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Quad Roller Skating Forum Discussions about quad roller skates and any other quad skating discussions that do not seem appropriate for one of our other forums.

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Old April 13th, 2017, 02:50 AM   #1
amohrfeld
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Default King Pin Angle vs. Axis of Rotation: finally understood.

I've struggled with understanding how king pin angle effects skate behavior. It never made sense. When I twist the trucks by hand, the movement feels the same on either a 15 degree king pin or a 30 degree.
I finally did some analysis using a 3D model and Finite Element Analysis.

Here are my conclusions:
The cushions behave similar when loaded with a "twist action" (steep king-pin) as they do when loaded more directly on the corner (flatter king-pin like DA45). The difference between the two scenarios is so small that the variable can be neglected. Especially given the tight range of king pin angles that are commercially available 5 degree to 30 degree. Hence, king pin angle, by itself, has almost no impact on skate behavior.

My analysis consists of comparing two plates, each with a different king pin angle. For both scenarios the axis of rotation is fixed at 45 degree. The model's input values are the same in both scenarios. The only difference is the king pin angle and truck.

First is a Royal truck with a 30 degree king pin. The movement is .060in



Second is the Advantage truck with a 18 degree king pin. The movement is .061in, almost identical to the Royal.



The angle of the axis of rotation appears to be the primary geometry factor for skate behavior. The reason plates with flatter king-pins turn more is the geometry allows for a steeper axis of rotation. For example, in the following picture I created a 45 degree axis of rotation with a 0 degree king pin. You can see the elevation difference between the pivot ball and the top cushion. This geometry is impractical. Instead, the actual plates with 0 degree king pin will be designed with a flatter axis of rotation. This results in less rotation per lean movement (or a stiffer feel).



So that's my thoughts to date. I still have some more analysis to do, so no telling what else I'll come up with. Feel free to question or comment.
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Old April 13th, 2017, 11:30 AM   #2
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Quote:
Originally Posted by amohrfeld View Post
I've struggled with understanding how king pin angle effects skate behavior. It never made sense. When I twist the trucks by hand, the movement feels the same on either a 15 degree king pin or a 30 degree.
I finally did some analysis using a 3D model and Finite Element Analysis.

Here are my conclusions:
The cushions behave similar when loaded with a "twist action" (steep king-pin) as they do when loaded more directly on the corner (flatter king-pin like DA45). The difference between the two scenarios is so small that the variable can be neglected. Especially given the tight range of king pin angles that are commercially available – 5 degree to 30 degree. Hence, king pin angle, by itself, has almost no impact on skate behavior.

My analysis consists of comparing two plates, each with a different king pin angle. For both scenarios the axis of rotation is fixed at 45 degree. The model's input values are the same in both scenarios. The only difference is the king pin angle and truck.

First is a Royal truck with a 30 degree king pin. The movement is .060in



Second is the Advantage truck with a 18 degree king pin. The movement is .061in, almost identical to the Royal.



The angle of the axis of rotation appears to be the primary geometry factor for skate behavior. The reason plates with flatter king-pins turn more is the geometry allows for a steeper axis of rotation. For example, in the following picture I created a 45 degree axis of rotation with a 0 degree king pin. You can see the elevation difference between the pivot ball and the top cushion. This geometry is impractical. Instead, the actual plates with 0 degree king pin will be designed with a flatter axis of rotation. This results in less rotation per lean movement (or a stiffer feel).



So that's my thoughts to date. I still have some more analysis to do, so no telling what else I'll come up with. Feel free to question or comment.
I agree. I just like the extra flexibility of tuning that the DA45s offer. I went through a bunch of skates about 7 years ago to figure it out. The geometry of the DA45 type plate lends itself to personal handling tastes that others can't do. Basically you have to buy the plate that works for you in the other kingpin angles that can comply with your wishes to get what you want because of the geometry.
ITs great having someone on here that can do computer analysis of the "unknown" elements. Thanks
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Old April 18th, 2017, 04:52 PM   #3
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Very interesting, thanks for doing this. I've been wondering if the trucks geometry was designed by "tradition" or "seat of the pants" and if so, if there are creative new ways to design trucks.
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Old April 18th, 2017, 11:42 PM   #4
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Okay, i'm gonna start with this, lol, I don't know sheep **** from wild honey when it comes to breaking all these numbers down and blah blah.... well may I do a little BUT ...
I skated on a borrowed Royal once ... let's let that sink in. I skate a Proline or U III. My friend that let me borrow his skates told me that if I needed to tighten the action to go ahead and do so. Now, normally I prefer a happy medium between tight and loose. Oh yeah, I also skated a Roll Line at the time too. But anyways, I actually had to loosen his action a little to get close to what I was used to with my Proline and when I did I felt more stability from the Royal because the axle is directly over the pivot pin. Let that last few words sink in ...
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Old April 19th, 2017, 11:39 AM   #5
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Default Time and experience.

