Fat Design Question (input wanted!)- Mtbr.com
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  1. #1
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    Fat Design Question (input wanted!)

    I was toying with the idea of building a one-off Fat bike frame for myself, but haven't been able to try one, and it's rare that we have snow for long, so...

    I'm thinking about chainstay length and its affect on flotation. Ideally, I'd think you'd want the front and rear wheels equal distance from your CG so that neither wheel was overly weighted and prone to breaking through the surface. On standard bikes, the weight bias is toward the rear, which is good for traction and wheelying and handling quickness, but probably not best for flotation.

    I'd like to hear folks' feedback on this. Do you find the rear wheel sinking in and breaking through? If so, would you give up handling quickness and climbing traction to help solve this? or is traction on steep climbs more of an issue? Does snow prowess mean crappier dirt performance, and vice versa?

    I'm contemplating something a bit different, and would love to hear all y'alls' input.

    Thanks!

  2. #2
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    random, poorly edited, but mostly on topic thoughts:

    I built my frame with the shortest possible effective chainstay length which would clear a triple front derailleur (I switched to a double specific to keep the crud on the tire from constantly buzzing the front der cage). We are not talking ultra short. Something around 17" (don't remember off the top of my head). I like a laid back seat angle/post/seat position. All this puts a lot more weight on the rear tire on flat ground.

    observations:

    I find I can run lower pressure in the rear than I can get away with in the front regardless of weight distribution, which helps with flotation given the rear weight bias.

    Front break throughs in snow are much worse than rear break throughs (general observation over about 30 months cumulative winter riding on various bikes in all conceivable conditions).

    a loss of front flotation is harder and less intuitive to recover from than rear, especially on singletrack. For a rear loss of flotation, just shift your weight forward slightly and pedal smoothly.

    drive traction management is not a big deal on my fat bike in the snow conditions I have encountered thus far. high speed lateral grip in the front sucks, but that is the tires. uphill on warm glare ice will probably stop it, but other than that, it is usually just a matter of keeping some pressure on the seat. Cold ice with some texture seems to have plenty of traction, for example.

    I wouldn't be afraid of a little longer stays hurting traction, just handling.

    It would be easier to keep some pressure on the seat on climbs with longer chainstays.

    the short stays seem to make remounts in iffy conditions easier to get moving (based on comparisons to long stay traditional tire bikes).

    You say it won't see much snow (was that a joke? aren't you in Colorado?). My bike sees a lot of snow, but I built it with short stays for good deer path singletrack handling. My bike sees a lot of snow. I don't regret the short stays in the slightest, even though they do represent a compromise. If you are building it for tired, plodding along marathon rides over barely rideable snow, go a little longer in the stays. If you are building it to try out the fat, and for MTB rides in the winter, build it with short (for a 29er) chainstays, and giggle like a little kid every time you ride it!

    Somebody with a 9:zero:7 can chime in and you will have it from both ends of the spectrum.

  3. #3
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    The only time I really think about chainstay length on an obesebike is when climbing. The slickeriness of most snow covered surfaces combined with the lack of perfect packing means that you really can't climb steep stuff like you can on dirt. Not even close. You either spin out or punch through, and it always happens a lot sooner than you'd think.

    That said, the shorter the stays are the easier it is to maintain traction as the grade tips up. So, in effect, short stays mean you stay on the bike longer when the trail is firm. CS length doesn't really make any difference when it's super punchy--body english can account for more than 1.5" of stay length, and all the body english in the world won't keep you floating on many/most trail surfaces.

    That said, too much of a front-wheel-weight bias is baaaaaad on snow. You can learn to deal with jacked up/slowed down steering geo but it's hard to unlearn torn thumbs, busted radii, broken ribs, and tweaked shoulders--all of which happen when the front wheel punches through first and you get ejected before you can react. I've only had the good fortune to experience the first three, and (coincidence? I think not) they all happened on a 19" CS bike.

    Can't say that I notice too many other differences in handling, given what we're talking about and where/how they get ridden.

    FWIW, my experience on the subject comes from 4 different (with progressively improved geo, including shorter stays on each) custom obesebikes ridden with Remolino, Gazzaloddi, Endo, and Larry tires, on bikes with chainstays ranging from ~17.3" to 19".

    Keep us posted, please--sounds interesting and worth learning from.

    MC

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    I'm going to say the most overlooked snow bike frame parameter that is more critical on a snow bike than a summer bike is standover. I built in a little over 6 inches of standover, and it is usually enough.

  5. #5
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    well had you delivered my new brown bike to Connecticut over the Holidays, you could have tried my snow bike with no shortage of snow!

    if I were to make changes to my Pugsley, I'd make a taller headtube for a more upright riding position.. this would also making it easier to alter front/rear weighting with upper body movements. I don't see much need for a lot of standover.. even on the most technical trails or climbs, most dismounts are to one side or another...but that's just me.

