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Old 03-24-2010, 01:14 PM   #1
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Default RC Shock Design -- why so premitive?

I'm new to the RC world, just started back in November. Ever since I opened up my first RC shock, I've been wondering why the design is so primitive.

I have dirt bikes, and I do all of my own mechanical maintenance, including suspension work. Dirt bike suspension has really evolved over the years, and most are very tunable. Of course you have pistons with holes in them designed to flow oil in a specific manner, but there are also shim stacks that help control the amount of force it takes (usually progressively) to have the suspension move a certain distance in the stroke. These shims look like very thin metal washers, that basically act like a spring. Stack them up on top of each other in various diameters and thicknesses and you create an environment that is very tunable.

For example, you can have a suspension that really floats nicely over the small chop, but can also really take a big hard hit too. Traditional springs are still used to get the desired ride height. The entire design is actually more complicated than this, but I wanted to paint the picture.

So, why don't RC shock designs take more advantage of the technology in the suspension industry? All of the basic pieces are there, just seems to me that someone has to take the next step.

Having just a small disk with a few holes in it to act as the piston and control the way shock oil flows and/or various oil thickness just doesn't seem efficient to me. I know making shims to scale for RC would very challenging, so maybe this is why you don't see them in use. You can't easily tune compression stroke and rebound stroke to act independently from each other. I know some guys will bevel the hole in the piston on one side to allow it to flow oil differently in that direction, but that seems so simple with no real way of measuring the result.

Just curious, had some thoughts and wanted to share. What do you think?

ben
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Old 03-24-2010, 01:31 PM   #2
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First company to make shocks with independant rebound and compression settings will be huge.

Not sure we can fit variable speed damping though.

I'm working on some position sensing shocks that get stiffer as they compress allowing really soft damping settings for quick rebound and soft small bump response.

But the advanced metal I have to use is $30 per piston and cracks easy.
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Old 03-24-2010, 01:38 PM   #3
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Originally Posted by Zerodefect View Post
First company to make shocks with independant rebound and compression settings will be huge.

Not sure we can fit variable speed damping though.

I'm working on some position sensing shocks that get stiffer as they compress allowing really soft damping settings for quick rebound and soft small bump response.

But the advanced metal I have to use is $30 per piston and cracks easy.
RPM was the first, traxxas was next... I havent seen any others.
http://www.rpmrcproducts.com/product...cks.htm#Medium


http://www.traxxas.com/products/nitr...04_details.htm
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Variable Damping Shock Pistons
For the ultimate in off-road performance, new variable damping shock pistons improve traction and bump handling. Slower compression rates matched with faster shock rebound speeds add stability and superior control through rough track sections.
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Old 03-24-2010, 01:39 PM   #4
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Yes, they are very simple, I agree. But there are two thing that keep it that way imo. One, cost control, a more sophisticated shock would be much more expensive to produce. Two, the ability of the average rc consumer to work on them. Some people have a hard time setting up a speed control let alone tuning a more sophisticated shock, sure there are people that can tune a shock, but in my experience most can't.
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Old 03-24-2010, 01:43 PM   #5
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If I had more resources, such as a mill and experience using one, I have an idea for a design that I'd try.

Basically create a check-plate design on both or on side of the pistons. Let me see if I can describe what I'm thinking....

You'd have the traditional shock piston with holes in it secured to the shock shaft. However, the piston would have the holes strategically positioned, some close to the center, some close to the edge. The nut that holds the piston to the shaft would have a short collar to enable a small/thin washer to float up and down on it. A small spring could be placed on this washer as well to help force it into a closed position against the piston face until the flow of oil causes it to open again. This could be true for both sides of the piston.

As the piston moves up (compression), the oil flow would cause the washer (shim) to lift from the face of the piston and allow oil to flow around it. Meanwhile, the washer on the other side would close off the holes that are dedicated to the rebound circuit. When the piston moved down, the same process happens in reverse.

This way, you could have one or two holes for rebound and 2-3 for compression, or whatever combination ends up working the best.

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Old 03-24-2010, 01:50 PM   #6
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This picture may be what I'm talking about. If that metal piece under the nut is allowed to open up when the piston moves one direction and close when it moves the other. This would allow the piston to move more freely in one direction, and slower in the other.



As far as things getting more complicated, this is somewhat true. It all boils down to how much more complicated the design becomes. And those who want to or who can apply themselves to learning how to tune their suspension better would stand to benefit.

I'm not thinking about a complete replacement shock. Maybe just a piston kit that would likely include a new shock shaft as well.

ben
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Old 03-24-2010, 02:28 PM   #7
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The biggest issue with something of this nature is that you lose your range of motion if you have too much going on. You basicly have to choose if you want to add 10mm to the top of your shocks or not do it. This also increases your shock towers length too. You can stick a small piece of foam above the piston which can help you to run a softer setup for small rough stuff & absorb that big blow if your shocks get crunched.
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Old 03-24-2010, 05:21 PM   #8
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That's also the idea in the RPM shock pistons, except the moving part that closes some holes on the compression stroke is made of plastic.
I have those on my Slash and the quick rebound really helps keep the wheels on the ground at high speed in the rough front straight we have on the track while at the same time providing enough damping to absorb jump landing.
Too bad they don't make those in larger diameter to accommodate my other rides. A set of 1/10 or 1/8 scale Ohlins shocks,that would be cool !
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Old 03-24-2010, 05:26 PM   #9
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I believe its to keep things "simple". I don't believe that the added complexity of mimicking 1:1 shocks would net a significant benefit to justify the cost, complexity, and price they'd ask for a set or engineering required.

