Every builder brings a certain philosophy to their creations, I thought it might be useful to share a little of mine:
- Fit is key: Both fit for the rider and fit for purpose. With a custom build there is no constraint around incremental sizing or fixed angles – my process starts with locating the contact points (handlebars, saddle and bottom bracket). Next the bottom bracket height is set, to match up with wheel/tire size, crank length, pedal type and type of riding. Then the chainstay length is adjusted to give correct weight distribution (taking the front center and riding position into account). Steering feel is largely a factor of the trail (achieved through a combination of headtube angle, fork offset and wheel size), so I will use the variables to give the required handling whilst avoiding any toe overlap with the front wheel. Every aspect of the geometry is carefully adjusted with consideration for the fit and handling of the bicycle.
- The ‘vertical compliance’ myth: A triangle is by definition a rigid structure. Unless a manufacturer is willing to give actual deflection data for their frame, then I am skeptical about these claims. And subjectively it is very difficult to accurately ‘feel’ the frame as it is not isloated from the saddle, seatpost, wheels and tires. However, a cantilevered structure can truly flex – and be tuned to do so. This is why I like to build around long exposed seatposts – the resulting small, compact frame is light and stiff, whilst the seatpost can flex to provide comfort at the saddle. I always used to use titanium seatposts for this application, but with several manufacturers now producing carbon seatposts designed to flex (such as Niner, Syntace, Ergon and Ritchey), there are plenty of options to tune the desired comfort level.
- The ‘stiffness’ myth: Along with ‘vertical compliance’ stiffness is another greatly touted attribute of modern frames. ‘Our frame is 24% stiffer than last year’ is a common marketing statement. But what does this mean and how does it affect the ride? When it comes to transmission of power to the rear wheel and a well functioning drivetrain, the rear triangle needs to be ‘stiff’. What I mean by this is having a rigid structure made by the triangulation of the chainstays across the rear hub axle, so that the bottom bracket shell and rear hub remain fixed in relation to each other during the asymmetric loading of the cyclist pedaling. This can be referred to as lateral stiffness, and a short set of steel chainstays are hard to beat – just in material science terms, steel is twice as stiff as titanium, three times as stiff as aluminium, and only the very high modulus carbon composites come close. The other attribute of frame stiffness that can greatly affect the ride is the torsional stiffness of the front triangle. This refers to the toptube and downtube trying to keep the headtube in the same plane as the rear wheel during twisting forces such as sprinting or hard cornering. If you are a track sprinter, riding only on a very smooth surface, then you want as near to zero flex in this area as possible. However, for the rest of us riding on real roads and trails, ultimate stiffness is perhaps not the best goal. We don’t want the bike to feel like a noodle when sprinting, or vague during cornering, but we do want just a little bit of give in that front end. This is because it allows the bike to conform to the road just a little, which gives better traction and will hold the line better on less than perfect surfaces. I select each tube individually to match the weight of the rider and the frame sizing to provide that sweet spot of feel. The tubes shape, dimensions and wall thickness all effect the stiffness contributed to the frame. This is a topic that could have a lot more written – some great articles on the subject are by Jan Heine.
- Those seatstays….: Following on from the rear triangle discussion above, if we have a nice stiff set of chainstays triangulated across the rear axle, then the seatstays are just there to provide a ‘prop’ for this structure under vertical loads. This means that the seatstays are only loaded in compression. The failure mode for a tube under a compressive load is buckling – and steel tubing has a very high resistance to buckling. Therefore I can select very small and light tubing for this part of the structure, whilst maintaining the rear triangle stiffness required.
Hopefully that gives a bit of insight into the thought process that goes into designing a frame; please let me know if you have further questions.
-Rob, December 2013.