Gear ratios and speed help?

[copmagnet80cc]

I've been looking around on this site and I saw a post that showed the gear ratios and speed on a motorbike and said that the lower the teeth are on the gear, the fast it will go to a certain limit..how is that possible? Isn't a bigger teeth count better for top speed? I'm a little confused about this and would like some tips for a gear I could get for top speed for my own motorbike.

 

[copmagnet80cc]

This is the post I read..

Gear ratios & speeds are as follows:
12T-44T
24T-9 speed cluster
8625 rpm's-26" wheel diameter-5:1 PB transmission
--------------------------------------------------
34T=25 mph
30T=29 mph
26T=33 mph
23T=38 mph
20T=43 mph
17T=51 mph

 

[mapbike]

Nope, larger the driven/sprocket on the wheel is the lower the gear ratio is and this will increase low speed power but reduce. Maximum speed.

A smaller rear wheel sprocket reduces low speed power but increases top speed potential as long as the ratio is not so high the engine cant pull it.

Look at a multi speed bikes rear sprocket set up and you'll notice that when you change gears so that chain is on largest gear closest to spokes, the bike gets easier to pedal but even though you pedal very fast your speed is limited, now change gear to smallest and you'll notice that bike is much harder to pedal and even though you pedal much slower than before you can easily move the bike faster than before even though you're pedalling much slower, gearing on our China girl engine kits works the same way.

Even if a bike only has a top speed of 30mph a smaller rear sprocket will lower engine rpm at that speed compared to having a larger sprocket.

Hope this helps.

Map

 

[mapbike]

This is the post I read..

Gear ratios & speeds are as follows:
12T-44T
24T-9 speed cluster
8625 rpm's-26" wheel diameter-5:1 PB transmission
--------------------------------------------------
34T=25 mph
30T=29 mph
26T=33 mph
23T=38 mph
20T=43 mph
17T=51 mph

This doesn't really apply to what you will have as top speed on a china girl engine kit build on a 26" wheel, I run 36 tooth or smaller rear sprockets on my bikes and they typicall cruise real nice @ 30+ mph my fastest bike will cruise @ 40+ mph with a 36T rear sprocket and I weigh 220 lbs, some people claim 35 mph with a 44T rear sprocket, in my experience a 36 - 40T serves most rides about right and either will give you a 30 mph+ bike with a well tuned engine.

Map

 

[rogergendron1]

with a 26 in tire and a 44t rear = 7,500rpm to get 32.2 mph

with 26in tire and a 40t rear = 7500rpm to get 35.4 mph

with a 26in tire and a 36t rear = 7500rpm to get to 39.3 mph

a 50 tooth on a 26in tire will net you 25 mph

as you see the smaller the rear sprocket the faster you go but the more torque you loose
the larger the rear sprocket the slower you go but the more power you get

i used 7,500 rpms as the example because thats the stock china girl max RPMs


if your bike is a 66cc china girl you need to evaluate what kind of terrane you have ... are there steep hills ? is it flat ground ? and how fast do you want to go ? if you want to go 40mph and live in an aria that has no hill what so ever a 36t will get you there, but if you live in a real hilly aria with steep inclines i sugjest you dont go any lower than a 40 t rear ! or you wont be able to climb the hills.

if you want to go slow and keep it under 25 like some people then you need to pick up a 50 t rear, your bike will have amazing power !

now you can get some of your torque back and gain speed with mods, but you have to mod your motor for a spacific driving style and gear it acordingly !

for instance i have a fully modded race motor that is capable of short 9,500 rpm bursts ! with a 44 tooth rear i can push 40mph ! that is the speed limit i set for my bike and i geared acordingly ! i can climb even the steepest new england hills at high rpms going nearly 30mph !!!! unfortunatly i have no bottom end what so ever LOL so i always ride in the upper rpm range and stay between 15 and 35mph sometimes short burst of faster speeds but it makes me nervouse !

with a stock china girl and a 44 t rear dont expect more than 30mph ! a larger gear will slow you down but give you more power a smaller gear will speed you up but rob you of power.

 

[rogergendron1]

This doesn't really apply to what you will have as top speed on a china girl engine kit build on a 26" wheel, I run 36 tooth or smaller rear sprockets on my bikes and they typicall cruise real nice @ 30+ mph my fastest bike will cruise @ 40+ mph with a 36T rear sprocket and I weigh 220 lbs, some people claim 35 mph with a 44T rear sprocket, in my experience a 36 - 40T serves most rides about right and either will give you a 30 mph+ bike with a well tuned engine.

