SRAM/Shimano specific RD hangers and dropouts?

[UpWrong]

I thought SRAM and Shimano top normal RDs used the same hangers or dropouts, but I've been finding differences. So I have a 9-speed X5 and a 9-speed Deore RD both long cage and with a 34T largest cog specification. I tried fitting the Deore to a Tern with the Neos derailleur adapter. Even with the B-screw all the way in the upper jockey wheel won't clear the two large cogs on an 11-34 cassette. Tried it with the X5, no problem and the B-screw is only about half way in. The opposite appears to be true on my Nazca recumbent. Using an SRAM x9 I can't get it to clear a 34T cassette but I'm told stock Shimano RDs can cope with that and even 36T. Clearly the hanger or dropout affects the largest cassette that an RD can manage, even when keeping within the spec of the RD. I've not seen reference to this issue before. Have others experienced this kind of problem?

[Brucey]

I think the differences may originate in the exact design of the hanger, in particular how long it is, its position w.r.t. the axle and where the tab is. If the RD is one with two spring pivots (many shimano), winding the b tension screw in has only a relatively weak effect on the top pulley position (*), whereas it is usually much greater for a single sprung pivot mech (all SRAM), on a standard hanger.

However if the mech is spaced out rightwards on some kind of different hanger, a SRAM mech may be almost in a straight line when it is in bottom gear, which greatly lessens the effect of the adjusting screw on the reach of the mech. It is not always simple to see what is going on.

It isn't unusual to find that you have to fit the B tension screw backwards in some shimano RDs to make it work on large cassettes.

(*) and this varies with the exact position of the B pivot. Basically the position of the top pulley is determined by the forces acting on the RD; there are two chain pulls and two springs at work. The two springs fight one another, so increasing the b tension has much the same effect as decreasing the A-tension. You can see that the effect of the upper chain pull (from the cassette to the top pulley) will vary with the sprocket size and the b pivot location. If the upper chain pull is (when viewed from the side) above the b pivot it pulls the mech one way and if it is below it, it pulls it the other way.

Fortunately it is rare that standard hangers are so different that they will only accept one type of mech.

cheers

[Valbrona]

And when frame designers check to see if what they have designed actually works they typically do so with just Shimano parts, hence the phenomenon of 'Shimano-friendly' bikes, at least in the case of high-end road machines.