Rear Derailleur Confusion

[Skywatcher]

Hi,

I'm building a Dawes Super Galaxy up from parts and have got to the stage where I need a rear derailleur. I've been looking at Shimano 105 models, in particular the GS (medium cage) one.

The chainset I have is 46-36-24 and I have 10-speed Dura Ace bar-end shifters so need something that will work with these.

I note that the 105 derailleur is listed as "Maximum front difference 22T, total capacity 40T" so I guess this means either finding a new chainset or am I reading this wrong?

[LWaB]

Total capacity is difference in biggest and smallest chainring added to difference in largest and smallest sprocket.

[Skywatcher]

Thanks, so my front difference is 22T and if I bought a 12-25 cassette then I'd be well within the limit but a 12-30 cassette would be at the limit?

[Brucey]

yup, but if you run 12-30 with a chain that is long enough to do the large-large combination and a GS mech all that will happen is that the chain will run slack on the small-small combinations.

Provided you can avoid using those (and you should be able to, they are pretty well duplicate ratios in most cases) then it'll work just fine. I've run bikes with gears like that for years.

cheers

[freeflow]

A 9 speed mtb rear mech would also work and allow cassettes upto 36 tooth maximum. My old super galaxy (1992 ish) still has the original deore lx rear mech.

[recordacefromnew]

Thanks, so my front difference is 22T and if I bought a 12-25 cassette then I'd be well within the limit but a 12-30 cassette would be at the limit?

I think you are talking about the 105 RD-5701-GS, which like you said has a 22T max front difference, 40T total capacity and also 30T max rear sprocket as officially specified, so I don't see why it should not work perfectly, keeping chain tensioned in all front / rear combinations, with a 12-30T cassette.

In fact given these official specs are nearly always conservative, and that you can adjust the B screw to increase clearance for larger rear sprockets, it is likely to work fine with a 11-32T too.

I am running a RD-7700 with a 32T large sprocket with no issue, its official max rear sprocket is 26T.

[9494arnold]

Hijacking the thread slightly, will 9/10 speed mechs work on 6/7 shimano casette with a HG Chain or are they designed for narrower chains?

[nez]

A 9 speed mtb rear mech would also work and allow cassettes upto 36 tooth maximum. My old super galaxy (1992 ish) still has the original deore lx rear mech.

Mine is ten years newer but has had Deore lx derailler from new.

[Brucey]

Hijacking the thread slightly, will 9/10 speed mechs work on 6/7 shimano casette with a HG Chain or are they designed for narrower chains?

9s mechs certainly work with 8s chain.

cheers

[Audax67]

A 9 speed mtb rear mech would also work and allow cassettes upto 36 tooth maximum.

I've mixed Deore with 105 shifters: fine but there was no adjuster on the mech so I had to fit in-line twiddlers in the cable. They work but I'm not fond of them - they're a bore to fit.

[CREPELLO]

A 9 speed mtb rear mech would also work and allow cassettes upto 36 tooth maximum. My old super galaxy (1992 ish) still has the original deore lx rear mech.

Just to clear up any "Rear Derailleur Confusion" - which is what this thread is about , are you saying that any old 9 speed MTB mech (SGS) will work with a 36T cassette? Because I was under the impression that you would need a shadow mech to achieve good shifting with such a set up.

Just interested.

[recordacefromnew]

A 9 speed mtb rear mech would also work and allow cassettes upto 36 tooth maximum. My old super galaxy (1992 ish) still has the original deore lx rear mech.

Just to clear up any "Rear Derailleur Confusion" - which is what this thread is about , are you saying that any old 9 speed MTB mech (SGS) will work with a 36T cassette? Because I was under the impression that you would need a shadow mech to achieve good shifting with such a set up.

Just interested.

I believe it would be unusual for any fairly recent Shimano/sram mtb mech to not being able to handle a 36T large sprocket.

Shadow mechs are designed to have a profile closer to the cassette to cut the risk of striking rocks, not to increase maximum sprocket size. Most if not all Shimano GS and SGS variants of the same mech have identical parallelograms, so only the cage length and therefore total capacity to keep the chain tensioned differ, not max sprocket size.

However, hanger geometry does affect max sprocket size - for reference sram recommends a length (distance between the centre of the rear axle and the centre of the rear mech fixing bolt) of between 24-28mm for road and 28-30mm for mtbs, while Shimano specifies 24-26mm for road and 28-30mm for mtb - for each 2mm there will be a 1T max sprocket size difference. Max sprocket size is of course also influenced by B screw adjustment/modification, chain length, top jockey wheel tooth count and manufacturer spec conservatism etc.

A good indication of such conservatism is demonstrated by the fact that Shimano/sram compatible large sprockets up to 41T and 42T are readily available, from e.g. http://www.ebay.co.uk/itm/41-Tooth-Cog-for-Mountain-Bike-Cassette-41t-Sprocket-/251408161635 and OneUp respectively. The suppliers say most mtb rear mechs (with the exception of the downhill/freeride Saint/Zee/Hone models) work fine. I believe them, I certainly have no trouble creating a gap in excess of 20mm between the 32T and the top jockey wheel of a fd-m972 on my mtb.

[RickH]

I believe it would be unusual for any fairly recent Shimano/sram mtb mech to not being able to handle a 36T large sprocket...

...I believe them, I certainly have no trouble creating a gap in excess of 20mm between the 32T and the top jockey wheel of a fd-m972 on my mtb.

According to my calculations you need roughly 2mm clearance for each extra tooth on the largest sprocket that you want to add, so in the above case, you should be able to add 10 teeth. Adjusting the B screw to see how much you can get it in any given setup will give some indication of what is possible.

Rick.

[Skywatcher]

Thanks for the advice I got at the start of this thread. I've now finished the build and I'm very happy with the results but need to tinker with the rear derailleur as it's skipping one gear.

I started by buying a Dawes Super Galaxy frame and forks which are early 1990s but had been used for a LEJOG run in the early 1990s, taken apart and stored since. Thankfully it was stored well and so it's in great condition, almost like new. Everything else I'm picked up as bits from Ebay, except the tyres, brake pads, rear derailleur, chain and cassette. It's not one for the purists but it rides beautifully and, much to my surprise, weights under 11.5kg.

[Brucey]

when you say you are skipping one gear, what do you mean?

Which bits did you end up with exactly?

BTW are you sure you have the cable correctly mounted in the rear mech pinch bolt? There is a right way and several 'wrong ways' for doing this, and it makes a difference.

cheers