So how do the inline skates turn?

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So how do the inline skates turn?

Postby piggyslayer » Wed May 05, 2004 8:30 pm

What is the physics of a parallel turn on inline skates.
Same question obviously holds for Harb Carvers.

I think this is interesting topic for all of us who are
into physics and want to know how things happen.

I found this link, it links 2 articles on the subject:
http://home1.gte.net/pjbemail/Turning.html

It becomes obvious after few minutes thinking about the physics of inline skate turn that what is happening here is far less obvious compared to skis. It is all in the wheels.
A more formal treatment of the subject probably does not even exist. There is a bunch of questions I do not know the answer to: like the level of impact of wheel density rating on the turn, the weight of the skater, etc.

I am interested in your thoughts.
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My son is an expert inline skater

Postby John Mason » Wed May 05, 2004 9:37 pm

It's interesting in that when he first started working on PMTS stuff, he said it's the same way he blades.

He has a hocky pair of roller blades. These will turn a very short radius when tipped. It's obvious to see how these turn. In hocky blades, the wheels are mounted on a frame that is slightly curved. So even a little tipping makes the arc of a circle.

In regular blades, it's the shape of the wheels that make them cut an arc when tipped.

On the carvers, I was trying railroad turns on them. I find them not as responsive as my skis. At this point I'm still much more comfortable on my skis than the carvers. One the plus side inline or carvers sure will get you fore aft balance in shape (or your going down like RIGHT NOW!). I do not have a roller blading background, so I find the carvers very challenging. Like - how does one stop? I'm still sorting that out.

Also, (though I haven't fallen yet) concrete is just a tad "firmer" than snow.
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Postby piggyslayer » Thu May 06, 2004 6:22 am

He has a hocky pair of roller blades. These will turn a very short radius when tipped. It's obvious to see how these turn. In hocky blades, the wheels are mounted on a frame that is slightly curved. So even a little tipping makes the arc of a circle.

I do not have inlines in the hocky variety, most pairs I have seen are not mounted on a curved frame, all wheels are touching ground, so the blade is straight, still you can turn them.

In regular blades, it's the shape of the wheels that make them cut an arc when tipped.

Well, yes and no, obviously round cross-section helps, but the same physics, as I read, is exhibited by, for example, car wheels (cars do not tip their wheels :( , so the forces in question are much smaller).
Do an experiment, take a pair of Harb Carvers or inline skates in your hand, put them on one edge and roll. They will roll straight. Now apply a lot of pressure (press down hard) and they will start to turn ? what kind of wheel shape can do that!
The physics is quite involved and is sort-of described in the link I gave.

Now I think that this phenomena could have been studied as part of engineering of the motorcycle wheels (similar forces can be at play, since motorcycle can be tipped to a side).
Does anyone know a study/theory which would be relevant?
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Postby Mr.t who forgot to sign i » Thu May 06, 2004 7:11 am

I do not have a roller blading background, so I find the carvers very challenging. Like - how does one stop? I'm still sorting that out


Stopping on inline skates starts by using the brake. So you push your skate with the brake forward after having moved all your weight on the other skate. In this way the skate after travelling a few inches goes up and the brake pad touches the ground. However, it is very hard to brake like that going downhill. It works only if never let your speed become more than a modest one. Carvers have to work the same in that respect.

On inline you use a T-stop, but I would be cautious about bringing one skate behind the other skate (where all your weight lies) at a 90 degree angle with respect to the skate you keep straight and apply gradually pressure with the wheels on the asphalt. Carvers are more cumbersome than inline skates for this purpose.

An idea is to use a snowplow which in inline jergon is called a slowplow. It works like on skis. But, I must confess, I have not tried it on carvers.

Finally, one old and fairly safe last measure kind of approach, is to run into the grass. In this way at least, if you fall, it ain't going to hurt badly.


If I get the carvers I want to try the cross-over move. It would be fun if instead of carving one could actually take turns by using the cross over mechanics which provides the fastest way to travel through a turn. I have a feeling that as your speed is low crossing-over will result in fast times on a slalom course, but if the speed exceeds a certain threshold then carving on carvers take over and wins.

I love wheels under my feet. :wink:
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Postby jclayton » Thu May 06, 2004 2:05 pm

I find the best way to slow down when at speed on inlines is to do quick slalom turns then turn up hill by putting your weight on both skates and bringing your heels together i.e. splaying your feet but keeping them almost on the same line . This turn is very sharp but you have to lean well into the centre of the turn .

I skate with a not so well trained husky who likes to stop suddenly and have a sniff etc so I have to be on the ball

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Postby piggyslayer » Thu May 06, 2004 3:53 pm

This is all very interesting discussion fitting the subject ?So what do I do with inline skates to turn them?, I hope someone will post some thoughts about ?So how do the inline skates turn??.

