Using the existing drivetrain and shift mechanism?

Discussion in 'Frame Mounted Engines' started by svejkovat, May 16, 2009.

  1. svejkovat

    svejkovat New Member

    New here. I posted in the welcome section. Just wanted to get a lead on any discussion here of the following..

    I'd like to connect a motor via a freewheeling sproket on left side of the pedal crank. It would be a very low torque (26cc Ryobi) 4 cycle. Just musing over the options in my head, this seems like it would be a pretty foolproof way to make use of the existing gear ratios and derailleurs of my present mountain bike.

    What would be stellar would be a freewheel single sprocket that is actually dedicated to this task. If not available, I might try to build the thing. But basically it would have to engage the shaft in reverse of the standard right hand mounted freewheel, and it would have some simple cable actuated clutch within to start the motor.

    Please tell me that this is actually an off-the-shelf item!
    Last edited: May 16, 2009

  2. svejkovat

    svejkovat New Member

    Ok, you can tell I'm new at this. Upon second thought, I realized that I'd need to have some centrifugal engagement at the motor unless I incorporated a second hand-clutch for idling (more complication). With a crank mounted motor engagement, that screws the pooch for the centrifugal clutch. If I cable-engage the motor nearer the motor's shaft, that means cranking up all that gear/chain reduction when pedaling while the motor is not in use... not really very good. Hmmmmm.

    Hang it all! That's evidently why my Puch moped has the motor engagement mechanism inside the transmission. It would be much better placed at the back sprocket on the wheel. In that case, you could actally pedal the thing without having to spin the motor drive chain and half of the motor transmission gearing (truly exhausting and impractial).

    S'ppose I could stick with my original idea of the freewheel on the left side of the pedal crank, keep the stock centrifugal clutch on the trimmer shaft, and just use the pull cord! It'd be less useful than a simple hand lever on the bars, but pretty straightforward I guess.

    I just know this has been done. How did you do it? It's got to be simple, simple, simple. And I'm not going to put an air prop on the back!
    Last edited: May 16, 2009
  3. duivendyk

    duivendyk Guest

    Simple things don't necessarily work,with most engines you need around a 20 :1 reduction from engine crankshaft to rear wheel.That includes the speedup between the normal crank (1to3 or so) and it, so we're talking about a reduction of at least 20+ to 1 to that chain wheel on the left .Go figure that one,at least two reductions in tandem are reqd.
  4. svejkovat

    svejkovat New Member

    I was also reminded in another forum that any arrangement that makes use of the bikes transmission (powers the pedal crankset) would necessarily be very inefficent. The motor would have to be geared 'down' to to match a human cadence (which, obviously, is what bike transmissions seek to do) and since the ratio from crankset to rear hub is generally 'up' and therefore wasting most of the motor's potential through the transmission. Too bad.
    What basic options have been pursued for integrating the motor with pedal power? Looking through the photographs here, it looks like 90 percent or more are pretty straightforward fixed chain drive to the rear with some sort of clutch in the motor to engage, or pull start, or simply friction drive. Is anything more elegant discussed here?
  5. superbee

    superbee Member


    Have you not seen the kits that have a shaft with two gears on each side. Chain from engine to that gear, which tuns that shaft and the gear on the right side. Now you have right hand power, then simply a chain to the crank gear and you now have gears..

    Getting a setup to work on the left side is just not worth it being that you have the wrong rotation in the rear gear setup.

    sickbikeparts makes a shifter kit. Not that much power is lost through simple chain and gears, and no, your not gearing it down to human pedal speed and loosing all the engines ability. you can have it geared to go upwards of 40 if you want.
  6. svejkovat

    svejkovat New Member

    I was just hit by the motor-bicycle bug a week ago. I have not seen anything really yet.

    But in fact I did finally see that page from sickbikeparts in the time since I last posted. Have any of you come up with anything similar on your own? I'd strongly prefer a small 4 cycle engine from subaru/robin or honda, and these are lacking the internal gear reduction as well as the starting mechanism of the 2 cycle kit-bike motors that sickbikeparts kit requires.

    Otherwise, that looks like the right direction to go. Since I'm not using a chinese two cycle, I may as well just try to duplicate the idea that the kit presents. The only thing I couldn't do in my shop is the front freewheel for 65USD.

