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Thread: Fat Bike front end geometry woes

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    Default Re: Fat Bike front end geometry woes

    Hello Kris,

    I'm the OP. I very much appreciate your contributions here. I certainly understand not wanting to give specific 'down to the millimeter' recommendations simply because there are so many variables. I will try to focus it a bit here - In my particular situation, on my own last winter I lowered the saddle 2 cm vs. my road bike setup - searching for stability. Obviously in my current bike ownership lowering the BB/adding drop isn't an option - I've exhausted my variables there.

    The Salsa actually has an adjustable rear drop out - I have a good 15mm of adjustability (447-464mm chainstay) - push the wheel forward? Back?

    Is messing with the front end going to help at all? I'm not asking for specific numbers, but based on what you've seen work, is the current 68.5 degree head, 51 mm offset in the ballpark? In that case, going to the trouble of having a fork made it isn't going to appreciably help. And I'm guessing that is the rub, predicting if something will "appreciably help"... Simply put, the bike wanders and once the steering is off center, it wants to continue falling/turning.

    To Jono Church - The stem is currently 9cm - I'm moving to 11cm and lowering it amidst the spacers as well.

    Thanks!

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    Default Re: Fat Bike front end geometry woes

    A wider bar could help more instead of the stem, your problem sounds a lot like what it can feel when I'm on a frame/cockpit that is too short. The trail is at the longer end but I don't think that makes enough difference for what s going on, trail on its own isn't that important

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    Default Re: Fat Bike front end geometry woes

    I have ridden my Mukluk 2 for two winters now, and had a similar initial experience to the OP. Definitely lower your tire pressure, and I also found stretching out the rear adjustable drop outs to lengthen the CS really made a difference. Lastly, to Kris's point, lowering my normal saddle height helps me get down into the bike and lower my overall center of gravity.

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    Default Re: Fat Bike front end geometry woes

    Quote Originally Posted by fortyfour View Post
    There are differing opinions on what should happen up front in terms of rake/head tube angle. But I started steep and things were a mess. I slackened things up over the course of a few builds incrementally and things were back on track and I was not wrestling the front end any more. But how slack or steep to go I think is up for debate? Notice in both extremes more or less rake was involved. I know from my own experience that a steeper head angle with slightly more rake acted kind of like a shovel or a plow but snow was not being moved out of the way. Slackening the head angle allowed for the front end to more easily float up and on top of the snow - again, moving that bubble of air out in front of you. Ideally, you want the bike to get up and onto the top of the snow. It still sinks but you're attempting to induce the front end to float UP.
    Have you tried your favorite angles and a *lot* more rake? 5mm is subtle, 20mm is not ;)

    Jeff Jones has been experimenting with it for a long time, and his new production design is 67.5° / 76mm

    lets him have a slack fatbike with the trail of a WC XC racer instead of matching a WC DH sled

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    Default Re: Fat Bike front end geometry woes

    Quote Originally Posted by fortyfour View Post
    There are differing opinions on what should happen up front in terms of rake/head tube angle. But I started steep and things were a mess. I slackened things up over the course of a few builds incrementally and things were back on track and I was not wrestling the front end any more. But how slack or steep to go I think is up for debate?
    Do you have any idea how much your front center changed over these iterations?

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    Default Re: Fat Bike front end geometry woes

    Quote Originally Posted by lakeside View Post
    The typical situation has me being dumped in the snow at slow speeds, sawing at the handlebars as the front tire gets tossed back and forth by the ruts. The culprit? Based on discussion with two frame builders; the 68.5 degree head tube angle coupled with the 51mm offset. By one online calculation, this results in over 100mm of trail. One offered up an analogy; “This is an exaggeration, but it’s like a chopper motorcycle - the front wheel isn’t steering, it’s flopping. As the front fork/tire moves off center it wants to continue turning - flopping - and you have to make ever more concerted corrections - and ultimately, down you go.”
    If the culprit really is wheel flop, taking weight off the front end should help. That would mean slamming the alternator dropouts all the way forwards to get the stays as short as possible, and then moving you center of gravity back with some combination of sliding your seat back and a taller and/or shorter stem. Wider bars could help too.

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    Default Re: Fat Bike front end geometry woes

    This is an old thread but it asks the exact question I have, that is how to make my fat bike steer and handle better in heavy snow and ruts.

    I have been riding fat bikes for a few winters and mountain bikes for 30+ years so I am pretty adept at off-road bike handling.

    My bike is a current model Trek Farley carbon size XL with 27.5x4.0 studded tires that I typically run at 6psi front and 8psi rear (190lb rider w/gear).

