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Discussion Starter · #1 ·
I recently built my 2021 Yeti Arc and I'm struggling to understand the chainline of this bike.

The dealer told me that none of my existing boost- or non-boost cranks would work on the new 2021 Yetis and Santa Cruz bikes as they are now designed around 55mm chainlines!? What?

Yeti states on their website that indeed, the Arc is designed for 55mm.

I went ahead and mounted my SRAM Stylo DUB boost crankset and low and behold, the chainring (32t) did NOT properly clear the chainstays, it barely had 1mm space. The Q-factor was fine though. Even with crankboots on, the cranks easily clear.

So I did some elementary school maths and figured out that a 0 offset chainring would effectively give me a 55mm chainline. (Boost is 3mm, non-boost is 6mm, and 0 offset is, well 0mm ;-)

Now, that sort of worked: my chainline now exactly, as specced, 55mm.
However: when backpedaling the chain still comes off, and the chainring is still very close to the chainstays (perhaps 2mm clearance), even though I went a size down, to 30t.

What's going on here? Another standard? The 2020 SRAM catalog mentions 55 CL versions of their cranks? Why is my drivetrain still now smooth even though I made it exactly as specced?
 

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since 4/10/2009
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Guerrilla Gravity has been using a 55mm chainline for a long time. They use regular boost hubs with a 3mm DS offset. Since I'm on a 28t chainring, I'm able to get away with 53.5mm chainline on mine. The Yeti website has too much flashiness and not enough technical spec, so a quick look didn't turn up enough info to me about why.

Backpedal chain drop isn't going to go away. The sideways deflection required to cover an 11+ cog rear cassette is just too much to overcome.
 

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Discussion Starter · #3 ·
Thanks, I feel that the chainline regular boost cranks are (52mm) actually better than what my modified setup (0mm offset chainring on boost cranks) gives me, despite the fact that my modified setup results in the ideal (55mm) chainline according to Yeti (info is in the FAQ section of their flashy site).

I would run regular boost and be done with it however it simply brings the chainring to close to the chainstay. Might have to go to 28t... but than I'd like a 9t cassette... 🙈

I feel this stuff should be properly sorted out by the industry, it's just too messy and confusing.
 

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Can you post pics of your BB, crank, and chainring setup as its installed?

At that wide of a chainline I wouldn't think you'd have clearance problems.

The bike comes standard with a 32t chainring, I'd be surprised if it didn't have clearance for a 34t. (well, maybe not surprised)

Which 0 offset chainring did you install? I know most DM chainrings for Sram cranks have 3mm offset.
 

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However: when backpedaling the chain still comes off, and the chainring is still very close to the chainstays (perhaps 2mm clearance), even though I went a size down, to 30t.

What's going on here? Another standard? The 2020 SRAM catalog mentions 55 CL versions of their cranks? Why is my drivetrain still now smooth even though I made it exactly as specced?
I found your post because I was wondering if anyone else had a similar experience to me. In my case I built up a hardtail (Surly Karate Monkey) and I bought the new SLX 12-speed group from Colorado Cyclist. When selecting the cranks, I opted for the one labeled "Boost" by Colorado Cyclist (FC-M7120) because I have a 148mm rear hub. MISTAKE! Once I had everything assembled I looked at the angle of the chain when it was on the largest cassette cog and it looked unhealthy. Sure enough, back pedaling would cause the chain to fall off the gear it was on. I didn't even try riding it.

I started investigating and realized the (FC-M7120) cranks have a 55 mm chainline which isn't ideal for boost hubs (148 mm) because it puts the chainring too far off center from the cassette. I guess there are some frames where anything less won't allow the chainring to clear the frame or the q-factor won't work. But now we have an example of a guy building his new Yeti and the manufacturer recommended chainline doesn't make the components very happy because things aren't lined up. This is weird. Didn't anyone test this stuff before they designed the bikes?

In my case, I got on Colorado Cyclist for not labeling the crank models correctly. Sure enough they checked and got back to me and confirmed that what they label on their website is misleading. I suggested they fix it or don't make any mention of "boost/super boost" so then customers would have to figure it out for themselves rather than be led to make a wrong choice.

So I really need the FC-M7100 with a 52mm chainline. Will CC exchange the M7120 for the M7100? Nope - because it has already been installed on the bike. But they have misleading labels on their website!!! I guess that doesn't matter because policy is policy. That seems like bad customer service. I asked them to please at least fix the website so other people don't go through what I went through. I won't buy from them in the future. I just ordered the FC-M7100, but it wasn't from them. At this time they still haven't fixed the website.

In the meantime, I had a pair of SRAM NX Eagle cranks (non-boost) on another bike and I temporarily installed them on the Surly. They seem to work fine, the chainring just clears the frame, and the chainring sits nicely in the middle of the cassette.