Map

map i will have to make a video to prove my 40mph claim on a 44 tooth stock sprocket !!!!!

i have 26 in irc avantage pro tires that are a bit taller than 26in and my motor is FULLY MODDED LOL we are talking everythin but he kitchen sink !

so with a 20t drive gear and a 82t driven gear for a 4.1 to 1 reduction and a 10 tooth drive sprocket to a 44 t driven sprocket on a 26in tire = 9,500 rpm for 40.7 mph !!!

i know of quite a few motors that turned 9,500rpms with just a few mods !

i only do 40 in short bursts though it makes me nervous i feel like its themotors upper limit or red line, i usually cruise aand keep it between 25 to 35 with the 44t sprocket thats from 6,000 rpms to about 8,500 rpms (my power band !)

 

[rogergendron1]

now to answer your question lol what i recomend for you to get to top speed on your stock china girl is to install a manic mechanic style sprocket adapter and a 40t rear sprocket, this should get you up in speed from 28 to 30 max to about 35 or more depending on the quality of you motors ports and how well your intake and exhaust anre port matched.

if you want to go faster and you own a dremal tool you can do a full port and polish and then port match the intake and exhaust, then order some jets for your carb and a high flow filter, if your good you can do some piston mods and install a titainium wrist pin and upgraded needle bearing, mabey even mod your stock exhaust to flow freely... the fuel air mixflow will drasticly improve and increase the amount of RPMs your motor is capable of, and as you know more rpms = more speed.

a fully moded motor with a 44 t rear can be even faster than a stock motor with a 36 t rear if built right ! and the best part is you have all the power of the 44 with out the sacrafice !

 

[Nashville Kat]

Try a 36 rear sproket- you'll probably love it.
Unless you are very heavy or riding a lot of very steep long hills, it'll be faster and won't bog down with a normal China girl carbureted fairly normally.

I have a 50cc bike with a 39 alloy sprock- made from an GT front chainwheel
and a 66cc with an alloy 34 - evolved from the original 44 to a steel 36 and eventually to the 34- in flat Florida, it pulls me along quite well 30 mph -plus cruise, 35 plus top speed with normal kit speed carbs

The one thing you'll notice right off is that you get to a nice speed without the motor racing wide open and so there's a lot less vibration- it just feels like you are more in control- you could still open it up screaming, but may not want, or need to.

light narrow alloy wheels help too. It's usually not that hard to add a 700 C skinny wheel to the front like a chopper motorcycle, but you'll probably have to change the caliper brake. Once I tried that, I had to have one on the back too, and now all I have on both bikes. But you have to more carefully ride within limits- I raced road cycling for a long time so I'm comfortable with the fast wheels.

good luck

The photos are of the evolution of the same bike my 66 cc Micargi-

with original beach tires back in '09 (far right), with a front 27" wheel (indoors)about 2011, and with the baskets on last summer and painted silver

 

[mapbike]

Rg1

The main reason for achieving your ideal top speed with a smaller sprocket is as Nashcat said, you cwn hwve a nice 30+mph cruise speed and keep engine rpm down, less wear on engine, better fuel mileage, wnd most importantly to myself and most others...... it will keep the typical China Girl vibes down to a more comfortable level, I dont dispute your 40mph claim with a 44T sprocket but because I've had bone stock engines run 38-39mph with nothing done to them but a smaller main jet in the NT carb and running a 40T sprocket, vibes were through to roof and the engines would have blown apart after a few miles of riding that way because of the level of vibes at those high rpms.

I'm over 200lbs currently and have a few fairly good hills to ride up and down on here in the Texas hill country as its called and 36T sprockets get me up all my hills good enough I never have to pedal at the top and can maintain 25+mph all the way to top and then I can cruise in the 30's as long as for as many miles as the fuel take will take me and not vibrate my guts out, I'm sure I can get to the 40mph mark on my fastest bike with a 44T sprocket also but if I can have that same speed and not be ripping my engine at max rpms which translate into more felt vibes in hands and back side, I'd rather have my cruise speedsmore comfortabl.

