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Rear Sprocket Sizes for Frame Mounted 4 Stroke

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Irish John

Guest
I have built my Grubee HuangSheng from the kit supplied by Bicycle-Engines.com 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.
 


H

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#
 
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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.
 
F

fetor56

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.
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.
 
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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:

ocscully

Member
Joined
Jun 23, 2007
Messages
661
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 bicycle-engines.com 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.

ocscully
 
H

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.
 
H

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 ;)
 
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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.
 
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