Here are a couple of pictures of the tool I used to spread the rear forks on our Bromptons.
1 x piece of thick walled mild steel tube, outside diameter 12mm
1 x piece of M8 mild steel threaded bar (minimum length 40 cm)
4 x large M8 washers
2 x small M8 washers
6 x M8 nuts
Hacksaw or other metal cutting tool
File or sand paper
1) Cut two 20 cm lengths of threaded bar. (I halved a 50 cm long piece of threaded bar)
2) Take the steel tube and cut of a piece 7.5 cm long.
3) Dress the cut end of the tube and threaded bar, if necessary
4) Screw on the screws and place the washers in the order shown in the third picture.
5) Slide the ends of the threaded bar into the section of tube as show in the first and second pictures
You are now ready to go!
How far do I need to spread my forks?
This is a bit difficult to say. You need to spread the forks further (under tension) than the 122 mm required to fit a modified Nexus 8 hub. The steel that the forks are made of will bend elastically before it starts to deform permanently.
With the tool in place I needed to wind it out to 138 mm on my Brompton which, when the tool was removed gave me a 122 mm opening. Damae’s Brompton needed the tool to be wound out to 140 mm before the Nexus hub would fit.
If in doubt start with a lower value, say 130 mm. Remove the tool and measure the distance between the dropouts. Then adjust as required.
You might need to bend the drop outs after spreading the forks so that they are parallel. Measure the gap when the dropouts are parallel again and adjust as needed.
Disclaimer – what happens if something goes wrong?
Use of this tool could damage you and your Brompton/bike. You are responsible for your bike and the modifications you make to it. I accept no responsibility for any damages or losses incurred by using the tool described in this post.
I do not recommend opening out the forks to accept a hub with OLD of 135 mm. You will get clearance problems between the tread of the tyre and the cross piece on the fork near to the rear hinge. To fix this problem you will need to modify/replace the dropouts or find another way of increasing clearance between the rear tyre and the cross-piece.