How does weight affect friction

I knew this thread would bring out the geeks vs. the realists.  I am almost out of popcorn at this point, so here's my 2 cents.

My practical experience has been that weight:sole area ratio appears to play a huge role.  Big guys are big, but rarely is the contact patch of their shoe on the slab proportionally as much larger.  So Mr. Beefcake (220 lbs) probably isn't putting twice the rubber on the rock and Ms. Butterfly (110 lb).  I see very often that the gals out-smear the boys, and I think this is the reason.

Clearly rubber matters. But it's "what you do with that rubber" that counts too, and I'm not talking about technique.  Specifically - I had a chance a few years ago to compare "stiff" shoes (Boreal Aces.  Great all-around shoe, but by today's standards they are really stiff.  TC Pros are becoming hugely popular as folks re-discover the benefits of a stiff shoe, but that's for another thread.) with 5.10 Mocs.  Same Stealth rubber on them. The Aces stuck better than the Mocs, for reasons that a few in this thread have already alluded to. In short, the little points sticking up from the rock surface were able to bite into the stiffer surface of the Ace, while the Mocs "draped over" these (see note) and weren't able to generate the pressure to get the bite going as well.

Note - this difference was most noticeable when standing on spots charahcterized by a single large (think pea sized) crystal. The Mocs slid down over these with ease, unless/until I got the crytal up under my big toe.  With the  Aces, I just placed my foot on the crystal, and it stayed put.

The problem here is that the simple physics models deal with fairly shallow slopes, constant contact areas, and no surface deformation changing the contact parameters.  Climbers are not like that.  If you were walking up a slope with super hard rubber soles, that would be one thing, but we are generally not walking up a slope.  We are using our hands to support some weight, and how we position our bodies can drastically ipact where the weight is on our feet.  As we stretch up for a hold, we are going to lever up on our toes, reducing the contact patch.  As we put more or less weight on our hands, we drastically change the force of gravity that the rubber needs to counteract.  Our bodies also end up being a set of levers, so lowering your center of gravity on a steep wall ends up putting more weight into the wall, increasing the normal force.  And since the rubber deforms under load, more weight will end up with better contact with the rock and more friction.

You can always run your own experiment.  Take your shoe and put in 20 pounds of weight or so and place it on a board.  Start lifting the board and measure the angle when the shoe starts to slip.  There are easy formulas where you can compute the coefficient of friction based on the angle and mass.  Then change the weight and repeat.  If the angle where the shoe slips changes, then weight impact the force of friction, if it doesn't, then it doesn't.

Dumbest geek thread yet to date. Stop making excuses for your poor footwork.

The classical model of 2 sliding bodies overlooks a whole bunch of really important effects.

Pure friction slab is really rare, typically there's some feature that's bearing more weight than the rest: crystal, micro-edge, lump, divot, whatever. For those cases, the shear/creep properties of the rubber or more important than it's stickiness, and a heavier climber is going to put more strain on the bit of rubber doing the most work . 

ViperScale wrote:

42

But what is the question?

JFM wrote:

But what is the question?

How many roads must a man walk down.

rgold wrote:

I think Coulomb was the first to propose that the frictional force is proportional to the normal force, and also that kinetic friction was independent of velocity.  The proportionality laws are also ascribed to Amonton.

cyclestupor wrote:

Also...  Don't call the friction equation "Coulomb's Law".  While Coulomb is responsible for the equation, he is also responsible for "Coulomb's Law" which is something completely different...

Coulomb's law states that: The magnitude of the electrostatic force of attraction between two point charges is directly proportional to the product of the magnitudes of charges and inversely proportional to the square of the distance between them (from wikipedia).

The static friction equation is far from a law, think of it more like a trend-line on a statistical plot.  There are lots of outliers, and as rgold mentioned, climbing rubber is certainly an outlier.

Thanks for the correction!

My guess is that the Bolivian you speak of would probably just prefer that you play it safe and let the lighter person lead that pitch...  Or let them pass.

Sean Peter wrote:

My guess is that the Bolivian you speak of would probably just prefer that you play it safe and let the lighter person lead that pitch...  Or let them pass.

I think he meant "Oblivion" dang you predictive text................. ;) 

Edit to add; I just googled "Bolivian food", being sent into Bolivian does not look too bad.............. ;) 

ZB 4yr wrote:

Help 

I'm wondering how my weight (195lbs at 6'4")affects the friction between my shoes and the surface while climbing (smearing)

Let's say my girlfriend (125lbs at 5'5") and I are wearing the same shoes, have the same stance on the same slope, with the same amount of rubber engaged on the same kind of rock.  Am I more likely to come off because I weigh more?  If I weigh more, does than mean that the friction between my foot and the rock will be higher than that of my girlfriend, thus keeping me on?  Or is there a threashold of some sort that once reached will send me into Bolivian?

Thanks 

Now, how do you people know he doesn't swear in other languages when he peels??? Am I the only one who curses when climbing???

Sheesh people, don't be so judgmental!

Best, OLH

ZB 4yr wrote:

Let's say my girlfriend (125lbs at 5'5") and I are wearing the same shoes, have the same stance on the same slope, with the same amount of rubber engaged on the same kind of rock. is there a threashold of some sort that once reached will send me into Bolivian?

Thanks 

Maybe his girlfriend is "Bolivian".................... ;)  Or her name is "Bolivian".................. ;) Either way you have to be standing pretty close to be wearing the same shoes.......... ;)

John Barritt wrote:

Maybe his girlfriend is "Bolivian".................... ;)  Or her name is "Bolivian".................. ;) Either way you have to be standing pretty close to be wearing the same shoes.......... ;)

Aargh. I should know better! I haven't had my coffee yet.

OP, our "smart" devices are idiots. Me too, but not always as entertainingly.

My favorite so far is "biner". But, at least now I know far more people type "boner".

Best, OLH

It's a quote from Iron Mike Tyson 

Look up "bolivion Mike Tyson".  You'll be in for a treat. 

Old lady H wrote:

Aargh. I should know better! I haven't had my coffee yet.

OP, our "smart" devices are idiots. Me too, but not always as entertainingly.

My favorite so far is "biner". But, at least now I know far more people type "boner".

Best, OLH

climbing friend,

why you search tubes of internet for "boner?" much better you make special friends in real life yes?