DR. BOB’S METHOD FOR MEASURING RAMP ANGLE


Introducing Robert Colborne, Ph.D

Dr. Bob, as we called him, was a consultant for the MACPOD ski boot project that conducted on-snow research with the Birdcage in 1991. His area of expertise is the human lower limbs and the quantification of moments of force or torques across the joints.

Dr. Bob’s method of measuring Net Ramp Angle offers some advantages over the method of calculating the individual angle of each component and then summing the angles to arrive at Net Ramp Angle. The drawback to Dr. Bob’s method is that it may not accurately capture the effect of toe spring, dual plane boot boards and heel lifts. I will describe the various aspects of ramp in my future post, THE MANY FACES OF BOOT/BINDING RAMP.


Dr. Bob on Calculating Net Ramp Angle

I had a little play with my skis/boots on the weekend. I built a ‘precision’ square (90 degrees) from a couple of pieces of wood and placed it in my Lange boot, mounted on the ski with the bottom arm of the square sitting on the bottom of the base assembly (boot board plus insole and heel lift).

In the photo below, the ski is decambered by the weight of the boot so the base is resting flat on the table. The slight curve created by the thickness of the ski that tapers towards the end that Michael mentioned can be seen. Moving the bindings forward as a unit lowers the toe boot interface and raises the heel boot interface. This increases binding ramp angle. Moving the bindings rearward as a unit has rhe opposite effect.

The angle of the vertical leg of the square can be measured with a precision inclinometer and subtracted from 90 degrees to arrive at NRA.

Skiboot

My Marker bindings are level heel/toe, so no ramp angle to account for there.  I had some problems with the shell of the boot coming in laterally below the hinge and rubbing on my lateral malleolus, so to get my malleolus up above that I had to put a small heel lift in…otherwise I’d have no skin left on my malleolus after a couple of runs. Plus, I have a ‘Happy Feet’ insole with a slightly raised heel… I carved out the medial arch support, so otherwise, the insole is flat and doesn’t interfere with pronation.

So, when I put my precision measurement tool in the boot I get a measurement of 9 degrees of total boot board angle and ski binding angle or NRA !!!!!!!

Back to the drawing board…

I have an additional issue with crash space above my instep in front of my ankle. I’ve dealt with that by increasing the tongue padding on the dorsum of my foot. Also, I’ve put a 6mm high-density flat rubber insole on top of the boot board, to raise my whole foot up in the boot. I can’t go any more than that without taking out the liner padding above my toes, so I guess that’s next. If I take out the heel lift, and put in some more high density rubber from heel to toe to lift my whole foot, that will reduce the total boot board angle, raise my foot to reduce the crash space dorsally, and also get my ankle bones up above the irritating shell deviation.

I have to wait for the ski season here to give you feedback…  In the meantime, I’m going to play with varying boot board angle on my force platform. I can put markers on my leg and boot and record the trajectory of the centre of pressure under my boot, relative to my limb segments, as I flex and extend my knee and hip over the boot. I’ll let you know what I see……

……… to be continued


FOOTNOTE

Dr. Bob: “I had some problems with the shell of the boot coming in laterally below the hinge and rubbing on my lateral malleolus, so to get my malleolus up above that I had to put a small heel lift in…otherwise I’d have no skin left on my malleolus after a couple of runs”.

My Comment: It is important to expand the boot shell, if necessary, to ensure the foot is fully supported on the boot board. Heel lifts can create a whole new set of issues.

2 comments

  1. There’s a much quicker & easier method to measure ramp angle & nra. The Clinometer iphone app measures planar level in two planes (ie along & across the zeppa) to 0.1deg. The app can be set to lock once the measured level stabilizes, making it perfect for evaluating levrl inside an object.
    If you first calibrate the app on whatever surface the boot or ski base rests on, you eliminate the effect of an unlevel surface.
    Then just throw the phone into the boot, let it sit for 2-3 seconds to stabilise and bingo!!

    1. I have tried using this app. The problem is that boot boards or zeppas are seldom flat in one or both planes even when they appear as if they were supposed to be. When a zeppa has what is called toe spring and especially a raised contour under the longitudinal arch, the ramp angle will be all over the place depending on where the phone is placed. I find it easier and more accurate to calculate the height of the heel end of the zeppa then plane it flat if flat and to specification. I will post on this issue soon.

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