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Originally Posted by gotsk8s View Post
Okay, i'm gonna start with this, lol, I don't know sheep **** from wild honey when it comes to breaking all these numbers down and blah blah.... well may I do a little BUT ...
I skated on a borrowed Royal once ... let's let that sink in. I skate a Proline or U III. My friend that let me borrow his skates told me that if I needed to tighten the action to go ahead and do so. Now, normally I prefer a happy medium between tight and loose. Oh yeah, I also skated a Roll Line at the time too. But anyways, I actually had to loosen his action a little to get close to what I was used to with my Proline and when I did I felt more stability from the Royal because the axle is directly over the pivot pin. Let that last few words sink in ...
It all still comes back to preference. Everyone is different and no one skates exactly the same as the next person. Finding the one plate out there that reacts very closely to your preference is the key to all of this. It seems you found your plate by trial and error as most people do. Finding the right plate for a person can be challenging(money) so most people never find it and just skate what they have on hand not knowing there is something better for them out there. IT would take a huge amount of money to set up a variety of skates for people to skate at a rink and then fit and test until they find what they are looking for.
And then some people are not at the level of skate experience(new skaters) where testing would make a difference.
Going through the process, learning to skate, getting experience, getting fluid in your movements, gaining power in you skating and movements, then you find your beginning skates are not up to par. The better you get, the more your skating exposes the weaknesses of your skates or preferences of your equipment.
Basically, if you are determined to keep on skating, you will look for the perfect skates for yourself. Given time and money, you will find them. Or you will analyze them after skating a few and make(force ) your skates to comply... by modifying them to do as you wish, after understanding what makes them tick..
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Old April 19th, 2017, 03:11 PM   #6
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Originally Posted by gotsk8s View Post
... because the axle is directly over the pivot pin.
Bingo.

That's the critical difference between a kingpin that is more near vertical (Like a Pro Line) than one with more of an angle to it (like an Imperial). It has to do with how the skater's weight is carried.

With a vertical kingpin, the upper cushion bears most of the load, which changes the way the suspension behaves. The upper cushion is under a lot of preload, and the lower not so much. And the amount will vary with the downforce generated by the skater.

With an angled kingpin, the pivot bears most of the load, with the Imperial 5 pivot angle being the best example of that. That leaves the suspension more unloaded.

I always find it interesting when someone tries to definitively characterize a skate's behavior with hard numbers. While it should be technically possible, the reality of it is that there are so many variables that it becomes impossible to accurately solve for x.

Here's a couple of actions that defy conventional wisdom.

Heier Skate-Rite


Rol-O-Way Rocket


I own and skate both. And they don't behave like you think.

Here's a hint - the Rocket has a secret. Here's a teardown I did years ago.
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Old April 19th, 2017, 03:30 PM   #7
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Default Same w/ these





And these:

The more of the oddball actions I play with, the less I understand it seems.
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Old April 22nd, 2017, 02:33 AM   #8
amohrfeld
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Originally Posted by gotsk8s View Post
Okay, i'm gonna start with this, lol, I don't know sheep **** from wild honey when it comes to breaking all these numbers down and blah blah.... well may I do a little BUT ...
I skated on a borrowed Royal once ... let's let that sink in. I skate a Proline or U III. My friend that let me borrow his skates told me that if I needed to tighten the action to go ahead and do so. Now, normally I prefer a happy medium between tight and loose. Oh yeah, I also skated a Roll Line at the time too. But anyways, I actually had to loosen his action a little to get close to what I was used to with my Proline and when I did I felt more stability from the Royal because the axle is directly over the pivot pin. Let that last few words sink in ...
Position of the rotation axis relative to the axle might be very important. I've been trying to find a good way to test that theory.

I am convinced that position of the axle in relation to the pivot point is very important. Since it determines how much of the force goes through the pivot and how much goes through the cushions.

Also, I don't want to neglect the importance of cushions and pre-load. But its not my point right now.
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Old April 22nd, 2017, 03:04 AM   #9
amohrfeld
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Originally Posted by dvw View Post

That's the critical difference between a kingpin that is more near vertical (Like a Pro Line) than one with more of an angle to it (like an Imperial). It has to do with how the skater's weight is carried.

With a vertical kingpin, the upper cushion bears most of the load, which changes the way the suspension behaves. The upper cushion is under a lot of preload, and the lower not so much. And the amount will vary with the downforce generated by the skater.

With an angled kingpin, the pivot bears most of the load, with the Imperial 45 pivot angle being the best example of that. That leaves the suspension more unloaded.




The pivot pin takes most the load in any plate design (70-90%).

In a static scenario (rolling without turning) the king pin angle determined how the load is distributed between cushion and pivot pin. As you mentioned, more angle results in more load on the pivot pin.

For a "turning the skate" scenario, you add turning forces to the system. The more angle on the king pin, the more angle you get on axis of rotation. It is this axis angle that governs how much additional load is distributed between the cushion and pivot pin.




