29-er Gravel Bike with Boost Spacing - What Drivetrain?!

HI. I’m totally new here and not aware of the rules, so please feel free to correct me if I am making a mistake with posting this.

I have been thinking about a custom built gravel bike, which can take 29-er tyres, but also accomodate a 2x drivertrain. From what I can tell, the Salsa Cutthroat offers that option with a GRX Drivertrain. But I just cannot understand how it works? They use MTB boost spacing but a road cassette and what is essentially a road drivertrain. Isn’t that not supposed to work? What am I missing?

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I’m no expert and not familiar with the Salsa range, but GRX pushes the chainline out by 2.5mm over road groups to accommodate larger tyres. If Salsa have used Boost spacing then that should push chainline out by another 2.5mm over standard, as I understand it. Shimano road and MTB cassettes have the same inter-sprocket spacing and are interchangeable - it’s the hub spacing which determines the chainline. As long as you put your front mech on the same chainline as the rest of the drivetrain, it should all work fine.

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Thank you for the reply. So, let me see if I understand this. If the rear spacing is boost, and a mountain bike bottom bracket is used, it should work?

The Salsa bike uses Raceface cranks paired with Easton chainrings.
The published chainline for the Raceface cranks is ~52mm (I dunno what this means on a double though).

If the Raceface/Easton combo keeps the large chainring at 52mm, that’s ~3.5mm further out than the large chainring on the GRX (48.5mm for FC-RX820-2). And optimal for 148mm spacing.

FWIW, it seems like you might be slightly challenged fitting wide tires with a GRX crankset. The two big questions: “How big of tires?” “How long do you want the chainstays?”

Also consider that doing a 2x with wide tires on a gravel bike might move your Q-factor out. For me, that’s a no-go on road/gravel (but not an issue on MTB).

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I can’t answer all your questions, as I’m primarily focused on 1x drivetrains, but I have a couple things to contribute. GRX is compatible with Shimano MTB cassettes. The Evil Chamois Hagar has been sold with an 11-46 XT cassette, which is technically beyond the max tooth count for the GRX rear der - but it works fine. I’ve done it on a gravel bike.

I believe the Cutthroat is designed with a MTB bottom bracket, so needs an MTB crank/spindle. This limits your options as road cranks are made for a narrower bb. As @ElysianBikeCo pointed out, Salsa gets around this by using RaceFace cranks with Easton rings (same company and compatible). I like White Industries because you can mix and match rings, crank arms, and spindles to fit your application.

I have personally been trying to push chainline out a bit for tire/ring clearance. You can do this without affecting Q-factor on a 1x setup with direct mount rings by using a ring with less offset. On my latest bike, I use a 49.5 mm chainline up front and boost rear hub with a SRAM Eagle 12-speed cassette. This last bit is probably not too helpful for you, but my message is to do a deep dive into how all the components and frame work together to get your desired result. You’ll find a solution.


As @ElysianBikeCo points out, the salsa uses RaceFace/Easton cranks. RF makes an array of spindle widths so you can mix and match to create any combination you want. If you really want to get weird, you can flip flop their “boost” chainring on 1x setups to get weird combinations.


This is not up-to-date documentation, nor does it have road/gravel stuff:
RF_Crank_Q_factors_and_chainlines___16th_April_2020.pdf (797.1 KB)

Digging deeper

With GRX/SRAM road-wide 2x crankset, the most you can realistically clear is a 29x2.1 tire. You need to bump out to MTB 2x chain lines to clear 29x2.3.

From Shimano:
Road 2x: 43.5mm
Gravel 2x: 46.9mm
Mtb “Boost” 2x: 48.8mm

I believe the primary reason Salsa uses boost 148x12 is because they use 110x15 spacing in the front to easily swap to a suspension fork.

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Very useful table! However, one thing to note. BMX rear hub spacing is 110mm. I am not aware if a 120mm rear hub spacing is a thing in any type of bike.


It’s for track hubs. Not sure how common it still is but frames like the Surly Steamroller still have 120mm rear spacing.

You are correct. I just happened to be researching BMX standards today. I think the chainlines are also 43-44mm, not 45mm on that chart. Oddly, enough, RaceFace does not make BMX stuff, so I don’t know why they went out of their way to put it on that chart!

