Factory Tour – Guru Cycles, Part 2: Building a Carbon Bicycle Frame
In Part One of our Guru Cycles Factory Tour, we showed how the design of the bikes goes from customer order through to a pile of tubes ready for assembly. Mostly, anyway.
Here in Part 2 we’ll finish the process and show how everything comes together to form a complete frame. I’ve seen plenty of carbon fiber bicycle manufacturing in our 5+ years of Bikerumor’s existence. Each time I head to a factory, I wonder what could be new? What could the next builder possibly do that’s different from all the rest. What’s amazing is that virtually everyone manages to surprise, and Guru’s no exception.
First, there’s the unique cut of the carbon swatches that make up the frame. It’s the most visible feature of their Photon bikes and gives it a slick sea snake appearance. Under all that carbon, though, is another very cool construction process I’d not seen before…
Guru’s engineers are constantly playing with different layups, seeing what improves the frame, what drops weight and, ideally, what will do both. Their bladder molding process leaves nothing inside except carbon, and this tube shows the patterned layups that help them achieve some incredibly light frames.
Most of the Photon’s tubes are made using a silicone bladder. The exception is the downtube/bottom bracket assembly. It’s put together with metal mandrels so they can create a single piece for better strength and stiffness while still getting the tolerances right for pressfit BBs.
Interior tabs between the BB shell and downtube add an additional point of contact for a stronger connection once it’s all heated and pressed, fusing the tubes together with a bit more structure. You know what it’s called when someone takes the time to add little touches like these? Love.
The layered BB shell wrap is folded over the top (click to enlarge and see the graduated layers), then more Love (carbon tabs) is applied over the sides.
The finished product looks like this.
Once all the tubes are molded, they’re binned with geometry charts and sent to the middle of the shop floor for mitering, then on to the tacking room:
This is where everything starts coming together in a recognizable form.
They also mold their own carbon dropouts, all with tooling made in house. What’s particularly cool about these is they allow for geometry adjustment without resorting to metal inserts. Simple solution to avoid mixed materials; better, lighter weight result.
With geometry sheets in hand, the jigs are set up to each bike’s specs.
The tubes are “tacked” into place with epoxy, bonding them together enough to hold their shape during the rest of the assembly process. Here’s where it starts to divert from most other builders.
With many carbon builders, the tubes start to get wrapped with the outer layers of carbon as soon as they’re positioned. Guru, however, is just getting started. First, they paint on a bonding agent leading up to each major joint.
That ensures the putty will stay stuck on the carbon during the sanding and shaping process. Wait, what? Putty?!?
Technically, it’s an epoxy molding compound, and it dries hard as a rock as its baked, which happens immediately after the frame is tacked together. That lets them easily sand and shape it.
Lest you worry about an expensive custom bike being weighed down by putty, it only adds about 6g to a finished bike.
Once it’s baked, this guy sands off most of it to create shapely tube junctions.
Looks good, but it’s not just for aesthetics…
Back in the clean room, the bikes are assembled using tube to tube construction. For the Photon series, that nonstructural putty adds functional shape to the headtube junctions. Since they’re only using UD fibers, the broader rounded shapes allow the fibers to be run over a bigger cross section without sharp bends.
Since UD fibers only do their job of stiffening the frames when in tension, this lets them spread forces across a larger area and put the strength in plane with how riding forces are actually acting on the frame. Olivier Lavigueur is the carbon wrapper and has been doing it here for more than five years, and has been at Guru for more than a decade.
After the overwrap is complete, a textile called Peel Ply is wrapped over it, then it’s vacuum bagged and put back in the oven to compress and mold the wrap to the frame. As with all carbon construction, the heat and pressure melts the resin between the fibers to effectively turn it into a single, solid piece. Many of these techniques are borrowed from the aerospace industry, which is a huge part of the Montreal area’s livelihood.
Between tacking and wrapping, it gets checked for alignment, brake placement and tire clearance. Then it gets checked again when it comes out of the oven for the last time.
If everything checks out, which it almost always does, the frames get sanded to remove the Peel Ply texture (click to enlarge).
They also remove any excess carbon from the insides of the headtube or BB shell to make sure they’re all in tolerance. The holes are cut for the cable stops and bottle bosses. Those parts are pressed or bonded in place, then it’s racked to await paint and final finishing…which we’ll show you tomorrow in Part Three.