New Build Question With SBP/5:1 Gearbox

[Deleted Member 4613]

I am thinking about building a shift mb using the dax 5:1 gearbox connected to a Tanaka 32cc 2 stroke engine. It's a simple idea of turning the motor and gearbox to face the front sprocket which will drive seven gears on the rear wheel. The question I would like to know is about shifting: Would shifting be smooth and would I be able to shift down and up skipping some gears? Could I shift smoothly from a high gear to a low gear without running through all the gears? I would like to have definite answers before doing all the work. Other wise, I'll just build a single gear rack mount, which I know will work. Mike

 

[5-7HEAVEN]

Yes to all your answers. I have a Tanaka 47R engine connected to a pocket bike 5:1 transmission. With a series of other sprockets, first gear is 46.32:1 and eighth gear is 15:1. The reason for extreme gears is because there is very little pedal-assisting you can physically do with a shift kit system. Shifting is silky smooth from any gear to any other. I always pedal and release the throttle when shifting. One thing you must remember is that the bike has to be in motion before you can shift gears, just like a normal multi-speed bike with rear cassette.

Remember 5:1 gear ratio is only the beginning; more gears and sprockets need to be installed to achieve super-low gear ratios.

 

[Deleted Member 4613]

What gear would I use on the motor and front sprocket? Maybe 10 tooth on the gearbox and 44 on the front sprocket? I need to learn how to select these sprockets. What do you have on your bike?

 

[5-7HEAVEN]

Since 8mm pocket bike sprockets are available in larger sizes than bicycle sprockets, I decided to use those. An 11-tooth drive sprocket slips onto the 5:1 gearbox spline, and chains up to a 72t chainring driven sprocket. Multiply the tranny's 5:1 to get.32.7272:1.
A 24t bicycle chainring sprocket chains to an 11t-34t rear cassette. That calculates to 46.36:1 granny gear and highway gear of 15:1.

These gears are good for steep hills or ramps. That's when I use every gear. On the flats, I skip every two gears, like from 1st to 4th, then to 7th gear, which is a responsive 17.72:1.

If you like bike sprockets, you could run 10t/48t engine drive, then 22t @ the chainring to get 37.09:1 first and 12:1 in final gear.

 

[Deleted Member 4613]

Thanks much.I too prefer 8mm and have built my rackmount cvt with 8mm. Now to get started.

 

[5-7HEAVEN]

The best thing is that there is a vast assortment of pocket bike components available. When you split the gearbox, you'll be surprised how tiny the gears are. Some boxes have helical gears, others have straight-cut gears. I have never had a tranny fail me, although they are quite noisy at times. I use general-purpose grease instead of oil in there.

The Achilles' heel is the $12 4-bolt freewheel which screws onto the crank arm. Runout is extreme, but I've never had one fail. If that bothers you, a HD $70 5-bolt freewheel is available from SBP. You just have to redrill the chainring sprocket's bolt pattern.

Good luck.

 

[Deleted Member 4613]

This has to be the lightest system ever: The combination

of the Tanaka Purefire 32cc and the Dax 5:1 gearbox weighs

in at 8 pounds, the motor 6 pounds and the gearbox 2 lbs.

The total width is 9 inches, great for a frame mount. I'm

very excited about either a shift frame mount or a single

gear rackmount using treewks double rightsided chain drive.

Better then a GEBE system, cheaper, more reliable, and

better than a SBP shift kit which is far more complicated

because it requires a jackshaft. Using Treewks double

rightsided drive for a rackmount, the engine is centered

over the rear wheel for better balance and a better

look.

 

[Deleted Member 4613]

5-7, there's just one thing I don't understand about your drive system. You're mixing 8mm and bicycle sprockets on the front but these take different freewheels with different hole patterns. What freewheel are you using that can take both types? You must be drilling new pattern holes in the 24t bicycle
drive sprocket so it can fit on the 4 hole 8mm freewheel. Right?

 

[5-7HEAVEN]

Yes I DID drill the bike chainring sprockets to match the 4-bolt freewheel and the 70t 4-bolt pocket bike chainring sprocket. My original bike chainring sprocket was 22t. That cracked, so it was replaced by a 30t. When I needed lower gearing, I had a 24t sprocket.

I like the 24t the best.

The system is light; don't forget to add some for the motor mount, and the freewheel, and the extra chain, maybe the thicker crank arms and the HD wheels.

One nicety is that OEM-size spokes can be used. They can take the abuse. Cassette sprockets wear prematurely from abusive powershifting. Chains break from chain jumping, then jam between inner sprocket and bottom bracket. Normal motorized bicycling is possible without constant breakage. I've only replaced one SS spoke, which broke at the head tip.

 

[Deleted Member 4613]

I am a little concerned about chain problems. With my cvt rackmount, the build was relatively easy, no drilling of sprockets necessary, and the final result is a very reliable system, smooth acceleration. I don't think the shifting can match a cvt, it's the ultimate in a motorized bicycle, totally automatic continously variable transmission.