91 Merlin retro/modern/custom build (aka the Merlingham)

[classen]

Working it up on the lathe:

The finished piece:

A very snug fit over the existing brake post/boss:

 

[classen]

Checking all the bits to make sure they fit:

Making some new brass bushings:

 

[classen]

Checking my bushing press setup. I machined the bearing to 0.500-0.501" OD and the brake arm has a nominal ID of 0.498-0.499"

Getting ready to plunge the bushings into liquid nitrogen to shrink em up:

Shrinking em up:

Ahhhh nice and snug:

 

[classen]

The brass bushing now provides a very nice fit to the cap with essentially no slop. The new cap also provides a nice flat 90 degree surface for the brake arm to rest on.

 

[halaburt]

The original WTB rollercams had brass bushings press fit into the brake arms and came with a stainless steel cap that fit over the brake post. I'm no engineer, but as far as I can tell this had two benefits.

1. The tolerance between the brake post cap and the brass bushing could be very tight fit since both pieces were provided with the brakes. No need to rely on a good fit between the brake and brake post (likely some variability between frame/fork manufacturers). The brake post cap essentially becomes part of the bike.

2. The cap over the brake post has larger diameter than the brake post/boss reducing lateral compliance (back and forth jiggle). i.e. larger diameter rotating axis provides better stability.

I'm neither an engineer or a machinist, but let me offer a few corrections to some your points here.
- What you're referring to as the "original WTB roller cams" are actually the later/final WTB models with the DKG-made arms. The true original WTB rollercam brake arms (sometimes referred to as "swiss cheese" arms for the drillium holes in them) pre-dated the models you're referring to. And before that there were numerous handmade prototypes by Charlie Cunningham.
- The prototypes and earlier WTB models used approximately 0.375" OD x 0.75" tall brake studs. When Suntour licensed Cunningham's design, they established a new, smaller stud size of approximately 8.9mm OD by 16.5mm. That Suntour size became the de-facto industry standard for rollercam (and U-brake) studs. Some Cunningham and Potts bicycles continued to use the original larger (and arguably superior) size studs. Later WTB rollercams included the stainless cap you refer to simply to convert from the Suntour size studs back to the original WTB size (which the WTB's bronze bushings were sized for).

The other thing I'd observe in your drawings and photos is that original WTB bronze bushings don't have a step in them. They are a very tight press-fit and the fact they don't have a step allows the arm to be moved slightly up/down in relation to the mounting to allow for arm clearance on frames and forks where the brake stud isn't mounted exactly perpendicular and/or to allow headset clearance, etc.

 

[Rich]

I tip my hat sir, that is a truly lovely machine, and kudos for having the jaffas to make your own brake set

How ever did I miss this thread I'll never know :S

Rich

 

[classen]

Thanks for all the useful information and the clarifications. I truly appreciate your insights.

- The prototypes and earlier WTB models used approximately 0.375" OD x 0.75" tall brake studs. When Suntour licensed Cunningham's design, they established a new, smaller stud size of approximately 8.9mm OD by 16.5mm. That Suntour size became the de-facto industry standard for rollercam (and U-brake) studs. Some Cunningham and Potts bicycles continued to use the original larger (and arguably superior) size studs. Later WTB rollercams included the stainless cap you refer to simply to convert from the Suntour size studs back to the original WTB size (which the WTB's bronze bushings were sized for).

This makes complete sense. The odd thing with the Merlin rear brake studs is the 30 degree angle at the base. This makes it difficult to get the brakes setup without jiggle between the arms and the frame.

The other thing I'd observe in your drawings and photos is that original WTB bronze bushings don't have a step in them. They are a very tight press-fit and the fact they don't have a step allows the arm to be moved slightly up/down in relation to the mounting to allow for arm clearance on frames and forks where the brake stud isn't mounted exactly perpendicular and/or to allow headset clearance, etc.

This also makes sense. I think my powers of observation are severely lacking . Now that I go back and look at various pictures of the DKG rollercams and the few that I have of Charlie's new roller/toggle cams I see that the brass bushing does not have a step. My rational for putting the step in was to keep the brake arm from sliding up and down on the brass bushing (which was a slip fit in my original prototype).

Now the brass bushing is press fit, but I still retained the step. Other than adding a little extra weight, and not allowing me the adjustability that you refer to, I think it will work quite well.

 

[halaburt]

This makes complete sense. The odd thing with the Merlin rear brake studs is the 30 degree angle at the base. This makes it difficult to get the brakes setup without jiggle between the arms and the frame.

At first I couldn't figure out what this 30deg problem you were talking about was... I thought you might mean the posts weren't quite perpendicular to the seat stays (no where near 30deg angle there though). Then I realized you meant the "shoulder" on the brake stud itself, right?

That shouldn't matter at all. There's enough of a lip there to prevent any "jiggle" if you've got the right tolerances and length for the bushing. Below is a pic of the rear brake on my Newsboy -- which appears to use the same post as your frame. On mine the arms are held very close frame (i.e., the bushing is almost flush with the back side of the brake arms). The only "jiggle" is maybe a half mm or less of play that the arm (and bushing) have purely inline with the post.

 

[pw_pw_la]

I hate epic old threads with broken photos.

HATE them!