Testing: sag and frame flex on the fatbike

I tested some things out on the fatbike the other night.

Fatbike Sag:  For one, nobody had a great idea of how much ‘sag’ there is when using fatbike tires only at 5psi or even 10psi.  Most thought it’d be about an inch of sag, so whatever BB height you have on the bike would decrease by an inch.  That is a lot of drop in BB height and could result in hitting your pedals on rocks and roots or just snow if you didn’t plan for it by getting shorter cranks or building the bike with a higher BB to offset that sag!   So anyways, my very hack way of figuring this out involved using the ‘tools’ above in the photo.  The tire pressure for both front and rear tires was set to 5 lbs. The long metal ruler was as vertical as I could get it while sitting on the bike, and the bike was as vertical as I could get it.  I leaned the bike against the wall against the handlebars to keep it vertical and in one place while I measured. I taped the end of a pencil in the drive-side crank bolt hole, and ‘swiped’ the ruler against the pencil by (1) moving the ruler back and forth, and (2) rocking the bike gently forward and back.  There was no difference in those two methods of measurement BTW.  What I came up with was surprising.  I weigh about 165 or 170 lbs soaking wet.  The bike weights about 36 lbs with the stuff I have on it right now (tube, allen keys, pump, etc.).  The BB height I designed the bike to have was 12″.  When I sat on the bike the BB height only dropped to approximately 11.75″ – about 5 -7mm less than when I wasn’t sitting on the bike.  For heavier riders, there will obviously be more sag, but for me it only sagged about 1/4″.  Much less than I was expecting. What this means is that I’ll use this number to ’round up’ on future fatbikes.  If the frame calls for a 12.5″ BB height I’ll make sure it comes out of BikeCAD with at least a 12.75″ BB height unless the rider is significantly heavier than me.

Fatbike Flex: My fatbike’s rear triangle seems to have excessive flex from side-to-side (laterally).  Most bikes are made to be stiff vertically so when you’re pedaling and providing pressure in the vertical plane, it doesn’t flex and responds to the power input with minimal power loss.  If you try to look down while pedaling on most bikes (don’t crash) you will not see the wheel or tire move at all in between the chainstays unless there is something wrong.  Some people gauge how stiff a frame is by pushing on the non-driveside pedal while in the down position and seeing whether the wheel can be seen to get closer to the inside of the chainstayThis isn’t the greatest test since it doesn’t replicate the force you put on a bike while riding but still does show some degree of what you’re looking for.

Now check out this video.

You can see that the majority of the flex comes from the hugeass tire moving as you push against the frame, but there is a bunch of flex in the rear triangle as well seen if you closely look at the little ruler and the edge of the rim.  I’m guessing it was around 4-5mm of flex! I was pretty much cranking on the frame but it does move pretty easily.  I’ve heard from other builders that this is common and nothing to worry about unless there is chain skip or if you’re actually rub the tire against the frame while riding (neither of which happen to me while riding this bike).  Apparently, almost this amount of flex occurs in a Salsa Mukluk (aluminum frame), then I’m not too worried about it on my steel frame.  I tried this on my singlespeed and geared 29ers and they didn’t flex as much as my fatbike.  I think it has something to do with (1) the 100mm BB shell – such a wide shell and BB axle that it has more leverage to torque on the rear triangle of the bike, (2) I dimpled the chainstays of this fatbike frame way too much.  I wanted to make sure I had plenty of clearance for snow and ice buildup and even though I got that I think i overdid it.  This is fine for the longevity of the frame but I believe that these s-bend stays are now a bit thin in cross-section so they move more in the lateral plane.  Just a theory, but seems like it may be the case.  (3) I used a thin seat tube on this bike since I was using a seat tube collar insert up top.  It’s a heat treated tube so it’s plenty strong, but the bottom of the seat tube is very thin so that could have something to do with the flex as well.

Each frame is a lesson and this one has been a whole new ballpark of info.  Not just the fixturing issues because everything is fatter, but along with fatness comes the need to beef up the tubing so these incredibly heavy wheels don’t have their own way with you.

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