frame construction: comfort, stiffness, layup, etc

spectastic

I'd like to understand more about carbon frames, what makes them comfortable, stiff, and light weight.

my understanding with the main difference between regular and higher end carbon frames is the grade of carbon material being used. Say most manufacturers use Toray 700 for their standard road model, and they use T800 for their higher end models. My question is what are all the advantages of using a higher grade carbon on a road bike? Is it just for weight, or everything else as well (stiffness/compliance)?

I've been told that the specialized and sworks only differ in their weight, that everything else aside from the pj is the same. So where does comfort come in? generally, a higher mod carbon will be stiffer, but it will also transfer more vibration to the rider, contributing to greater discomfort on bumpy roads. So how does the super light bikes like the tarmac, supersix, r-series, get to use such a small amount of material, remain stiff, yet be more comfortable than their heavier aero counterparts? what do they do in the design to dampen the vibrations? thanks

MisterMuncher

I'm not an expert, but...

To the best of my understanding, your higher end frame will use the stronger grades of CF for critical areas, with the proportion increasing as you go up the price band. Lower grades are that bit more compliant, so get used for areas where that is critical. A lower end frame will be all or mostly of a lower grade CF, and strength is added by simply adding more material. So say in the bottom bracket junction, you could have 3 laminates of really tough CF, or 6 or 7 laminates of less tough CF laid in the mould. The latter will achieve about the same stiffness and strength, but it's invariably going to be heavier.

Aero bikes by necessity of design have bigger tubes which are more formed. The shapes aren't designed for stiffness or compliance per se, as they would be on a more classic/lightweight design, so they must use more material to achieve the both the shape and the stiffness required. If a downtube, say, is designed for stiffness and low weight, it's a simple matter of increasing the section width, and proportionally decreasing the tube wall thickness. Add in the consideration of a Kammtail shape and things get more tricky.

Regarding confort, I'm again not 100%, but I'd say the bigger part of the problem with Aero tubes is that the shapes are less well understood than classic designs, and the best ways to add compliance haven't been fully worked out yet. The shaping just might not lend itself to the same methods. See also early aluminium bikes, teeth-rattling and super-stiff because they basically applied steel-derived engineering to oversized alu tubing, and compare them to modern, more developed alloy bikes.

ai_1

In brief, higher modulus carbon fibre means it's stiffer and stronger. That means you can use less to do the same job. So you can have lower weight and/or higher stiffness & strength. The primary thing to remember about carbon fibre is that the characteristics of a structure can be controlled by how you arrange the orientation of the carbon fibres. So, for example, if you were to build a frame and then decided you wanted to increase strength or stiffness on some specific axis, you wouldn't just use more carbon fibre or a higher modulus of carbon fibre and arrange it in the same way. That would stiffen or strengthen everything, not just the specific characteristic you want to influence. So, what you do is add fibres only in the direction and location needed. So for example if a top tube was too flexible laterally you might add unidirectional fibres running lengthwise on either side of the top tube but leave the top and bottom alone. This would have a much bigger impact on lateral flex than vertical flex.
Essentially the modulus of the carbon dictates how much material is needed, the lay-up dictates how the structure behaves.

You mention aero bikes being heavier and less comfortable. A non-aero frame can be optimised specifically to provide the required strength/stiffness with the minimum of material. The design of an aero frame is more restricted as everything must be a compromise between aerodynamics and strength/stiffness and weight. The optimum solution for strength/stiffness and weight is what you used for the non-aero frame. The aero bike will have to compromise because the best structural shape will never be the best aerodynamic shape. Thus to get similar strength/stiffness with a less structurally perfect shape you'll need more material. The aero shape restrictions also limit your options for introducing flexibility to aid comfort for the exact same reason.

So an aero bike may be stiff enough, comfortable enough and light enough but it won't be as good on these fronts as a bike that dispenses with the limitations of being "aero". However, if the compromises are not large then the aero advantages may outweigh the compromises and make the aero bike the better options. Of course this will be dependent on the rider, the route and the event as much as anything else.

Hope this helps.