The question is why if tipped to an edge the inline skate turns as opposed to going straight?
If you tip your bike to one side forcing the front wheel to be straight then (besides falling) would the bike turn a bit or would it go straight? Your intuition may say it would go straight, so how come inline skate does not?

I have seen so much good discussion on this forum about ski turning radius and what it means from theoretical stand point, so how do you measure inline skate theoretical turn radius or Harb Carver turn radius?

Any ideas?
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Postby Mr. T » Thu May 13, 2004 2:07 pm

Piggyslayer, if you turn your bike to one side and you move your body in the opposite direction enough you can still go straight. It all depends on where the center of mass of the system bike + biker is.
On inline skates, you do the same, if you can tip your skates and move your body in the opposite direction you can still go straight as well. However, it is a trickier thing to do for you do not have handlebars to help you with it.

Basically, whenever the imaginary line (which represents gravity) through the center of mass of the system bike + biker or skates+skater is no longer passing through the mid axis of the system, a momentum is created which causes the system itself to turn.

If you'd like some sophisticated treatment on this part of mechanics, get yourself a copy of some advanced textbook in Physics at the master's level at least.


Inline turning radius is very variable and basically it depends on how much to the side you can lean and/or tilt the skate and this depends among other things on how much lateral stiffness the boot or the shoe offers. Plus, skaters change it completely by performing the cross-over which actually allows them to accelerate through the turn and change the radius of the turn almost continuously for practical purposes. It also depends on how long the frame is. 5 84mm wheels skates have longer frames than 5 80mm wheels or 4 100mm wheels skates, for example.

The hardness of the wheels also can contribute some to the turning radius.

A very interesting and complicated issue. Carvers are probably in between skis and skates and hence present probably a hybrid problem.

One could write easily a Master's thesis on the topic.
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Postby piggyslayer » Thu May 13, 2004 6:45 pm

Mt T thanks for taking on the issue. Here are some comments:

Piggyslayer, if you turn your bike to one side and you move your body in the opposite direction enough you can still go straight. It all depends on where the center of mass of the system bike + biker is.

Darn it, if I only new it is that simple I would not get all these bruises trying. :wink:

On inline skates, you do the same, if you can tip your skates and move your body in the opposite direction you can still go straight as well. However, it is a trickier thing to do for you do not have handlebars to help you with it.


NO you will NOT go straight, you will turn! And so will you on the bike just the radius on the bike will be bigger. This is the essence of my question: what is happening to cause the skate turning when tipped onto an edge.

If you'd like some sophisticated treatment on this part of mechanics, get yourself a copy of some advanced textbook in Physics at the master's level at least.


The relevant branch of science would be, I think, some branch of material?s science/continuum mechanics. This lies on crossroads of physics and mechanical engineering (closer to engineering) while continuum mechanics is more mathematized version of it. I do not think, a graduate physics textbooks would cover any of this. I my previous life I have done some of the math around continuum mechanics so this is how I know.

The hardness of the wheels also can contribute some to the turning radius.


I agree, and I think this is the essence of the parallel carved turn on inline skates or carvers. On a very high level the inline skate or carvers turn because the wheels are soft and get distorted around the contact area where they touch the pavement. The wheel partially reshapes itself to ?undo? the distortion and to a smaller extent parts of the contact area shift/move around.
This is how the turn happens but it is only hand waving explanation.


One could write easily a Master's thesis on the topic.


I think it can easily be a PhD thesis. I think some relevant work exist, not in textbooks, but is a part of the engineering study of motorcycle tires. Since tipping to one side or the other is how you ride a motorcycle engineers must have studied turning forces resulting from tire distortion due to the tipping action. I bet that whatever scientists/engineers have come up with is not much more than a finite element method to get some numerical assessments.

I guess I wrote the post to accentuate the fact that when we make our happy turns on skis we ride on top of easy to understand and comprehend physics (at least most of it), when we make our happy turns on inline skates or carvers we ride on top of a mystery.
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Postby Mr. T » Fri May 14, 2004 4:06 am

\quote]
The relevant branch of science would be, I think, some branch of material?s science/continuum mechanics. This lies on crossroads of physics and mechanical engineering (closer to engineering) while continuum mechanics is more mathematized version of it. I do not think, a graduate physics textbooks would cover any of this. I my previous life I have done some of the math around continuum mechanics so this is how I know.
[/quote]

Piggyslayer,
You could always go into quantum mechanics if you want for in the end all is a consequence of atoms, but really there is no need. About bycicle turning I remember my honors courses in Physics making us study this topic in classical mechanics. In most problems objects are simplified to be
a point, i.e. they have no dimensions. In this case you consider the dimension of the object and hence you get extra complications. Of course, if you want to compute the radius within fractions of an inch, then probably you are right and you need to add materials to the equations.