    Anyone done this with a 4 cycle? What did you do about starting if you didn't just pull start?
    Last edited: May 18, 2009
  7. superbee

    superbee Member

    Whats wrong with the 2 stroke? I agree, a powerfull 4 stroke would be nice, but the 2 strokes are small and really dont have bad performance with exhaust.

    Im getting ready in the next week or two to build mine to use gears.

    The chinese ones do come with a pull start (on some) and have a dry friction clutch (like an automobile) that you can dump when moving to start the bike.

    That clutch is good for shifting gears, its going to be alot more work using a 4 stroke.
  8. duivendyk

    duivendyk Guest

    Your assertion that such a system would be very inefficient is incorrect,that is a misconception.The fact that it requires more 'gearing down' to overcome the gearing between crank&rear wheel does NOT make it necessarily inefficient.Inefficient implies large loss but the extra frictional loss is actually quite small only a few percent,while the improvement in performance due to the ability with the variable gearing to match the engine power to the speed required can be quite large.This shows up when climbing hills,or fighting headwinds and is the reason bikes are equipped with multispeed hubs or deraileurs.Doing this is an elegant and efficient way to use the existing drive system,the downside is that it requires a freewheeling crank and that your cadence is likely to be rather high if you choose to pedal along at high speed.
    Last edited by a moderator: May 18, 2009
  9. svejkovat

    svejkovat New Member

    It would only be inefficient relative to applying it more directly to the rear. But you are correct. It would only be less efficient within a narrow range and outside of that range (which is what using the shifters allows you to reach) it would more than compensate for that.

    2 cycle? I don't know. Like everyone else I guess, I've had lots of two cycles, lots of four. The twos were/are motocross, weed trimmers, mopeds, etc. The fours were/are cars, road bikes, lawn mowers, etc, and (a) weed trimmer. Having lived with all these machines I'm leaning toward a four for this project. My 4cycle trimmer is about 1/3 as noisy, about 1/6 as noxious smelling, and capable of doing work at mid rpms. All of these attributes, especially the low noise, I'd value highly in a motorized bike.

    One thing that playing around with mopeds has shown me is the marvelous advantage of being less conspicuous. My Puchs are, despite being two strokes, pretty quiet and so much more bicycle than motorcycle that I can go many, many more public places on them than I can on my 650 BMW without raising eyebrows or cops. In fact, this experience led me to the thought of motorizing my mountainbike and for exactly these reasons. If I can get some of the same performance out of my mountainbike (with a trimmer motor it ought to weigh in at about 1/3 to even 1/4 of the Puch) and at the same time make it even quieter and less smelly, I'd have the PERFECT urban fun machine!

    As far as starting goes, the pull is always an option. What would be truly ideal would be to add a little handlebar actuated clutch/freewheel to the right side of the jackshaft immediately before the front freewheel in a system similar to the shift kit from sickbikeparts. Then, the centrifugal clutch could be dispensed with entirely, and as an added bonus, the bike could be pedaled free of the engine transmission and jackshaft (you'd need to have a locking tab on the clutch lever for pure pedaling). I doubt if such a thing exists. Yeah, starting the 4stroke without a cord is going to be a bit of a challenge, and yanking it all the time would be a pain.

    I wasn't going to mess with my mountainbike until I came across a Specialized Hardrock for practially free the other day and the dream started anew. The frame is twice as beefy as my cheap mountainbike and it has a very nice front suspension (far stronger/lighter in fact than my Puch's!). I'd also like to integrate a spring and shock under the seat somehow. Any ideas there?
    Last edited: May 18, 2009
  10. duivendyk

    duivendyk Guest

    I agree that a 4 stroke is preferable unless you are looking for power&speed in that category the 43ccc 2 stroke GP 460 putting out over 4HP (11k rpm) is hard to beat,.It fits a standard 78mm cent.clutch.The 4 stroke Subaru/Robin are quality engines,easyto start with automatic compression release.Check for info.Ihave no direct experience with SBP kit,but I do know that pedaling along with it is not in the cards (cadence over 100rpm I seem to remember).So its use is for getting started and qualifying as a MB or moped,as far as the law is concened.A single speed system is limited to flat or mildly hilly terrain unless the engine is in the 50-100 cc displacement range.A CVT setup is attractive has about a 2:1 range,a lot better than nothing .TheChinese ones have the usual problem: poor bearings.You might also look around in the pocket bike engine arena.Those engines have real clutches I think.
    Last edited by a moderator: May 19, 2009