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Old March 28th, 2017, 05:06 PM   #21
bigred875
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Originally Posted by snailmont5oh View Post
The guy and his buddies were all fantastic skaters. The one dude was jumping into the air and smacking the shiny paper stars hanging from the ceiling. The dude I was talking to did this thing where he skated on the edge of one wheel with his other foot wrapped around his leg for like a half a lap, at speed, dodging slower skaters. I talked to him because he could've been me 20 years ago, as far as body type, glasses, etc. The main difference is that I was *never* that good. So, about the time I tell him that, his buddy (the high-jumping one) says, "Show him your skates!" He did.









Yes. That's a two pieces of square tubing welded to a piece of round, and homebuilt trucks with 5/16" grade 8 bolts welded on for axles. And yes, one of the truck pivots is almost gone, with about 1/8" of material left. When I pointed it out, he admitted that he needs to add some metal there. That was the truck that he was skating on the one wheel on! I told him that I needed some pics, because you guys would never believe it.

On edit: Mods, if this doesn't belong here, feel free to break it out into it's own thread.
amazing.... do you think he runs these because he's broke/cheap or because he breaks everything?
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Old March 28th, 2017, 05:21 PM   #22
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By Doc: "That has a big impact on how the sk8r interfaces with the plates. It does not change how the mechanics of the sk8 work."

Yup, that makes sense. But I believe we are interested in a combination of both the skater interaction and plate mechanics.

Obviously, just don't expect a plate to change its behavior by the way it is sk8ed. The results from sk8r to sk8r will vary on the same sk8, but that is the sk8r, not the sk8s.


By Doc:
Be careful with the pivot angle thing. There are some odd ball action geometries that do NO sk8 as you would expect by looking @ them."

Again I agree. As you move the pivot angle more flat you quickly move the caster point away from the wheel. This makes a very unstable setup on the front wheel. It's like pushing a rope.

What I am wondering now is how far can I move the pivot angle and still remain stable. Since I am using 2 cushions, my damper should improve. So I think I can sacrifice a little more caster position.

The more I think about this, the more I am wondering if I am misunderstanding your pivot angle terminology. Are you talking about the imaginary line (axis of rotation) that the truck rotates around or swings on (most trucks don't actually rotate on that axis)?? Richard and I had a huge "discussion" about this years back. Search irregular tetrahedron... Should turn up.

Damping? Again, I'm not following. Urethane has hysteresis which certainly acts like a spring w/a damper, but I don't see how more would be better.

I know for a fact that for any given durometer, taller cushions behave like the durometer is less, even though they measure the same. NOTE: the retainer cups in your diagrams will make the cushions act shorter than they actually are.

Here is a ROUGH measurement of pivot angles for some plates I have lying around:

My existing double cone custom setup with Royal trucks: 45
Snyder SD: 53
Avanti: 50
Laser SA: 48
Atlas: 43 My daughter's - I have no idea how it skates. But she does well with them.
The Groove as shown above: 40
I have no basis for understanding your numbers. Can you pleased show me a pic of what these angles are in relation to??

ATLAS plates sk8 about like 15 Roll Lines. Their urethane cushions are essentially dead, the rubber ones are pretty sweet but don't last very long. I was getting 6 months out of mine.
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Old March 28th, 2017, 10:33 PM   #23
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Doc:
I am referring to the angle of the imaginary line. I'll start calling it angle of rotation. I was crudely recording approximate angles from other skates to see where my design fits. The DA45 is approximately 45 degree. But the others are not much different.

Essentially I am worried about wheel wobble (or other unpredictable behavior) that results from the unstable nature of the forward-most wheel. This video is a great reference.

https://www.youtube.com/watch?v=Gh7gWJAvOvs

Flattening the imaginary angle makes the front wheel more unstable.

I'm actually misspoke on the dampening. As the cushion length increases the dampen rate reduces. The same is true for the spring (as you stated). Both these together with the mass determine the resonance frequency - or wheel wobble point as we might call it. Ideally we want a setup with the natural frequency outside the normal operating range. Please allow me to revisit this once I work out the math.

