Opinion or science? Power band vs gearing

mew905

mew905

New Member
So I got some more parts and am able to put wheels on my green bike. I plan on making it a race bike but the question is, is it better to gear high (44t) and aim for a high RPM power band? or gear low (36T) and aim for low end torque?

I'm betting it'd be better for the high RPM power because the higher RPM's usually means you have more room to port, and higher RPM's make power gains easier. I dont plan on making it go 80kph like race bikes in cali, right now my target is 55kph with a ton of power for hole shots and corner exits. But I'm not sure if this is the way to go. 55kph on a 44T is roughly 8500 RPM, which is fairly high for an HT and the gearing would allow you to go through the revs faster, but 55kph on a 36T is a more conservative 6300 RPM, allowing you to keep a relatively broad power band
 
My opinion: low gearing (44t) is for low end grunt , acceleration AND high rpm.

High gearing (36t) is for sacrifice of low end power and more highway speed.
 
The majority of the bikes in raced in the So-Cal races use rear sprockets near the 40T range.

If you're a heavy guy?, you might want a few more teeth; If your a light weight guy?, you can run a few less. ;)

Baird weighs 130 lbs... He uses a 29T sprocket... I weigh 200lbs, I use a 40T.
 
My personal observations of direct drive motorized bicycles is 2-strokes rock at lower RPM bands while 4-strokes rock when you wind 'em up to the high RPM band.
I don't know if that helps but worth mentioning I think.
 
8500 is considered "mid range" for a 2 stroke. Right around that is a good target to shoot for when modifying these engines for performance.
 
mew905 said:
So I got some more parts and am able to put wheels on my green bike. I plan on making it a race bike but the question is, is it better to gear high (44t) and aim for a high RPM power band? or gear low (36T) and aim for low end torque?
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It is entirely up to what kind of track you are racing on, to determine what kind of gearing you should be running. For example, a tight technical track with lots of turns and very few straightaways, you would want lower gearing. A high speed track with few turns and long straights or perhaps a 1/4 mile drag strip you would want higher gearing for top-end speed.

Venice Motor Bikes said:
Baird weighs 130 lbs... He uses a 29T sprocket... I weigh 200lbs, I use a 40T.
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And I weigh 175lbs. and I run a 48t sprocket on the rear and a few sprockets in between, plus 20hp!
 
So I've come to the conclusion porting for high power and choosing a bigger sprocket is the way to go, because 2-stroke motors love revs, the more you get, the more power they're willing to make. basically port for whatever RPM you're comfortable with, then gear for your chosen top speed. a motor that makes 9hp at 10k RPM and geared for 50mph (a 44T) should easily outrun a motor that makes 5hp at 6k RPM and geared for the same speed (~32T) given the same weight, even if the 9hp makes less power than the 5hp at the same, lower speeds

So, I've ordered a new chain and I'll put my 44T sprocket back on and port for high RPM's, hoping that my RSE reeds and Runtong carb (Dellorto clone) wont be limiting me too much. As much as I love my top speed and being able to use less gas for the same speed, the motors dont use much gas as is and the extra speed could be alot of fun.
 
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You can tune your engine in either direction. Entirely up to you.

What you can't do is just slap high gears on (smaller amount of gear reduction/low tooth count rear sprocket) and expect to go faster if your engine can't handle the load. This is fundamental for newbies.
 
I know, but I think its better to go for the high RPM power, as 2 strokes are naturally rev happy, and its hard to suck a ton of low-end power outta them, its much easier to get the revs up and gear accordingly, I imagine small mods will make a bigger impact at high revs as well. My roommates bike is geared pretty low by comparison, a 20" rear tire (Schwinn stingray) and a 44T sprocket with a SBP pipe and long header throws all his power at the low end, he's got enough he can actually do burnouts with the HT, but my bike is tuned for higher speeds, 26" tires with a 36T sprocket and higher porting, but I lack a good pipe, but mine outruns his from 25kph and on, but I think my 65kph top speed is power limited, not rev limited, so if I got a bigger sprocket and ported higher I may be able to beat it. I'd love to have a bike like Scotto's 50mph 41T beast
 
I agree with making a 2T engine as free rev happy as possible for any type of HS racing.
 
mew905 said:
So, I've ordered a new chain and I'll put my 44T sprocket back on and port for high RPM's, hoping that my RSE reeds and Runtong carb (Dellorto clone) wont be limiting me too much. As much as I love my top speed and being able to use less gas for the same speed, the motors dont use much gas as is and the extra speed could be alot of fun.
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Rock Solid reeds and box are sized for about a 25 cc motor.

Skyhawk GT-5, Dax RT, Reeds, 36T sprocket, 70cc Puch Hi head, modified stock exhaust, Jag CDI. Top speed 61.1kph
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61.1 kph is only about 38 mph which is really sloooow for a 36 tooth sprocket.

if that is as fast as you can go with a 36 toother your performance mods are
doing nothing because a box stock motor will go that fast.
 
MotorBicycleRacing said:
61.1 kph is only about 38 mph which is really sloooow for a 36 tooth sprocket.

if that is as fast as you can go with a 36 toother your performance mods are
doing nothing because a box stock motor will go that fast.
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does that mean u need to tune it better to go faster on a 36t? dont mean to hijack the thread, but i max out around 40mph depending on weather and how much weight im carrying for tools. Now ive got to 46 on a downhill
 
40mph is pretty good actually. To get beyond 45mph with most bikes takes some money/time. When I go that fast on a bike, I consume underwear in a brown haze at an amazing rate.
 
Seems power limited at ~38mph. It sucks, it does pull harder than an out-of-the-box motor, but for some reason just cant push past that limit. Plus I four-stroke going much faster, if I go WOT, it four-strokes anything faster than 61kph, however if I back off on the throttle a bit I can get an extra 4kph, I think my reeds are limiting my flow too much, 36T @ 40mph is 7300 RPM which I figured was still fairly good. I may order a set of 40mm arrow reeds and grab a GT5-a cylinder.

I still need a good pipe to really draw a good amount of power out of it, it's quicker for sure, but a drilled stock muffler is not only really loud, but not that great.

Pablo said:
40mph is pretty good actually. To get beyond 45mph with most bikes takes some money/time. When I go that fast on a bike, I consume underwear in a brown haze at an amazing rate.
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Yeah, but it's true, 40mph is pretty good, I've seen people on here struggling to get that, but as MBR mentioned, motors have come out of the box that fast. My very first personal build was that quick, no mods, straight outta the box. I imagine the stock CDI had something to do with it too, the stock CDI produces waaayyyy more power than a jag/2 stroke CDI does, but the jag/2 stroke saves your bearings.
 
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Pablo said:
40mph is pretty good actually. To get beyond 45mph with most bikes takes some money/time. When I go that fast on a bike, I consume underwear in a brown haze at an amazing rate.
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well my motor isnt stock so i would of figured it should go faster if your guys are saying a stock motor should hit 40 dependin on rider weight and sprocket size.











Mew, how are you figuring out the rpms based on mph? jus curious. im guessing thats how most people figure out rpm without actually having an rpm gauge
 
you guys definetly got my attention and ill keep an eye on this one and I am sure I will lern a lot form you all and who knows mabe ill be able to help somewhere
 
dumpstercrusher said:
well my motor isnt stock so i would of figured it should go faster if your guys are saying a stock motor should hit 40 dependin on rider weight and sprocket size.

Mew, how are you figuring out the rpms based on mph? jus curious. im guessing thats how most people figure out rpm without actually having an rpm gauge
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I use my gps for my speed then input all the factors into the gear ratio app, and play with the RPM till i match my speed.
 
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