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Suspension design with air shocks

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Unproductive View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Unproductive Quote  Post ReplyReply Direct Link To This Post Topic: Suspension design with air shocks
    Posted: Nov/18/2009 at 11:46pm
Last year we ran ryde fx air 2.0 shocks with a motion ratio between .4 and .3(decreasing with compression) throughout the travel, in order to get the suspension to act how we wanted we had to run them reasonably well under the recommended range.

This year we're running fox evol's, I was wondering what kind of motion ratio you guys are running with them. Is there any information out there on calculating effective spring rate for them?

I'd also like to ask the guys at USF about one of their swingarm cars, is there a reason you guys leaned the rear shocks over so far, other than MR and packaging? Is there anything else I'm missing?



Edited by Unproductive - Nov/19/2009 at 6:59pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote jeiB Quote  Post ReplyReply Direct Link To This Post Posted: Nov/19/2009 at 12:09am
Hey,

we've been running the FOX floats for 2 years now, we might be using them this year as well depending on budget. We usually keep the rising rate characteristic of an air shock because it prevents the car from bottoming out. In previous years, we've had ratios of 0.6 - 0.7. I looked at the damping curves (rebound and compression) and i wanted to run higher motion ratios this year to get the shock to "work" harder...kinda abandonned that though for now.
Anyway, good topic Thumbs Up
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Post Options Post Options   Thanks (0) Thanks(0)   Quote j-man Quote  Post ReplyReply Direct Link To This Post Posted: Nov/30/2009 at 6:18pm
You need to know the damping curve of the shock and the unsprung mass speed...
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Post Options Post Options   Thanks (0) Thanks(0)   Quote kenneth.mandeville Quote  Post ReplyReply Direct Link To This Post Posted: Dec/01/2009 at 8:28am
        http://constans.pbworks.com/f/Fox+Shox+Specs+and+Sponsor+Info.pdf

That will give you some damping coefficients to work with.

         http://www.mathworks.com/matlabcentral/fileexchange/11199-247-dofs-suspension-model-for-simulink?controller=file_infos&download=true

       http://www.mathworks.com/matlabcentral/fileexchange/authors/15500

These have quarter, half, and full models so you can see how your vehicle will respond to whatever disturbance you put at the wheel.


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Post Options Post Options   Thanks (0) Thanks(0)   Quote DrewT Quote  Post ReplyReply Direct Link To This Post Posted: Dec/01/2009 at 9:34am
Originally posted by Unproductive Unproductive wrote:


I'd also like to ask the guys at USF about one of their swingarm cars, is there a reason you guys leaned the rear shocks over so far, other than MR and packaging? Is there anything else I'm missing?
 
I can't speak for USF, but we do something similar with our trailing arm cars, and from the picture you put up it looks like they intended to do this as well:  Notice that the upper shock mount for the rear is directed into the node where the SIM and rear bracing join the RRH; that's probably the strongest, best supported, point on that entire car.  And by placing the suspension loads on the RRH (where you have to use heavy tubing), you eliminate suspension loads on any of the rear bracing behind the RRH (so you can make it out of the lightest possible tubing without worry).
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Post Options Post Options   Thanks (1) Thanks(1)   Quote Unproductive Quote  Post ReplyReply Direct Link To This Post Posted: Dec/02/2009 at 11:54am
Originally posted by kenneth.mandeville kenneth.mandeville wrote:

        http://constans.pbworks.com/f/Fox+Shox+Specs+and+Sponsor+Info.pdf

That will give you some damping coefficients to work with.

         http://www.mathworks.com/matlabcentral/fileexchange/11199-247-dofs-suspension-model-for-simulink?controller=file_infos&download=true

       http://www.mathworks.com/matlabcentral/fileexchange/authors/15500

These have quarter, half, and full models so you can see how your vehicle will respond to whatever disturbance you put at the wheel.




