Rack Trailer? Addressing gravity center issue

Discussion in 'Rack Mounted Engines' started by NeoY2k, Aug 28, 2009.

  1. NeoY2k

    NeoY2k New Member


    I once went there when I wanted electrical for silence, but never built it because it was expensive and batteries were too much of a problem.
    Gas makes more noise, so I couldn't take the shortcuts at night anymore. But running longer distances, and probably faster, It would probably end up as quick.

    So I'm back, but this time, I'll go gas!
    I just got an idea that may work on my bike, and I'd like to know what you think about it.

    Ok, what are the requirements?
    - Low noise
    - Steep hill climbing
    - Cheapest
    - Ultra simple
    - Shouldn't look aggressive.

    So I want a 4 cycles. To go cheap, I'd buy the Harbor Freight 2.5HP 80cc or 6.5HP 200cc. Which one should I choose? I have very steep roads where I live (up to 15% easily and maybe more!).

    I don't want a frame mount because I'd like to avoid the "motorcycle" look, and avoid policemen questions too.

    I though about rack mount. These motor are not that small and light (especially the 6.5HP!) and I fear having 35lbs just right behind my back for gravity center reasons.

    The last solution was push trailers. They require way more metal work, not that easy to make. They are too large and ugly to my taste.

    I then realized I had 2 mounting holes near my rear wheel hub, symetrical. Maybe for rack mounting.
    I would then just have to screw a tube on each side, and make a push trailer directly attached there, saving this big ugly curved tube attached to the seat, and making build WAY more easy and good looking.

    Then I simply thought: why not just make a trailer, with a non-motored wheel? Put the motor on it, a gear of the left of the rear wheel, and run a chain between both?

    This way, I have something working like a rack mount, but on a trailer, with low gravity center. I can have the motor very close to the wheel saving space, metal work is ultra simple, and the build looks rather innocent.

    My questions are then:

    Should I choose the 2.5HP or the 6.5HP motor?

    Should I mount this trailer on an horizontal axle or not? Not having one may cause problems on irregular roads. Having one may let the motor assembly bumps. The mounting axle being 2-3 cm appart the rear wheel axle, may it cause problem with chain lenght? Mounting without an axle but using suspension on the trailer wheel wouldn't be a (complicated) solution?

    The ultimate solution to the last question may be not to add a wheel for this trailer, in fact, not make a trailer, just make a rack at the place of a trailer. Do you think tubes attached on each side, with two mounting points 40cm appart, would handle motor weight and vibrations?

    Is there something special about this build that you think I should think about?

    I have no experience, I just am thinking at how I could create it to reach my goal. In fact, the solution I stressed in bold would be my preferred: costs nothing, ultra simple, should work well.

    Oh last question: Would using a belt instead of a chain reduce noise much? Are they easily available and cheap?

    Thank you,

  2. loquin

    loquin Active Member

    Something like the attached sketch? (red is added rack mount 'stuff.'

    It should work, and it would address the center-of-gravity issue. Having said that, IMO, the C.O.G. 'issue' is greatly overstated. I've had a staton rack mount friction drive for years, and the first day I rode it, I noticed it. After that, nothing.

    Note that you will need to add a strut from the motor rack to up near the seat. Otherwise, you'll have too much stress where the 'rack' mounts.

    Now, I would not run the 6.5 HP motor there. It's too heavy. The HF 2.5 HO should be fine.

    You might want to consider a CVT/Jackshaft approach though. You'll need to get the motor output shifted over from the right to the left anyway, with a jackshaft. (IMO, you don't want the motor chain and the bicycle chain on the same side of the rear hub. If the motor driven chain jumps, it will get all tangled up with your existing chain, as well as (probably) the spokes.) If you add a reversing gearbox so you can turn the engine around backwards, the muffler will be pointing at you...

