Guru factory tour - carbon fiber bike construction

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 factory tour - inside a carbon tube

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.

Guru Factory Tour - bottom bracket and downtube assembly molds

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.

Guru Factory Tour - bottom bracket and downtube assembly molds

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.

Guru Factory Tour - bottom bracket and downtube assembly molds

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.

Guru Factory Tour - bottom bracket and downtube assembly molds

The finished product looks like this.

Guru factory tour - carbon bike tubes ready for assembly

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:

Guru Factory Tour -frame tacking jigs

This is where everything starts coming together in a recognizable form.

Guru Factory Tour - adjustable carbon dropouts

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.

Guru Factory Tour -frame tacking jigs

With geometry sheets in hand, the jigs are set up to each bike’s specs.

Guru Factory Tour -frame tacking jigs

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.

Guru Factory Tour -frame tacking jigs and putty prep

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.

Guru Factory Tour -putty molding compound

That ensures the putty will stay stuck on the carbon during the sanding and shaping process. Wait, what? Putty?!?

Guru Factory Tour -putty molding compound

Yep, all work and no play here at Bikerumor and Guru. Season’s Greetings!

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.

Guru Factory Tour -putty molding compound

Lest you worry about an expensive custom bike being weighed down by putty, it only adds about 6g to a finished bike.

Guru Factory Tour -putty molding compound sanded smooth

Once it’s baked, this guy sands off most of it to create shapely tube junctions.

Guru Factory Tour -putty molding compound sanded smooth

Guru Factory Tour -putty molding compound sanded smooth

Looks good, but it’s not just for aesthetics…

Guru Factory Tour -putty molding compound

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.

Guru Factory Tour - outer structural carbon wrap

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.

Guru Factory Tour - bottom bracket and downtube assembly molds

Guru Factory Tour - bottom bracket and downtube assembly molds

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.

Guru factory tour - frame alignment checks

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.

Guru factory tour - final assembly state before finishing

If everything checks out, which it almost always does, the frames get sanded to remove the Peel Ply texture (click to enlarge).

Guru factory tour - final assembly state before finishing

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.


  1. Adifferentmike on

    The foam itself wouldn’t add strength of the frame but as stated in the article it can place the outer plys of carbon in a more optimal location and thus increase the structural intesgrity of the frame.

  2. Devin on

    All of that is looking good Guru. Those fillets are gigantic, that’s gotta make it super easy to lay the UD (if you’ve never done it- it’s not a fun, straightforward process, LOTS of tricks to making it take curves nicely.)

    I’m looking at using carbon dropouts of similar design, but I question the long-term durability against some of the super aggressively knurled end caps on wheels. No matter how much compression you give the carbon, without some metal interface there’s going to be a slow migration of carbon from the dropout to your floor. Through-axles, anyone?

    Looking forward to Round 3!

  3. Ted on

    Someone please explain to me how they get the frames to be the same each and every time when all that they are doing is arbitrarily sanding away at what is essentially Bondo.

    Also the mitering on those tubes is atrocious.

  4. Tom on

    re the mitering, I’m pretty certain you can’t miter carbon tubes the same way you would metal – too shallow a cut through the ply could lead to delamination, which would destroy the tube.

    That said, doing tube to tube construction is not the most efficient use of carbon. All those lovely load paths along the fiber have to be interrupted, which is why they have to bondo and wrap afterwards.

    Also, is all forgiven in terms of prior photon frames cracking?

  5. Devin on

    @Ted + @Tom: Carbon tubes need a bond line- you’re not looking for Ti tubing fit-up with your copes (which, by the way, is the more correct term for tubes interacting with tubes in this fashion- mitering has too much market traction though, coping will never catch on.)

    Carbon tubes can absolutely be coped with microscopically small cuts- you just need to be using a super high quality diamond hole saw and proper feed/speed. Other very basic tricks apply.

    Tube-to-tube construction is one of many different methods to build frames- definitely the one I consider “best” for custom frames. Yes, you’re not able to load carbon in a totally continuous fashion, but NO carbon frame truly does this. Using female molds and bonding triangles together does functionally the same thing- so unless you’re doing an entire frame as-one (which is UNBELIEVABLY difficult to do without problems re: proper compaction in the entire structure,) you’re getting to the same place, just down a different road.

  6. 1Pro on

    Devin knows something. But I’ll add that even the 1 piece front triangles or full one piece molded frames are an assembly of rolled tubes(raw) and joint work pulled together, threaded with bladders and molded. Point being fibers are not continuous like one might imagine. In cases where you might argue they are continuous then it is likely a “net lap” where each side is layed up in each mold half and an excess lap on one half is molded to the ID of the other half. Less than ideal IMO.

  7. Bre Rue on

    Nice tour, I love to see how the different companies come up with different solutions to problems! Mitering could definitely be a bit better though. We used high grade diamond saws at a slow speed with liquid lubrication to get perfect miters every time. I know that some other larger companies use lasers to precisely cut tubes as well. They may want to add a couple of tube mold sizes with swagged ends so you would not have to filler them, which would mean a better anchor and less material needed for the same strength. Plus why not co-mold mold the dropouts into the stays since the distance is fixed. Less glue and better power transfer. Lastly, try not to drill holes into a tube for bottle cages and cable guides, mold them in and you will have a stronger overall structure and no cut fibers 😉

    There is a lot to be learned out of the hundreds of light frames we made, and until I get back into the building side of things, I love to help other companies improve their game.

  8. Devin on

    @Umbria- sometimes (not always, or even often) the people leaving comments DO know more than companies like Guru. I know a lot about building custom carbon bikes, and I’d wager that Bre Rue knows more than me.

    Internet tough-guys are one thing, people who actually make the same kind of products (so they know what they’re looking at) are quite another.

  9. Devin on

    I can’t speak for Bre, but it looks like I’ll be going to NAHBS. Probably with something sensible (kind of) and with something ridiculous ($5,000 scoot bike?….)


COMMENT HERE: (For best results, log in through Wordpress or your social media account. Anonymous/fake email comments may be unapproved or deleted. ALL first-time commenter's posts are held for moderation. Check our Comment Policy for full details.)

This site uses Akismet to reduce spam. Learn how your comment data is processed.