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# The Effect of Terrain Angle on Center of Mass

I have been rereading the threads about forward lean, zeppa ramp angle and binding delta combining to position the point where a persons center of mass (COM) acts through his/her foot. There seems to be a consensus that the combination of angles should result in the force vector from your COM acting somewhere from the ball of your foot back to the middle of your arch more or less. The boot experts say that moving your COM outside this "sweet spot" would be detrimental to your skiing. There are various drawings and calculations to support this. I'm pretty average size and using the calculations my net forward lean from zeppa ramp and binding delta should be between 3.2 and 4.6 degrees. This to me seems reasonable for standing on a flat but wouldn't the force vector of my COM intersect my skis in front of my boot tips when I was bombing the fall line on a 30 degree ski hill unless I got very much in the back seat? As a skier makes a turn from one traverse through the fall line to the other traverse on the same 30 degree hill they encounter a range of inclination from 0 to 30 back to 0 and adjust their COM to suit. How important then is the ramp angle and binding delta when the ski slope inclination is almost always very much (several times) more?

Quote:
Originally Posted by Speeder  This to me seems reasonable for standing on a flat but wouldn't the force vector of my COM intersect my skis in front of my boot tips when I was bombing the fall line on a 30 degree ski hill unless I got very much in the back seat?

No. No matter what the slope angle is the fact remains that friction is largely absent (or at least greatly reduced), relative to an everyday "normal" situation we find ourselves in (flat, grippy surfaces, like in a ski shop ). But on the slopes, as our skis move down the fall line we must keep up with, or stay over them so that we may keep our balance axis over the balls/arches of our feet. Such fore/aft adjustments are most easily accomplished by sliding the feet forward or pulling them back underneath our CM's, thereby adjusting the position of our Cm's over our feet--we are free to do this because snow is slippery.

Also of importance here is that the skis always travel faster than our CM's (they always want to move away from us), so in order to have our balance axis that far forward when straight-lining the fall line we would have to be in an extreme and constant state of feet pullback. BUT, since we would be on flat skis at this point AND there would subsequently be little force from the snow pressing back at us such a movement would have little effect anyway (other than to keep us really over the forebodies of our skis) until that is, we encounter some sort of surface irregularity whilst in our excessively forward position.

IF we were standing statically (where our CM's are free to move but our feet are not) facing downhill on a 30* high friction surface and were then ramped an additional 3.2-4.6* the effect would be rather different.

Or perhaps I misunderstood your question.........

p.s. This is may be the wrong forum for this type of question.

zenny

Edited by zentune - 10/16/13 at 10:11pm

This is why you must move forward to stay perpendicular to the slope. so you must first be balanced statically and then the skier must adjust balance as the slope changes!

I am not as smart as Zenny or as clever . I am a simple man, hence, a simple answer!

Boot balance is more of an anatomical geometry issue.  Where does your body feel the most balanced, efficient and comfortable over the sliding platform.

Quote:
Originally Posted by Speeder

I have been rereading the threads about forward lean, zeppa ramp angle and binding delta combining to position the point where a persons center of mass (COM) acts through his/her foot. There seems to be a consensus that the combination of angles should result in the force vector from your COM acting somewhere from the ball of your foot back to the middle of your arch more or less. The boot experts say that moving your COM outside this "sweet spot" would be detrimental to your skiing. There are various drawings and calculations to support this. I'm pretty average size and using the calculations my net forward lean from zeppa ramp and binding delta should be between 3.2 and 4.6 degrees. This to me seems reasonable for standing on a flat but wouldn't the force vector of my COM intersect my skis in front of my boot tips when I was bombing the fall line on a 30 degree ski hill unless I got very much in the back seat? As a skier makes a turn from one traverse through the fall line to the other traverse on the same 30 degree hill they encounter a range of inclination from 0 to 30 back to 0 and adjust their COM to suit. How important then is the ramp angle and binding delta when the ski slope inclination is almost always very much (several times) more?

Speeder,

You might want to rethink that 0-30-0 thing.  As you are skiing down the 30 degree slope, you need to conceptually keep your body at 30 degrees to true vertical the whole turn, every turn.  If you get vertical at the top and bottom of each turn, that means you are banking your turn and leaning back at the finish and through the transition.   That's why Atomicman says stay perpendicular to the slope.

Figuring out what ramp angle and forward lean and so on that you need by thinking it out doesn't work so hot.  Trial and error work better.  Unfortunately, that takes \$\$ and time.

Are you just analyzing this for the pleasure of it, or looking to buy boots?

FWIW, I think what speeder meant by "inclination" in this case was the change in effective slope angle we encounter as we move through different phases of the turn (from slope 0* across the fall line to slope 30* in the fall line.....) as opposed, or at least that's how I took it.

