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Pairing a Shimano 105 Rear Derailleur

Old 02-12-17, 09:24 PM
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Pairing a Shimano 105 Rear Derailleur

Hi,
I'm in the market for a new road bike, and I do quite a few large hills for my size, so I've been very happy with my current 11-34T rear cassette. 34 (or more!) is a must for my new bike.

Thing is, if I whittle the world of new endurance bikes down to those that come standard with 34T, its a very small selection. And I'd really like to go with 105 groupset (not Tiagra or lower).

There are a few standard models I like that come with an 11-32, and a slew that come with 11-28.

So a couple things I'm trying to figure out:
a) If I get a bike with the 11-32 (105 GS), I presume all I need to do is slap on a new 11-speed 11-34 cassette, I'll be good to go? Maybe need a longer chain too? The Shimano website says the GS is 32T compatible, but then also says total capacity is 37T. That would be great. I would think the crank size would have an impact on this too (at least chain length). Crank likely to be compact 50/34. The Shimano 105 GS webpage

b) If I get a bike with an 11-28 (105 SS), the Shimano website says that is 28T compatible, but yet again says its total capacity is 33T. Does that imply an 11-32 would work? The Shimano 105 SS webpage

I'd really like to get an idea of how wide I can cast my net for a new bike, and how much extra $$ I'd need to spend on rear derailleur, cassette, and anything else (chain, etc?).

TLDR - I am trying to figure out the parameters for pairing a rear derailleur, between cassette/crank/chain and anything else.

Many thanks for any help!
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Old 02-12-17, 09:50 PM
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For Shimano rear derailleurs "maximum sprocket" is the largest rear cog it will shift to on an average derailleur hanger. "Total capacity" is the total difference between small and large cogs plus the difference between small and large chainrings. So in your first example:
(32-11) + (50-34) = 37t total difference, which is right at Shimano's maximum recommendation.

Now, Shimano is very conservative. Installing a cassette with a 34 tooth large cog should work fine if it's adjusted properly by a good mechanic who knows that he's going beyond the recommended limits.

My day-to-day bike has a 11-34 cassette and a 24-42-53 triple, so my "total capacity" should be 52. Shimano rates the derailleur at 43 teeth... but I don't ride in the small chainring and small cogs, so I don't worry about the chain hanging loose (much). I don't use the small chainring often, but today's ride had a couple miles of 7 to 8% climb... and I'm old, out of shape, and I've been off the bike for a couple months. I needed those gears.
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Old 02-12-17, 09:56 PM
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The total capacity is the amount of chain which can be taken up, not the largest cog size which can be accommodated. It is calculated by taking the difference between the big and small chainrings and the big and small cogs and summing the two together. So for example "a" it is (50-34)+(32-11)=16+21=37.

The largest cog capacity is a separate specification and is simply the largest cog which is guaranteed to be accommodated. This can often be exceeded but it is not certain that it can, nor by how much.
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Old 02-13-17, 05:01 AM
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I would get the bike that fits me the best and modify the gears to suit my riding needs. Gearing is easy to fix, bike fit can be problematic.

If the bike you get has 11-32 gearing I would put your existing 11-34 (from you old bike) on to the new one and see how it works. If its good happy days, if not fine tuning may help.

If the new bike comes with something like an 11-28 you will probable need to replace the cassette (11-34), increase the chain length (to accomodate more teeth) and the rear derailleur (longer cage). I would (did) put a mountain bike derailleur on your new bike. It would give you a much greater range of gearing options.

My current road bike runs 105 (5703) 10 speed shifters, a 105 triple crankset, a 9 speed XT rear derailleur and either an ultegra 12-30 or XT 11-36 (depending on how hilly and heavy I'm loaded and what wheels I'm using) For me this system allows great flexibility and works very well for my bikes purpose. Some people might suggest that the shifting performance is not as good as it could be but it is still very good and it excedes my expectations.

If you do choose to go down this road there are a lot of details you will need to ensure before buying components. Eg pull ratios between shifters and derailleurs; 10 speed road shifters are not compatible with 10 speed rear mtb derailleurs but are with 9 speed. I'm not sure how compatible 11 speed road shifters are with 9 speed rear mtb derailleurs.

Last edited by Donnie Johnson; 02-13-17 at 05:04 AM.
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Old 02-13-17, 06:22 AM
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Originally Posted by Lightchop
a) If I get a bike with the 11-32 (105 GS), I presume all I need to do is slap on a new 11-speed 11-34 cassette, I'll be good to go?
There are no 11-34t 11spd cassettes available from Shimano or Sram. The only company that seems to be making them is IRD and they cost as much as a Dura Ace cassette. They also seem to be scarce (possibly vapourware) and shops either do not list them or are out of stock.
Originally Posted by Lightchop
b) If I get a bike with an 11-28 (105 SS), the Shimano website says that is 28T compatible, but yet again says its total capacity is 33T. Does that imply an 11-32 would work? The Shimano 105 SS webpage
An SS short cage RD is really designed for a 28t cassette maximum. Depending on how long your derailleur hanger is, you may be able to make it work with a 32t by adjusting the b-screw. But it's not ideal. Another option to give you more clearance is a Wolftooth Road Link that attaches to your hanger and extends the RD to give more clearance for a 32t.

