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Boot sole physics help

post #1 of 20
Thread Starter 
Genious I am not. I can't figure out if my thinking is correct on this so I thought perhaps someone can help.

I got new boots (292 mm) and only my heel peice can be moved. When I move the heel peice to fit the new boot size the boot center mark ends up ahead of the OEM mounting point on the ski. Boot center mark ends up 7.5 mm ahead of OEM mark on ski.

So I have to remount my toe peice regardless. Heel peice can remain mounted as is as there is enough adjustment to accomadate either boot in the future.

If I remained with my old boots (307 mm) and I had to remount I would want my center of boot remounted to be 3 mm behind the OEM mark on the ski for better deep powder performance,

My question is:

If I remount the new boots (292 mm) with center of boot lined up to center of OEM mark will I have less pressure to the front of the ski than with the 307 mm boot simply because the new boot is 15 mm shorter (7.5 mm each end)

Leverage laws would say it would require more input to the 292 boot to equall the same reaction that I used to get from my 307 mm.

but if my weight is distrubuted the same with either boot (IE stance remains almost exact) then there would be no change to how the ski performs.

What say yee?

Diagram is cheezy but perhaps it will help in the discussion.

post #2 of 20
Quote:
Originally Posted by Marmot mb View Post
If I remount the new boots (292 mm) with center of boot lined up to center of OEM mark will I have less pressure to the front of the ski than with the 307 mm boot simply because the new boot is 15 mm shorter (7.5 mm each end)
Ah, no, you have more leverage to the tip of the ski because your ankle is now 7.5 mm closer.

The fulcrum is under your heel, not at the ski tip.
post #3 of 20
Thread Starter 
Quote:
Originally Posted by comprex View Post
Ah, no, you have more leverage to the tip of the ski because your ankle is now 7.5 mm closer.

The fulcrum is under your heel, not at the ski tip.
I'm a plumber so I will use a plumbing or tool analogy.

If I have a pipe wrench and I use a snipe on it.

I will have more leverage (power) from a 10" snipe than from a 7" snipe.

The end of the snipe would then be my ankle and as the ankle is closer with the 292 mm boot it would have less effect with the same amount of force, as previously with the 307 mm boot, as it would be closer now to the front of the ski.

or does this analogy not work in this application of a ski.

Is leverage really even at play?

What I am trying to acheive is move back the point on my ski where the majority of my mass rests. At present with the 307 mm boots I find "sometimes" I have to baby the tips a tiny bit in really deep snow when going fast and I would rather not have to do that but rather just stand normally and go hard.
post #4 of 20
If I know what a snipe is, it is an extension that slips over the handle of the wrench.

If so, your theory is correct, it's just that you are looking at it the wrong way around.

The tip is either a 7" extension with boots forwards or 10" extension with boots back. The 10" extension wil lift your toes MUCH easier than the 7" extension. The tips will be much more difficult to pressure when the boot is further back.
post #5 of 20
Thread Starter 
Quote:
Originally Posted by BigE View Post
If I know what a snipe is, it is an extension that slips over the handle of the wrench.
yes it is.

Quote:
Originally Posted by BigE View Post
If so, your theory is correct, it's just that you are looking at it the wrong way around.

The tip is either a 7" extension with boots forwards or 10" extension with boots back.
I think we loose each other here. I beleive there is two points of leverage at play but for now lets just discuss the one I was refering too. The back of the boot or the point that the rear of the boot makes contact with the binding.

If the back of the boot is closer to the front of the ski them the back of the boot would have less upward leverage effect to pressuring the tips if one was to lean forward, causing the rear of the boot to rise.

Also this works in reverse when one leans forward on the boot then the front of the boot gets pressed downward and if the front of the boot is farther away from the tip it will have less downward leverage effect on the tip.

Perhaps I should clarify or state what I beleive causes my tips to dive a bit.

#1 the point that at present my mass rests on the running length of the ski, and that this point is "possibly slightly more forward than I would like.

As referance I know my boots are mounted center of boot matched to OEM center of boot mark on the ski. Therefore I would want my mass resting slightly more rearward on the ski.

#2 the amount of leverage force that is applied by leaning forward or back against the boot which in turn exerts pressure or force to the bindings which in turn effect the pressuring of the ski against the snow.

I think these two factors cause the very slight amount tip dive that I experiance in deeper snow.

