Building a touring bike - road and mountain rear derailleurs
 

I'm building a touring bike around an 88 miyata 615 and am almost done collecting my parts. I have an Ultegra 6500 rear derailleur and a cassette to match it in my parts bin I could use but the cog limit on that is 27t. Is it worth getting a new rd that accepts larger cassettes even with my 48-38-28 crank?

Since your crank goes all the way to 28t, just use that one. Ultegra is great quality. Spending another $200 to get the equivalent in a mtb derailleur for a gear that might not even be noticeably lower seems like a waste of money.

If you do find you struggle on hills, you'll know exactly where to invest next.

I would keep what you have. If hills become an issue I would see if I could get a smaller chainring, maybe something like 26 or even 24 tooth, a whole lot cheaper than a new derailleur.

The problem isn't that Ultegra rear has a 27 tooth limit & the cassette you will use, it's the 20 tooth difference on the fron chainrings. If you size a chain for the large front cog & large rear cog, in mana\y of the gears on the small ring the chain will sag a lot. If you size the chain to small cog/small chaing ring, you rip the derailleur off somewhere in the large chain ring. An ultegra rear der can't handle the chain wrap needed on a triple.

So I definitely need a mountain derailleur with this crank?

depends on what you're climbing and for how long, seems like you are keeping vintage anyway, a 9spd XT rear with matching 34t cassette does not fetch much now a days, $50 gets both pieces

I have a pretty wide range front (52/42/24) and wide range rear (11-32 eight speed). I find that if I am on my smallest front chainring, I can't use my 11 or 12 tooth rear cogs on my cassette. But those gears are pretty cross chained, so I don't use them anyway. The issue is that the rear vintage XT derailleur (I think it is a M739?) won't take up all the slack.

Bottom line is that if you want every possible gear to be useable, then it matters. If you do not use the highly cross chained gears, you can run wider ranges than the derailleur can take up.

http://bikeforums.net/attachment.php...hmentid=414413

But you are using an XT rear der (off road/touring), aquateen wants to use an Ultegra der, designed for road gearing (tighter/smaller cogs)

Running a 53-39-30 with a 9 speed 105 FD and SRAM X9 (old tech 9 speed) with 11-34 on the back I have to compromise. I can have 53-34 safe (as in oops I cross chained not ^&*^&* ride over) and 30-15 functional but anything under just sagging. There simply isn't enough chain wrap. Doesn't bother me as who uses 30-13 and 11 anyhow?

OK looks like I'm going to pick up a new derailleur. Thanks for all the info folks

I thought that the Ultegra 6500 was a "mid cage" RD that was designed for double or triple cranks?

I have a 6500 on my Bianchi with a 52/42/30 and it works perfect. Stock RD.

Yes, it looks like the 6500 can handle a 22 gear jump but I ended up just ordering a Deore derailleur for more cassette accessibility.

IMO that was a good decision. I see that you have a Trek 1000. I also have one that I used for commuting and have also used it for light touring. The bike is equipped with a Deore rear derailleur with a 12-34 cassette, and a 48/36/24 crankset. This is an excellent combination for where I live. You can probably imagine how that bike climbs with those gears. The Deore RD has been on the bike for 10 years, eight of those commuting. It is reliable and a good value. I can't say I've ever regretted having those low gears.

That derailleur will also set you up for some chainring changes when you find out that a 28 tooth chainring is not small enough:)

Bike, low geared Trek 1000, at the top of Lava Butte, near Bend, Oregon. I took this picture to prove to my daugter-in-law that an old guy could also get to the top. She did it the day before.
http://i783.photobucket.com/albums/y...182fe23a1d.jpg

Lava Butte:
Lava Butte - Wikipedia, the free encyclopedia

I was simply responding to a previous comment that sounded like having a derailleur that could not take up all the slack was a bigger problem than it really is.

Probably the best course of action as you didn't state whether you had a GS or a SS cage RD-6500. https://www.bike-components.de/bedie.../rd-6500.2.pdf

Brad