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measuring dissipation of force in bouldering mats

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12-May-2006
4:08:43 PM

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Chalk Free

12-May-2006
4:33:33 PM

Pasco scientific, texas instruments and others have accelerometers, force probes and a stack of other stuff that may be helpful.

G

12-May-2006
9:10:26 PM

From a theoretical perspective the force on the boulderer could be estimated from their rate of change in momentum (a rearrangement of the impulse equation which can be derived from F=ma).

Force = (mv2 (momentum at point 2) - mv1 (momentum at point 1))/time
Tthe longer the time it takes to reduce the boulderer's momentum to zero (i.e. come to a stop) the less force - think a very soft spongy mat that you sink into without bottoming out compared to a hard pile of closed cell foam.

In terms of numbers:
mv2 = 0 because it is after they have landed and the mat is compressed.
mv1 could be just before they hit the mat. From a theoretical point of view it would be easier to model the climber as a solid ball to remove the complication of legs absorbing any impact. Assume m=75kg (my mass), and v = 4 m/s (I fall from about 1 m and land flat on my arse - the 4 comes from v^2=u^2-2as). So mv1=300
time I will assume is 0.2 seconds. I guestimate this based on the fact that landing on my arse seems to be over fairly quickly.

Therefore Force = (0 - 300)/0.2 = 1500 N which is a force equivalent to sitting a 150 kg weight on top of me.

In terms of measuring the force you could either:
1. place the bouldering mat on top of some sort of load cell which measures force, then have someone fall on it.
2. measure someones velocity just before they hit the mat and measure the time it takes from when they hit the mat until they come to a stop, then use the equation above.

Hope this helps

cheesehead

12-May-2006
9:51:51 PM

You hit the spot, G
Fast rolling cameras are possibly the best way to measure this. With a consitent film speed, you can calculate time and velocity pretty easily, regardless of how complex your shape (climber, dog, jelly).
Accelerometers strapped to the climbers ankle is another solution.

mousey

12-May-2006
10:02:21 PM

>You hit the spot, G
hahahaha

gremlin

12-May-2006
11:12:51 PM

Wouldn't it be better to test the impact force on the climber rather than the ground under the mat?
Think of a mat with a layer of wood sitting on top a foam cell mat.
Dissipates force well but it might hurt to land on...

tmarsh

13-May-2006
9:37:29 AM

The sports sciency departments at Vic Uni would have access to high-speed video gor things like gait analysis.

tim

mikl law

15-May-2006
8:03:08 AM

A small accelarometer is cheap (<$1000) and pretty bulletproof, gathers data in microseconds (it's those 3G peaks that break bones). I can put you in touch with someone that knows a lot about them.

Otherwise an upper bound value can be guesstimated thus (out with the physics books guys):-
Distance fallen to top of mat gives you the velocity.
Max possible AVERAGE force comes from the time it takes to deccelarate over this ditance

cheesehead

15-May-2006
9:28:01 AM

I think both experiments are valuable - average force on ground and force through climbers landing gear. The difference will help you determine how much the force is absorbed versus disipated. I think there might be an important difference (will have to think about this)

Dalai

15-May-2006
9:49:40 AM

VU Biomechanics Unit - located in the Basement of the VU building in Flinders Lane in Melbourne.

I went into the labratory making inquiries a few years ago, as I was looking into going in that direction for a career. They have a very good set up!

Equipment on site

shmalec

15-May-2006
5:54:22 PM

I think this is probably a bit of a waste of time if you don't account for the wobbly nature of a person falling. I think the ratio of the falling distance divided by the thickness of the mat will give you the number of G for a solid body ie a rock. If you don't take into account the springiness in your legs, rotation about your cg, squishiness of your backside etc etc the numbers will suggest bouldering mats should be made like 50cm thick gym mats. Guess you could do some sort of relative comparison of foam - looking for particular acceleration/stiffness characteristics as you squash it, eg constant acceleration/stiffness up to some predefined load level.
Better go get some dinner...

cheesehead

17-May-2006
9:17:30 AM

>G for a solid body ie a rock. If you don't take into account the springiness
>in your legs, rotation about your cg, squishiness of your backside etc
>etc the numbers will suggest bouldering mats should be made like 50cm thick

I suspect they might. As far as the expeiment is concerned, I'd suggest you do several control runs (big weight), As stack of runs of mates falling onto the pad(s), and also some calculations/questions to physio/biomech guys as to what forces people can withstand before busting bits*
The interesting/valuable thing (thus the real point of this experiment) would be to see how these different values differ and relate.

*perhaps an intersting sideline would be to model Mikl's test to failure - see how spot on he is about 3g...

mousey

17-May-2006
4:59:51 PM

what kind of foam?
hot wires are pretty darn good for the job and easy to make

tmarsh

17-May-2006
5:05:52 PM

Not surprisingly, there is a tool for everything.

Bosch foam cutter

And if you have $500 or so to blow on a dedicated foam cutter, then it can be yours too.

The fumes that will come off the foam will be horrendous if you hot cut it, although it's easy to make a hot knife. The basic recepie is low voltage, high current - preferably via a variable transformer - and some kind of wire. It needs to be tensioned a little so you get a straight cut, but tension it too much and it'll want to stretch and break. The Nati mountain shop uses a neat set-up like this for cutting accessory cord and the like.

Personally, I'd give an electric knife a go first. As it happens I even have one you can borrow.
tim

deadpoint

18-May-2006
11:56:19 AM

To make a foam cutter get some stainless steel piano wire, a coping saw and a variable power supply, 5A at 0-5volts, works a treat, I use a setup like this for cutting foam wings for radio controlled gliders. I have made wings up to 3m in length like this

take!

1-Jun-2006
9:27:09 PM

Sorry to reply late (exams etc...)
I think that finding the amount of force which is suitable for the body is a starting point. Then sampling a small amount of your chosen material(s) to see which ones give the lowest readings with in the max force. The test sample can be any standardised size to suit your anvil/drop weight test rig.
Have a look into the way that motorcycle clothing impact protection is tested to gain CE approval in Europe (standard en1621-1 and en1621-2).
From memory the torso (thoracic cage) is good for 4kN, the major limb joints 35kN. The 4kN is also used for equestrian protection vests. Note, these are values for one-off impacts and are maximums to limit serious trauma, not forces for repeated bouldering falls. I remember having a book that had impacts using cadavers in simulated car accidents and recorded the forces and different 'injuries'. There should be similar lists of acceptable forces for human limb closed kinematic chains somewhere - try chiro, human movement and physio faculties etc.
Other thoughts on the test rig would be to contact Australian Standards for the number of a crash helmet test facility (can't be more than 1 or 2). They should be happy to walk a student through their place (maybe) and you can get ideas from them.
Look at gymnastic suppliers and see if their stuff refers to a standard, this standard may have useful force parameters (?).

Hope this helps. Good luck!

Oh and a new stanley knife (box cutter) blade should do most of the stuff, you should only need sample sized foam pieces.

JamesMc

13-Jun-2006
7:06:00 PM

Is there interest in getting a crash test lab to drop a dummy on a mat. Would be able to measure accelerations, neck forces etc........... ?

I MIGHT be able to arrange it

JamesMc

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