An Aerospace Perspective on Carbon

It seems that carbon fibre has begun to make sense to mountain bikers. And I've always been drawn by its charms. It's used gratuitously on F1 cars and on the most sophisticated aircraft in the world, so it must be the Best. Stuff. Ever. It can be made incredibly light, and yet we are told it can also be laid up to be stiffer than aluminum or steel. And we hear the fatigue life of carbon can be virtually infinite and certainly longer than steel, aluminum or even titanium when done right. And things seem to be getting better. Epoxies and resins are improving and processes are becoming more sophisticated and streamlined so the internal structures no longer looks like backyard fibreglass repairs. The price however, continues to grow with the technology. 

Does riding a carbon bike churn out more endorphins? Can we get more rad on carbon? Will carbon frames trim Strava times? Will composites attract potential mates? Marketing tells us yes. And marketing never, ever lies. 

One of the most compelling elements of frames laid up with pre-preg is how damned beautiful they look. Velvety lines convey strength and efficiency while inducing irrational lust, but what about the value proposition? As we continue to work on our back to back Knolly Al vs C evaluation, we thought it was appropriate to harness a potentially dissenting opinion on the merits of the other black gold. 

Joe McEwan is the man behind Starling Cycles, makers of bespoke steel frames in Bristol, U.K. The Starling Murmur has impressed and surprised testers, including those at DIRT who called it one of their favourite bikes of 2017 saying, "we put it back to back with many of the carbon elite and it more than held its own." Joe didn't get his start in steel but in Aerospace (see below) and his deep experience with carbon comes from an entirely different realm. While his opinions aren't free of bias, they are infused with a healthy skepticism that counters the influence of large marketing budgets.*

Here's what he had to say. 

*I don't doubt anything Joe told me, but there is certainly a perceived bias because Joe doesn't sell carbon bikes!

Cam McRae - Would you please tell me a little about your background and the sort of experience you have working with (analyzing etc.) materials like aluminum, chromoly steel and carbon?

Joe McEwen - Before Starling Cycles, I spent nearly 20 years (Since 1999) as a Stress Engineering for the Aerospace Industry. My job was to perform calculations/FEM analysis and occasionally testing to design and check aircraft components. I started as graduate, but progressed to Lead Stress roles and department heads.

Much of my time was spent working on new carbon composite materials; developing new analysis methods and technologies. I spent 4 years as technical head for a composite research department. However, I also did a large amount of aluminium analysis. But typically in aerospace you use industry agreed methods rather than analysing from first principles. I’ve only done limited steel analysis, mostly for bolts in aerospace. But the stress analysis of most homogenous materials is similar. Composites are a different matter!

You have chosen to build bikes out of steel despite your experiecne with composites. Do you think there is a case at all for making carbon mountain bikes?

Yes, my biggest concern with carbon fibre is the damage tolerance. If sufficient damage tolerant capability can be demonstrated, then yes composites can potentially make good bikes. As with anything, correct design is much more important than what material is used. Although material choice is a key part of the design process. It should be noted that even aerospace has been known to use composites for political and marketing reasons rather than engineering reasons.

Just a quick aside on damage tolerance. Carbon fibre is prone to something called BVID (Barely Visible Impact Damage), although this may be an aerospace-only terminology. Essentially, a carbon fibre laminated structure (how most bikes and aeroplanes are constructed) can sustain a lot of internal damage from an impact. Typically the lamina (layers) are caused to separate at the soft epoxy matrix. The issue is that there might be significant internal damage, but only a tiny scratch on the outside. I.e. you do not know the damage is there.

There are solutions in the form of matrix material composition or special design features, but I’m not familiar with what is done in the bike industry. Conversely, the multiple fibres in carbon composites are considered a damage tolerant structure, the fibres do hang together somewhat after failure.

Do you think there is a company in the industry who is doing it well?

Yes, the big companies with sufficient resources and experience should be able to build good composite bikes. A good friend of mine, who I very much respect, spent some time working for Specialized on behalf of McLaren to design the Venge road bike. So they are definitely getting the right people.

How important do you think frame weight is in mountain bike performance both uphill and down all other things being equal?

Weight is obviously important going up. More weight means more potential energy needs to be put into the system as you gain height. Going down is more complicated! Let’s consider the system as a whole (bike plus rider) to get some ideas. On some tracks, a big system (Peaty, Rennie, Moir, Gutierrez) will do better, on others a small system (Hart, Brosnan) will do better.

