Jackshaft Yay or Nay

Gupa Nupa

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Twin Cities, MN
Ive seen a lot of things arguing both directions. What are the thoughts/opinions of some of the more experienced builders, and why?

(this would be for a bike that I want to be capable of both road cruising and light trail use)
 
I love mine. I can cruise a nice clip and still master the muddy monsters. It's a lot more work to up keep and install than a single though. Pain in the arse to get right sometimes. Better gas mileage, longer engine life, lots of benefits, and some down sides too. Chain breaks, your done and walking, you now have a more dangerous right side, two chains whipping around right by your feet. You have to maintain the bearings in the crank, only so many miles then replace. Failure to do so can be catastrophic and possible injure you and cause you to crash.
 
More information as requested. Perhaps more than you wanted....

My current plan is to use sixthreezero's Evryjourney as my frame (https://www.sixthreezero.com/customizer/mens-evryjourney-steel-touring-hybrid-bike). My goal for the bike is to have a solid all-a-rounder. Ideally the bike is solid on trails and roads, and has a cruising speed of 40-45 mph, top speed around 50 (low end is more important than anything over that - I dont plan on ever going over 60). Torque isn't all that important to me as I'm only 120 lbs. and don't mind pedaling on start up and don't plan on towing anything, but a dead stop start would be nice. Right now I have a flying horse engine (BBR tuning), but I plan on using the Phantom 85 for this next build.

Everything I have read about jackshaft kits so far has been very mixed. Some people have found lots of success with them and promote them, while others claim that there the power loss that they have makes them useless.

Before I get too far gone I'd like to make sure I actually understand jackshafts correctly. My understanding is that jackshafts are basically a transmission, allowing you to run you engine at lower RPMs while at higher speeds. It does this by using a chain to connect the 10 tooth clutch sprocket (that would normally power your rear wheel directly), to your jackshaft input sprocket (usually a 17 tooth sprocket). This is transferred through the actual jackshaft to the other side of the bike where there is another sprocket (usually a 10 tooth sprocket) that connects to the actual free wheel down by the cranks. This free wheel lets you pedal the bike to start without having to pedal at the same speed as the engine. The power goes into the freewheel at a 44 tooth sprocket and exits at a 36 tooth sprocket. At this point the power is transferred through the standard rear cassette which you can shift like normal.

10 tooth -> 17 tooth in (jackshaft here) 10 tooth out -> 44 tooth free wheel (connected via free wheel) 36 tooth out -> cassette.

Is this understanding correct? If so how does one find the best gearing for what they need (balancing torque and top speed)? Can the actual bike chain (that connects the free wheel to the cassette) withstand the torque and abuse associated with a jackshaft?

Thanks for your patience in explaining this to me!
 
I'm gone be honest, I won't tell you what cassette I use, I don't want anyone buying them up and they are becoming more rare by the day. lol

Now a jackshaft will get you a motor cycle feel, but all tht complex gear junk in the cog set can go if you use an igh. I plan on switching when I'm not so lazy. lol. I have a bunch of SA hubs, some of them will become a transmission and others a wheel. all a jack shaft does is make the pedal side a gear train. Now it uses the rear cog and whatever set up you like back there. A mega range works pretty good. you will skip gears a lot. no need for a cadence like when you pedal. 24-28 is really the lowest gear you will need if not hauling. I climb anything with a 24, a 34 is awesome for tree climbing. lol
 
More information as requested. Perhaps more than you wanted....

My current plan is to use sixthreezero's Evryjourney as my frame (https://www.sixthreezero.com/customizer/mens-evryjourney-steel-touring-hybrid-bike). My goal for the bike is to have a solid all-a-rounder. Ideally the bike is solid on trails and roads, and has a cruising speed of 40-45 mph, top speed around 50 (low end is more important than anything over that - I dont plan on ever going over 60). Torque isn't all that important to me as I'm only 120 lbs. and don't mind pedaling on start up and don't plan on towing anything, but a dead stop start would be nice. Right now I have a flying horse engine (BBR tuning), but I plan on using the Phantom 85 for this next build.

Everything I have read about jackshaft kits so far has been very mixed. Some people have found lots of success with them and promote them, while others claim that there the power loss that they have makes them useless.

Before I get too far gone I'd like to make sure I actually understand jackshafts correctly. My understanding is that jackshafts are basically a transmission, allowing you to run you engine at lower RPMs while at higher speeds. It does this by using a chain to connect the 10 tooth clutch sprocket (that would normally power your rear wheel directly), to your jackshaft input sprocket (usually a 17 tooth sprocket). This is transferred through the actual jackshaft to the other side of the bike where there is another sprocket (usually a 10 tooth sprocket) that connects to the actual free wheel down by the cranks. This free wheel lets you pedal the bike to start without having to pedal at the same speed as the engine. The power goes into the freewheel at a 44 tooth sprocket and exits at a 36 tooth sprocket. At this point the power is transferred through the standard rear cassette which you can shift like normal.

10 tooth -> 17 tooth in (jackshaft here) 10 tooth out -> 44 tooth free wheel (connected via free wheel) 36 tooth out -> cassette.

Is this understanding correct? If so how does one find the best gearing for what they need (balancing torque and top speed)? Can the actual bike chain (that connects the free wheel to the cassette) withstand the torque and abuse associated with a jackshaft?

Thanks for your patience in explaining this to me!
Yes that's it perfectly. I have a Mongoose Dolomite (7 speed) with a shift kit. If you want a shift kit (like I did, and do) then you want one. However it does complicate things in that it's more stuff to do, more stuff to maintain, and more stuff to go wrong. If, like me, you want what you want, then you get what you want. But you may also get some stuff you don't want, like more cost (shift kit cost more than my engine,) more tinkering, more maintenance and more down time. If you don't have a super small triangle on your bike you won't have to make custom parts like I did.

In my opinion a shift kit isn't necessary if you are riding the average roads or trails, and in fact, you may be able to go faster without one. The nice thing is I've gone 18 mph up the steepest hill in town in first gear. The downside is the shifting. Also I noticed my two highest gears (gear six - 18T and gear seven -14T) are all but useless. I'd say if you have a lot of hills you are better off with one. In the end though, it's really up to what you want.

Another reason to go with one is if you have a bike with a rear disc brake so that you don't have to use the rag joint needed on a standard build. There are ways around that, but, ... it's all a trade off one way or the other. Where do you want to spend your time and money? It becomes quite the rabbit hole and so I would suggest you go with your gut. This is my first build so I don't have tons of MB experience, but I do know some things about the shift kit having gone through it myself. Don't hesitate to ask. I've gotten so much help from this site I'm certainly ready to give some back where I can.
 
I would agree if your using a old 3 speed, but with a good hub it is well worth it.
 
A few tid bits...

Only company still producing 3 speed internally geared hubs is Sturmey Archer.

Best freewheels are the Trials "Green" White Industries ones.

Sick Bike Parts had White Industries produce the freewheels they sell with their Jackshaft kits.
 
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