    Attached Files:

    Last edited: Aug 28, 2009
  3. NeoY2k

    NeoY2k New Member

    Thank you
    Yes, it would be like the drawing. But do you think I'd really need an attach near the seat? Making the horizontal bar longer and attaching both near the pedal and the rear hub wouldn't be enough? I have a bicycle that has a very long rear arm... In fact, as it is a fully suspended bike, the second attach couldn't be very high anyway.

    Do you think 2.5HP would be enough to run very steep hill at a decent speed? I didn't measured it, but it is very steep... Of course a CVT would help. I was also interested in the 196cc because of its wider range without using a CVT, to save on cost and keep it simpler, and maybe to keep a low pitched noise at low speeds.

    Looking at videos on youtube, these motors seem to run counter clockwise seen from the output:

    So putting it on the rear would allow me to have a chain on the left of the rear wheel (not on the deraileur side). I then don't see the need to shift it from right to left?

    I was also interested in putting the motor there to keep hot parts further apart from me.

    Thank you very much
    Last edited: Aug 29, 2009

    TREEWK Member

    Neo, If It Is Not Illegal, You May Like Looking At "motormaker" Build Posts On The Other Forum. You Go Into The Members List, Then Click On Posts By Him. The 2 1/2 Ho Is All You Need. I Seen A Scooter Wheel/tire, Sprocket, Disc Rotor, Axle, Nuts And All For $23. On Ebay, Yesterday.. Whatever You Do, You Will Have Fun. Ron
  5. loquin

    loquin Active Member

    Depends on the location you'll be using it. In many places, a 6.5 hp motor would cause the bike to be classified as a motorcyce, pedals or not.

    Now, do you HAVE to have the angled support? No. Not if you make the horizontals very heavy, to support the cantilevered weight. Remember, hit a pothole at speed, and the motor could end up on the ground! At a minimum, you would be looking at 2 inch by half-inch steel channel for the horizontal member, if there's no diagonal. There can be a lot of force in a dynamic situation like that. The strongest approach, and one that would be lighter than a simple cantilever, would be to add a diagonal up as high as you can reasonably get, which ends up forming a triangle. (*similar to a roof truss on a house.) The diagonal member will end up under tension most of the time, so, it can be fairly light - almost a strap - to support the weight of the engine.
    Last edited: Aug 28, 2009
  6. NeoY2k

    NeoY2k New Member

    Well, not wanting to debate over "legal" issues, but here in France it seems that NOTHING could ever be legal (motored bikes, or anything else).
    Example? Even for a Solex, you now need an ID plate and insurance that can easily go to 300$ a year. For a Solex, yeah. Just to add a bed on a van to sleep at night when stopped needs to be examinated and will cost you 4500$ to get the new vehicle agreement. And it's illegal to build your own vehicle. So any motor you use, you are totally illegal.

    That said, french policemen have quite a sense of humor, and seeing a motored bicycle that doesn't look like a motorcycle/scooter wannabe probably won't cause you any problem, given that it is low noise/not piercing. No way you could ever run these 2 strokes happy time here, you'd be caught within an hour!

    Treewk, by the other forum, you mean motorbicycling.com? I'm not registered on it. However I seen the trike and the side-car from him, and indeed, he made really great work!!!! Is that what you are suggesting me? I'll register on this other forum to see how he made it and if I technically can do it. However, I'd prefer to make it simpler. For the 2.5hp vs 6.5hp, I'll develop a bit later.


    I understand that completing the triangle really would be better, so that's what I'll do (if I do not use an extra wheel... but you understood I'd prefer to go without that extra wheel). It will be a rather flat triangle, but after all, this triangle supports the bike and biker's wheight and road irregularities, so it should handle this extra weigh. My rear fork is made by Newman, who makes motorcycle parts... I don't think they would have put their name on it if it wasn't rugged. Well, all this bicycle is very rugged indeed...

    Now on the 2.5hp vs 6.5hp, you understood I'd prefer the 6.5hp, but you say I'd better go with the 2.5hp. So I'm wondering.

