Chain tension problem

exavid · Dec 31, 2009

I'm having problems getting the drive chain adjusted on my Grubee 49cc kit. When the bike wheel rotates the chain goes from about 1/2" free play to very tight every 180 degrees. I checked the driven sprocket and found the radial runout to be less than .003" with lateral runout of around .04". The only thing I can figure out is that I might have a bad sprocket. The chain rollers don't seem to seat in the bottom of the sprocket tooth gullets like they do on the pedal driven chain wheel on the other side. Anyone here have a similar experience and if you corrected it what was the cause?
This conversion has been a pain due to overly close crank arms to the engine which caused me to mount the engine a bit higher than I'd wanted to which resulted in the need for me to heat and bend the exhaust pipe a good bit to get the muffler to clear the frame and crank arms. I haven't worked on the muffler problem yet due to the chain difficulty. :-||

2door · Dec 31, 2009

You've checked the run out on the engine sprocket but have you checked the rear wheel sprocket. If it is not perfectly centered on the rear hub then you will see the problem you describe. Lift the bike so you can spin the rear wheel and watch for proper concentricity of the rear sprocket. That's probably where your problem is. Another place to look is the kit supplied chain. It is #415 and many of them have been found to have tight rollers, kinks and twists. Many of us replace it with a good quality #41 industrial chain available at Ace Hardware, Tractor Supply Co. and other industrial supply outlets. Let us know what you discover.
Tom

Venice Motor Bikes · Dec 31, 2009

Why wouldn't you look at YOUR REAR SPROCKET INSTALLATION first???? wtf?

*edit* Also.... you could've just bought a new wider crank for only $10 & solved all those clearance issues...

exavid · Jan 1, 2010

The rear sprocket is the driven sprocket the one on the engine is the driving sprocket. The rear sprocket is the one that is about .003" off center (radial runout) which is pretty insignificant. Likewise lateral or axial runout (wobble) is very small. As I posted above there doesn't seem to be any problem with centering. The tightening and slacking of the chain occurs with each turn of the rear wheel so it doesn't seem to be the chain, I can't see any stiff links and the chain looks good off the bike. It runs smoothly and no side curvature. The only suspicion I have right now is it doesn't look like the chain fits the sprocket well since the chain rollers don't seat in the bottom of the sprocket tooth gullets. Has anyone had any problem with the drive sprocket supplied with their engine kits?

Venice Motor Bikes · Jan 1, 2010

Yes, it seems that your rear sprocket is the problem...
If you want the easiest fix, buy a good sprocket & hub adapter from Jim @ creative engineering. His ad is on the side of this page.

HoughMade · Jan 1, 2010

Try grinding the corners off of the teeth first. If the run out numbers are as you say (when measured mounted on the bike????), I don't see how it can be the sprocket itself. You gave a significant clue when you mentioned the chain not seating on the sprocket. Grind those teeth and see if you can get the chain to seat with no interference.

Venice Motor Bikes · Jan 1, 2010

I was just wondering how you measured the sprocket to find out it is centered? From the center hole?, tips of the teeth??
Because I have seen many sprockets that have the center hole not quite centered, & on other sprockets the tips of the teeth are uneven!!

The way I mount the stock sprockets these days is to wrap the sprocket with 44 links of chain & then spin it to center it off the chain!!

& to be very honest with you, If you only have 1/2" of difference in play in your chain? You did a very good job!

exavid · Jan 1, 2010

I measured the radial runout with a dial gauge clamped to a bracket which was clamped onto the bike frame. A thin strip of aluminum cut from a pop can was attached to the tip of the dial gauge to allow the sprocket teeth to pass by the gauge without snagging on the sprocket tips. The dial gauge was also used for the lateral or wobble measurement. With the gauge in place I just turned the rear wheel and watched the variation on the gauge. The wheel bearings had just been cleaned, lubed and adjusted prior to the check to make sure there was no play in the wheel itself. I agree with the suggestion that the problem is a bad chain to sprocket fit in the sprocket teeth. It does look like the only real solution will be to get a new sprocket and/or chain.

