Rear Sprocket Sizes for Frame Mounted 4 Stroke

Discussion in 'Frame Mounted Engines' started by Irish John, Apr 20, 2008.

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  1. Irish John

    Irish John Guest

    I have built my Grubee HuangSheng from the kit supplied by but I kept the 44T sprocket I had on the rear wheel. This goes beautifully on the flat at about 30 mph but is no good on hills and the engine splutters and struggles when I have to pedal at speeds of less than about 10mph. I could put a 48T sprocket on or the 56T sprocket that came with the kit but I fear that the 56T will be very slow on the flat.
    Has anyone got experience with different sprocket sizes on the rear wheel. I've read about people using 38T sprockets but that would surely struggle on a slight incline. It is a long and messy process to change sprockets and then lengthen or shorted the chain to suit so I am interested to know from others experience before I go ahead and use a 48T and then the 56T if I have to.
    I am not that heavy - about 85 kilos. The bike is a Schwinn cruiser.

  2. HoughMade

    HoughMade Guest

    I had mine with the 56 tooth over 30 mph without much drama. I just put the 48 on (haven't ridden with it- the things is in pieces right now), but I am inclined to take that off and go with the 56 as it worked fine and I am not small at all.... 6'3" over 220#
  3. Irish John

    Irish John Guest

    Thanks so much HoughMade, On the strength of that I'll forego the 48T and put the 56T sprocket on. I really hope it will go over 30 mph which is 48 km\hr - a good enough speed for any bike. I hope I don't have to turn back on hilly roads with that one. The KMC chain I use has to be cut in order to add links. Chain splitters won't push the pins out of the links.
  4. fetor56

    fetor56 Guest

    Don't use a chain splitter...use a nail punch and hammer & ONLY push the pin far enough through to get the link out........NEVER push the pin right through or you'll never get it in again.
  5. Irish John

    Irish John Guest

    Thanks Fetor. I will do that for ever after but I bet I punch the pin straight through. Another name for a heavy hammer is an Irishman's Spanner!
    I do hope people with sprocket knowledge will contribute to this thread cos it's an important subject for 4 strokes.
    Last edited by a moderator: Apr 20, 2008
  6. ocscully

    ocscully Member

    I'm going to wade in here with some math/numbers on various 4-stroke kits that are currently available. I've been researching and collecting this information as part of putting together a drivetrain for my first motored bike project using Metric Timing Belts and Pulleys.

    The basic Grubee Kit as supplied by has a total reduction of 18.85:1 using a 56t sprocket which gives you a minmum speed of 9 mph @ 2200 rpm and a max speed of 29.6 mph @ 7200 rpm. Now Houghmade has reported speeds approaching 40 mph using this set up and seems to be more than happy with its performance. The numbers for a 44t sprocket are as follows 14.8:1 total reduction 11.2 mph @ 2200 rpm and 37.6 mph @ 7200 rpm. A 48t sprocket equals 16.15:1 total reduction 10.5 mph @ 2200 rpm and 34.5mph @ 7200 rpm

    The Dax Titan Rack Mount as it was tested by Augie and I believe currently being sold has a 44t sprocket on the rear for 22:1 total reduction 7.7 mph @ 2200 rpm and 25.3 mph @ 7200 rpm The titan has a higher rev limit though so I'll add a max speed of 28.1 mph @ 8000 rpm. During Augie's testing of the prototype he reported a max speed of just over 38mph and seemd to say that it could cruise in the middle 30's efforlessly. And Duane at Dax claims that this set up will cruise at 35 mph all day long.

    The GEBE Kit with the Subaru/Robin 35cc with the 12t driver has a total reduction of 22.25:1 7.6 mph @ 2200 rpm and 25 mph @ 7200 rpm. Members here are reporting speeds approaching 35 mph with this set up and GEBE claims 28-35 mph for ths system.

    One of the first things you will notice is that the math vs the reported speeds are far from the same or even close. So how do you make a decision? It seems to me that you want to have a total reduction in the high 18:1 area if you want to do a minium amount of pedaling to start your bike and assisting on climbs. And you seem to have come to this conclusion as well. The 48t and 44t are both pretty much flatland set ups and even on the the flats both of these sprockets would require a lot of pedaling to get them going with your engine and gearbox set-up. What I find interesting about your experience is that with the 44t sprocket that you started with I would have expected you to be reporting speeds in excess of 40mph seeing as how Houghmade is just under 40 mph with a 56t sprocket?

    I'm still trying to understand why the math of the ratio calculator is so different from peoples actual experience with these various drive systems?

    Note: the rpm's used for these calculations are 2200 rpm because that is the rpm where most auto clutches engage and 7200 rpm because that seems to be the max rpm for the Honda GXH50/or Clone with a load.

