What RPM am I Turning at?

E

Edd

New Member
I have a 26" hardtail MB, stock engine, and a 44 tooth sprocket. I usually ride at 30-32 mph, 32 being at the max RPM I feel comfortable taking the HT to. I could twist the throttle more and rev higher, but I feel the engine will grenade if I rev any faster. So, what RPM am I turning at? If you give me the transmission gear ratio (small engine pinion and big clutch gear) I can figure it out, but I haven't been able to find it. I'm just curious, but money's a tad tight right now (college next year) so I can't buy a Tachometer.
 
Thanks for the replies guys! The Aleman, I have been looking at a smaller sprocket, considering there are no hills here in Cypress Texas. If i bought an expansion chamber, what would be the smallest sprocket I could get? I only weigh about 120lbs, no hills, and usually carry no cargo (i.e. groceries, backpack, etc.).
 
probly a 27 tooth maybe less.. depends what ya want
personally with your weight i would go a 36 as they are easy to get and you'll still have plenty of grunt and yeh grab ur self a good quality pipe then match port the intake pipe to the intake port, same deal with the exhaust.

I'm literally double your weight (ya lucky bugger :)) and have no issues with running a 36 tooth on all my ht's. hope that helps.
 
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I was thinking somewhere between 32-36 tooth sprocket. I'm looking for a little more top speed so that i can keep up with 40-ish mph traffic. BTW Breno, I am 17 and really skinny, which might explain why I weigh so little (probably slightly anorexic lol!).
 
I weigh 215. My bike has a 36T sprocket, SBP Xchamber, custom made Boost-bottle and a Dax RT carb and I can hit 36MPH an cruise at 30MPH like nothing. So yah, I think a 36T would get you into the 40's just you'd have to run it pretty hard. A 32T sprocket would get in the 40's for sure with throttle to spare but your bike would lose a bit of torque. And greetings from Rosebud dude.
 
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I had a 34T for a couple years, it was my sprocket of choice. Capable of 40+, but very quiet around town under 30MPH.

I weighed about 220-225, never afraid to pedal. I never had an expansion chamber, just a boost bottle.
 
Thanks Texhun! BTW, It seems like both you and The Aleman use Boost Bottles, but on every other thread I encounter on Motorbicycling.com I hear they are absolute trash; a waste of money. So, are they any good (and yes, I realize the topic has gone way off track from the original thread topic, but it's nice to chat with people who know what they're doing, unlike me :[ )? Oh, and I'll probably be going with a 34 or 32 tooth (like I've mentioned, don't need the torque). Thanks to 'ratiocalc', I know that with either of those sprockets, I can reach 40mph.
 
I made my bottle before I was a member of this forum, only cost about $3. I saw a small gain in midrange torque, about 18-28MPH with my 34T.

Where I lived (ND) didn't have many hills, but there was lots of wind, especially from the North. Sometimes it was hard to keep it over 20!
 
My bike will go a little faster than 39 mph My gearing is 5.17 mph per 1000 rpm with a 26x1.95 tire and a 36T sprocket. It'll pull 7700 rpm on a flat with me on it. I weigh 220 lb and I don't care. I'm pretty happy with this gearing, but the motor could handle more. A 34T would probably be perfect for best top speed, but will cause problems being too tall for cruising on trail under 20 mph. With my 36T it's almost a hassell riding below 20. It's easier to gas it up to 20 and then coast down to 15, or 16 then repeat. That's only ever a problem if you're trying to go slow enough to ride with bicycles, and they better be fast bicycles. Acceleration from 20 to 30 is pretty good with the 36T, and you don't have to peddle too fast to start off. It doesn't feel like it hurting it to cruise 35 mph. A fatter tire would be even better.


RPM MPH
1000 5.2
1200 6.2
1400 7.2
1600 8.3
1800 9.3
2000 10.3
2200 11.4
2400 12.4
2600 13.4
2800 14.5
3000 15.5
3200 16.5
3400 17.6
3600 18.6
3800 19.6
4000 20.7
4200 21.7
4400 22.7
4600 23.8
4800 24.8
5000 25.8
5200 26.9
5400 27.9
5600 28.9
5800 30.0
6000 31.0
6200 32.0
6400 33.1
6600 34.1
6800 35.1
7000 36.2
 
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Boost bottles are ok if you know what your doing. You get a little extra torque and smoother acceleration but you lose a little top end, that's what happened to me anyway.
 
I have a top speed of 29, with my gearing, it looks like my rpm maxes out around 7000. wondering if weight affects the relationship of rpms to speed? in other words, could a heavier rider have his engine turning at higher rpms, yet still be going slower than a lighter rider at the same rpms. seems to make sense, considering an engine would need more power to pull more weight.
 
motor_bike_fanatic said:
I have a top speed of 29, with my gearing, it looks like my rpm maxes out around 7000. wondering if weight affects the relationship of rpms to speed? in other words, could a heavier rider have his engine turning at higher rpms, yet still be going slower than a lighter rider at the same rpms. seems to make sense, considering an engine would need more power to pull more weight.
Click to expand...

I've tried this. More often than i really care to mention. And on average nothing really changes. Sure the bike has a harder time getting going with a heavier rider. Once going though the engine actually doesn't work as hard to keep up the same speed. Once mass is in motion it tends to stay there. You can even try this on bikes without a motor. Usually the heavy rider will always end up going faster. Has to work harder to get there, but once up to speed doesn't have to work as hard. Anyway it all averages out.
 
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