    Bike geometry is
    Head tube angle: 69
    Fork offset: 51mm
    Seat tube angle: 73
    Effective top tube: 643mm
    BB height: 315mm
    Chainstay length: 440mm
    Wheelbase: 1159mm
    Trail/Flop/Mech Trail: 95/32/89 (need to double-check these numbers)
    Stem length: 70mm
    Bar width: 800mm

    Does anything with this set-up jump out as a cause for poor steering and handling at low speed in deep snow and ruts?

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    Default Re: Fat Bike front end geometry woes

    I'd tend to think that powermeter data should be provided with those kinds of questions.

    Regardless of their balance and bike handling ability, it is always much easier for example for an elite cx racer to ride in the sand and deep mud than the average joe and I'm always more comfortable in tricky section when I am fit and can deliver the power I need to to navigate those rock/ruts/sand/snowy sections on any kind of bike.
    --
    T h o m a s

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    Default Re: Fat Bike front end geometry woes

    Quote Originally Posted by compulsive View Post
    Does anything with this set-up jump out as a cause for poor steering and handling at low speed in deep snow and ruts?
    Quote Originally Posted by compulsive View Post
    27.5x4.0 studded tires that I typically run at 6psi front and 8psi rear
    Try 3 and 4 1/2. Then try 2 and 3 1/2. The latter might be too soft, but try it anyway.

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    Default Re: Fat Bike front end geometry woes

    I'm not trolling but I figure when I'm in deep snow or ruts that the bike wouldn't be handling that great no matter what. I have tried different pressures but that is about it.

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    Default Re: Fat Bike front end geometry woes

    @compulsive
    Just for clarity, are you referring to when you are chugging along at a slower speed the wheel feels like it is "flopping" or is it that the wheel is getting "pulled" in one direction or another?

    The reason I ask is that what I have to keep reminding myself is that the big ol' fat front tire likes to grab traction on everything it encounters. Whether it is the side of rut or even different snow conditions on the left/right side of the tire when moving through it. And when the speed is lower this feels more exaggerated.
    Whereas a "traditional" width tire just knifes through it, as you well know.

    Like yourself, I have 30+ years riding in the woods but the goofiness of the fat bike is always a bit of an adventure.

    Perhaps that is not what you are referring to, just a thought.
    Brian McLaughlin

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    Default Re: Fat Bike front end geometry woes

    @compulsive:

    One of the reasons you feel that "tugging" or "grabbing" is because of the shape of the tire contact patch. Most are long and relatively narrow. But with a 4-5" tire? It gets shorter and wider which is good because that's what produces float. Lowering air pressure of course changes that contact patch shape as does adding it. More float, more traction? Let air out. Want less float, faster spinning/less drag? Add tire pressure.

    Since you're tied into your geo set by the actual frame, fork and forks offset, the cliff notes version is the little bit you can do is mess with tire size, tire type/brand/model and tire pressure. How much power you're producing will drastically change depending on base firmness/softness, snow conditions, how much powder is fresh over a packed trail, what type of snow that is (wet vs dry powder), temperature (above or below freezing) as you attempt to maintain traction in varied conditions without cutting the rear end loose or sawing through the base/crust. So I'm not so sure how that may be helpful. Depending on conditions at slow speeds front end handling characteristics are going to be more pronounced vs at speed, they're going to quiet down. Also it will depend on the shape and condition of the trail/packed base. Deeply rutted? Big carved and narrow track? Wide, smooth packed trail? That's all going to change how the front end handles, feels and reacts. But at some point, there's not much you can do and it's just a characteristic of that large of a tire and bubble of air up front, except if you change geo of the front end...

    And what I've recently found is that forward geo really helps to quiet that "pull" and wander down by a long shot. It's still present but not to the degree it used to be with similar geo of your Farley. So one option could be (if you have room and the fork steerer is long enough) is to try an angleset and get that front end out to 67° from 69°. My own personal build is running 67° HT with a 35mm stem so the front center is awfully long as is the wheelbase (which is great for float, traction and handling when making first tracks). Not sure what your stem length is, but one of the characteristics I've noticed about forward geo and front ends is your hands in relation to the steering axis when you're seated on the bike. In profile when viewed from the side, a longer stem places your hands in front of the steering axis vs with a short stem/long top tube and same cockpit length (center of bars to saddle tip), that places your hands behind the steering axis. So instead of pulling on the front end you're essentially pushing on the front end. All that aside, my current fatty handles completely different than previous builds because of this. The front end is quite predictable and that pulling of the front end isn't as severe. It also takes less effort to keep the bike on-line so there's less under/oversteering going on under hard efforts at slow speed when you're trying not to break traction and I think this has something to do with where your hands are in relation to the steering axis.
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    Default Re: Fat Bike front end geometry woes

