Fat Bike front end geometry woes

[lakeside]

I have a Salsa Mukluk Fat Bike that I'm not happy with. The front end geometry is not well suited for it’s intended use - crawling along snowy rutted environs.

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.”

At higher speeds in smoother conditions it’s seemingly okay, I have no mountain bike and use this whenever my ‘cross bike is out of it’s element. I have ridden 2 other Fat Bikes, a Pugsley and a 9zero7, both traded for test rides with their owners and both handed my Salsa back to me with thumbs down. My rides of their bikes were uneventful.

I spent some time e-mailing Salsa last winter and their polite response could be boiled down to; you’re a roadie, it’s not a road bike, get used to it. I beg to differ.

Geometry wizards - am I imagining this issue? Solutions? I realize changing the head tube angle isn’t realistic - I suppose one could build a shorter front fork (there’s 2” of clearance between the tire and the bottom of the fork crown), but that would certainly mess up the bb height and I don’t need a 75 degree seat tube angle… A fork with more offset? Bring the trail down? If so, how much? Or is this merely a "band aid" to cover up the head tube angle "wound"? Any recommendations for a frame builder to construct such a fork? (Framebuilder #1 does road only and #2 hasn’t returned my call) This Fat Bike geometry is a different animal than the road bike specs I’m used to.

I’m also not adverse to putting this thing out on Craigslist and starting over...

Pictures attached below: (Click on them to enlarge and make readable)
Salsa Geometry (My bike is a medium)
Screen grab from an online trail calculator
Pic of the bike out in the wild - the VW Bus is actually converted into an ice fishing house - no engine - just tow it out onto the ice!

Thanks everyone!

[thollandpe]

I find that tire pressure has a HUGE effect on handling in the snow, and that it takes a while to be comfortable with how low you can go. Too firm results in the dreaded sawing, because the tire sinks in and the contact patch moves ahead of the steering axis (negative trail). But that's the opposite of what you expect.

Do you have a normal 1-1/8" steerer? Cane Creek makes a headset that let's you adjust the angle, that could test your hypothesis.

PS Happy to test that beast out for you this winter.

[lakeside]

thollandpe,

For snowy conditions I typically run 6-8 psi in the front, 9-10 in the back. Is this too much? (using a dial indicator gauge calibrated in single psi from 0-30)

The steerer is tapered, so the adjustable angle headset isn't an option.

Thanks for your insights.

[Peter Polack]

Checlk the specs and run the trail numbers for the Pugsley and the 9Zero7 and see how they compare. If the trail and rake numbers are considerably different, you may have your answer there. I'm under the impression your comment, "My rides of their bikes were uneventful." means they didn't have weird handling like your Salsa.

As far as having a replacement fork built, I think you either need to take the leap yourself and just spec it out and find a builder, or discuss it with a builder familiar with those types of fat tire bikes. I recommend 44 Bikes. I think he can understand your problem and if a replacement fork is the solution, Kris is the right builder for the job since he has experience with extra-fat bikes.

I also argee with thollandpe's comment about tire pressure. At very low pressures, the tire will deform and squirm and you apply steering corrections. To prove whether that's your problem, try inflating the tires to max pressure vs. your normal pressure and see if there's a beneficial handling improvement. You may save a lot of money on a fork.

[nahtnoj]

Yep, if you are looking for a VSalon builder to help, this has Kris written all over it...

[artfrank]

You should check out this very recent post across the hall at mtbr by Mike Curiak who is probably one of the most knowleadble folks around on this subject through his thousands of miles riding the Iditarod trail in Alaska.

[lakeside]

Thank you!

You should check out this very recent post across the hall at mtbr by Mike Curiak who is probably one of the most knowleadble folks around on this subject through his thousands of miles riding the Iditarod trail in Alaska.

[palincss]

You should check out this very recent post across the hall at mtbr by Mike Curiak who is probably one of the most knowleadble folks around on this subject through his thousands of miles riding the Iditarod trail in Alaska.

I read that post. His observations are devoted exclusively to a specific kind of dry powdery snow that won't stick to itself. Does snow like that get the ruts that are the issue here?

