# 49cc Engine RPM Question

Discussion in 'Transmission / Drivetrain' started by cbecker301, Dec 30, 2013.

1. ### cbecker301New Member

Hi! I have a quick question for you all. I was wondering if there was a way to determine engine RPM based off of speed and the gear ratio that I am running on my 2 stroke 49cc motor bike. I would like to start some experiments to see which gear ratio would give me the optimal speed (around 35 - 40 mph) but in order for me to determine which sprocket I should buy, I need to know the engine RPM. If any of you know of any mathematical formulas or advice to help me solve this problem that would be great! I was tinkering around on the gear ratio calculator posted in the performance section as well. Thank you for your help!!

2. ### bluegoatwoodsWell-Known Member

Well. I suppose that you could figure out your rear wheel rpm at a given speed pretty easy. But you'd have to know the gear ratios from rear drive sprocket to front drive sprocket to clutch to crank to calculate engine rpm. Do-able, but tedious.

Maybe you ought to take your bike out and get up to a speed where the engine is sounding just the way you like it for cruising. In other words some comfortable, fairly high rpm. Measure your road speed at that point.

Let's imagine you've measured your road speed as 20 mph and you want 35. Let's also imagine that right now you have a 44 tooth rear sprocket. The standard size.

20 divided by 35 is going to equal X divided by 44. X is the sprocket you want.

So. 20 times 44 divided by 35 equals.....mumble, mumble.....What??!!...infinity???...no...get the calculator....I'll be right back.....

20 times 44 divided by 35 equals 25. (roughly) Do they even make a 25 tooth sprocket?

I might be in over my head here. But if I haven't made any mistakes, then you want a 25 tooth sprocket.

And I know you didn't ask for this advice. But I'll give it anyway. I really wouldn't recommend doing 35 to 40 on an MB.

3. ### cbecker301New Member

Ok, thank you very much for the tip! I was just wondering if others had found a formula or experienced the same problem and solved it. Thank you very much for the advice though!

4. ### LR JerryWell-Known Member

I've got a very simple answer to your problem and will give you exact results. Northern Tools and Staton Inc both sell a Tachometer/hour counter for \$50.

5. ### butterbeanWell-Known Member

If it's a kit engine, they typically run about 7k or so redline. A 32t will give you close to 40 mph with a 26" wheel. Keep in mind, when you change gear ratios to increase speed, you lose torque. Especially if you are a bigger person at all, you will struggle up hills immensely. If you're smaller or there are not a lot of hills where you live, maybe not such a big deal. But you may also need to pedal a bit more before letting the clutch out when taking off. Just some things to keep in mind.

6. ### bluegoatwoodsWell-Known Member

butterbean brings up a good point, by the way.

These MBs are a bit more heavy than a pedal bike. And if you're carrying any cargo, then this becomes worse. Stops and starts can get a bit clumsy.

It's not a big deal if you have plenty of space to work with. But if you're starting up from a red light with cars going by just to your left and others wanting to left turn right at you, and so on, then this gets a bit uncomfortable.

There are a few ways of dealing with this. Having good low speed torque is one of them.

7. ### butterbeanWell-Known Member

I recommend bringing the engine into a wider powerband (powerband = range of effective rpm's). Stock, the 2 stroke kit engines have a fairly narrow powerband. But there are a lot of options for increasing power, some very mild, some for the more technically advanced and skilled. If you don't have a lot of riding experience, I don't recommend starting with heavy modification if you value your own safety or that of others. To start, there are better carburetors, tuned pipes, high compression heads, different ignition modules, different air/fuel ratios (be careful changing your fuel mix, too lean will blow it up in a short time). For the more skilled rider and builder, there are reed kits and boost porting. There is a plethora of information here about all these things, take your time and do your research, then decide what's best for you. Good luck!

8. ### butterbeanWell-Known Member

Another option, though not as comfortable on the legal side, is a larger 4stroke engine. I have a 98cc 4 stroke minibike engine, no mods done other than removing the governor, and shedoes30 easy and will climb walls. I get 80mpg roughly.

9. ### MotorBicycleRacingWell-Known Member

The gear ratio calculator will do it for you.

Put 20 in the first box
82 in the 2nd

That is the 4.1 to 1 ratio built into the clutch side of 48 cc and 66 cc motors.

Measure the diameter of your wheel and you have a 10 tooth front sprocket.

that's all you need to figure out which rear sprocket size will work for you.

6600 rpm is A, two octaves below middle C...

this will teach you for throwing spiders at girls during math! forget the cutie pie, you need just PI! and some basics on ratios...

ramble time!

"left turn right" i love english at times.... cars feel much nicer when they pass on the right, BTW

wait a minute! 48cc, 49 cc or 50 cc?

is this actually a CAG engine? or just the plain old standard HT kit? you failed to mention that.

because, it sounds to me like youre BUILDING something, not just tweaking something you already ride?

if its a CAG.... calculate the rpm at around 10,000.... HT kit...6-7,000rpm