Maximum Torque

mew905

mew905

New Member
My roommate is planning a build on a Jesse James chopper frame and wants maximum torque while still being able to keep up with traffic (30mph/50kph). He's a pretty heavy dude at 360lbs and doesnt want to pedal (surprise surprise).

Anyway, he wants to keep the motor as close to stock as possible (so minimal cutting the cylinder or piston). I told him the only way he's going to get more torque is a high compression head, intake extensions and a properly tuned expansion pipe or torque pipe. I also told him to keep the 44T sprocket and maybe even go one size bigger (56T is the next size up where we'll be buying it). He's interested in my second set of Reeds here but I told him he would have to make a boost port to get any gain.

Am I missing anything here? He thought about the Runtong/dellorto carb, but I dont think it'll give him much extra. Only other way I can think of him getting what he wants is with some extensive modification such as crank balancing, piston weight reduction to gain top end RPM then use a 56T sprocket.
 
Hi, if he's after maximum torque & interested in ur spare reeds, this is where he'll get it but it requires some serious modding to realise the power gains they allow. The boost port isnt essential but porting the piston along with removing a portion of the skirt adjacent to the intake port & lowering the intake port floor is. The idea with reeds is to allow the motor to intake thru 180 degrees rather than 60 degrees. U then need to seriously upjet ur carb as the intake velocity is considerably reduced. A boost port can be ok also but must not be oversized & must exit to the combustion chamber properly (towards the rear of the head) or it interferes with the loop scavenging in the combustion chamber. Properly set up reeds do give a substantial torque increase tho. To give u an idea, I have a reeded 66cc that has an RSE billet head, 16mm mikuni carb, SBP chamber & home made Jag type CDI (modified), that runs thru an SBP shift kit to a shimano hyperglide cassette. B4 fitting the reeds & doing the piston port/intake port/small boost mods port & rejetting I could get to 6th gear on the cassette at about 66kph (on a steel framed 26" mountain bike) where it would run out of torque. Now I can use all 8 gears easily & tho the motor doesnt rev quite as high as b4 I can get it up around 76kph (it'll wind out more). The greatest gains are definitely in the low to mid throttle area. Without doing the piston porting etc, all the reeds are doing is eliminating reversion blowback thru the carb. Hope this is of some help. Cheers
 
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I totally agree with the shift kit. Reeds can give decent gains but some serious modification is nescessary to achieve these gains. A shift kit on the other hand offer great gearing flexibility & are a bolt on installation. The SBP shift kit also gives triangulated mounting of the motor to the frame. They do take a very small portion of available power in that it must overcome the added resistance of 3 extra bearings (2 on the jackshaft & 1 in the freewheel), an extra 2 chains plus 2 small radius bends in the chains but this resistance is neglible compared to the gains they give with well chosen cassette ratios. My recommendations for him would be a decent manifold, carb & air cleaner, an SBP shift kit, an SBP expansion Chamber & if he an put 1 together, a CDI that has a proper timing curve. Leave the motor stock otherwise & he'd b right. Cheers
 
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here are some major drawbacks to your roommate's plan.

1.the JJ chopper has 20" wheels.

that's good for torque, but requires a lot more power to get up to the speeds that a 26" wheel will. it's simple physics, as a smaller wheel will get rolling much easier than a larger one, but will need to roll faster to keep up with a larger wheel.

make sense?

i built a 20" race bike that will kick butts off the line, but get's passed in the straights by the bigger bikes.

trying to get torque off the line and for climbing hills as well as a higher cruising speed is a very difficult thing to do with a china 2 stroke. it's usually either one or the other, especially with a stock(ish) motor.

2. the JJ chopper has the aerodynamics of a refrigerator box. the upright position of the bike works great with a 100hp motorcycle, but not too well with 2.5.

3. the JJ chopper has the ergonomics of a broken excercycle. those bikes are rough to pedal. standing up is almost impossible, and you're roommate is gonna have a tough time taking off from every stop.

4. to be blunt, but with no offense intended, at 350lbs, your roommate is asking a lot from a bicycle, and much more from a motorized bicycle. to get good torque and top speed from a good bike and an average size rider is tough enough. add 200lbs to that, and... i'm sure you get the picture.

but don't let me stop you from trying. just being realistic...
 
yeah for sure, I 'doubled' a friend on my bike, our combined weights happened to equal his, and I saw the troubles the bike had. I had to pedal to get up a hill the bike normally flies up on its own, stopping was straight up near impossible, I see his plight.

I was thinking if he used the stock sprocket or even went one bigger, the smaller tires should help with takeoffs (he wants a centrifugal clutch too at some point). I plan on doing the porting for him to maximise low end gains, and he's buying my SBP pipe off me (I cant figure out how to mount it, so I'm just gunna get a voodoo or a jag torque pipe) and I told him to grab a puch head for $45. However with a crank balance and piston weight reduction, it's apparently possible for some of these motors to hit 11k RPM, which should give him the top speed too (if it can even get up to that speed). I think it was foureasy with a youtube video of doing 50mph on a 41T sprocket. Other than reeds, I think he's at the limits in terms of power. I'll also recommend he grab quality bearings so he doesnt have a lockup (I imagine with all that weight right on top of the back tire, a siezure would be catastrophic)
 
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He is asking a lot but it is doable. You can design an engine for torque but still get some decent revs out of it although the acceleration for the last half of the rpm range is weak. If I were him I would keep the piston port intake, extend it, and also extend the exhaust 5-6 inches. Extending the intake and exhaust have magical results for raising torque. I have tried this setup vrs a reed valve intake with torque pipe and I prefer the piston port intake because it gives more power from a stop. He wont burn the engine up unless he keeps the std CDI on it. He will need to turn the flower nut in more on his clutch to keep it from slipping.
 
There is no substitute for cc's. At his weight he needs the maximum displacement motor that will fit to offset his weight. The extra weight of the motor is a mute point at this point if the rider weighs 350+ lbs. A jackshaft kit to any type of multi-geared hub wouldn't hurt either.

He picked the right bike for him with that massive rear wheel and that heavy duty bike frame. Just keep the back tire at it's maximum PSI to avoid pinch flats and keep the rolling resistance as low as possible. I also would drive an extra seat post deep into the seat tube into reinforce it. Also stay on as smooth a road as possible or at least slow down a lot when the road conditions fall apart.
 
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Calls for a custom large displacement four stroke build, with plenty of low-end and able to take off from a stop under throttle alone.
Realistically, a standard frame will NOT hold up at 20+ mph carrying that kind of load, and chopper style frames are uncomfortable to ride anyway.
Your results may vary.
 
I agree, a big 4 stroke is needed here. He will probably burn up that little 2 stroke revving it high at low speeds. It should b pretty easy to fit a 212 predator in that frame.
 
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