50/34 crank and 11-36 cassette for gravel/road bike - changing out my 46/30

34 replies · 4,908 views

dodge77
15 posts
#1Sep 17, 2019, 5:06 AM
Hey guys,

Before I order the Shimano 105 crank 50/34 and Sram 11-36 cassette just after any thoughts on this combo.

Roadwheel set with GP5000, and a gravel wheelset with GravelKing SK 38. Currently on a FSA 46/30 + shimano 11-34, and use the granny gear a lot on the steep peak district hills.

I’m looking to change so I can fit my Stages 105 power meter, but don’t want to lose my climbing gear, and want the range and step up through gears to still be good.

Currently run the 46 up front mainly, and tend to be in the 11, 13, 15 or 17, 99% of the time. Then on the steep hills tend to be in 30 up front, with 30 or 34 out back.

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pkram
42 posts
#2Sep 17, 2019, 5:57 AMedited
My thoughts are less about the gearing, more about the components. I just want to highlight two things just in case you hadn't considered it: 1. There are 11 speed 11-36 cassette's from third party that are compatible with Shimano free hubs, so you don't need to change the free hub (unlike with the SRAM cassette). EDIT: Sorry I hadn't realised SRAM made cassette's compatible with Shimano free hubs.

2. I don't know what rear mech you are using but it's likely the chain wrap specification would be below what you will be doing with the new setup (16T + 25T = 41T, typical medium cage Shimano rear mech officially supports up 39T). It might work perfectly well, but officially you are outside the manufacturer tolerance.
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Brother Shamus
8 posts
#3Sep 17, 2019, 11:58 AMedited
Exactly as pkram alerts to. A 36 max low sprocket seems to be in the no-man's land between the RX800 and RX812, with a max 34t sprocket for the former and minimum 40t sprocket on the latter.
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dodge77
15 posts
#4Sep 17, 2019, 5:24 PM
Thanks guys, I went off many forums and sites which have tested the 105 medium rear mech up to 40t, without any kind of goatlink or messing. At most just a b-screw fettle.

https://road.cc/content/feature/246424-how-get-ultra-low-gearing-gravel-bike-adventures

Pkram hers the cassette, slides straight on Shimano 11 spd road hub

https://www.cxmagazine.com/sram-11-36-1170-cx1-cassette-new-product-cyclocross-gravel-2015
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pkram
42 posts
#5Sep 17, 2019, 9:50 PMedited
The chain wrap is different from the max sprocket supported. It relates to cross chaining: if you would ride 34Tx11T (and a have chain length that works with 50Tx36T) there is a lot slack on the chain that the rear mech needs to pick up. Again, most likely not an issue in real usage (especially if you avoid cross chaining). But make sure you cut the chain with this in mind though: maybe you can optimise the chain length.

Another thing on my mind: you plan to buy a left crank arm with power meter? or you will send in your own left crank arm? I ask because you might be able to use the 105 crank arm with power meter on a GRX crankset. In that case you can keep to the same chain ring sizes (46-30, or 48-31), in turn also the crank arm length (if you would prefer) and use your current cassette.

Lastly: make sure you lower you saddle slightly with the new crank arm length as the pedal will be lower on the down stroke.

Just sharing all my thoughts, good luck!
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pkram
42 posts
#6Sep 17, 2019, 9:55 PMedited
Brother wrote:RX800 and RX812, with a max 34t sprocket for the former and minimum 40t sprocket on the latter.

In addition the RX812 is intended to be used in a 1x system. It doesn't have the chain wrap (also known as chain capacity) to be able to support a 2x system. So although it would support a 40T sprocket it would not work with the intended cranksets as that would be 2x.
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dodge77
15 posts
#7Sep 18, 2019, 3:51 AM
pkram wrote:The chain wrap is different from the max sprocket supported. It relates to cross chaining: if you would ride 34Tx11T (and a have chain length that works with 50Tx36T) there is a lot slack on the chain that the rear mech needs to pick up. Again, most likely not an issue in real usage (especially if you avoid cross chaining). But make sure you cut the chain with this in mind though: maybe you can optimise the chain length.

Another thing on my mind: you plan to buy a left crank arm with power meter? or you will send in your own left crank arm? I ask because you might be able to use the 105 crank arm with power meter on a GRX crankset. In that case you can keep to the same chain ring sizes (46-30, or 48-31), in turn also the crank arm length (if you would prefer) and use your current cassette.

Lastly: make sure you lower you saddle slightly with the new crank arm length as the pedal will be lower on the down stroke.

