 Poster: A snowHead
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Why doesn't rated boot flex effect rear binding din settings?
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Obviously A snowHead isn't a real person
Obviously A snowHead isn't a real person
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because it's tested on a pull release which has the toe of the boot as the fulcrum. The boot angle should be consistent as the flex rating of the boot will suit the weight/ability/strength of the skier.
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Well, the person's real but it's just a made up name, see?
Well, the person's real but it's just a made up name, see?
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So a small light skier that prefers a stiff flex boot?
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feef, you see, we often change people's boots for them, they sometimes fit directly into the binding with minimal fuss, if we then take them from say a 100 to a 130, they should up the rear DIN?..
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Anyway, snowHeads is much more fun if you do.
Anyway, snowHeads is much more fun if you do.
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a vertical pull is a vertical pull. The boot will be pulled upright whether it's a soft boot or a stiff boot, and then the heel gets lifted up with a slight pivot around the toe until it comes free.
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If they are changing to a stiffer boot, then would it not be reasonable to suggest they are doing so because they are skiing more aggressively and so feeling thighburn when overpowering a softer boot. In that case, you'd be upping the DIN according to their ability anyway?
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The purpose of the release point is to prevent injury. If the release point is not hit because of the flex of the boot then the flex is just doing some of the job of the binding. Remember, what you are doing in adjusting the binding is adjusting the flex on a spring. As feef point out, a pull is a pull and it will take a certain amount to force to get the release point regardless of the boot (or binding, for that matter) flex. I would not necessarily agree that boot flex is solely (yup, coat in hand) determined by weight/ability/strength – ‘purpose’ would be my overriding (walking towards the door) concern. You want flex in the park and lack thereof on the race course {closes door after passing through to ensure lack of contact with buttocks}.
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The resistance comes in the most part from the spine of the boot, before the "pull" is effective, it begins at the heel of the binding, this is the same force that will eventually cause the binding to release you from the heel, causing "walk out". Try dropping the din very low on your heel, now flex your boots and see how it rips out, now imagine, with more force required to move the boot, higher flex rating, how much more force you will apply and how much more easily the binding releases. We've experimenting at the shop, same boots, just upped the flex, 100-130, it's alarming!
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You'll get to see more forums and be part of the best ski club on the net.
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It's a long time since I did any mechanics but;
A softer boot will delay the moment that the release pressure is reached as the boot absorbs the initial part of the fall by flexing forward, damping the fall (taking more time). A boot with a stiffer flex would transmit this force to the binding earlier in the fall arc, thus releasing the binding before the skier's body has moved so far forward (less time).
Newton's second law says F=ma
Where F = the horizontal force on the skier's leg or the upward force on the rear binding.
m (mass of skier) is a constant, so F is therefore proportional to a (acceleration).
a = dv (change of velocity)/dt (change of time)
I think that dv is the same for both boots ie from 0 to accident speed, only the time to accelerate is different (greater for soft flex boots)
Thus F is greater for stiffer boots where the same skier has the same accident.
This would seem to indicate that DIN settings need to be increased for stiffer boots in the same accident scenario?
All a bit O level I know (and I may've made an incorrect assumption with dv), I'm sure somebody can do better! 
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ww166no, I frickin love you Barleymow.
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snowHeads are a friendly bunch.
snowHeads are a friendly bunch.
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Seems to me that a stiffer boots allows but does not require a higher release setting?
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And love to help out and answer questions and of course, read each other's snow reports.
And love to help out and answer questions and of course, read each other's snow reports.
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ww166no, the assumption with that is that the ski remains flat on the ground, stops moving, and the skier exits forward, pivoting about the toe.
In reality, the heel piece exits with a pull up, perpendicular to the ski, in which case the flex of the boot isn't relevant as it will straighten to the perpendicular before pulling up on the heel piece. There will be cases in between your total forward motion, and my total vertical, but I suspect that the incidence will tend towards a vertical force.
