Author Topic: Road links and derailleur capacity  (Read 7212 times)

quixoticgeek

  • Mostly Harmless
Road links and derailleur capacity
« on: 08 October, 2018, 08:12:49 pm »

On my new bike I'd like to get some really low gears for easy spinning up hills when I've got so much distance in my legs that grinding is not an option.

My current plan is a FSA 46/30 chainset, with a 14-34 rear cassette. But I'd really like even lower. Thing is Shimano don't seem to do any cassettes between 34 and 40, at least in 11 speed, that I can find. I have a Wolf tooth components road link, so I can move the derailure out a bit to fit round the 40t dinner plate. But I then run into issues with derailure capacity. I'm looking at the Shimano RX805 dereilure, that has a capacity of 39t. But, (46-30)+(40-14) is 42.

What happens if you go over the capacity of a derailure?

J
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mcshroom

  • Mushroom
Re: Road links and derailleur capacity
« Reply #1 on: 08 October, 2018, 08:22:44 pm »
I don't use any 11sp stuff, but assuming that Shimano/SRAM are still as interchangeable as they were 7-10sp, then this would give you an 11-36
http://www.chainreactioncycles.com/sram-pg1130-11-speed-road-cassette/rp-prod123725

As a side note, having bought mainly 8sp stuff for ages, 11sp isn't half expensive!
Climbs like a sprinter, sprints like a climber!

quixoticgeek

  • Mostly Harmless
Re: Road links and derailleur capacity
« Reply #2 on: 08 October, 2018, 08:24:40 pm »
I don't use any 11sp stuff, but assuming that Shimano/SRAM are still as interchangeable as they were 7-10sp, then this would give you an 11-36
http://www.chainreactioncycles.com/sram-pg1130-11-speed-road-cassette/rp-prod123725

As a side note, having bought mainly 8sp stuff for ages, 11sp isn't half expensive!

Are SRAM and shimano cassettes interchangeable?

It's not that expensive in the grand scheme of things, and it's the only option if you want di2.

J
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zigzag

  • unfuckwithable
Re: Road links and derailleur capacity
« Reply #3 on: 08 October, 2018, 08:38:41 pm »
it should be alright, but i would go for a really low ratio. i ran 30-36 as my bottom gear and was wishing for lower on several steep climbs.

quixoticgeek

  • Mostly Harmless
Re: Road links and derailleur capacity
« Reply #4 on: 08 October, 2018, 08:40:10 pm »
it should be alright, but i would go for a really low ratio. i ran 30-36 as my bottom gear and was wishing for lower on several steep climbs.

Lower than 30 front 40 rear?

J
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zigzag

  • unfuckwithable
Re: Road links and derailleur capacity
« Reply #5 on: 08 October, 2018, 08:46:49 pm »
30-40 is fine, i was wishing for similar ratio. super low gears also provide more control when climbing on loose surfaces.

Kim

  • Timelord
    • Fediverse
Re: Road links and derailleur capacity
« Reply #6 on: 08 October, 2018, 08:47:33 pm »
What happens if you go over the capacity of a derailure?

The tensioning function of the rear derailleur runs out of travel and the non-drive-side chain goes slack when combinations of smaller sprockets/chainrings are selected.  You can get away with this on recumbents (where the weight of the length of non-drive-side chain can serve to maintain tension, and idlers/tubes keep things lined up), but I've not tried it on a bike with a more normal chain length.

If you exceed the capacity of a front derailleur, eventually the chain ends up rubbing against the bottom of the cage in the small chainring.  You can usually exceed the manufacturer's spec a bit.  I do on my tourer, to the effect that I get a little rubbing if the chain is turning but not under load (ie. when soft pedalling to change gear or backpedalling), but there's just enough clearance that this disappears when actually winching up a bastard hill in the undersized granny ring.

Re: Road links and derailleur capacity
« Reply #7 on: 08 October, 2018, 09:09:47 pm »
One other thing to worry about is that once you've jacked a small mech out for a dinner plate it'll end up too far from the small sprockets to shift between them effectively. That was certainly my experience with a SRAM 11-36 and a Shimano medium cage road mech.

With Di2 you also have the option of fitting an XT rear mech, which is designed for cassettes 11-46t! The downside is you have to also get an MTB front mech, since the front and rear mech need to be the same type (MTB vs road) as each other.

quixoticgeek

  • Mostly Harmless
Re: Road links and derailleur capacity
« Reply #8 on: 08 October, 2018, 09:29:43 pm »
One other thing to worry about is that once you've jacked a small mech out for a dinner plate it'll end up too far from the small sprockets to shift between them effectively. That was certainly my experience with a SRAM 11-36 and a Shimano medium cage road mech.

With Di2 you also have the option of fitting an XT rear mech, which is designed for cassettes 11-46t! The downside is you have to also get an MTB front mech, since the front and rear mech need to be the same type (MTB vs road) as each other.

I considered this, but alas, the Shimano XT front mech has max capacity of 10 teeth. So I wouldn't be able to use the 30/46 front end.

