I think I understand derailleur compatibility now but would appreciate someone to check for me to prevent an expensive mistake. I'd like to buy a Shimano Deore XT Di2 M8250 which I've seen for a good price to fit to either a 2016 Trek Stache 7 or a 2023 Trek Fuel 9.8 XT. I noticed looking at the specs for the derailleur it said Direct attachment (conventional) which I thought might be a new standard the bike's not compatible with however I think it's the other way round and the older derailleurs support direct mount and a normal hangar but the new ones only support a conventional hanger.
Looking at the 10 speed Deore XT derailleur on one bike and the 11 speed M8050 on the Stache, they both have a small link where it connects to the bike. I'm assuming that this link is there to enable compatibility with a normal hanger and if my bikes have this link present, they are not direct mount and therefore would work with the 8250?
The Fuel lists the frame spec as follows:
OCLV Mountain Carbon, internal storage, angle-adjust headset, Mino Link adjustable geometry, adjustable leverage rate, guided internal routing, magnesium rocker link, 34.9 mm seat tube, ISCG 05, 55 mm chain line, BSA 73, downtube guard, shuttle guard, ABP, UDH, Boost148, 140 mm travel
Again I mistakenly thought UDH might mean something to do with direct mount but on reading up on that, it's standard hanger design that's not frame specific so would work fine with this derailleur?
by JohnMcL7
3 Comments
It attaches to a hanger like a normal derailleur.
There are a couple different standards at play here.
Standard (direct) attachment: The age-old derailleur hanger that *nearly* all derailleurs are in some way, shape, or form compatible with (with the exception of Transmission). **The RD-M8250 follows this standard.**
Shimano Direct Mount: This is what you are asking about. Shimano direct mount derailleurs can either attach directly to Shimano Direct Mount hangers, or to standard attachment hangers by the use of a B-link. Modern Shimano road derailleurs follow this standard, as did older Shimano mountain bike derailleurs, but they have since dropped it for new mountain bike derailleurs. **The RD-M8250 does** ***not*** **follow this standard.**
SRAM UDH: This is a standardized format of **derailleur hangers**, and all UDH bikes share the same UDH hanger. UDH can be adapted to either standard/direct attachment using a standard UDH hanger, or it can also be adapted to Shimano Direct Mount with an UDH-to-Direct Mount hanger (less common), meaning it can fit either standard derailleurs or Direct Mount derailleurs with the right hanger. Furthermore, Direct Mount hangers can also fit on the standard UDH hanger using the B-link (this may be how your current 10-speed XT or 11-speed M8050 is currently attached on your Fuel in particular).
SRAM Transmission: This is a new format of **derailleur** that skips the hanger entirely and bolts straight to a UDH frame, meaning it does not work on anything other than UDH frames. Not particularly relevant here, but it might help to resolve your understanding of UDH.
One of your frames is a UDH frame using a standard UDH hanger, the other is not but appears to be using a standard attachment hanger. Both will work fine with the RD-M8250. It should be able to bolt in directly so long as you remove the B-link.
There is no B-link. But you can’t use a direct mount hanger. You just attach straight to the normal derailleur hanger.