A higher system weight means there is more kinetic energy in the system. So a small bump will take a lesser proportion of the energy from the system and the rider will carry more speed through rough sections. A lower system weight allows quicker acceleration, braking and direction changes.

But there seems to be a growing set of experience from E-Bikes that show even with motors turned off, they are quicker than traditional bikes. The additional weight seems to allow the suspension to work better and more grip is maintained.

We have not answered the question yet of whether weight is good going down!

You mentioned earlier that some frames appear to be just a carbon version of the original design. What are the limitations of that assuming that is the case?

In aerospace we used the term ‘black metal,’ where we analysed, or even designed composite components in the same way it had always been done for aluminium. Early on at my time at Airbus I spent some time looking at old drawings of the De Havilland Mosquito, an aircraft made from plywood. This is a much better starting point to design a composite aircraft, rather than the contemporary aluminium aeroplanes.

The same applies to bikes, why should a carbon bike look anything like an aluminium bike. The component interfaces are another weak point of carbon bikes. Metallic threaded inserts are notoriously hard to get to stay in place. A different solution here is needed.

You have chosen to use steel to build your bikes. Why did you disqualify carbon and aluminum?

Primarily I used steel as it is the easiest material to work with in your shed. But I have always had steel bikes; hardtails and some DH bikes. I liked the way they rode, so was happy with this as my material of choice. Maybe a political rather than engineering choice!

I’m yet to define what gives steel bikes the certain ride quality, but I think it’s a function of two things. Firstly, the density and stiffness of steel means it builds best (strength and weight) using thinner diameter tubes. These thinner diameter tubes are more flexible than large diameter aluminium or carbon tubes. Secondly, I think the dynamic behaviour of the material after it has bent may be a factor. But I have minimal evidence for this.

We've all heard stories of cast off frames being tossed into the sea, not to mention toxic working conditions for those in manufacturing overseas. Are these avoidable? Is it possible to manufacture using carbon in a way that is sustainable either in the west of in Asia?

The environmental question is massively complicated! Unless you can follow every step of the manufacturing process for everything involved, even down the amount of rags that they use in the factory that makes the ships that move the materials around, you will never get an answer. How we make our bikes is the least of our problems, there a lot more shit stuff to solve first.

Using a thermoplastic (i.e. it melts, rather than thermoset which is chemically set) matrix can allow carbon fibre to be recycled.

What are the long term performance prospects of each material? Does carbon, or the epoxy used to hold it together rather, have longevity issues? Where does that leave steel and aluminum?

As part of the design process you need to consider long term performance. Steel rusts, aluminium suffers from fatigue (often at welds or bolt holes), carbon is damaged and also retains moisture which reduces its strength (a big issue in aerospace).

All can be solved with correct design solutions.

Are there areas on a bike where you think carbon is a good choice?

See all of above, yes if used properly. But in most cases it is primarily used because of the perceived benefits, rather than the real requirements.

I run carbon bars and wheels, but only because I have been given them from free. If it was my money, alloy wheels and ti bars are good.

Have you ridden a carbon-framed MTB that you felt performed well? If so was it in spite of or because of the material choice in your view?

I have never ridden a carbon frame in anger! I don’t have the money or access!

Missing in virtually all the informed opinions and suppositions about the merits of carbon versus metals is real data, aside from weight. Smart companies track their reported failure rates from both impacts and manufacturing defects, but that info isn't made public. And information about waste, worker safety and other environmental concerns is even more impenetrable. Consumers are left to corral marketing-free information wherever possible; anecdotally from bike shop employees (who often have the best information despite being subject to bias), internet comments and from riders we meet on the trail. And hopefully those of us in the media help on occasion as well. 

While many of us have been drawn by shiny carbon things, the allure of metal is starting to regain its pull. Aluminum frames continue to improve as new processes for forming and butting tubes allow structures that begin to approach the design freedom carbon allows. And steel has an allure and feel that perhaps only titanium can compete with. We are fortunate to live in a time when tiny concerns like Starling Cycles, making custom steel duallies, can coexist with huge bike brands. While we are indeed spoiled for choice, the process of selecting the perfect bike, and the right frame material, has never been more difficult.*

*See Uncle Dave for some help there