Max capacity at the front with XT/XTR is 10t, meaning a 38/28 front. Coupled with upto an 11-42 back. Which, running the gears gives:

Lowest gear: 1.5m
Highest gear: 7.4m

Which, at 90rpm, gives me 39.5kph top speed, and 8kph at the bottom. But if I am OK grinding at 60rpm, 5.3kph.

which compares to the 6.3kph at 60rpm on my current setup (40/28, 11-34)

J

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Re: Road links and derailleur capacity
« Reply #9 on: 08 October, 2018, 09:31:36 pm »
Ignoring Di2 for a moment - you can usually get Shimano derailleurs to work quite satisfactorily a long way outside it's rated capacity - even without the wolf-tooth.
On an old Ultegra with a rated capacity of 11-27 cassette and a 22T difference at the front, I've quite happily running an 11-32 cassette and a 24T difference at the front (customised triple) 

I read somewhere the new Shadow style road mechs (R7000 and R8000) rated for 11-34 are happy using the SRAM 11-36 although you do need to wind the B-screw in a bit, however I can tell you that with an 11-34 at the back and a 52-39-30 triple up front the chain does go slack as Kim describes on small-small combinations  :facepalm:

You can tell I've been exploring low gear options for while can't you ?
 

quixoticgeek

  • Mostly Harmless
Re: Road links and derailleur capacity
« Reply #10 on: 08 October, 2018, 09:38:56 pm »


Holy crap, I've just seen the price of the XTR rear mech. It's twice the price of the Ultegra... maybe that isn't an option...

J
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Beer, bikes, and backpacking
http://b.42q.eu/

jiberjaber

  • ... Fancy Pants \o/ ...
  • ACME S&M^2
Re: Road links and derailleur capacity
« Reply #11 on: 08 October, 2018, 10:22:37 pm »
Ignoring Di2 for a moment - you can usually get Shimano derailleurs to work quite satisfactorily a long way outside it's rated capacity - even without the wolf-tooth.
On an old Ultegra with a rated capacity of 11-27 cassette and a 22T difference at the front, I've quite happily running an 11-32 cassette and a 24T difference at the front (customised triple) 

I read somewhere the new Shadow style road mechs (R7000 and R8000) rated for 11-34 are happy using the SRAM 11-36 although you do need to wind the B-screw in a bit, however I can tell you that with an 11-34 at the back and a 52-39-30 triple up front the chain does go slack as Kim describes on small-small combinations  :facepalm:

You can tell I've been exploring low gear options for while can't you ?
 

I wonder if the clutch versions of RD now available (shadow-plus on DI2 etc) stop that chain slap you describe?
Regards,

Joergen

Re: Road links and derailleur capacity
« Reply #12 on: 08 October, 2018, 10:55:26 pm »
Holy crap, I've just seen the price of the XTR rear mech. It's twice the price of the Ultegra... maybe that isn't an option...

XTR = Dura-Ace
XT = Ultegra

Both are available in Di2. The only thing unique to XTR is the Di2 triple front mech.

quixoticgeek

  • Mostly Harmless
Re: Road links and derailleur capacity
« Reply #13 on: 08 October, 2018, 10:57:30 pm »
Holy crap, I've just seen the price of the XTR rear mech. It's twice the price of the Ultegra... maybe that isn't an option...

XTR = Dura-Ace
XT = Ultegra

Both are available in Di2. The only thing unique to XTR is the Di2 triple front mech.

XTR comes in SGS and GS versions. XT comes in GS only. XT GS has max capacity of 35, XTR SGS has max capacity of 41.

J
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Beer, bikes, and backpacking
http://b.42q.eu/

Re: Road links and derailleur capacity
« Reply #14 on: 09 October, 2018, 01:01:59 am »
What happens if you go over the capacity of a derailure?

The tensioning function of the rear derailleur runs out of travel and the non-drive-side chain goes slack when combinations of smaller sprockets/chainrings are selected. ....   

if you set the chain long enough to use big-big, like you should do. 

I've only worked on a few setups using a hanger extender but one that worked really well used a long cage 105 mech (5800 series) ; this shifted really well on the smaller sprockets even though the guide pulley was miles away from them (the mech was set to clear a 40 or 42 or something).  I suspect that

a) the 'long teeth' on the guide pulley of that mech (which are becoming more widespread on shimano road mechs now, but then were rare, eg not on the short cage version of the 5800 series RD) were in good part responsible and

b) that the shifting performance would be likely to degrade more quickly than normal once everything starts to get a bit worn.

But with fresh parts it all worked really well.

cheers

Re: Road links and derailleur capacity
« Reply #15 on: 09 October, 2018, 10:23:28 am »
Stable doors and all that, but...
One would have thought that the correct action would have been to stump up for a proper super-compact chainset like the Sugino OX601.

This uses standard 110/74 chainrings, which allow an inner down to 24T for steep hills, and which are both relatively cheap and fairly easy to find.
Using a smaller chainring keeps the intervals between gears smaller, and doesn't require expensive cassettes.

