Mountain bike with chainsaw motor help

Discussion in 'General Questions' started by JEEP, Sep 3, 2013.

  1. JEEP

    JEEP New Member

    Hey I'm new here and completely new to motorized bikes too (not dirt bikes though I have owned lots of them) I had this random idea cutting wood at the cabin with my friend as he was running the saw and I was hauling brush but I realized how that look like it would work so good in the bike I have in storage (he bought a new saw and said I can have the old one at the end of the year). I'm a small engine mechanic so I know how to make it run real nice :D. I have a welder (miller 210) and a like and 18 speed bike, the saw is a 40cc poulan wild thing that's only three years old. Ok now on to the good important stuff, I'm going to make it a shiftable chain drive or I will forget this project completely! So I was planing to put the motor above the pedal jack shaft and run the chain off the motor to the big sprocket on the jack shaft of the bike and also remove the pedals, then keep the regular chain drive of the bike in the lowest gear in front and shift the what ever like 7 gears in back. The wheels are probably like 20-24's? Will my gearing even be close? I'm just going to buy as small of sprocket fleet farm has to weld on the clutch if the saw. Give me some some input I want to know if my gearing will be even close, on the way into town there's a not super steep but long hill I'm going to have to make it up too but its ok if I have to go to 1st gear for that. Maybe I will have to remove the jack shaft and weld an even smaller gear to it for the stock chain. Thanks

  2. butterbean

    butterbean Well-Known Member

    There is a program that can help you determine gear ratios and speed at any given ratio based on engine rpm. Before you buy any sprockets, and most especially before you weld them on, use the program and it will take all the guesswork out of it. Enter the number of teeth on each gear in sets of two, starting at the engine and working your way back to the rear wheel. Try to find out the max rpm of the engine you're using, and enter the number of teeth on each gear on your rear cassette one at a time to know your ratio and top speed in each gear. The program can be found here:
  3. JEEP

    JEEP New Member

    well like i said im 100% new to this stuff. just wondering if anybody has done this yet. the gear ratio link is really cool, im going to look at my sprockets and figure it out, what would be a good speed with that tool, like maybe 7 in first and 30 in high gear.
  4. professor

    professor Active Member

    When you weld a sprocket onto the saw clutch, make sure it is super concentric, please put a decent muffler on it too. Mounting the saw motor is the difficult part because it hasn't provision for that, unlike normal engines.
    Which is why you do not see it done much- if at all.
    A good old 4 stroke is infinitely easier to do and no gas to mix, quieter and much longer engine life.
  5. HeadSmess

    HeadSmess Well-Known Member

    one. select the correct saw to use. separate engine from oiltank etc, like most stihls do. you end up with a neat engine, with solid mounting lugs, thats small and compact.

    the flywheel needs shrouding. some type of cylinder cowl can help.

    starting can be tricky once you strip the engine. either it will need a manual clutch with no freewheel, or some sort of pulley for pull starting with a rope.

    if you dont strip the engine, its wide and ugly. fins look cool, flouro green plastic doesnt :) personal taste really.

    chainsaw does 10,000rpm approx.

    your legs... 88rpm is scientifically calculated as the optimum pedalling "cadence".

    so, you need 10000/88 for the reduction to the pedals.

    its about 113:1.... because the pedals gear UP. most engine transmissions gear DOWN.

    100:1 would make it sound a bit happier instead of like a mozzie.

    whereas, if you forget the shifter, and just set it for 40mph, youll only want about 16:1 reduction... use the calculator ;) 8-9000 rpm is a nice happy medium on a rev happy engine that can take WOT unloaded.

    a chainsaw will do it. whack a real pipe on one with anything over 30cc, and youll be finding out exactly what breaks and what needs beefing up pretty quickly.

    comparison... a ktm 50 versus a peewee 50... one goes fast, one just barely goes, :jester:
    an ms180 with a pipe for 12k is making around 5hp. geared right, with shifting...thats enough to break 70mph. without shifting, it might do it but will take a long flat to do so!
    Last edited: Sep 7, 2013
  6. HeadSmess

    HeadSmess Well-Known Member

    if this is the poulan wild thing you have....

    then its ideal. the engine is just a plain 40cc engine, and will look a bit like this baby...

