Top pull / Bottom pull mech.. which is which?

[ermsamermsam]

Maybe this can go in the F.A.Qs. Assuming it gets answered.
I am a bit confused about the difference between top pull and bottom pull front mechs.

A bottom pull mech has the cable running under the bottom bracket. That is clear.
A top pull: the cable runs from the top and the sleeve runs down to the top of the mech with the cable extending through the mech and the cable is held by a bolt at the base of the seat tube.

Is that all there is to it?
Why did it seem so confusing?

 

[MikeD]

Not quite. Bottom pull is as you describe. Top pull has the cable from above clamped to the mech in the usual way. The thing you're calling "top pull" actually uses a bottom pull mech and weirdy cable routing to make it kind of "top push", as found on some old Treks and other stuff - the mech clamp bolt was replaced by a cable stop and there was an extra boss on the seat tube.

 

[ermsamermsam]

Aha.
I am actually thinking about a '91 trek, so you have it bang on. Thanks for the clarification.

 

[MikeD]

It was a '91 Trek that I encountered it on. I might even still have the little cable stop widget somewhere. Your best bet would be a "proper" top pull mech, though, unless you're going for 100% catalogue spec ;-)

 

[neilll]

Sorry to butt in but if anyone has one of these spare I would like to know : http://www.retrobike.co.uk/forum/viewtopic.php?t=64885

 

[MikeD]

Having read that, ignore my top pull mech suggestion unless you can create some sort of cable stop higher up the seat tube (or do that V-brake noodle bodge as in the thread).

 

[ermsamermsam]

If you did have the cable stop widget around I would be interested. I was thinking "I'm going to have to find one of those cable stop widgets from somewhere" and you might be able to save the day..