Another option. If any of the bikes come with a compact 5-arm 110 bcd crankset. You could fit a 33t inner chainring instead of a 34t cassette. It wont be quite a low gear as a 34t cassette but it does make a difference.

An 11-36t cassette (Sram) is the next option and the same applies. the GS long cage is really designed for a 32t cassette maximum. But you can experiment and/or use a wolf link.

Last edited by trailflow1; 02-13-17 at 07:35 AM.
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Old 02-13-17, 08:02 AM
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Originally Posted by Lightchop
TLDR - I am trying to figure out the parameters for pairing a rear derailleur, between cassette/crank/chain and anything else.
It was mentioned already, so ill just 2nd the wolftooth roadlink. google it. $20 for a piece of nicely finished aluminum which will take care of your problem.

Get a SRAM PG1130 cassette which is 11-36, use the wolftooth, and make sure its mated to a 5800GS derailleur. the GS is Shimano's mid-cage length and will be able to handle the chain necessary for all this.


One disclaimer with all this is that I use a 46-34 crankset and you want to use a 50-34 crankset which puts the total tooth capacity at 41t. That is 4t over the states 37t limit.
My guess is that everything would be good. I know for sure everything is good at the max 37t which is what I run.
With that said- perhaps ask yourself if you need a 50t ring. How often are you on the 50/11 combo?
A 46/11 combo is the same gear inches as a 50/12 combo. Ride for 15min straight in the 50/12 combo and see if you really need that 50/11 gearing. If you dont, you could use the 46/34 chainring combo, not miss anything, and get a sweet setup for sure.
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Old 02-13-17, 08:31 AM
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Many thanks for the tuition everyone! This makes sense.

I'm starting to wonder if I'll just never have enough gears. I'd like to avoid a front triple. And I would prefer to stay within the blessed cassette size. I'll be too upset the day a mech tells me my problem is because I'm going past the product specs.

I might just be jaded from having bottom-of-the line components on my existing bike (Claris everything, except for my climbing package I put on which is Alivio RD and 11/34 cassette). It's been a constant struggle keeping that running smooth, hence my desire to go to a better, blessed groupset out of the box and just keep it that way.

Oddly the Shimano Tiagra GS allows for 39T total capacity, so seems I could pair an 11-34 with compact 50/34. Granted its 10 speed. That might be my perfect mix of "blessed setup" with top gearing.

Final question - the Tiagra GS page says 39T capacity - but how do you interpret these specs:
Maximum Sprocket: Low sprocket: 34T / Top sprocket:12T (front double)
Minimum Sprocket: Low sprocket: 28T / Top sprocket:11T (front double)
Maximum Front Difference: 16T (front double)
Total Capacity: 39T (front double)

Does that imply with a compact 50/34 that I can only (in a blessed fashion) use a 12/34, or does it allow for an 11/34? The Specialized Roubaix SL4 comes standard with a Tiagra groupset, compact 50/34 & 11/34 cassette. I do love the looks of the Roubaix, I just think at 250 pounds I might crack some carbon, hence my general desire for Aluminum.
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Old 02-13-17, 08:45 AM
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Originally Posted by Lightchop
I'm starting to wonder if I'll just never have enough gears. I'd like to avoid a front triple.

The triple would certainly give you the lower gears you want, without the large jumps that an 11-34 cassette will have.
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Old 02-13-17, 08:58 AM
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Originally Posted by mstateglfr
It was mentioned already, so ill just 2nd the wolftooth roadlink. google it. $20 for a piece of nicely finished aluminum which will take care of your problem.

Get a SRAM PG1130 cassette which is 11-36, use the wolftooth, and make sure its mated to a 5800GS derailleur. the GS is Shimano's mid-cage length and will be able to handle the chain necessary for all this.


One disclaimer with all this is that I use a 46-34 crankset and you want to use a 50-34 crankset which puts the total tooth capacity at 41t. That is 4t over the states 37t limit.
My guess is that everything would be good. I know for sure everything is good at the max 37t which is what I run.
With that said- perhaps ask yourself if you need a 50t ring. How often are you on the 50/11 combo?
A 46/11 combo is the same gear inches as a 50/12 combo. Ride for 15min straight in the 50/12 combo and see if you really need that 50/11 gearing. If you dont, you could use the 46/34 chainring combo, not miss anything, and get a sweet setup for sure.
Yes that product got me excited. However the 14T max (as per their product page), as you point out, will not mate with any of the standard road bike cranks. Literally every crank I see on my hit list of 40+ bikes is 16T (52/36, 50/34, 48/32, 46/30). I get your point that I could slap on a smaller width crank, but I'm back to tinkering around with the setup a lot.

