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rgold
Trad climber
Poughkeepsie, NY
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Feb 28, 2009 - 01:50am PT
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Rubber friction is a very complicated thing. Have a look at
http://www.tr.ctw.utwente.nl/Research/Publications/PhDTheses/thesis_Deladi.pdf
for example. There's just a tad more to it than the naive model obtained from Amonton's two laws, which are probably best for two different polished metals. For example, the thesis notes that the microgeometry of the rubber plays a substantial role in the observed coefficient of friction. We don't know whether the variations in this microgeometry are bigger within one particular brand or between brands of rubber. And that microgeometry is influenced by the size of the normal load, with the result that different frictional "laws" may apply at different loads.
To begin to get anywhere, such tests probably have to be conducted with a human being (the same human being) in the shoe(s). And it isn't clear to me that a static slab friction test, even if more properly carried out, is the best measure of anything beyond absolutely pure friction climbing, which was probably never a central ingredient of climbing practice and certainly isn't today. It may well be, as JB emphasized, that the stickiest rubber in the slab test will have qualities of flexibility, conformability, edging ability, and durability that make it a poor performer overall.
From my very old-school perspective, the thing that makes modern rubber significantly different from the older rubbers is that in modern shoes you can move your feet on the holds while the holds are weighted, and indeed a certain amount of modern technique involves pivoting on weighted holds. Try some of those maneuvers in the shoes of yore and you'd be off the footholds and out in space in an instant. So some of what makes modern rubber "sticky" is its ability to conform over and adhere to macro features while moving under a load, and this may well be quite different than the ability to adhere to the micro features of a relatively smooth slab. Unfortunately, this type of conformable adhesion is almost certainly influenced by shoe construction, so it may be hard to discern the role that the rubber is playing, even if a suitable test could be created.
Finally, a word about statistics. The tests do measure something, namely the ratio of one particular fixed normal force to the corresponding frictional force for one particular size and shape of rubber on one particular material. But without some measures of variation and confidence intervals to go with the means, we have absolutely no idea if we are looking at significant differences.
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Omot
Trad climber
The here and now
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Feb 28, 2009 - 02:51am PT
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Dr. JRW,
Go for it! But please consult with a polymer chemist or two. Physicists tend to simplify the situation, as the NWU/CERN guy did. Every tire company must have a bunch of chemists and material scientists studying rubber friction on a bunch of surfaces and there must be a vast literature on the subject. Not the exact same application, but a good place to start to see what's been done and what is understood.
Also, please add error bars (a pet peeve of mine)! Looking forward to seeing the results.
Enjoy,
Tomo
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sb4
climber
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Mar 12, 2009 - 05:54pm PT
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Bottom line is I'd like to know if certain shoes would help me climb. The problem is we can't afford money or time to try every shoe for a few weeks, and even then your energy level could change, making comparison difficult. It's a tough problem. But a realistic rubber test might be a rough guide to shoes that perform a little better. I'd prefer to hear that all shoes are about equal and forget about it. But if some shoes significantly reduce slipping on rock, I'd have to be interested.
If I had to rig a test, I might try this. Get a vertical slab of granite, or go outside and find a suitable rock.
Rig up a 175 pound weight on top of a pole. At the other end, put the rubber in the shape of a piece of a large ball (sphere section) to keep the same contact area as we change the angle. Maybe some kind of a swivel joint would allow a flat section.
Attach a rope to the weight end, with the ability to keep it approximately perpendicular to the pole. Place the end with the rubber piece against the granite wall.
Lean the weight out at an angle to where friction keeps it from falling down. It should resemble a climber on a rappel.
Slowly pull in the rope to reduce the angle to the wall. At some point, the rubber will slip off. Measure the angle. Repeat many times.
Problem: how to keep the pole from swinging sideways toward the wall. Maybe instead of a single pole, make a fork with two poles each with a rubber "foot" against the wall. Or some kind of low-friction guide.
That's my 2 cents.
Regards,
-SB
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tolman_paul
Trad climber
Anchorage, AK
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Mar 12, 2009 - 06:09pm PT
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The funny thing is you mentioned what can have an effect, the shoe, and then you get hung up testing rubber which IMHO doesn't have that much of an effect, with what is currently available.
Shoes do make a difference, sometimes. I've had most itterations over the years from the glove like boreal ninjas which you could smooch into things nothing else would fit, the scarpa lemenstral which you can stand on nickle edges because they are so incredibly stiff, to many in between.
Seldom is the rubber the limiting factor, and how well a given rubber performs depends on how clean it is, how fresh (not oxidized) ambient temp, type of rock, etc.
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donini
Trad climber
Ouray, Colorado
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Mar 12, 2009 - 07:26pm PT
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I think the friction characteristics of rubber is overrated. If rubber is too soft it rolls instead of holding an edge.
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bachar
Gym climber
Mammoth Lakes, CA
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Mar 12, 2009 - 07:41pm PT
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All I can say is that if the rubber doesn't feel right to me, I ain't gonna be doing any free soloing with it.... just saying.
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donini
Trad climber
Ouray, Colorado
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Mar 12, 2009 - 08:15pm PT
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John, I agree with you. Obviously, you and I were around for the advent of sticky rubber. I'm saying that sometimes climbers get into micro differences about the stickiness of rubber and that, for me , there are other characteristics of rubber that are also important in making the shoe "feel right."
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bachar
Gym climber
Mammoth Lakes, CA
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Mar 12, 2009 - 08:29pm PT
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Definitely. Hardness is one and the other is "static" vs. "dynamic" coefficient of friction.
Some rubbers have a high static coefficient of friction but low dynamic coefficient of friction - they hold but when they pop, they pop quickly. Other rubber that has a little higher coefficient of dynamic friction will hold but slide slowly when it pops. The latter has a lot more "feel" to me and I can definitely tell a difference between the two.
It's very hard to test the differences between most rubber now days. I have my own testing system that I trust, others have theirs. When you get ten or twenty different rubber samples from different chemists it's good to have a test that will tell you what is best!
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tolman_paul
Trad climber
Anchorage, AK
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Mar 12, 2009 - 08:34pm PT
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The only time I've been unhappy with a climbing rubber is when it has rolled off of edges. I can't think of any time where I felt that a slightly stickier rubber would have helped signifigantly.
I'd rather have 1/2 dozen different types of shoes that excelled at different footwark, then 6 pairs of the same boots or shoes with 6 differen types of rubber.
I started climbing in the Fire age, so I've never known a non sticky climbing shoe.
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TradIsGood
Chalkless climber
the Gunks end of the country
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Mar 12, 2009 - 08:45pm PT
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Good reactions here.
To the physics prof. As Ed mentions, trust no climber to believe your results.
There was a test that showed conclusively that chalk reduced the coefficient of friction between skin and rock. That does not sell chalk. It does not get industry funding.
Skin properties are quite well studied in industry and support the conclusions of the friction test.
Your even above average climber is not educated enough to understand the studies, and he is also biased enough that even if he firmly believes that he understand climate change to be perfect science will pooh-pooh the study that shows chalk to be a hindrance.
Conformance, as rgold mentions, is an important property, not only for rubber but skin as well.
I bought a pair of Evolv's a few year's back. They apparently demonstrated to sales people to show that shoes were so "sticky" they would adhere to vertical surfaces. Trust me, under load, it does not happen! Shear, or deformation, plays an important role.
There is a lot more to the "equation" than friction coefficient. That is most important on slab climbing and least important on edging. But even slabs are not really linoleum or polished granite.
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jstan
climber
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Mar 12, 2009 - 08:47pm PT
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First the coefficients of both static and dynamic friction are of interest, unless your name is Croft.
Secondly, the rubber's hardness helps determine its purchase on edges. Here you need to design a surface texture you think is representative of the rock on which you will be using it and you have to use a representative normal force. There may be several textures and you may have different normal forces with different people. Face it. You have to design and prove each texture represents the designated rock and you have to cover the full range of expected normal forces.
An approach to designing the representative texture:
The presence of a foot in the shoe, I suspect, can be reduced to agreeing that the normal force is not constant over the entire area in contact with the rock. So come up with a model texture (above) that sort of agrees with what a person in the shoe can do on an inclined plane of the rock in question. Once the model is qualified forget about putting feet into the equation. Then vary normal force over all of these textures.
In defense of rationality a final comment. Shoes are cheap enough and they wear out fast enough so that everyone goes through many pair. Whatever the study shows I suspect the majority of people will still be going out and trying different shoes. They will be doing this even after all the manufacturers have independently done this study to make their product better than that of their competitors'.
I don't bother to mention the manufacturers will also be designing both smearing and edging models.
EDIT:
I did one of the early tests on the effect chalk has on effective coefficient of friction between the rock and skin. Early 70's as I remember.
When there is little chalk on the rock, chalk does indeed promote increased friction.
However when the remnant layer gets too thick the friction is reduced.
This is why for decades now people have carried toothbrushes so excess chalk can be brushed off, when encountered. I would say anytime you see the surface has a very consistent white appearance you are getting into the region where friction has been decreased. If you can see obviously chalked holds in photographs you are probably already in this regime.
When your hand is extremely sweaty and you grab a heavily chalked hold, the chalk itself becomes slippery.
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Splater
climber
Grey Matter
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Mar 12, 2009 - 10:12pm PT
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"There was a test that showed conclusively that chalk reduced the coefficient of friction between skin and rock."
It was a nearly useless test.
The main purpose of chalk is to minimize the reduction in grip caused by moisture/sweat/greasy skin. The reduction in friction due to chalk is much less than the reduction in friction due to sweat.
Try this test sometime:
Go climbing on a hot day.
Grease off handhold without chalk.
Hold on by using chalk.
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TradIsGood
Chalkless climber
the Gunks end of the country
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Mar 12, 2009 - 10:22pm PT
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Apparently you did not read it, Splater.
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bachar
Gym climber
Mammoth Lakes, CA
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Mar 12, 2009 - 10:25pm PT
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"The main purpose of chalk is to minimize the reduction in grip caused by moisture/sweat/greasy skin. The reduction in friction due to chalk is much less than the reduction in friction due to sweat. "
Got to agree with Splater...
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Splater
climber
Grey Matter
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Mar 12, 2009 - 10:56pm PT
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jstan,
Yes caked holds are greasy.
That doesn't mean they'd be less greasy without chalk.
When I climbed at a place that didn't allow chalk, the holds were even worse. They were covered in an oily slime mixture of sweat, dead skin cells, and dirt.
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jstan
climber
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Mar 13, 2009 - 01:14pm PT
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Since it was so difficult to quantify I did not in the 70's attempt to simulate various degrees of sweatiness.
At Carderock in DC we used to have 95/95 weather which did in fact make an observable difference in the sweat on one's hand. Even under those conditions and with unbroken streams of people going up and down routes using mica crystals all sloped the wrong way I don't remember chalk being used particularly. If by some accident someone did something amazing at Carderock we all were pretty sure Ken Wilson would not be there the next day giving interviews. I worked right next door to the Blue Plains Sewage Plant. If we all had been using chalk I might have decided simply to work nights at the Lab. Carderock would have been uglier than Blue Plains.
On a more interesting note. When I went to my first Facelift, out of nostalgia I went over to look at Sunny Side Bench following an absence of almost forty years. Its surface texture has been changed totally. The natural texture is entirely gone and it is smooth and coated deeply with rubberized DNA from all the bodies that have been dragged over it.
This is not to say that is altogether bad. After all climbing is only an activity wherein you walk up to whatever problem is there, and you amuse yourself by seeing if you can figure it out. Nothing deep or significant is going on. When we chalk up a problem all we are saying is that we are not interested in doing the problem that was there when we first walked up.
Other than the ugliness that is created I am not even saying it is wrong to avoid the problem that is really there.
Rubberized DNA can present some really interesting problems.
If you happen to be one of the first 100 parties to do a route that week chalk did help get you up it. No question.
An idle observation. The OW threads have apparently identified Basket Case as being perhaps one of the hardest off widths. With all of this notoriety and given judicious placement of several anxiety reducers(SAR's) rubberized DNA may even find its way to that august location.
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noshoesnoshirt
climber
dangling off a wind turbine in a town near you
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Mar 13, 2009 - 01:45pm PT
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Interesting article from the January American Society of Mechanical Engineers journal.
Highlights:
"• Friction force is NOT proportional to load, NOT independent of area, and NOT independent of speed.
• Friction coefficients from tables represent specific conditions. Your results may vary.
• High friction and high wear do not always equate."
http://memagazine.asme.org/Articles/2009/January/Theres_Rub.cfm
(I'm such a geek I have this on the back of my toilet with the Playboys, a copy of Rock'n'Road, and a book on numerical methods called ironically, "Relaxation Methods for Scientists and Engineers")
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MH2
climber
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Mar 13, 2009 - 02:29pm PT
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A guy let me try a pair of the new Firé shoes at UW rock. There was a concrete slab with embedded granite rocks that varied in angle. I just stood in my EBs on a section steep enough for them to slip slightly and then tried the Firés in the same place. The Fires were clearly stickier. Sticky isn't the best word for what we want, though, because you want the shoe to lift up without having to scrape bubble gum off the bottom.
I've been waiting almost 30 years for a better rubber but haven't seen any great leap like the EB/Firé horizon.
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noshoesnoshirt
climber
dangling off a wind turbine in a town near you
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Mar 13, 2009 - 02:52pm PT
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I'm holding out for this
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