rear sprocket swap...what can i expect?

Discussion in 'Frame Mounted Engines' started by motorpsycho, Apr 8, 2010.

  1. motorpsycho

    motorpsycho Active Member

    i currently have the stock 44 tooth rear sprocket on my bike.
    49 c.c., slant head, expansion chamber, high flow air filter, 20" frame with 20" wheels.
    right now, i can get about 25-27 mph out of my bike on flat ground and it has plenty of take off power.
    If i go to a 41 tooth sprocket, how much difference should i see in the top end?
    how much bottom end will i lose? howmuch will the mid range rpm's drop?
    If there will be a 2-3 mph increase, and little to no loss of bottom end, then i might make the swap.
    I just want to see if i can get a couple more mph out of it, and lower the rpms when i'm just cruising at 15-20 mph.
    I would just drop to a 36-38 tooth,but i'm afraid that i would lose too much bottom end.
    I do not peddle my bike, i ride it like a motorcycle. since i am running 20" wheels, the rpms are pretty high at 15-20 mph.
    would going to a 41 tooth sprocket be worth it? or will i not really notice a difference?
    i read that you can expect a 1-2 mph increase for every tooth smaller you go. so, if that's true, going from a 44 tooth to a 41 tooth should give me about 3-4 mph more, and it MIGHT lower the mid range rpms a tiny it worth it? swapping a rear sprocket isn't a 10 minute job, so i don't want to waste my time if it won't make a difference.
    my main concern is lowering the mid range rpms, rather than gaining top speed.....but a few more top end mph would be cool too.
    Last edited: Apr 8, 2010

  2. Neon

    Neon Member

    Have you considered investing in a Shift Kit? That solves sprocket swaps. Costs a lot more than a sprocket though.
  3. retromike3

    retromike3 Member

    smaller is better

    I went from a 44 to a 36 and it made a big difference in my top speed. I do have to start out pedaling and then engage the clutch but I can get up to about forty M.P.H. on the flats. I do have a 26" wheel so you might want to get a bit smaller cog. It made a big difference in how useful the bike is in getting around the suburbs. I have a S.B.P. pipe and a good plug and shielded wire so it does have a bit more horse power than stock.

    I think if I had enough money I would love to have a shift kit, but a smaller cog is about 40 bucks and a shift kit is a lot more. so I go with what I can get on the cheap. It would be hard to say anything bad about S.B.P. they have treated me quite well with all of my questions and like a human.

    My two cents Mike Frye
    Last edited: Apr 8, 2010
  4. motorpsycho

    motorpsycho Active Member

    well, the problem with a shift kit is that i can not use one.
    I have a single speed rear hub(because i am using a 20" schwinn stingray with 20" wheels) and i do not have the room to add a shift kit / jack shaft to my bike. i would not be able to add say a 5 speed rear hub because the rear frame is too narrow.
    yes, with a 20" wheel and a 44 tooth sprocket i can still get 25-27 mph out of my bike which is pretty good if you ask me.
    Considering the fact that my rear wheel is only 20" i do not have the advantage that most guys have by running a 26" wheel.
    I don't want to have to peddle on the take off tho, i just want to let the clutch out and go.
    i think the most common available sprockets (without having one custom made) is 60, 56, 50, 44, 41, 38, 36 tooth.
    i know that going from 44 teeth to 41 isn't a huge difference, but i'm afraid to go with the 38 for fear of losing too much bottom end take off.
    The only alternative i have is to swap rear sprockets, or leave it as it is with the 44 tooth.
    i do not ride in traffic, and i do not commute on my bike..i just buzz around my neighborhood and the back roads here for fun.
  5. retromike3

    retromike3 Member

    Ok but what next?

    The question is now is what can you do Vs. what can you get away with? Since you do not have a freewheel or a rear brake and you don't want to pedal to start your kind of limited to your top speed. I have built up multi speed hubs on twenty inch wheels and I think you can set up a Stingray with a BMX brake if you want to, But it will take a little more work and a bit of pedaling when you start. That is the compromise. I myself like to go fast (what I think is fast anyway) and so I am willing to pedal for about four strokes and then drop the clutch. I like the extra range I can get out of my motorbike(since I live in the suburbs and everything is so far away from everything else here) and the speed I get out of a taller gear. A 41 might be the compromise you are looking for, it would go a little bit faster and you might get away without pedaling (although I need to ride my pedal bike more or buy fat man clothes) but even if you did I don't think you would ever pedal hard enough to rase a sweat, I don't.

    good luck, and keep riding!:bandana:

    mike frye
    Last edited: Apr 10, 2010
  6. motorpsycho

    motorpsycho Active Member

    well, i really can't afford to buy a shift kit, and then have the added expense to do a multi speed rear hub and then find out that it doesn;t fit. I pretty much know that a jackshaft set up will not fit on my bike, plus, like you said, i would have to come up with a rear brake if i run a free wheel rear hub.
    I like my coaster brake and i don;t like the clutter of a derailuer and all that.
    I wouldn;t mind peddling if it means that i can make the mid range rpm.s drop a bit, and possibly raise my top speed a bit.
    I'm still thinking on the whole thing....not sure if i want to just leave it as it is, or do the sprocket swap.
    another thing is that there are a lot of hills here, so that's another thing to consider.
  7. happycheapskate

    happycheapskate Active Member

    I wondered how a 44t would do on a 20" wheel. 27mph you say?
    Not bad.

    I'm sure someone here has a gear calculator for MB's with happy time type engines.

