Gear ratio: Specific Torgue Question

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GreedyRogue

New Member
I just finished my first build and am loving it so far. I have a 48cc BGF motor with a 41 tooth rear sprocket. I weigh 260 lbs and get a top speed of about 28ish, which is fine for me. I am still breaking on the motor and have only put in about 3 liters of fuel, so maybe I need to finish breaking it in before I change the sprocket?

The problem I have is any incline in the road seems to kill the speed/power. I don't mean like small hill or mountain trail degrees, I mean like any increase at all. Even a small 1-2 degree seems to drop me to 15-20 mph mark.

What I need is a suggestion on how much bigger a sprocket I can get. I prefer to stay around the 25 mph mark but I definitly need some higher torque. I know my first concern is the weight issue but until I get that fixed I need help with how big a sprocket I can go before I start dropping below the 25mph mark. Is there a calculator that I can use to see the different variables?

Unfortunatly the shift kit is out of the question, both financially and because they way I interpret the law, they aren't allowed in Idaho. Open to any recommendations.
 
just modify your exhaust... the exhaust spout should be at around 16mm... You can drill an additional hole and install another spout making a dual pipe. You'll notice an increase in torque by at least 50% and can possibly use a 36 tooth sprocket. Although, it will be loader but still tolerable... A tuned pipe would actually be better if you got the $$$.
 
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I actually took the exhaust into consideration. I removed the entire baffle and end cap and just "straight piped?" it. There was a minimal increase in power. Went up the same street and still felt the motor slowing down. I don't think the power increase vs. noise level is worth it though.

I live in Twin Falls Idaho, and there are a couple places in the canyon I would like to be able to go to with my bike, but it does not have the power to go back up the road, even with me trying to peddle. Mind you, I am out of shape but still. I don't mind losing speed if I can get the hill climbing out of it that I need.

I'm not sure how to figure out the balance between speed and climbing I need without just buying all the different sprockets and test driving them but that is not practicle, nor in my budget.

unfortunatly there are only a few other MB's in the area. I met 2 of them at the gas station the other day. We had a nice chat, compared bikes and such. They said I was they only other person they have seen or heard of.

If there was more of aMB community here, I could see about maybe borrowing a few sprockets to test but no such luck.
 
Before you do anything wait until you get some more miles on the motor. It will start to run better after about 2-3 gallons of gas. For the first two gallons use 24:1 oil mix and check the gap on your plug. About .020 - .028 will be fine unless your using the white wire for lights, then use .015 - .020. After break in get yourself an SBP expansion chamber and go to 32:1 on the oil mix unless you want to use synthetic oil then follow the manufacturers recommended mix and use just a little bit more. After that then you can play with gear ratios but I would stick with the 41 tooth on a 48cc, anything more and your asking for too much.
 
i think waiting may be the best answer ans just ride a few tanks through it like you suggest. I put the baffle back into the exhaust but I had to remove it to line up the screw hole. Now my gasket is shot and the only stuff they have in stock locally is 1/4 as thick as the stock gasket was.

Ill have to check the spark plug gap next as I never bothered/thought to check it when I installed the motor.

Maybe I should be surprised it works at all.

EDIT:
I looked up 4 stroking and think that may be one of the problems I am having. I "think" that it is 4 stroking in the mid range. I watched a youtube video that demonstrated regular 2 stroke and the 4 stroke sound using a "choke trick." I checked my c clip and it was on the 3/5 mark so I moved it up to the 2/5 to make it a bit leaner. Not as much of the 4 stroke sound but it is there in the mid range still.

Comparing the throttle on a range 1-10, before it was 4 stroking at the 4-8/10 range, now it is in the 5-6/10 range. What happens it I raise the c clip again to make it even leaner? Can I damage my motor with it too lean?
 
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as long as you're getting a little 4-stroking, you don't have to worry about be ing too lean.
You should do a plug chop and check for the right color to verify that your mixture is right.
After it's warmed up good, run it hard for about a munite with lots of throttle.
Then, hit the kill button and the clutch at the same time, and coast to a stop. After it cools down, remove the plug and look at the inner insulator. Light brown is perfect. Whiteish is too lean, and darker is too rich.

I always try to tune mineto where I get a tiny bit of 4-stroking at WOT, so I can knowthat sound that it's not too lean anytime, by the sound.

Sounds like you could raise the clip one more notch.

BTW, I run 50/1 pennzoil mix in all my chinagirls. after break in. never any problems yet with 4different motors. I use 32/1 during break-in, and run it hard woth lots ofull throttle, but not letting the rpms get too high
 
Plug chops are going to be a little more difficult on my bike. I don't have the clearance to remove the plug without removing the engine. Today may be the only rain free day to ride it for the next week. I'm considering getting an NBK plug for it and playing with the fuel mixture before I play with chops.

Right now I'm still going through my first gal. of fuel mixed at a 16:1. I think next gal. ill change it to 32:1.

Since I'm no longer worried about the sprocket issue, I'll do some testing and if I still have problems with the carb/fuel/plugs, I'll make a new post in the right forum.

Thanks for the replies here though.
 
I have a 48cc Flying Horse motor that I've tuned since last September. I have a 40T sprocket. My bike will run between 28 and 31 mph on a flat depending on wind direction. It does pretty good on hills, but any hill slows it down some. It takes a steep hill to slow it down below 20. usually it can maintain about 23 uphill. I weigh about 220.
This is what I've done to get this kind of performance.

I got a slight improvment in power and throttle response by switching to an extended reach spark plug NGK BP6HS

I replaced the spark plug cap with a NGK and installed copper core wire.