[Dancingbear]

I'm giving this a bump and a suggestion, because Lakeside is one of the good ones. So.....bump. And if Lakeside added his name to meet the posting requirements for the frame forum section, and the Mods could see fit to move the thread there ....then maybe we could resolve his conundrum.

[fortyfour]

Thanks for the mention fellas. I don't claim to be an expert, but I suppose I can say that given my experience, I am a little more in tune with how to build fat bikes more than most perhaps?

My comments only are speaking in regards to a fat bike on snow - a fat bike on a dirt is a much different beast. Snow and the type of snow can play a lot of different roles.

So the handling of a fat bike can really be a layer of different factors. Conditions play a huge part as the type of snow plays a factor and temperatures can change what is going on beneath you if it's a long ride. The shape of a fat bike tires contact patch can play a part. Tire pressure plays a part. Lastly, the actual bike geometry plays a part. The one giant factor that you need to know is that because the tire is so large, it will grab at any little rut and toss you or drag you in one direction or the other resulting in you having to under or over steer, which leads to an extreme of either one and then you're in the cabbage... I know with all of Mike's knowledge/experience, he is on a very similar trajectory for fat bike setup and geometry as I have been following, with really not speaking with him to this regards (I've read that article, and it's pretty much what I have been doing with my experience). To a certain degree, the front end will wander a bit no matter what you do. But the degree of how much it actually wanders starts to get sorted in the construction of the fat bike itself. At speed, like you have found, all of that kind of just melts away. It's at slow speed when things get amplified (and many times, slow speed is the majority of the ride when out in the snow).

The preparation of the trail can play a role, how much or how little of a base you have, how packed the snow is or how rutted up it is... I'd say up to 8" of fresh powder you're most likely fine and rolling steady. I've found you start to get bogged down anything over that. Wet heavier snow really can be a problem. But this is where a tires lug depth comes into play as well as when temps start to rise-you want meat and you want heavy lugs that bite deep into the snow. Hard pack it doesn't really matter. I know for me, a 5" tire like Surly's Bud and Lou combination on an 80mm+ rim are game changers. If there is any sort of base, a 5" tire makes a HUGE difference. 4" just does not provide enough float.

Two important factors I have found that play off of each other are head tube angle and bottom bracket drop. You want that front end out in front of you but not too far out in front of you. The "air bubble" is rolling over the snow and not pushing at or into the snow if I am explaining that right. I've felt at speed, the bike actually will rise up on top of the snow to a degree - so if you can maintain some speed, you will benefit.

The next factor being that you want that bottom bracket low. Really low - this lowers the riders center of gravity, getting the rider "in" the bike and really helps to stabilize the ride. I'd say just from glancing at the geometry sheet for your Salsa, if it had more drop, you MIGHT not be having some of the issues you're having, or at least not as much? At 60mm of drop, from my experience, that's just not enough for snowy conditions be it dry powder, hardback or soft/wet snow. Some rides I will actually drop my saddle height up to and beyond 1" to gain traction and stability and this is why in the winter, I run a seat post collar with a lever.

I've also found that too short of a rear end does not really benefit a fat bike. A little more length in the rear end helps, but not so much that things get sluggish and feel like a tank.

I'd say though with certainty: there is no single factor that is the cure - it's really a recipe of all of these above factors that sets the stage for a fat bike that handles well.

Head tube angle, bottom bracket drop, chain stay length, tire width, tire pressure and the riders center of gravity balanced between the wheels and built IN the bike are the biggest things to focus on. You heard it here first... I mention nothing about that word "Trail".

[j44ke]

Yeah ^^that^^ should be duplicated somewhere like Kris' Smoked Out or on the Framebuilding forum. Interesting stuff. Not what I expected.

[fortyfour]

Yeah ^^that^^ should be duplicated somewhere like Kris' Smoked Out or on the Framebuilding forum. Interesting stuff. Not what I expected.

I will post this in my Smoked Out as well - thanks for the suggestion. I'm currently working on detailing my own personal fat bike like I have been doing with all my other personal builds so there will be a lot of information above and beyond what you may have read above. My current fat bike set up underwent some changes which are of some note (and those were based on last season's riding in the snow), and good to compare with where I have been - I'm waiting for that frame/fork to come back from powder so I can include it in the article as the next generation. So the article may be a 2 part article as there is just that much information to put down. If there are any questions or if I have not done a good enough job of explaining something, let me know and I'll do my best to answer as best as I can. Fat Bikes are very different animals.