    Why would I prefer the 6.5hp? Not to reach extreme speeds (I don't think I would ever go faster than 30mph). I won't remove the governor, I don't want to (you'll see later why)

    First I'd like to climb up steep hills.
    Gearing the 2.5HP for lower speed/higher torque would do it, but what would be the speed I could then reach on these hills? Wouldn't the speed on flat suffer too much? OK, using a CVT would allow me to adjust for it and have torque to go up the hill, and speed to go on flats.

    But then is my greatest problem: I would be with a rather wide open throttle most of the time, thus emitting a higher pitched noise. Especially riding that residencial area hill that goes from town to where I live, full of old persons who easily reach their phone to complain about the noise. Plus I am a sound engineer myself, and I prefer low pitch rumble to piercing noise...

    Using this 196cc 6.5hp, I could keep low rpm even on these hills, and have enough efficiency range thanks to the 196cc to avoid a CVT, making it simpler, and cheaper.

    Plus I secretly wish I could find a way (a trailer?) to take a passenger, because I have a few friends living in the same area, and going back from town is a real pain climbing this hill xD

    Now maybe you have some other arguments in favor of the 2.5HP, I'm open to discussion: it won't be built this week, I have the time to become reasonable and opt for 2.5HP.

    Thank you,
    Last edited: Aug 29, 2009
  7. NeoY2k

    NeoY2k New Member

    Hum I'm looking at videos on youtube, it's hard to tell, but maybe does the 6.5HP makes more noise anyway? So I'd be better with the 2.5HP?

  8. loquin

    loquin Active Member

    I'm not talking about an extra wheel.

    What I'm talking about is the strain on the new portion of the bike - the rack.

    Your existing rear support is probably going to handle the extra weight just fine.

    However, the new rack that you're proposing is a cantilever structure. You would need to anchor it at the rear axle, and forward, on the chain stay, near the peddles.

    So, you would end up with something like the attached image below. You have a downward force being applied in the vicinity of the motor, which consists of the static force cause by the weight of the engine, and the dynamic forces when you hit the bottom of potholes, etc.

    The upwards forces at the front of the support are caused by the support trying to rotate around the axle. (I'm assuming that you securely fasten the new side members to the chain stay) The red and blue areas of the sketch indicate the tension and compression that is taking place in the side member. They are concentrated in the area near the axle.

    Think about a diving board. This is essentially what that motor support will be doing - flexing near the axle. Since there's a large component of the force due to dynamic forces, the side members will need to be made relatively massive to resist cracking/bending near the axle.

    However, if you can make the supporting structure triangular, the individual pieces can be substantially lighter, yet be as able to resist the forces that are trying to pull the structure apart.

    Attached Files:

    Last edited: Aug 31, 2009
  9. NeoY2k

    NeoY2k New Member

    Well, I was reacting to treewk's answer who said I could buy an extra wheel for cheap on ebay.

    I understood your point on adding a diagonal tube to relieve the flexion on the horizontal one, and totally agree with it. But thank you for the schematics! It kind of remind me of my technical BsC lol (and I came up with such a stupid idea though I studied mechanics... why is my brain made of chewing gum?).

    On my bike, the horizontal tubes will have to be strong even with this diagonal, because the triangle then completed won't have a very open angle (due to the rear shock absorber). But yes, a rather closed triangle is better than a flat triangle (horizontal tube with two mounting points).

    Thank you for pointing it to me, otherwise I may have built it, ran a few hundred meters and heard my engine crash on the floor...

    Regarding the engine, my main concern is the noise.
    Is the 2.5HP quieter than the 6.5HP? Does anyone have both and could tell me about it/maybe capture the sound in similar conditions?

    Is the 2.5HP running WOT to go up a hill louder or quieter than the 6.5HP running lower rpm to do the same task?

    I find CVTs very expensive :( even if they are an optimal solution.

    Regarding this "rear rack mount" idea, I think it won't be good in the end. Why? If the front wheel lifts (a stair, a bump, who knows) the engine would crush on the floor... :(
    Last edited: Sep 1, 2009