Design Review - 3DP MTB Yoke

Hey all!

I’m trying my hand at a mtb yoke and would love some feedback on it.

Clearance for 29x2.5" tires at a 420mm chainstay length and T47 BB. I was looking to do 2.6" at 410mm but those dang Shimano cranks have a little nubbin that eats into clearance something fierce. I’m tempted to just say “no Shimano for you.” Feel free to use/borrow this design but know that it is made by someone who has only a slight idea of what they’re doing!


CS-Yoke-V2.step (1.3 MB)
CS-Yoke-V2.f3d (2.2 MB)

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Thank you, I may use it once Daniel gives it his blessing. :smile:

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Looks nice and tidy. Death to the Shimano nubbin, I missed that on my latest frame and I can’t see why their lower end chainrings are designed that way. I tend to move the loam shelf closer to the tire so that the yoke profile looks close to an offset of the tire from the top view, my thought is that it gives it better lateral bracing but I haven’t done fea on it. On the non drive side it’ll probably be tough to get the torch in there at a good angle, but you’re a better welder than I and there isn’t much room to thin it out with the tire right there. Id say go for it! Printing a test in plastic goes a long way too

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I know! Same issue. Apparently, this is why they had to make their 55mm chain line cranks…

I know its not ideal, but if people wanted to use Shimano cranks, you can start with the 55mm chain line cranks, then use spacers to get the right clearance to clear that rivet. Also, its worth noting that the 30t chainring rivet is closer, and should clear easier.

The design looks great. it is very printable and the clearances are well-designed. I have a few questions/pointers.

Are you designing around a specific tire? I have found the knob location is critical for the whole min chainstay length question. Here are some tire diagrams from WTB. You can see how weird tire sizing is.

If you can get exact tire dimensions, you can optimize the yoke quite a bit to make it stronger:

Also, a side note, I am happy that as a community, we have grown to be able to talk about chainstay lengths cordially :rofl:

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two more things I would check:

  • the NDS crank arm clearance
  • clearance for the reamer/BB tool

My yoke JUST clears this BB tool:

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@Daniel_Y excellent point! I bet there are some sad stories about tool clearance out there

I didn’t even think to look at tire profiles! I was just going for a generic 2.5" tire, but I’ll try to dig up some data. It doesn’t look like Maxxis publishes that info so I may be stuck measuring the old fashioned way.

That is so smart! Is this speaking from experience? :wink: I think the T47 tools are pretty similar profile to the cup size so I’m not too worried about that, but I’ll go measure them today!

Ditto what Daniel said re tyre size/shape. Your tyre drawing looks like a circle with corner knobs added, but in reality a tyre has knobs not only on the sides but down the centre - see the WTB drawings above. In other words you may end up with less clearance than you think for a given CS length, as the tyre is taller than your drawing.

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Did a bit of revising - it looks like the 2.5" WTB model has great clearance and the 2.6" WTB model has decent clearance. NDS crank has plenty of space too!

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I do not have experience with this type of printed part but this design looks nice and tidy to me! Very cool😎 Nice work!

I have a curiosity that could apply to chainstay yokes in general. Obviously they have to be much thinner than the chainstay tubing in the width direction but it doesn’t seem popular to increase the height of the thinned section. It seems like increasing the height could help retain horizontal bending stiffness in the thinned area. Naturally FEA can help make comparisons but comparing the second polar moment of inertia of the cross-sections is also pretty good.

Be forewarned chainstay stiffness seems to be one of my idiosyncrasies but I figured I’d mention this incase it helps or makes anyone else curious.

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It’s hard to see from the screencaps I posted, but this design tapers from 25x19mm to 19mm. I played around with a more dramatic starting shape but it just made the shell command angry.

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Right on. You’re right I thought it was about the same height the whole length. It will be fun to see how it turns out!

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This is what I ended up with! I’m going to be placing an order for a few this Monday. If anyone wants in on this very-much-a-prototype run, I’d happily resell them for $100 shipped each. No warranty expressed or implied :wink:

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I will PM you. I will be in for one.

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