Best engine and gearing for a very heavy bike?

Discussion in 'Spare Parts, Tools & Product Developement' started by SvdSinner, Sep 28, 2009.

  1. SvdSinner

    SvdSinner New Member

    I'm in the process of putting together a rather large bike around a Micargi Triplet Tandem, a trail-a-bike and a double child trailer.

    The project is part of a crazy dream of mine where my entire family of 6 is going to ride across Iowa in RAGBRAI 2010. (450-500miles over 7 days)

    I want to use a bike motor in a very different way than most people. I plan on not using it very often, and mainly only if the kids tire out on a big hill or during the last portion of the day's ride. (A $200 motor is a great insurance plan against a family rebellion) Also, in Iowa, I have to keep the maximum attainable top speed on level ground to < 25Mph to operate under the slow moving vehicle laws. (Since I will carry more than 1 person, I will fall into slow moving vehicle laws, rather than bicycle or moped laws.)

    Additionally, long term reliability is not a concern. The whole thing will probably not be used much after the big RAGBRAI event is successfully completed. For this reason, I'd prefer to go as cheap as reasonable since this is a one shot wonder vehicle.

    So, the questions for this thread are:
    1) Are there any particular engines I should look into knowing that it will be pushing a 600# vehicle weight (Bike + 6 riders)?
    2) Does anyone know the red-line RPMs on these little bike motors? I might need that to document that the engine and gearing can never go past 25Mph in normal operation. (I'd prefer a cruising speed of 16-20mph)
    3) How big of a sprocket should I be using for something this heavy? (From a pulling standpoint? I realize the max speed requirement might force me to a higher gear than pulling requirements)
    4) Is there anything specific I need to consider if I want to spend 80-90% of the time pedaling with the motor off?

  2. loquin

    loquin Active Member

    You're looking at a lot of weight...

    I was thinking about this last night, and your best bet may be a Mitsubishi TLE43 with a Staton friction drive and a 7/8 inch roller. The motor sits to the side of the tire, instead of on top if it, as a rack mount chain drive does. You could also get a second u-bracket, and shift the motor back more than standard, which should allow it to clear everything. The motor would be angled up somewhat, so, you might think about tossing out the RS35 as a possible engine choice.)

    (note - the GEBE approach would sit even lower, but, it's also substantially more cost than the Staton friction drive, and more 'finicky' in installation.)

    (also note - the BMP kit would cost about $100 less than the staton, but, the smallest drive roller available with it is a 1 inch roller)

    The only question will be the trail-a-bike connection arm, but, if you can mount it a little higher on the seat post, it may fit. This approach is certainly the simplest and is very reliable.

    The redline RPM varies by motor - the Honda 35cc 4-stroke cat get up to 7000 rpm or so; the TLE43 up to 7500-8000 (stock)

    7500RPM with a 7/8 inch roller equates to about 19.5 MPH; the 4-stroke (@7000 RPM) about 18 MPH.

    While the Mitsubishi motor would give you the best power (with the small profile) in a friction drive, you could also go with the Honda or RobinsSuburu 35cc 4 stroke class - they're quieter, have a lower pitch, and produce much lower emissions. (plus, you don't have to worry about pre-mixing the gas.)

    When you're not going to be using the motor, turn it off and lift it up a bit so that the roller isn't in contact with the tire. (this eliminates the bearing and roller friction drag.)
    Last edited: Oct 3, 2009
  3. Were I you, I would consider a frame mounted, 50cc, Chinese Honda replica engine with a clutch hold button on the clutch lever. The sprocket that comes with the kit will be too high geared for your needs, so when you order the kit see if they can substitute the largest sprocket available to gear it down. You need the most torque you can get for the money and a larger 4 stroke engine will deliver that. The hold down button on the clutch lever will give you hands free operation untill you need the engine. The Harbor Freight (etc) Honda replicas are fairly inexpensive and are well made enough to serve your needs. A frame mount will allow you to have any trailer hook up made for bicycles that attaches to the seat post with no clearance proplems. An even bigger engine would be better, but will be difficult to frame mount and may exceed what is allowed under the law. A variable speed transmision might also help, but the cost and extra weight might be a drawback. Since this setup will be putting the rear wheel and spokes under a very heavy load, both with the weight carried and the torque transferred through the spokes, I would get the strongest wheel with the heaviest American made SS spokes I could possibly afford.
    Last edited: Oct 3, 2009
  4. loquin

    loquin Active Member

    IMO, the problem with the frame mount 4 stroke in this case would be added complexity of getting it mounted on the tandem bike - either a very long run with clutch/throttle cables, or a very long chain run, the width of the motor interfering with peddling (unless you get the crank replaced,) the issues with getting the chain aligned/the rag-joint sprocket, and the fact that unless you remove the chain, there will be some additional drag at all times. Since the stated goal is to have a unit that will be used rarely, the simpler, the better. It doesn't get much simpler than a friction drive, and getting chains aligned and tensioned can be a finicky business...