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  1. #1
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    Shortening a Surly fatbike seat tube.

    The looks of my Moonlander frame is bothering me. I feel that the seat tube is too long and the extra reinforcement bit is superfluous.

    I'm thinking of cutting it shorter and removing the extra reinforcement piece of tubing at the front. While I'm at it, I'd cut the slot for clamping on the front side to protect it against dirt flying from the rear tire.

    I'm a bit over 71 kg (156 lbs) and seat post insertion after shortening would still be 9 cm, so durability should not be a concern. There would be a slight weight saving from removing steel, but my primary motivation is vain aesthetics.

    Has anyone done this? Any comments?

  2. #2
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    I would shy away - the tubes are probably butted and/or not designed for the crimping force down lower.

  3. #3
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    Your best bet is to go into a horse supply shop and buy a set of blinders, blocking your view of the offending part of bike.
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  4. #4
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    Op, I agree completely, those braced frames are ****ing heinous. Glad I have my first gen. pug!

  5. #5
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    rälläkkää vaan, kyl se kestää

  6. #6
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    Quote Originally Posted by gregclimbs View Post
    I would shy away - the tubes are probably butted and/or not designed for the crimping force down lower.
    Very good point - the frame is constructed of butted steel. I assume the top part of the seat tube is not butted, because the seat post should fit snugly into the tube for a certain length. I'll measure the inner diameter at the point I intend to cut and a few cm below it.

  7. #7
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    Sounds like a good idea to me.

  8. #8
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    Quote Originally Posted by Saul Lumikko View Post
    Very good point - the frame is constructed of butted steel. I assume the top part of the seat tube is not butted, because the seat post should fit snugly into the tube for a certain length. I'll measure the inner diameter at the point I intend to cut and a few cm below it.
    For the amount you want to take off, I doubt the butting will cause any problems.

  9. #9
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    I'm doing this!

  10. #10
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    A bunch of the Pugsleys have had issues with cracks at the seatstay/seattube junction; there are numerous threads on this forum discussing this. By most accounts, Salsa has been very good at providing replacement frames under warranty. They have also revised the frame design a bit.

    Why on earth would you want to weaken your Moonie frame in the same area that the Pugs is breaking? And you will be invalidating the warranty, as well. I can't see this ending well!

    If it looks that bad to you, then you should have bought a different frame, a custom one if need be...

  11. #11
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    Quote Originally Posted by JoeG View Post
    A bunch of the Pugsleys have had issues with cracks at the seatstay/seattube junction; there are numerous threads on this forum discussing this. By most accounts, Salsa has been very good at providing replacement frames under warranty. They have also revised the frame design a bit.

    Why on earth would you want to weaken your Moonie frame in the same area that the Pugs is breaking? And you will be invalidating the warranty, as well. I can't see this ending well!

    If it looks that bad to you, then you should have bought a different frame, a custom one if need be...

    Exactly voids warranty.
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  12. #12
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    Quote Originally Posted by JoeG View Post
    A bunch of the Pugsleys have had issues with cracks at the seatstay/seattube junction; there are numerous threads on this forum discussing this. By most accounts, Salsa has been very good at providing replacement frames under warranty. They have also revised the frame design a bit.

    Why on earth would you want to weaken your Moonie frame in the same area that the Pugs is breaking? And you will be invalidating the warranty, as well. I can't see this ending well!

    If it looks that bad to you, then you should have bought a different frame, a custom one if need be...
    What he's talking about doing shouldn't weaken the frame in any meaningful way, if done correctly. Shortening the seat tube extension slightly might possibly even lessen the chances of cracks in the seatstay junction area, as there will be slightly less leverage on that area. Probably not any sort of meaningful difference, but worth noting.

    But yes, it will void the warranty.

  13. #13
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    Saul

    While I agree with Andy that what you have planned should not hurt anything, why buy any future trouble? There is a small chance that by removing the small support tube more stress will be transfered to the junction of the seat and top tube.

    Wait until the three year warranty runs out, play it safe for a while longer.

  14. #14
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    Come on guys, this is a terrible idea.

    Hacking off parts of a butted steel frame, around an area of weakness (seat tube cracking)? And only because you don't like the way the gussett tube looks? Worst idea ever.

    Solution: Sell or trade your moonie for an earlier Pugsley without the gusset tube. Or just ride your bike without cutting parts of the frame off.

  15. #15
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    Thanks for the heads-up about cracked Pugsley frames! I'm looking into it and here's what I found so far:



    Seems like the cracks you mention are not affected by the gusset between the seat tube and top tube. The problem lies elsewhere.

    I am well aware that modifications like this will void warranty and affect possible resale value. I'm not rushing into this head first, otherwise I would have broken out a hacksaw without posting this thread. Good input for the most part, but please let me make my own choices. Valuable insight is appreciated.

  16. #16
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    Quote Originally Posted by ultraspontane View Post
    Come on guys, this is a terrible idea.

    Hacking off parts of a butted steel frame, around an area of weakness (seat tube cracking)? And only because you don't like the way the gussett tube looks? Worst idea ever.

    Solution: Sell or trade your moonie for an earlier Pugsley without the gusset tube. Or just ride your bike without cutting parts of the frame off.
    this.

  17. #17
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    Quote Originally Posted by Gigantic View Post
    this.
    the gusset was added because of the cracks at the seat tube. Horrible idea to remove it. If you're so offended by it, get a different bike, one that you can pose better on.