  7. HoughMade

    HoughMade Guest

    I suspect that my speedometer was off to some extext when I reported close to 40 mph- largely based on your figures, that is why I reduced my speed report from 40 to over 30- I do not see how my speedometer could be that far off. One of these days, I'll verify everything another way and see just how off it is. Obviously math does not lie, but I am not going to assume the reduction on the Grubee box until I have actually counted the teeth in my unit.
  8. HoughMade

    HoughMade Guest

    One other thing- this is what you will find in the FAQ section of
  9. HoughMade

    HoughMade Guest

    OK- I went to bicycles-engine, blew up the pictures of the gears for the Skyhawk II and counted the teeth.

    Here is what I got:

    Primary gear on the centrifugal clutch: 35

    Large intermediate gear the primary gear engages: 54

    Small intermediate gear directly keyed to the large intermediate: 20

    Large final gear keyed to the output gear: 48

    The ratio of the box itself is therefore 2.7:1.

    Let's say the 35 tooth from the engine goes around 1000 times. How many times does the 54 tooth go around? 648 times. The 20 tooth keyed to the 54 tooth also goes around 648 times. If the 20 goes around 648 times, how many times does the 48 engaged with the 20 go around? 270 times. The 48 tooth is keyed to the output gear. Therefore, for every 1000rpm that goes in, 270 come out= 2.70:1.

    Now, plug that into the gear ratio calculator for a 26" bike.

    6000 rpm- 33.8 mph

    7000 rpm- 39.4 mph

    Run it over 7000 rpm and you can have over 40 mph- but I would not do that much.

    I'm a lawyer, not a mathemagician, so if I screwed that up, show me and I will admit my mistake ;)
  10. HoughMade

    HoughMade Guest

    My error has already been pointed out- top speed is a little over 30 mph if you do not overrev it. whoops- simple math error- but apparently not as simple as I am.
  11. Irish John

    Irish John Guest

    If you measure your wheel by rolling it along the ground for a complete revolution and measure that length on the ground etc. and then put that into the digital speedo you will get a speed reading of much greater accuracy than a car. Bike speedos are very very accurate if they are set up with the correct wheel measurement.
    I hit 58 km\hr yesterday with my 44T sprocket but there was a tail wind - in more ways than one cos 58 is pretty frightening on a Schwinn cruiser! My 44T sprocket can't climb inclines at all well and will have to be changed. Great on the flat - very fast at low revs but useless on any hill. Pity we don't have gears on frame mounted engines.
  12. HoughMade

    HoughMade Guest

    I could not bring myself to use a digital speedometer....probably part of the issue.
  13. Irish John

    Irish John Guest

    Yes HoughMade, I know what you mean. I used to have MG sports cars until they replaced the flickety Smith speedo & tacho with those ones that show a red line progressing horizontally like a bar chart. I was disgusted with them.
    Another thing about digital bike speedos is they take too long to adjust to changing speeds because they need the wheel to revolve a few times before they can figure out the speed.
  14. Herrmanator8

    Herrmanator8 Guest

    hey, im not a real expert on sprockets but im always looking for ways to make things faster so im just learning.. but i had seen a website posted on here before thats really helpful it: it helps determine the top speed of a gear ratio.

  15. Irish John

    Irish John Guest

    This discussion isn't really about max speed its about optimal performance under differening circumstances. If you never go up hills its totally different to if you do go up hills. If you want to thrash the guts out of a small motor then it won't last long. It's about finding a good speed over the average of road conditions where the motor isn't running at full revs all the time. If I want maximum speed I'd buy a Ducati or a Suzuki.
  16. fredbert

    fredbert Guest

    What size of sprocket would be best for going up hills, I want torque and about 10-15mph max.

    I have a 20" wheel using a gxh50 with grubee bear box.
  17. HoughMade

    HoughMade Guest

    The higher the tooth count, the more low end torque and the less top end speed. If you want 10-15 mph max, even with a 20" wheel, I would try the 56 which is the highest commonly out there.
  18. Irish John

    Irish John Guest

    56 tooth seems to be the largest I've seen and even with that I had clearance problems because the chain was hitting the chain stays on my Schwinn cruiser if it stretched a bit. It does work OK but the clearances are very exact and I found I was always adjusting it to make sure it had an unimpeded run. The 48T was faster overall but I do have to pedal hard on certain hills - so much so that the muscles in my thighs get sore. Some hills are to be avoided altogether. Even the 56T couldn't get up the really steep hills. The 4 stroke isn't as good on pulling power as the 70cc 2-stroke but there are so many other advantages.
  19. HoughMade

    HoughMade Guest

    With his 20 inch wheels, the clearance will be the issue, but it will pull much better than on a 26 inch wheels- it's like having something like a 65 or 70 tooth on a 26".
  20. fredbert

    fredbert Guest

    Thanks for the advice I was thinking along the line off this sprocket.

    I should have said that I am making/trying to make a trike setup.

    What about a mini moto sprocket?

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