Where I get lost is: would simply having a shorter boot sole cause my mass to be moved back slightly onto the ski & if the change in available leverage force have an end result the same as if I simply moved my old boots back 3 mm or so.
post #6 of 20
Quote:
Originally Posted by Marmot mb View Post
. The back of the boot or the point that the rear of the boot makes contact with the binding.

If the back of the boot is closer to the front of the ski them the back of the boot would have less upward leverage effect to pressuring the tips if one was to lean forward
No. The fulcrum is underfoot. Of major importance is the lenght on either side of the fulcrum. You are thining tip to back of boot is one side, but, as comprex said, the fulcrum is under your ankle.

In fact, when you dorsiflex and plantar flex the foot (use a gas pedal for eg.) there is a spot on the sole that does not move up or down. It is stationary. THAT is where the fulcrum is located. You will have the same amount of leverage in the new boots as the old boots if you line that spot up in the same place.

Moving the boots back gives the tips even greater dominance in lifting your toes -- the result is that the shovels can lift your toes more.

IMO, there is a far greater effect in moving youre entire weight forwards than this subtle change in fulcrum position.
post #7 of 20
Just went through this process. First, I would not rely on the OEM mark. You want to use the middle of the running length of the ski. Put both skis together and mark where they touch at tip and tail. Measure the distance, divide by two and subtract the result from one of the marks. Make a mark on the ski.

Then put your boot in the binding. Measure the height of the heel and toe. The heel will be higher. Make a shim out of layers of duct tape on the heels equal to the difference.

Make a balance board, a board with a dowel or PVC pipe on the bottom. With your boots on stand on the board. move forward and back, until you find the point where tipping forward and backward takes the same effort. Have someone mark your boots at the center of the dowel/pipe. If no helper is available, mark the front of the boots, measure the distance to the dowel/pipe, measure the same distance on your boots and make a mark.

Mount the bindings, so the mark of the boot lines up with the mark on the ski. For powder, you may want to move slightly back, however the powder will make the front portion of the ski effectively longer, since the powder wil contact more of the upturned tip than the tail.
post #8 of 20
Thread Starter 
Quote:
Originally Posted by BigE View Post
No. The fulcrum is underfoot. Of major importance is the lenght on either side of the fulcrum. You are thining tip to back of boot is one side, but, as comprex said, the fulcrum is under your ankle. .

You will have the same amount of leverage in the new boots as the old boots if you line that spot up in the same place.
I now agree, after a long discussion with my buddy on the phone I was thinking about this wrong.

I agree because if one looks at the forces from binding leverage the front and back cancell each other out.

Also my previous statement is incorrect, the front binding would have more downward force to the tip not less.

"Also this works in reverse when one leans forward on the boot then the front of the boot gets pressed downward and if the front of the boot is farther away from the tip it will have less downward leverage effect on the tip."

I was not thinking about the forces being applied against the fulcrum point but rather always to the tip. Forces against the fulcrum point I agree is correct.

Quote:
Originally Posted by BigE View Post
Moving the boots back gives the tips even greater dominance in lifting your toes -- the result is that the shovels can lift your toes more.

IMO, there is a far greater effect in moving youre entire weight forwards than this subtle change in fulcrum position.
Are you saying (and I would agree) that with the front toe peice moved back some (which HAS to be done to get 292 boot center mark lined back up with ski mark) then produces more ski area infront of the binding to effect more upward forces to the toes.

I don't get what you are saying about greater effect in moving you entire weight forwards than this subtle change in fulcrum postion.
post #9 of 20
Thread Starter 
Quote:
Originally Posted by fischermh View Post
Just went through this process. First, I would not rely on the OEM mark. You want to use the middle of the running length of the ski.
Thanks for you post, I do really want to try this method on my next pair of skis but..... I have many days on these skis using that mark and 95% of the time the weighting is great, the ski performs well and I have no complaints.

It is only in deep Powder that I want my weight just ever so slightly back a bit, so that I don't have to weight back to go crazy fast in powder and feel like I am water skiing.

I can't say the ski wouldn't perform even better with your method the 95% of the time, maybe it would.

It's just that I don't want to effect that large of a change from my base knowledge that I know from actuall experiance using the ski for a season and a bit.

I am going to however do the first part just to see where the mark lines up in relation to your method.

Question when you went through this did you change from using it at OEM mark to the new method? what change did you notice?
post #10 of 20
Thread Starter 
Quote:
Originally Posted by comprex View Post
Ah, no, you have more leverage to the tip of the ski because your ankle is now 7.5 mm closer.
sorry to re-reply but after better understanding the fulcrum concept, I want to question this point.