    SUNP0007 (Large).jpg SUNP0008 (Large).jpg

    sorry the pics are :poop:, i just grabbed the first cam i found. needs refocussing :(

    good white balance though... wouldnt believe its nighttime with one lousy flouro tube on it...

    that, personally, is the type of engine a HT should look like. a 42cc talon. ive been debating over whether to use it for a bike or a boat...
  7. JEEP

    JEEP New Member

    Ok well my wild thing does not have a compression release but its stratified carburator, I was going to leave it in the saw case so I could use the bar nuts and handle bolt to mount it
  8. 5-7HEAVEN

    5-7HEAVEN Member

    Chainsaw engines have been installed on bicycles. Almost all of them have been friction drive where the engine dies off at a stop.
    In my opinion, no one has used this engine to its fullest potential. The key to a successful chain drive motorized bicycle is using an effective clutch or drive belt.
    Once you have that problem licked, it's a matter of selecting proper gears. Let's try to imitate what I'm using on my Tanaka 47R-equipped bike.
    In first gear, my ratio is 46.36:1. This will allow you to climb ANY hill or ramp. Second gear is 38.18:1. This works well from a standing start, even on inclines.
    Final drive is 15:1, good for highway speeds of 40+mph.
    How do you get such low ratios? Start with 11t/34t 8-speed cassette on the back wheel.
    However, it will be VERY difficult obtaining low gearing with only one jackshaft.
    If you were to somehow install a 9-tooth sprocket as your right-side chainring, you'd get 3.78:1.
    Then, if you could install a 9-tooth engine sprocket, you'd have to get a 12.27:1 ratio between left-side chainring sprocket and engine sprocket.
    To get 12.27:1 gearing, you'd need a 110-tooth chainring sprocket. This is gigantic and would hit the chainstay (frame), if using bicycle chain.
    Getting all this accomplished happens, IFFF your engine sprocket aligns with left-side chainring sprocket.
    If you can't align these sprockets, you might as well install a second set of sprockets on a jackshaft.
    First, it'll allow you to center your engine on the bike. Secondly, it'll help in building the proper gear ratio to get your chainsaw engine running properly.
    Mounting an 11-tooth engine sprocket with a 54-tooth jackshaft sprocket and 8mm T8F chain gets you 4.909:1.
    Bolting on a 72-tooth chain ring sprocket with an 11-tooth jackshaft sprocket and 8mm T8F chain makes 6.545:1.
    With a 34t cassette sprocket and a 24t right-side chain ring sprocket, you'll have 1.417:1.
    Multiplying your combined ratio ( 4.909 x 6.545 x 1.417) will calculate to 45.53:1 in 1st gear and 14.73 in 8th gear.
    Your bike will fly up steep hills and on flat ground with these gears.
    Now all you have to do is build it.
    ANNND, with the correct-length bottom bracket, you can still have working pedals.
    Last edited: Sep 8, 2013
  9. HeadSmess

    HeadSmess Well-Known Member

    each to their own... they just look really rough left in the case. neat auto chain oiler if you can keep it... fuelling can be a bit nasty, unless you use a seperate tank.

    sprockets... #25 chain is pretty common, and certain scooters use flats on the shaft. pretty easy to file a flat on the crankshaft. the finer the chain, the easier it is to get large reductions in a small space. and they run quieter.

    you retain stock air filter, but need to change throttle mechanism. ditto on choke, possibly. the stock exhaust will do if you like snorting twostroke residues... otherwise, tuck it away somewhere neat with an extansion. if it comes out too close to your back, you get a dirty big stain on every shirt you wear. the exhaust clings to your back like its slipstreaming :) well, it is, really... so get the outlet low to the ground as possible or youll stink.

    and lastly... the handles isolated from case.... forget it. just mount up off the bar bolts. if they can hold a bar still, hopefully theyll keep the engine still...

    back on previous post...when i said 100:1 ratio...thats from engine to CRANKS. as mentioned, the bikes gears then gear UP.

    so, in granny gear, youll have 1:1 on the pedals to rear wheel. or 100:1 for the engine, give or take a few decimal places given sprockets available etc.

    in high gear, if you have a 55 chainring and an 11 rear... or 1:5... your engine will have 20:1 by the time it reaches the rear wheel.

    even 20:1 as a final ratio is pretty low geared.

    meaning 50:1 would be more than adequate. 50:1 and you could tow a bus...

    10:1 and if the engine will wind out enough...hope you have brakes!

    15:1-18:1 is a nice "fixed gear" ratio, the HT can get away with 12:1 if you dont mind pedalling hills a lot...

    my only gripes here...

    chainsaws output on the right hand side.

    9t chainring to a 34 rear sprocket may be good reduction for the ENGINE, but when it dies and you have to pedal home... youll be spinning those pedals and not going anywhere. think MTB in super extra low 89 degree gradient climbing granny gear...

    and that is how fast theyll be spinning when youre riding under power....

    work on the idea that you do not, ever, want to pedal more than 90rpm. unless you like knee surgery.

    its either a lot of reduction before the cranks, or seperate to the cranks altogether, ie, the other side...

