So how do the inline skates turn?

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Postby piggyslayer » Sun May 16, 2004 1:21 pm

Hobbit,
Thanks a lot, this is a very feasible explanation.
It is sort of like saying that friction is much higher on the part of the contact area closer to the inside of the turn, thus the wheel turns moving more on the side that the friction is smaller (the same idea as in the infamous wedge turn).

Thinking in this direction was also pointed out to me by one of my friends. The only possible doubt follows from the observation that wheels grip very well, there is almost no give in the grip (this was observed by authors of the link from my first post). If this is so, then the deformation of the whole wheel seems like a more feasible explanation rather than the contact area sliding against the surface (different parts of it at different rates due to different levels of friction) ... I hope I am making sense in explaining the difference.

I think there is some truth in both explanations and both contribute to the turn.
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Postby Hobbit » Sat May 22, 2004 5:29 pm

piggyslayer,

I think that the wheel grip is a static measurement or criteria vs. the wheel rolling in the direction of the minimal resistance is a dynamic system behavior. A similar discussion was going on on Epic where John Mason had to argue about the ski being able to bend dynamically due to the ski linear motion while the other guy was saying that you can't bend the ski more because of the surface resistance ( for example while standing on the carpet ).

Could we also discuss some other Carvers features?
For example, I'd like to understand the answers to following questions:

1. There are two wheels in the back and just one wheel in front.
I would expect the symmetrical wheel placement. Isn?t the ski curve shape more or less symmetrical?

2. Why does the front wheel has a smaller diameter?

I would speculate that the single wheel in the front provides more pressure and more wheel deformation, which probably causes a sharper steering.
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Postby piggyslayer » Sun May 23, 2004 10:24 am

Hobbit,

Since what we have as far as physics of the turn is some hands waving and nothing more it is hard to argue answers to your questions in any rigorous way.
Reading Harald?s answer would be really educational.

For now, what I propose is to make some assumptions and try to build our understanding based on these assumptions. At the same time we need to keep eye on empirical evidence to make sure that our assumptions are correct and they do not contradict reality (and we do not fall in logical traps similar to an argument that you cannot create an arc with straight ski).
Hey empirical testing will be fun!

Here is what I think we can assume:
A1) Increasing the pressure on the wheels reduces turn radius.
(if you tip inline skate holding them in you hand and roll they do not turn, if you put them on, put them on the edge and start rolling they will turn).

A2) Increasing the tipping angle reduces turn radius.

A3) Softer wheel will make shorter radius turns (??)
I think this is true based on (A1) but I have not experimented with it.

A4) I am not stating it since I am not sure if it is true. But A4) would describe relationship between wheel size and turn radius.

Here is my understanding of why there is one wheel in front and two in the back of Harb Carver, it is hand waving still, and I would love people commenting on it, and I would love to learn if am anywhere close to why the inventor wanted wheels designed in that way.

WHY 1 IN THE FRONT:
Moving your fore-aft balance in the fore direction will put more pressure on the SINGLE front wheel and according to A1) will shorten the turn radius. This is because front wheel will (according to A1) follow the tighter arc and the two wheels in the back are close together and will be somewhat unweighted. They probably ?skid? a bit.
NOTE: when you move your balance fore on the skis you will reduce turn radius so Carvers behave the same way as skis do!

WHY 2 IN THE BACK:
Moving your fore-aft balance on the aft side positions you over the two wheels. So there is far less pressure inflicted on any single wheel and you speed up and reduce turning effect.
NOTE: the same happens on your skis!

The same reasoning may explain why comp model has smaller wheel in front. You want to accelerate if you CM moves over large wheels and make slower turn when your balance is more fore. This mimics ski behavior.

MY EXPERIMENTS:
My experiments seem to confirm that I get easier turns when I move a bit forward on the carvers.
I have to admit that I still have hard time applying all skiing movements to Carvers. In particular being able to pull the free leg back in the upper part of ?C? generates a lot of pressure on the single front wheel. I think I am subconsciously afraid that any little stone or uneven pavement and the front wheel will refuse to move forward and I will face plant on the asphalt. So I end up moving my new ?free? foot a bit forward early in the ?C?. I am working on fighting this wrong movement.
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Postby tommy » Mon May 24, 2004 7:06 am

I have to admit that I still have hard time applying all skiing movements to Carvers. In particular being able to pull the free leg back in the upper part of ?C? generates a lot of pressure on the single front wheel. I think I am subconsciously afraid that any little stone or uneven pavement and the front wheel will refuse to move forward and I will face plant on the asphalt. So I end up moving my new ?free? foot a bit forward early in the ?C?. I am working on fighting this wrong movement.


I can relate to that: On inlines I've noticed that any "sharp" turn feels much more stable if the free "ski" has some lead, even to the point that the inside skate is fully in front of the outer one. On the carvers I really have to fight this tendency to inside tip lead as well, turning without it feels a bit shaky.

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Postby piggyslayer » Mon May 31, 2004 11:20 am

I have to admit that I still have hard time applying all skiing movements to Carvers. In particular being able to pull the free leg back in the upper part of ?C? generates a lot of pressure on the single front wheel. I think I am subconsciously afraid that any little stone or uneven pavement and the front wheel will refuse to move forward and I will face plant on the asphalt. So I end up moving my new ?free? foot a bit forward early in the ?C?. I am working on fighting this wrong movement.

Hey, I am quoting myself :oops:

It is strange but the movement has disappeared, it was there on Saturday, and it disappeared on Sunday. If I only knew why.
I suspect moving the foot forward may have been a result of trying upper body counter too much and moving my inside hip forward in the upper C.
I guess what is great about the carvers is that they let my body discover range of motion which is beneficial and pin-point movements which are too much.

