Stuff I've learned from research! (New material)
Posted: Tue Jun 10, 2008 7:54 pm
So I've been scouring this site for months -- reading, learning, expanding my understanding of ski construction and design. I want to thank Kelvin, Kam and Kam and everyone who has contributed to this site. The wealth of knowledge we have here is truly amazing!
That said, I have come to realize that this site is not the end-all be-all when it comes to ski building. I recently got my hands on a copy of Lind and Sanders' book "The Physics of Skiing: Skiing at the Triple Point." All of you should have a copy of it. The chapter on equipment is completely invaluable.
The book talks about a lot of things that are not discussed on this site, or that have only been mentioned in passing. I'm gonna introduce those ideas here -- I hope that this thread blossoms into others, so that we can have some good in-depth discussions about this stuff.
1. The role of laminates
I'm not sure how many people on this site understand just how the strength of the fiberglass layers comes around. A sheet of cured fiberglass on its own is pretty flimsy. Skis work sort of like steel I-beams, whereby the stiffness comes from the *separation* between two flat pieces of steel (or fiberglass). I think the stiffness goes up cubically with separation, so a small change in core thickness can make a big difference. One model I made showed that the fiberglass was responsible for about 50% of the stiffness of the ski, and the core the other 50%. Lind and Sanders go so far as to describe the core as to say "the core of a ski serves chiefly as a separator for the ski's structural elements."
2. Pressure distribution
When you stand on a ski that has camber, more of your bodyweight is supported at the tips and tails than at the center. Same when you're turning. This pressure distribution changes when we lean forward or back, and it has huge implications for ski performance. So camber has a lot more to do with the dynamic properties of a ski than we thought before. The book doesn't say too much about calculating this distribution though. I'm working on it.
3. Taper (you guys probably already knew this one, but what the heck)
Taper is defined as the shovel (width - tail width)/2. It's bigger when the tip is fat and the tail is skinny. A ski with low taper will want to hook up and carve through the whole turn. One with high taper will be easier to release at the end of the turn. This is why some skis (my K2s) have "progressive sidecut" -- because it raises the taper, making the tails easy to release.
4. Tail stiffness
You can measure the stiffness of a ski tip or tail by clamping it at the center, loading the tip/tail, and measuring the deflection. The stiffness depends on the deflection and the distance between the tip/tail and the clamping point. Most skis have about the same stiffness in the tip and tail, but racers will want a stiffer tail so that it will continue to carve as they lean back at the end of the turn.
5. Mount point (this one is interesting)
The above-mentioned fore/aft spring constants vary cubically with distance from the clamping point (which models the boot/binding pretty well). So the turn-finishing properties of the ski are extremely sensitive to small changes in mount point. The book says a 3 cm change in mount point can make the tail's spring constant 25% higher.
6. Damping
This is a big one. I'm starting another thread for it.
7. Flexure and Vibration Modes
I started another thread for this one too.
That said, I have come to realize that this site is not the end-all be-all when it comes to ski building. I recently got my hands on a copy of Lind and Sanders' book "The Physics of Skiing: Skiing at the Triple Point." All of you should have a copy of it. The chapter on equipment is completely invaluable.
The book talks about a lot of things that are not discussed on this site, or that have only been mentioned in passing. I'm gonna introduce those ideas here -- I hope that this thread blossoms into others, so that we can have some good in-depth discussions about this stuff.
1. The role of laminates
I'm not sure how many people on this site understand just how the strength of the fiberglass layers comes around. A sheet of cured fiberglass on its own is pretty flimsy. Skis work sort of like steel I-beams, whereby the stiffness comes from the *separation* between two flat pieces of steel (or fiberglass). I think the stiffness goes up cubically with separation, so a small change in core thickness can make a big difference. One model I made showed that the fiberglass was responsible for about 50% of the stiffness of the ski, and the core the other 50%. Lind and Sanders go so far as to describe the core as to say "the core of a ski serves chiefly as a separator for the ski's structural elements."
2. Pressure distribution
When you stand on a ski that has camber, more of your bodyweight is supported at the tips and tails than at the center. Same when you're turning. This pressure distribution changes when we lean forward or back, and it has huge implications for ski performance. So camber has a lot more to do with the dynamic properties of a ski than we thought before. The book doesn't say too much about calculating this distribution though. I'm working on it.
3. Taper (you guys probably already knew this one, but what the heck)
Taper is defined as the shovel (width - tail width)/2. It's bigger when the tip is fat and the tail is skinny. A ski with low taper will want to hook up and carve through the whole turn. One with high taper will be easier to release at the end of the turn. This is why some skis (my K2s) have "progressive sidecut" -- because it raises the taper, making the tails easy to release.
4. Tail stiffness
You can measure the stiffness of a ski tip or tail by clamping it at the center, loading the tip/tail, and measuring the deflection. The stiffness depends on the deflection and the distance between the tip/tail and the clamping point. Most skis have about the same stiffness in the tip and tail, but racers will want a stiffer tail so that it will continue to carve as they lean back at the end of the turn.
5. Mount point (this one is interesting)
The above-mentioned fore/aft spring constants vary cubically with distance from the clamping point (which models the boot/binding pretty well). So the turn-finishing properties of the ski are extremely sensitive to small changes in mount point. The book says a 3 cm change in mount point can make the tail's spring constant 25% higher.
6. Damping
This is a big one. I'm starting another thread for it.
7. Flexure and Vibration Modes
I started another thread for this one too.