SBP Shift Kit - Ratios, Speeds and Pedal-starting

Discussion in 'Frame Mounted Engines' started by AussieSteve, Feb 7, 2010.

  1. AussieSteve

    AussieSteve Active Member

    I've been playing around calculating gear ratios, speeds in each gear and pedal-starting ratios for my shift kit.
    With the right combination, it should be easier to pedal-start while still having a good range of speeds.

    I'm basing all calculations below on a max 7400RPM, (my engine pulls to 7400), with a 25.57" wheel and a 2-stroke HT engine with standard primary drive and countershaft sprocket.
    The gear cluster is 6-speed: 28,24,21,18,16,14.
    Also, I've only done calculations for some of the range of freewheel chainrings available from SBP.

    Stock shift-kit ratio for the RHS is 10T>44T/36T, with a top speed in 1st of 23.6mph, (37.8kph) and a top speed in 6th of 47.2mph, (75.5kph).
    The pedal-start ratio is 4.4:1 - very hard to turn over with my hi-comp billet head.

    For myself, I don't need excessive speed, but want easy starting and a fairly low first gear for take-off, so I'm going for an 11T jackshaft sprocket driving a 36T sprocket, with a 24T sprocket driving the rear gear cluster. (The last setup in the list below.)
    This gives a pedal-start ratio of 3.3:1, a top speed in 1st of 21.2mph, (33.9kph) and a top speed in 6th of 42.3mph, (67.7kph).
    A 10T jackshaft sprocket would provide lower drive gearing for a lower 1st gear, but would increase the pedal-start ratio to 3.6:1, still 20% easier than the stock 10T/44T, but a little hard for me, I think.

    The downside of using a driven RHS sprocket smaller than 44T is that the sprocket/chainring speed increases, possibly shortening the life of the freewheel bearing.

    All speed figures below are calculated, not measured, so wind resistance etc will play a big part in top gear speeds.
    Also, the higher final drive gearings might be near impossible to pedal-start.

    I hope someone finds this info useful - I did.

    ... Steve

    Below are figures for:-
    10T>44T/36T, (standard shift kit ratio)

    10T>44T/36T, (standard shift kit ratio)
    1........23.6mph / 37.8kph
    2........27.5mph / 44kph
    3........31.5mph / 50.4kph
    4........36.7mph / 58.7kph
    5........41.3mph / 66.1kph
    6........47.2mph / 75.5kph
    Pedal-start ratio = 44:10 (4.4:1) - hard - (standard).
    Chainring speed = 241RPM @ 7400RPM. (4 revs/sec)

    1........24mph / 38.4kph
    2........28mph / 44.8kph
    3........32mph / 51.2kph
    4........37.4mph / 59.8kph
    5........42.1mph / 67.4kph
    6........48.1mph / 77kph
    Pedal-start ratio = 36:10 (3.6:1) - medium-hard - (18% easier than standard).
    Chainring speed = 295RPM @ 7400RPM. (4.9 revs/sec)

    1........23.1mph / 37kph
    2........26.9mph / 43kph
    3........30.8mph / 49.3kph
    4........35.9mph / 57.4kph
    5........40.4mph / 64.6kph
    6........46.2mph / 73.9kph
    Pedal-start ratio = 30:10 (3:1) - easiest - (32% easier than standard).
    Chainring speed = 354RPM @ 7400RPM. (5.9 revs/sec)

    1........31.7mph / 50.7kph
    2........37mph / 59.2kph
    3........42.3mph / 67.7kph
    4........49.4mph / 79kph
    5........55.5mph / 88.8kph
    6........63.5mph / 101.6kph
    Pedal-start ratio = 36:11 (3.3:1) - medium - (25% easier than standard).
    Chainring speed = 325RPM @ 7400RPM. (5.4 revs/sec)

    1........26.4mph / 42.2kph
    2........30.8mph / 49.3kph
    3........35.3mph / 56.5kph
    4........41.1mph / 65.8kph
    5........46.3mph / 74.1kph
    6........52.9mph / 84.6kph
    Pedal-start ratio = 36:11 (3.3:1) - medium - (25% easier than standard).
    Chainring speed = 325RPM @ 7400RPM. (5.4 revs/sec)

    1........21.2mph / 33.9kph
    2........24.7mph / 39.5kph
    3........28.2mph / 45.1kph
    4........32.9mph / 52.6kph
    5........37mph / 59.2kph
    6........42.3mph / 67.7kph
    Pedal-start ratio = 36:11 (3.3:1) - medium - (25% easier than standard).
    Chainring speed = 325RPM @ 7400RPM. (5.4 revs/sec)

    I stopped here because this is the best balance I'll get for my intended use.

    P.S. I should add that the SBP kit's standard ratio is probably fine for most people, as far as starting the engine goes. When I describe pedal-starting as 'hard', above, that's largely due to the hi-compression billet head and my low body-weight, 50kg, (110lbs).

