Many owners remove the rear rack of their F-frame and then cut off the “spike” of the main frame that supports it. This certainly makes the bike lighter and gives it a more modern look (possibly even more so than a spaceframe), but is it a good thing for the long-term health of the frame?
Alex Moulton never approved of docked frames, but didn’t give a reason for this. The record-winning Moulton Speed ridden by John Woodburn from Cardiff to London was docked, and Alex Moulton was reportedly not happy when he found out, possibly because he wanted to show what could be done with something looking like a bike that the public could buy; he was trying to sell them, after all.
This is more of an engineering examination rather than a marketing one but first, a little history. The very earliest 1963 F-frames had a rack with only a top strut, from the seat cluster to the front of the rack. These suffered from bent rear spikes if the rack was heavily loaded, so the lower, adjustable strut and “foot” were soon added. In this case it appears that the top strut was mainly intended to support the rack. The spike itself is hardly of a shape and size designed to help support the extended seat tube and prevent it from bending backwards. This means that the top strut was designed to act in tension, and the thin flattened ends and lightweight tubing used also bear this out.
The lower strut, when added, was clearly designed to act in compression to support the rack and transfer some of the load to the main frame. The lower strut is also of heavier tubing and with more substantial ends.
So the upper strut was not designed to act in compression, which is the only way that it can help support the seat tube. However, we need to answer two further questions: can it act in compression, and does it?
The answer to the first question is yes. The strut is not slender enough to suffer from Euler buckling (although the flattened ends complicate any accurate calculation of the critical buckling load) and even the light gauge mild steel tube used is strong enough in compression not to suffer from shear failure, even by a heavy rider pushing hard on an ascent. The safety factor is several times the likely load.
The answer to the second question is also yes. Although the top strut may have been designed to act in tension, a simple experiment shows that it is in compression when being ridden. Tap the strut with something non-marking, like a wooden spoon, and note the sound it makes. Now lean hard on the saddle, or have a friend sit on the bike, tap it again, and notice how the tone changes. The strut is clearly compressed by the seat tube trying to bend backwards.
The original bikes with only a top strut may not often have put the strut into compression with a heavy load on the rack, since the spike would have bent (hopefully elastically) away from the seat tube and the top strut would have started in tension, pulling on the seat tube rather than supporting it. Only when the seat tube bent substantially would the strut have gone into compression. Add the lower strut, however, and the spike cannot bend at all at its midpoint, meaning the top strut is pushing against something fixed.
So the final question is: does it matter? With no reinforcing struts, the seat tube will undoubtedly flex backwards to a greater extent if a rider sits in the saddle and pedals hard, for instance when climbing a steep hill in bottom gear. Does this increase the likelihood of a fatigue fracture at the pierced seat tube joint?
My view is that it does increase the risk but (as with all risks) it may be one worth taking, if there are mitigating factors such as:
- A well-built frame will have a better-brazed joint than a sloppily-built Kirkby special
- A low mileage frame will not have accumulated fatigue at the joint (steel, in theory, has a fatigue limit, but in practice it is of little use if the tubing is overheated in construction or the frame design is not very stiff)
- A light rider will cause less stress than a heavy rider
- A standing climbing style puts no stress on the joint
- Series 1 bikes have a seat tube which is stiffer fore-and-aft (the seat tube was turned through 90 degrees for series 2, presumably to increase bottom bracket stiffness) and therefore resists the weight of the rider better
If you do decide to dock the frame, I have two recommendations:
- Don’t cut off the whole spike – leave two inches of it in place. People who have cut the spike right up to the seat tube have experienced early failure of the frame, because the brazed joint is weakened. The joint must remain fully intact.
- Have the cut end capped with a brazed-in steel plate. This does three things: it keeps water and dirt out of the tube, it stiffens it by resisting squashing of the oval section, and it looks a lot better.
Oh, and don’t forget to replace the two self-tapping screws that used to hold the foot of the lower strut to the main frame spar; they also retain the rear suspension block!
Moultons and other endangered species 