    I've got a 44t on a 26"x1.75 ATB wheel. It goes about 32-35mph WOT as fast as possible. I usually go 20-28.

    I have been thinking about going to a 20" wheel with a dual cog BMX hub using 20t fixed left side and 20t freewheel right side. This should give a taller overall gear but help with the acceleration of the bike overall, and give more space for a wide checker tread tire. I am not sure if my pedals will hit the ground though.

    Maybe on my next build!
  8. motorpsycho

    motorpsycho Active Member

  9. motorpsycho

    motorpsycho Active Member

    rear sprocket swap...

    I decided to do a rear sprocket swap. I went from a 44 tooth sprocket to a 41 tooth sprocket. Even tho there is only 3 teeth difference, the 41 tooth is quite a bit smaller in diameter than the 44 tooth.
    One obsticle for me was the chain because i had my chain cut to fit the 44 tooth, and with the 41 tooth, the chain was too long. I cut out 1 link, and then the chain was a bit shorter than i would like it to be. So, i ordered a couple of 1/2 links from sbp to fix this.
    Right now, i have the rear wheel mounted on my bike, and the chain on, so it is rideable.
    the thing is, i just don;t like how close the axle bolts are to the openings in the drop outs. I need to slide the wheel back a little more for peace of mind.
    The other obsticle was my rear fender. I had to chop off part of the fender where it mounts to the frame(by the cranks) because the tire was rubbing on the fender mounting screw.
    With all that out of the way, it took me about 4 hours to do the axle swap, cut the chain(s), cut the fender, and get it back together.
    I have not ridden it yet to see how much of a difference there is with the 41 tooth sprocket, but all i can say is that it better be worth it.
    I am not going to ride it wide open until i get the 1/2 links and get the rear wheel slid back a little more.
    I will take it for a test ride this afternoon to see if i can notice any difference in mid range rpms.
    remember, i have 20" wheels and with a 44 tooth sprocket i was getting about 25 mph out of the top end.
    we'll see how much top end i gain by going to the 41 tooth...but my main concern was lowering the mid range rpm's a little (while just crusing @ 15-20 mph.)
  10. imjakethesnake

    imjakethesnake New Member

    im riding on a 20 inch rim with 44t to 18t and im getting 47.7 mph with a 43cc motor... but i have a diff setup
  11. motorpsycho

    motorpsycho Active Member

    well my set up now is the stock 10 tooth gear on the motor, with a 41 tooth sprocket on the rear wheel. My bike takes off from a stop the same as it did with the 44 tooth, but now it has more acceleration.
    It's like by raising the gear ratio, the motor comes into it's mid range power band later, and it holds it longer. It pulls better than it ever has because the engine torque is not peaking and dropping off as soon as it was with the 44 tooth.
    I only took it up to 21 mph yesterday, but at 21 mph, there's still plenty of rpms left to go more.
    With the 44 tooth, it felt as if the engine was wound pretty tight at 21 mph, and it would struggle to get up to 25 mph.
  12. happycheapskate

    happycheapskate Active Member

    Thats cool. I'm ordering the 36t from Bikeberry today. I will have to drill my own 9 holes though, bummer. Still it seems the cheapest option.

    For those running a 20" wheel, has anyone used a track hub with a locking fixed sprocket (say 18t ?) and a BMX freewheel or 6 speed freewheel on the pedal side?

    This ought to solve the problem of the rag joint and give a seriously fast gear/wheel combo.
  13. motorpsycho

    motorpsycho Active Member

    you could use your old 44 tooth sprocket as a template to drill the 9 holes on the 36 tooth sprocket.

    Right now my pedal side sprockets are 36/16 which makes peddling pretty tough with 20" wheels and 4" long cranks (like starting off in 5th gear on a ten speed.) so i am going to swap out the 16 tooth sprocket and replace it with a 19 tooth, which will lower the gear ratio and make it much easier to pedal.
    I don't pedal it very much, but it will be easier when i need to.
  14. happycheapskate

    happycheapskate Active Member

    Thats what I expect to do. I plan to either make a stencil with it, or clamp them together and mark them, drill tiny pilot holes, then bigger holes. I haven't seen the pre-drilled 9 hole 36t sprockets that also have the offset teeth like the original cog. All I've seen is flat "racing sprockets". I guess those are good if you can use them, because you can flip them over when they start getting "shark teeth" and extend the life of the sprocket.
  15. happycheapskate

    happycheapskate Active Member

    I got a 38tooth 5 hole offset sprocket from Bikeberry and made it 9hole and widened the center hole with sandpaper. It works well.

    BBerry sells 9hole flat 36t now for 25$

    I am still thinking about the track hub with BMX rim though. Has anybody done that?

    I know the pedals will drop about 2" and steering angle will slacken. That would be ok.
  16. Slowbalt

    Slowbalt New Member

    By my best guess 'I'd say 32mph. It should still have good mid range. Best thing for those 50cc engines is a pochet bike tuned pipe.
    Last edited: Mar 20, 2011
  17. Reader

    Reader Guest

    A smaller rear wheel is gonna give you more top end speed not vice is just the same as having a smaller rear gear. The front spricket is opposite.
    Last edited by a moderator: Jun 24, 2017
  18. Jwestie15

    Jwestie15 Member

    Currently waiting on a shift kit and my new wheelset, but I'm building 9speed shifter on 24x1.75in rims with 1.75 tires
    Should have a good combo of speed and acceleration with the shift kit and the smallish tires, will report back with real world experience after my parts get here