Chain keeps falling off drive sprocket???

Discussion in 'Frame Mounted Engines' started by RalliartRsX, Aug 22, 2008.

  1. RalliartRsX

    RalliartRsX New Member

    Well, this is just frustrating. When I got the bike, the tensioner was absolutely shot so I decided to go ahead and and make my own tensioner.

    1st a pulley was used from a rope pulley setup but was tightened too tight and basically destroyed the pulley

    2nd & 3rd were actually ideas I researched from this site. Using one of the cogs from the derailer of a regular multi speed bike. Well, the supposidly grade 8 bolt snapped when the chain gained tension when the clutch was let out to start the bike. The 3rd wheel just basically snapped in half!!!

    Now, hopefully on my finally iteration, a spring loaded tensioner I think its all sorted..............Or is it??

    Well, I finally put everything together and now the chain refuses to stay on the drive sprocket inside the housing. I think that after the many times the chain has either been caught in the rear sprocket and it jamming in the drive sprocket housing from all the broken tensioners that its finally given up the ghost (but I have no idea on how to check a chain to make sure its right)

    NOw it seems the top part of the chain coming from the rear sprocket to teh drive sprocket is sagging and I think it just doesnt have enough tension to keep it from moving around in the housing and getting caught up inside it.

    Any suggestions?? Pics below of how it looks:

    (P.S dont worry about the bottom part as I removed it from my tensioner. That is exactly how the top part of the chain is even when the tensioner is in place)


  2. Hmmmm.....On most installs the tensioner roller goes below the bottom of the chain nearest the bike's chainstay. (see pic).

    If you try to run your bike as it is in the pics you are just asking for trouble (jambed up chain or worse and it could cause an accident so please be careful).

    If you can set the tensioner up properly and post additional pics that would be great.

    Hope this helps you.


    Attached Files:

  3. HoughMade

    HoughMade Guest

    If your chain droops like that with the tensioner in place, that is the problem. When setting up the tensioner, you either have to have the rear wheel off the ground or the clutch in so either the front or rear sprocket turns freely. Otherwise, you can put as much tension on the bottom as you want and the top will still droop.
  4. Oh....If you "tension up" the chain the way you have it set up, it would tend to "lift" the chain off the engine drive sprocket which would tend to aggravate the situation more (i.e. make the chain want to come off since you are lifting it up and off the engine sprocket).
  5. HoughMade

    HoughMade Guest

    I guess I was assuming that normally the OP had the chain over the tensioner wheel on the bottom like most of us- perhaps he could clarify by showing how he normally sets it up.
  6. Skyliner70cc

    Skyliner70cc Active Member

    That isn't a stock chain is it? it looks smaller/thinner than OEM change. I have found that the bike chains are more likely to jump if sprocket isn't perfectly round, if teeth on sprocket have too much chrome on them, and if things aren't just perfectly lined up. That's why I use the stock chain, never failed and works great after it stretches a bunch when new.
  7. graucho

    graucho Active Member

    Absolutely! All of my bikes and mini choppers had and have the roller on
    the bottom chain return as in andyinchville's photo. If the bottom chain
    section gets to close to the top chain..... remove a link and readjust.
  8. HoughMade

    HoughMade Guest

    Good point- one reason i use a #41 is because of the internal width, there is more of a fudge factor for sprocket alignment...and I have never, ever, had a chain problem.
  9. RalliartRsX

    RalliartRsX New Member

    Sorry for the length in getting all updated. Out of town for the weekend

    Anyways, on to the update

    The chain is not stock but a bmx chain. Specs I think are................ 1/4inch #41 chain?? Not too certain.

    As far as the top slack, I ended up moving the rear wheel down maybe 1/8th of an inch to tighten up the slack, which it did on the top part of the chain. Went out for a test and the throttle stucks................Threw me (or more like I bailed) off as I grabbed a ton of front brake.............:roll:

    Walked back to the bike and luckily the only thing bent was the headlight.

    Went and filled her up and she was good to go for a few miles. Then, the chain started to jump the drive sprocket again..............:sigh:

    So, I take her back home to check alignment of rear wheel and everything seems to be as they were when I first received the bike. Strange. It has gotten to the point now where the chain jumps the drive sprocket when its not even under power!!! I will just be coasting down the hill to get her started and it jumps the sprocket!!

    A few guesses

    1) The chain has bunched up/ tangled soo much it has just stretched from all the stress hence why the top part is sagging and not 100 percent lined up

    2) Drive sprocket bent (I sure as **** hope not!!)

    3) Rear free wheel sprocket is bent (I sure as **** hope not # 2!!)

    I will get pics of everything sorted when I get home this evening with tensioner and all, but the top part of the chain still looks the same

    Thanks for yall's patience

  10. Skyliner70cc

    Skyliner70cc Active Member

    I had same problem once on stock chain before I realized the need to knock the chrome off the rear sprocket. I now walk each bike to make sure sprocket and chain work well together.
  11. RalliartRsX

    RalliartRsX New Member

    Just took rear wheel apart and taking it into work tomorrow for proper inspection with my friend. I be taking a pic of the drive sprocket to show here and see what yall think of it. I will also take pics of the sprocket/wheel to see what yall think of that

    Also, on close inspection, the chain seems to have a master link!! Will be taking it tomorrow to get it shortened after the wheel is checked.

    More detailed pics up tomorrow!! Thanks for all the response
  12. spad4me

    spad4me Member

    Chain Line is important, is your Engine straight and tightly attached to the frame.
    Use a wire pointer attached to the rear seat stay or chain stay to see if your rear sprocket is wobbly. I set the wire pointer end to be as close as possible to the rear sprocket and turn the wheel to check for wobble. Tighten the bolt to compress the rag joint .
    I was able to stretch the ****** out of a bmx chain. I was actually ripping the links apart. ( trying a cvt on a happy time engine) too much torque.
  13. RalliartRsX

    RalliartRsX New Member


    Rear sprocket is bent. It has a slight kickout and I think thats what making the chain fall off the drive sprocket (which kinda makes sense since the chain would just fall off just by pedaling before even starting the thing) :rolleyes: Thats what I get for trying to be genius ;)

    Anyways, new sprocket on the way and hope, just hope, I will have this ready for the first day of classes next tuesday.

    Keep your fingers crossed folks. I really wanna ride this thing!!
  14. Huntington

    Huntington Member

    I didn't see if you fixed your chain tensioner yet but like everyone already said, your set up is incorrect. regardless of your rear wheel being true or bent, your chain will come off if your tensioner is pushing it up.
  15. graucho

    graucho Active Member

    Yup, have the roller under the bottom chain. See photo below.

    Attached Files:

  16. Huntington

    Huntington Member

    And here is another

    Attached Files: