How does weight affect friction

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 

Your question is a physics problem! With all variables being equal except weight (force applied to the system) the friction force opposing the applied force will be EQUAL. When the applied force exceeds the maximum static friction force, you will slip! If any other variable changes, then the friction force will change. If you change the surface area of contact, the type of rubber, the angle of applied force, etc. then the friction force holding you on will change. 

A factor that can be overlooked is how well the foot is placed. With everything being equal (including applied force), you may find that the fluidity of movement of the body becomes a factor. Sometimes climbing can be like a dance, especially on friction slabs. To increase the amount of contact area, you can literally "smear" your sole to the rock. With practice you can feel the static friction giving way to dynamic friction which signals your body to release and re-adjust your foot, or get more weight on your hands.....It is subtle technique here....

Try out different shoes (flexible soles have worked well for me on slabs). The more slabs and friction you climb the better you will get at it.

Tim McGivern wrote:

Your question is a physics problem! With all variables being equal except weight (force applied to the system) the friction force opposing the applied force will be EQUAL. 

Maybe on a completely smooth surface like glass (most friction slabs have imperfections), but otherwise doesn't apply to rubber. The amount of deformity, which varies with weight applied, alters the amount of friction. Which is why some same model of men's & women's climbing shoes have different rubber (with the men's having a harder compound). 

With that said, it's harder for people w/ larger feet to be as precise w/ their foot work.

From a physics point of view, the amount of friction present is the product of the normal force and the coefficient of static friction.. Same shoes, same surface means the coefficient of friction is equal for both of you. The normal force is essentially how much weight is put on the surface.  So, all other things equal, twice the weight means twice the amount of friction.  So I would argue how much friction you both experience is equal, you weigh more but you have a proportional amount more friction supporting your weight.

From a climber perspective, how much friction you experience, and how likely your foot is to pop, depends on your technique, the geometry of your body, the distribution of your weight, and the surface area contact of your shoes.  

IMO, in reality reading a bit about slab climbing technique and experience are really the most important factors in getting the most friction, you can break out the trig and free body diagrams if you really want to technical about it(If sin(X) > u*cos(X), where u is coefficient of static friction, and X is the angle of incline, then slippage will occur, the "threshold") but again I trust my intuition and experience to tell me I'm going to stick.

reboot wrote:

Maybe on a completely smooth surface like glass (most friction slabs have imperfections), but otherwise doesn't apply to rubber. The amount of deformity, which varies with weight applied, alters the amount of friction. Which is why some same model of men's & women's climbing shoes have different rubber (with the men's having a harder compound). 

With that said, it's harder for people w/ larger feet to be as precise w/ their foot work.

I see your point. However, If the person is not slipping, the counteracting Friction Force is equal to their force that they are applying to it. It doesn't matter what the weight is, the friction force will be equal to that weight. Maybe I wasn't super clear....I believe you are talking about when it blows.....

All surfaces (including glass) are rough when you look at them close enough. As the deformity you mention brings the two materials closer together, the potential friction force will go up. 

As for the larger surface area question posed second, it does matter what the surface area is. The more surface area you have the higher the potential friction force is going to be, thus allowing more "weight" to be loaded onto the system! As discussed above, the more weight you put on the system, the closer the materials get, increasing the surface area, providing more friction, allowing more "weight" force into the system!

Brandon.S wrote:

From a physics point of view, the amount of friction present is the product of the normal force and the coefficient of static friction.. Same shoes, same surface means the coefficient of friction is equal for both of you. The normal force is essentially how much weight is put on the surface.  So, all other things equal, twice the weight means twice the amount of friction.  So I would argue how much friction you both experience is equal, you weigh more but you have a proportional amount more friction supporting your weight.

From a climber perspective, how much friction you experience, and how likely your foot is to pop, depends on your technique, the geometry of your body, the distribution of your weight, and the surface area contact of your shoes.  

