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42/28 double crankset with Tiagra 10 speed ST-405 shifters

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Cyclocross and Gravelbiking (Recreational) This has to be the most physically intense sport ever invented. It's high speed bicycle racing on a short off road course or riding the off pavement rides on gravel like : "Unbround Gravel". We also have a dedicated Racing forum for the Cyclocross Hard Core Racers.

42/28 double crankset with Tiagra 10 speed ST-405 shifters

Old 11-14-20, 04:36 PM
  #1  
paramount3
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42/28 double crankset with Tiagra 10 speed ST-405 shifters

I've done a fair bit of forum searching and have found some options to try, but I don't have high confidence in any of them. Hoping for some help. My situation:

I'm building up a road-biased gravel bike based on a steel Kona Rove ST frameset, 31.8 mm diameter seat tube, will run with 32-37mm tires. Kona sells the complete bike with a 1x11 drivetrain, chainline 45.5 mm. I want a wider gear range than the stock 11-42 range, and especially I want easier low gears. I have the first-generation Tiagra 2x2 hydraulic shift/brake levers ST-RS405 (which are functionally equivalent to the current ST-4720 levers). These levers are 10 speed, but with the same cable pull as current Shimano road 11 speed systems, which means that they can be used with current 11 speed front and rear derailleurs (eg the GRX RX810 and RX812), or with the few 10 speed derailleurs that use this pull, which as far as I know is restricted to the Tiagra 4700 series and the GRX 10 speed series (RX400 I think). Anecdotally, these levers don't work with Shimano MTB derailleurs due to differences in cable pull.

My question: What is my best option for a 2x10 setup to get my desired gearing? Specifically, which crankset and front derailleur should I use?

Ideally I'd have something like 26/36 low gear, and 44/11 top gear, but I'm willing to give more off the top if pressed. After trying a 1x10 setup, I have decided that a good compromise would be 2x10 with an 11-36 cassette with 42/28 or 40/26 double crankset. Maybe even 40/24 if I could get that to work.

I have a Shimano GRX RX812 rear derailleur that I will probably have to trade for an RX810, and I'd like to stick with one of these clutch rear derailleurs if possible.

Note: the two front derailleurs that should work with the shifters have chainline of 43.5 mm (FD-4700b) and 46 mm (FD-RX400). Note also that an FD-4700b is on its way to my mailbox now, while it seems impossible to get an RX400 front derailleur at the moment. FD-4700-b specifies a large chainring of 46-52t and a max capacity of 16t

Here's what I can try using (mostly) parts that I will have on hand:
1. Existing old square taper FC-M900 XTR triple crankset (47.5 mm chainline with 107mm bb spindle), running 42/28 on the middle/inner positions, which should give an effective chainline of about 44mm. Paired with FD-4700-b front derailleur. Possible issues: front derailleur might not work with the smaller-than-specified chainrings (42 vs 46t large, 28 vs 30t small). Possible chainline issues. Possible chainring spacing issues (or at least will be hard to choose the correct chainrings). Could I try chainring spacers?
2. Existing Shimano Deore FC-M615, 170 mm, 40/28, chainline 48.8 mm. Will try with the FD-4700-b even with the chainline mismatch. Might try one of various MTB FDs that I have. Possible issues: Same as option 1, but maybe more chainline issues. Could I try chainring spacers?
3. Sram (or RaceFace, etc) direct mount crankset with a 2x spider. I happen to have a brand new X0 (I think) set of 42/28 rings mounted on a spider, 120/80 BCD. I don't have a Sram direct mount crankset, however. Which crankset would I have to buy to use this ring/spider set to get the desired chainline?
4. I do have a SRAM X7 crankset that is not direct-mount, but I could mount the 42/28 chainrings on this to try. However, this crankset has a chainline of 48.5 or 49.5 mm.

Long post, lots of details, but I know others have faced this situation, and I'm hoping there's an optimal solution out there. Thanks.
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Old 11-14-20, 08:10 PM
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You should be able to run a 46/30t crank with a Sunrace (possibly others?) 11-40t cassette pretty easily using the GRX 400 (10 speed) rear mech. There's a thread on the riding gravel forum about using 42t cassettes with that mech with mixed results (probably OK), but the 40t would probably be safer w/o the b-screw maxed out.
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Old 11-15-20, 12:25 AM
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I'm no chainline figures expert, so I'm just going to suggest a couple of things which may help you:-

The old Sram Force crankset with removable spider. NorthShoreBillet does a 104/64 spider which fits that crank.
Engin Cycles does a 120/80?bcd spider ($$$) which also fits that crank.

Easton EA90 (alu version) or the stupid expensive carbon EA90SL crank. RaceFace 104/64bcd 2x ('10') spider mates with that crank.

The old Cannondale Hollowgram crank also works with a certain 104/64? spider, but I can't remember which one at the moment.

One option to get around different cable-pull between road shifters and mtb (front only?) is to slap on that gizmo (forget the name) which the cable wraps around, thus changing pull.