Whats funny about all of this is that there surely is a market for standard, larger gearing on a road bike. I cant be alone, and I have passed many (granted probably novice) riders that have to stop on large climbs as they try to push their 11/28 up a 10-12% steady grade.

FWIW - the hill that I am due to climb in 2+ months that is freaking me out. Check out the climb from mile 29. Its 1,200 feet in 2 miles!

Maybe I just need to lose some weight That might just be the path of least resistance!
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Old 02-13-17, 09:01 AM
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Originally Posted by Lightchop
Oddly the Shimano Tiagra GS allows for 39T total capacity, so seems I could pair an 11-34 with compact 50/34. Granted its 10 speed. That might be my perfect mix of "blessed setup" with top gearing.

Final question - the Tiagra GS page says 39T capacity - but how do you interpret these specs:
Maximum Sprocket: Low sprocket: 34T / Top sprocket:12T (front double)
Minimum Sprocket: Low sprocket: 28T / Top sprocket:11T (front double)
Maximum Front Difference: 16T (front double)
Total Capacity: 39T (front double)

Does that imply with a compact 50/34 that I can only (in a blessed fashion) use a 12/34, or does it allow for an 11/34?
A different way to read this is-
- It needs a small cog that is 11t or 12t.
- It needs a large cog that is between 28t and 34t.

Now as you can guess by the theme in this thread, 'need' is very loosely defined as Shimano(understandably) is conservative with its limitations.
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Old 02-13-17, 09:50 AM
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Originally Posted by Lightchop
Yes that product got me excited. However the 14T max (as per their product page), as you point out, will not mate with any of the standard road bike cranks. Literally every crank I see on my hit list of 40+ bikes is 16T (52/36, 50/34, 48/32, 46/30). I get your point that I could slap on a smaller width crank, but I'm back to tinkering around with the setup a lot.

Whats funny about all of this is that there surely is a market for standard, larger gearing on a road bike. I cant be alone, and I have passed many (granted probably novice) riders that have to stop on large climbs as they try to push their 11/28 up a 10-12% steady grade.
Yes, there is a big market for cranksets with a smaller large ring. And perhaps a smaller small ring too.
Thing is- a lot of riders(all being recreational, I would guess) dont realize it. They spin slowly at 50rpm in some tough gearing even though that isnt ideal. Many never actually need 50/11 as they cant sustain that for more than a minute or two on the flats at an ideal RPM.

I have 46/34 on a gravel bike with 11/36 cassette, 50/34 on a road bike with 11-30 cassette, 48/34 on a road bike with 13/23 cassette, and a couple of triple cranks on 2 touring bikes.
I ride a lot, but am also bigger than average in height and weight. I dont need and wouldnt miss the 50t ring. I could easily use 46t on my roadbikes and be perfectly fine without missing that extra little bit that I use for a very very short amount of time.

Same applies on the rear end- you can make the gear ratio smaller with a smaller chainring or a larger cassette(or both). 34/28 works for most avid cyclists on the road. But certainly not everyone.



I think this is all from a couple reasons-
- want to be like the pros.
- fewer changes mean lower costs when spec'ing a bike. 50/34 and 11/28 works for most, so that is what's spec'd at best on the higher end groupsets. Those who then pursue a better individual setup are in the vast minority and work to figure out how to customize their setup(tinker).
A Shimano CX50 crankset with 46/36 rings can be had for $70. A 34t ring can be had for $12. That crankset is 105 level quality. Its a good option to use for changing to being able to use a full cassette more often.
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Old 02-13-17, 08:26 PM
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Originally Posted by Lightchop
The Specialized Roubaix SL4 comes standard with a Tiagra groupset, compact 50/34 & 11/34 cassette. I do love the looks of the Roubaix, I just think at 250 pounds I might crack some carbon, hence my general desire for Aluminum.
I doubt you will have any problems with the Tiagra group set, granted its ONLY 10 speed! It has the gear range you are interested in and the Tiagra group set is reliable. I will probably put Tiagra on my everyday (most ridden) bike when the time comes. I would doubt that in a Double Blind Randomised Control Trial that the average cyclist would notice any difference between 10 and 11 speed across the range of gears you are considering (if they didn't count).
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Old 02-13-17, 10:00 PM
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Originally Posted by Lightchop

FWIW - the hill that I am due to climb in 2+ months that is freaking me out. Check out the climb from mile 29. Its 1,200 feet in 2 miles!

Maybe I just need to lose some weight That might just be the path of least resistance!
I don't know- that big hill is pretty close to the beginning. It's the little 10-12% spike at mile 79 that would kill me.

I did this a few years back: https://ridewithgps.com/routes/1055578 . Check out mile 4.3... 20%!
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