qt50 gear ratios and rpm explained

It's 4:38am and I can't sleep. Yesterday was the same so I decided to tear down my spare motor and count gear teeth.

There are three sets or gear reductions in the qt50 motor, one on the clutch side, one one the output side and the last in the rear end. The clutch side is 33 to 63 teeth, the output side is 15 to 19 teeth and the stock rear is 10 to 57 teeth. With the stock wheel circumference of 1,422mm the rear wheel does 565.6rpm at 30mph. Engine rpm is 7,791 at 30mph. I'd heard stories that this little motor is capable of 13k rpm. At 13k rpm you'd be going almost exactly 50mph which I don't understand how cuz before swapping rears to the pw I was only hitting 32mph with a bunch of mods but still running my 50cc jug. I'm wondering what's the fastest anyone has got a qt50 while still running the 50cc jug and stock gears? And what they did to achieve it. Also I ordered my yt60 jug and piston etc. and they're on the way but I'm fabing a pipe and need the exhaust flange's port diameter for my pipe calculator, any one know what the exhaust flange's port diameter is or is it the same as the stock qt jug? Thanks and I hoped I helped some one by posting this info. Ryan.

Re: qt50 gear ratios and rpm explained

Yeah, I was trying to find the max RPM of a QT50. Thanks. Paul

Re: qt50 gear ratios and rpm explained

I have a Qt50 that I have completed a carburetor bore which required adding small amounts of material to the casting. Carb bore is now over 14mm opposed to just over 12mm stock. I've bored a stock 49cc jug (40mm stock) out to 44mm and installed a stock 44 mm Yamaha piston. I think the 49cc jug is a complete different set up and to obtain more power you need the YT60 jug. The reason being when you bore the stock 49cc jug you change port timing in the wrong direction and thereby inhibit full rpm/but at the same time increase displacement which can be deceiving. What I do know is that you can make a (weakly timed) Qt60 do a HARD 38 mph with rear gears and a 60cc set up. I've bought a digital tack and unfortunately have not set it up to read the rpm's at top speed but I realize that math does not lie. If your calculations are correct I"d like to think 50mph is do able with stock gears. The simple truth is I make this QT50 scream with PW50 gears and I don't need a tachometer to tell me it won't spin much faster. It will only do 38-40 mph. Mind you thats fast and you can cover a whole lot of ground in a quick hurry. But if anyone can make this engine do 50mph with stock gears I'd refer to them as the BAD A$$ MF'er I know and you don't.

Re: qt50 gear ratios and rpm explained

Don't get me wrong though! I think the QT50 is the best of the best. There is no other moped/scooter that was so well thought out! Every detail has been thought of and corrected. I could go on and on about the superior design of this moped but you would never realize it until you took one apart or sized it up against a competitor. The simple fact it uses a drive shaft with a ring gear similar to a BMW motor bike is reason enough. This bike goes thru dirt, muck and grime and is totally sealed. Period. This drivetrain has been in use for over 20 years and is still the top competitor in PeeWee racing. If you thought that was funny your an IDIOT. This is a serious machine/setup and can run with the best of them! The only downfall is brakes and this is the worst problem on all mopeds/scooters.

Re: qt50 gear ratios and rpm explained

Before you order a pipe based on the exhaust outlet you might want to make the outlet larger. One to 1 and a sixteenth inch is real common for moped performance headers. I just got a QT and will be creating a parallel performance reality right along side yours

Re: qt50 gear ratios and rpm explained

what difference does it make running pw50 gears over stock qt50 gears? thanks

Re: qt50 gear ratios and rpm explained

Because the wheels are smaller on the PWs the rear ring gear is smaller in order to gear it up to compensate for te smaller rear tire circumference. Putting those gears in the QT gears it higher. A lot of PW people like to put the QT gears in theirs to get low speed grunt for the dirt and trails.

Re: qt50 gear ratios and rpm explained

so it gives you more top end. do you think i would get from my 27 now to 30mph? im jst lookin to get a little more top for the long rides(25miles) to college.

Re: qt50 gear ratios and rpm explained

yea. you could also move your compression ratio up to 10:1 or a psi reading of 147

Re: qt50 gear ratios and rpm explained

ok im a little stumped here how you could do those things. im still kind of newb to mopeds so i know how to take apart and build but i don't know those details. could you give me a little more detail to how you do those things like ratio or psi reading

Re: qt50 gear ratios and rpm explained

ratio is how big your cylinder is, should be 50ccish, to how much space is above that ,the empty space in your head. so if you cc is 50 then you cram it into your head which has 5cc of space you just got a 10-1 ratio. to up the compression ratio and compression you make your head smaller usually done by milling or lathing the head making it shorter.