These days there are so many standards and options, all continuously changing, that it's difficult to keep up. The resellers can't even keep things straight so it's really the customer that has to figure this out for themselves.
 

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Yeah, you really got to watch it with this proliferation of equipment standards and meathead bike sellers. I caught Universal Cycles saying wrong things about their XTR crank chain lines just before they shipped it so I ended up with the correct one. I guess now you have to go on Shimano's site and cross reference the item model # you're buying. CC should exchange that. I'd complain more.
 

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Discussion Starter · #8 ·
Can you post pics of your BB, crank, and chainring setup as its installed?

At that wide of a chainline I wouldn't think you'd have clearance problems.

The bike comes standard with a 32t chainring, I'd be surprised if it didn't have clearance for a 34t. (well, maybe not surprised)

Which 0 offset chainring did you install? I know most DM chainrings for Sram cranks have 3mm offset.
Sorry took a while to get the pics. But here they are.

So to summarise:

- Truativ Stylo carbon cranks
- SRAM BB92
- Wolftooth 0 offset ring

- I measured the chainline twice and I'm 100% confident that it's exactly 55mm.

- I don't think this is the ideal chainline. I tried with a boost ring (3mm offset, giving me a 52mm chainline) and it ran better and I was able to backpedal in ALL gears without the chain coming off. However as you can see in the pictures there is simply not sufficient space to run a 3mm offset ring, it's already tight.
 

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I tried with a boost ring (3mm offset, giving me a 52mm chainline) and it ran better and I was able to backpedal in ALL gears without the chain coming off.
If it runs better with a 52mm chainline, why not set it up that way? The chainring either touches the chainstays or it doesn't, correct? Do you really need the extra clearance between chainring and frame? By running 55 mm it seems like you're going to stress and prematurely wear out your drivetrain components by using your lower gears. I just had a brilliant idea: adjust your low limit screw on your derailleur so you can't shift into your lowest 2 or 3 gears. Problem solved! Your drivetrain will last longer and it forces you to really build some fitness on those steep climbs. ;)
 

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Here's a current picture of mine for reference. Remember I said I'm temporarily running SRAM cranks intended for 142 mm hub even though this Karate Monkey has 148 mm hub installed. I don't know what this chainline is, but I'm assuming it's less than 52 mm and it puts the chainring in a good position with respect to the cassette.

Notice how little clearance there is between chainring and chainstay - maybe 1 mm? I've ridden it a couple of times like this and so far no problems. What could go wrong if the chainring gets too close to the frame? Are you worried about mud building up between chainring and frame? Could frame flex actually result in the chainring scraping the frame?
 

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Hey ecracker,

isn't the only difference between 7120 and 7100 cranksets the amount of those 3mm spacers you install? I just installed a 8120 on my bike and I think that what it is. Just leave out one of those on the drive side and you basically downgraded to a 7100.

Assuming it works for SLX 7120 the same way it does for XT 8120.

HTH.
 

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Hey ecracker,

isn't the only difference between 7120 and 7100 cranksets the amount of those 3mm spacers you install? I just installed a 8120 on my bike and I think that what it is. Just leave out one of those on the drive side and you basically downgraded to a 7100.

Assuming it works for SLX 7120 the same way it does for XT 8120.

HTH.
I thought about this when I was installing, but I don't think that's the only difference. I suspect the shaft/spindle length is different too. I tried leaving out a spacer on the drive side, but that required I add an additional spacer to the non-drive side to take up the slack in the spindle. That put the chainring in a good position, however it placed my crankarms off-center (the drive side crankarm was closer to the chainstay than the non-drive side). This meant the pedals would not be equally spaced from the centerline of the bicycle. I briefly considered riding like that. After all, how much are my legs going to notice a few millimeters of asymmetry in the pedal location? Ultimately I decided against that since long term comfort depends on proper fit and I have to assume that each of my legs is equidistant from the centerline of my body (even though I've never measured :p).

I ordered the FC-M7100 and it should arrive today. I'll compare it to the FC-M7120 with some calipers and let you know what the differences are.
 

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Could frame flex actually result in the chainring scraping the frame?
yes. it can, and has been documented on this forum many times, so we have real world examples.

you want a few mm of clearance. I think the recommendation is for 3mm to account for frame flex. a lot of factors will determine how much flex is likely to happen, so it's not something a consumer can predict.
 

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I think the recommendation is for 3mm to account for frame flex.
If I were a manufacturer I would think twice about designing a bike with 148 mm hub spacing and 55 mm chainline since 1x12 drivetrains are becoming common. So what do you do now? Be careful not to back pedal and replace your chain, ring, and cassette more often.
 

