Part 2—Flex matters

Last week's blog post outlined the fundamental approach that I take to determine the best design for my bikes. This week I want to go into a little more detail as to how this approach is then used to refine frame performance. I will focus on the specific topic of frame stiffness as this has become something of a hot topic in the industry and something that you will no doubt be familiar with.

There are a number of different areas where the ‘stiffness’ of the bicycle frame have an influence over what the rider experiences.  The most prevalent, that you will no doubt be aware of are; bottom bracket stiffness, head tube stiffness and ‘vertical compliance’.  The simplest of these to appreciate is ‘vertical compliance’ which is the term widely adopted by the industry to describe how much a bike’s saddle will deflect so as to reduce the transmission of shock and bumps to the rider.  It is now commonplace for manufacturers to adorn their bikes with many fancy acronyms for the various technologies that are in place to improve the ‘vertical compliance’ of the frame.  Whilst it is true that the nature and shape of the material of the frame in certain areas may have an influence over vibration transmission, it is fundamentally unavoidable is that the ‘double triangle’ shape adopted by almost all bicycles is inherently extremely stiff with multiple members supporting the others at all junctions.  As such the most effective way to influence saddle deflection is to use the seat post or mast that extends above the top-tube/seat-stay junction as a cantilevered ‘beam’; i.e the longer you make the beam the more it will deflect at the loaded end.  You will notice that, unless requested otherwise by a customer, I design most frames to have a reasonable amount of exposed seat-post/mast in order to get the desired deflection at the saddle according to rider weight and riding style.  You may also notice that I tend to use very slender seat-stays on my signature road frames, you could be forgiven for thinking that this is to promote increased comfort. In fact this design decision is driven by material choice, which will be covered in a later post.

The next area that I want to focus on in detail is that of bottom bracket stiffness, or to be more accurate ‘lateral’ bottom bracket stiffness.  this refers to the ability of a frame to maintain its centreline when you apply a force to the pedal (which is not on the centreline and so creates a torque tending to displace the bottom bracket away from the applied force). It is widely considered that a frame that is able to resist this torque, thus deflecting less under load, is a more efficient frame to ride because no energy has been wasted in moving the bottom bracket.  Indeed this is true from a purely mechanical perspective but, as you will have gathered from my first post, making a good bike is not quite as simple as making the most mechanically efficient one.

My investigations into bottom brackets stiffness started with personal curiosity:  I had several bikes, some modern ‘super stiff’ frames and some older, heavier ‘not so stiff’ frames.  I was puzzled as to how I could ride an older ‘not so stiff’ frames as fast as a modern one over my regular training routes.  After much theorising and testing an answer began to become clear;  the key was that, contrary to popular belief, there is in fact a mechanism that allows the energy used to ‘flex’ the bottom bracket sideways to be returned to the drivetrain as useful energy.  The key to this is that as the bottom bracket moves sideways it also moves vertically downwards. As you continue through the pedals stroke and are less able to apply pressure on the pedal the bottom bracket moves back to the centerline and it’s axis moves vertically upwards accordingly.  However, thanks in part to inertia, your foot continues to move through the pedal stroke and not upwards with the bottom bracket axis, this creates a positive (clockwise) torque with your foot now being the pivot and so contributes to the rotation of the cranks.  The easiest way to appreciate this is to try it yourself using this simple demo:  

  • Hook your bike up to a turbo trainer but leave the resistance unit off the rear wheel so that it can spin freely.  
  • Find a solid, incompressible object that is tall enough to support a pedal when the cranks are roughly horizontal.
  • Rotate the wheel backwards until there is about 10-15mm of free space between the pedal and the support that you found.
  • Clamp the rear brake on so that the wheel cannot rotate and stand on the pedal (it will move downwards to rest on the support as the bottom bracket flexes)
  • Once the pedal is resting on the support release the rear brake.
  • You will see that the rear wheel spins despite the fact that the pedal has not moved at all as it remains resting on the support.

It is all well and good that some of this ‘wasted’ energy is returned but in the pursuit of marginal gains even an energy transfer of 98-99% does not cut it.  The key to the importance of this flex is in what it means for the biomechancis of the rider.  In my first post I identified how reaction forces are key to a rider’s experience of the bike.  In order to appreciate this imagine running up a flight of old wooden stairs and compare this to  the very different sensation of running up a flight on concrete stairs.  This is because the reaction force going through your legs from the stairs is different in each case; the solid steps create a sharp spike of reaction force (as they do not deflect under your weight) whilst the wooden steps create a more gentle ramp (as initially some of your weight deflects the step and so less reaction force is transmitted to your foot).  Overall the total ‘energy transfer’ is the same, as you don’t sink into the stair in either case, but the nature of the transfer is different.  Relating this example back to the bike we can begin to form a real understanding of how a different amount of bottom bracket flex can influence what the rider experiences.  In simplistic terms the shape of the reaction force is 'kinder' to your legs over the course of a ride and gives you that feeling of really being 'on top of a gear in a much wider range of gears than on a stiff frame. Essentially with an understanding of the mechanism which returns energy to the drivetrain, it is possible to optimise this particular frame performance characteristic to retain the fabled ride quality of a ‘good old steel’ bike without sacrificing the exciting performance that is expected of today’s latest and greatest.

It is important to point out here that that is really only a fraction of the story; not all types of frame need this feature, in fact on some it would be detrimental.  The relationship is also more complicated than simply allowing the bottom bracket to flex; whether the flex comes from the front triangle or rear triangle is critical (why some flexy frames are better than others for certain things, more on that in a future post). Nevertheless I hope that it has offered you an insight as to how the frame can be used to manipulate the biomechanical experience discussed last week.  More importantly I hope that it has given you an appreciation that all is not what it may seem when it comes to the somewhat simplistic list of desirable performance features championed by the mainstream industry!

Design PhilosophySturdy Cycles