Just sharing all my thoughts, good luck!

See this is the kind of thinking i'm after 😃 You may have just blown my mind with mixing 105 and GRX suggestion! lol

Would the Q-factor be the same on the 105 power meter left crank as the GRX? or is q factor set by BB? As in I wouldn't want my left foot to be closer inboard than my right etc. bar that its just hollowtech II 105 power, on hollowtech II grx crank i guess?

I much prefer the GRX 46-30 + 48-31! Could leave my rear cassettes alone then, which I have on 2 wheelsets.

I'll adjust the saddle, didn't think about that 🙂

I think my OCD would let me see past mismatching cranks, for a 48/31 which i'd probably swing for in GRX, for that nice long road stomping gear, and 31 much nicer than 34 for climbing still.

I'd probably need a new front mech if GRX works, but again prefer that to 50/34 + 11/36, rubbish chain wrap compromise, which i'm only doing to get my Power back.
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HollyBoni
333 posts
#8Sep 18, 2019, 3:58 AMedited
The problem with the RX812 is that it uses an offset upper pulley. While Shimano did move to this design even on 2x rear derailleurs in the MTB world, i'm not sure how it would work in a 2x setup.

IMO the 36T is a no-man's land because Shimano doesn't have an 11spd 11-36... When they come up with their specs, they most likely only consider their own cassettes... The 10spd GRX uses an 11-36, because Shimano does have (had) 10spd 11-36 cassettes.

OP, take a look at the 10spd GRX RD.
https://bike.shimano.com/en-AU/product/component/grx-10-speed/RD-RX400.html
It's just an old MTB RD from the 10spd days with a new logo. It uses the 11spd pull ratio like Tiagra 4700.
It's cheap, it has a clutch, 41T capacity, 36T max cog.

But really, plenty of people have exceeded the factory specs on a lot of RDs.
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dodge77
15 posts
#9Sep 18, 2019, 4:04 AM
So options

Q factor 146 weight 713.4g (50/34)
https://bike.shimano.com/en-EU/product/component/shimano105-r7000/FC-R7000.html

Q factor 151 weight 710g (48/31)
https://bike.shimano.com/en-EU/product/component/grx-11-speed/FC-RX810-2.html

Q factor 151 weight 806g (46/30) why so heavy - guessing 6xx is budget series?
https://bike.shimano.com/en-EU/product/component/grx-11-speed/FC-RX600-11.html

So 5mm Q Factor difference in 105 VS Grx, would that mean my 105 power meter would sit 2.5mm closer to chainstays/wheel? or 5mm? Wonder if Q factor is set by actual spindle length, or curvature in crank arm?

Sorry for rambling!

PS. if this works out they don't look that bad a match

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HollyBoni
333 posts
#10Sep 18, 2019, 4:08 AMedited
I have no experience with this, but Shimano moved the chainrings outward on GRX for more tyre clearance. You might not have enough travel in your current front derailleur. I've heard people running GRX cranks with road FDs, but they had to max out the adjustment screws.
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dodge77
15 posts
#11Sep 18, 2019, 4:19 AM
HollyBoni wrote:
The problem with the RX812 is that it uses an offset upper pulley. While Shimano did move to this design even on 2x rear derailleurs in the MTB world, i'm not sure how it would work in a 2x setup.

IMO the 36T is a no-man's land because Shimano doesn't have an 11spd 11-36... When they come up with their specs, they most likely only consider their own cassettes... The 10spd GRX uses an 11-36, because Shimano does have (had) 10spd 11-36 cassettes.

OP, take a look at the 10spd GRX RD.
https://bike.shimano.com/en-AU/product/component/grx-10-speed/RD-RX400.html
It's just an old MTB RD from the 10spd days with a new logo. It uses the 11spd pull ratio like Tiagra 4700.
It's cheap, it has a clutch, 41T capacity, 36T max cog.

But really, plenty of people have exceeded the factory specs on a lot of RDs.

Thanks Holly,

My plan would be to leave the 105 rear mech and ultegra 11-34 cassette alone, if I can get GRX 810-2 crank working.

So i'd hopefully plan to change to just GRX 48/31 crank, GRX front mech. / add 105 power meter.

I'd possibly need a new chain as my current is cut exact for 46/30.
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pkram
42 posts
#12Sep 18, 2019, 5:50 AMedited
dodge77 wrote:

Would the Q-factor be the same on the 105 power meter left crank as the GRX? or is q factor set by BB? As in I wouldn't want my left foot to be closer inboard than my right etc. bar that its just hollowtech II 105 power, on hollowtech II grx crank i guess?

