Hi all: I am doing a mild bit of gear freaking, and want to change my Norco's stock 52/40 x 13/15/17/19/21/24 gearing to add a lower low: I've decided on 52/38 and have sourced a 27T Uniglide cog for the back.

Question #1 - bike has 600EX rear derailleur, and according to Velobase will wrap 26T and take a max of 28T cog. Chain wrap using the parts I have above is 28T - is this significant enough to worry about?

Question #2 - thinking the best spacing comes with a 13/17/19/21/24/27 rear (big jump to high gear!) but would welcome other opinions.

Thoughts?

Mark

ps. Velobase gets better and better all the time - thanks to all who are contributing.

I have friends who jump 24-28 all the time in the western NC mountains. They definitely slow-spin while doing so, at least 2-3 revs to let it jump.

Generally, if you bump up one end of the range, you may be able to jump up on the bottom, too, depending on if you have a lock ring or use the small cog.

Thanks Robbie: I need the 13T small cog as it is the locking cog for the cassette. Hardly ever use it and may look for a 14/15T lock cog.....

Do I need to worry about the chain wrap capacity of the derailleur though? I am exceeding the spec by 2 teeth....

Mark

Why not get a long-cage 600EX? I see them occasionally on eBay.

Definitely an option, but looking at what I can do without replacing the RD. My thinking is that I will never use Large/Large or Small/Small, so exceeding the rating by a couple of teeth will be OK.....but I wanted to check with others who may have done this.

Mark

Size the chain for large-large in case of "accident".
Small-small will just give you chain sag, but won't bend/break the DER like too short a chain can.

I really dislike the proposed 13-17 jump at the top end. If you are currently using a 6-speed freewheel with standard cog spacing and a 126mm rear axle overlock width, consider a readily availble Shimano 14-16-18-20-22-24-28 seven-speed, which will give you a nice tight ratiometric progression and the wall-climbing low gear you crave.

I have a long-cage 600 (a BR6207, not necessarily an EX), and it shifts TERRIBLY! That's why not to get one. A Huret Duopar or Ecopar is a far better choice. Even a Campy Rally, and I'd bet a Campy Gran Turismo, would be better choices.

To teh OP: you outline a very odd cluster: 13/17 .... 27. Can you actually make that combination? Stores that can sell you cogs are today few and far between.

Road Fan

I concur completely, with the possible addition that the wider Megaranges, 13-34 7-speed, give a highly usable spread in the first 6 positions. With a Duopar, the difficult-seeming shift from the 24T in position 6 to the 34 T in position 7 is a piece of cake. It's not a piece of cake with a Shimano 600 long cage. With that cluster and Duopar derailleur your 52-40 crankset does not have to be changed.

I'm using one of those Megarange 7 speed freewheels on an '88 Schwinn LeTour. I put a Shimano XT rear der.(not sure what vintage, it's silver colored) and it handles the 24-34 shift effortlessly.

John: thanks for the comments - from a ratio perspective I too dislike the 13-17 jump but was trying to avoid buying yet another cog for the uniglide cassette.....and since I rarely if ever use top gear I was not so worried.....but you have opened up another possibility - I had not considered the 7 speed cluster so I will look at that. Is that cassette compatible with the 600EX hub (circa 1980?).

I have a 38T front ring on the way now....I will try that with the standard rear cluster first, and I have a 27 tooth Uniglide cog on its way as well to experiment with.

Road Fan: you pointed out something I had missed - I was keeping the 13T small as it is the lock ring, but just realized after your post that eliminating the 15T second cog (I was attempting to shift everything down by one to make room for the 27T low gear) may not be possible as the 17T can't go in the second position on the uniglide freehub....Perhaps I just need to spring for the 15T lock ring for the first position and keep the original progression, eliminating the 13T and adding the 27....

Regarding the RD: I have a mint SunTour V-GT Luxe to use if necessary,though I was trying to keep to the all arabesque group for looks.....

Finally - my original question was "what happens if I exceed the rated chain wrap by 2 teeth"? I think the answer I got is possible chain sag in the 13/38 combination - is that the only impact? Or do I need to get the Suntour ready to mount?

And just in case there is any doubt - I am doing this as a learning experience, so please keep the suggestions coming - I am not set on any particular direction and the bike is quite enjoyable with the stock gearing.....just wanted to play around to see the results in the "real" world.

There was a long cage Arabesque rear der. I am using one on a Schwinn Voyageur 11.8 that I put a triple crank on.

I think that is right. Bill Kapaun provided good advice. As long as the jockey wheel and inner cage plate will clear your 27t cog you should be good to go. As for the chain sag, you are never going to use the small/small combo anyway, so it doesn't matter.

Sierra: thanks for the thoughts - the specs on the short cage 600 RD says 28T max so should be OK there, and you confirmed my thoughts on the chain wrap question......

So current plan is to try the 38 front ring with the stock cassette on the back - a 5 minute change that yields some immediately testable results for my roads.

Next, try pulling apart the cassette and fitting the 27 tooth low gear. I will probably, for the sake of the first test, just replace the 24T low gear with the 27 just to see if I really need the "granny" (sort of) low gear. If not, probably just leave things with 38/52 front....

If the granny turns out to be the bee's knees, will experiment with different combos of cogs I have in hand.....if there are shifting problems with the RD I'll likely try out the SunTour just to see (it is a wonderful RD), but probably this is as far as I'll go.....unless, I get obsessive about proper progression and then I'll look into John's suggestion of the 7-speed cluster (to get optimal ratios) and sourcing a long cage 600EX.....

Thanks everyone for the suggestions!

Yes, that should work fine. In late 1973 I bought a bare 1970 Peugeot UO-8 frame (see my signature) and built up a bike for my wife. Since this preceded ultra-spaced 6-speed freewheels, I pretty much had to optimize around a 10-speed setup with a standard 52-42 chainring set (TA Professional 3-bolt). Starting with a very popular road racing combination, 52-42 / 14-16-18-21-24, and not wanting to disrupt that very nice 1.5-step ratio progression, I decided I could best get her the low gear she wanted by sacrificing the 14 and adding a large cog. I tried 16-18-21-24-32, which worked superbly with my old SunTour V-GT derailleur. The 88 gear inch top end was high enough for her needs, and the 35-inch bottom gear got her up Franklin Canyon, Benedict Canyon, and our other favorite neighborhood hill climbs in west Los Angeles.

Update: finally received all the pieces and thought I would report on what I ended up doing:

Using a 52/38 front combo, and I created a rear cluster of 27/21/19/17/15/13, I ended up with a nicely effective 9 speed (out of 12 possible), with nice even jumps of 11-13% except for the 1st to 2nd jump of 29% (like a granny) and the jump to highest gear at 15%.

The 600EX Arabesque RD had no problem with the 27 tooth rear, and actually could handle the 52/27 combo (not that I'd use it!). No issue with chain sag on the 38/13 combo either.

Thanks all for your thoughts and advice!

Mark

Nice going, Mark. That reminds me of one of my early experiments, 54-47 / 14-15-19-21-28, which would have been better with 14-15-17-19-21-24-28, had 7-speed cogsets existed back then (early 1970s).

54T front!! What is your real name, Eddy Merckx?

Thanks for the comment John!

Mark

FWIW, I run a 14-22-24-26-28-30-32-34 eight-speed w/ an old long cage 600 in the winter on my it-snowed-two-feet-last-night bike. No problems even with the 8-pin dump on the high end.