[G-reg]

My few cents as an ND Dude that's been "fatbikin" for a while now.

For a mostly "dirt" bike look at solid 29er geometry, DON'T use suspension/suspension corrected forks, think really hard about if 29+ wheels will work for you.

For a proper snow/sand bike:

Relative to a "racy" 29er

Longer CS / Shorter Front Center: Closer to 50/50 weight balance between wheels.

Steeper HT angle, longer fork rake, FFS NOT suspension corrected: Keeps some feel/nimble-ness while decreasing the self-steer that the leverage of a wide tire can generate.

Low BB: The lower you can tolerate the steeper/shorter the front end can be while still being relatively stable.

Use a Surly Bud for a front tire, I know there's other weight weenie things coming out. Just don't, Bud is THE front tire at the moment.

GO TUBELESS!! All of the marketing BS you've been fed about the benefits of tubeless MTB tires.....Well they are really evident on the big wheels. Either you have a massive heavy tube in a 5in tire on 100mm rims, or a 26in stretched within an inch of it's life simply making contact with the casing. In either circumstance the tube is fighting with the tire casing as you roll along.

[Will Neide]

I find the discussion of chainstay length interesting. The linked review recommends shorter, with what appears to be a rearward bias. Any thoughts on that because it looks like two recommendations on here for longer chainstays. I have plenty of snow riding experience, but oddly never on a fat bike. With riding on snow being so dynamic and diverse, I think it would be hard to pinpoint any one variable as making the ride better or worse.

[fortyfour]

I find the discussion of chainstay length interesting. The linked review recommends shorter, with what appears to be a rearward bias. Any thoughts on that because it looks like two recommendations on here for longer chainstays. I have plenty of snow riding experience, but oddly never on a fat bike. With riding on snow being so dynamic and diverse, I think it would be hard to pinpoint any one variable as making the ride better or worse.

Correct and that is why I mention there is no single factor: it is the entire recipe. (That also applies to any bicycle.) But I think even MORE so for a fat bike as the terrain literally changes (goes from solid to a liquid and then back again) and moves beneath you.

It is interesting that the article points towards shortening the rear end, but I think that is to see what the effects are when shortening the rear end. I can say I've done that and I've been there. I started long, went shorter and then went back to slightly longer (but not as long as I first started). Shortening the rear end also shortens your wheelbase, which doesn't do so well in the control column. I found the shorter wheelbase made the ride a little choppy and a little too sensitive? Like I said above: you want stability but not so much where it feels like a tank. That is just something you need to figure out through build, ride and repeat. I build in saddle set back as part of the rider compartment-so I am already biased a little backwards:

Here is my current set up slammed at 16.75 with 5" tires. Through out the season I tried different lengths and settle just around 17" to be just the right amount of everything - but that is in relation to all the other factors too. That is an important note to point out. I'm making decisions based on the whole recipe.

Here's the build just before with 16.5" and 4" tires.

Here's the original version with around 17.25" and 4" tires. However much steeper head angle than the above 2 - I built it very much like I would a 29er for trail and that was a mistake. Head angle all wrong and bottom bracket drop all wrong.

IF there is any one item on that recipe that has the most impact I personally think it's bottom bracket height - I'm currently in the high 70's for drop and that takes the tire pressure and displacement of the tire when seated into account - so when seated, I'm even lower. And depending on the conditions, I'll even drop the saddle some too. From what I have experienced over the course of 3 prototypes and 4 winters, lowering the riders center of gravity really grounds the bike. As soon as you move the riders center of gravity up into space, that's when the associated problems begin to get amplified. You shift the weight up and forward naturally doing this. Shifting that weight down, biased backwards slightly and IN the bike, helps to stabilize/quiet things down. I've found that also is a big factor on a 29er too, but not to the extreme with regards to a fat bike. There are other different factors I need to consider when designing a mountain bike.

