linberl

I just need to know if the Shimano RD-6401, 600 tri color Ultegra 8 speed rear derailleur has any compatibility issues with the Sram MRX twist shifter. It's alot lighter weight than the claris rear der that came with my Bike Friday and a neighbor has one in great condition he is willing to part with for a very reasonable price. I just am
leary because I know some of the older stuff worked differently. Let me know if I should jump on it or not, and thanks!

Andrew R Stewart 

IIRC the 6401 was a very early indexed der. While the cable pull ratio was the same across many years there are some der geometry differences that can come into play when more cogs are used. Shimano use to say that their old units were not forward compatible but the new was backward compatible. Like any "off chart" application the proof is in the trial. Maybe your neighbor will let you test it out and you'll know for sure. Andy

cpach

Andrew is accurate on this, but Claris is also 8 speed so I would bet good money this works just fine.

linberl

Yeah, my bike is 8 speed. It will be a "final sale" because he has to dig it out of his garage and, understandbly, doesn't want to spend the time doing that for nothing, lol. Seems like you think it should work, so I'm going to buy it. That Claris is 300 grams and this one is just over 200. Big difference, especially when you're carrying the bike.
Thanks!

fietsbob

Maybe add drillium, if lighter is what you seek..






...

Andrew R Stewart 

The gains from the bike's "upgrades" are far less productive then those from working on the engine and command center. Andy

linberl

When one is carrying their folding bike around, weight matters. Working on the engine isn't going to do anything to make the bike lighter to carry. Dropping even a quarter of a pound makes a difference when holding the bike for more than 15 minutes (which I do).

Andrew R Stewart 

Perhaps the aprox 1% weight loss due to the 100grams difference will make a difference. Good luck. Andy

GrainBrain

What's your largest rear cog? I was able to run a 26 tooth gear on my 6401. 28 might be pushing it. Mine is the short cage version.

linberl

Bike has 11-28. Chainring is 53t (folding bike). With 16" wheels I can't use a long cage so I hope my neighbor has the short cage - he can't remember.

GrainBrain

Next time try searching Google with this: bikeforums.net: rd-6401

It's what I do all the time. Looks like you're good to go up to 28.

linberl

Hey that's really helpful; never thought to do a specific google search like that. Thanks!

linberl

It's cumulative with other options to drop the weight more. 100 grams x 4 does make a difference.

Andrew R Stewart 

I understand the way to get a bike lighter very well. But as only the der was mentioned I restricted my reply to the topic at hand. Andy

linberl

I get that. I was just explaining it was part of more than a 1% change, lol. The bike is 19.25 lbs and I'm a tiny old lady, so even half a pound makes a difference carrying it =).

fietsbob

Top of the component cost tiers XTR & Dura Ace has more light Ti parts...
maybe look for a lot of Titanium bolts and so forth.. seat post, straight MTB bars in carbon?

SEA has small companies making Ti & Aluminum aftermarket parts for Brompton,
which has more unique parts, but sells well globally.

Bi Fri uses more mainstream components in their designs , hubs crank seatpost , bars .. etc..

linberl

My seatmast and stemmast are titanium. I specced out very light weight components and have already swapped into a 78 gram stem and 92 gram handlebars. The weakest component on the bike is the claris derailleur because no one makes a high quality 8 speed anymore. So I'm looking at NOS or slightly used. XT/XTR are possiblities if my neighbor's rear der is long cage and not usable for me. Dura Ace older (8/9speed) don't translate to my shifter.

fietsbob

The actual RD stroke just moves, continuously from one limit screw stop to the other..

speeds is the detends in the shifter, moving the cable.. X far.. like the spacers in the cassette are in thickness.

dual pulleys like JTek converts the cable pull input to a different output, by being 2 different diameters..

but thats adding stuff..

linberl

I have no idea what that means....

smashndash

A derailleur doesn't inherently have a "number of speeds", only a pull-to-movement ratio. If you have a shifter you want to use, you could theoretically use it with any derailleur using a ratio adapter. But since you want to reduce weight, adding stuff isn't ideal.

linberl

Ah, yes, but I believe from what I've read that you also then have to take into account chain width and cog spacing as well, yes? It's not just the pull ratio? Btw, thanks for the "translation" lol.

veganbikes

Yes but if you stick with 9 speed and below you should be good. I doubt the 600 is going to be long cage but if so and you get it for a reasonable price and want something different I am happy to make a trade or possibly buy it if the price is right. I love my Tricolor.

linberl

Well it just got interesting. He has a HUGE parts bucket. I now have the choice for the same price of either the 600 (short cage) OR a 105 rd5501 short cage. The 105 seems barelyused and the jockey wheels are pristine; the 600 is definitely used. I'm assuming both would work to replace my claris 8 speed rear der. Any thoughts from anyone?

veganbikes

600 will look better and perform like a champ but the 105 is a fine derailleur and will give you plenty of good service. It is a tough choice. Both should work fine replacing Claris, pretty much anything will except Tourney but that shouldn't even exist.

fietsbob

there is a measurement exercise ..

compare the inward motion, of the RD, as you pull the cable, to the amount of cable pulled ..

Go !..