7800 Medium Cage, so close!

[Manc33]

7800 medium cage rear derailleur got sold for over £70 tonight. Short cage ones can be had for £35.

I bid more than £60 on it (with postage it would have made it close to £70) but I ain't prepared to go higher than that so didn't win. It was already "horrendous" at the price I was willing to pay!

Makes me wonder what the winning bidder must have put on it. £120? £140? £160?

A guy in the US has a NOS one on sale of a GS... its only £175.

How many of these 7800-GS come up in say, one year? Not even one? One every 2 years? 4 years?

I think I will stick with my 6500-GS. It shifts like lightening but I just wondered if the DA is better. The funny thing is the GS can be had for about £15-£25 and like the GS Dura-Ace, are less common than the SS ones, just nowhere near as rare as the Dura-Ace GS.

Even the 7700-GS won't do because it can only take a 26t (same as the SS). The 7800-GS takes a 27t, meaning it can be pushed to 28t and still be reasonably close to the cassette sprockets. You can push a 7700-SS to 28t and who knows 30t but the mech is pulled away from the cassette then and when you're doing that it is totally defeating the object of putting the best equipment on, you're forcing it so it cannot shift as fast. You might as well just use a MTB derailleur.

The closest I got was bolting an XTR M971 SGS swingarm onto the DA-7700, that leaves a gap of about 4mm between the mech and swingarm, because the XTR mech is a lot wider than the DA, where its swingarm is thicker around that area than the XTR. I would think that setup would be too weak anyway, apart from the fact that you'd have to regrease the p-tension after every ride in the wet. I have seen pics of it done though. Who knows how they sealed that gap up, maybe with some sort of fancy washer but it would have to be custom made to allow play in the p-tension while not letting the elements in.

[Brucey]

or



will ye not have a drop of XT....?



cheers

[Valbrona]

So how often do you use you 50-something chainring?

[Manc33]

How often... the 52t... only downhill probably, but what if it is a long shallow downhill with a tail wind.

Brucey, been through all that, the M772 was the best shifting rear mech I used on the 11-32t but it was brand new.

I'm fussy and want the mech shifting fast.

On 8-speed on a 11-32t I couldn't get on with that jump from 15t to 18t (20%).

This led to weeks of messing about with cassettes up to 23t/26t but then of course I missed that big sprocket on the middle ring. Even more than that I missed the 11t.

In the end I am just going to get a mech that the Shimano doc says is a 27t and push that on a 28t sprocket.

On my 6500 GS (max 27t) I can screw the b-tension in so its a bit tight and it seems to be OK on a 28t in the lowest gear. It behaves like the DA one did - if shifting from sprocket #2 to #1 while on the granny, its fine, if on the middle ring and low sprocket, then drop to the granny, you get the jockey wheel rub.

On a x-26t cassette there's just no 11t option unless you use an SRAM (blergh!) so... the 28t being a 11-28t gives me back that 11t I was also missing, even though some say "you don't need an 11t" they tend to live in Holland.

[Brucey]

Brucey, been through all that, the M772 was the best shifting rear mech I used on the 11-32t but it was brand new.

No, no you haven't, not at all.

RD-M772 is a completely different mech, different capacity for a different cassette, single pivot, different everything.

RD-M750 GS or RD-M751 GS (not SGS) are basically stronger, better versions of the ultegra/DA 9s medium cage dual pivot mechs with a better capacity than either. If you have not tried one then you have not 'been through all that' at all.

Best of all they are about £20 for a really good one on flea bay because you are not bidding against idiots who think that they must have 'dura-ace' written on everything....

So do yerself a favour and buy something sensible instead of trying to cobble together some 'frankenmech' or other, eh?

cheers

[Valbrona]

How often... the 52t... only downhill probably, but what if it is a long shallow downhill with a tail wind.

Well if you are short of capacity in your rear mech a 50-something chainring that only has the chain on it when you go downhill is not very logical.

[Manc33]

Brucey sorry I meant I have "been through all that" having cassettes on up to 32t and realizing road mechs sit nearer the sprockets than MTB mechs do. Some MTB riders are using Dura-Ace mechs claiming it shifts quicker than their XTR did, but forcing them to run on 30t and even 32t.

That was why I said someone managed to fit a short cage XTR cage to a Dura-Ace, because the cage on the XTR (despite being a SS itself) was slightly longer than the DA SS cage. I just don't know what models they used to do that, because with the two I tried to merge (7700 body / M971 cage) it leaves a big gap at the p-tension.