Of course I agree with what you said though, LF

zenny

Edited by zentune - 10/23/13 at 6:04am

Thank you, zenny.  I agree with you too!

Speeder, the virtual bump idea is worth investigating. The idea mostly talks about tip drop though. Which helps explain how slope angle and balance in the fore art plane changes. Trouble is we ski in three dimensions. This means we need to look at how things occur in more than one plane, or perhaps since this is about alignment relative to the balance axis, it might make sense to speak about the three axis rather than planes.

Forces acting upon us get identified as vectors and the balance axis is a sum vector that shifts around sort of similar to how a top wobbles. Not exactly because it can swing from the bottom like a pendulum. Nor do we ski on level ground. That is why considering only one axis, or one plane only explains a small part of what is occurring as we turn. But how does all this relate to ski and boot alignment?

Ramp angles and equipment set ups get you lined up along the balance axis prior to moving onto inclined terrain. Which may seem insignificant but consider how a few degrees of wheel alignment significantly affects a car's stability and ability to either go straight, or track cleanly through a turn. We can compensate of course but why not align the wheels so we don't have to compensate for that bad alignment? Like ski set ups it is a static adjustment done in the shop but when adjusted well, the results out on the road are quite profound. Hope that makes sense...
Edited by justanotherskipro - 10/25/13 at 12:46pm

Speeder,

There is an accepted methodology for dialing in your saggital plane alignment which begins with assessing the ankle's dorsiflexion range to determine appropriate "ramp" angle and "cuff forward lean" angle then we look at the skier clicked into the bindings to determine a ball park static angle for the "delta" angle created by the stand height differential created by your particular binding and boot sole length.  The last parameter to consider is the binding mount position which affects where you stand over the ski's sweet spot.  By coordinating and optimizing these four parameters you create a solid "detent" or home position where you are the most balanced and can return to after leveraging either forward or aft.  We don't bother measuring angles as the actual numbers don't mean much.  Ultimately once these angles have been set it is fine tuned by experimenting with some fine tuning on the hill in a dynamic state.  Getting it right has a huge benefit to your skiing!

Quote:
Originally Posted by LiquidFeet

Speeder,

You might want to rethink that 0-30-0 thing.  As you are skiing down the 30 degree slope, you need to conceptually keep your body at 30 degrees to true vertical the whole turn, every turn.  If you get vertical at the top and bottom of each turn, that means you are banking your turn and leaning back at the finish and through the transition.   That's why Atomicman says stay perpendicular to the slope.

Figuring out what ramp angle and forward lean and so on that you need by thinking it out doesn't work so hot.  Trial and error work better.  Unfortunately, that takes \$\$ and time.

Are you just analyzing this for the pleasure of it, or looking to buy boots?

I am trying to rationalize if forward lean, binding delta and boot board ramp together are so important why I don't notice much if any difference between skiing on an Atomic 1018 with 1.5mm of delta and Tyrolia 17 with 5 mm of delta when both are mounted on similar skis.

Quote:
Originally Posted by bud heishman

Speeder,

There is an accepted methodology for dialing in your saggital plane alignment which begins with assessing the ankle's dorsiflexion range to determine appropriate "ramp" angle and "cuff forward lean" angle then we look at the skier clicked into the bindings to determine a ball park static angle for the "delta" angle created by the stand height differential created by your particular binding and boot sole length.  The last parameter to consider is the binding mount position which affects where you stand over the ski's sweet spot.  By coordinating and optimizing these four parameters you create a solid "detent" or home position where you are the most balanced and can return to after leveraging either forward or aft.  We don't bother measuring angles as the actual numbers don't mean much.  Ultimately once these angles have been set it is fine tuned by experimenting with some fine tuning on the hill in a dynamic state.  Getting it right has a huge benefit to your skiing!

Thanks for your reply and you can believe me when I tell you that I've spent way more time than I should have reading your (and others) detailed posts re the interrelationship between body morphology, the two boot angles, binding delta and binding position. My angst arises firstly from the fundamental importance of these angles and secondly from the constraints on adjusting these variables. I want a race/ high performance boot so my choice is limited to one or two models from each manufacturer none have adjustable ramp or forward lean. Presumably each manufacturer has their own apparently secret combination of ramp and forward lean that are unlikely to be too much different one to another which for some reason they prefer to not publish. It's clearly not to keep them secret because they have the tools to measure and for sure know the forward lean / ramp of their competitors boots.

Here's my personal heresy, I wonder how important dorsiflexion and boot ramp are in the whole equation, I'm not particularly flexible and when standing I can crouch and move my lower leg to an angle of 60 degrees to the floor without any binding or discomfort and there is no hiperf/race boot with 30 degrees of forward lean even when fully flexed. Boot ramp angle would serve to shorten the effective base that you stand on but only slightly and it would ameliorate slightly the negative effect of any forward lean on ankle dorsiflexion.

