phorn

I just picked up a nice "new old stock" steel MTB frame from the 90s and I don't know what this braze-on is for just above the bottom bracket. Any ideas?

jsharr

I am going to quess that is for a replaceable braze on deraileur mount.

fietsbob

a cable reversal pulley to use a bottom pull derailleur on a frame with top tube routed cables.
It's still done for Cyclocrossers, as STI Brifter compatible FD
are just bottom pull
so the grooved pulleys are available thru like QBP, IE any Bike shop,
since so many have a QBP wholesale account .

Or, you could just get a top pull front derailleur and ignore it.

phorn

I was wondering that - but I have only ever seen braze-on derailleur tabs that braze-on mount derailleurs attach to. Have you ever seen those typical tab mounts screwed in to a boss like this instead of brazed on directly? I'd consider using it instead of a clamp derailleur if I could find one. Otherwise this might be in the way...

phorn

Thanks - that sounds like a definitive answer.

LarDasse74

I agree it might be a top/bottom pull adapter pulley mount, but only if there is a cable stop somewhere above that would guide a cable into the pulley.

I am suspicious because every pulley I have seen designed for that function are on the back of the seat tube.

fietsbob

When Keith Bontrager was still a guy in Santa Cruz making bike frames, himself,
he routed the cable down the right side of the seat tube of his Cyclocross Bike Frames.

Bezalel

It's for an E-type derailleur.

fietsbob

Mount a Chain/trouser guard on it ... perhaps ??

OK Phorn let's see the rest of the elephant .. more information needed..
where are the cable routing braze-ons?

phorn

I'll throw some more photos up later tonight as I'm at work right now... Thanks.

BCRider

For a mountain bike it almost seems like any pulley mounted onto that boss would be in the right place to fould the FD cage. At least that's my feeling given the scale provided by the BB shell diameter. I tend to agree with fietsbob that this is likely what it is but it seems like it's too high up. My other guess what going to be a chain guide mount point if this is a frame that had aspirations of serious big drop woods riding as opposed to light trail or cross country racing.

wesmamyke

I'm thinking the same thing, been awhile since I ran across anything etype, but that's in the right spot.

Edit: Is the frame a Dirt Research?

phorn

One cable stop on the left side of the top tube, two on the right side of the top tube, and one on the back of the seat tube, about two inches below the junction of the seat tube and seat stays:

phorn

Edit: Is the frame a Dirt Research?[/QUOTE]

Yes, it is a Dirt Research Pecos model, made with Columbus Thron tubing. By the way, it is my third Dirt Research frame for various family members and we have been very pleased with them overall, but this is the first one that had this particular configuration of cable stops and braze-ons. That left side cable stop actually is handier for the v brake setup I'm installing (on my other DR frames I've had the noodle wrapping around the seatpost for standard cantis, so I used a Problem Solvers clamp-on cable stop for the V setup).

If it is an e-type derailleur mount, as it appears it could be (now that I have looked at schematics of those online), is there any reason to go with an e-type for this build or would you still opt for a top-pull clamp mount "normal" derailleur. I have read that the e types have limited adjustability (and more limited choices in chainring combinations), but are there any advantages to the e type I'm not thinking about?

Bezalel

Is your seattube one of the standard sizes? I noticed that the tube size changes below the seat cluster If your seatpost clamp is a standard size than you can't use a clamp-on derailleur.

wesmamyke

The E-type is nice in that it can't really go out of adjustment. The BB length needs to be pretty close to perfect, and it needs to either be a E-type BB or use a 73mm in a 68mm shell so the derailleur has room to mount. Also like you said limited chainring choice as you have no up/down adjustment to play with.

vettefrc2000

https://techdocs.shimano.com/media/te...9830648095.pdf


Direct Attachment Type

Bezalel

No, the direct attachement type requires a rectangular mounting plate on the seattube. The E-type attaches to the bottom bracket and just uses the braze-on to stablize the derailleur.

phorn

Yes, the seat tube is 28.6, so no issues preventing using a standard clamp design derailleur.

LarDasse74

The one issue I can think of is that the damn e-type braze-on might interfere with the clamp of a top-swing model derailleur. Nothing a hacksaw and a file can't solve, though.

fietsbob

Just put a Top pull/top swing FD on and ignore the braze on,
or get a SRAM 9x3 dual drive and with one chainring you can fit a chainguard on.

E types came on with Full suspension bikes
with no seat tube to mount the FD onto.

Bezalel

Do you have a valid reason why you don't want to use an E-type derailleur? What size chainring do you intend to use? I don't know why the frame designers decided on this type or derailleur and unless I know why, I can't discount their reasoning.