Jackshaft Jackshaft Kit U-bolts

Discussion in 'Transmission / Drivetrain' started by Fabian, Sep 10, 2009.

  1. Fabian

    Fabian Well-Known Member


    I have a Jackshaft kit installed on my bike - i love it; it works great and after having gears to play with, i'll never go back to a fixed drive system.

    There is only one small problem to be addressed in the attachment of the Jackshaft kit to a bicycle frame and it's a saftey related issue.

    Basically you can end up with very serious injuries if your down tube cracks and breaks open due to metal fatigue from a crush point/pinch point or just plain cracks from excessive pressure being applied on the U-bolt clamp.

    When purchasing my 2-stroke engine kit, the manager advised that i use thick rubber under the U-bolts to prevent the frame cracking, particularly thin walled aluminium frames.
    One of his customers over tightened the down tube U-bolt, cracking the frame and then rode the bike down the road.
    A short time later the downtube seperated in half and the bike opened up beneath him, throwing him, head first into the bitumen.
    He doesn't have a face left or any front teeth and is missing a good portion of his nose.

    After hearing that story, i decided to create a simple fix for the problem so that scenario would not happen to me from a metal fatigue issue.
    In the photos attached you will see that i have used 1mm sheet steel, formed around the lower downtube U-bolt and the face of the smaller seat tube U-bolt carrier.
    The seat tube U-bolts can have the original plate steel Chinese engine mount modified so as to be a backing plate for the U-bolt, doing a similar job to forming sheet steel around the curved section of the U-bolt; saving you time.

    For 1 & 7/8 U-bolt clamps, i used a sectional size of 60mm x 25mm on both the U-bolt and the backing plate.
    You can't go all the way around and cover 180 degrees on both sides as the straight part of the U-bolt will then crush the plate steel into the frame tube on the sides.

    For the smaller seat tube clamp i used a sectional size of 40mm x 25mm for the backing plate.

    Once sheet steel is formed to the curvature off the U-bolts and backing plates, you need to secure them so they sit flat and square and evenly spaced along the radius.
    I don't have a welder so i just used superglue to initally hold the plates in place (the surfaces being glued must be absolutely clean, spotlessly clean for the glue to hold).
    Then i placed the clamps on the frame and tightened them up, so forcing the plates to make a firm connection with the U-bolts and backing plates.
    Once the superglue dried (that stuff is incredibly stong) i removed the plates & U-bolts and applied 4 layers of gaffa tape to provide a level of smoothing for any subtle differences in the curvature and also to increase the friction level between frame surface and u-bolt assembly.
    The glue is only holding the plates in place, it adds no structural integrity.

    This method has big advantages as the chain tension clamps can easily be adjusted as the clamps don't dig into any tape wrapped around the frame tubes.
    I found that prior to making the backing plate modification, the two sections of the plate simply forced their way through the gaffa tape wrapped around the frame and started cutting into the aluminium.
    When trying to adjust the motor to get correct chain tension, the plates wanted to remain stuck in the groves they cut into the gaffa tape.

    Someone with good welding skills should make up a batch of these U-bolts and approach manufacturers of Jackshaft kits to have them included in the package, on the basis of saftey.

    Hope this post provides someone with (at very least) peace of mind when installing a jackshaft kit.


    Attached Files:

  2. Al.Fisherman

    Al.Fisherman Active Member

    Where was the kit purchased from.... I'm looking for all ways to mount these engines. Depending on the application, one way may work better then another.
  3. Fabian

    Fabian Well-Known Member

    which kit in particular?

    The engine kit or the jackshaft kit.

    If it's the Jackshaft kit, it was purchased from SickBikeParts.
    The Jackshaft kit has turned out to be completely reliable.

    The only thing i had to do was stabilise the original pushbike chain that drives the rear cassette/cluster with the idler rollers supplied in the Chinese engine kit.
    I have put up a post about that and it's a simple modification that has nothing to do with the operation of the Jackshaft kit.

  4. Ghost0

    Ghost0 Guest

    Just for clarification:

    "Our Shifter kit uses the stock front mount, and the guy who cracked his frame and lost his face was not using our product and was not our customer".

    As usual you bring up some valid points. We do touch on this subject in the manual and especially caution when using aluminum frames. We do recommend using the stock strap from the engine kit but when it comes to safety you can never be too cautious.

    The functional issue here is that when you increase the surface area of the clamps they no longer offer as much clamp force and can allow the engine to slip on the seat tube putting too much pressure on the chain adjuster. It is a fine line there. Typically what I do is increase the support of the lower clamp because it does not have the added support of the seat post but leave the upper clamp unmodified so that enough clamp force can be applied.
  5. Fabian

    Fabian Well-Known Member

    Hi Ghost

    In essence, the Jackshaft kit does not pose any problem - i've mentioned that before.

    The greatest potential for frame cracking comes with the original Chinese engine kit on the down tube (tube from headstock to bottom bracket) and in the case of large diameter (approx 2 inch) down tubes, as is the case in most bicycle frames today.

    Those tubes are large in diameter but paper thin in sectional thickness and crush with the slightest of ease.
    I should take a photo of the next door neighbour's frame when he tried installing a chinese engine kit and would not listen to me when cautioning him against using a U-bolt directly on the frame.
    Needless to say, he fastened the U-bolt and it cut straight through the frame - quote "just wanted to make sure it was done up nice and tight".
    What's worse is he intended to cut open a beer can and slide it around the frame to hold epoxy resin in place as it dried - this is just sheer stupidity as he doesn't want to buy a new frame; his bike is kind-of pricey.

    A load spreading modification is absolutely imperitive on the lower downtube.