Ultegra RX short cage

Discuss light weight issues concerning road bikes & parts.
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TheOnlyAaron
Posts: 15
Joined: Wed Mar 13, 2019 8:28 am
Location: Singapore

by TheOnlyAaron

Hi all, I'm looking to get an ultegra rx800 derailleur to complete my 1x build (42,11-30).

I was wondering since I am planning to use a 11-30 cassette, is it possible to swap out the RX800 GS (Med) cafe to a R8000 SS (short) cage. As per the Shimano EV, the RX800 shares the same outer plates as RD8050, so is it safe to presume that all the cages are interchangeable?

Also if it's possible to swap to a R8000 SS cage, does that mean by extension I can swap to a R9100 cage since they are interchangeable?

Below is the rx800 outer cage for reference.
E10FEF96-392B-46FF-B5FB-9391A6A8706B.jpeg
Thank you.

MagicShite
Posts: 55
Joined: Sun Jun 26, 2016 3:33 pm

by MagicShite

The knuckle is different. That is why you can't just change a new mid cage to the short cage and also expect it to do 11-32 smoothly.

The long cage is fine and intended for 11-30. Plus you get a better chain wrap. Why not just use the stock cage?

ooo
Posts: 884
Joined: Sat May 21, 2016 12:59 pm

by ooo

Try Shimano Zee RD-M640 (SS) short cage.
Spring fixing holes are closer to axle on RX, but that is easy to fix (it's just a holes without thread).
Stopper pin position distance to axle needs to be checked, if it is different - it will be harder to fix (it have thread inside).
If you can measure stopper pin distance to axle on your RX cage (center-to-center), I can compare it to mtb cage distance.
Attachments
Zee RD-M640 (SS)
Zee RD-M640 (SS)
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hlvd
Posts: 314
Joined: Sat Mar 05, 2016 4:54 pm

by hlvd

I tried the opposite on a 6800 going from small to medium, it doesn’t work, the derailleur pivots differently.


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ooo
Posts: 884
Joined: Sat May 21, 2016 12:59 pm

by ooo

Changing cage doesn't change cassette size compatibility, but it can improve chain slack. Photos & info @ handsonbike.
I have installed short cage (SS) on body of RD-6800-GS so I can use the same 11-32 cassette in 1x11 setup with less chain slack.

RX with short cage will be best for 1x11 with cassette up to 11-36, and for 2x11 up to 11-28T with front difference up to 10T.
Numbers may be different for di2 RX RD and mechanical RX RD.

For @TheOnlyAaron 1x build (42,11-30) it makes sense to try this.
Last edited by ooo on Tue Oct 08, 2019 12:57 pm, edited 2 times in total.
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WorkonSunday
Posts: 163
Joined: Tue Jan 16, 2018 4:39 pm

by WorkonSunday

does it need to short cage? i thought R8000 and RX800 series med cage starts from 11-28 to 11-34 ?
Some say pour 10ml water out of your bottle to save that last bit of the weight. Sorry, i go one step further, i tend to the rider off my bikes. :thumbup:

ooo
Posts: 884
Joined: Sat May 21, 2016 12:59 pm

by ooo

it doesn't need it, but for 1x11 you can use lower clutch tension with shorter cage, performance may be better, plus extra clearance.
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TheOnlyAaron
Posts: 15
Joined: Wed Mar 13, 2019 8:28 am
Location: Singapore

by TheOnlyAaron

ooo wrote:
Tue Oct 08, 2019 6:15 am
Try Shimano Zee RD-M640 (SS) short cage.
Spring fixing holes are closer to axle on RX, but that is easy to fix (it's just a holes without thread).
Stopper pin position distance to axle needs to be checked, if it is different - it will be harder to fix (it have thread inside).
If you can measure stopper pin distance to axle on your RX cage (center-to-center), I can compare it to mtb cage distance.
Righto will measure it when I recieve my RX derailleur.

As to why I want to change to a short cage, just for the minor weight savings (if changing to R8000 SS) and a little more weight savings if changing to (R9100).
Also cause I like the look of a smaller derailleur better as I ride on an XXS bike.
ooo wrote:
Tue Oct 08, 2019 9:10 am
Changing cage doesn't change cassette size compatibility, but it can improve chain slack. Photos & info @ handsonbike.
I have installed short cage (SS) on body of RD-6800-GS so I can use the same 11-32 cassette in 1x11 setup with less chain slack.
Thanks for the guide. Will try ASAP.

Thank you all for the feedback.
Will order a R8000 SS cage and update once I install it.

ooo
Posts: 884
Joined: Sat May 21, 2016 12:59 pm

by ooo

handsonbike guide can help if you use SS cage from RD-R8000-SS on RD-R8000-GS (not RD-RX800-GS)
For RX RD you need to use cage from Clutch RD, like Zee. I can give you some numbers from Di2 RX and Zee:

Zee distance from pulley to spring hole 11.15 mm
RX distance from pulley to spring hole 8.55 mm

aliexpress link = zee (saint) ss cage: "M640 M820 Inner SS" + "M820 Outer SS"

Image Image

Zee distance from pulley to stopper ping 17.83 mm
RX distance from pulley to stopper ping 16.93 mm

Image Image

Stopper pin hole on Zee cage is 0.5-1 mm too far from pulley axle - very likely compatible with RX RD body
Spring hole on Zee cage is 2.5 mm too far from pulley axle, you need to extend existing hole (closer to axle) or drill a new hole (better way)
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