Frictions differences between Ultegra RX DI2 derailleur and Ultegra DI2 classic derailleur

Discuss light weight issues concerning road bikes & parts.

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by romanmoser

Hi !
Technical question for a change :unbelievable:

I need a DI2 Ultegra long cage derailleur
It is for my aero disc road bike, I rode in the south part of Belgium , roads are 50% bads , holes etc then 25 % okayish and 25 % good roads .
I hate cobbles and try to avoid them because of my light weight even if I did the Ronde Van Vlanderen Gran fondo twice the middle distance and the longer one .

I've read that intersting article about frictions ( and power loss thus ) in a 1X transmission .
The writter is sponsored by Sram but I guess he could use 2X ETAP or something too ...

The conclusion is that the minimal losses in power consecutives to the clutch
are compensated by the fact that the chains don't dance everywhere and stays put
It's on a 1X tough

What's your opinion about me using a ultegra rx with clutch on my 2X road bike ( 26 to 30mm measured tires ) ?
Will I loose some energy ?
I guess maybe pros sponsored by shimano would use them in others races if they was no disadvantages
But sometimes pros team don't do the best choice and if they used the rx all season they wouldn't be on dura ace :!: ... rivetrains

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by spdntrxi

I cant notice anything while riding with it on or off. I use the RX Di2 RD with a 2x system. I feel a little more resistance when turning the cranks by hand, but that pressure is nothing compared to riding leg power.
2019 BMC TM01 Road UCI config 7.36kg

by Weenie

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by Multebear

romanmoser wrote:
Mon Sep 17, 2018 8:09 pm

What's your opinion about me using a ultegra rx with clutch on my 2X road bike ( 26 to 30mm measured tires ) ?
Will I loose some energy ?
There's something wrong with these two questions.

It's not about opinion. Either you will lose some watts or you wont. It's not something one could have an opinion about. It's something, that can be tested. Pretty sure someone tested it and posted it somewhere.

I ride with Ultegra RX rear mech on a 2x system. I don't feel any loss of watts. But if there is a loss, it probably wouldn't be more than 1-2 watts. And how would I ever feel that difference.

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by IrrelevantD

I have no actual data to support this, I'm just thinking this out logically here....

I would be very surprised if you saw more than a Watt difference between different deraileurs. AFAIK, the pulleys are the same, so the only place you really should lose power would be because you have a longer chain. All of the power transfer is hapening on the top side between the chainring and the cogs. All of the extra links of chain for the longer deraileur are on the slack side as the wheelbase doesn't change. Even at that, I don't know that simply having the extra links would inherently create additional friction as the pulleys being the same size aren't going to create any additional movement. Perhaps less due to the longer cages creating more open angles.

Now, if there is more friction in the bearings/bushings in the pulleys, that obviously would make a difference, but I think you would see more of a difference in power loss between 26mm and 30mm tires than in the length of a deraileur cage.
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by mattr

There was some work on the difference between non-clutched and clutched MTB mechs. It's bugger all *unless* you wind the tension right up. Even then, it's only a couple.

On my MTBs, i experimented and wound the tension off until the mech stopped working (bouncing chain) then added a tiny bit of clutch. Now they work (no bouncing chain) and barely noticeable drag (even spinning by hand).

The big tell tale is the extra shifting loads when you have lots of clutch tension.......

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by romanmoser

@Multebear : In the first question , I don't include any word about power loss , friction : right ?
In the second I did

It may seems paradoxal
Thanks to all for yours returns anyway :)

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by weiwentg

Resurrecting this thread because a) Cyclingtips did some testing that attempts to answer the general question if clutch RDs add drivetrain friction and b) I actually got a gravel bike with Ultegra Di2, and I got a deal on an Ultegra RX Di2 RD.

First, Cylingtips did some testing with Friction Facts, video version of the article here. Basically, Shimano clutch RDs seem to add about 3W of friction when you downshift to an easier gear, but not when you upshift. The issue is that their clutch gets partly loaded and doesn't reset itself. While partly loaded, it adds that 3W of friction. SRAM's RDs don't perform in this way, and they added no friction in either direction in testing.

The video chalked this up to an issue of manufacturing tolerances. Also, both the video and the article explain that your clutch can get reset if you downshift enough (about 5-6 teeth total) or if you hit enough of a bump. If you are on gravel or cobbles, then chances are that you'll hit such a bump fairly soon.

The article also seemed to suggest that the ideal setup has a high clutch tension but a low derailleur spring tension. You can adjust the clutch tension easily on an Ultegra RX RD. If you try this, do scroll down to the comment in this article by jamesb and the reply from Il Falcone. Jamesb said that his RX Di2 R was struggling to shift to larger cogs. Il Falcone suggested his shop had seen similar experience, that it could be corrected by reducing the clutch tension from the stock setting, and that the Ultegra RX Di2 RD's motor wasn't strong enough to shift with increased spring tension.

This is not actually my experience. When I put on my RX Di2 RD, I increased the clutch tension to near maximum. It does seem a bit slower to downshift. However, in a recent gravel event, it was able to downshift to all cogs, even in the big chainring. The guy I bought it from on the Paceline forum said that the motor wasn't strong enough for his 1x setup with a ~40t or larger max cog. I have a 2x setup with a 34t max.

I could not notice an increase in friction overall when I installed the RX RD. This probably just means I'm not very sensitive. I didn't do a full gravel event on the original Ultegra Di2 RD.

The original post is about an RX RD on a 2x road bike. The OP is likely to lose some watts in the scenario I specified, and I'm not sure I can see a strong rationale for this on most road bikes. Having my chain not bounce around was helpful on a gravel bike. On a road bike, you'd have to mostly be riding on cobbles to have the costs outweight the benefits, in my opinion.

by Weenie

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