I hadn't thought of the change in Q-factor... The BB does not affect the q-factor: GRX and 105 are recommended with the same BB anyway.
I will review and compare my GRX and 105 crankset to show you how the difference in q-factor is achieved. I expect that there will be a gap between the left crank arm and the BB if you would install the 105 left arm crank. You could potentially put a spacer in there, but I wouldn't personally go that route.

I'll see if I have some time tonight or tomorrow to make the comparison.
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pkram
42 posts
#13Sep 18, 2019, 6:13 AM
Also...

https://store.stagescycling.com/Stages-Power-L-Shimano-GRX-POWER-METER-Factory-Install
1
dodge77
15 posts
#14Sep 18, 2019, 9:17 AM
pkram wrote:Also...

https://store.stagescycling.com/Stages-Power-L-Shimano-GRX-POWER-METER-Factory-Install

I'm getting the stages shipped to a US address then shipped across to the UK, so don't want to have to post a new crank arm to them, but from what you've said i'm hopeful about the 105/grx mix 😃

Really appreciate the help, and look forward to your comparisons.

My current FSA Omega cranks are 146 Qfactor, with good rear chainstay clearance, so hoping the wider 151 GRX will let power meter easily.
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Cmtgravel
83 posts
#15Sep 18, 2019, 5:15 PM
Easy solution: keep the gearing, lose the power meter.
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dodge77
15 posts
#16Sep 19, 2019, 2:19 AM
Cmtgravel wrote:Easy solution: keep the gearing, lose the power meter.

I've ridden with power on my full sus for 4 years, invaluable to me now. Since getting a gravel bike, the full sus doesn't get a look in 😃 But I sorely miss my power meter lol
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Brother Shamus
8 posts
#17Sep 19, 2019, 9:36 AM
dodge77 wrote:
I sorely miss my power meter

Just curious: do you race?
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pkram
42 posts
#18Sep 19, 2019, 10:58 AMedited
dodge77 wrote:

I'm getting the stages shipped to a US address then shipped across to the UK, so don't want to have to post a new crank arm to them, but from what you've said i'm hopeful about the 105/grx mix 😃

i made a comparison and posted some pictures below. The GRX left crank arm has a different angle outward compared to the 105 arm. So that’s how the Q factor is achieved on the left side. Which means you can’t solve it with a spacer between the arm and BB...

it does mount on the spindle as per usual, so you could ride it but then the Q factor would be different on the left side compared the right side.

I am sorry if I got you too excited by giving an invalid suggestion...

-----
The following photos of the crank arms show the 2.5mm difference in outward position, while both cranks were installed flush against the BB. So the difference in Q-factor is simply achieved by having a more angled crank arm. (ignore the absolute values on the ruler and just compare the difference)
GRX:
105:
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pkram
42 posts
#19Sep 20, 2019, 12:06 AM
The problem can always be solved with spending more money though... Absolute black has a set of 46/30 chainrings that can fit the 105 7000 crankset. https://absoluteblack.cc/oval-road-chainrings-30-46-and-32-48-for-110-4bcd/

These are oval chainrings, but I assume there are also round sets from other suppliers available.
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dodge77
15 posts
#20Sep 21, 2019, 3:18 AM
pkram wrote:

i made a comparison and posted some pictures below. The GRX left crank arm has a different angle outward compared to the 105 arm. So that’s how the Q factor is achieved on the left side. Which means you can’t solve it with a spacer between the arm and BB...

it does mount on the spindle as per usual, so you could ride it but then the Q factor would be different on the left side compared the right side.

I am sorry if I got you too excited by giving an invalid suggestion...

-----
The following photos of the crank arms show the 2.5mm difference in outward position, while both cranks were installed flush against the BB. So the difference in Q-factor is simply achieved by having a more angled crank arm. (ignore the absolute values on the ruler and just compare the difference)
GRX:
105:

Thanks amazing pics and detail! I've done more digging and many say its impossible to feel the 2.5mm difference when riding as in the grand scheme of legs/knees etc its minuscule. So this could still be an option.

Another update Stages have replied to me and said they'll be offering the GRX crankarm proper for sale off the shelf in a few weeks 😃

I've done some sums on whether to go 46/30 or 48/31 and the latter seems to offer better stomping gear which i'd like for the road, but an exactly equal climbing gear as my current FSA. Is this due to longer crank arm length?

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