When Cedric Eveleigh told us about his Supre Drivetrain
which splits apart the two functions of derailleurs (shifting gears and tensioning the chain), thereby dramatically reducing the chances of breaking derailleurs on the trail, we wondered why nobody had thought of that before. But in a sense, they had. Cedric himself showed me this patent from 1937
, which describes a drivetrain with an arm that selects the gear on the cassette, with a separate tensioner arm and pulley tucked behind the chainring. So why haven't we been using drivetrains like this since the late thirties? And what specifically is it that allowed Eveleigh's design to be granted this patent,
which was just published? I got on the phone with Cedric to find out.What's new?
The problem with the design from 1937 is that there isn't much space for the tensioner arm to move clockwise before it hits the upper chain span. That means the arm can only take up so much slack in the chain, so it was only compatible with very narrow-range cassettes, and certainly wouldn't work with modern wide-range cassettes combined with suspension systems that require even more chain growth.
The Supre drivetrain works with modern cassettes because it's designed around a high-pivot and idler suspension design. The idler pulley moves the upper chain span out of the way, making room for a tensioner arm with a much wider range of motion.
Even so, Supre Drive requires the idler pulley to be positioned further forwards than on most high-pivot bikes to make room for the tensioner arm. And when we first reported on the Supre Drive
, we showed a prototype bike that was only capable of running a 10-45t 11-speed cassette. But the drivetrain pictured above accommodates a 10-51t 12-speed cassette, which will make it more appealing to the mass market and therefore to bike brands.
This requires the idler pulley to be even further forward, but as the Supre drivetrain is aimed at OEMs who want to design bikes around it, Cedric sees this as a positive rather than an issue. "Fortunately, moving the idler pulley forward has the advantage of reduced cross-chaining angles," Cedric explains. "This reduces drag and wear, especially in the highest and lowest gears." It's certainly true that cross-chaining significantly increases drivetrain drag at the most extreme angles, so increasing the length of the upper chain span should help alleviate this problem.
Cedric's patent stipulates that the chain remains above the "clearance line", labelled here with number 121.
Eveleigh's patent application
was granted on the basis that it was a novel combination of the frame-mounted tensioner, rear derailleur and high-pivot idler, plus a stipulation that his design kept the lower chain line above what's called the clearance line - a line drawn from the bottom of the chainring to the bottom of the largest cassette sprocket.
Cedric thinks the patent might have been granted without this stipulation, and patent documents are usually designed to be as vague as possible to keep the inventors' options open, but Cedric added this detail to make his application more watertight and because he couldn't foresee the need to ever make a drivetrain with a lower chain clearance, since getting the fragile bits higher up off the ground is kind of the point.
"It's possible that I might have been able to get that claim accepted by the examiner with just the tensioner separate from the derailleur and the idler pulley," Cedric explains. "But just for the reason of obviousness and to avoid unnecessary lawsuits [laughs] I decided to narrow my claim a little bit. When you go to lawyers and ask them to write a patent application, their reflex is to make the claim as broad as possible .... it could be applied to a lawnmower or something ... they're just trying to make as much money as possible. I'm motivated to just protect what I invented as it's specific to bikes. That's along the lines of why I narrowed the claim to have the chain remain over that clearance line because that makes it a bit more specific to what I actually contributed."Clutch development
In our conversation, Cedric went into more detail on his damper design for the tensioner arm. In most MTB derailleurs, the clutch (which is a form of damper) uses a one-way ratchet to apply friction via sliding surfaces to resist the movement of the cage in the forward direction. As we all know, this greatly reduces chain slap, noise and dropped chains, but it can make it harder to shift, particularly when moving into a larger sprocket. This means the clutch friction has to be a tradeoff between shifting performance and chain slap. Also, this test from Cycling Tips
showed that the clutch in Shimano derailleurs can temporarily increase chain tension after shifting into a larger sprocket, and this measurably increases drivetrain drag in some situations.
An early version of the Supre drivetrain used a torsion spring and a one-way sliding friction damper in the tensioner arm, as you'd find in the cage in a derailleur. Cedric was struggling to find the right amount of friction to allow for consistent shifting while also providing enough resistance to reduce chain slap. His solution was to do away with the sliding friction damper ("clutch"), with its high static friction, and replace it with a speed-sensitive hydraulic damper, like you'd find in a shock.
That means the resistance during slow movements like shifting is very low, but the resistance to faster movements associated with chain slap is very high. According to Eveleigh, that means there's less need to fine-tune the friction because there's much more damping bandwidth that works for both shifting and chain slap - you can have your cake and eat it too.
Another innovation is the spring which tensions the chain via the tensioner arm. In a conventional derailleur, shifting into the larger sprockets winds up a torsion spring in the cage's pivot, which increases the chain tension because the spring provides more force.
This means the chain tension is significantly higher in the larger sprockets, and more chain tension means more drivetrain drag because chain links under higher tension have more friction when they articulate to pass through the jockey wheels.
In Eveleigh's design, the tensioner arm is driven by a cable that engages with a cam-shaped ramp on the arm with a variable radius - (this article explains a similar concept in more depth
). This changes the leverage ratio between the tensioner arm and a linear coil spring in a cartridge that is hidden in the downtube. The change in leverage ratio is engineered to compensate for the increase in spring tension as it is stretched, keeping the chain tension roughly constant through all the gears.
I say "roughly constant" because Eveleigh actually designed the tension to drop off slightly in the larger sprockets (the opposite of a conventional derailleur). Eveleigh reasons that you don't need as much chain tension when you're climbing because chain slap and derailment are less of an issue at slower speeds.
Compared to typical derailleurs, his system has much less chain tension in the low gears, but it also has slightly less chain tension in the high gears. Normally, this would allow the chain to slap about through a larger range of motion, making it noisier and more likely to fall off. But the hydraulic damper is designed to solve that issue by applying more damping force at high speeds than a conventional clutch derailleur. The advantage of lower chain tension is reduced drivetrain drag. This, combined with extra-large jockey wheels and idler pulley, plus smaller cross-chain angles thanks to the longer upper chain span, are designed to minimise the drivetrain drag which is a drawback of idler drivetrains generally
When asked about the packaging of the spring/damper cartridge, which in its current form is designed to be mounted in the downtube, Eveleigh was understandably tight-lipped. "I can tell you I'm working on a version that's much more compact."
Cedric told me he is working with "one major mountain bike company, three more nimble local manufacturing companies and a few small scale frame builders." Many of them are currently at the 2D design stage, working out their suspension kinematics around the Supre drivetrain and its unusual idler location.
Cedric says the system is pretty straightforward to design into a bike with a high-single-pivot and the idler connected to the swingarm. He has an information package including 3D models of the chain clearance in each gear, so bike brands can design their bikes to fit around it more or less as they would with a gearbox or e-bike motor. Bike companies who want a different suspension design (such s a four-bar linkage or a frame mounted idler pulley) will have a bit more back and forth with Lal Bikes to make sure it's compatible.
Cedric thinks that one of the more nimble brands with in-house production will likely be the first to market with the Supre drivetrain.
After being unable to buy a single Pinion gearbox for a bike he designed around it in 2012, Cedric says he'll work with anyone who wants to use his system, no matter the scale. "Anyone who wants to can go ahead ... even the kids who are reaching out to me for a high school project, I'm like here are the specs let me know what you come up with."