Monday, November 26, 2012

Fat Bike Questions Answered

Because I like the B&W version of this.
 A while back, I wrote this missive concerning fat bikes. At the end of that post I received several comments and the last was filled with some questions that I will attempt to answer here. First up is the following:

"What pressure do you run on XC? "

Fat bikes are really, really air pressure sensitive machines. A difference of 1 psi will be felt and will make a difference in ride feel at the range a fat bike should be ridden at for air pressure.

Range? Yes, air pressure should be changed depending upon conditions. First of all, I never run my fat bikes at anything higher than about 10psi. That's paved commuting/hard packed XC riding pressure right there. Oh sure, you can convince yourself that a higher pressure will roll faster if you want to, but in reality, it makes the tire bounce more, which is actually losing your battle against rolling resistance, not winning it. At much over 10psi, I start to bounce. There- that is why I do not air up any higher than that.

On the low end, I go down till the side walls wrinkle, but no lower. This only for maximum traction. That's about 4psi for me. Usually I will range between 7-9psi, which covers most ride types. Told ya these were air pressure sensitive beasts! 

"Is there a pressure gauge that Fatties use?"

Well, as you can see from the above, a pressure gauge that reads really low psi and reads it accurately is best. Slime makes some readily available low pressure pencil type and dial gauge low pressure readout tools to help you get the job done. (Here's an Amazon link to the dial gauge)  There are other gauges too, but the point is, they are available, and almost always Schrader Valve types, which can work if you have a Presta Valve, (like most fat bikes), with an adapter.

Some floor pumps can get you by. They won't be really precise as to correct psi at that low a range, but some pumps can get you great "relative readings" which may be all you need. I have used a Topeak Joe Blow for just such a reason with good results.

"Front tire pulls to the left when hitting the brake. Is that normal?"

This type of question is something I've been asked a lot of lately. Not necessarily having to do with braking, but "pulling to one side" issues in general. This has to do with two things unique to fat bikes.

First, the tire footprint, if you will, is huge in comparison to anything else most people have ever ridden on in terms of  a bicycle before.  That means there is a lot more grip, and the dynamics of how that wide footprint interacts with the terrain is entirely different than any other off road bicycle. You will especially feel this amplified in ruts or on "lensed out" trails.

Imagine that the single track is a "U" shaped path. Your fat bike tire will want to grab the sides of that "U" shape and crawl up the walls of the trail/rut which will feel like a pull to one side. A fat bike is doing this incrementally on dry trail all the time. I feel it in my handle bars. It feels like the bar gets "heavier"- then "lighter"- on one side or the other. In fact, if you pay attention to this, you'll feel the tires twisting the entire frame ever so slightly at times.

While riding, your fat bike tires may encounter thousands of these little ruts, "U" shapes, and other things which are felt as "pulls" to one side or another. Depending upon the severity of the trail irregularity, this can be felt as a substantial pull, or just an odd feel.

Suspension: Not quite ready for Prime Time
Secondly, your tire pressure and overall tire profile can enhance or detract from this "pulling" sensation. Less air pressure enhances "pull"/odd feelings. More tire pressure lessens this feel. (And as you recall from above, a 1psi change may be all it takes to get rid of this feel for the most part.) Tire profile shape has an effect on this also. The "flatter" the tire profile, the worse the "pulling" effect gets. Surly Endomorphs are famous for this feel. Their flatter profile compared to more current designs makes the feel of an Endo much different than say, a Larry. Width of a tire also increases this feel, so a BFL compared to a Larry will feel differently, and the BFL will exhibit more of this "pulling" feel than a 3.8"er does.

Considering Lefty fork, any thoughts?

Ahh! Suspension forks!! They seem to be the "holy grail" right now when speaking of fat bikes for single track/off road use.

First of all, there are no really good solutions for a suspension fork for a fat bike. The Lefty will "get the job done", but it isn't optimized for the job, and it is very expensive to do a Lefty for a fat bike. Riders that have them report that they work well and that a "purpose built" fat bike fork should be considered for fat bikes as a whole. If you have the money and patience to track down the component pieces to make a Lefty work for your fat bike, it isn't a bad idea. Personally, I will wait for "the real deal". That's just me though. Nothing against Leftys in particular here. Oh, and I should mention that the European company, German Answer, has a fat bike suspension fork available. Might also be worth a look.

Any steps to remove the grin form your face when you get out off the bike??

Well.......yer on yer own there, Buddy! 


spruceboy said... sells a nice low presure presta guage that works pretty well ->

Ari said...

I use a schwalbe digital gauge that works awesome. I also received a Michelin digital gauge at a trade show that is also awesome. My pump gauges are way off.

Guitar Ted said...

@spruceboy, @Ari: I understand that there are other gauges- that's why the post is written the way it is- but I suggested what I did as a way for those who do not/don't want to deal with getting a gauge on-line or have to track one down at a store to have a way to get out there and have better experiences on fat bikes.

No barriers sort of suggesting, in other words. Is using a floor pump ideal? Of course not- but you don't have to have a precise Presta valve gauge to enjoy fat biking either. That's where I was coming from with my answer here.

Irishtsunami said...

This was one of your most informative posts. I have had my Muk for a month or so and I have been using it to commute to work (far from efficient but fun). I have been running at 10-11 PSI and the ride is suprisingly harsh. You are spot on with the bouncing, but it never occured to me that one or two PSI would make that big of a difference. I will be playing with it one or two PSI at a time.


Tim Songster Sr. said...

Is Triple D for fat bikes only?I don't own a fat bike but am interested in doing the Triple D. What tires do you recommend for my 29er?

rideonpurpose said...

I've ridden triple D and finished well on schwalbe racing ralphs, not even the 2.4s... conditions could make it very very hard though!

Steve Fuller said...

Personally, I tend to run my Husker Du's at about 20 PSI on dirt. For singletrack, I don't like the sideways squirm I feel at lower PSI. Just how I am. Everyone is different. :)

Guitar Ted said...

@Tim Songster Sr: No, and as a matter of fact, most riders were using mountain bikes until recently.