Flat Belt Final Drive Set Up

Discussion in 'Transmission / Drivetrain' started by Janders, Jul 13, 2015.

  1. Janders

    Janders New Member

    Hi All,

    I am in the final stages of planning for my first build and have several question regarding final belt drive systems, similar to those used on the 1911/12 Indian Belt Drive Single and the 1910 Harley Davidson.

    Basic Components

    The bike will be powered by Honda GX160 or clone. The governor will remain as 3600 rpm is more than enough for this style of bike. This bike is intended to be for cruising and minimal stop / start riding. Having said that, there will be times where slow speeds are required in busy traffic areas. I will use wide 26 inch rims with 90mm sturmey archer front drum brake hub and either a 70mm sturmey archer drum hub or coaster brake for the rear.

    The engine will be mounted with the cylinder more upright than standard to add to the appearance of the bike and to better fit into the vintage styled loop frame. I have searched the internet and cannot find any negative comments about increasing the angle of the cylinder, apart from a slightly higher oil level in the sump.

    I intend to use a belt drive directly from the engine crankshaft to the rear rim. From my research, the flat belt would be a better option that the A or B section V belts. The reasoning is twofold; a flat belt has better grip and flexibility around a smaller drive sprocket (less than 3 inches), and the flat belt may provide better slip for when the clutch is disengaged. V belts would provide superior grip, but free-wheeling when the idler clutch is released may be an issue.

    Thanks to JPilot's gear calculator, I propose to use one of the following sprocket options to obtain the desired gearing.

    Option 1 - 9.60:1

    Front sprocket - 2.5 inches
    Rear sprocket - 24 inches
    Clutch engagement speed (1500 rpm) at 12 mph
    Top speed (governed 3600 rpm) at 29 mph

    Option 2 - 8.00:1

    Front sprocket - 3 inches
    Rear sprocket - 24 inches
    Clutch engagement speed (1500 rpm) at 14.5 mph
    Top speed (governed 3600 rpm) at 34.8 mph

    A review of the 1912 Indian discusses the clutch as follows, "if the jockey wheel was dropped completely down, the belt would slip over the pulleys, just like being in neutral, and grip could be gradually increased as necessary, or a partial-slip condition used in case of difficult terrain".

    My questions refer to the slippage of this style of clutch. Fully released and fully locked should be easily achieved by the use of a lever and spring loaded mechanism that provides sufficient tension without stressing the belt. The partial slip setting is where I am uncertain about the effectiveness of the drivability. Partial engagement would enable some slip and a setting could be made to lock the clutch in 50% engagement for slow riding. In theory engine idle rpm ( or slightly above) and an amount of clutch slip below the 100% clutch lockup should enable slower bike speed.

    Do any members have experience with this style of clutch and provide some information on the nature of how this would work in real conditions? If this slipping of the clutch is effective, could it also be used as a standard form of clutch / drive engagement from a standing start? Would the engine idle rpm need to be raised to provide a rpm buffer to compensate for the additional load from the engagement?

    Any information would be greatly appreciated.
    Last edited by a moderator: Dec 18, 2015

  2. HeadSmess

    HeadSmess Well-Known Member

    your best sources are old machinery texts..."webers workshop companion, ed 1926" etc...

    um, flat leather belts... a small percentage of slip, about 5% , increases friction and therefore power transmission(?!) joining em requires practice :p

    theres a minimum size pulley required for a given thickness.

    v belts are much the same but can be a bit grabbier... dont require the width...and seem to work fine for ride on mowers which is about the most common use of belt drives... yep, they work fine, but dont expect to use the 50% for too long or youll glaze things up, wear belts out, etc. FAR FAR BETTER TO FIND a variable pulley...or consider how to make one? then you get full power transmission within the ratio ranges... with no steps in the shift.

    pretty cheap...
    a handy person with a lathe could modify that loose side to run on a slider with a lever to vary the ratio... or make the whole darn thing from scratch.
    Janders likes this.
  3. HeadSmess

    HeadSmess Well-Known Member

    one day i REALLY have to try this fluid coupling (not quite a torque convertor) from a washing machine on a gokart...
  4. Janders

    Janders New Member

    Hi HeadSmess,

    Thanks for the idea. I had not consider this concept at all. This is definitely worth exploring further. Overall width of the engine / pulley drive configuration is important and the narrower it can be, the better. One other advantage of a belt clutch was the removal of the pull starter from the other Honda engine; therefore another 4-5cm saving in engine width. The engine could be started in the same manner as an vintage belt driven board tracker.

    I have a few questions.

    1. Would this variable pulley be a single engine driven pulley that could control the ratio by one side of the pulley sliding sideways?

    2. Could this pulley also act as a clutch by fully releasing the belt drive tension, or would an idler pulley be required to act as the clutch (tensioning) mechanism?

    Thank you again for your assistance.
  5. HeadSmess

    HeadSmess Well-Known Member

    with you on that...the narrower the better, my pet hate of four strokes and brushcutters... and if you could replace the flywheel with something lighter and not so, erm...bulky...? better again...