wjliebhauser · Jan 1, 2010

My rear sprocket had teeth that were inconsistent. Luckily, even the install instructions suggested checking them and grinding them down. If you have access to a bench grinder and a file, you can make short work of this. Besides, I'm not sure the steel in that sprocket is exactly 'mil spec' and chain riding up on the teeth could bend or break a tooth, or throw the chain, under the right torque situation. At first I was muttering rude comments about Chinese quality control, but then I thought, 'for $120, whadya expect?!' If I want Honda or BMW motorcycle quality, I need to buy Honda or BMW motorcycle.

exavid · Jan 1, 2010

For the price of the kit I think the quality is quite acceptable. I could have bolted the engine on and had the bike running the next day but a couple of things slowed me down. Mainly the garage is pretty cold the past couple weeks so I only work on the kit an hour or so in the late afternoon when it gets a bit warmer in there. The other thing is I get kind persnickity about how things work. The two remaining things I have to do are to figure out and correct the cause of the chain tension varying and the other is a simple job of getting out the acetylene torch and reshaping the exhaust pipe above the muffler so it clears the bike frame and crank arm. I'm in no hurry, when the winter weather is nice I get the urge to take the Silverwing or Goldwing out to keep them limbered up.

Nashville Kat · Jan 1, 2010

Have you checked for a stiff link in the chain? Have you cut it recently? A stiff link might go around the back sprocket fine, but then stick out and hang up at the small engine sprocket where it's outta sight.

exavid · Jan 7, 2010

Well I ground some of the driven sprocket teeth and got the chain to run with pretty much the same amount of slack all the way around. Yesterday I cut the header pipe on the muffler and spliced in 2" of tubing to get clearance from the bike frame and buttoned her up. I was really surprised how quickly the engine started up. It was a lot quieter than I expected. I made three separate rides on the bike in the afternoon but on the third ride apparently the idler pulley slipped and the resulting slack caused the chain to come off the sprocket and lock the wheel. I was making a short u-turn so the sudden unexpected lock up dumped me on my butt. Luckily not any real damage. So tomorrow I'll modify that idler so it can't move. Discounting my inelegant dismount it was a lot of fun.

wjliebhauser · Jan 7, 2010

Did the idler rotate inwards towards the wheel? That is pretty classic. These little engines vibrate like the dickens. Things need to be kept TIGHT-TIGHT-TIGHT. Good lock washers, lock nuts, and Loc-Tite adhesive get to be our serious friends. Glad you weren't hurt and had fun!

I highly recommend a spring-loaded self-adjusting tensioner. There are after-market ones on the web, or you can easily make your own. Just a thought. Good Luck!

exavid · Jan 8, 2010

What happened is the idler slid downward. It was about as tight as I could get it but it looks like there is only a small shoulder (ala carraige bolt) that keeps it from turning when your tighten it. I pulled the thing apart and cut a slot in it's head so I can hold it without it's turning. I do like the idea of a spring loaded idler, I might just modify this one or just build my own. I'd prefer to have an idler sprocket any way that would be better than the pulley. It would probably need to be on a welded mount on the bike for alignment reasons but would be more accurate. Which ever way I go on this it'll be better than the original, I don't like to get off the bike that fast.

BarelyAWake · Jan 8, 2010

Hey there exavid - I dunno ifn you've seen it, but there's a buncha good tensioner/idler ideas throughout this thread;

http://motorbicycling.com/f11/chain-tensioner-suggestions-11815.html

exavid · Jan 8, 2010

Getting things right now. I revamped the chain setup and now have the chain from the engine to the rear sprocket direct with no idler. Much better, quieter and a lot easier to pedal too. The chain just touches the bike frame so I'll braze a reinforcement on the outside of the frame in that area and heat and thin the frame tube at that point to get clearance. I got about five miles test riding today before the rain started. There's no rattle or other noise othe than the motor which has a very quiet exhaust and noticeable gear whine. So other than the slight change to the rear frame it's time to clean up the bike. I'm going to disassemble, sand and paint it along with some cleaning and polishing. I want to weld a lug to the frame and the muffler so I can get rid of the clamp type muffler bracket. I think that will look a bit cleaner. Once I get the cosmetics done this should be a fun little bike.

michaelgrav · Jan 8, 2010

measure your engine sproket distance from the frame. Your engine sproket should be 1/8th inch further outside than your lower sproket!

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Chain tension problem

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