The older ones you mention "RD-M750 GS or RD-M751 GS" I have never tried and if you say they shift like lightening I believe it - but do you mean use that on a cassette up to 28t or 32t? If it is a MTB mech, doesn't it mean the parallelogram sits slightly further away than say an Ultegra, even if the XT mech is based on it?

This is why if I am on a 28t low sprocket I don't really want to use a MTB mech.

I remember trying the M772 on a 13-26t cassette and the distance from all the sprockets (with the b-tension screw removed) was quite a lot and caused a delay between shifting and the chain moving.

Then again the RD-5701-GS I tried was also like that on a 23t.

One thing I have learnt is always check what cassettes were available in the groupset of the mech, not that it means much, but its a guide.

Then the max capacity... people say mechs meant for up to 26t "can be pushed" to a 32t or whatever, but that means bringing the mech away from the sprockets so far it might as well have a MTB mech on it, or like you say Brucey it ends up a MTB mech works better like a certain XT one.

So I am sticking to 28t lol. It seems putting on a 32t or 30t is always going to give you slightly worse shifting, regardless of using a road mech with its b-tension way past what it should be, or a MTB mech with its b-tension removed completely.

Road mechs probably "expect" a 25t. MTB mechs probably expect a 32t or 34t. Something like 28t is in no man's land and "works" on both, but I reckon if you can push a road mech to run smoothly in the lowest gear on a 28t (when that mech's spec sheet says 27t, not 26t!) you can have the best of both worlds, snappy shifting and only the very smallest amount of jockey wheel interference - which considering I would be in the lowest gear in rare situations it is worth using it that way. If you change from middle to granny then go up to the 28t there isn't any interference, so you could sort of work around it and never have it interfering.

Brucey I will probably end up on that exact one you said but... I have to faff about.

[Brucey]

I've run several MTBs on RD-M750-GS with an 11-28 cassette and it worked pretty well; almost identical shifting to my RD-6500 mech (from new, on a 12-23 cassette). [The max for the RD-M750-GS mech was 32T BTW and that is a bit of a stretch.]

The only flaw was that the chain would run slack on a small-small combination on a triple using this mech, because the overall capacity isn't really enough.

This mech would be a ~£20 punt and it should come with a set of ultegra quality pulleys in it; how bad can it be?

cheers

[Manc33]

Gotta show it off

http://i.imgur.com/O0bEpXC.jpg

Yep, the coveted GS version.

You wouldn't believe what (little) I paid for it if I told you.

One thing between this and the RD-6500 (and its a strange one to me) is that the RD-7700-GS can take slightly more chain wrap than the RD-6500-GS can.

I thought about this and just assumed with it being the top stuff, rich middle aged overweight men would complain if you couldn't have a big gear range with it, I don't know, but it handles the chain wrap slightly better because my chain is the same length and I can get an extra small sprocket when on the granny, compared to the 6500 GS.

This is also what the spec docs say, 37t capacity on the 6500-GS and 38t capacity on the 7700-GS.

The 7700-GS is a bit of a one off mech though with 13t jockey wheels when the SS and other GS models below all have 11t jockey wheels. The only other mech I have seen with 13t wheels is the MTB Alivio 8-Speed.

Does it shift gear any quicker than the 6500-GS did.... no, not really!

Here's another thing about it - the b-tension is a lot more forgiving than the 6500-GS was, I have got the screw about half way out on the DA and it is still OK in the lowest gear on the 28t sprocket. Again I think because Shimano made it to handle whatever is thrown at it because people would complain if it didn't?!

I don't even know how I got hold of it, someone must be watching over me (half serious).

Its turning into something special, Dura-Ace parts: rear mech (7700), rear hub (7700), brake calipers (7800). XTR parts: 8-speed shifter pods M951. Ultegra parts: front mech because they don't make a DA one and brake levers because they don't make DA flat bar brake levers.

Everything else on the bike I don't really care.

I suppose you could say the frame needs upgrading and those parts have no place on a frame like this... but I have had a carbon frame and just went back to the metal one again.

[Brucey]

13T jockey pulleys are theoretically more efficient as well as giving more mech capacity (extra chain wrap round). However this is not without cost; any amount of wear in the bushings has a more detrimental effect on shifting preceision and if you don't know any better it does, ahem, look like you have built your own mech using alivio parts....

cheers

[Manc33]

lol, that's just it, on a bike that was £300 brand new no one is going to bother looking at what's on it.