I think these motors come jetted a little rich from the factory. I rejetted to a 68 from a 70. It still runs a little rich but close enough.

I moved the slide needle to the leanest position.

I run regular gas.

I did open up the exhaust a little, but only enough to increase noise very slightly.

I use Red Line synthetic 2 stroke oil @ 50:1

If you have a lot of hills consider going to a 44T sprocket.
 
My problem isn't even hills, any dip in the road and I slow down to the point of stopping. I put in a ngk b6hs yesterday and I'm looking at replacing the spark plug wire as well, soon.

I have already put my slide needle at the leanest with little effect other than my idle is too high now. I have a couple hills in town but only if I go down into the canyon. The rest of town is mostly level, just the normal up and down that all cities have.
 
GreedyRogue said:
My problem isn't even hills, any dip in the road and I slow down to the point of stopping. I put in a ngk b6hs yesterday and I'm looking at replacing the spark plug wire as well, soon.

I have already put my slide needle at the leanest with little effect other than my idle is too high now. I have a couple hills in town but only if I go down into the canyon. The rest of town is mostly level, just the normal up and down that all cities have.
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It sounds a lot like you motor isn't running very well. I'm always surprised how well my little 48cc motor can drag my hefty butt up hills. You should be able to at least maintain 20 mph on a fairly steep hill especially with a 41T sprocket. Mine is a 40T. Some of the steepest hills I've gone up have dropped me down to about 18 for a few seconds.
 
The roads we have around here are just your basic up and downs, but like I said they still kill me. Driving up a steep hill like up the canyon road we have here in Twin Falls, it just can't do it at all.

I did replace the spark plug wire and boot and got much better power out of it, but I think I'm still running too rich a fuel.
 
How fast does you bike go on a flat road. It should be doing about 25 or a little more. Elevation could have a little to do with it. I'm at about 650 ft and my bike came jetted really rich.
 
Exact speed, I'm not sure. But 23-27 sounds about right. I got a front flat and my speedo wire got cut. Until I get that fixed, I'm just guessing.

I need to figure out how to measure my jet so I can see where I am currently. Who has the best prices for jets if I need to buy one? I don't have the tools to do it myself.
 
GreedyRogue said:
Exact speed, I'm not sure. But 23-27 sounds about right. I got a front flat and my speedo wire got cut. Until I get that fixed, I'm just guessing.

I need to figure out how to measure my jet so I can see where I am currently. Who has the best prices for jets if I need to buy one? I don't have the tools to do it myself.
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You can buy jets at sick bike.
 
HI,

If a 41T sprocket gets you 28 MPH, then a 44T (a common "stock" sprocket will put you about 25.6 MPH roughly (on a 26" wheel typically you will gain or lose about 1 MPH for every tooth you gain or lose....I think the actual number is actually about .8 MPH FWIW)...

If you go to about a 45 T or 46 T sprocket you will gain a little more hill climbing power while still being close to your 25 MPH goal.

While going with a larger rear sprocket will help you in climbing hills, I would recommend you gear for the for the terrain you ride 95% or more of the time....no sense in "buzzing" the engine unnecessarily if your main concern is only a hill or two.....On the other hand...I'm not sure why a very small incline would slow you down so much but there were a number of good suggestions on what to look for in the earlier posts.

Hope this helps you.

Andrew
 
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I got my speedo running and top speed was alot lower than expected. 19-20 seems to be my top speed on flat ground, even at wot. The type of "hills" I'm referring to are more like dips in the road.

For example, 1 road that I have problems on. The road dips in then climbs out on each side. Approxamitly 4 blocks long. I can sit on my bike at the bottom of the dip and see the road level on each side from a sitting position. Using my speedo I got a top speed of 18.6 going down one side and dropped to 10.4 going up the other with no peddling.

I'm not riding up San Fransisco sized slopes here, just general dips you find in any town. I ordered a new .66 main jet Wednesday so I'll see how much better it does then.

Does anyone know what the internal gearing is in a BGF motor? I heard like the Grubee motors have 5:1. I'm trying to figure out my overal gear ratio including internal gearing.
 
GreedyRogue said:
I got my speedo running and top speed was alot lower than expected. 19-20 seems to be my top speed on flat ground, even at wot. The type of "hills" I'm referring to are more like dips in the road.

For example, 1 road that I have problems on. The road dips in then climbs out on each side. Approxamitly 4 blocks long. I can sit on my bike at the bottom of the dip and see the road level on each side from a sitting position. Using my speedo I got a top speed of 18.6 going down one side and dropped to 10.4 going up the other with no peddling.

I'm not riding up San Fransisco sized slopes here, just general dips you find in any town. I ordered a new .66 main jet Wednesday so I'll see how much better it does then.

Does anyone know what the internal gearing is in a BGF motor? I heard like the Grubee motors have 5:1. I'm trying to figure out my overal gear ratio including internal gearing.
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My Flying Horse motor is 20-82
 
I weigh in at a little over 300, My stock 44t was decent, but I lacked hill climbing on a few of the steeper inclines, and on a prolonged gradual incline acceleration was impossible. I kicked my rear sprocket up to a 50t and my problems went away. My top speed really didn't suffer might even had gone up a little, as the extra torque was able to move a little more mass. I have read a few posts that top speed actually goes down with a 36 t as a result of not being able to wind out to take advantage of smaller gear.
 
Is there any reference chart to compare tooth count? For example a 44t does xx mph and xx torque. Just generalities is what I am looking for. I know that in general for every tooth you drop you gain .8-1 mph but what about torque numbers?
 
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