    Tttthat's All Folks.
    Last edited: Feb 7, 2010

  2. Fabian

    Fabian Well-Known Member

    Yikes Steve

    You have been doing your homework!
  3. AussieSteve

    AussieSteve Active Member

    Yeah, this bloody rain. Nothing else to do.
    I'd much rather be riding.
    At least I think I know what gearing I want.
    I like that 11T/36T/24T combo.

    ... Steve
  4. Fabian

    Fabian Well-Known Member

    Typically my method of pedal starting is to get up a reasonable amount of speed with high cadence, then release the clutch whilst the bike is rolling and pedal start the engine.

    This takes load away from my legs and all the force is used to start the motor.
    If pedal starting on a hill, i turn around and roll down the hill, then pedal start the engine and make my way back up the hill.

    I've got a 9 speed hyperglide cassette with the following custom sprocket arrangement.

    11 - 12 - 13 - 14 - 16 - 19 - 23 - 28 - 36

    With an 11T jackshaft sprocket driving a 44T chainwheel sprocket and (dished) 30T chainwheel sprocket driving a 36T cassette sprocket, i'm able to travel just a fraction over 4 mp/h (6km/h) at 1850 rpm

    With the 36T chainwheel driving the 11T cassette sprocket, i can theoretically travel at speeds around 50 mp/h (80 km/h), but as a good boy, i would never exceed the legislated maximum speed of 25 km/h.

    Last edited: Feb 7, 2010
  5. AussieSteve

    AussieSteve Active Member

    Love that low gearing. I might eventually make up a custom 6-speed cluster, or go to new 7 or 9 speed gears, (and a custom cluster).

    Is that the law in Victoria?
    In NSW, there's no speed limit for power-assisted bicycles, besides those posted beside the road. Note that for mopeds, though, the speed limit is 50kph.

    The real problem here is the 200W max power output.
    No petrol engines go that low. At least, our's certainly don't.
    Therefore, in NSW, our bikes are mopeds, limited to 50kph and requiring license and registration.

    I've attached the current, (2008), NSW legislation below, for anyone that's interested. (Not sure about Victoria)

    ... Steve

    Attached Files:

    Last edited: Feb 7, 2010
  6. professor

    professor Active Member

    Isn't the shift kit starting like popping the clutch in a car?
    When I did that I used second gear, to keep the inertia of the car up and the drag of the engine (as it starts to spin) lower- but still fast enough to start.
    So translated to the bike- get into a higher gear before dumping the clutch?
  7. AussieSteve

    AussieSteve Active Member

    Hello Professor. I wish it was that easy. The freewheel prevents bump-starting by inertia.
    Also, the engine-jackshaft-chainwheel gearing is fixed.
    You must pedal the motor over with a shift kit and the starting ratio is the same regardless of the gear that you're in. If you're pedalling the bike forward at the same time, a low gear helps, rather than a high gear like when bump-starting.

    Fabian's method works fine - get rolling then effectively kick-start the engine, although it places more stress on the freewheel at the moment of 'take-up'.

    I've opted for a centre-stand, so I can kick-start on the stand, then ride off.
    That's why a reasonably low first gear appeals to me.
    There's no problem with stressing the freewheel. Since the back wheel is not already turning, the slack can be taken out of the freewheel before kick-starting.
    (Let's see how long my centre-stand lasts.)

    ... Steve
    Last edited: Feb 7, 2010
  8. Ghost0

    Ghost0 Guest

    I would suggest against your kick start method and here is why. Because you can only rotate the engine as many times as a pedal stroke from 12 to 6 o'clock it will take more "attempts" to start the engine. Using Fabians, and most of the rest of us with Shift Kits, you can continue to pedal and turn the engine over. Thus you are really only putting the maximum amount of stress on the freewheel once per start attempt and not repeatedly until the engine starts as with your method.

    One way that has worked for others at least on initial start up is to remove the mag cover and use a 14mm socket on the mag nut with a drill to start the engine. Once started and warmed up pedal starting becomes easier.
  9. AussieSteve

    AussieSteve Active Member

    I don't quite understand the problem.
    Whether I'm kick-starting on the stand or pedal-starting, (pre- shift kit), it always takes 2 "attempts" - one with the choke on to 'prime', then one with the choke off and the engine always starts immediately.
    I don't 'stab' the pedal for starting but let the engine accelerate during the half-rotation, the same as if I was pedalling the bike forward, but without the added weight.

    Also, since the rear wheel is not already turning, the slack in the freewheel is taken up smoothly at the beginning of the kick stroke.

    Either way, I can't pedal it over for more than half a pedal rotation, so have no choice. I'm not going to pull the side-cover off every time I want to start the engine.
    I bought the centre-stand in advance, made sure the bike would kick-start OK, then went ahead and ordered the shift kit with heavy-duty freewheel especially for the purpose.
    I'll just have to replace the freewheel if/when it fails.
    The other option - remove the shift kit. If I have to do that, at this stage, I'll get rid of the bike altogether.