IMO, in reality reading a bit about slab climbing technique and experience are really the most important factors in getting the most friction, you can break out the trig and free body diagrams if you really want to technical about it(If sin(X) > u*cos(X), where u is coefficient of static friction, and X is the angle of incline, then slippage will occur, the "threshold") but again I trust my intuition and experience to tell me I'm going to stick.

What he said!

42

Your question is an interesting one, and not one I've ever thought of, but I'd be interested to see the actual mathematical answer. I think someone above alluded to this, but in practical means there is a LOT of technique in maintaining friction on slab climbing. I remember struggling again and again and again to get up this 5.7X (on tope rope) slab in City of Rocks. I could only make it half way then pop. Kim Miller walks up borrows my shoes and proceeds to just walk up it like it was nothing, didn't even look like he was trying. We were of a similar weight and he even wore my shoes. Technique was the big factor that day. 

I think your height plays a bigger role than weight.  Shorter people have a lower center of gravity, making it easier for them to keep their balance, and they also have smaller hands/feet, making it easier to crank on tiny features.  Although taller people do have more reach, this doesn't usually help much on friction slabs.  What you are experiencing is probably more a factor of balance/technique than friction or biology.  Shoes make a really big difference, as does shoe rubber; not all are created equal.  Stay away from Vibram Edge rubber; it's horrible for this sort of thing.  My first time on the slabs at ERock was in TC Pros and it felt like glass; came back in Anasazi Pinks and it was considerably easier because C4 sticks and also tends to be more sensitive.  Edge is great for vertical edging, but on slabs Stealth is king.

The classical model of friction, i.e. Coulomb's Law, implies that surface area and weight are irrelevant. Without getting into the math (even though it's really quite simple), more area means that there are more little "patches" of rubber holding you, but each patch has proportionately less force pushing it into the rock to hold it in place, so the two effects cancel. Similarly, the more you weigh, the more force there is pushing the rubber into the rock, but the rubber also has to support more weight. Again, the effects cancel exactly. 

However, the Coulomb model is not very accurate for materials like rubber on rock. Car tires, for instance, experience load sensitivity; the coefficient of friction decreases with normal force (the force pushing into the road). So, a heavy car, which Coulomb's Law says could brake or corner just as fast as a light one, can't do so. I expect, but would not be at all surprised if I were wrong, that climbing shoes behave similarly.

Very different scenario.  Coulomb's Law doesn't take inertia into consideration, which would explain the difference.  Balancing on a slab is a very static activity; the effects of inertia would be negligible unless you were doing something wrong.

Ted Pinson wrote:

Very different scenario.  Coulomb's Law doesn't take inertia into consideration, which would explain the difference.  Balancing on a slab is a very static activity; the effects of inertia would be negligible unless you were doing something wrong.

Tire load sensitivity has nothing to do with inertia. It is, however, a dynamic loading condition, and I totally agree that hot rubber deforming at high speeds is a very different from the loading situation in slab climbing (especially at the rate I climb slabs :) I mentioned it more as an illustration that Coulomb's law applies poorly in many situations, rather than a direct model for slab climbing. In fact, given the conventional wisdom that "weighed rubber doesn't slip", the opposite might be true, that the coefficient of friction for shoes tends to increase with load.

climbing friend,

the weight it a makes you fat, must pull down with more crushingstrength

Friction is a very complicated thing.   The classic physics equation for static friction is really more of a guideline.  After all,  it tells is that the frictional force doesn't depend on surface area at all.  This is mostly true for some material combinations and is pretty wrong for others.   

Just take a look at the size of slicks on a drag car.   They aren't that wide just too look cool.  On the other hand sometimes you want less surface area, and a more jagged surface that will bite into the other material.   E. G.  Knobby tires on a dirt bike,  or mud tires on a truck actually sacrifice the amount of rubber in contact with the ground to allow the tire to bite into the dirt.   But of course those same tires suck on asphalt. 