Note, I'm not saying that everyone is catapulted upwards, vertically out the ski, but a heel release of often when the ski digs in, it then flexes before the binding releases, and so the actual exit force is more perpendicular to the ski than longitudinal to the ski.
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feef, The superman double eject was my theroretical model! I hadn't allowed for ski flex so I agree with you, except to say that there will be almost no incidents of either pure vertical or pure horizontal motion. All falls will be a mix of both, leading to a mixed ratio of your principal (where boot flex doesn't matter) and mine (where it does) in every fall. It may be that your model is the dominant one, but depending how you fall the boot flex will have a greater or lesser effect (but always some) on the binding release. From the sound of it, CH2O's shop test was more akin to my pure horizontal model, hence a more notable result?
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You know it makes sense.
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I'd of thought that a stiffer boot would transfer the force of the fall to the binding more quickly but wouldn't affect the force at which the binding actually released. Whether the slight difference in elasticity in that part of the system is actual significant is the important factor though. I'd suggest in most falls it probably isn't due to the majority of the mass of the system being contained in something inherently very floppy in comparison (a human body).
Anyway DIN settings are a standard based on some reasonable approximations to put people into a reasonably safe range of release hence all the fudge factors about ability and age. Adding another variable into the setting isn't going to make it more accurate particularly as you then need to standardise the measurement of "boot stiffness" (e.g. how does one treat a boot with a smooth limited range of movement that ends on a hard stop versus one that progressively increases over a wider range of motion).
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Otherwise you'll just go on seeing the one name:
Otherwise you'll just go on seeing the one name:
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| Quote: |
I'd of thought that a stiffer boot would transfer the force of the fall to the binding more quickly but wouldn't affect the force at which the binding actually released
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That's what I assumed too, but if F=ma then 'more quickly' = >a = >F doesn't it?
The rest of what you say is completely correct, this is all a load of theoretical nonsense which has an almost negligible effect on an actual fall, however it's an interesting exercise and the OP has stated that his original question stemmed from an extremely artificial experiment on his shop floor which limited most of the variables to something like the situation discussed above.
Nobody is suggesting that you ramp up your DINs because you've bought new boots, but it might be one of the many factors you consider when you change your kit, lose weight or improve your skiing?
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Poster: A snowHead
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lose weight?? perish the thought
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Obviously A snowHead isn't a real person
Obviously A snowHead isn't a real person
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Well, the person's real but it's just a made up name, see?
Well, the person's real but it's just a made up name, see?
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The forward release function is, presumably, there to protect the knee joint in a forward fall.
If the binding is operating properly then it will release when there is a certain forward turning moment on the clog of the boot.
It is irrelevant where the leg is in relation to the clog UNLESS somewhow, there are some ergonomics that say that a knee is weeker when the foot is flexed upwards.
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ww166no, in reality we're talking about impulses as the forces involved are not instantaneously applied. In this case more quickly means with a higher impulse which could be the same force transferred in a shorter period of time.
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Anyway, snowHeads is much more fun if you do.
Anyway, snowHeads is much more fun if you do.
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| altis wrote: |
| The forward release function is, presumably, there to protect the knee joint in a forward fall. |
Having torn one of the muscles in my calf in a forward fall on (failing) to land a jump, I can confirm that it should also protect against that.
So, would a stiffer boot have protected me against that injury more than a softer boot; possibly, as I may not have flexed as far forward as to tear something. That follows that you could be as safe ramping up the DIN in a stiffer boot.
But was it over flexing that caused the tear, or me trying to pull myself upright?
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| meh wrote: |
| ww166no, in reality we're talking about impulses as the forces involved are not instantaneously applied. In this case more quickly means with a higher impulse which could be the same force transferred in a shorter period of time. |
OK, that makes sense, so what does higher impulse mean for either the skier or the DIN setting?
