Yesterday I crashed on a tarmac pump track because my front tyre, with low pressure, squirmed sideways. It was a 2.3 tyre, on a rim with 29mm internal width, it didn’t leave the rim and it wasn’t tubless so didn’t burp but you can see the wear where the sidewall contacted the ground.

This has got me thinking about the downsides of low pressures…

What is it that stops fat bikes doing this all the time?

Force = pressure x area so a 4″ tyre will take far lower pressure to maintain its shape compared to a 2″ tyre. 12psi on a fat bike feels really solid for example, 7-8 feels about as soft as a normal tyre/pressure.

Agreed but I thought the point of fat tyres were to replace your suspension and absorb trail chatter. So you’ll want to run them lower than whatever feels like “normal tyre pressure” otherwise you might as well have normal tyres on right?

Put another way if they’re doing their job of replacing your suspension then they must be deforming more than a normal tyre?

The tyres don’t replace suspension because there is no damping. My rigid fat bike feels like a rigid bike whatever pressure is running, which is why the Bluto fork is popular.

My understanding is that they are deforming no more in a percentage than a normal tyre but due to the enlarged sidewalls that percentage just translates to a bit more comfort.

I’ve never ridden a fat bike in anger but that doesn’t square with my experience riding a rigid bike, which I have done. Rigid bike with normal tyre pressures feels like a rigid bike. Rigid bike with low pressures feels nice as the high frequency chatter is gone.

Presumably there is damping on the scale of small chatter, it’s what gives your tyres rolling resistance?

somouk: that makes sense I think. Trail chatter absorbtion is a function of absolute tyre deflection. Squirm is a function of relative tyre deflection compared to the size of the tyre.

Anyone care to differ?

And would procore solve this for smaller tyres?

The 80mm rim helps control squirm.

Run a 50mm rim with your 2.3″ tyre and you won’t get much squirm.

Hard cornering at high speed on tarmac isn’t really the forte for a big volume – low pressure tyre though really… 😕

The b+/29+ combo on mine can squirm all over if I push it on the road, but it spend most of its life on mixed trails, grass, the odd tow path etc. For that its about as perfect a tyre set up as I’ve ever tried.