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isn't the only difference between 7120 and 7100 cranksets the amount of those 3mm spacers you install?
HTH.
The difference between the M7120 (right) and M7100 (left) is obvious. With calipers I measure about a 6 mm difference in length. So changing recommended spacers on one side means you have to add spacers on the other side and then the crankarms won't be equally spaced from centerline of bicycle.
 

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If I were a manufacturer I would think twice about designing a bike with 148 mm hub spacing and 55 mm chainline since 1x12 drivetrains are becoming common. So what do you do now? Be careful not to back pedal and replace your chain, ring, and cassette more often.
Well, I think some mfrs are doing it smarter than others (as is always the case).

As I mentioned before, my bike uses a 55mm chainline, but the hub is also offset 3mm to the drive side to accommodate that.

I simply wouldn't buy a bike that's got a worse chainline. Seems to me that buyers should shoulder some of the responsibility here. If people didn't buy the bikes with terrible features, then the manufacturers would be more careful about making sure that they do things buyers like and avoid things that buyers scoff at in the future.

But, in this case, people will buy them because they're Yeti bikes, regardless of what Yeti actually does.
 

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As I mentioned before, my bike uses a 55mm chainline, but the hub is also offset 3mm to the drive side to accommodate that.
I'm looking at the Guerrilla Gravity website because I'm curious about the hub offset. It doesn't say much other than the hub is 3 mm offset to the drive side. So this means your wheel centerline is also 3 mm offset from center. Or is it a special wheel build that sets the rim 3 mm off center from the center of the hub?
 

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As I mentioned before, my bike uses a 55mm chainline, but the hub is also offset 3mm to the drive side to accommodate that.
I'm looking at the Guerrilla Gravity website because I'm curious about the hub offset. It doesn't say much other than the hub is 3 mm offset to the drive side. So this means your wheel centerline is also 3 mm offset from center. Or is it a special wheel build that sets the rim 3 mm off center from the center of the hub?
 

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Discussion Starter · #19 ·
Here's a current picture of mine for reference. Remember I said I'm temporarily running SRAM cranks intended for 142 mm hub even though this Karate Monkey has 148 mm hub installed. I don't know what this chainline is, but I'm assuming it's less than 52 mm and it puts the chainring in a good position with respect to the cassette.

Notice how little clearance there is between chainring and chainstay - maybe 1 mm? I've ridden it a couple of times like this and so far no problems. What could go wrong if the chainring gets too close to the frame? Are you worried about mud building up between chainring and frame? Could frame flex actually result in the chainring scraping the frame?
I guess it would make sense perhaps to just run the boost ring. It's as close as your pic but but more area as it sits more flat alongside the chainstay. I'm not that worried about flex but having spend this much money on a frame I would have expected more.

I'm happy running a smaller chainring as well (28t) but this exposes the frame to rocks and other hard edges due to the design. I've already damaged the brakehose underneath the frame hopping onto a concrete ledge as it's too much exposed now with a 30t chainring.
And Yeti has been so stupid to only provide 2 ISGG bolts instead of 3 so I cannot add a bashguard, only a chainguide, wtf!?

I'm frankly just dissatisfied with how this area of the frame is designed. It's not well thought through and corners have been cut to make it compatible with all modern trends. At this level I wouldn't expect comprises like this being made.

I might try to find a 1mm spacer and run a boost chainring, that would settle in the middle then space wise. Hope the axle is long enough.
 

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I'm looking at the Guerrilla Gravity website because I'm curious about the hub offset. It doesn't say much other than the hub is 3 mm offset to the drive side. So this means your wheel centerline is also 3 mm offset from center. Or is it a special wheel build that sets the rim 3 mm off center from the center of the hub?
The manual for my frame has more details about the wheel offset.

It's actually not a big deal and apparently lots of folks have been able to take wheels built with a centered rim and redish them to GG's spec. The offset isn't generally so much that it requires different length spokes (that is, assuming the spokes are the correct length to begin with). I built my wheels specifically for this bike, so I accounted for the offset in my spoke length calcs. In order to get the rim offset correct, you put 6mm of spacers on the truing stand at the NDS of the hub, and then build for a centered rim.

So what that does is push the cassette of the hub 3mm towards the drive side (keeping the chainline reasonable when using a 55mm chainline for the chainring), and the rim offsets the other direction in order to keep it centered relative the centerline of the frame.

It's not altogether different from Cannondale's Si offset they did for awhile. Except Cannondale did it with a 142 hub to basically get a boost chainline just before boost became a thing. Other smaller brands have done similar, too. The instructor who taught me wheelbuilding owns a custom frame that uses a similar offset so was familiar with what I needed to do. I certainly did a good job of choosing something a bit more difficult for my first wheel build, eh?
 
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