-black-wheel/
https://www.monstercraftsman.com/universal-bolt-on-mini-bike-tensioner-green-wheel/
Good point, I think I was assuming it "worked before" the way it is, on second look that tensioner/mount doesn't look factory. 67 Rupps had a very similar setup and chain length but had a spring loaded tensioner.
 
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#5
If you can fit one, a CVT/torque converter could be used to get the sprocket closer to the swingarm pivot but it looks like you may not have room for one in that frame, looks pretty tight. Relocating the swingarn pivot to the rear of the tube would open things up a bit but maybe not enough, can't tell from pictures. Nice bike, looks like a Puma/Gemini/Keystone/Deltek, made in Taiwan and constructed better than most popular USA minibikes (I'll probably get flamed for that but it's true, I have a '69 Rupp Roadster to compare it to, the Taiwanese frame is better made). I'd like to have one of those.
 
#6
If you can fit one, a CVT/torque converter could be used to get the sprocket closer to the swingarm pivot but it looks like you may not have room for one in that frame, looks pretty tight. Relocating the swingarn pivot to the rear of the tube would open things up a bit but maybe not enough, can't tell from pictures. Nice bike, looks like a Puma/Gemini/Keystone/Deltek, made in Taiwan and constructed better than most popular USA minibikes (I'll probably get flamed for that but it's true, I have a '69 Rupp Roadster to compare it to, the Taiwanese frame is better made). I'd like to have one of those.

Thanks! Im gonna give the spring tensioner a try. I am hoping that will help solve the problem. I don't think a jackshaft would fit with the honda engine that is currently on the bike.
 

desert rat

Well-Known Member
#8
One more thing that will help is to get the roller closer to the rear sprocket. You have a lot of chain run south of the roller right now so it is really only protecting the frame not controlling the chain. That is a really long chain run on the top side too so it may be jumping the clutch first then jumping the rear. Just my $0.02.
 
#9
One more thing that will help is to get the roller closer to the rear sprocket. You have a lot of chain run south of the roller right now so it is really only protecting the frame not controlling the chain. That is a really long chain run on the top side too so it may be jumping the clutch first then jumping the rear. Just my $0.02.
One more thing that will help is to get the roller closer to the rear sprocket. You have a lot of chain run south of the roller right now so it is really only protecting the frame not controlling the chain. That is a really long chain run on the top side too so it may be jumping the clutch first then jumping the rear. Just my $0.02.

Im just wondering where I would mount the roller to bring it closer to the rear.
 

desert rat

Well-Known Member
#13
That is shock linkage, looks like there is a small roller at the pivot to prevent the chain from rubbing the link.
Look all the way to the rear sprocket it has a chain guide. The chain can not come off the rear no matter how loose it gets. My 250 "works bike" probably would have an 8" slack droop in the chain at full compression if it was not set up right (I'm 5'9"and the seat is in my arm pit when I'm standing next to it) and I have never lost a chain.
 
#14
Im just wondering where I would mount the roller to bring it closer to the rear.
I think it would be most effective near the middle of the chain run. Your swingarm makes things more complicated than installing one on a hardtail bike. The simplest thing would be to mount it to the engine plate with the roller between the clutch and swingarm pivot. If it's a swinging arm type of tensioner, as most are, it should be a trailing arm (relative to chain travel). I think that expensive Monstercraftsman tensioner would be nearly ideal for your application so I would suggest something similar. You could get creative and mount it to the bottom of the engine plate, placing the roller where the fixed roller is, or mount it to the swingarm, but either of those would be more complicated.

I just now got an idea that should have been my first thought. The existing fixed roller mount could be used as the pivot point of a spring-loaded tensioner. Post a straight-on side-shot I can study, see what I can come up with.
 
#15
Look all the way to the rear sprocket it has a chain guide. The chain can not come off the rear no matter how loose it gets. My 250 "works bike" probably would have an 8" slack droop in the chain at full compression if it was not set up right (I'm 5'9"and the seat is in my arm pit when I'm standing next to it) and I have never lost a chain.
Gotcha, I thought you were still on the subject of a tensioner. There are also disc-type sprocket-mounted chain guides, I've seen them on go-kart / minibike parts sites.
 
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