|Originally posted by webboy:
...Can someone summarize the differences between forward mounting and adding a little ramp angle please?
"Summarize" may not be the right word, but here goes ...
To answer your question in a non-superficial way, we first have to look at something called the PDF. This is the distribution of pressure that the ski exerts on the snow along its length, and is one of the most important things in determining how a ski will behave. Take a look at the graphs referred to in Note #2, below, for some examples of different pressure distribution functions or PDF’s. Unfortunately, ramp angle, delta angle, cuff forward lean angle and fore-aft binding location all interact in a fairly complicated way with intentional and unintentional changes in the skier’s joint angles to produce changes in the PDF.
A skier’s boot simultaneously exerts a combination of a force AND a torque on the middle section of the ski. The force is not always pointed straight down, and the torque is not always exactly fore-aft (i.e., around an axis lying in the plane of the snow, perpendicular to the ski – sticking out of the sidewalls of the ski). However, to simplify the discussion, lets consider the basic case of a flat ski going straight on flat terrain, so the force and torque point exactly in these directions.
Considering this force in absence of torque, pure downward force from the boot always causes a peak in the PDF that is almost directly underfoot no matter where the binding is mounted on the ski. Move the boot/binding forward by a certain amount (and make no other changes), and the peak in the PDF will move forward on the ski by the same amount. Moving the binding also changes the tip-heavy / tail-heavy balance of the ski. It changes the polar moment of inertia of the ski. For example, moving it forward makes the ski easier to pivot. It also changes the relative amount of flex of the ski in front of and in back of the boot. For example, moving the binding forward will decrease the length of the ski in front of the boot and increase it behind the boot. This effectively stiffens the forward part of the ski and softens the rear part of the ski.
OTOH, the fore-aft torque that the boot can exert on the ski can drive the tip or tail of a stiff ski into the snow like a long lever, and hence change the PDF in regions of the ski far away from the underfoot area. This can produce major changes in the behavior of the ski without inducing any of the other effects mentioned in the previous paragraph. On softer skis, fore-aft torque will produce more localized effects in the PDF and reduced effects on the performance (ie, they are more "forgiving").
The way I like to think about ramp, cuff forward lean, and delta angles is any particular combination of these variables sets the minimum, midpoint, and maximum fore-aft angle that the lower leg can achieve relative to the snow. These variables also do other things such as set the min, mid and max angle of the ankle, but with respect to the pressure distribution of the ski on the snow, the first effect is more important. The reason this is so is that if the angle of the lower leg (relative to the snow) is changed, AND the angle of all other body joints remains unchanged, the fore-aft position of the CM will change, and this will generate the fore-aft torque that we discussed in the previous paragraph. As described earlier, a heavily pressured tip versus a heavily pressured tail produce very different behavior in the ski.
Obviously, the “fly in the ointment” is that when the angle of the lower leg changes, people tend to react to this in different ways by changes in the rest of their posture. Some will under-compensate, some will compensate, and some will over-compensate. For example, if the boot/binding anglea are changed to force the knee more forward, some people may try to keep their upper leg and torso almost vertical. Doing this would be under-compensation and will move their CM forward. Other people may react by closing their knee angle too much and may even wind up sticking their butts out to the rear. Depending on what they do with their pelvis and spine, this could move their CM backwards. This would be over-compensation.
The “bad” side is that these changes are not under the control of the ski/boot technician. The “good” side is that they ARE under the control of the skier (within biomechanical and psychological limits).
In summary, the way I look at these adjustments is that moving the binding produces changes in the performance of the equipment that are well understood, and which do not so quickly result in compensatory behavior from the skier. The changes can be as small as desired. OTOH, adjusting the boot & binding angles can produce significant changes in high end skiing, but because all skiers react differently, these changes are not quite as controllable / predictable, so their effects really should be backed up by on-snow experimentation / observation.
Hope this helped.
Tom / PM
1) There have been a fair number of previous threads on Epic which have discussed binding location, for example:http://www.epicski.com/cgi-bin/ultim...=002120#000002http://www.epicski.com/cgi-bin/ultim...=003266#000001
2) There have been an even larger number of threads describing ramp, forward lean of the cuff, delta, etc. It's probably best to search on them using these terms.
3) Examples of different pressure distribution functions (PDF’s) can be seen in a message of mine posted August 31, 2003 08:30 AM in this thread:http://www.epicski.com/cgi-bin/ultim...=004446#000008
4) In an old post http://www.epicski.com/cgi-bin/ultimatebb.cgi?ubb=get_topic;f=4;t=001321;p=2#0000 37
, I described a very easy way (ie, bathroom scale and a couple of telephone books) to measure the effect of various bodily movements and equipment tweaks like adjusting cuff or delta angles). While this measurement is static and not actual skiing, I find that a direct experimental approach like looking at the reading on a scale to be very compelling when people are unconvinced about the basic effect of various posture changes or boot/binding adjustments.
----[ January 31, 2004, 01:48 AM: Message edited by: PhysicsMan ]