So in my ignorance I say as long as a person has an average range of motion any forward lean and ramp within reason will do.

Where I see forward lean being important is having enough angle to position your knees over your toes and that's about it. I have skied very aggressive forward lean boots (Caber) and very upright boots (Lange) and have been able to adjust to both. I have found I prefer the forward lean and ramp in the 70's model Grand Prix or Astral Slalom (Banana) if I knew what those numbers were I'd buy boots with similar numbers today. I have bindings with delta from 1.5mm to 6mm  (about 1 degree difference with my BSL) and can switch back and forth between them without any problem. So clearly one degree of accuracy isn't significant to my skiing.

In a nutshell I am unconvinced that boot ramp angle is an important variable to modify because the amount of change possible is insignificant when compared to the forward lean and the binding delta. If a boot has sufficient forward lean to position a persons knees over their toes no more adjustment is required.

The Emperor has no clothes! It's apparent to me that the boot manufacturers don't think that adjustability of these variables is important either or they would offer boot boards with different angles and forward lean adjustability. They do seem to agree that some limited shaft cant adjustability is a good thing.

I don't believe that they are depending on the skill of local boot fitters to fine tune their creations they know better than that.

not clear why you want your knees over the toes?

Why wouldn't you?

You can load the tips of the skis easier with a stiffer, more upright boot, if thats your goal.

If the knees bend so far that they are over your toes, your butt will have to stick out pretty far, and youre relying too much on quads to keep you upright. Or your boots are too soft.
Quote:
Originally Posted by rod9301

not clear why you want your knees over the toes?

In one or several (I can't remember) posts concerning how much forward lean is too much boot experts postulated that in order to have your CM over the mid arch area of the foot the front of your knee should be no farther forward than your toes. Thus if you have a long lower leg relative to your foot length you need a more upright boot and vice versa. It was also put forth that exaggerated forward lean leads to excessive quad burn but may provide better performance depending on the situation. In this case the experts suggested the only way determine the optimum forward lean was to vary the binding delta to find the "sweet spot". As I said in a previous post, I routinely vary my binding delta by 4.5mm without any perceptible effect. That being said the bindings are on different skis with very different characteristics and each has been tweaked for binding location which it has been suggested would compensate somewhat for the different binding deltas.

I'm still trying to come to terms with the boot manufacturers not providing adjustability in their boot angles in fact it would be difficult if not impossible to flatten or raise the boot board by even 3mm in a close fitting race boot and reducing the forward lean any amount requires some pretty major surgery.

The experts at Epic ski have also stated that softening the flex of a boot by removing the spine fasteners is not an acceptable method yet the manufacturers say in their manuals that this is the "spine rivets" purpose. I have a couple pair of boots with "FLEX ADJUST" printed beside the spine fasteners.

So on one side BOOT EXPERTS say adjust boot board angle, adjust forward lean, don't remove spine rivets, the manufacturers provide no ajustability and recommend the removal of spine rivets to adjust flex.

The experts also suggest your boot fitter should ski with you to asses your needs and verify their corrections....YEAH RIGHT! Like that's going to happen!

Yes, it's frustrating.

Thus the trial and error.  If you perceive no difference, then you must be making appropriate adjustments in your skiing when you switch.

Why mess with something if it ain't broke?

If we move our knees fore aft over our feet (standing in our shoes) we can find a place where we have most of our weight on our met heads (fore foot) with our heels lightly touching in our shoes.

This is the place (Sagittal plane, Tibia angle) we need to stand in our ski boots--- knees over toes is a kind of open ended idea, I mean how far over the toes is good?  Now when you are in this

It turns out this is a very finite "little" position----"Home base".

This Tibia angle position is controlled by the forward lean of our ski boots and will vary according to the size (Circumference) of your calf muscle in the boot.

I agree with a previous statement that boots should come with forward lean adjustments, I don't agree that a ski boots forward lean can't be economically adjusted, we do it all the time, on almost

By the way, this "home base" position is very finite---too little forward lean in the boot will stand you up on your heels and make the boots seem too stiff---too much forward lean will push the

knees forward too far and make you sit back.

Getting this just right is worth the effort, I hate adapting to overcome these issues, it cost too much.

mike

Quote:
Originally Posted by rod9301

You can load the tips of the skis easier with a stiffer, more upright boot, if thats your goal.

If the knees bend so far that they are over your toes, your butt will have to stick out pretty far, and youre relying too much on quads to keep you upright. Or your boots are too soft.

^^^ in front of your toes^^^^^

Quote:
Originally Posted by markojp

^^^ in front of your toes^^^^^

I think you are thinking of when you load the front of the skis you would have your mass moved forward somewhat, which would bring the knees in front of the toes.

I am referencing a "centered up" position, prior to initiation.

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