I did find when putting together my 2x build that using a 'road' front derailleur (for 50/34 for example); the cage was too long, and thus the lower end of the cage hit the chainstay when shifting down to my small 23t ring. So switched to a front mtb derailleur with shorter cage length.

Playing around with chainring spacers a bit would tweak chainline a little. Rings will also have a certain offset-figure to them.

Unfortunately for you I started with 10sp Sram road shifters, so am using Sram mtb derailleurs which have the same pull.

40/23 or 24 at times, using Easton crank with RaceFace 104/64 and TA Specialites 10/11sp rings, which by memory go up to 43t? in 104bcd.
Wolftooth chainrings and TA spacers.

Note some of the TA rings come in two different tab widths to best visually match up with spider arm-end width.

Currently on a 12-27 cassette with mid-length cage rear clutched derailleur. That's enough low end for the time being.

Gearoop Taiwan has a direct-mount 44/28 ringset, but that's not your ideal 26t (or lower). Not sure if its pattern mates with that Force crank though. https://weightweenies.starbike.com/f...p?f=4&t=164058

Last edited by tangerineowl; 11-15-20 at 01:00 AM. Reason: txt
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Old 11-15-20, 09:20 AM
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Originally Posted by tangerineowl View Post
The old Sram Force crankset with removable spider. NorthShoreBillet does a 104/64 spider which fits that crank.
Engin Cycles does a 120/80?bcd spider ($$$) which also fits that crank.
I'm hoping to get this to work. I'm mostly worried about the crank arms clearing. Currently I have a SRAM mountain crank on my gravel bike with spacers. FD is a CX70, which is discontinued. I'm pretty sure a GRX FD would work. SRAM Apex road shifters.
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Old 11-15-20, 10:04 AM
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tangerineowl If I found a good 42/28 or 40/28 system that worked well, that would be fine.
zen Thanks--46/30 GRX crankset with GRX FD would be the plug-and play solution (if I could actually buy that FD). But the 46 is higher than I need, and 30/36 is not a low enough gear. I'm hesitant to go to goatlink-dependent solutions for the RD. With the RD-RX812 that I have, any 2x system will be going beyond the stated total chain capacity (but I would try it anyway), so yes, I might have to switch to the RX400 or the RX810.

For reference, here is a forum post that discusses some of these questions.
https://forums.thepaceline.net/showthread.php?t=232810
especially the post by Mark McM on the first page
Lots of discussion of the SRAM direct mount solutions.
unterhausen I have the 42/28 rings mounted on the SRAM spider, but even on the road cranks like a SRAM Force 22, I think this would give a chainline that is too far out (4 mm or more?). What is your experience with spacers with the mountain crank, and why are you planning to ditch your current setup? If the CX70 FD was truly the right one to use, I would try to find one used somewhere.
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Old 11-15-20, 10:23 AM
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I think there is very little difference between the CX70 and the GRX FD other than the extra 2.5mm on the GRX. The big difference between them and a more normal road derailleur is the curvature of the cage. I haven't had any issues with chain line on the mountain crank. I think I might have all the spacers on the drive side, which is worse case for chain line. I occasionally ride it big-big because I'm not paying attention and it works fine. Modern chains do okay in extreme angles. I had some problems getting things to work that I'm pretty sure are idiosyncratic to my bike.

4mm doesn't seem like a lot, people are riding worse. If the crank/chain rings fit and shift, I would ride it.

The reason I'm switching is that I have a relatively low-end SRAM mtb crank and it weighs quite a bit. No other reason.
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Old 11-15-20, 11:31 AM
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Originally Posted by paramount3 View Post
zen Thanks--46/30 GRX crankset with GRX FD would be the plug-and play solution (if I could actually buy that FD). But the 46 is higher than I need, and 30/36 is not a low enough gear. I'm hesitant to go to goatlink-dependent solutions for the RD. With the RD-RX812 that I have, any 2x system will be going beyond the stated total chain capacity (but I would try it anyway), so yes, I might have to switch to the RX400 or the RX810.
You should be able to run an 11-40 cassette w/o an extension on the GRX derailleur. Here is a video of the setup with an 11 speed GRX system using the 48/31 crank. With that setup the only caveat is not using big-big (48-40).

https://www.youtube.com/watch?v=wNeYYM9_aeM

Not trying to sway you either way (I like to fiddle with things too), but the GRX 600 46/30 crank, 400 derailleurs, and a Sunrace 11-40 cassette would most likely work well.
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Old 11-16-20, 09:43 PM
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Originally Posted by zen_ View Post
You should be able to run an 11-40 cassette w/o an extension on the GRX derailleur. Here is a video of the setup with an 11 speed GRX system using the 48/31 crank. With that setup the only caveat is not using big-big (48-40).

