We've reported on Trinity before. Their Prototype 1 downhill bike, with its steel frame, high-virtual-pivot suspension and gearbox or singlespeed drivetrain is more than interesting enough to warrant its own article. But now the three-man brand has upped the ante on innovation.
Drivetrain: Flipped crank drives a cassette via a derailleur on LHS, jackshaft drives rear hub via idler pulley and tensioner on RHS. Shifting: Six-speed, SRAM AXS, 11-22T Weight: "398g heavier than a normal drivetrain.. unsprung mass reduced by 506g" Frame material: 4130 Chromoly front triangle, billet aluminium rear triangle Rear Wheel Travel:170mm Wheelsize: Mullet More info:@trinity_mtb
The concept of a frame-mounted derailleur and cassette isn't new. It was used to great effect by Honda in the mid-2000s, where moving the derailleur out of harm's way and shifting its mass from the wheel to the mainframe was said to offer some tangible benefits to the RN01. And compared to a gearbox, the derailleur retains the crucial ability to shift while sprinting and is arguably lighter and more efficient too. Shimano recently filed a patent for a similar concept, but the idea hasn't seen daylight since Honda canned their MTB endevours in 2007.
I got in touch with Mick Williams, Trinity MTB’s engineer, to find out more about what he calls exploring an old idea with new tech.
The plan is for the gearing to be encased in a lightweight box.
What inspired the project and what's the idea behind the drivetrain?
The idea behind the drivetrain was not being 100% satisfied with the current gearboxes on the market that our bike is designed to spec. From an engineering and efficiency standpoint, the best box so far has been the Honda RN-01 (in our opinion), so we wanted to bring the Honda back to life with new generation off-the-shelf parts, including AXS shifting. It’s far from complete and we’ve only experimented with component locations for a few days, but it’s looking promising.
Is it a modification of your previous gearbox bike or a whole new bike?
No, this is the original Prototype 1 which was featured on Pinkbike a few months ago. This external gearbox has been retro-fitted to Proto 1 past tense of the build - we had zero plans of this drivetrain when building it (kind of goes to show how fast the R&D of a project like this progresses), so geometrically it isn’t perfect as the frame wasn’t designed around it. We’re working on provisions in V2 to be designed around this gearbox to make the set-up bang-on.
The derailleur is essentially run back-to-front (the chain is fed from what's normally the lower jockey wheel to the cassette), but it seems to work.
Can you explain how the drivetrain works?
Mechanically it works the same as your normal drivetrain with all the same running gear; we’re using a SRAM AXS derailleur and shifter with the bottom half of an Eagle cassette (so a 6 speed 11-22T), the stock crankset, and everything else - this means that the “box” can be taken off and reconfigured to its original drivetrain form (all with the original length RHD chain), or with a Pinion/Effigear or other standards, depending on your preference. We want this to be versatile. Everything is rotating the same direction as normal by way of a jack-shaft that connects the cassette to a single speed cog on the right side of the bike, so in that sense and contrary to other gearboxes, we’re maintaining the working order and efficacy of a normal chain and cassette drivetrain.
Is the cassette on a slider to maintain the chain line?
It’s not. This is something we mucked around with and would be great, but to do so the above point of using stock parts was not possible - we’d need custom stepper motors and the like. We really wanted to utilise the stock groupset equipment to make the box able to be re-assembled to its original form and vice-versa. We’re open to exploring this again though.
What's the efficiency like?
Equivalent to a current standard drivetrain and much better than any other traditional gearbox on the market due to their backwards rotating gears, their nature of being submerged in oil and generally having poor clutches.
Where efficiency really goes out the window with a standard drivetrain is when dirt and contaminants are introduced; there may be a little more friction added by the additional idler pulley/cog and the incident angle of the chain about the cassette, but in a real world setting this minimal addition will be offset by having the transmission more easily protected.
Also, talking of efficiency, many suspension kinematics of a bike are altered depending what gear the rider is in, due to the effect the chain linking the swing arm to the mainframe has (altering output efficiency) - our box is single speed driven from the main frame to the rear wheel, so no matter the gear the kinematic efficiency is always the same.
We’ll dyno all this in the future to fully quantify for potential customers.
What's the weight like?
It only adds 398g compared to a normal drivetrain. This is made up by an extra short chain (LHD), the jack-shaft and 2x single speed cogs. What’s more important to note is that it decreases the unsprung mass by 506g by moving the derailleur and cassette to the main frame (sprung body), so dynamically it’ll feel lighter for the rider to handle when on the trail.
We also reckon we can get the 398g figure down by purpose building the V2 frame around the box rather than retro-fitting. Minimal weight will be added by the protective shrouds, but we want to do these from a super-light material (carbon fibre, etc).
The WRP CentreHub is another of Mick's projects. It allows the bike to shift while coasting, which could be useful in downhill racing, but the bike is compatible with a normal freehub and fixed chainring.
I'm intrigued by the CentreHub crank. Would the design work with a regular crank and freehub? Presumably, you've had to modify the CentreHub to work anticlockwise? If you're using a fixed hub, why didn't you have the singlespeed chain on the left, the chairing and cassette on the right and the cranks on the conventional way round - then you could use a normal CentreHub and pedals on the normal side?
Yep, a normal chainring [and freehub] can be run too.
As for the CentreHub, it’s fully modular so the clutch unit can just be flipped around to covert it between a RHD and a LHD in a matter of seconds.
You’re right about having the transmission on the right and we thought about it a bit - but this means changing the rear brake to the opposite side. We have bolt-on dropouts so we could achieve this easily, but it’d eliminate the option for a rear-wheel mounted clutch [freehub] as normal, so this way is more versatile. We also have a spec where it’s all on the RHD, but it’s busy, very busy. We’ll give this one a crack and you’ll probably see a few of the configurations mentioned above come out to play in the near future.
Have you ridden it yet?
Only around the car park; we’re making these shrouds first to enclose some of the moving parts before hucking any triples. It's had vast improvements since the vid was taken last week, consisting of swapping the derailleur's idler and the indexing cogs around (considering it’s mounted upside down), which made a huge difference. Along with an altered hanger and some other things.
What's next - are you planning to sell it?
We’d love to have this as an optional upgrade when pre-ordering a Trinity MTB for the DH lords among us.