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!

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!

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.

 

Leave a Reply

Your email address will not be published. Required fields are marked *