rubber on engine mounts slows you down.

Discussion in 'Frame Mounted Engines' started by craig7, Jan 22, 2015.

  1. craig7

    craig7 New Member

    i know i'm new and all but i'd like to pass on some knowledge which i myself have just learned and experienced.

    i first built my motorized bike about 3 weeks ago, but it needed a few electrical bits before it would run, once i got it running i decided to put rubber on the engine mounts. it made sense at the time as i'm sure it does to many of you. the rubber should just absorb some of the vibrations right? not at all and i've experienced it.

    with the rubber on my engine mounts (about 3/16" thick strips and cut up tires) i couldn't get past 37km/h(police radar sign verified) without serious (painful) vibration and noise from the engine. also the rubber made the engine a bit loose, i'd constantly be adjusting the chain because the engine would be moving a little to the left side of the bike every time i drop the clutch to start it.

    so i just took out the rubber. engine is definitely secure now, i can't wiggle it from side to side. vibration actually feels a noticeable bit better, or at least there is no vibration stopping me from going faster like there used to be. i think it accelerates slightly faster, but it definitely has a high top speed now, i was able to really give it some throttle now and it took it just fine and kept up with traffic in a 50km/h zone without holding anyone back much.

    the difference was honestly night and day. my engine is revving higher and running better without rubber on my mounts, it's that simple. yaay! take this knowledge and move forward with your bikes. dont ask me why it is but it is and it works. onto the real mods now i suppose

  2. butre

    butre Well-Known Member

    yep, I ignore the opinions of anyone who advocates rubber mounts on a bicycle
  3. HeadSmess

    HeadSmess Well-Known Member

    most people on here are all the happier to give a thorough flaming to all rubber clamping advocates as anyone with either experience or common sense knows rubber is a flaming silly thing to use!

    theres a few explanations to the issue, but basically its this...

    its a two mount system, so the engine is only marginally held at best.

    to prevent engine moving under load requires the clamps to almost squash the tubes.

    inserting rubber here just either compresses the rubber past its elastic limit, or requires one to leave the clamps loose, which then lets the engine move, and or the bolts to come loose, whichever comes first :)

    this means the engine is free to do what it wants, and what it wants to do most of all is vibrate!

    clamping directly to the tube does a few things... it secures the engine properly.
    it adds mass to the part of the engine that shouldnt move...the crankcase.
    the longer the tubes in a frame for a given diameter, the more vibration is reduced.

    ie, big long tubes are as stiff as shorter, skinnier tubes. so long skinny tubes are best.

    best frame for vibration ive had was a monster 24" shogun frame with skinny 1" tubes... worst has been a old school bmx/chopper.

    easy :)
    QGolden likes this.
  4. rodnmo

    rodnmo New Member

    I used the rubber insulator from car/truck exhaust pipe hangers, works really well for the vibration and when I had my motor rigid mounted my mounts broke off the frame, and since using the hanger rubber I've had no problems 77cc 4hp 1957 chainsaw motor.

    Attached Files:

  5. bluegoatwoods

    bluegoatwoods Well-Known Member

    "i know i'm new and all but i'd like to pass on some knowledge which i myself have just learned and experienced." --craig7

    There is nothing wrong at all with a newbie thinking, making observations, theorizing and sharing all of this with the community.

    It's good for you and it's good for us. Keep it coming.

    I've had some decent results using leather and a material that was like tough neoprene as a shim in the motor mounts. But in both cases, it wasn't superior to using nothing at all.

    And using the wrong material at the wrong tension will cause troubles like you've described.

    So I don't get excited about using any kind of buffer material. It doesn't seem to be of any use and it can be downright bad.
  6. joemaddog

    joemaddog New Member

    Seat post tube

    Has anybody tried filling the seat post with liquid rubber to reduce vibration?
    Tom from Rubicon likes this.
  7. butterbean

    butterbean Well-Known Member

    You are absolutely correct, craig7. Any motorized vehicle is transferring energy from the engine to the rear wheel. Using any resilient material to mount the engine will allow some of the energy to be transferred to the frame instead of the wheel. The vibrations you experienced were mostly due to that factor which most who use rubber mounts fail to consider. At least you noticed it and rectified it.
  8. Lead shotgun pellets, lead in various ways is used as active vibration dampening.
    In a sealed tube would pose no toxic threat.
  9. butre

    butre Well-Known Member

    I have a heavy steel cable rammed down my seat tube. it was a scrap cutting that I got for free and the junk that's free is the junk for me. I would rather use lead pellets.
  10. bcredneck

    bcredneck New Member

    take a cut of saw and cut the cable 1/4"at a time the little cable chunks would be like pellets
  11. Redneck,
    Post the results of your suggestion, curious minds need to know. :)
  12. Once my Sportsman Flyer build is complete...... (long pause ) I want to re-think and redo the engine mounting system used in my Huffy -Davidson (see my avatar ) rubber blocks to isolate the engine. Logically motor mounts as used in vehicles provide vibration isolation and dampen drivetrain torque relief. But empirical data from major MB builders and my own experience restoring vintage Harleys tells me that a hard mounted engine, firmly attached to the frame is if not perfect survived and were used for decades . I may have to make my own engine mount for the Huffy-Davidson as it gets reworked, I promise to give you all a comprehensive evaluation.
    Forgive my digression, I have no empirical data to verify rubber mounting an engine could slow or impair motive force. Might be rough on the drivetrain as I think I am experiencing
    Tom from Rubicon, WI
    Last edited: Jul 19, 2016
  13. bcredneck

    bcredneck New Member

    I'd rather stay light with my bike
    But you ever see the brace that connects the top and down tube on a motorcycle and creates a triangle right behind the head tube that's there to cancel out frame harmonics
    Tom from Rubicon likes this.
  14. jaguar

    jaguar Well-Known Member

    rubber allows the engine to vibrate more which can froth the fuel in the carburetor which affects how it runs

    best to balance the crank. Two 3/8" holes in the crank wheels will change night to day!
    john morrow and Tom from Rubicon like this.
  15. Great suggestion Jag,
    Do you put the crankshaft between centers ? I did a crankshaft turning job for a guy. It was a engine from a log splitter which was mated a hydraulic pump via tapered keyed shaft. My customers application was for a Go-Cart so I turned the out-put side to fit a centrifugal clutch. I spun the crankshaft to 1000 rpm for a fine finish with no imbalance. Do you weigh the con rod piston assy. and the remove the same from the counter balance?
    Jag., forgot to ask, 2 stroke or 4 stroke?
  16. Frame harmonics? Never would have given it consideration till you mentioned it. Learn something new every day and live a long happy life. Thanks.
  17. jaguar

    jaguar Well-Known Member

    Tom from Rubicon likes this.
  18. gary55

    gary55 Well-Known Member

    After all the various methods I have seen so far nothing has convinced me of anything other than solid mounting. I get a lot of guys saying the bike speeds up then dies down then speeds up then dies down. They think I'm crazy when I tell them it's loose motor mounts allowing the motor vibrate enough to create bubbles in the carb, but tighten them up and there believers. On the large frame mounts I double the frame where the mount attaches so the flat engine mount won't stress the frame where they connect.
  19. john morrow

    john morrow Member

    Jag. Has the best solution to stopping engine vibration. I drilled the 2 hole's at 3/8 and like magic vibration gone.
    zippinaround likes this.