Jim Colegrove - Trek's inimitable master - Dirt

Mountain Biking Magazine



Jim Colegrove – Trek’s inimitable master

Trek's Senior Composites Engineer talks through the Session structure

Trek’s carbon fibre boffin has been with the company for over a quarter of a century, having an intimate knowledge about the material that has played a major part in making the bikes what they are today – world championship winning ones.

Colegrove arrived at Trek from a small engineering company in Salt Lake City that worked with aerospace brands such as Boeing, Lockheed and Northrop.

When he touched down at Trek he was the man that brought carbon fibre manufacturing into the Madison headquarters building a facility that has gone onto produce such bikes as the Madone road bike and the Session 9.9 that Rachel Atherton and Aaron Gwin have had so much success on.

Unlike nearly every other carbon downhill bike, the carbon Trek Session is still made here today, and is one of the few (if only) carbon downhill bikes to be made in the US. In a recent trip to the Trek factory Jim introduced me to the people and the processes behind making the Session. I asked him a few questions about the material he knows so well:-

Words and Images by Steven Jones

Dirt: Give me some reasons why (if) you believe carbon fibre performs better on a downhill bike than other materials? What are the performance advantages?

Colegrove: Weight, stiffness, and tune-ability of the laminate to provide best performance and feel. Speed and rider capability have increased, and the fact that we can adjust the attributes of all areas of the frame is a huge advantage.

Ride feel is extremely important, and again, the ability to adjust the laminate allows us to give the rider the best advantage with a bike that that is nimble, tracks precisely, is light weight, and helps to enhance the abilities of the rider. No other material gives our designers and engineers this level of flexibility.

What material and why? Percentages of UD vs fabric?

We can build a lighter stiffer bike with UD, however it is less damage tolerant than cloth. So we find a balance between the two materials and where we use them on the frames. Top tubes see less damage than the down tube. Little, if any, cloth is used in the TT, but we have selective areas with cloth reinforcement in the DT among other areas.

Do different size bikes have varying amounts of material?

Yes. Each frame has a blend of unique and common laminate. Our Session DH frames are size specific to allow us to tailor the laminate to the general size and weight of the riders.

It would be great if every frame could be custom for each rider’s size, weight, and riding style, but the cost of the frames increase greatly with the extra work of developing the best laminate for each rider. We have taken great time and effort to develop the best all-around laminate for each size bike focusing on the performance and ride feel.

The stiffness doesn’t always come from the material type or lay-up but shape of the frame correct?

Correct. While carbon fibre is a very stiff and strong material, much of the ride performance stiffness comes from the section properties of the frame (frame shape). We use sophisticated 3D modelling in conjunction with high-level composite-based Finite Element Analysis (FEA) in developing the frame, swing arm, rocker link, and stays. In the virtual world of the computer, we can run hundreds of simulations subjecting the bike to all kinds of different load scenarios to see how the bike performs under different loading.

Once we have this information, we can build bikes that show the most promise, then test them in our lab and with our pro riders. In doing so, we can correlate the information from the real world and the virtual world to make sure that we are designing as accurately as we can and producing the highest level of performance that our customers expect.

You’ve been chasing lighter, stiffer for some time now. Is it just about stiffness and have you reached the limit?

We’re very close to reaching a theoretical lower limit on weight. I do not believe we are at that point yet, but we’re getting very close. Years ago, it was easy to take a lot of weight out of the frame by simply changing from aluminium or steel to carbon fibre. Today, we are struggling to find grams to cut. I am continually amazed by what our designers, engineers, and analysis teams can do to find ways to reduce weight and maintain ride performance.


But is lighter, stiffer the most important factor? Surely the aim is about feeling? How can you tailor ‘feel’ within a frame? How does the R&D work with that?

Throughout the industry, the mantra has been “lighter and stiffer.” We’ve known for a while that we were reaching that theoretical lower limit on weight, and so for several years now we have been focusing on ride performance and feel over just making the lightest and stiffest bikes.

One of the beauties of carbon fibre is that you can vary the material type, the number of plies, the ply angles, as well as where the plies begin and end. Unlike other materials, like aluminium or steel, where you have a uniform wall thickness, carbon fibre can be varied everywhere throughout the frame. This allows us to fine tune all those characteristics that give us a better-handling higher-performance bike.

Do people get carried away with carbon fibre, after all it’s not about the material, it’s about what the designers/engineers do with it?

This is one of the biggest challenges we face. It seems that everyone wants to make everything out of carbon fibre. It’s a wonderful material, but it is not the best material in all applications. Many times we want the isotropic properties of aluminium or steel. Putting threads into carbon is pretty sketchy, so the BB and other similar areas get either bonded or co-moulded inserts. It is always about how you design and use all the materials.

How many hours/material costs per frame?

We can’t share exact costing info, but the cost of carbon fibre ranges from a low of around $50 per pound all the way up to over $900 per pound for our highest-performance material. We use a mixture of all these materials in each frame.

From Angel and Audrey on preforms and lay up to Sabrina on assembly, the number of people involved in manufacture (US) is about 20 right?

If you’re talking about people who directly touch the bike, 20 to 25 is a fair number. There are a lot of people in support roles behind the scenes, like in warehouse and logistics that you may not have accounted for. Building bikes is an incredibly labour-intensive venture, and this is one of the root challenges and main drivers of moving production to Asia.

What’s number of people involved in manufacture of say a Remedy in Asia?

The number of people is higher than in the US. Asia has labor as a resource and so they will use it. The US tries to focus on automation and lean manufacturing to help minimise labour costs.

What’s the reason the Session has been made in US?

We have the all the key components like design, engineering, materials, and manufacturing expertise. It is also because we are so highly connected with the product. We are also able to respond very quickly to requests from our different teams. In one case we were able to go from a rider request to carbon fibre test bike in about six weeks. This is crazy fast.


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