    A quick question - when the freewheel fails, does it fail 'safe', (ie lose all grip) or does it lock up on failure and spin the pedals with the engine?

    ... Steve
    Last edited: Feb 7, 2010
  10. Ghost0

    Ghost0 Guest

    The standard FW can fail in either mode, free or locked. The HD, which I believe you have has not failed due to the pawls failing. We have only seen 2 fail and both were bearing failures.

    I guess I will have to try one of those heads to see how difficult it is to turn over. I forget that not everyones engine, bike etc are like mine. Mine is so easy to turn over it is ridiculous.

    I really don't think you will have any issues with your starting technique. I just wanted to point out to the rest of the audience that it is not the preferred method.
  11. AussieSteve

    AussieSteve Active Member

    That's more or less what I thought. The HD type would probably be the same, if it does fail in the pawls.

    That's comforting, from my kick-start point of view. I'll get a spare soon and keep it on hand, just in case. At the first sign of potential failure, I'll fit the new one.

    I am an extreme case, at 50 kg, (110lbs), and not much puff, but yeah, the 66cc with billet head is hard to turn over. The stock head was 115psi, the billet is 142psi. Doesn't sound like a huge increase, but even a 23yo friend of mine was shocked by the compression and has trouble turning it over and he's a lot younger, stronger and fitter than I am.

    No worries, Jim, and as I mentioned, I'll just buy a spare HD freewheel when I buy the sprockets etc and keep it on hand, just in case. The cost is worth the ability to kick-start.

    The rain has finally stopped, so I'm off for a test-ride shortly.

    ... Steve
  12. You know they sell a pull start dont you,i have one and my engine starts with the first pull.
  13. Pablo

    Pablo Motored Bikes Sponsor

    I may just get a pull start!

    Unlike Jim, I run a pretty extreme compression ratio. I have a shaved slant plug head. Mid winter, my engine is pretty tight first turn over after sitting a week or three. But once I crank it, the next crank is not so bad and then it fires up.

    When I put the regular head back on, it's like butter!!
  14. Fabian

    Fabian Well-Known Member

    Hi Paul

    Is there any chance of SickBikeParts making a decent slimline billet pull start mechanism for the Chinese 2-stroke engines.
    This would allow me to change to a much desired 9T - 48T jackshaft to chainring gear ratio.
    I would not have the issues of trying to pedal start the engine.

    I've got my credit card at the ready should it be put into production.

    Last edited: Feb 9, 2010
  15. Pablo

    Pablo Motored Bikes Sponsor

    Thinking about it.

    Why would it need to be from a "billet"?
  16. Fabian

    Fabian Well-Known Member

    The pull-start doesn't have to be made from billet.

    I was just thinking from the perspective of a totally redesigned slim line system made up in CAD, keeping the thing nice and compact and free from pedal crank interference with standard width cranks.

    I guess it would be easy to punch the parts out on a CNC mill if it's designed as a CAD file.

    I've heard that the standard Chinese pull-start mechanism is not very well engineered and prone to breakage.

  17. Skyliner70cc

    Skyliner70cc Active Member

    I've got 2 bikes with pull start. The only failure I've had with them is the cheap pull rope. Once the pull rope was replaced, both have been very reliablable with no issues.

    As for pedal starting, I had one freewheel fail on a high compression head. I then upgraded to the heavy duty freewheel as a precaution. I usually pull start on first start of day and then pedal start when I'm riding-I shut motor off and pedal by pedestrians and other bikes on the trail and restart after I pass them.
  18. Ghost0

    Ghost0 Guest

    I agree that a quality, compact pull start would be cool. The problem is the SBP R&D department already has a full plate. Maybe the Manic Mechanic can take on the challenge. He sounds like he is buys too though.
  19. AussieSteve

    AussieSteve Active Member

    I considered a pull-start, but had heard that they fall apart quickly.
    Also, I wasn't sure how a pull-start would go with the hi-comp head.
    The other issue, as mentioned above, is the width.
    If there was a good slim one on the market, with a decent ratio for starting with the billet head, then like Fabian, my credit-card is at the ready.

    Had my first spin with the shift kit yesterday. No problems, everything is working well. Wow, what a difference. Only had a short ride, to let things settle a bit. A quick chain adjust etc and I'll have a decent ride later today and give it a real test.

    ... Steve
  20. Fabian

    Fabian Well-Known Member

    Ok, so there is some consensus that manufacturing a compact slim line pull start is economically viable - 2 people with credit cards at the ready!

    Someone, please manufacture a decent pull start and get it happening ASAP!

    I'm going to create a new thread, calling for manufacturers to create a compact slim line pull start and a new design multi plate wet clutch.

    Last edited: Feb 10, 2010