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is buachail foighneach me
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Discussion Starter · #1 ·
Now shake your head back and forth a little bit like you can't believe someone would ask something so xxxx. Some of the more frustrated among you might want to put your helmet on in preparation for doing this: :madman: And I fully expect at least three responses with nothing other than this: :nono: OK, here we go:


What I have:
I have a Surly Karate Monkey. I can run a 26 x 3" tire in the frame, on up to 70mm rims, but just barely. (I see some of you are already commencing with the :madman: ). By barely, I mean the tire rubs just an eensy little bit due to your standard issue tire wobble. All of my current wheelsets fit this frame. Nicely.

What I want:
As if you haven't guessed, I want to widen the chainstays(plenty of clearance up top for my immediate goal). Last winter, I tried putting a hub in the dropouts and ramming a 2 x 4 in there. It sprung back. Oddly, it seemed to spring back only once the temperature got really cold(-30F cold). I want at least 5 mm clearance on either side of the tire, which measure out to about 77mm, or just over three inches.

How I want to do it:
In the following Microsoft Paint Masterpiece, you will note my rendering of my chainstays and BB as they are now on your left. On the right, you will notice something different. Particularly, that green thing. I would like to cut the chainstays, braze(or have brazed or welded) a small 4130 tube to the chainstay nubs still attached to the BB, and then bend the section(s) still attached to the dropout outward so that they line up adequately with everything else(yes I know that there a lot of everything elses in this equation) before brazing them to said tube. Ideally that 'green' tube would also be bent to accommodate the same summer 29" tires I currently run.

<a href="https://www.flickr.com/photos/[email protected]/4116210627/" title="stooopidea by anrothardonn, on Flickr"><img src="https://farm3.static.flickr.com/2545/4116210627_562b2a2ee9.jpg" width="500" height="313" alt="stooopidea" /></a>

What I want to know:
Most chainstay and chainstay yoke setups I've seen have the yoke directly attached to the BB shell. Assuming the joining is done to the appropriate standards, and assuming the measurements have all been worked out; Is a chainstay interrupted by a perpendicular tube(or any object for that matter), structurally sound?
Assuming you yourself welded it, or fillet brazed it, would you trust a tube intersection of that arrangement to hold up under your own riding? The reason I ask, is that it seems to me like I would be adding a sort of 'pivot' point. Kinda like tying a knot in rope drastically reduces the strength of the rope at that point...

Thank you, thank you, thank you for actually reading all that crap. :thumbsup:

EDIT: I do have frame building 'experience'. I took the full two-week UBI tig welded chro mo course. I have, however, likely forgetten everything since...
 

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Student of the Bike
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Thinking outside the box, I like it!

The 2 issues that I see are:

1) Do you plan to run gears up front? Widening that part of the CS area will almost guarantee chainring interference with normal combinations.

2) Not knowing what the tube selection is for the CS. We are never fans of modification to a bike that we don't know what the tubes are that we are cutting. However, considering its a relatively stout KM, I'd guess that the stays would be fine with this mod.

I think its a rather creative and simple solution especially if you are running SS and could manipulate the chainline to allow for this widening.

Keep us informed of how it goes.
CJB
 

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I've seen a TON of Monkeys with broken DS CSs right in that area you're thinking about cutting yours. That may be a good or a bad thing I suppose, but something to consider. I'll see if I can find pics, a buddy broke two or three in the same spot and he's usually good with pictures.
 

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I'm responding politely because you did give fair warning and preface you task as if you understand reality:

Why not just remove the stays entirely and replace with ones with better clearance? Should be just as easy and should look and be a lot better in the end.
 

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is buachail foighneach me
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Discussion Starter · #5 ·
Thanks for the responses!

CBaron: I do not intend to ever run more than a single ring up front, so that shouldn't be an issue. Thanks for the encouragement.

Jay: I would really be interested in seeing those. Thanks!

PVD: The chainstays on the Karate Monkey are pretty heavily manipulated and bent to achieve a large amount of clearance. Not too sure I could actually do any better with full length stays. But maybe I could. I mean, the designers were taking into account three rings and a front derailluer when originally working out the numbers... I'll have to think about that solution a bit more. Thank you!
 

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Who turned out the lights
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I think Pete has it right. I'd look into some 3/4" or 5/8" straight gauge and bend up your own stays just the way you want them. I think you're going to end up with chainring clearance issues at the 'front' side of your intermediate tube, and you may also get into clearance issues with your 29er summer tire with that tube, even if you bend it.

5/8" 0.058 4130 can't really ride any rougher than the rest of the frame.
 

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sean salach said:
Jay: I would really be interested in seeing those. Thanks!
Sent a link to my buddy. I know he has a couple of them (broken). Wants me to weld your plate fix in his now... He'll post up pictures if he has them I suppose.
 

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shandcycles said:
You're lengthening the chainstays. How do you intend dealing with the seatstays?
You can easily section the stays by the amount that you're adding in with the cross-yoke. I'd even think that they (SS) would flex enough to put in a small plate yoke in the area being discussed.
-CJB
 

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look for pictures of stickel bikes. he did a yoke similar to what you want to do but used some flat stock instead of a tube. this could help with clearance and make mitering easier. on the other hand, if not supported correctly it could tend to twist or be flexy. stickel's was done well, with little unsupported flat stock length.
 

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is buachail foighneach me
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Discussion Starter · #13 ·
Yep, flat stock is looking to be the most likely route for me. I understand that replacing the entire chainstays would look better and probably be more structurally sound, but it seems like doing so would only add more complication and expense, when I (theoretically) could make a relatively simple mod that should hold up fine. Thanks again for all the input.


Slightly less ghetto 'sketch'.
 

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Don't do it.

You're only going to ruin your bike doing this. I'm all for DIY, but what you're proposing here is considerably more difficult (both in terms of mitering, keeping everything in alignment, and joining) than just replacing the chainstays, or even the whole rear end.

Get some scrap tubes, make some random joints, and put them together. Then try to break them. See how that goes, then think more about how you want to do this. No more drawings or crazy ideas until you've got some idea of what you're up against.

-Walt
 

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is buachail foighneach me
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Discussion Starter · #15 ·
If I were to try to braze it myself, I would definitely do several practice and test pieces ahead of time....
 

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OOps. For some reason I thought I had clicked on the General Forum to find this thread, where my funny would be OK. My bad dog.
 
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