https://www.youtube.com/watch?v=wNeYYM9_aeM

Not trying to sway you either way (I like to fiddle with things too), but the GRX 600 46/30 crank, 400 derailleurs, and a Sunrace 11-40 cassette would most likely work well.
That does look convincing. However, after some more digging, I learned that my ST-405 left shifter pulls more cable than MTB left shifters, meaning that with proper limit screw settings, I should be able to use an MTB double front derailleur, e.g. 10 speed XT FD-M786 (if it pulled less cable, I'd be screwed). Therefore I can use an MTB crankset, e.g. 10 speed XT FC-M785 (or SLX, etc from same era) which came as 40/28, 38/26, and 38/24. Chainrings are conveniently available for these cranks still. Although I was kind of hoping for a "road" build, I've pretty much only ridden MTB cranks for the past 15 years or so, and it's fine. Better yet, I have the 40/28 Shimano crankset and FD sitting here on another bike, as well as a SRAM X7 39/26, so I can swap over the parts and try these out with 11-36 cassette. Later I can decide whether to try for the 11-40 (would be Sunrace CSMX3 1140t), which would be good especially if I went 42/28 with the SRAM crankset.
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Old 11-21-20, 12:30 AM
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Originally Posted by paramount3 View Post
That does look convincing. However, after some more digging, I learned that my ST-405 left shifter pulls more cable than MTB left shifters, meaning that with proper limit screw settings, I should be able to use an MTB double front derailleur, e.g. 10 speed XT FD-M786 (if it pulled less cable, I'd be screwed). Therefore I can use an MTB crankset, e.g. 10 speed XT FC-M785 (or SLX, etc from same era) which came as 40/28, 38/26, and 38/24. Chainrings are conveniently available for these cranks still. Although I was kind of hoping for a "road" build, I've pretty much only ridden MTB cranks for the past 15 years or so, and it's fine. Better yet, I have the 40/28 Shimano crankset and FD sitting here on another bike, as well as a SRAM X7 39/26, so I can swap over the parts and try these out with 11-36 cassette. Later I can decide whether to try for the 11-40 (would be Sunrace CSMX3 1140t), which would be good especially if I went 42/28 with the SRAM crankset.
OK, that doesn't work. SRAM X7 Crankset with 42/28 almost works with XT FD-M785, but the derailleur cage jams against the big ring sometimes. I can make it shift on the workstand, but I doubt it would work in the real world. I think the problem is that this derailleur is for chainstay angles 66-69 degrees. I calculated the chainstay angle of my frame as 63.8 degrees. I don't have a braze-on so there's no way to shim the derailleur mount at an angle (no easy way, anyway, that I know of).

I then tried the Tiagra 4700 derailleur with the SRAM X7 42/28. Here, the chainstay angle is right, and I think there's a chance it would shift the 42T ring (specified min 46T for the big ring). However, the chainline for this derailleur is 43.5 mm, and it need another 4-5 mm to reach the big ring. I could try a GRX FD-RX400, which has a 46mm chainline, but that might not be far enough, and I can't find an FD-RX400 anywhere. I am considering chainring spacers. unterhausen it sounds as if you have done this with a SRAM MTB crankset. Do you have the details for how to make this work?

The other thing I will try is a Shimano trekking front derailleur. I have another bike with a FD-T610, which I believe is the FD-T610-3, designed for 63-66 degree chainstay angle. I will try this as it should match my chainstay angle of 63.8 degrees. However, this is a triple mech, so I'm not sure it will handle the 14T drop from 42 to 28.

I really thought all of this crap would be solved with the GRX groups, yet here I am hacking again. 42/26 crankset with 11-36 cassette is a wonderful range for all sorts of road/gravel/touring cyclists, there is no engineering reason for this not to exist, yet it doesn't exist.
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Old 11-21-20, 03:08 PM
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[QUOTE=... However, this is a triple mech, so I'm not sure it will handle the 14T drop from 42 to 28.[/QUOTE]

I have been following this thread with interest, however I'm no drivetrain expert so i have little to offer in the way of real help. I run a 3503 triple road crank with 50, 39, 24 rings and a 12 -36 Cassette. I have no serious issue with the 15T drop, though i had to adjust the front mech more precisely than when i ran a 26T inner ring. but I'm also running a 50T ring, so I'm guessing my mech is having to do more of a reach. The choice of a trekking front mech makes me wonder if you might have success with a triple road crank like a 5703, minus big ring. I could be way off the mark and its just a wild guess. internet shows a 45mm chain line if that is helpful to you
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Old 11-21-20, 11:30 PM
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Bingod Thanks, that's an interesting thought. My first attempt was with a triple FC-M900 (old XTR MTB) crankset set up as a 1x with a 36T in the middle position. A road triple with 42 in the middle position and 26 or 28 in the inner position is worth a thought. Can I ask what FD and RD you are using that let's you use 50/39/24 with 11-36?
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Old 11-22-20, 12:24 AM
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Originally Posted by paramount3 View Post
Bingod Thanks, that's an interesting thought. My first attempt was with a triple FC-M900 (old XTR MTB) crankset set up as a 1x with a 36T in the middle position. A road triple with 42 in the middle position and 26 or 28 in the inner position is worth a thought. Can I ask what FD and RD you are using that let's you use 50/39/24 with 11-36?
My drivetrain is 9 speed. I use a Deore XT SGS RD which works with my 3503 levers and i have the Sora 3503 triple front mech. I used a long cage Sora RD with a 26T ring, It worked adequately. BTW my cassette is 12-36. I also use an 11-32 for road
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