    Quote Originally Posted by fortyfour View Post
    @compulsive:


    And what I've recently found is that forward geo really helps to quiet that "pull" and wander down by a long shot. It's still present but not to the degree it used to be with similar geo of your Farley. So one option could be (if you have room and the fork steerer is long enough) is to try an angleset and get that front end out to 67° from 69°. My own personal build is running 67° HT with a 35mm stem so the front center is awfully long as is the wheelbase (which is great for float, traction and handling when making first tracks). Not sure what your stem length is, but one of the characteristics I've noticed about forward geo and front ends is your hands in relation to the steering axis when you're seated on the bike. In profile when viewed from the side, a longer stem places your hands in front of the steering axis vs with a short stem/long top tube and same cockpit length (center of bars to saddle tip), that places your hands behind the steering axis. So instead of pulling on the front end you're essentially pushing on the front end. All that aside, my current fatty handles completely different than previous builds because of this. The front end is quite predictable and that pulling of the front end isn't as severe. It also takes less effort to keep the bike on-line so there's less under/oversteering going on under hard efforts at slow speed when you're trying not to break traction and I think this has something to do with where your hands are in relation to the steering axis.
    There is so much good info flowing in this thread!

    @sk_tle I agree that power and overall fitness plays a large role here. The handling of my bike never bothers me during casual rides but when riding at 10/10th during a race or a tricky narrow DH segment the bike feels unwieldy and more difficult to control that I think it should be.

    I can try lower tire pressures but at 190lbs w/gear I am worried that a tire might roll if I go much lower. I have carbon wheels running tubeless. Another thought was to move up from 27.5 x 4.0 to a 4.5" front tire to see if added flotation might improve the handling and grip.

    The angleset idea is brilliant! Even dropping back from the stock 69 HTA to a 68 HTA should calm things down a bit without making it wander too badly on steep climbs.

    I have 800mm bars and don't want to go less than 780mm but in terms of stem length, assuming that I could fit comfortable within a reasonable range, would a shorter or longer stem be beneficial ?

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    Default Re: Fat Bike front end geometry woes

    I’ve got Farley and it works great. It’s just that snow riding can really suck and you can end up riding in conditions you would otherwise never bring a bike to.
    Rutted out soft snow? Who would ride a bike there for fun normally? It’s the equivalent of riding on the beach but worse, and that’s all about tire pressure to do well.

    Don’t be afraid to go down to 1 PSI or 2 PSI in certain conditions. I usually start at 4 PSI as a high upper limit if it’s not hard packed. It sounds crazy, but it’s what everyone does to make it work. Your tire won’t roll, and you’ll be moving too slow for anything really bad to happen anyway. Get one of the 0 - 15 psi or 0 - 30 psi Meiser gauges and test everything.

    Shorter stems slow down steering reaction and give you more leverage. My Farley is 780 w 55mm stem. They work right together. Longer stems are the wrong way to go if you want more stability.

    Fat biking is just weird biking. You end up in so many un-rideable situations it never seems worth getting too hung up on the bike tech having a chance at fixing the problem. It’s fun, mostly, but probably because otherwise you might be sitting inside. Otherwise you can usually go for a run and move faster.

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    Default Re: Fat Bike front end geometry woes

    Quote Originally Posted by compulsive View Post
    There is so much good info flowing in this thread!

    I can try lower tire pressures but at 190lbs w/gear I am worried that a tire might roll if I go much lower. I have carbon wheels running tubeless. Another thought was to move up from 27.5 x 4.0 to a 4.5" front tire to see if added flotation might improve the handling and grip.

    The angleset idea is brilliant! Even dropping back from the stock 69 HTA to a 68 HTA should calm things down a bit without making it wander too badly on steep climbs.