    1, yes, 2,yes AND no...remember you would require a spring tensioner to keep tension...

    but...they "usually" have a centre bearing, so when separated to maximum, or the smallest pulley size, the belt runs on the bearing...ie, neutral. as soon as it starts contracting, pinch goes the belt and youre off in low gear... if its the DRIVER.

    therefore no need to release the tension...except during servicing :)

    depends what type pulley you find really.

    they sorta often run in pairs so the belt runs dead straight but thats only really an issue with LOTS of HP... gotta keep in mind that the angle of the "cones" isnt a random number which limits the ratios available (belt can only be so wide and still allow halves to get close enough together for max diameter..)... but still 1 1/2" to 3 1/2" or so is a pretty decent range! all depends on what belt they use, etc etc...

    try reading through the CVT pages... thats exactly what it is. may be something there?

    the tricky part is to actually adjust them while in use... the one i pictured you set and lock, technically it has steps, as it requires one rotation for the set screw to line back up. but... using a fork/yoke similar to the clutch bearing in a normal car, or a lead screw/ramp of some type... they are available. $$$ versus need...

    if you had a lathe... :)
    Janders likes this.
  6. Janders

    Janders New Member

    Yep, if I only had a lathe. :).

    I have been reading up on variable pulleys since your post. Belt tension would seem to be as much of an issue as the pulley itself. I can now understand why the CVTs are a commonly used solution. However the width of the CVT on the Honda could be a problem. It is not so much the width of the engine pulley, but the 2nd pulley that runs off the jack shaft. Mounted far enough forward in the frame, the peddles would sit comfortably behind and clear from obstruction, but this may require a longer wheelbase.

    The GX160 width with the pull starter removed is approximately 26cm. I agree the standard flywheel could be replaced with a thinner version and that may reduce the width by another 2-3 cm. The crank output drive shaft on the other side extends about 6 cm and therefore to take advantage of the reduced overall width, the engine may need to be mounted slightly to one side within the frame.

    I am trying to plan this build out fully before jumping in head first. Therefore, I have not purchased many specific parts, except for the engine. I think there is much more planning to be completed. One issue of living in Thailand is that it is near impossible to walk into a local shop and ask detailed questions.

    Thanks again for your input.

  7. HeadSmess

    HeadSmess Well-Known Member

    i wouldnt say its too hard. you dont need too much tension, and it isnt exactly industrial or anything...it doesnt have to last for ten years of constant use? remember that idler bearing in the pulley voids any reason to slacken off...you just need a jockey wheel on a hefty spring with enough movement to take up the slack. you also dont really need it to autoshift, and THAT is the hard part. matching torque to expansion with springs and stuff...ouch. a really complex subject.

    you cant really walk into a shop anywhere and ask detailed questions :rofl: im an occasional mystery shopper :) and oh boy, do i put them to the test... best one was walking into a PAPER shop and having the girl tell me they didnt have tissue paper... cough cough.

    thailand.... sounds like you need to learn the lingo? one day ill go over and find an engine for my tuk tuk...my poor lil tuk tuk...

    labors cheap there...you could find someone that would make something like this with a modified sewing machine because he doesnt own a lathe either... :)

    alternative is run the belt in the secondary drive, chain on the primary and a chain will always be narrower than any belt and pulley...

    or have a look at two speed belt drives?

    food for thought :) nothing about it is easy...even with a lathe...and mill...and stuff...
    Janders likes this.
  8. Janders

    Janders New Member

    Thanks again HeadSmess,

    I believe this is the ideal solution. The split, variable ratio pulley with a flat bearing as an idler (neutral) and a spring tensioner to provide continuous felt tension.

    I will probably run a chain drive from the engine to a jack shaft configuration, in a 2 to 1 reduction. This would enable a smaller rear wheel pulley to be used; therefore requiring less rear frame modifications and a more compact set up and provide better final drive ratios. Primary drive would be a 10 tooth to 20 tooth chain driven sprockets and the secondary drive (located more inline with the engine and behind the engine) would be v belt from the split pulley to a standard v belt pulley.

    Commonly available v belts could be used ; maybe B section would be better for a slightly wider belt. The spring tensioner would run on the outside of the belt, from the under side.

    I actually have a contact for somebody here with a lathe, so it should be okay :)

    Ahhh yes, learning the Thai language. I have tried for years and failed to master the intricate details of the tonal changes. I ask for the bill at a restaurant and receive a chicken burger (that is a story for another day). Singer sewing machines are everywhere on the streets and I brought one here from my country years ago. A full service and adjustment was $10. When I picked it up, I made sure the man had tested it. He told me yes. He then asked what this large, funny shaped steel lever was for. I told him that was how one controlled the electric motor......

    Nevertheless, he got this old machine to run silently and smoothly without even starting it. It is quite amazing how creative some locals can be here.