MTB boost frame with road crankset...can I do that?

[simplemind]

Because the largest chainring I can run on my Mountaineer is a 32T, I “think” I want to run a 2X road (Force AXS) crankset (46-33), which is only available in a road configuration. These run the DUB BB, so I’m not concerned with fitment, but have no idea how a boost frame would work with a road crankset. Any idea?

[jtbadge]

No, you specifically need a "boost" MTB crankset.

A road crank won't mount on a 73mm BB shell.

A "regular" MTB crank might not clear your chainstays.

If the max chainring spec is 32t, a ring with more than 32 teeth won't clear your chainstays.

[sparky33]

Lets say you have a SRAM Eagle crankset that is sold in Boost and non-Boost formats. Is there any difference aside from the 3mm and 6mm chainring offset... are the crank arms any different?
Taking this a step further, can one take a conventional non-boost mtb crank and mount up a 3mm offset chainring and have it work in a Boost frame?

and vise versa? This reverse scenario came up with a friend who happened into an extra Eagle Boost crankset and was thinking on whether it could be used for an old non-Boost frame if paired with a 6mm offset chainring.

[jtbadge]

Lets say you have a SRAM Eagle crankset that is sold in Boost and non-Boost formats. Is there any difference aside from the 3mm and 6mm chainring offset... are the crank arms any different?
Taking this a step further, can one take a conventional non-boost mtb crank and mount up a 3mm offset chainring and have it work in a Boost frame?

and vise versa? This reverse scenario came up with a friend who happened into an extra Eagle Boost crankset and was thinking on whether it could be used for an old non-Boost frame if paired with a 6mm offset chainring.

I checked out XT M8000, for example. The Q-factor is the same for boost and standard (non-boost) but the chainline is offset by 3mm outboard for the boost. Unclear if this is built into the arms or rings, but Shimano doesn't appear to have separate SKUs for boost vs. non-boost chainrings, so it seems like the arms themselves are different.

So I guess a regular MTB crank should fit this frame, but the chainline will be stressed in the smaller cogs.

[sparky33]

A road crank won't mount on a 73mm BB shell.
Agree.

I checked out XT M8000, for example. The Q-factor is the same for boost and standard (non-boost) but the chainline is offset by 3mm outboard for the boost. Unclear if this is built into the arms or rings, but Shimano doesn't appear to have separate SKUs for boost vs. non-boost chainrings, so it seems like the arms themselves are different.

So I guess a regular MTB crank should fit this frame, but the chainline will be stressed in the smaller cogs.
Very interesting wrt to not separate SKUs.
Crazy, right?
There is more and more cross-standard borrowing in bikes right now. And it is challenging to figure out what is actually compatible with what, both within and across brands. The marketing material doesn't get into these corner compatibility questions.

[simplemind]

No kidding Steve...hence the OP question. I figured it would be a Q-factor issue, but my eyes glaze over when I try to look at all the iterations.

So, what causes a road (a SRAM Force crankset in this case) crank to not fit a boost frame?

[Burning Pines]

No kidding Steve...hence the OP question. I figured it would be a Q-factor issue, but my eyes glaze over when I try to look at all the iterations.

So, what causes a road (a SRAM Force crankset in this case) crank to not fit a boost frame?

The bottom bracket shells are different widths, this has been the case with mountain bikes for decades (68 vs 73mm).

[PeregrineA1]

https://uploads.tapatalk-cdn.com/20191106/5f41bc31fc1fb1f4acfad07eb658c2cb.jpghttps://uploads.tapatalk-cdn.com/20191106/87f222cf0c45bed1379dfb85ff9a0f27.jpg I

It can be done. Custom spider and a shin on the FD.


Sent from my iPhone using Tapatalk

[simplemind]

It can be done. Custom spider and a shin on the FD.

Sent from my iPhone using Tapatalk

So that’s how it’s done. :beer:

[jtbadge]

So that’s how it’s done. :beer:

But it won't work with a SRAM crankset because the spindle isn't a separate part from the crankarms. The White Industries crankset is three pieces (each arm separate from the spindle) and is a special case in that regard.

And this frame was designed to clear the larger chainrings.

Why don't you call up Moots or your dealer and have them tell you what crank to buy.

[kingpin75s]

Why don't you call up Moots or your dealer and have them tell you what crank to buy.

The only answer that would work for his frame is a SqT crankset.

Long spindle to move the chain line out from road to Boost and hopefully that would also be enough for the rings to clear the chain stays.

[kingpin75s]

And this frame was designed to clear the larger chainrings.


I would also add that beyond keeping current with standards, what was the point of building the Black Cat as Boost if you are only running clearance for a 2.35.

Again, just do not see why Boost is needed if you are not running mid-fat. Not wrong, just not solving anything I would think.

Good point on the frame design as the Mountaineer was built to be a traditional Boost mountain bike and the Black Cat very much a niche design as is reasonable with full custom. Very different situations but does illustrate that far more to consider than hub standard.

[simplemind]

I would also add that beyond keeping current with standards, what was the point of building the Black Cat as Boost if you are only running clearance for a 2.35.

Again, just do not see why Boost is needed if you are not running mid-fat. Not wrong, just not solving anything I would think.

Good point on the frame design as the Mountaineer was built to be a traditional Boost mountain bike and the Black Cat very much a niche design as is reasonable with full custom. Very different situations but does illustrate that far more to consider than hub standard.

Although I doubt I would do it, the Mountaineer frame will accommodate 2.8 width.

[simplemind]

The only answer that would work for his frame is a SqT crankset.

Long spindle to move the chain line out from road to Boost and hopefully that would also be enough for the rings to clear the chain stays.

Now it’s getting ugly! ;)

[Spoker]

I ran / run road compacts on non boost frames without problem as 1x.
I have very little need for the small cogs so I don't worry about chainline.
I takes some trials or being good on auto cad to see if it will work.

[d_douglas]

https://uploads.tapatalk-cdn.com/20191106/87f222cf0c45bed1379dfb85ff9a0f27.jpg I

It can be done. Custom spider and a shin on the FD.


Sent from my iPhone using Tapatalk

Damn, that’s a Grail Bike. In my size. Amazing!

[commonguy001]

Depending on chainstay clearance with the rings, nothing is stopping you from running Easton 46/30 direct mount chainrings on a race face boost spaced set of crank arms. That’s what they’re doing on the new 2020 Cutthroat.

[SpeedyChix]

Lets say you have a SRAM Eagle crankset that is sold in Boost and non-Boost formats. Is there any difference aside from the 3mm and 6mm chainring offset... are the crank arms any different?
Taking this a step further, can one take a conventional non-boost mtb crank and mount up a 3mm offset chainring and have it work in a Boost frame?

and vise versa? This reverse scenario came up with a friend who happened into an extra Eagle Boost crankset and was thinking on whether it could be used for an old non-Boost frame if paired with a 6mm offset chainring.

First item:
SRAM: the Eagle crank is the same, the offset of the ring = boost (3 offset), non-boost (6 offset), fat (0 offset), etc.
Shimano: the cranks are different and the chain rings are same.

Second item: it depends what crank. See first item above.

Third: since it is SRAM Eagle, yes.

[Jamie Medeiros]

Here are some numbers if you want to do the math. Both the SRAM road and mountain frame requirements. Pay attention not only to the chain ring numbers but also the crank arm itself.

https://www.servicearchive.sram.com/service/sram/437