questions about gearing, top speed and pipe length

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bigbutterbean

Active Member
I have a 68cc 2 stroke bicycle engine purchased from boygofast on ebay. I am considering several different setups, but one involves changing the gearing with my current engine for higher top speed and changing my pedal gear to the fewest number of teeth possible for easier pedaling up hills. currently I have a 44t rear, and at about 27mph, my ratio calc tells me my engine is spinning at about 6250 rpm. I want to increase top speed to improve gas mileage. a shift kit is not currently an option as I have a single speed cruiser. I really like the look and feel of this bike and the way it rides. I weigh around 280. So here are my questions.

Does anyone know what is the fewest number of teeth available on a pedal gear for a single speed bike? I currently have 28 teeth on my pedal gear but would like less.

I am considering a 36t sprocket. I want to cruise at around at least 30 and get there fairly quickly, as I live in the city and there are traffic lights to deal with. From what I understand, the key to achieving speed quickly with taller gearing is the right pipe. I would be purchasing the sbp expansion chamber. What is an approximate estimation of a proper head pipe length to achieve a cruisable speed quickly?

I know that hills will be even more of an issue now than they are with my current gearing, but I believe I can overcome this obstacle by making it as easy as possible to pedal uphill. My only concern is putting extra stress on the engine, even though it will be far easier to assist the engine with a smaller pedaling gear. Will this be an issue regarding the life of the engine?
 
I usually run a 36T on my 80s. Still climbs hills pretty good, and easily runs up to 35-36.

On the peddle side, a 28T is pretty small already. The sprocket sizes you can find depend on what kind of crank you have. If you can't find a smaller chainring, you might be able to get a bigger sprocket for the rear wheel.
 
I have to help the motor out on hills with a 44. I am sure that a 36 will not climb hills well for me. gonna stop at the local bike shop and ask about chainrings. how about a suggested pipe length for the sbp expansion chamber?
 
bigbutterbean said:
I have to help the motor out on hills with a 44. I am sure that a 36 will not climb hills well for me. gonna stop at the local bike shop and ask about chainrings. how about a suggested pipe length for the sbp expansion chamber?
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You're outweighing me by 60 lbs, but I have 48cc with a 40T. It will do 30 mph and chug up the steepest hills around here at 20 mph.

They say one tooth equals 1 mph.
 
yeah my weight is probably the main thing that keeps me from climbing hills. Im compensating for drag by making it easier to pedal. the guy at the local bike shop is retarded. he is trying to tell me the smallest chain ring I can get for a beach cruiser is 43t for a one piece crank, and 25t for a 3 piece crank. he said that the largest back gear I can get is 21t. of course, this is the guy who told me that they havent made drum brakes for bicycles in about 30 years. i dont even know why i go there. so we have hill climbing covered. how about pipe length?
 
bigbutterbean said:
yeah my weight is probably the main thing that keeps me from climbing hills. Im compensating for drag by making it easier to pedal. the guy at the local bike shop is retarded. he is trying to tell me the smallest chain ring I can get for a beach cruiser is 43t for a one piece crank, and 25t for a 3 piece crank. he said that the largest back gear I can get is 21t. of course, this is the guy who told me that they havent made drum brakes for bicycles in about 30 years. i dont even know why i go there. so we have hill climbing covered. how about pipe length?
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You can always google the parts in question to find out what's available.

I've run a SBP pipe with a short 9"ish, and a long 22"ish header pipe on one of my bikes.

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I thought the top speed was about the same either way. About 38 mph. With the long header it had much better bottom end.
 
You're going to need a large chainring if you plan to pedal while going up hill at a reasonable speed. Maybe a 44t chainring?
 
sorry venice, you are mistaken. the chain ring is the gear on the pedals. larger chain ring = faster speed. smaller chain ring = easier to pedal. already went down from 44t to 28t and is easier to pedal than it was before. not sure if I will find a smaller chain ring, so I will have to get a bigger rear cog. husky sells a 22t rear cog, gonna order that eventually.
 
I know the top speed is going to be about the same no matter what the length of the header is. but from what I understand about expansion chamber is that a different length head pipe changes how quickly the engine can achieve peak rpm, and thats what I am looking for. I want to achieve peak rpm as quickly as possible after taking off from a dead stop, so what is a good starting point for pipe length for achieving peak rpm quickly?
 
actually, you sort of already answered my question. you said with a long header it had much better bottom end. I take it acceleration was much smoother and quicker with a better bottom end? so when I get the sbp expansion chamber kit, use the entire length of pipe that it comes with, and that will help out acceleration?
 
bigbutterbean said:
actually, you sort of already answered my question. you said with a long header it had much better bottom end. I take it acceleration was much smoother and quicker with a better bottom end? so when I get the sbp expansion chamber kit, use the entire length of pipe that it comes with, and that will help out acceleration?
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Yeah a longer header moves the powerband lower in the rev range. How long the header is depends on where you can mount the pipe, a lot of times.
 
I may possibly get a mountain bike and do a shift kit, but we will have to see. a friend of mine just got a scooter, so maybe I can talk him into giving me his mountain bike. we shall see.
 
The reason I said to go with a large chainring is because the best way to go up hill on these is to get a running start & go fast up the hill. A small chainring is good for pedaling at low speed.

Your title for this thread asked for help with 'gearing for TOP SPEED'. :-{
 
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re-read the title of my post, which is, "questions about gearing, top speed, and pipe length". That means I have questions about gearing AND top speed, not gearing FOR top speed.
 
you have your preferred method of climbing hills, I have mine. a smaller chain ring will make it EASIER to pedal than a large chain ring.
 
There is really a lot more involved in changing the powerband on your engine than just the length of the head pipe. Port timing in any given 2-stroke using an expansion chamber has to be calculated to be done properly. A bolt-on expansion chamber such as the SBP pipe is not specifically tuned to your particular engine. To get it where you want it is going to take a little experimentation and some calculation on your part, it's not just a matter of using an 8" or 20" head pipe and viola.....instant low end torque monster is produced.

I don't mean to discourage you, but one pipe may work great on one bike and not so great on another with a nearly identical engine. A larger displacement engine will give you more of what you're looking for but it may not be legal in your state as we all know. Just a little food for thought.

Experiment, take in other peoples input and don't get discouraged......you'll get it in time. Ride like the wind!
 
I have a 68.5cc engine from bgf. Its not legal in my state, but I see cops all the time and they dont stop me. I am going to get an sbp expansion chamber and just experiment till I get it performing the way I want. I know now that a single speed bike is not easy to pedal up hills, no matter what pedal gear ratio you have. At least not for me. I have decided that I need to step up the game. I thought about installing a 99cc predator, but after adding up the cost of the engine and all the parts I would need, everything came to about $600. Of course, that is including a front wheel with a drum brake. I know that no matter what, I need better brakes if I want to go faster. I then realized that I can get a mountain bike at wallyworld with a front disc brake for around $200. shift kits are less expensive than they used to be, so for a little over $400, I can have a bike with a multi-speed transmission. I bet with a shift kit, the performance range is very decent. I can also control my top speed by switching gears, which is going to be safer for city driving. So I am going to go with the shift kit setup. It took me a long time to come to this decision, being that the reliability of these engines isnt all that great compared to 4 strokes, but I feel its the most economical for me.
 
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