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    Quote Originally Posted by A. Nony Moose
    well had you delivered my new brown bike to Connecticut over the Holidays, you could have tried my snow bike with no shortage of snow!

    if I were to make changes to my Pugsley, I'd make a taller headtube for a more upright riding position.. this would also making it easier to alter front/rear weighting with upper body movements. I don't see much need for a lot of standover.. even on the most technical trails or climbs, most dismounts are to one side or another...but that's just me.

    You've never made a quick dab only to find that the snow that would hold up the bike tire wouldn't hold your weight? It has only happened to me once recently in a way that caused me to remember, but I have a lot more standover than a pugsley. I also have an 8" head tube

  7. #7
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    I'd like to see a picture of this bike roc!

    I'm of the proclivity to keep the stays shorter, with a taller to tallish headtube relative to your normal bikes. Its just not easy to get into summer positioning what with, all that gear and gut hanging around in winter time(damned dark beers...). I totally agree on the wieght bias thing, as well as the standover issue. That's why I went with a 20" Pug, and not a 22".
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  8. #8
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    I appreciate all the input, so far!

    I'm in western North Carolina, so we don't get/keep a lot of snow. Higher elevations stay covered longer and we have the blue Ridge Parkway that shuts down at the first sign of a flake, so it'd be great to have a bike for that, but It'll probably see more dirt duty than actual snow. Besides, snow is sorta cold

    My inclination is to bias the design for dirt, but want to make sure that shorter stays and a more rearward weight bias isn't going to have the rear wheel breaking through constantly. That got me to thinking about the typical rearward weight bias of bicycles and if a more even distribution would be an advantage to some. The feedback about the front breaking through is just the sort of thing I was hoping to hear.

    I'm not sure when/if this project will get going, but you'll be the first to know.

  9. #9
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    not sure...

    ...about the other tech-geek (used lovingly) stuff, but don't ditto the Sandman top-tube thing unless you are up for a tryout with the Mormon Tabernacle Choir...Standover may seem underrated-until you need it...
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  10. #10
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    So in general, shorter stays within reason are preferred as Longer stays do little to increase effective traction, and longer stays inherently put more weight on the front wheel. I'll throw a third vote for more stand over as I have recently dabbed exactly as stated above with the obvious result.

    It seems that with the latest crop that a touring-esq front end is kind of the norm. Somewhere around 69.5 degrees of head angle with 50-55mm of rake. I have a bike with similar steering geo and really find that I like the neutral nature of it on snow and particularly ice. What are thoughts on that becoming more a of standard?
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  11. #11
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    Quote Originally Posted by ~martini~
    I'd like to see a picture of this bike roc!

    I'm of the proclivity to keep the stays shorter, with a taller to tallish headtube relative to your normal bikes. Its just not easy to get into summer positioning what with, all that gear and gut hanging around in winter time(damned dark beers...). I totally agree on the wieght bias thing, as well as the standover issue. That's why I went with a 20" Pug, and not a 22".
    http://forums.mtbr.com/showthread.php?t=507694

    Part of my justification for building my own was not being able to get enough standover on a pug with a long enough top tube. Juggernaut has about a 25.5" top tube, depending on how you measure, and more standover than the 18" pug.

  12. #12
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    Quote Originally Posted by benwitt11
    So in general, shorter stays within reason are preferred as Longer stays do little to increase effective traction, and longer stays inherently put more weight on the front wheel. I'll throw a third vote for more stand over as I have recently dabbed exactly as stated above with the obvious result.

    It seems that with the latest crop that a touring-esq front end is kind of the norm. Somewhere around 69.5 degrees of head angle with 50-55mm of rake. I have a bike with similar steering geo and really find that I like the neutral nature of it on snow and particularly ice. What are thoughts on that becoming more a of standard?
    Not nearly so obvious as on a 36er, but the 29" rolling diameter wheel needs more rake for the same trail compared to a standard 26" tire. To get about the same trail as a 26" tire with 45mm rake, the 29er needs more like 70mm rake (depending on head angle and tire pressure).

    So unless you were talking about 29er touring bikes, my point is that they may not be as similar as they look.

  13. #13
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    Steve, maybe build it with an old Schwinn banana seat on it to give a lot of room for body english for the snow goers.

    For the dirt / sand riding I would say the same as the snow folks, you do not want the front end plowing or breaking through the sand first. Let the front compact the sand a little for the rear and then the rear does not sink as far anyway. These babies have a lot of gyro effect due to wheel weight and a slow handling bike would be bad in my book.

  14. #14
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    I was talking about a Fargo, should have been more specific.
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  15. #15
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    Does anybody know the outside diameter of the Endo ? 740 ? Less or more ?

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