If I keep the center marks lined up then shouldn't my ankle really stay in exactly the same location regardless of boot sole length?

I would agree with you if I did not remount the front toe peice as the center of the boot would then be 7.5 mm more forward then before, but so then would the center mark of the boot as well. Which I know is going to make matters far worse for powder.
post #11 of 20
Quote:
Originally Posted by Marmot mb View Post
sorry to re-reply but after better understanding the fulcrum concept, I want to question this point.

If I keep the center marks lined up then shouldn't my ankle really stay in exactly the same location regardless of boot sole length?
You are quite correct, if the foot is equally centered in both boots.

(it may not be, but that's outside of the scope of discussion here)


In writing ankle I may have been guilty of an oversimplification and we might look at where that pivot is more closely.
post #12 of 20
How about this answer: if you are concerned about leverage applied to the bindings, a shorter boot give you more force, not less, because it is a type 2 lever, like a wheelbarrow. The force applied upward by your boot is the torque applid by your foot divided by the moment arm, and the shorter the moment arm, the more the force. This is why shorter boots get higher DIN stteings. As long as the boot finding interface doesn't move, the torque applied by your foot to the ski is constant, and the ski's length is constant, so there is no change in force applied. If you change the mounting point of the boot center that would change the moment arm of the ski, and given the same torque applied by the foot, you would decrease force on the ski tip if you move the mounting point rearward, and increase tip force if you move the mounting point forward.
post #13 of 20

try them the way they are without bothering to move the binding

you may not notice 7mm, or you may be closer to the center of the running length and like it better.
post #14 of 20
I have RX 8's with Railflex bindings. Last year, I used the method and mounted them as far forward as possible. They skied fine.

This year I got a new pair of boots. I replaced my Tecnicas with Heads. The Tecnicas were too big and I wanted to try a more upright boot. I went through the process, but did not shim the heel to account for the binding height/delta angle. I took them out skiing and could not keep my tails from washing out. Being too prideful to attribute it to poor technique, I spent a week going over in my head what could be wrong.

I posted a question on the forum think the the CRL theory may not be right, due to the structural members in the ski and the plate. Finally, I decided to try shimming the heel and using the CRL. Shimming the heel moved my balance point forward. I adjusted the binding to match the CRL with the new point. Skied on Monday night and it made a huge difference, the tails tracked the tips. I am amazed how moving a binding an inch makes such a large difference.

Hope this helps a little.
Quote:
Originally Posted by Marmot mb View Post
Thanks for you post, I do really want to try this method on my next pair of skis but..... I have many days on these skis using that mark and 95% of the time the weighting is great, the ski performs well and I have no complaints.

It is only in deep Powder that I want my weight just ever so slightly back a bit, so that I don't have to weight back to go crazy fast in powder and feel like I am water skiing.

I can't say the ski wouldn't perform even better with your method the 95% of the time, maybe it would.

It's just that I don't want to effect that large of a change from my base knowledge that I know from actuall experiance using the ski for a season and a bit.

I am going to however do the first part just to see where the mark lines up in relation to your method.

Question when you went through this did you change from using it at OEM mark to the new method? what change did you notice?
post #15 of 20
Quote:
Originally Posted by Marmot mb View Post
Are you saying (and I would agree) that with the front toe peice moved back some (which HAS to be done to get 292 boot center mark lined back up with ski mark) then produces more ski area infront of the binding to effect more upward forces to the toes.

I don't get what you are saying about greater effect in moving you entire weight forwards than this subtle change in fulcrum postion.
I am saying that the 292 long boot sole in a 302 binding moved your entire weight fowards. That effect is huge, while the 7mm change in the position of the fulcrum is very small in comparison.

While moving back 7mm will add to the leverage that the shovel uses to lift the toes, moving back will reduce the tip dive simply because there is less weight on the tip. Having less weight on the tip is the bigger factor.
post #16 of 20
Thread Starter 
So I think there is some consensus that simply changing boot sole length as long as the center of boot mark is continued to be lined up with the center of ski mark & that the ankle or fulcrum point is not altered by that change that no noticeable change will be found.

However in my case the toe peice will be moved back 7 - 9 mm effectively exposing or creating more flexible ski in front of the toe peice.

Do you all think this will effect a change in the tip float or dive that would possibly equal or be near the same as moving the center of boot back 3 mm if I were to continue using my old 307 mm boots?
post #17 of 20
So long as your foot lines up at the same point on the ski, the performance ought to be identical.