For heavier riders and extremely hilly areas, I do recommend a 40-44T for more power but I'd tell someone to expect a comfortable cruose speed of 28-30mph and not to expect that they will achieve 40mph without having many engine mods and a big loss of economy in fuel and longevity of the engine, all this said I plwn to puta 44T on a 24" wheel on one of my bikes for the sole purpose of having a lower speed torque machine that I can play with more on dirt trails at lower speeds, if I can get a max speed of 25 mph and have a pile of pull at lower speed Ill be happy, a well tuned RT carb and high compression with my latest expansion exhaust that works great in the low to medium rpm range should get me a nice little hump jumper and trail ripper combo......

Peace all and safe happy riding

Map

 

[rogergendron1]

yeh my motors dont last long becaus of the consistant high rmp driving all the time and compared to a stock motor my milage is nearly cut in half lol !!!

i recently upped the comp to slightly over 13:1 and now i run 93 oct mixed 30% with xylene or toluene (witch is 117 oct) or mmt concentrate if i can find it lol...to get 100.2 octane, then i mix in upper cylinder lube and full synthetic 2 stroke race oil, like amsiol or lucas to get a final mix of 32 to 1, 100 octane !

aaaaah yeh my bike has never ran better or had more power ! i like it a lot and i am willing to pay the 6$ a gallon to make the race fuel and i am willing to cut my engine life in half to gain a ton of power ! after all a new dax 70cc motor is only 120$ and with the mods i have they last at least a year of super hard every day riding so i do get my moneys worth ! for all the power i can suck from thease things now i think 120$ a year and 10 hours of my time is worth it !

i love my torque machine's too man !

 

[mapbike]

yeh my motors dont last long becaus of the consistant high rmp driving all the time and compared to a stock motor my milage is nearly cut in half lol !!!

i recently upped the comp to slightly over 13:1 and now i run 93 oct mixed 30% with xylene or toluene (witch is 117 oct) or mmt concentrate if i can find it lol...to get 100.2 octane, then i mix in upper cylinder lube and full synthetic 2 stroke race oil, like amsiol or lucas to get a final mix of 32 to 1, 100 octane !

aaaaah yeh my bike has never ran better or had more power ! i like it a lot and i am willing to pay the 6$ a gallon to make the race fuel and i am willing to cut my engine life in half to gain a ton of power ! after all a new dax 70cc motor is only 120$ and with the mods i have they last at least a year of super hard every day riding so i do get my moneys worth ! for all the power i can suck from thease things now i think 120$ a year and 10 hours of my time is worth it !

i love my torque machine's too man !

Sounds cun to me bro...... torque is king and if I rode everyday I'd probably do mine different, it's just a fun hobby among my other hobbies and living in rural central Texas im about 10 miles from the closest small city and about 7 miles from my local little town population 422 I ride in spring, summer and fall wben weather is good usually in the early morning or late evening for mostly just a relaxing joy ride, I luke to ride the dirt back roads in my area and look around at what the other country folks around me are doing and watch the deer and wild turkey either look at me like what the heck is that or just run like crazy as if their life depended on it, my riding and use of my bikes is way different than y'all city folks, and my bikes take a real beating on these rough very dusty roads around here, since I ride lots of open road and several miles a a time at cruise speeds I enjoy a high gear ratio that will let me cruise at 30+ mph fairly comfortably, when I ride a bike into town that is roughly a 30 mile round tdip everytime so I want my bike to be solid, dependable and have a nice cruise speed so it doesnt take me half a day just to ride to town and back, so all the advice I give people is based on my experience with what I have found to work good for my riding conditions since I built my first bike back in 2009 from a elcheapo rough running red bat ebay engine kit from boygofast, it still runsgreat but is the weakest engine in my herd now and the vibes are horrible above 28mph even with a 32T rear sprocket on a 24" wheel, this engine will be a project in the future for a better trued crank to smooth it up and some port work to get it breathing better....blah blah blah.....!

If you are getting such good power you should consid r at least trying a 36T on the rear, your power will drop a tad on the lower end but, your cruise speed will jump several mph or at least if you keep it at around 40 like I do your rpms will come down and add life to the engine and smooth up the ride a bit, heck even a 40T is a really good overall gear in my opinion not to low and not to high if someone wants a comfy 30 mph cruiser, personally I like to have at least 35mph cruise speed if I can but some people dont feel comfortable over 25-30 mph so a 40T or even 44T works nice for that if there engine doesnt have really severe vibe issues.

Happy safe riding...

Map

 

[Austritch]

Hey guys, I'm building my 150cc fat tire bike. I have used the ratio/speed calculator and got a 8:1 ratio for ~48mph. Is this a realistic speed for my bike with 26" rim?