    I have 800mm bars and don't want to go less than 780mm but in terms of stem length, assuming that I could fit comfortable within a reasonable range, would a shorter or longer stem be beneficial ?
    RE Tires: This is your contact with the terrain. It is literally one of the single biggest factors I'd argue when riding on snow. It's less so when you have a nice solid groomed base or even really well packed wide base. But when I moved from 4 up to 4.8" tires, there was a demonstrable difference in float and traction and hence handling. Over time, grip/traction and float are split between both wheels but biased towards the rear. So often I'm letting more air out of the rear than the front. When I moved up to a set of Terrene Johnny 5's that are studded, I noticed because that tire is just so big, I could run higher pressure. But it's never been out of the ordinary for me to run PSI down in the 5-7psi range. I don't know how fast you can get going in your new of the woods, but here its a slow slog. I'm constantly breaking trail most rides. But if your frame has room for tires above 4", and they will fit... do it. Depending on the width of your rim, I'd also not even worry about rolling a tire. The wider the rim, the more vertical the sidewalls become, and the "fuller" the tire profile is. That adds up to more stability and with a fuller tire, more float.

    RE Wander on climbs:
    I've found this is exasperated by too much power to the pedals if you are also trying to maintain traction at slower speeds. Reason being, you're also simultaneously attempting to maintain your balance on a surface that can move beneath you and if you're on a narrow track, you have soft now to the left/right. So there's a lot of balance and too much effort can make the bike wander left/right and to correct that, you need to under/over steer all while maintaining your balance. At slow speed, that's tricky. On pavement or dirt, that's easy. Plus you have to remember that all that traction also translates to movement - so if you steer left/right to correct yourself, the direct opposite will occur: You'll over compensate. Light on the bars, try to use the hips/legs and less input from the upper body - just keep that loose and relaxed. Small reactions make big changes up hill on a fat bike. But a wider base of contact, i.e. a wider tire up front, yeah, that's going to make a difference.

    RE Bars/Stem Length:
    What length stem are you running now?
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    Default Re: Fat Bike front end geometry woes

    I'm going on my first fat bike ride tomorrow and am really enjoying all of this insight. I don't think the snow will be deep enough to matter much but better than inside. Thanks!
    DJ Boyd

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    Default Re: Fat Bike front end geometry woes

    Just got my first fatbike a couple of months ago, but have only had one ride in snow. This weekend was a lot of very technical rock gardens, have definitely went over the bars once, and come close a couple of other times. This is actually my first time riding any "newer" geometry. Some days I really like it, and my XC bike feels strange, but then after a few minutes I fall in love with the XC geo again.

    My regular XC bike is a Ragley TD:1 (2009, 24 TT + 60mm stem, 72° HA) and the fatbike as you can imagine is a completely different beast (Felt DD30, 25" TT, 40mm stem). I know my weight is more rearward on the fatbike (25in TT w/ 60mm stem, 69° HA) but struggling a little bit with some aspects of the handling.

    Seems really easy to put too much weight forward when riding (maybe out of habit from the other bike, it definitely handles better when my weight is back but doesn't always feel natural), like trying to lean forward and back to lift the heavier front end over rocks. Also in the extended technical sections I got a lot of hand numbess which I rarely got in the past (maybe a bit if I did a 1.5 continuous effort at the velodrome where I'm not moving around on the bike). Not exactly where to start. May try some Ergon grips but wondering if I should play around with stem length (I definitely don't feel that stretched out).

    This thread is awesome btw!
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    Default Re: Fat Bike front end geometry woes

    There's some great advice here and I will pass on some I got. This was offered to me by a multi-time Iditabike competitor. There was a snow race and he asked about my tire pressure. I shrugged and he squeezed a tire. His eyes went wide and he said that the tires should be soft enough for him to be able to contact the rim by pressing sharp and hard with the heel of his hand. He asked if I had tubular cement, and suggested I glue one bead to the rim (and remember which side in case you flat). That would keep the tire from slipping under torque and shearing off the valve stem. This was dark ages with 26" x 2.2" tires, 20-mm wide rims, way before tubeless.

    Race day was near freezing and the snow was soft, slippery, and granular. Guess who won?

    It was fun following the few tracks ahead, the grooves would become wild s-turns on every little rise, and then become footprints. And soon enough you'd see another rider to reel in and pass. Some guy running in ankle-deep crud wondering how the hell is that asshole still riding?
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    Default Re: Fat Bike front end geometry woes

    He asked if I had tubular cement, and suggested I glue one bead to the rim (and remember which side in case you flat).

    That is good advice for those of us who still run tubes, and have tires that are prone to slipping at low pressure. A little tubasti is all it takes. I glue directly using the tube of cement against the installed (and deflated) tire. Put a dot on the tire bead where it normally sits under the rim lip, and string it out a few inches; dab another dot and string it out, etc, until you are all the way around. Pulling it off the glued side when necessary is not difficult. When reinstalled it should retain most of the grippy characteristics. The tire will not slip with this stuff!

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