Brucey wrote:the spacing front and rear is different which means you should either use a set of cantis with post-mount brake blocks or use BR-CX50/70 and expect to use a different spacer front and rear. [Brakes with 'V' type blocks will cope with about 5-6mm variation in boss spacing, e.g. from 78mm (centre to centre) to 83mm, tops.]
cheers
Are there any particular post-mount cantis you would recommend? I am fitting brakes to a bike with the same geometry as the OP's, as per my post above. I'd prefer post-mount so that I have the same spare pads for all the household canti & v braked bikes.
Thank you to all . Going for a set of Shimano CX50's once the frame has had a repaint. £80 to get the bosses replaced/moved .Think I will have to pass .
I have also just bought a Raleigh Randonneur frame that I am building up. It came with the original Canti brakes but one set were damaged and I could not get them to work properly. I replaced them with Dia Comp 999 brakes which I thought looked more in keeping with the originals: https://www.sjscycles.co.uk/brakes/dia- ... ver-brake/
The bosses on my frame are 65mm apart at the rear and they just about fit.
The performance is nowhere near as good as the originals that are on the front.
fitting is one thing, mechanical advantage (MA) is quite another. Rear brakes are often not as good as fronts because the cable is longer, draggier, and spongier. Past that the differences may lie in brake blocks (type and whether or not they are bedded in) and the MA. Here there may be several things at work but the MA is certainly different.
The original brake is a 'mid arm' cantilever. The Dia Compe is a 'wide arm' cantilever. You can adjust the MA of the former whereas the MA of the latter is both almost fixed and a lower value. In both cases the virtual arm length (VAL) of the brake is given by projecting the line of the straddle cable and seeing where it crosses a horizontal line that passes through the bosses.
When the dia compe brake arms are horizontal (i.e. so that the ends of the straddle are about the same height as the bosses) it doesn't matter how you set the straddle, you cannot change the VAL; it is always very close to the actual arm length, so the MA is fixed.
With the mid arm design as pictured the VAL is about double that of the Dia compe brakes so the MA is about double too. With a mid arm canti you can alter the MA; setting the straddle cable flatter increases the brake MA and making it longer reduces both VAL and the MA.
One of the reasons I suggested BR-CX50 is that this brake works with both high and adjustable MA. Cantis like the dia compe one are great for mud clearance (low MA also brings with it increased running clearance) but the fixed MA is also quite low which doesn't suit everyone or every brake lever. They especially don't suit recent STIs.
.Think I will have to pass .