Cantilever brakes for a 1985 Trek 620 or 720 is a tricky issue. On modern bikes, cantilever posts are spaced at about 80mm, whereas on older road bikes the spacing is narrower, particularly so in the front. On the 1985 Trek 620, the posts are space at around 65mm.
This narrower spacing was abandoned in the late 80's on cantilever equipped road bikes as the then new-fangled mountain bikes used a wider spacing. This was a much larger market and the brake manufacturers began designing cantilever brakes to this wider spacing.
This situation is further complicated by converting to 700c wheels as is being done in The Phoenix Project.
Sifting through which brakes may work is a chancy undertaking as it is nearly impossible to determine from photographs and published specifications. I've been through this drill on my previous Trek 620 and on my wife's 1985 Trek 720.
Fortunately, the Shimano BR-MC70 brakes that came stock on the 1985 Trek 620 will work for such conversions. These brakes were included in the Deore XT and M700 "Deerhead" groups. This is the set that came on the Trek 620 that is the subject of The Phoenix Project:
These are very nice cantilever brakes and are sought after by collectors. They have quite a bit of up and down adjustment in addition to the radial adjustment of the post arm. They also have an integral toe-in adjuster, the black rings seen in the picture above.
As this set was in very good condition, I closely considered just rolling with them and avoiding a cantilever compatibility quagmire. However, they have no provision for balancing the spring tension between the arms. The cantilever brazeons on the Trek 620 only have one hole for the spring unlike others that have several for adjusting tension. That left only the unappetizing workaround of bending springs to balance tension.
Fortunately, the follow-on model, the Shimano M732 cantilever brakes, included in the Deore XT group, is nearly identical in geometry and features and also includes a tension adjustment. They have a slightly different look:
They also have two spring settings, one for normal brake levers and one for SLR (Shimano Linear Response) levers. They also have a tension adjustment on one arm per pair:
Otherwise they are nearly identical other than form to their Shimano BR-MC70 predecessors.
They do have a beefier look, especially around the cantilever mounting posts:
Selecting these brakes did infringe upon the "no vintage components" guideline of this project. However, the dire situation posed by the Trek 620 setup issues more than warranted a waiver. The fact that I was also able to purchase a new bike's worth from benscycle.net, a regular bike part vendor, made me feel a little better about this deviation.
Shimano Deore XT has decades worth of a well-deserved reputation great reputation for good operation and reliability under the toughest of situations.
These are very versatile brakes with their integrated toe-in adjusters, up and down adjustibility, and tension balancing. Furthermore, these features are incorporated in a shiny, elegant, uncluttered form.
Nobody has ever done time for selecting Deore XT.
We come up with a pretty low Tweed Factor of one rain poncho. Generally, these type of folk avoid NOS parts, especially somewhat expensive ones. Much more likely choices would include Paul Neo-Retro cantis on the high end, a choice I considered but didn't want to take the compatibility chance:
Even more likely, as these sorts of riders like to adorn their $2000 USD (or more) frames with cheap components that discreetly underscores their superior wisdom about cheapo components, is the Shimano CT91:
These don't look like anything to me but a pair of late-80's Deore XT brakes. If that has become a fashionable pose, it is news to me.
These were not too expensive for a set of quality cantilever brakes. However, it could be argued that it is a high price to pay simply for tension balancing, as that is all I gained over the existing set of brakes.
We carry forward $2698 USD. These brakes, a bike's worth, were $89.99 USD, call it $90 USD, from benscycle.net inclusive of shipping.
The Sausage Factory