  18. #18
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    Quote Originally Posted by Saul Lumikko View Post
    Valuable insight is appreciated.
    But valuable insight can come in many different forms. Like in all those old movies where in the middle of some catastrophe one guy would be freaking out, saying "we're all going to die", scaring all the women and children. At that point the calm, cool, collected leading man would walk over, take him by the shirt collar, give him a good smack across the face and say "Get ahold of yourself man!" The guy that was freaking out would immediately calm down, thank the leading man for slapping him and not cut his seat post, er... I mean not cause any more problems for the rest of the movie.

  19. #19
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    Ask a few frame builders for their opinion. I'd post this question on the Frame Builder forum and see what some people with relevant experience think of your idea.

  20. #20
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    Hey;

    Frame guy talk - A seat post puts a lot of stress into the top area of the ST, especially if that post is short on insertion. The general rule of thumb is to not have more than 2" of unsupported ST above the TT. One means of reducing this stress is to sleeve the top of the ST. Another is with a brace like this. In order to get good standover height for a snow bike, the TT was dropped a bit, leaving more ST unsupported. The ST brace used on these frames would lessen the forces acting on the busy TT/ST/SS tube junction.

    This instance sort of validates my thinking on the ST/SS cracking issue. I did not think the cracking was seat post related. My theory is that it is caused by compression forces acting on the SS's. Because of the side positioning of the SS's where they weld to the ST, and the wide angle from which they approach this junction, this compression bows the stays outward a bit, causing the material of the ST between the SS's to be stretched. The point just outside the weld is the weakest area of any weld joint, and that is where you see the cracks. It's also possible that they have missed the thicker end of the butted tube. There... how's that?

    Under no circumstances would I increase the stress at the TT/ST/SS junction by removing the brace and shortening the ST. Get your frame either warrantied or welded, but leave the design alone. Even those who "know what they are doing" don't always get it right.
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  21. #21
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    Quote Originally Posted by Gigantic View Post
    the gusset was added because of the cracks at the seat tube. Horrible idea to remove it. If you're so offended by it, get a different bike, one that you can pose better on.
    Please look at the picture I posted: cracked seat stay welds and a gusset. If the gusset was added to stop cracks, it's doing a horrible job at it. Doesn't sound like Surly to first make a frame that cracks, then revise the design and still have it crack at the same spot.

    TrailMaker: In the pic I posted (which is not my bike) it seems that you hit the nail on the head when it comes to the cause of cracking: the seat stays do approach the seat tube from very wide angles. The gusset seems to have done absolutely nothing to stop the cracking. On my Moonlander the seat stay tops are straight and meet the seat tube at a far smaller angle.

    Here's what my frame looks like:



    TM, your explanation reg. the purpose of the gusset is more in line with information from Surly: the top tube was lowered to provide a lower standover height, and the gusset was added to support the seat tube, which now extends higher above the seat stays and top tube.

    Smithcreek, I like your analogy. But it seems to me that a few other people here are the panicing ones, while I'm carefully considering whether or not to do this, asking and weighing opinions. So far I've gotten great comments about some things I overlooked, but also poorly founded fear-mongering.

  22. #22
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    The short tube that you want to remove almost certainly reduces the stress on the seatstay/toptube junction.

    With a saddle in a normal position, the rider's weight will cause the seatpost to want to bend to the rear. That bending stress puts the gusset tube under tension and transfers the force to the top tube and away from the seatstay/toptube junction. The gusset tube is there for a reason.

  23. #23
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    From the Pugsley frame failure thread:



    Looks like Surly has addressed the issue by reducing the angle of approach, so the compression forces don't tear the welds apart. A quote from a fatbike aficionado regarding the picture:
    Quote Originally Posted by nvphatty View Post
    the C stay tubes on both sides are attached to the sides of the seat tube as opposed to behind it AND with support from the TT.
    The gusset tube is there to support the exposed seat tube, but does not seem to have anything to do with the seat stay cracks. The reason not to shorten the seat tube and omit the gusset from the factory could be as simple as not wanting to change the frame size nomenclature (a 20" frame must have a 20" seat tube), or to allow people to use a wider selection of short seat posts instead of requiring a really long one.

    JoeG, your observation of the gusset tube is 100% correct. However, it does not address compression loads from weight on the pedals, transfered via the rear wheel into the seat stays. I'm willing to bet the pre-2013 Pugsley seat stays would crack no matter how stiff the front triangle is made.

  24. #24
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    Hey;

    SS bridges exist to decrease the amount of unsupported/un-triangulated stay length, and to mount brakes or fenders. In this case, they would also help with the torsional load being entered into the ST-to-SS welds. You can clearly see in the first pic that the triangle formed by the bridge and stays is very short from peak (at ST) to the base (bridge). The angle the SSs approach the ST from is also pretty wide.

    The revised frame begins the SS bend toward the ST far earlier, sending the compression forces into the ST in a more linear (straight line) fashion. Also, the bridge being farther back likely creates more triangulation and stiffness. The SSs running more directly into the TT also takes stress away from the ST.

    All in all, a very subtle, but likely very effective redesign. Who knew that those fat tires could transfer that much force through the frame? Maybe we fat riders better push ourselves away from the table more often?!
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  25. #25
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    For reference;

    This is the SS/ST junction of my first Fatbike.



    Most people ply the Well Trodden Path. A few seek a different way, and leave a Trail behind.
    - John Hajny, a.k.a. TrailMaker

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