With the Cobbled Classics now behind us, and the distant memories of my own time racing on Belgium cobbles fading again, I felt inspired to touch on a topic that is very important for those roads, but also for all of the less-than-perfect surfaces that road bikes get ridden on.
In my introductory article, I dove into the stiffness of various frame materials and some of the implications those properties have on designing a bicycle. I touched on the torsional stiffness of the frame and the impact that has on the way the bike handles. Of course, the frame needs to have suitable geometry for the purpose in order for that optimized stiffness to be effective. Which I will come back to! Today I would like to discuss another property of frames that I feel is sometimes misrepresented: the oft-touted “vertical compliance”.
Speaking specifically in the sense of a traditional non-suspended bicycle, my understanding of the common usage of the term “vertical compliance” is to imply some sort of passive suspension to remove impacts and/or vibrations from being transmitted to the rider, mostly through the saddle. Occasionally this term is used for transmission at the handlebars too.
I think it is clear why making the rider more comfortable is something worth pursuing – for recreational riding, more comfort makes riding more enjoyable. For performance riding, it is likely that a more comfortable rider can ride at a high output for longer. There have been some studies suggesting that reducing shocks and vibrations to the muscles can aid in performance. And, whilst we are mainly talking about suspending the rider rather than the bike here, if the bike can in some way absorb bumps so that the whole bike and rider package doesn’t get lifted up, then there is the potential to save energy over rough surfaces.
Between the road surface under the rear wheel and the rider we have a connection of systems that will transmit the impacts and vibrations, each with some potential ability to reduce that transmission. Firstly, we have the tire – or tire and tube together. This is perhaps the most active suspension in the system, featuring an adjustable air spring, with a somewhat adjustable volume (by changing tire size). This is by far the best way to add comfort, and the relatively recent acceptance of larger tires and lower pressures has been a great step forward.
Secondly, we have the wheel. A wire-spoked wheel is a tension structure – an engineering marvel because of the load it can carry relative to its weight. The spokes must always remain under tension as they have little compressive strength. If a wheel was to absorb any impacts, that would mean the rim at the bottom moving towards the hub, so the tension on those lower spokes would reduce. This is only possible within a narrow band without inducing a fatigue load on those spokes that will lead to failure.
So typically, the wheel is considered a rigid structure within our system. If there is a claim that a wheel is offering vertical compliance, then I would suggest asking for deflection data to support this. Note that Zipp’s adoption of a single-wall, motorcross-style rim for mountain biking that allows “ankling” is a different type of compliance with the goal of allowing the tire to better track off-road terrain.
From the wheel the loads move into the rear dropouts and thus the frame. For the classic double-diamond bicycle frame, we have a triangle at the rear. A triangle is by definition a rigid structure (think of three rods connected by pin joints – it is constrained and cannot change shape). The only way for any movement to occur is if at least one of the sides of the triangle can either axially compress or laterally bow. Conceptually this can be engineered by the design and material choice. In practice with the steel frames I build, I have found it difficult to realise any measurable deflection, even when I have curved the seatstays to encourage that bowing movement and built them from very small 8 mm tubing.
Maybe this is a good time for a quick aside about the seatstay design I typically build with; an explanation was requested in the comments on my first article. Approaching frame design by starting from basic principles, I could see that the chainstays, once triangulated across the rear hub axle, provide the lateral stiffness between the bottom bracket shell and the dropouts. The role of the seatstays is then to act as a prop, and to hold the dropouts level so the rear wheel can’t twist out of plane. The loading here is small, and is entirely a compressive force.
A steel tube loaded in compression has great strength – the failure mechanism would be buckling, and even a very small-diameter, thin-walled cromoly steel tube will sustain a loading far beyond that seen in a seatstay before that would occur. With this in mind, I built my first frames with 3/8” (9.5 mm) seatstays. Subjectively I couldn’t feel any difference to a frame with larger seatstays; it created a lighter-weight frame and gave a unique look. I now also have some static loading data to confirm that this design, when tested for lateral and vertical deflection, performs the same as a more conventional seatstay design.
In recent production performance bikes we’ve seen an almost wholesale move to dropped seatstays. The idea here is that, with correct material selection and design of the seat tube, more flex can be encouraged (essentially the seat tube bowing forwards and allowing the saddle to move backward on that arc accordingly). So far I only have one data point that supports this from my testing, and whilst it does confirm that there is increased vertical deflection with dropped seatstays, the difference is very small. Again, I would encourage asking for comparative deflection data to back up claims that this design feature provides a measurable ride quality benefit. From a consideration of frontal area though, lowering the seatstays has good potential to improve the aerodynamics of the frame.
We can now move away from these triangles which are going to struggle to offer too much in the way of actual compliance. Instead, let’s look at the seatpost which, as a cantilevered structure, can truly offer visible and beneficial flex. One of the first road frames I built for myself was designed around a thin-walled titanium seatpost at maximum extension. I can stand next to the bike and lean on the saddle and see it move. Subjectively, this improves comfort at the saddle when riding on chip-seal and other rougher surfaces.
There are now several seatposts designed to take advantage of being in this cantilevered position – from the extreme of the Ergon leaf-spring design, to others from the likes of Ritchey and Syntace where the carbon layup has been designed to reduce the stiffness in the fore-aft loading direction.
The saddle is the final part of the system here. Traditional leather saddles, with the leather stretched over the frame, can have a hammock effect and aid in reducing vibrations transmitted. This extends to plastic saddles where the base is designed to flex in a similar way. Then of course there is the foam padding, and sometimes other features aiming to increase comfort.
My conclusion from reviewing these connected systems is that the best way to add this mystical “vertical compliance” is by adding air volume and lowering pressure at the tire, and with careful selection of the seatpost. We see this with modern road bikes that are generally able to fit 32 mm or even 35 mm tires. These are typically installed on a rim with a wide internal channel, which gives more support to the sidewalls (as they are closer to vertical, rather than ballooning out from a narrow rim), helping to allow for lower pressure – which is further assisted by using tubeless tires which reduce the danger of pinch-flatting.
Whilst some riders may prefer the look of a classic level top tube, by having a sloping top tube to increase the length of exposed seatpost, the selection of a compliant seatpost can be effective. Meanwhile, a more traditional square frame with minimally exposed seatpost length is unlikely to significantly improve comfort through a different seatpost.
True “blind” testing of bicycles is difficult to do, but back when Josh Poertner was working with Zipp and professional teams, he did his best to formulate this type of testing. That testing realized a similar result – the only differences the riders could reliably feel were for tire pressure and seatpost type. The frame design and construction could not be detected.
It would be wonderful if the industry in general would provide data when making claims; humans are pretty terrible at being able to subjectively feel differences. Often when we are told that a bike is going to be “stiffer” or “more compliant”, that is precisely what we report back.
Rob English is a world-renowned custom framebuilder with an education in mechanical engineering. ‘The whys of bike tech’ is Rob’s technical column for Escape Collective that delves into engineering principles, frame design, material selection, and whatever else inspires discussion. You can see more of Rob’s creations at EnglishCycles.com.