Retro-Direct Driven Gravel Bike

Too apt a backdrop not to use it.

I must confess I have a soft spot for wildly impractical bicycles. Utilitarian bikes may be my bread and butter, but every once in a while I’ll get a wild hair to build something totally different, like the belt-driven fixed-gear cargo bike I built to help Gates test their new longtail belt back in the day. Around that same time a colleague of mine brought up a similarly strange project he had taken on, a retro-direct driven bicycle.

Three sets of pawls, there's always one disengaged.

It's odd for a bike with a short rear end to need two chains, but this is an odd bike.

The idler mount sits in the sweet spot between the two main runs of chain.

“What is a retro-direct drive?” you may be asking yourself - I know I was. There was a time (a liminal space, if you will) after freewheels were added to bicycles but before derailleurs were a thing. All kinds of novel solutions to the gearing challenges of the European countryside were being developed as bicycles exploded in popularity. One such solution, dubbed the retro-direct drive, utilized two freewheels and an idler pulley for reversing the chain direction. This allowed the rider to engage one freewheel via the usual method of pedaling forwards and the other via pedaling backwards. This gave the rider two gear ratios to choose from, one for hills and one for cruising on the flat.

I built my first retro-direct the way most have been built post-1900. I found the cheapest, most beaten up frame I could (a Schwinn Super Sport that weighed at least 8 pounds) and brazed an idler mount on the drive side chainstay. This solved the chain reversal problem, but mounting two freewheels on a single hub presented another hurdle.

Again, I followed the conventional wisdom of the time, which was a clever repurposing of the adjustable cup of a square taper bottom bracket (remember those?). Traditional “BSA” threaded bottom brackets have the same threading as most freewheels at 1.375” x 24 TPI. You can thread one single speed freewheel onto a multi-speed freewheel hub, thread the adjustable cup into the remaining threads on that first freewheel, and finally thread a second freewheel on to the adjustable cup.

This worked, but had a number of downsides. Chief among them was the fact that you would have to lengthen the hub axle to get the end cap out past that second freewheel, usually to tandem spec (145mm OLD). As anyone who rode tandems in the 90’s can tell you, this makes it really easy to bend your axle, as the bearing sits under the freewheel threads, about two inches inboard of the frame. I rode my retro-direct on the many rocky dirt cutoffs along the Willamette river in Eugene, and lost track of how many axles I bent.

The original retro-direct circa 2016. I also added bottle bosses and cantilever studs, which was a lot back then.

Another fun “feature“ of retro-directs is that pedaling backwards can and will unthread your pedals if you don’t use a thread retaining compound and periodically check that they remain tight. These issues along with the difficulty of pedaling backwards led me to abandon the retro-direct and make room for the next project, but I’ve always wondered what a retro direct made around modern standards would look like.

An early prototype of the freehub adapter.

Fast forward to 2024 and I’m looking at the specs for SRAM’s XD driver for another product I’m working on, and I’m reminded that the driver has a unique M32x1 thread for attaching the cassette. This is smaller than the ID of a freewheel thread, leaving just enough room for some sort of thread adaptor. After some quality time in CAD-land, I had a 3D model of a sleeved thread adaptor that would solve all of my hub complaints about retro-directs. The XD driver has a bearing much closer to the end cap, so along with a quality thru-axle the axle-bending issue would be solved. There was also just enough room for both freewheels without having to re-space the hub or the frame. I milled a test part in aluminum to confirm it would work, then ordered some burly 17-4 stainless steel for the production version.

While brazing another idler mount to a frame’s chainstay would be easier, I wanted to develop an idler mount that would attach to a standard derailleur hanger, that way any like-minded kooks could order their own and share in the backpedaling life. To that end, I decided to test the drive on a frame I designed well before the retro-direct project came into focus.

A lot going on in not a lot of space.

I designed and machined a relatively simple tensioner that would place the idler pulley at the midline between the two freewheels with enough projection to make the chain angle low to ensure a smooth running drivetrain. I decided to keep to my preferred setup of running the higher gear in reverse - I find maintaining a cadence over 90-ish RPM difficult pedaling backwards, and getting started pedaling forwards in the lower gear is much easier.

With all the retro-direct parts and normal bike parts hung on the frame I saddled up for my first backpedaling in almost a decade. Thankfully, it doesn’t seem to matter whether you’re pedaling forwards or backwards, it’s just like riding a bike, albeit a little different…