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| Quote: |
ww166no, in reality we're talking about impulses as the forces involved are not instantaneously applied. In this case more quickly means with a higher impulse which could be the same force transferred in a shorter period of time.
OK, that makes sense, so what does higher impulse mean for either the skier or the DIN setting?
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does that make sense? surely the binding is designed to release at a certain measure of force not when that force has been applied for a minimum amount of time as an impulse would suggest. (the binding has no way of knowing how much mass is being brought to a standstill and how long it will take, it only knows how much force it is feling right now)...
whole thread makes sense to me, harder boot = higher instantaneous force = easier release....
i think it's getting over thought a bit too much
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dulcamara, the problem with physics is quite often what makes sense is "not even wrong". 
I actually agreed with the conclusion above as well just with a bit more nuance.
ww166no, releasing more quickly. Although as above I doubt it's a significant when mixed into all the other shenanigans in setting the DIN.
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You'll get to see more forums and be part of the best ski club on the net.
You'll get to see more forums and be part of the best ski club on the net.
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| dulcamara wrote: |
| surely the binding is designed to release at a certain measure of force not when that force has been applied for a minimum amount of time as an impulse would suggest. (the binding has no way of knowing how much mass is being brought to a standstill and how long it will take, it only knows how much force it is feling right now)... |
Don't bindings have a certain amount of elasticity? You must apply sufficient force to overcome the retaining springs, and then move the boot far enough to reach the point of no return. A vastly higher force than the spring could oppose applied for a very small amount of time will not displace the boot far enough for a release to occur, and when the force ceases the springs will return the boot into place.
If the retaining devices were made of some sort of glass that turned to dust as soon as sufficient force were applied, the duration of that force would be quite irrelevant. But they ain't, right?
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Surely the DIN setting system, in a round-a-bout way, does compensate for stiffer boots by having Type 1/2/3/3+ skiers? The assumption being that better skiers have stiffer boots?
A Type 1 beginner is going to be in a 70ish flex boot so will be at the bottom end of the setting scale for his BSL/height/weight. Equally, an expert Type 3/3+ skier will most likely be in a 120+ flex boot so moves up the scale?
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snowHeads are a friendly bunch.
snowHeads are a friendly bunch.
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This all came about with a boot we were stiffening for a client, we have a rental binding on a false ski that we have fixed to the work bench, we then started going through different boots looking at the spine flex interface. This is where the boot is riveted in it's back. Into which we put various boots, and with our hands around the back of the spine pull the boot towards us and look t the range of motion and test the flex. I noticed that With the stiffer boots the rear binding was moving towards prerelease. We rechecked and made sure all adjustments were true. They were, so we dropped the din as per marker and then Salomon charts, With the stiffer boots and the same force the boots pulled out of the bindings, heel first of course. Hence the thinking. Similarly we sara video recently, a company had been testing boots with remove able soles, ie Quest, Cochise etc. at a din of 7, their tests saw the sole units rip out. Now, there's something fishy, as we have clients using these boots at din of 17, just more stuff to keep us thinking.
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And love to help out and answer questions and of course, read each other's snow reports.
And love to help out and answer questions and of course, read each other's snow reports.
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Also the skier type stuff has no interest to us, we regularly put intermediates in 120 flex boots, skier level and boot flex is a massive red herring as far as we're concerned.
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Also the skier type stuff has no interest to us, we regularly put intermediates in 120 flex boots, skier level and boot flex is a massive red herring as far as we're concerned.
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totally agree but for the point I made above, "Tesco burgers .... "sorry, start again, "horses for courses" - purpose is the overriding defining reason for choice of flex, not level of performance.
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You know it makes sense.
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Oh, and the maths above is a bit tosh - no offence - you need to use the functions related to elastic deformation, 'f=ma' assumes all parts of the body acted on are moving as one unit in a vacuum. The problem CH2O is addressing does not relate to static bodies in space.
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