Motor Mount Issues

Discussion in '2-Stroke Engines' started by Drunkskunk, Feb 10, 2010.

  1. Drunkskunk

    Drunkskunk Member

    I have an old Columbia, and a new Spookytooth HT motor. It sits well in the frame, and clears everything except the sprocket. I can easily move the motor either up or forward by lengthening either the front or rear mount. either is just as easy to do.

    But whats better for the frame? A longer front mount, or a longer rear mount?

  2. retromike3

    retromike3 Member

    longer mount

    I moved the rear mount by adding washers to the bolts between the motor and the seat tube. I did that so I could get the chain length correct. I did have problems but it was not caused by that modification. The problem I had was that my seat tube was too big and I filed my rear mount wider so it would fit.

    It broke and then my mounting bolts broke so I drilled and tapped two new ones and then I built a new mount out of square steel tubing. I have put quite a few miles on this current set up and it seems to work well.

    Mike the bike guy

    Attached Files:

  3. Add a longer front mount.
  4. AussieSteve

    AussieSteve Active Member

    Yep, what cabinfever1977 said, from a mechanical/engineering point of view. The highest stresses will be on the rear mount, so keep it more solid. The front mount only really stops side-to-side movement and doesn't need to be as sturdy. (The chain tends to pull the engine backward, so stay up against the frame at the back if possible.)

    ... Steve
  5. Scotchmo

    Scotchmo Member

    That may be true for resisting drive chain loads. For vibration resistance, the front mount is more important. The front downtube is the stiffest member of a bicycle frame. It also has the shorted coupling distance to the reciprocating and rotating components of the engine.

    The HT engine was not designed to use a front spacer. It comes with a rear spacer. If you can attach the front mount directly to the downtube, do it. Lengthen the rear spacer to make up the difference. I reasoned that the front mount was so important that I went to the effort to replace the dowtube of my Schwinn Delmar with one of the correct diameter and angle.

    Sometimes it is easier to add a spacer, plate, or u-bolt assembly to the front. But then you have to live with the consequences. They may be minor or you may end up having your front mount or exhaust studs break from vibration. It can work fine, but it is not the best way.

    Attached Files:

  6. actually the engine tries to move to the left,so add a muffler strap to help hold the engine to the frame.And that rear spacer also fits the front. I have 1,000 miles and nothing broke and have been riding WOT for a year now.
  7. Scotchmo

    Scotchmo Member

    How can the engine move to the left? Look at the picture I posted. The engine nests onto the front downtube. It would have to break off a large piece of the engine mount and casing. I don’t see how a properly mounted engine can move to the left, so no need for an additional strap to hold the engine.

    I guess if you added a spacer to the front mount, it could allow the engine to move to the left. That is another reason for not putting a spacer on the front mount. Put the spacer on the back mount only if at all possible.
  8. AussieSteve

    AussieSteve Active Member

    Scotchmo, for many, if not most of us here, a nice snug fit like yours is impossible. Our downtube is too thick. In my case, it's thicker than the stock muffler. An adaptor, (spacer), of some description must be used. Then, the engine can very easily pivot to the left.

    ... Steve
    Last edited by a moderator: Dec 15, 2015
  9. Scotchmo

    Scotchmo Member

    I understand. The original downtube on my bike was also too large.
  10. AussieSteve

    AussieSteve Active Member

    You'd absolutely hate my front mount, Scotchmo. I drilled my frame originally, so when I fitted my shift kit last week, I had to make a slotted, spaced mount, to allow for upward adjustment of the engine along the seat tube to tension the RHS chain. It's a 2" square piece of 1/8" steel, with two holes for the original bolts and a slot for my long one. (An 8mm x 90mm high-tensile bolt.) To adjust, I back off the mounting nut, do the seat tube adjustment, then add washers, allow the bolt to slide in the slot and re-tighten. There are 3 layers of thin rubber in the equation and small 1/8" curved steel reinforcing plates each side of the alloy downtube, but still...
    I won't mention how hard it is to add washers right now. I had planned to use a wedge-shaped piece of nylon from a cutting board, with a slot, so that I could slide it in to take up slack, but there wasn't room.
    One advantage, though - it's in exactly the right place for my muffler strap, so when the engine mount is tight, I can slide on the strap with a pair of washers and tighten another nut, with Permatex Blue to lock the thread.

    I just uploaded some pics of the mount in the my 'My Rides' album on my profile page.
    Also added pics of the whole bike with the new shift kit, 3L tank, centre-stand for kick-starting, etc a few days ago.

    ... Steve

    Here's one shot of the mount:-


    Attached Files:

    Last edited: Feb 11, 2010
  11. When accelerating all the engines are forced to the left by the force of the chain and many have had there studs or mounts break(these are also the people who didn't use a muffler strap).My engine has a pullstart and centrifugel clutch and is atleast 9"s wide. So to clear the peddals i had to raise my engine higher and so i mounted it to the rear first then made a custom front mount. But even with a regular engine the frame may be to big for the front mounting studs and so to get the engine seated i would mount it to the rear first then add front mount and strap. But if you can get it mounted good with the front mount then just add a extra spacer on the rear mount.