I only beg to differ. I can lean both the bycicle and the skates and lean my body in the opposite direction and still go straight. With a bicycle is easy, with skates a little less. We used to play that game with bycicles when me and my friends were 10-12, about
a quarter of a century ago. On skates, just hold one of your skates with your hand softly and lean it and push it from behind trying to stay central.
It will go straight. I tried it last night.
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Postby piggyslayer » Fri May 14, 2004 1:25 pm

Mr T.
I disagree. The turn is caused by a continuum mechanics phenomena and not a classic mechanics phenomena. You can study classic mechanics all you want and it will not explain what is happening. Continuum mechanics, materials science are part of mechanical engineering and are for solving common engineering problems dealing with bending, stress, elasticity, etc.

If you read my earlier reply to John Mason (3rd post in this thread), I did describe exactly the same experiment that you are describing, the skate when rolled by hand goes straight. Now put the skates on your feet and stand on the edge (by the way this is a great carving exercise) by tipping your feet, counterbalance with upper body. Your CM should be above the skates and they should be tipped to a good angle. Take ski poles and push YOU WILL BE TURING!
Explain that with classic mechanics! This is plain proof that something interesting is happening that goes beyond classical mechanics.

This phenomena cannot be explained with classic mechanics, it is what happens to the material in your wheels that causes the turn.

By the same argument, your bike is also turning, however, the radius is big enough so you do not notice it.

The whole conversation is just proving to me that this stuff is not trivial and gets people confused. Maybe I am confused as well, who knows. If so I would be grateful for setting me straight (hey wait, no, I want to be turning!).
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Postby Guest » Fri May 14, 2004 2:12 pm

It surely is an interesting issue Piggyslayer. If you push with poles I agree you will be turn on your skates. No doubt. You will create a momentum right there for which I cannot counter-balance in any way I know. I agree with you as well that if you put the deformation of wheels the equations get more complicated and clearly pushing with your hand from behind applies a force through the axis of the skate and does not deform the wheels to create the famous momentum. And, yes, I also agree about the continuum mechanics. That is what I meant by saying that you cannot consider the skate just like a point with all the mass concentrated at that point. Just did not use the right term, sorry. It is indeed the kind of mechanics I studied in my Mechanics honor course in Physics.
As for the bycicle you could be right. Give me a straight stretch of road and I think I can go counter-leaning for hundreds of yards until I get tired to stay in that position.

Have a nice week-end.
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Postby piggyslayer » Fri May 14, 2004 6:25 pm

I think I came up with a cleaner reformulation of the question so here it is:

Skis turn by bending, skidding, and by combination of the two.

Skates/carvers turn by ???

Please fill out the question marks.

I have omitted step and jump turns as they are possible for both skis and skates and are less interesting from the physics/mechanics stand point.

Guest, thanks for your post and thanks for agreeing with me.
About the bike: I think what is happening when the bike is tipped a small
correction is applied to the front wheel so that it goes straight.
If the radius is big such correction is hard to notice and happens in a natural way (in the same way we correct the direction when road slopes to one side).
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Postby *SCSA » Sat May 15, 2004 5:44 am

piggy,

So how do you like the carvers? Think they'll help your skiing? What's the word?
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Postby piggyslayer » Sat May 15, 2004 1:00 pm

I love them
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Postby Hobbit » Sat May 15, 2004 10:56 pm

I?d like to put my 2 cents on how the skate might be turning. My approach is rather simplistic but it may hold some water anyway. Also I find the illustrations on the reference page from post 1 in this thread a little bit misleading (just read on :) and you'll see why).

So if you think about the shape of the section of the wheel surface by straight plain when the plain is at 90 degrees to the wheel ( the wheel is rolling straight and the plain is oriented as the surface of the ground) you would expect it to be in the shape of an ellipse.

If the plain section would be made now at some angle (similar to the wheel tipping position), the ellipse would evolve into a more asymmetrical ?triangular? shape because there is less material in the wheel closer to the wheel side edge. When the gripping spot has an asymmetrical ?triangular? ellipse shape the wheel would try to roll in the direction of the lesser resistance and will be turning in the direction of tipping. The more you tip the wheel the more asymmetrical the gripping area would became and the turning would be sharper.

This logic would also explain the experiment from the same article describing how the little arrow drawn on the wheel is turning to the inside when the gripping area is under the glass which is tipped. The illustrations in the article show the gripping area to be symmetrical and that?s what I find misleading.
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