Monday, September 19, 2011

Direct mount 411

This is for the folks out there who are patiently waiting (or not so patiently waiting) for Mark at Paragon to get some more direct mount front derailleur mounts made. In other words, the framebuilders will be more interested than the rest of y'all, probably.

You don't have to wait, folks. In fact, you can very easily make your own mount from some bits of scrap. No special tools (other than normal framebuilder stuff) needed.

First, why use a direct mount front derailleur? I've written about it a bit in the past - suffice to say, if you're building full suspension bikes, short-chainstay bikes, or just want a bit crisper front shifting, direct mounts rock. You can do all kinds of weird and unconventional things with your seat tube, and still run a front derailleur. Good stuff.

Now that I've talked you into it, here's how I make a mount. There are many other ways to do it, you can get as creative as you want (I might note that the front side of the mount is a perfect spot for an always-popular *bottle opener* if you roll that way).

First, get some 1/4" plate. I used some cheap bar stock from the hardware store (some kind of mild steel); you could also use 4130 or whatever else you think is worthwhile. Remember that you've got to thread the derailleur onto this plate with an M6 bolt - so doing a lot thinner than 1/4" is probably a bad idea if you want to keep from stripping things or having the derailleur fall off.

The specs for where to drill/tap your mounting hole are here. You'll want a minimum of 25mm or so worth of edge for the derailleur to mount up to, anything shorter will allow the derailleur to rotate around the bolt and cause problems.

Once you've drilled and tapped your piece of plate, weld or braze an old piece of seatstay/chainstay/whatever to the back side. Make sure the mounting bolt can thread through the hole freely! Now miter to fit your seat tube and put the derailleur in the correct location. This is easy with a mill and a small fixture (if you want a picture of the fixture, post a comment and I'll get one up - I forgot to take any) or alternately it can pretty easily be done by hand. You'll need to do a little bit of trig to figure out what angle to cut the miter at (and how much, if at all, to offset it if you're doing something odd with the seat tube) but if you've gotten this far, you can probably handle that.

To mount the mount to the frame, I use an old piece of aluminum plate, bolted to the bb, with a second bolt to hold the mount in place on the seat tube (no picture, sorry). The only tricky bit here is to make sure you attach the mount parallel to the BB shell face, so that the derailleur will sit straight when you bolt it on.


Jesus_Castro said...

So do I have to use a hyperbolic substitution?

Meriwether said...

Thanks for posting that Walt!
Question: How do you choose how much to offset the mount by? Or what chainstay/seat tube angle to you shoot for when figuring out how much to offset the mount? Is there a 'better' angle between the 66-69 degrees that Shimano recommends or does it just depend on the shortness of the stays, etc?