4 stroke on a 29 cruiser - rear sprocket?

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worksuxxx

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Jul 16, 2013
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Omaha
Hey everyone. My build is complete and am out riding already. I have a question regarding the rear sprocket sizes. My kit came with a 52t. I am only getting about 22mph. I am assuming with the 29er tire size I am losing top end. Any suggestions on a good sprocket size to achieve a nice cruising speed of at least 30 mph. I found a 36t but think that would be geared too low, but not sure. Hoping to get an idea so that I can still place an order today.
 

mapbike

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Mar 14, 2010
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if your at 22 mph with the 52t. a 36t would get you at 33mph
Only if the engine has the grunt to pull that high of a gear I would think, sometimes a lower ratio will allow the engine to turn more rpms and actually increase the top speed if the gearing is so high that it is lugging the engine down instead of allowing it to wind up to it potential RPM's/Top HP.

Experienced this on some of the go Karts I've built in the past while experimenting with gearing for top speed.

Map
reddd
 

MotorBicycleRacing

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Jul 28, 2010
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if your at 22 mph with the 52t. a 36t would get you at 33mph
Your assumption is off because the 22 mph figure is low for that 52 tooth sprocket.

If the gearbox is 3 to 1 with a 10 tooth output his 4 stroke should be going 43 mph instead of 22 mph.
The bike is over geared especially with that 29" wheel.

With a 26" tire the speed would be about 39 mph.
 
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worksuxxx

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Jul 16, 2013
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I am not running a tach so I do not no where I am with the rpms. I just got back from another ride, seemed to go a little faster. I allowed it to rev up higher. It seems at one point there seems to be a little vibration and or noise, which is where I usually backed off the throttle assuming I was pushing the rpms too high. This time I allowed a higher rev, which it pushed through that vibration and clatter type noise, then smoothed out and gained more speed. Without seeing actual rpms, i am very worried riding it at the higher revs. I assumed the 4 stroke was more for low rpm riding, and 2 strokes were for high rpm riding. Will the 4 stroke handle high rpm riding for any distance?
 

worksuxxx

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Jul 16, 2013
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Omaha
Oh to correct a few things. The rear sprocket is actually a 50t. Front drive sprocket is a 8t. I do believe it is a 3 to 1 ratio.
 

MotorBicycleRacing

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I assumed the 4 stroke was more for low rpm riding, and 2 strokes were for high rpm riding. Will the 4 stroke handle high rpm riding for any distance?
clattering? very important to check your valve clearances and adjust if necessary.
Your motor won't perform if the clearances are way off.

The 50 cc Huasheng redlines at 7800 rpm and can be run at more but it
isn't recommended.

With your sprocket sizes you gave I wouldn't change anything.
6,500 rpm at 30 mph would be a good cruising speed and they will do that all day.

7800 rpm = 36 mph with a 29" tire.
 
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worksuxxx

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Jul 16, 2013
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OK. Checked speed with gps tonight. Fastest I got it on level ground was 25 mph. Not sure of the rpms because I don't have a tachometer. Where do you guys get yours? I weigh in at 200 lbs, bike with motor weighs 60 lbs. I have no problems pulling the hills, it even accelerates going up them. I am thinking of going to front=10t rear=44t. I would like to see a cruising speed of 30, max 35.
 

MotorBicycleRacing

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OK. Checked speed with gps tonight. Fastest I got it on level ground was 25 mph. Not sure of the rpms because I don't have a tachometer. Where do you guys get yours? I weigh in at 200 lbs, bike with motor weighs 60 lbs. I have no problems pulling the hills, it even accelerates going up them. I am thinking of going to front=10t rear=44t. I would like to see a cruising speed of 30, max 35.
25 mph with 29" wheels is 5600 rpm
something is wrong with your motor.
Check your valves, you will lose power if they are too loose.

10 tooth to 44 tooth would give you a theoretic top speed of 55 mph with 29ers at 7800 rpm
6500 rpm would be 42.5 mph
Never going to work!
if you can't go faster than 25 mph with a 18.75 to 1 ratio how is going to a 13 to 1 ratio going to help?

You don't need a tach if you have all the accurate gearing ratios and tire diameter.
The TTO tachs are the best, look on Amazon.

what do you base your decision on to go with front=10t rear=44t.????
 

Master-shake

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Feb 24, 2013
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Oh gee, I didn't know about all that different gearing stuff. Well, if anyone else needs to know, an onyx with a standard china girl kit will go thirty+ out of the box no mods at all.
 

worksuxxx

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Jul 16, 2013
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Omaha
Wow. Im an idiot. Front sprocket is a 10t. I miscounted the teeth. So how much does this throw us off in this discussion?
Front 10t Rear 50t.....15.1 ratio(not 18.75 to 1)....So again, WOT = 25mph w 29ers
Should I still look at the valves?
And how in the heck do you calculate rpm & speed with tire diameter?
 

shwnrttr

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Dec 2, 2009
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Wow. Im an idiot. Front sprocket is a 10t. I miscounted the teeth. So how much does this throw us off in this discussion?
Front 10t Rear 50t.....15.1 ratio(not 18.75 to 1)....So again, WOT = 25mph w 29ers
Should I still look at the valves?
And how in the heck do you calculate rpm & speed with tire diameter?
that would put you at 44.9 @ 78000rpm. i dont think the engine has enough power to pull that kinda speed. you can download the jim sitton calculator just search this site for it