(Assuming your FSA chainset uses an FSA-specific BCD, which was the case for the last one I looked at)


Re: Road links and derailleur capacity
« Reply #16 on: 09 October, 2018, 11:20:46 am »
Caveat - I haven't had the chance to read it all, but this article seems to be what you're looking for, albeit non-Di2, though the article concludes you could use the MTB mechs for a Di2 set-up.

https://road.cc/content/feature/246424-how-get-ultra-low-gearing-gravel-bike-adventures?qt-more_features_sidebar_tabs=0&page=1

We are making a New World (Paul Nash, 1918)

Kim

  • Timelord
    • Fediverse
Re: Road links and derailleur capacity
« Reply #17 on: 09 October, 2018, 12:22:23 pm »
What happens if you go over the capacity of a derailure?

The tensioning function of the rear derailleur runs out of travel and the non-drive-side chain goes slack when combinations of smaller sprockets/chainrings are selected. ....   

if you set the chain long enough to use big-big, like you should do.

Indeed.  Too short a chain is asking for trouble.  Even if your shifters are of a type that tell you what gear you're currently in, even if your chainrings are right there in front of your view of the road ahead, at some point you won't be paying attention and will make that shift.  Probably as you put the welly on at the start of a climb, or as you downshift on approach to a junction.

There are only two reasonable reasons for running too short a chain:  Either because you've had to remove links to fix it mid ride, or because you're test-riding someone else's recumbent.

Hm, wondering if the electronic systems can keep you out of big:big reliably enough to usefully shorten the chain?

Re: Road links and derailleur capacity
« Reply #18 on: 09 October, 2018, 12:49:09 pm »

Hm, wondering if the electronic systems can keep you out of big:big reliably enough to usefully shorten the chain?

that is an interesting thought...  if that is the only reason for using them it does seem like using a sledgehammer to crack a nut though...

cheers


Re: Road links and derailleur capacity
« Reply #19 on: 09 October, 2018, 01:54:18 pm »
Yes, they can be programmed to auto-shift at certain combinations. Well Di2 can.
We are making a New World (Paul Nash, 1918)

Re: Road links and derailleur capacity
« Reply #20 on: 09 October, 2018, 03:07:00 pm »
Mine (50-34 and 11-28) likes to shift to the second largest sprocket before dropping to the small ring (and a smaller sprocket), so you’d only be saving a little bit. I think you might be able to change the shift points with the Windows software though (I only have the phone app).

The front shifter switches remain in use in synchro mode (unless you globally remap them) and I don’t think you can disable full manual mode.

There’s also the problem that adjusting the front mech is done in big-big gear. I don’t think you *can* adjust it in any other gear.

Re: Road links and derailleur capacity
« Reply #21 on: 09 October, 2018, 03:42:56 pm »
Mine (50-34 and 11-28) likes to shift to the second largest sprocket before dropping to the small ring (and a smaller sprocket), so you’d only be saving a little bit. I think you might be able to change the shift points with the Windows software though (I only have the phone app).

You can use the iPad app to re-programme the shift points. Can't remember if the iPhone app will.

Quote
There’s also the problem that adjusting the front mech is done in big-big gear. I don’t think you *can* adjust it in any other gear.

You're correct that micro-adjust of the front mech canm only be done in big-big - which means overriding the syncro mode.  It's dead easy the change the modes from manual/semi-syncro/syncro though, just a few quick button presses.
We are making a New World (Paul Nash, 1918)

quixoticgeek

  • Mostly Harmless
Re: Road links and derailleur capacity
« Reply #22 on: 09 October, 2018, 06:09:08 pm »
Stable doors and all that, but...
One would have thought that the correct action would have been to stump up for a proper super-compact chainset like the Sugino OX601.

Except they have been discontinued.

Quote

This uses standard 110/74 chainrings, which allow an inner down to 24T for steep hills, and which are both relatively cheap and fairly easy to find.
Using a smaller chainring keeps the intervals between gears smaller, and doesn't require expensive cassettes.

(Assuming your FSA chainset uses an FSA-specific BCD, which was the case for the last one I looked at)

FSA have a direct mount thing for the outer ring, and a 90mm BCD for the inner ring.

J
--
Beer, bikes, and backpacking
http://b.42q.eu/

Re: Road links and derailleur capacity
« Reply #23 on: 09 October, 2018, 07:28:08 pm »
You can use the iPad app to re-programme the shift points. Can't remember if the iPhone app

I’ve just checked and again and it doesn’t appear to. I don’t currently have an iPad.

Quote
You're correct that micro-adjust of the front mech canm only be done in big-big - which means overriding the syncro mode. 

Not if your chain isn’t long enough for big-big, which is the scenario being posited!

Re: Road links and derailleur capacity
« Reply #24 on: 09 October, 2018, 07:39:53 pm »
Set up with a dummy chain then.
We are making a New World (Paul Nash, 1918)