I’ve got to that dicey part of a Brompton 8 speed conversion where I open up the rear triangle to take an Alfine 8 hub. As posted elsewhere, the dropouts on a Brompton are 112mm apart. Shimano hubs are typically 135mm, so that’s asking the tubes of the rear triangle to bend apart by 23mm – quite an ask when the tubes themselves are only 180mm long from the point where they are braced together, to their rear end at the dropout.
On this job I’m experimenting with swapping the large serrated 10.7mm locknut on the non-drive side for a much smaller 4.4mm one. (From SJS Cycles).
This saving of 6.3mm width brings down the requirement to widen the triangle to just under 129mm.
The good news is, I’ve achieved this already – well nearly. By opening up the triangle to 150mm under compression, I found it sprang back to 128mm – nearly there!
UPDATE: OK I feel a bit silly. I removed the large locknut and discovered it sits on a section of the axle that is wider than the visible part! So my narrower locknut will not work, because it won’t thread on! And even if I found one that did, the step-up in diameter would likely prevent the axle nut tightening onto the frame. So that ain’t gonna work. Back to widening the frame further…
However, I decided to try a little experiment. Intuitively, steel being bent a little will spring back to its starting position, providing the bend is not too severe. Beyond a certain point the structure of the steel will change, and it will retain some of the bend permanently. But is time a factor? I decided to reopen the triangle to a ‘safe’ 145mm and leave it for a few days. I’ll be interested to see if it springs back to 128mm, or whether maybe, after time, it gives me the extra 1mm I need. I’ll let you know!
UPDATE: Time + tension made no difference to the steel, which sprang back into position.
I say a ‘safe’ 145mm because I know I’ve taken the triangle on this new bike up to 150mm without any visible sign of damage. On previous conversions, I took it up to 155mm to achieve the 135mm spacing needed for an Alfine 8. On one of these occasions there was slight splitting of the paint around the brazing on the dropout. Hence going for the narrower target of 129mm this time.
UPDATE: Having widened to 135mm, all works fine. There is no paint splitting around the brazing. However, I did notice that the rear fork on one side had developed a very slight crease, suggesting it had not bent in a smooth arc. It seems to be no problem for the bike, and I imagine the steel will simply settle like this – but it left me a bit unsettled – and perhaps shows up the limitations of this kind of procedure. Widen your rear triangle at your own risk!