    Last edited: Sep 10, 2013
  10. HeadSmess

    HeadSmess Well-Known Member

  11. 5-7HEAVEN

    5-7HEAVEN Member

    If I'm understanding correctly, 100:1 without engine built-in gear reduction would be impossible with one jackshaft.
    That'd be like a 10t engine sprocket and a 1000t (one thousand teeth!) chainring. The sprocket would be taller than the bicycle.
    With conventional 10t/25t jackshaft combination, you'd need 5 jackshafts to get 97:1 gearing. 2.5 x 2.5 x 2.5 x 2.5 x 2.5 =97.66:1
    You'd need four jackshafts like that to get 39:1, which is lower than necessary. A 32.72:1 gear ratio at the chainring is a great combination, before it reaches the 8-speed cassette.
    That 9t/34t was suggested if the OP removed the pedals as he had mentioned.
    If the engine has reduction gears, then much higher gear ratios can be used in the calculations.
    If the engine has 5:1 reduction gearing, then a single jackshaft of 11t at the engine and 72t at the chainring gets you 32.73:1.
    I use standard Shimano or KTM 8-speed bicycle chain from 24t chainring to the cassette. It works well and reasonably priced.

    I'm beginning to realize why there aren't many 2-stroke engines running thru chain drive. :(
    If pedalling I'd suggest 24t at the chainring. That's what I have, and pedalling is easy going.
    Last edited: Sep 10, 2013
  12. HeadSmess

    HeadSmess Well-Known Member

    yep. i did say, 50:1 would be a lot better if the whole standard (18? 21? 24?) type setup is used. 100:1 does give just a bit more than necessary. as i realised with some thought.

    dont worry, ive been playing with gears and such forth nonsense for years. so i agree completely, when you do the maths, theres a reason not many twostrokers run on the chain drive. ht already has a 4:1 internal, added to the 4.4:1 standard gives a bit over 16:1 total, a good medium on those engines between sounding tortured and being overloaded... they do not rev very high at all :(

    whereas im finding a CAG engine gets grumpy with 18:1.

    you can get away with 4:1 to 5:1 in a single reduction. i believe 6:1 is the max allowed by most chain manufacturers. using even #35 chain makes for some large sprockets, and lining all those shafts up gets pretty tricky. plus covers cus who wants a chain breaking when turning at 8000rpm?

    sometimes, its actually easier to use gears for the primary reduction. much smaller, much safer. a much better option, as chains suffer centrifugal forces when spun at crankshaft speeds.... also, no tensioning etc required if the centres are laid out accurately.

    the only way to get 50:1 or higher in a single reduction? a harmonic drive. bit on the pricey side but. can be obtained with ratios over 1000:1 as standard :) got one on my cnc mill :) about the same size as a roll of masking tape... 4" diameter, 1" thick... they dont use gears.

    meh. i think we scared the OP :jester:
  13. 5-7HEAVEN

    5-7HEAVEN Member

    Can you explain harmonic drive reduction?
  14. HeadSmess

    HeadSmess Well-Known Member

    erm. yes and no.

    its sort of like a planetary gear system, with a sun gear and a ring gear.

    the sun gear has no fixed teeth, just spring loaded pins. (there are a few different methods such as swashplates etc, but they all achieve the same result)

    an eccentric rotates inside the internal (sun) gear, and the lobe of the eccentric pushes out a pin. the eccentric is the input shaft.

    this "pin" is basically a gear tooth. it engages with appropriate tooth in outer ring gear, and rotates it a small amount.

    the eccentric keeps rotating, and as one "pin" retracts, the next one emerges, engaging the next tooth, ad nauseum.

    really need a picture to explain it properly, but basically... the reduction ratio is set by the difference between number of pins and number of teeth in external ring gear.

    been around since at least 1960.

    theyre used in the 4th axis etc of mills, as they have no backlash (can take cuts while rotating in either direction) and make huge reductions in very tight spaces. belts, chains and gears always have some backlash. 1 degree of backlash on a 4 foot table makes for 10.5mm of inaccuracy at the circumference. most machines work to a 5 second or better accuracy. thats around a 700th of a degree...

    consider, a nautical mile is one minute of the equator. 5 seconds of arc at the equator makes for... 150 metres!

    sorry. i just double checked myself.

    what i described is known as a cycloidal drive.

    the harmonic drive uses a gear that deforms around a rotating eccentric but is much the same in that they both have huge ratios available in small spaces.

    the big difference is a cycloidal cant be, its always a reduction unit, input shaft is always the input shaft, while harmonic will go either way, increase or decrease.

    cycloidal is good on the mill, as cutting forces are not transmitted back to the motor.