Not to mention it actually feels more natural to be a little more upright and lower when riding in snow. The whole ride you are concentrating on maintaining control and traction. Your cadence also plays a very large role and big gears just don't make sense. 28t up front and at least 36t out back. Switching to a 1x11 this season in combo with a 28t up front. The key to traction is maintaining momentum, and having a constant/consistent pedal stroke. As soon as I try to get on it depending on the conditions, the rear tire breaks loose OR I push too hard and the front end starts to slide/wash to one side or the other. Heading down hill you can really feather the brakes and lay into turns doing moto style slides. Anyone who has not ridden a fat bike down a groomed single track trail in the woods at speed NEEDS to get out there and try this. It's a fucking blast hauling down a hill in snow and feathering your way through a good moto-style slide on snow.

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. I do know that with a Bud up front and a Nate out back, that was a huge difference but with the Nate out back there still was not enough float in some conditions so I'd be punching through. Now with a Lou out back, I did not experience the same problems. I rode single track in the dead of winter, un-groomed no problem (the previous build this would have been a problem):



I guess that's a lot more information.. I'll post this up in my Smoked Out too.

[thollandpe]

Kris, that is some terrific info and another reason why VSalon rocks (hint: it's about the know-how that's free to roam in here).

Do you ride much on churned-up trails? Good snow down here gets boot-tracked within days, and even though a bike can get you to the untracked stuff there's still some crap to ride through. I'm wondering how your bikes do in that crap. I can't snowshoe-groom every trail I ride on.

[fortyfour]

Do you ride much on churned-up trails? Good snow down here gets boot-tracked within days, and even though a bike can get you to the untracked stuff there's still some crap to ride through. I'm wondering how your bikes do in that crap. I can't snowshoe-groom every trail I ride on.

So it depends on the amount of churn. Snowshoe tracks in wet snow really don't make a big difference. The occasional hiker who decides they're not going to wear snow shoes is ok. But then you get the group of 10 who have decided late in the season they don't want to wear snowshoes and post hole their way to the top of the mountain and back. That's a complete mess - it is rideable but depending on the amount of post holing going on, it can be really tough going. So yes, I can ride on pretty messed up trails. It does not smooth it out though. I mentally adjust imagining myself in a really sweet LONG rock garden. Then it gets only slightly better. 5" tires makes a huge difference in this too. 4" tires these same trails in the above conditions was no fun at all.

Actually, here's a good example. This was totally rideable - a little bumpy on occasion, but a lot of fun:



I had some horseback riders late in the season decide it's a good idea to take their horses out onto the snowmobile trails. You'd think they would stick to one side and destroy that... Nope. They're on the left, right, center and every which way. I get it - it's their trail too as it's all multi use and spring was in the air. But it completely destroys a seasons worth of hard packed trail. I typically ride single track that I normally ride during the spring, summer and fall. NO ONE is out there pretty much and I have the place to myself until a really big storm hits and I have to get out there with snow shoes. Typically I'll ride that, the big storm finally hits and then I'm looking to go over to other areas that are multi use with snowshoers. Once that gets too post holed up, then I'm on the snowmobile trails. But those horses threw a monkey wrench at me late last winter..

[thollandpe]

Ugh, I hear you. It's a crying shame when a trail you've groomed for many weeks gets post-holed. Que sera sera on shared trails, but still.

You're bringing my 44 jones to a full boil, dammit. What I'm looking for is to ride all that crap, your photo recalls several gorgeous days where the riding was almost impossible. What's worse is that pushing was even more frustrating (I have not resorted to carrying snowshoes on the bike).

Rock garden? I try to imagine that the prostate massage is somehow good for me.

But some days it's just spectacular, even on the "skinnies" thanks to the snowmobubbles that haven't been trailered up north:


PS Breakable crust?

[xjoex]

I'm calling tire pressure. You should be at 4PSI, 5 MAX. 10 PSI totally defeats the fat bike benefit. Try it, you'll love it.

My wife rides a mukluk all winter with no complaints.


-Joe

[JonoChurch]

I'm going with something simple - handle bar height and stem length. Looking at the photo the salsa could be higher and shorter than the other 2 which would mean you have less weight on the front wheel and less stability. Try going longer and lower, by 10mm to start with

Also, tyre pressure