Whenever you have one material that is softer than the other,  the harder one bites into the softer one, even if only at the microscopic level.  That is why a SLCD will hold in a granite crack but can be tagged right out of a wooden or glassy crack.   Stone bites into the relatively soft aluminum,  but the smooth lobes of the cam won't bite into wood unless they have some pretty aggressive teeth. 

With climbing rubber it's all about deformation of the rubber around the rough features of the rock.  But there is a balance.   Too much deformation and the shoe slips, too little,  and the stone can't bite into the rubber enough.   That balance is dependent on weight and temperature and other variables of course.   This is why women's shoes have softer rubber than men's, and drag racers do burnouts.

If you are a big guy,  you should try shoes with a little stiffer rubber and more supportive midsole.

From the perspective of the Coulomb approach, which is almost certainly wrong for the case of rubber on rock, neither weight nor contact surface area matter.  You don't slip if the "coefficient of friction" (in quotes because of its inapplicability) is greater than the tangent of the angle of the slab.

Coulomb's "Law" (it isn't even remotely a law the way Newton's laws are) is most applicable to the case of two polished surfaces in contact, so that the forces retarding sliding are essentially atomic.  When you get something like rock, which even when smooth is rough at a level above microscopic, and rubber, which deforms and can be penetrated by all the surface rougosities of the rock, there is no reason to believe Coulomb's "law" will give meaningful results.  It seems plausible that contact surface area matters, since it allows for more things to stick up into the rubber, and the cimber's weight matters, since a heavier weight presses the rugousities further into the rubber.

As long as you are using the same shoe the friction will always be the same, the weight doesn't affect the friction.

What you are really I think trying to ask is about how much force it will take to move 2 objects across the rock if the only difference is weight. The weight of the object pressing down will require different levels of force to be able to slide across the surface the same.

In the case of smearing the weight difference will have little to no impact into the equation because when you are generally smearing you are using your muscles to press towards the wall. If you are smearing by pressing straight down the wall there is nothing that will keep your feet on the wall so if you weigh more if anything it requires you to press harder in the smear to avoid sliding down but highly unlikely to be able to notice the different between 2 climbers unless one is extremely over weight because the heavier climber is likely to have more muscles to push their feet into the wall compared to the increase in weight.

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.

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 

To heck with all the math and physics..........been a slab climber my whole life, I've been 6' tall since I was 15, climbed at 150lbs, 230lbs and everything in between. Climbed in fire's when C4 and stealth didn't exist. Weight has never had a bearing on being able to stick. It's about applying your weight (whatever it may be) to your feet (no matter what size) properly, period. Somebody once said "If you're drowning in a bathtub or having trouble on a slab, stand up" ........... JB

Ted Pinson wrote:

I think your height plays a bigger role than weight.  Shorter people have a lower center of gravity, making it easier for them to keep their balance, and they also have smaller hands/feet, making it easier to crank on tiny features.  Although taller people do have more reach, this doesn't usually help much on friction slabs.  What you are experiencing is probably more a factor of balance/technique than friction or biology.  Shoes make a really big difference, as does shoe rubber; not all are created equal.  Stay away from Vibram Edge rubber; it's horrible for this sort of thing.  My first time on the slabs at ERock was in TC Pros and it felt like glass; came back in Anasazi Pinks and it was considerably easier because C4 sticks and also tends to be more sensitive.  Edge is great for vertical edging, but on slabs Stealth is king.

The height can play out the other way on a slab, too... If this were a true friction slab of consistent angle and smoothness,  with no "better" spots for hands/feet, then height is not an issue at all, and your "shorter people have lower center of gravity" argument has merit.

However, in most cases there ARE specific holds/foothold locations on most "slab" climbs. And then height COULD come into play where height is an advantage. A key thing about climbing slabs is that you need to have your center of gravity over your feet ( the so-called "ass out" stance) but if you simultaneously need to reach up to a hold that is far enough to make a shorter person lean in close to the rock, the center of gravity moves out of the base of support, and the feet are now more prone to sliding... while the taller person can reach the hold and still keep the center of gravity over the feet, keeping optimal friction.