I don't think in some cases you can't simply "scale-down" a 1:1 shock, car, or something of that nature and have it work the same way...

Same thing as scaling up. It doesn't always work.....
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Old 03-24-2010, 05:41 PM   #10
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I'm for keeping things simple but I like the idea of a shock with differing compression and rebound rates, especially for tracks with washboard sections.
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Old 03-24-2010, 07:23 PM   #11
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Another full scale shock trick that I have thought of using in a 1/8 or 1/10 shock is the position relative damping used in Moroe Sense-A-Trac shocks and struts. They valve the piston a bit stiff for the hardest rate you want at the ends of travel, especially compression for when we jump. Then cut a groove in the side wall of the shock body to give a larger oil bypass passage when in the center of the ride height range. This gives a softer smoother ride over the small bumps and rough surfaces. When the piston moves past the ends of the groove, the oil now has to flow only through the piston holes for a the stiff rate. This could help slow the chassis as it crashes down off of a big jump.

The factory shocks on my 1/8 Z-Car buggy actually do have a progressive damping piston shim. This is backwards from most street car shocks. The piston starts in the soft mode, and the shim is bent to float away from the holes in the piston. Oil flow from rapid compression will force the shim to flex against the piston. The shim though also has a hole in it, just a little smaller than the one in the piston. This increases the damping a little bit under rapid shock compression. I think I would like a bit larger difference in hole sizes, between the piston and shim. The difference is quite small now, and I am certainly bottoming hard off of large jumps. So far it is not causing any big problems, it never bounces, just slaps and take a set, right now! I also thought about just adding a little bit of a bump stop with some fuel line around the shock shaft. I want to be in control until the chassis is 1/4 inch off the ground, then let the stops hit the tubing and slow the chassis hit .
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Old 03-24-2010, 07:38 PM   #12
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Most RC drivers cannot feel the difference in making changes to the shock pistons.

In a Losi 1/10 application, only very advanced drivers can feel the change from going from 30wt and 56 pistons, to 25wt and 57 pistons. They feel very similar on the bench, and drive very similar on the track, however from a fluid dyamics point of view, they are quite different.

As stated above, scaling down 1:1 technology doesn't always work, gravity is constant, and mass is reduced. This ratio does not remain. Back to the Losi pistons, there is a reason that they skip from 57 to 60 - as the holes get smaller, the 58 and 59 holes are so little difference in actually surface area reduction - it is negligible on a shock dyno - you have to skip two drill sizes to record any kind of change in the damper's performance.

So basically it isn't worth it.
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Old 03-24-2010, 08:26 PM   #13
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Didnít somebody make Jump Jets or something like that, Parma or RPM maybe? They had a long tapered/cone section that stuck through the piston hole, thus providing/attempting travel dependent damping?
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Old 03-24-2010, 08:45 PM   #14
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Didnít somebody make Jump Jets or something like that, Parma or RPM maybe? They had a long tapered/cone section that stuck through the piston hole, thus providing/attempting travel dependent damping?
dude you just totaly dated yourself! lol but so did i cause i had a set. lol. those were cool. very progressive dampening. the farther the piston traveled up the smaller the hole got. good design didnt last though. i use the rpm dual stage pistons and i can feel any change i make. i dont think ill ever go back to standard pistons. its just way more tunable.
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Old 03-24-2010, 08:59 PM   #15
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Originally Posted by armourbl View Post
I'm new to the RC world, just started back in November. Ever since I opened up my first RC shock, I've been wondering why the design is so primitive.

I have dirt bikes, and I do all of my own mechanical maintenance, including suspension work. Dirt bike suspension has really evolved over the years, and most are very tunable. Of course you have pistons with holes in them designed to flow oil in a specific manner, but there are also shim stacks that help control the amount of force it takes (usually progressively) to have the suspension move a certain distance in the stroke. These shims look like very thin metal washers, that basically act like a spring. Stack them up on top of each other in various diameters and thicknesses and you create an environment that is very tunable.

For example, you can have a suspension that really floats nicely over the small chop, but can also really take a big hard hit too. Traditional springs are still used to get the desired ride height. The entire design is actually more complicated than this, but I wanted to paint the picture.

So, why don't RC shock designs take more advantage of the technology in the suspension industry? All of the basic pieces are there, just seems to me that someone has to take the next step.

Having just a small disk with a few holes in it to act as the piston and control the way shock oil flows and/or various oil thickness just doesn't seem efficient to me. I know making shims to scale for RC would very challenging, so maybe this is why you don't see them in use. You can't easily tune compression stroke and rebound stroke to act independently from each other. I know some guys will bevel the hole in the piston on one side to allow it to flow oil differently in that direction, but that seems so simple with no real way of measuring the result.

Just curious, had some thoughts and wanted to share. What do you think?

ben
Dude! I race motocross too!!! Pitbikes though, but its still awesome!

They can't really do that because of the size! When you scale things down, the physics do not scale properly, especially fluid physics!!! I have been taking physics for 3 years, and scale size is a big deal! Thats why nitro motors are so big, the vehicle is 1/10th scale, but the motor is WAAAAAY bigger than that, due to fluid dynamics issues!!!

Check out the Traxxas E-Revo suspension, its the most advanced out there!!!
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