Sure, you may have a slightly smaller flat spot when the ski is pressed into an arc due to the 292 vs 307 bootsole, but I doubt that you will notice it.
post #18 of 20
Thread Starter 
Just an update after having my toe peice remounted 7mm further back and getting a few days on them.

They seem to be slightly better in powder. I feel more centered and can bang off turns quicker and more relaxed. I don't seem to need to worry so much about tip dive anymore. Going really fast it's still in my mind, but maybe it's just me. Not sure if this is just from the new 120 FR Langes or the mounting. Either way I am happy with the ride quality.

I did notice that in chop or crud the tips are slightly harder to keep straight. I beleive this is because I have less boot sole length to keep things in check IE less leverage. Not a big deal and isn't really worth mentioning as after a few days I don't even notice it now, but I did at first.
post #19 of 20

ski length in relation to boot length?

MARMOT, don't sell yourself short ("genius I am not")... you look pretty sharp to me! I think I had three levels of physics through high school, college and grad school... but you've taught me a lot, as you sorted through this.

FOG, this was a great answer below. Your last point has always seemed inuititive to me (more tip force with forward mount, less with rearward mount), but I haven't seen this clear an explanation for higher DIN settings with shorter boots. My related question (if anyone is still out there... I'm coming late to this thread)... is if there is a point at which sole length gets TOO big for given length of ski? Or conversely, where ski gets too short for given boot length? Either because of larger flat spot, forces, whatever. I've got big honking boots... size 12, 348 mm sole length. My current skis are 178cm, and I haven't had problems. But if I was considering shorter skis in the future, would there come a point where they were TOO short to function well in relation to those long soles?

Quote:
Originally Posted by FOG View Post
How about this answer: if you are concerned about leverage applied to the bindings, a shorter boot give you more force, not less, because it is a type 2 lever, like a wheelbarrow. The force applied upward by your boot is the torque applid by your foot divided by the moment arm, and the shorter the moment arm, the more the force. This is why shorter boots get higher DIN stteings. As long as the boot finding interface doesn't move, the torque applied by your foot to the ski is constant, and the ski's length is constant, so there is no change in force applied. If you change the mounting point of the boot center that would change the moment arm of the ski, and given the same torque applied by the foot, you would decrease force on the ski tip if you move the mounting point rearward, and increase tip force if you move the mounting point forward.
post #20 of 20
Quote:
Originally Posted by Marmot mb View Post
So I have to remount my toe peice regardless. Heel peice can remain mounted as is as there is enough adjustment to accomadate either boot in the future.
You don't have to do anything so drastic.
Quote:
If I remained with my old boots (307 mm) and I had to remount I would want my center of boot remounted to be 3 mm behind the OEM mark on the ski for better deep powder performance,
Where did you get the idea that 3mm was going to make a noticeable difference? I'd guess that a talented skier might be able to double blind figure out which ski was mounted that bit back, but if you want to mount yourself behind OEM you should choose a multiple of centimeters, not millimeters.
Quote:
Leverage laws would say it would require more input to the 292 boot to equall the same reaction that I used to get from my 307 mm.
No.
Quote:
but if my weight is distrubuted the same with either boot (IE stance remains almost exact) then there would be no change to how the ski performs.
Shorter boot=more leverage, just like FOG pointed out.

Empirical evidence of such: Your DIN setting and bootsole length are inversely proportional. The same torque applied by your leg (and the same bending forces in your lower leg) result in more force applied at the toe and heel of the boot.

I'll have to respectfully disagree with the consensus here and suggest that regardless of the placement-to-a-millimeter of the new boot, the new boot/ski will behave differently primarily because of the length difference.
Quote:
Originally Posted by fischermh
Just went through this process. First, I would not rely on the OEM mark. You want to use the middle of the running length of the ski...
This is, IMO, needlessly anal.

Of course, I don't believe in directional speaker wire either.
Quote:
Originally Posted by Marmot
I did notice that in chop or crud the tips are slightly harder to keep straight. I beleive this is because I have less boot sole length to keep things in check IE less leverage. Not a big deal and isn't really worth mentioning as after a few days I don't even notice it now, but I did at first.
You can believe whatever you want, but it would be unfortunate for anyone reading this to get the idea that it were true. As per FOG's explanation and the evidence presented in the ISO standard, you exert more leverage with the shorter boot in the twist axis as well. It would be mighty inconvenient if the twist and forward lean binding settings diverged as a function of sole length.
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