Bigger rings than 38/28 on MTB crank ??

Discuss light weight issues concerning mountain bikes & parts.

Moderator: Moderator Team

ooo
Posts: 884
Joined: Sat May 21, 2016 12:59 pm

by ooo

MichaelB wrote:
Fri Feb 14, 2020 2:30 am
ooo wrote:
Thu Feb 13, 2020 11:59 am
...
To use FD-CX70 with mtb chainline, you need to have braze-on version of FD-CX70 (IFDCX70TF) and "wide road braze-on adapater" (to increase chainline):
11.7618.000.002 = SRAM Wide Spacing Braze-on Adaptor
K13-030-34.9-BLK = K-Edge K13-030

..
Any idea of how much extra 'cahin line' is gained with the above ? None of the places have that info listed.
I have paired k-edge with ultegra fd and 104-64 chainrings (middle ring and small ring triple mtb crankset)
amazon review list k-edge offset as "0.5cm", ebay sellers list 2.75mm offset, may be it was different kits?

Sram pdf shows 47.5 mm chainline (it looks like 2.5mm offset, not sure about math behind this):
Attachments
wide-26.gif
'

NickJHP
Posts: 347
Joined: Wed Feb 14, 2018 2:22 am
Location: Canberra, Australia

by NickJHP

ooo wrote:
Thu Feb 13, 2020 11:59 am
There is no problem to use FD-CX70 with STI shifters. But if you want to shift FD-CX70 from a flat bar you will have to find specific road flat bar 2x10 shifter (not 2x mtb shifter, not 2x11,2x9 road shifter) compatible 2x10 road flat bar shifters: SL-4600, SL-R460, SL-R780 (not compatible 2x10 road flat bar shifters: SL-4700, SL-R770)
Actually, with two chainring setups I've found no problem using road FD with MTB shifters or vice versa. I've setup some of my wife's bikes with road FD and MTB shifters, because she uses road frames that take a braze-on FD with flat bars, and they shift fine between the chainrings.

Ditto for MTB FDs with road shifters. I have a couple of touring bikes using SRAM Force 22 shifters and Shimano 2x10 MTB FDs, and they shift fine as well.

MichaelB
Posts: 702
Joined: Tue Apr 22, 2008 4:31 am

by MichaelB

ooo wrote:
Fri Feb 14, 2020 2:39 am

I have paired k-edge with ultegra fd and 104-64 chainrings (middle ring and small ring triple mtb crankset)
amazon review list k-edge offset as "0.5cm", ebay sellers list 2.75mm offset, may be it was different kits?

Sram pdf shows 47.5 mm chainline (it looks like 2.5mm offset, not sure about math behind this):
Ta. Found the SRAM one online for $50 :shock: , so will see how I go 1st with the std setup.

Cheers Nick, I'll keep that option up my sleeve (MTB FD & Road shifters) if I need to.


MichaelB
Posts: 702
Joined: Tue Apr 22, 2008 4:31 am

by MichaelB

Don't forget, I'm in Austraya, and some either don't ship here or charge heaps.

E.g. that last one is charging US$14 shipping

Found the SRAM one here locally for A$55 (US$35). I'll wait and see how I go once the FD and rings get here and see if I need the extra width from the FD clamp

User avatar
MayhemSWE
Posts: 370
Joined: Thu Mar 12, 2015 12:44 pm
Location: Stockholm, Sweden

by MayhemSWE

MichaelB wrote:
Tue Feb 11, 2020 7:33 am
Long story, but reality is due to the 73mm BSA BB on my new frame, I'm limited to MTB cranks which are typically 38/28 or 36/26 rather than the gravel-ish ratio of 46/30 that I'm after.
What frame is this? Considering the frame is apparently designed to be used with MTB cranks, I hope you have verified that larger chainrings will be able to clear the chainstays…
ceilersen wrote:
Wed Feb 12, 2020 11:34 pm
Am I missing something? If your BSA frame is limiting your options, why dont you change the BB to an axle standard with with more options? A BSA to DUB BB is 30-40 euro and would give you plenty of SRAM options?
Yes, you are missing something. BSA is a bottom bracket shell standard. DUB is a crank axle standard. Two entirely different things.

MichaelB
Posts: 702
Joined: Tue Apr 22, 2008 4:31 am

by MichaelB

MayhemSWE wrote:
Fri Feb 14, 2020 3:53 pm
MichaelB wrote:
Tue Feb 11, 2020 7:33 am
Long story, but reality is due to the 73mm BSA BB on my new frame, I'm limited to MTB cranks which are typically 38/28 or 36/26 rather than the gravel-ish ratio of 46/30 that I'm after.
What frame is this? Considering the frame is apparently designed to be used with MTB cranks, I hope you have verified that larger chainrings will be able to clear the chainstays…
Frame is a GT Xizang 9er Ti.

Sticking with 28T inner and going to 42T outer, so should work.