Re: qt50 gear ratios and rpm explained

I meant to suggest the higher compression in conjunction with changing out the rear ring gear with a pw50 ring gear. You will need a press to change out the gear.

Re: qt50 gear ratios and rpm explained

Sorry to bring back one of my long dead threads but I wanted to add to what I said. The stock qt rear gears are 10/57 and the pw gears are 11/54. This gives you a 12.5% top speed increase at the same engine rpm. With the pw gears 30mph equates to the engine turning at 6,853rpm instead of 7,791rpm with the qt gears. I GPSed my bike today to check the acuracy of my 1422mm wheel circumferance measurement and I'm dead on, I'm actually really suprised that it wasn't off by atleast 1mph at 40+mph but it wasn't off at all. I GPSed my bike at 43mph in a half tuck on flat ground with no wind. I'm wanting more power to kick in at 40mph to push hard to 50mph and so I want to build a pipe for that rpm. 40mph with the pw gears equates to the engine running at 9,141rpm. What I want to know is is it smart to sustain my engine at that rpm? Before doing the math I didn't know I was sustaining 9.5Krpm! I want to hit 50mph but at that speed the engine will be rotating at 11,374rpm!(sounds like disaster to sustain it there but then again I don't know). Is it just smarter to lengthen my drive shaft and run a larger diameter rim and tire????? Thanks. Ryan.

Re: qt50 gear ratios and rpm explained

no help here, but your bikes a ripper dude! you were dominating out of the corners at the latebirds invitational. i was impressed.

Re: qt50 gear ratios and rpm explained

That was in no part thanks to original tires(new tubes), a very bent front fork(the top of my tire leans over a lot), a bent frame from crashing in practice that I still haven't straightened and running my tires at 32psi(I didn't even think of it till after that weekend). It was all about holding corner speed because my pos will shift into second at about 15mph but will only shift down into first below 5mph. I got a lot of crazy stuff left to do to handle corners even better but at least I got this done tonight.

This way I can lean it farther without scraping the pegs. But I implore anyone with the information I need. Is running a bike at 10K+rpm continuously a bad idea???? .......... where'd I put that dirtbike rim..........

Re: qt50 gear ratios and rpm explained

Downhill Harvey (OFMC) /

I'm no pro either, but I would say yes, it's bad for extended periods. I do think that running those rpm's and higher can be done, and be reliable, if all rotating parts are professionaly balanced. If you're racing it, get things balanced. That's what the racers do.

Re: qt50 gear ratios and rpm explained

My left side crank half is from a qt and the right side crank half is from a mj so I think balancing it would be a good idea.

Re: qt50 gear ratios and rpm explained

did you put two different cranks together? Generally speaking a stock moped crank can handle 10,000 especialy one with roller brgs. The Yam cranks come balanced at least externally balanced from the factory

Re: qt50 gear ratios and rpm explained

yep.

Re: qt50 gear ratios and rpm explained

yeah dude that thing was killing those corners at the track i was real sketched out cus i saw your tires before the race and was wondering when you were gonna slide right into me.....one out of a billion turns was not bad

Re: qt50 gear ratios and rpm explained

lol. were you the one that pushed me into the inside of that last real corner before the straight so hard my rear end stepped out and got caught in your bike? Hard racing for sure, I was trying to avoid the inside curb and you at the same time. lol if so I'm really sorry about that I should of just grabbed a handful of brake and let you over take me in that corner. I'm so very sorry. On a different note. This pipe I originally built on friday for my friends solo odyssey.

I designed it for 8Krpm but after it was built I remeasured it and it's more like a 9Krpm pipe. It was horrible on his bike. I doubt he could even free rev his engine to that speed, what was I thinking. It made his bike start four stroking like crazy and his take off was ill affected also so we took it off. So about an hour ago I cut of the head pipe that was just an extra stock head pipe for his solo and I welded on a stock head pipe from an extra qt pipe. At about 40mph it kicked in strong and was pulling past 45mph hard but I ran out of road, wasn't wearing a helmet and remembered a picture of a piston with a hole in the exhaust port side from running a high rpm pipe. The bad part is below 40mph it has half the power of my other pipe. At least it's fun for a speed run, time to get the gps and camera out....

Re: qt50 gear ratios and rpm explained

@STEVEW It's very very very important to me that you elaborate on what you said about the exhaust port to exhaust flange opening,header beginning diameter, ratio. My exhaust port is big enough but the area constricts at the flange to only 23+-mm in diameter. I really want to dremel it out larger but haven't been able to find any literature on the subject. Please help. Thanks. Ryan.

Re: qt50 gear ratios and rpm explained

nah dude its cool i had more fun racing you than anything, plus it was just a little nudge w/e's

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