But what I can say is this:

I played with different setups on the left and right foot and the double cone setup had far less wheel wobble than the single barrel. That's actually why I wanted to try this double barrel setup. That and skating the double cone is a blast.

Good point on the cushions. I can take them off the top barrel. I don't know the duro on the F-16 cushion. but I prefer the cup with the soft blue sure grip cone since it seems to give-out when tightened. (Or maybe my cushions were an old/bad batch)
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Old March 29th, 2017, 02:12 AM   #24
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Default Caster

Caster is the angle, just like rake on motorcycles. We have pivot pins/ball and sockets. I had been using the term pivot axis since the imaginary line starts at the center of the pivot ball.
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Old March 29th, 2017, 02:39 AM   #25
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Those are barbaric at best.

I will say this though. Anyone can machine a shape out of a piece of metal and put someone else's trucks on it. That guy gets mad props for making his own. When it comes to making plates, that's what separates the men from the boys right there.
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Old March 29th, 2017, 03:09 AM   #26
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OK, got my head around the math. I shall summarize:

It's all about the load that results in wheel shake (resonance frequency).
Using a longer or thinner cushion results in the shake occurring with less applied weight. This is preferred since more weight has shifted to the other foot before the wobble starts. Hence the wobble is less likely to impact the skater.

This matches my recent trials with different cushion shapes and lengths.

So, back to the project goals:

The plates are intended for session skating. I want a fluid surfing like feel to the skating. They must get to a reasonable speed, but max speed will be sacrificed for the movement gains.

The plan is to decrease the angle of the pivot axis to gain more turning for less foot movement. This will allow the skater to stay above his skate while making a tighter turn. However, the trade-off is a more unstable scenario for the leading wheels. I hope to gain back stability with the longer cushions.

At this point it's still all theory. And the best way to prove/disprove may be to build. But I'm still open to any more input.
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Old March 29th, 2017, 11:46 AM   #27
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Originally Posted by amohrfeld View Post
OK, got my head around the math. I shall summarize:

It's all about the load that results in wheel shake (resonance frequency).
Using a longer or thinner cushion results in the shake occurring with less applied weight. This is preferred since more weight has shifted to the other foot before the wobble starts. Hence the wobble is less likely to impact the skater.

This matches my recent trials with different cushion shapes and lengths.

So, back to the project goals:

The plates are intended for session skating. I want a fluid surfing like feel to the skating. They must get to a reasonable speed, but max speed will be sacrificed for the movement gains.

The plan is to decrease the angle of the pivot axis to gain more turning for less foot movement. This will allow the skater to stay above his skate while making a tighter turn. However, the trade-off is a more unstable scenario for the leading wheels. I hope to gain back stability with the longer cushions.

At this point it's still all theory. And the best way to prove/disprove may be to build. But I'm still open to any more input.
Making the angle more verticle, makes the skate turn more for less input. Too vertical and it won't matter, the cushions will have to be extremely hard to control the plate. It is a combination of skater, caster and cushion hardness. The longer cushion scenario eases the ramp up of resistance to deformation. This prevents traction breakaway on deep pushes while letting the skate handle well. Also axle centerline dimensions have to do with all this as well.
Reason to build. I was getting by on the test skates I had but, but I wanted more length than was available. So I built to the length I wanted. Using available trucks made it easy for me to build to the length I wanted. My White Magnums were 7 3/4 +". Stability is worked out by juggling the caster against the cushion hardness for acceptable stability. The only time my skates get the shades is when a yellow cone break down.

There is one other reason I am working on for building another set of plates. A lower CG of the pivot point. Stock factory trucks won't work in this scenario.
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Old March 29th, 2017, 02:11 PM   #28
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There is one other reason I am working on for building another set of plates. A lower CG of the pivot point. Stock factory trucks won't work in this scenario.
"lower CG of the pivot point" please explain. What is the goal?

Also, I'm going to do some finite element analysis before building. I need to make sure I am getting the movement I am predicting. I'll post pictures once complete. We can return to the conversation then. (I have been know to change my mind multiple timed during a design phase.)
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Old March 29th, 2017, 05:41 PM   #29
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"lower CG of the pivot point" please explain. What is the goal?

