by "bhtooefr" (bhtooefr)
Published 01/28/2017 at 10:03
by "bhtooefr" (bhtooefr)
Published 01/28/2017 at 10:03
Tags: bikelopnik
; transmissions
; retro-direct
; retrodirect
; hirondelle
; vintage
; sram
; xiongda
; e-bike
STARS: 3
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Just pedal backwards. Wait, that was yesterday’s gearing system . Today, just keep pedaling backwards.
What you see here is a retro-direct drivetrain, which dates back to at least the 1860s (in the single chain form here, it dates back to 1903). Two freewheels are used, but they’re rotated in different directions by the pedals, so when you’re in high gear, the low gear freewheel is being pedaled backwards and is freewheeling, and vice versa. And, because of the chain being crossed, it doesn’t matter which way you pedal - you’re going forward.
This particular configuration has the high gear as the forward gear, as vintage retro-directs tended to use, although low gear is sometimes used as the forward gear.
There are, however, some downsides. Pedaling in reverse is a rather unnatural motion that uses different muscle groups, so power and cadence are both compromised, which is why most setups use low as the reverse gear. Pedaling in reverse also tends to unscrew pedals from the cranks, to the point that some retro-directs from back in the day had welded pedals to prevent that from occurring (red Loctite is your friend if you build one). And, you can’t roll a retro-direct bicycle backwards - the freewheels will try to turn the cranks in opposite directions, and will lock the rear wheel. Oh, and of course, you only get two gears, although Hirondelle actually combined this with a front derailleur back in the day, expanding the range to 4 gears.
Now, if you’re familiar with how planetary gears work, you might be thinking of an improvement to this, that eliminates the nasty chain routing, because planetary gears have very easy ways to get reverse - use one-way clutches so that reverse only engages with a reverse input, and vice versa, and you’ve got a retro-direct. Unfortunately, the drawbacks of retro-directs are such that nobody’s bothered to make an internally geared retro-direct hub. However... two companies, SRAM and Xiongda, have made internally geared retro-direct electric bike motors . The rider won’t see a change in gear (I’m pretty sure that’s the case on the SRAM, and it’s definitely the case on the Xiongda), but the electric motor will . Also, these systems seem to have provisions for rolling the bike backwards in their clutch design.
Here’s how a Xiongda front hub motor works:
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"TheRealBicycleBuck" (therealbicyclebuck)
01/28/2017 at 11:53, STARS: 0
So yesterday’s post wasn’t all that unusual, but this... this , is something I’ve never seen before.
Neat.
"bhtooefr" (bhtooefr)
01/28/2017 at 12:00, STARS: 0
I almost posted this first, because it’s actually a lot easier to understand (and I was having trouble figuring out how low gear worked in the Sachs Duomatic, until I realized where the low gear drive pawls were)... but I wanted to build up to this.