All parts are on their way, so will soo find out !

ceilersen
Posts: 7
Joined: Fri Sep 16, 2016 11:08 pm

by ceilersen

MayhemSWE wrote:
Fri Feb 14, 2020 3:53 pm
MichaelB wrote:
Tue Feb 11, 2020 7:33 am
Long story, but reality is due to the 73mm BSA BB on my new frame, I'm limited to MTB cranks which are typically 38/28 or 36/26 rather than the gravel-ish ratio of 46/30 that I'm after.
What frame is this? Considering the frame is apparently designed to be used with MTB cranks, I hope you have verified that larger chainrings will be able to clear the chainstays…
ceilersen wrote:
Wed Feb 12, 2020 11:34 pm
Am I missing something? If your BSA frame is limiting your options, why dont you change the BB to an axle standard with with more options? A BSA to DUB BB is 30-40 euro and would give you plenty of SRAM options?
Yes, you are missing something. BSA is a bottom bracket shell standard. DUB is a crank axle standard. Two entirely different things.
Yeah... no.. I'm not missing that part.. that is why I suggest a BSA-to-DUB BB - it will allow you to use DUB crankset in a BSA frame.. look it up :)

MichaelB
Posts: 702
Joined: Tue Apr 22, 2008 4:31 am

by MichaelB

ceilersen wrote:
Sat Feb 15, 2020 11:33 pm


Yeah... no.. I'm not missing that part.. that is why I suggest a BSA-to-DUB BB - it will allow you to use DUB crankset in a BSA frame.. look it up :)
Indeed it theoretically does, but is there a 46/30 11sp crankset available ? (and affordable)

My quick googling seems to show they are 12sp, but only looking on mobile so could be wrong.

Anyway, will see where I end up with my Shimano MTB version and 42/28 ratios

Ta

ooo
Posts: 884
Joined: Sat May 21, 2016 12:59 pm

by ooo

Is it possible to modify bb shell to make it 68mm?
'

MichaelB
Posts: 702
Joined: Tue Apr 22, 2008 4:31 am

by MichaelB

ooo wrote:
Sun Feb 16, 2020 7:48 pm
Is it possible to modify bb shell to make it 68mm?
2.5mm off each side is not a simple job.

I did think about it though :unbelievable:

ooo
Posts: 884
Joined: Sat May 21, 2016 12:59 pm

by ooo

2x11 is not as good as 2x10 for mixing different types of chainrings, chains, cranksets.

I think it is simple job, good LBS should have bb shell facing tool to take off 0.2mm from new frames (2.5mm takes more time)
Make sure bb have enough threads inside and extra width outside, check that chainstay is ok for road/grx chainring clearance

Image
'

User avatar
MayhemSWE
Posts: 370
Joined: Thu Mar 12, 2015 12:44 pm
Location: Stockholm, Sweden

by MayhemSWE

ceilersen wrote:
Sat Feb 15, 2020 11:33 pm
MayhemSWE wrote:
Fri Feb 14, 2020 3:53 pm
ceilersen wrote:
Wed Feb 12, 2020 11:34 pm
Am I missing something? If your BSA frame is limiting your options, why dont you change the BB to an axle standard with with more options? A BSA to DUB BB is 30-40 euro and would give you plenty of SRAM options?
Yes, you are missing something. BSA is a bottom bracket shell standard. DUB is a crank axle standard. Two entirely different things.
Yeah... no.. I'm not missing that part.. that is why I suggest a BSA-to-DUB BB - it will allow you to use DUB crankset in a BSA frame.. look it up :)
In what world does putting a bottom bracket into a shell give you more options? It gives you less options as you are now restricted to one axle standard.

Also it sounds like you assume all DUB cranks are the same, but they are not. Just like most other standards (GXP, Hollowtech II, etc.) the axle diameter may be the same but the lengths are different between road, MTB, MTB boost etc. You cannot fit a DUB road crank into a DUB MTB bottom bracket, the axle is not long enough.

What one could potentially do however is fit a Sram road spider onto a Sram MTB crankarm, on the condition that the larger rings are able to clear the chainstays. Not sure how well it'll shift though with the chainline offset a good deal outward…
Last edited by MayhemSWE on Tue Feb 18, 2020 1:42 pm, edited 1 time in total.

MichaelB
Posts: 702
Joined: Tue Apr 22, 2008 4:31 am

by MichaelB

ooo wrote:
Mon Feb 17, 2020 10:23 am
2x11 is not as good as 2x10 for mixing different types of chainrings, chains, cranksets.

I think it is simple job, good LBS should have bb shell facing tool to take off 0.2mm from new frames (2.5mm takes more time)
Make sure bb have enough threads inside and extra width outside, check that chainstay is ok for road/grx chainring clearance

Image
Quickly mounted the crank last night. Can quickly see where there might be interefence between chainstay and rings, but don't get the rings till next week.
The Shimano BB comes with some spacers, so can play a bit with the offset to improve chainline.

Think facing the BB to remove the 5mm total is probably workable on some frames, but lack of dropped stays on the Gt frame make it not ideal. Have plenty of tyre clearance though (up to 2.35" or 60mm).

MichaelB
Posts: 702
Joined: Tue Apr 22, 2008 4:31 am

by MichaelB

So, since a few more bits came in the mail on Friday, time to start checking that it's gonna actually fit together.

1st was checking that the Project 321 Lefty adaptor fits.

Starting to come together. Lefty in it's new home :D
Image
free image hosting

It's a real shame that the Speed versions didnn't come in the 114mm clamp distance so I could get rid of that spacer stack. The only way to achieve that is to buy a Hybrid 2.0 or Ocho Lefty, which according to those in the know are akin to Satan's spawn and should be banished from earth. So a spacer stack it is. One day I may strip that white paint and polish the clamps to match the frame.

The other key part to trial was whether the 42T outer chain ring cleared the chainstay. 46T would definitely NOT have fitted, easily.

Sheesh, that's close :shock:
Image

There is about 1mm gap there, so may not need any other spacers, but pending how the FD-CX70 plays re the chain line, I may need to get the SRAM Wide clamp adaptor and play around there a bit more. 8)

I'll update when more bits get together :D

Post Reply