Also, I'm going to do some finite element analysis before building. I need to make sure I am getting the movement I am predicting. I'll post pictures once complete. We can return to the conversation then. (I have been know to change my mind multiple timed during a design phase.)
Most of the "truck type" skates have all the action going on above the axle, this puts force towards the outside of the outside wheels. This makes the inside wheels lighter in theory. It's theoretical but I think there may be a way to promote even better traction with harder wheels by bringing the center line of force closer to the center line of the truck, evening out the weight applied to the wheels while cornering. I would have to use 2 independent scales to test it on the analyzer. And a new plate with a completely new design. In my mind I see the outside wheels being loaded more in hard cornering. On the analyzer, I could measure it and prove it. Basically it is trying to move the center of gravity line of the plate closer to the center of gravity line of the trucks in contact with the floor. The pivoting of standard style plate pushes the centerline of the plate out. If the pivot point is lowered, the cg is brought closer to center.
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Old March 30th, 2017, 02:08 AM   #30
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Making the angle more verticle, makes the skate turn more for less input.
You are correct. Had to run 2 scenarios of my model to prove it. So my thought was backwards. To achieve the goals, the rotation angle will have to move vertical. This will actually work to my advantage with stability. But as you mentioned, controlling the movement might be difficult, especially with the lower spring-rate of the double cushions.

I'm not wanting a huge gain in the rotational movement. Maybe 10% more. I'm thinking this still keeps me in a range that can be controlled.
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Old March 30th, 2017, 04:18 AM   #31
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It just occurred to me that I have not considered an important concept: Load distribution between the Pivot Pin and Cushions. As I make the angle of rotation steeper, more load transfers through the pivot pin. In my experience this makes for a harder feel and a looser setup. Any thoughts? I'm going to have to do FEA to understand this concept better.
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Old March 30th, 2017, 04:41 AM   #32
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Most of the "truck type" skates have all the action going on above the axle, this puts force towards the outside of the outside wheels. This makes the inside wheels lighter in theory.

Interesting concept. I see where you are going with that. For speed: equal weight distribution to all wheels is ideal.

I've considered a similar design, but I'm more concern with stability. With the cushion and pivot above the truck, the system is naturally unstable. The Arius moves the cushion to a better location. And if it were easier to tune I might try it. Still there might be another way to do it. So good luck.
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Old March 30th, 2017, 11:14 AM   #33
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It just occurred to me that I have not considered an important concept: Load distribution between the Pivot Pin and Cushions. As I make the angle of rotation steeper, more load transfers through the pivot pin. In my experience this makes for a harder feel and a looser setup. Any thoughts? I'm going to have to do FEA to understand this concept better.
The sweet spot is not hard to find, and changes in angle will not affect loading between the pivot pin and cushions to any large affect. You are using stock trucks, so you will stay within their design range anyway.
If you are designing new trucks, that is a whole new thing.
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Old March 30th, 2017, 11:24 AM   #34
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You are correct. Had to run 2 scenarios of my model to prove it. So my thought was backwards. To achieve the goals, the rotation angle will have to move vertical. This will actually work to my advantage with stability. But as you mentioned, controlling the movement might be difficult, especially with the lower spring-rate of the double cushions.

I'm not wanting a huge gain in the rotational movement. Maybe 10% more. I'm thinking this still keeps me in a range that can be controlled.
You are going to find that the more steering you ask for as far as input/output ratio, the stiffer the cushions will have to be to control it. It is easier to ask for less and then lean the plate over slightly further to get the turning angle you are looking for. Steeper caster makes for twitchy handling and harder cushions.

The issue with harder cushions, harder on the frame/plate, boots and feet/ankles. If the plate is not allowed to lean over, the CG of the plate stays further to the outside of your turn radius, making the skate "top heavy". Steering can become very touchy, then go into immediate wobbles with too vertical of caster line and too soft of cushions. I experienced this with the SSTs. I was not going to harder cushions to stop it, so caster changes were in order. The two go hand in hand, caster and cushion hardness.
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Old March 30th, 2017, 11:32 AM   #35
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Interesting concept. I see where you are going with that. For speed: equal weight distribution to all wheels is ideal.