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Interestingly, Race Face had a special crank spider for their SixC cranks with 4/104mm bolt pattern and BMX-specific chainline many years ago. However, those are long gone and I wasn’t able to find a spare spider a couple of years ago. Even the manufacturer was helpless in trying to source one.

Thank you everyone for all your inputs. I think I finally understand what is happening with this setup.

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I’m currently building a gravel bike around a boost GRX double drivetrain, like the one discussed in this thread. To achieve the 52mm chainline, I use a Race Face Turbine crank with a 136mm axle. The chainrings are Easton double gravel rings:

My question is regarding the BB: Normally, I would just use a 73mm BSA and call it a day. But I would really like to use a T47 BB to gain a bit of chainstays welding area to boost the clearance. Can I use a 85.5-87.5mm internal T47 BB with a Turbine crank to achieve the 52mm chainline? From the compatibility sheet of Chris King BB, it says that the Turbine crank is not compatible with T47 BB (internal nor external):

Thank you!


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You’ll want to use a Paragon 92mm BB. Check out the Raceface Crank Data Sheet below

T47 External is equivalent to PF30 (68 Road / 73 MTB)
T47 Internal is equivalent to 386EVO Road / 392EVO MTB

It’s important to note that the T47 Internal BB spec is 1mm narrower than 386EVO/392EVO thanks to Trek. You can read more about this in the T47 Internal Spec Topic.

The BB Shell width should end up at 91.5mm. If you’re planning to use the King BB you may have to use a couple spindle spacers since it was originally designed for 92.5mm


Thank you so much @PineCycles!

I’ll report how it went when the frame is done.


Not sure if it helps but I’m running Turbines with a 136mm spindle and External T47 bottom bracket here (73mm wide) and it all works pefectly fine. Went with a Wheels MFG BB and ended up needing to use about 1.0mm of spacers to get the preload I wanted. The nice thing about Wheels BB’s is they include about 3.0mm of spindle spacers to get the fit dialed in if you end up a bit short.

Also is there a reason for goin for an inboard BB over outboard? In my experience the tool purchase on an outboard BB is far better making it the way to go.

A couple things to note. If you’re wanting to get a slightly narrower Q factor you can use the Easton Cranks with any raceface spindle. With Easton/Raceface the spindle dictates the chainline.

Speaking of chainline. The Stock Spindle on the Easton Crank is 129mm and the Easton states a 45mm chainline. The 134mm spindle you have will only move chainline over 2.5mm to achieve a 47.5mm Chainline. So you might consider getting a wider spindle or purchasing the Bingham Built Spider with Praxis 48/32 Chainrings

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Inboard offers more real estate for chainstays welds and allows bends to be less drastic. It can be especially useful if you’re using round chainstays and bending/denting them yourself. If you’re really pushing the limits of chainline it can make a big difference. IMO it’s the best use of the oversized shell.

I see T47 as offering 3 main benefits

  1. Chainstay Real Estate - This is the main benefit of T47 Internal over External (see photo below)
  2. Internal Routing Through BB - Both internal and external achieve this
  3. Offsetting Seat/Down Tubes and Chainstays - Both internal and external achieve this

If you’re not utilizing T47 for at least one of the benefits above, I think BSA is the way to go. Lighter and cheaper


Ahh that’s a good summary. I guess the main reason I moved to T47 is because I’ve had lots of issues with 30mm spindle cranks and BSA shells. In my experience the external cup of a BSA BB is doing too much work in that arrangement and a 73mm T47 solved those issues for me. I guess it’s worth stating that my experience is more of an end user/mechanic than builder.

Emphasis mine, but what do you mean by this?

I feel like there’s a relatively big step up in the cup to make it from the BSA shell (34mm) to the outside of the 30mm bearing (42mm). I’ve had lots of creaks in 30mm/BSA setups when the bikes were ridden hard off-road. I’ve even sheared off a White Industries Bottom Bracket at the point marked out below. I fully acknowledge that it could have been user error or even a faulty BB but it seems like a very weak point to be taking all the crank load.


So yeah, for gravel/road use it might be fine but I defo don’t like the arrangement for trail/touring. Ideally it would be good to have more options in high quality 24mm spindle cranks. It seems like everyone is goin 30mm so I decided on T47 to suit that. So far so good.