Sorry, this is obviously not relevant to the topic of this thread.
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Postby Harald » Mon May 31, 2004 2:41 pm

How Harb Carvers Turn

When a Harb Carver is tipped on one set of its wheels the skier?s Center of Gravity (CG) begins to fall toward the center of the instantaneous arc, if enough tipping force is applied. The front wheel sets a different radius than the back wheels; this is proved by the wheel marks left on the pavement. Each track left by the Harb Carvers has two lines, one from the front wheel and one from the back wheels. The front wheels describe a smaller arc; the back wheels a slightly larger arc. It?s the force developed from the front and back wheels working in unison that allows the Carvers to turn and skid through a turn. The force generated by the wheels is called ?lateral or side force? also called grip, produces the turning. Here grip and skid are closely related.

As the skier goes through a turn the wheels produce lateral force using slip angle. Slip angle is the difference between the direction of the wheels or where the wheels are pointed and the actual direction of the Carvers. This is produced when the wheels are tipped on to their sides. Since the wheels flex and deflect, slip angle is possible. The shape of the wheels helps to create tipping once forces are applied. The wheels are able to grip the surface, but they also slip, yielding somewhat to the pressure developed from the turn and body weight on the wheels.

When pressure is reduced or taken away by flexing the stance leg, the angle of the Carvers is reduced easily and the Carvers come out of the turn. Momentum from the act of releasing helps move the CG toward the downhill side of the lower Carver (downhill leg or stance foot) at the end of the turn. This helps to put the Carvers onto the new wheels, for the new direction. Lateral foot tipping and allowing the GC to move freely across the wheels, creates the turn in the new direction. Momentum is the secret to getting the skiing feeling on the carvers. If you release the edges (wheels) from one turn and let your CG move toward the new turn, your Cavers will easily develop tipping angles for that turn. If you are stiff or hold onto the turn too long, the momentum will disappear, which doesn?t help to tip the Carvers for the next turn.
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Postby Jeff Markham » Tue Jun 01, 2004 6:47 pm

I've been doing some research since this topic came up. Because of my motorcycling background, I have been searching the Internet for motorcycle-related explanations of turning (since there are precious few references to inline skate turning and the tire/wheel cross-sections are similar).

It appears that motorcycles and inline skates have something in common called "camber thrust". Basically, it is the "cone effect" that Mr. Baum dismisses in his article. If I understand his objection to the "cone effect", he is saying that the skate axles are fixed parallel and would, due to the wheels' grip, therefore resist the turning effect. If the axles could be turned toward the center of the turn, as on a motorcycle, then they would converge to the center of the turn. They are obviously prevented from doing so by the skate frame.

I disagree with this objection on the basis of 1) the relative shortness of the skate wheelbase and 2) the deformability and slipping action (i.e., non-perfect grip) of the contact patch. The shorter the wheelbase, the less wheel deformation/slipping required for a given radius turn. The converse is true for longer wheelbases, e.g., those for a 5-wheel racing speedskate

Example: If you consider a theoretical two-wheeled inline skate with the wheels very close together (i.e., almost touching), it would turn very easily in response to tipping. The angular difference between the parallel axles and the center radius of the turn would be relatively small. On the other hand, if the front and rear axles were separated by 36 inches, it would be comparatively difficult to make a tight radius turn. In this situation, the angular difference between the front/rear axles and the center radius would be relatively enormous. Obviously, a normal pair of skates is somewhere in between these two extremes.

By the way, the actual radius of the turn is not solely influenced by camber thrust, because centifugal forces tend to increase the radius dictated by camber thrust.

For those of you who are interested, google "camber thrust" and "motorcycle" to see some interesting references. In particular, the following links discuss camber thrust (as well as other motorcycle turning issues):

http://www.tonyfoale.com/Articles/Tyres/TYRES.htm
http://www.ommriders.com/read-ride/adva ... eering.htm

From what I can determine, camber thrust is consistent with Harald's description.

Comments welcome.
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Postby Harald » Wed Jun 02, 2004 9:50 am

Yes, this is consistent with my understanding.

I have resently designed a steering mechanism (very simple) for the front wheels of the Harb Carvers. It may make them too easy to turn, but it's worth a try. I hate applying for patents, but if this additional twist to the Harb Carvers could make them available to rank beginners, we may have something for everyone, not just dedicated skiers. I won't be able to start on this project until July.
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Postby Hobbit » Wed Jun 02, 2004 7:05 pm

Harald wrote:Yes, this is consistent with my understanding.

I have resently designed a steering mechanism (very simple) for the front wheels of the Harb Carvers. It may make them too easy to turn, but it's worth a try. I hate applying for patents, but if this additional twist to the Harb Carvers could make them available to rank beginners, we may have something for everyone, not just dedicated skiers. I won't be able to start on this project until July.


Just a thought...

I don't know if this is applicabale, but it would be nice to have just one model of carvers ( Comp ) with the steering mechanism.
If you would be able to enable / restrict / disable steering there would not be a need to step through the models as you progress on carvers.
Start with the steering enabled (beginner), than make it a more restricted steering if possible (intermediate) and finaly disable it (expert).
This would allow combining the features of all existing models in one.
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Steering may not be the solution

Postby Harald » Sun Jun 20, 2004 5:10 pm

This is true, but skiing improvement is dependant on tipping not steering. If the Carvers turn with such ease that tipping is no longer the focus, we may end up with the same problem we have with skiing on snow. Skis turn so easily that skiers don?t see the need to improve until they run into terrain and snow where steering no longer works and tipping is essential. Then they realize they didn?t spend enough time on the right basics (tipping). The amount of tipping required to make Carvers turn easily is minimal. Making the grade on cavers is like a right to passage into the performance skiing world.

We are instituting a trade in or up grade program for those who are ready to move up to Comps or Pros.
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