I always find it interesting when someone tries to definitively characterize a skate's behavior with hard numbers. While it should be technically possible, the reality of it is that there are so many variables that it becomes impossible to accurately solve for x.


The idea is to determine how a change in a particular variable effects performance. Once it is understood, move to the next variable.

I firmly believe that well designed skate may not give you the performance you want. But a poorly designed skate will never perform well.

Thanks for sharing the pictures. Cool designs. They still uses a similar axis of rotation (red lines).


Last edited by amohrfeld; April 22nd, 2017 at 05:11 AM.
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Old April 22nd, 2017, 04:56 AM   #10
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Originally Posted by amohrfeld View Post
As the top of the top cushion moves away from the axle (horizontal plane), the load transfers more to the pivot pin. However, DA45 trucks are designed to keep the top cushion as close as possible to the axle (horizontal plane),
I think you can safely ignore the relationship between the top of the cushion and the axle. I can show you actions that seem to violate your observation.

I'd focus on the weight distribution between the upper cushion and the pivot. The more weight the pivot carries, the more the cushions become unrestricted in their role in tempering the action.

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Originally Posted by amohrfeld View Post
What I find interesting is on most skates (15 deg to 30 degree king pin), the pivot pin actually takes the majority of the load (about 70-85%). It does not feel like this when you skate because the system is dynamic. The cushions smooth out the response.
Have you modeled this or is it just a swag? I do think you're onto something however, with the observation that the system is dynamic. That's precisely what makes this so hard to characterize.

Plus, don't forget that the dynamic doesn't end with the plate. The same plate set up the same way will behave completely different on two different skaters. Go model that.

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Originally Posted by amohrfeld View Post
The idea is to determine how a change in a particular variable effects performance. Once it is understood, move to the next variable.
Well sure. Variables are not islands, however. You can't change one element and not expect there to be changes in the others. There is a great degree of interaction.

Quote:
Originally Posted by amohrfeld View Post
Thanks for sharing the pictures. Cool designs. They still uses a similar axis of rotation (red lines).
You might want to spend some time wandering old threads. This subject has been beaten to a pulp long ago.
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Old April 22nd, 2017, 04:54 PM   #11
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Originally Posted by dvw View Post
I think you can safely ignore the relationship between the top of the cushion and the axle. I can show you actions that seem to violate your observation.

I'd focus on the weight distribution between the upper cushion and the pivot. The more weight the pivot carries, the more the cushions become unrestricted in their role in tempering the action.


I edited my comments right after I posted it. Because I was not explaining it correctly. (I have not completely wrapped my head around it. But I'm getting closer.) There are 2 scenarios to look at: rolling straight where the plate takes the skater's weight, and turning. The pivot pin takes most the load in the first scenario. The cushion takes most the load from the turning force.



Have you modeled this or is it just a swag? I do think you're onto something however, with the observation that the system is dynamic. That's precisely what makes this so hard to characterize.

I'll post calculations later. That's also why I had to break it into two scenarios. (supporting skater's weight, and turning)


Plus, don't forget that the dynamic doesn't end with the plate. The same plate set up the same way will behave completely different on two different skaters. Go model that.

The skate mechanics will still be the same, regardless of the skater. The models help me understand what variable to change to get a desired behavior. Once understood, the concepts can be used to design or tweek a plate for each individual's preferences.


Well sure. Variables are not islands, however. You can't change one element and not expect there to be changes in the others. There is a great degree of interaction.

The variables work together for the final feel. However, each variable does something different and I believe it is possible to understand the effect and side effects of making changes to the different variables.



You might want to spend some time wandering old threads. This subject has been beaten to a pulp long ago.
I have spent a lot of time looking at old threads. There is a lot of good information there. There is also a lot of bad information. Still many details exist that no one can explain (to my satisfaction). That's why I'm finally digging into it.
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Old April 23rd, 2017, 03:00 AM   #12
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Here is my load distribution analysis. I simplified it as much as possible.

It is broken into two parts.

Part 1:
First is the static load with no turning. As you can see, the majority of the load is on the pivot pin (80%). This analysis is interesting because small changes in the position of the king pin or pivot pin will have a huge impact on the load distribution.

For example, if we move the pivot pin back just .15" then 50% more load will transfer through the cushions. I am not sure how this will feel, buy I think it would be noticeable.


Part 2:
When turning, we have to add the foot force and wheel reaction that together "pinch" the side of the cushion. The force distribution between pivot ball and cushion changes based on the amount of angle movement. (So the result is an equation that I graphed) The more rotation, the more load on the pivot pin. However, the majority of the force remains on the cushion (about 70-100%).

These two parts need to be added together to get total behavior.

So what does all this mean? Not much. But I know this: All commercial skates that I own have very similar geometry and position ratios. I believe this is on purpose. The existing geometry must achieve a good balance of movement, control, strength. Testing other ratios will require making custom trucks. And I'm not ready to pay for that.



Turning diagram is a bit tricky to visualize. I had to center on the axis of rotation. This is the only way to get simplified equations.
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