What if I told you that there was a drivetrain available that had the following attributes:
- Completely sealed gearbox with oil-lubricated internal gears.
- Perfectly straight, unchanging chain line with a 174mm q-factor on a standard chainring.
- Mass of the drivetrain is centrally located on the bike.
- Perfect 11.5% gradient between gears, with 18 gears covering a 636% change in ratio.
- All of the gear reduction/multiplication at the cranks, permitting you to run a heavy duty, but very simple, singlespeed hub at the rear wheel.
- No rear dérailleur.
- Add or dump as many gears as you want, any time, including shifting under load.
Sounds intriguing, right? Pinion is working on just such a drivetrain. Details about it are on their website, where they also feature this gearing chart showing the range of gears:
What are the downsides of this setup?
Weight: They’re predicting about 3.2kg for the hub/transmission, cranks and shifter…that’s about 7 pounds. No matter how you cut it, that’s porky…a couple times the weight of a ‘standard’ drivetrain.
Complexity: Inner-working details are not yet fully known.
Compatibility: As you might expect, a bb-based transmission gearbox requires a custom frame. Carver, maker of the fat carbon fork I’ve previously talked about that I run on the Mukluk, is working on that issue, as their blog shows.
The cables, as you might guess, are for shifting…so you could run continuous housing, sealed at the gearbox, for a pretty crud-resistant drivetrain. From a crud-resistance viewpoint, it’s basically like a singlespeed.
And yes…the pedal and chainring are on separate bearings…the chainring spins at a different speed than the pedals, depending on the gear that you’re in. Pretty complicated…but it’s all internal and sealed.
You can subtract the weight of a BB bearing…but in addition to the 3+kg of the drivetrain, you also have the weight of the pretty substantial forged additions to the frame, shown in the pics above. Massive.
Warning: Totally difficult to follow discussion of questionable engineering principles follows. You may want to stop reading here.
Strain: One last potential negative to consider is strain in the drivetrain. This is going to be kind of hard for me to put into words (or at least into the right words). With a ‘normal’ drivetrain, you apply force at the crank–which is transmitted through the chainrings and chain. The chainring determines how much leverage you have on the chain…but the total ______ (power/torque/strain) that you’re putting through the chain is limited by the leverage factor between the crankarm length and chainring size. Power is transmitted through the chain to the cassette, where another torque adjustment occurs at the cassette (with larger or smaller gears changing the gear ratio and torque multiplication or division). The cassette is mounted directly to the hub, and transfers the force you’re generating, adjusted by whatever gear you’re pushing, to the wheel. With this drivetrain, all of the torque adjustment (multiplication or division) occurs at the bottom bracket, and is put through the chain.
Let me try to explain that better. Assume that you’re on a bike that is totally stationary, with the rear tire bolted to the floor. You apply 100 ft/lbs of torque to the cranks, which happen to be 1 foot long. The chainring is sized such that the chain is roughly halfway up the cranks–the chainring is 6″ in radius (12″ in diameter. I know, I know…but it simplifies the math). In that instance, the leverage of the cranks effectively doubles your torque…so the chain has to withstand 200 ft/lbs of torque. Much of the torque multiplication or division actually occurs at the cassette, so while you might generate more force in a given gear ratio, the chain doesn’t know it–it only knows the force you’re applying at the cranks. I can go to my very highest (slowest) gear, which actually applies more torque at the hub than I generate, because of the gearing advantage, but much of the torque multiplication is occurring at the cassette/hub. The chain doesn’t experience that torque multiplication.
In that same bike, same scenario, same effective gear ratio, the torque multiplication is occurring inside the gearbox. The increased torque is being output through the “chainring” after being multiplied through the bike’s gearing. The chain is being subjected to greatly increased force, because of where you’re doing the gear reduction. Sure…the hub feels the same force…but because the gearing is all before the chain, you’re putting a lot more potential strain on the chain.
I’m going to try one last analogy here. In the world of spec’ing semi-trucks for hauling gravel, you have 2 sets of ‘force adjustment’…just like on a bike with a traditional drivetrain. On a bike, the 2 sets of force adjustment are chainring and cassette–and they are joined by the chain. If you do all of your force adjustment at the chainring, the chain gets subjected to much greater force.
I don’t think I can explain it better without going into a discussion of the gearing of transmissions and final drives in semi-trucks…(from where my experience tells me that if you do too much of the gear reduction in the transmission, and not enough in the final drives, you’ll blow u-joints)…and it’s that experience that makes me think about where the force multiplication or division is occurring, and what effect that has on the poor ole’ chain. And this setup concerns me a bit–for that reason. It’s within the realm of a mortal man to break a chain on a fatbike without putting all of your ‘transmission multiplied force’ through the chain. I worry about what this will do to the poor old chain. Perhaps someone with more engineering background than me has already considered this. We shall see.
End questionable discussion of engineering here.
Fatbikes are going in different directions. From a race perspective, this makes little sense…you wouldn’t think of running it on a Beargrease (even if the frame did work). But for an expedition fatbike…it’s interesting. I’m not saying it’s a good idea (yet), but I like to see new ideas and experimentation. And yes, I know it’s not the first (or last) bb-mounted gearing setup. I’ll be curious to see how this turns out.
I think the inevitable outcome from this is simple: You take this, use it drive a XX1 rear end with 11 cassette gears, and run that on a Rohloff hub…and you’d have (18 x 11 x 14) 2,772 gears. That should be just about enough.