I've considered a similar design, but I'm more concern with stability. With the cushion and pivot above the truck, the system is naturally unstable. The Arius moves the cushion to a better location. And if it were easier to tune I might try it. Still there might be another way to do it. So good luck.
With the cushion and pivot above the truck it makes for more stable feel to the skater, but makes less than ideal conditions for traction. I have not skated Arius but would have to think that the traction would be better if only by a small amount in comparison because of the pivot area seems to be closer to the floor. Closer to the floor = CG of plate closer to center of "trucks" or verticle center line between the wheels during hard lean over of the plate.

Project is on paper at this time. We will see if it has promise. Got a way to go on it. Thanks.
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Old March 30th, 2017, 11:41 AM   #36
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Default Caster change and cushions,

Is the only way you can take your stock trucks to persuade your skates to modify their attitude. Caster rules steering, Cushions tame it. DA45s lend themselves easily to modifications. Others do not, the angles are wrong for caster changes. You can however increase cushion height IF you have adjustable pivot pins. Non-DA45s would have to have a redesign of the trucks on a particular plate to adjust the caster with out causing issues with the pivot positions and loading. If the angle is attempted to be manipulated by increasing the cushion height, then the pivot pin can be adjusted.But this only adjusts for the cushions height and you end up back at the same caster angle, the plate will be slightly further from the floor. So caster remains relatively the same as it was.

Caster is strictly controlled on non-DA45 type plates by truck design.
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Old March 31st, 2017, 03:02 AM   #37
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You are going to find that the more steering you ask for as far as input/output ratio, the stiffer the cushions will have to be to control it. It is easier to ask for less and then lean the plate over slightly further to get the turning angle you are looking for. Steeper caster makes for twitchy handling and harder cushions.

The issue with harder cushions, harder on the frame/plate, boots and feet/ankles. If the plate is not allowed to lean over, the CG of the plate stays further to the outside of your turn radius, making the skate "top heavy". Steering can become very touchy, then go into immediate wobbles with too vertical of caster line and too soft of cushions. I experienced this with the SSTs. I was not going to harder cushions to stop it, so caster changes were in order. The two go hand in hand, caster and cushion hardness.
I agree with all this. This relationship is a limitation of any design. And there is no way to get around it. Which is why I'm a little hesitant to pull the trigger just yet.

I also agree with you regarding caster. In fact I might create the option to shim under the pivot cup.
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Old March 31st, 2017, 07:11 PM   #38
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Default if you would like to try

Out my old white magnums. They will need custom length stepped kingpins(7/16-3/8). Then 7/16 threads cut into the plate. ~7 3/4 axle cl with sg trucks. 165mm mounting holes. You could borrow them and test all you want to. Ill bring them next time I come into town.
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Old March 31st, 2017, 09:21 PM   #39
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Out my old white magnums. They will need custom length stepped kingpins(7/16-3/8). Then 7/16 threads cut into the plate. ~7 3/4 axle cl with sg trucks. 165mm mounting holes. You could borrow them and test all you want to. Ill bring them next time I come into town.
Let me take a look at them.

Side note: I'm skating Saturday, so I can't meet this weekend.

I thought the best plate to modify would be the magnums. However I don't think there is enough meat in the plate once I mill out space for the second cushion.


Update on project: I was worried that I modeled parts correctly so I compared it to my Royals. In the model I set my kingpin angle at 60 deg. and updated the pivot position to match the actual hardware. I then held up the plate to my monitor to see if the profiles matched. (This is the super-scientific way of doing things) But it looks really close.



The most I can rotate the truck assembly is 5 degree. At that position the axle is directly under the pivot pin. That gives me an angle of rotation of 50 degrees. I wanted a little more. So I am considering the Advantage truck.

Last edited by amohrfeld; April 1st, 2017 at 02:13 AM. Reason: adding picture.
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Old March 31st, 2017, 09:31 PM   #40
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Why longer cushions?:

I have noticed the standard cushions bottom out at the end of the stroke. Some may like this behavior as it can feel stable, especially at high speed. However I want a linear relationship in force/displacement throughout the stroke. Going to longer spring will allow more travel before experiencing the bottoming-out effect.
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