Just to make sure I have it right, for the quarter model, k1 is the spring rate of the tire, m1 is unsprung weight, k2 is spring rate of the suspension, Co is the damping constant, m2 is vehicle weight, z1 and z2 are the displacements of the suspension and vehicle respectively.  Edit: Base metric units?

Edit again, this wasn't covered in the last matlab/computing class I took, but, is it possible to input a curve instead of a constant into simulink. I'd have to imagine there is....


Edited by Unproductive - Dec/02/2009 at 12:21pm
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Post Options Post Options   Thanks (1) Thanks(1)   Quote msctgb Quote  Post ReplyReply Direct Link To This Post Posted: Dec/02/2009 at 1:57pm
Originally posted by Unproductive Unproductive wrote:


Edit again, this wasn't covered in the last matlab/computing class I took, but, is it possible to input a curve instead of a constant into simulink. I'd have to imagine there is....


Make a source block with a variable in it, say "xr".  This block should be your position (or whatever) input.  Then, in your InitFcn (File>Model Properties>Callbacks>InitFcn) make something like:

t = linspace(0,10,10001)';
xr = [t 9*sign(sin(2.5*pi*t))];

When you execute the simulation, Simulink starts by executing the initfcn code, and then uses those variables where they're called throughout the simulation.  In this case, that function is now your input.

EDIT:  It's a bit of a mistake to call that a function.  It's an array of points created using the above function.  This also means that if you had some reference positions you could generate an array that does specifically what you want as an input.

I used to hate MATLAB, now I don't know how I ever worked without it.


Edited by msctgb - Dec/02/2009 at 2:23pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote kenneth.mandeville Quote  Post ReplyReply Direct Link To This Post Posted: Dec/02/2009 at 11:27pm
You can also make a random signal generator, sin wave, a step function, or pretty much whatever you want.  I will have to be in front of it to remember the exact steps, if I get a chance tomorrow I will.  

You have all the variables correct, there should be a diagram somewhere in the zip file for reference.  Just remember that the K and C for the suspension is relative to the wheel so you have to take into account the motion ratio.  

You don't have to use the program to determine your MR, it will give you a good idea of how the vehicle works with it though.  To determine my MR I used a transmissibility (driver displacement / wheel displacement) equation from a vibrations for a quarter model with a varying MR.  The damping coefficient changed with the MR, but the spring constant stayed the same because I knew exactly what ride frequency I wanted.  I plotted the MR vs. Transmissibility and chose the one that I felt was best.

Does this make sense?


Edited by kenneth.mandeville - Dec/02/2009 at 11:28pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote FWBcut Quote  Post ReplyReply Direct Link To This Post Posted: Dec/03/2009 at 10:33pm
Having the shock forces go into the SIM/RRH node as mentioned is one of the main reasons. The other reason is the progression we get from it. We can have a soft wheel rate at normal ride height and have an increase rate as the suspension compresses.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Rob71zilla Quote  Post ReplyReply Direct Link To This Post Posted: Dec/04/2009 at 12:35pm
What about your limited travel with the shock laid down so much?
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Kenneth Quote  Post ReplyReply Direct Link To This Post Posted: Dec/04/2009 at 1:33pm
I think you actually get more travel that way...
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Post Options Post Options   Thanks (0) Thanks(0)   Quote blue2kss Quote  Post ReplyReply Direct Link To This Post Posted: Dec/04/2009 at 11:53pm
We got more than 10" with it that way
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Shocker Racer Quote  Post ReplyReply Direct Link To This Post Posted: Dec/06/2009 at 1:05am
Has anyone ever found a way to calculate shock effiecency based on its angle for the fox evols?  I am assuming straight up and down to be 100% and completely horizontal to be 0%, but i was wondering what it looked like in between.  Basically trying to find the optimal angle for the shock, as well as save strength, get the right travel all that jazz....
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Rob71zilla Quote  Post ReplyReply Direct Link To This Post Posted: Dec/06/2009 at 6:47pm
That's what I thought too but apparently that's not right if USF is getting 10" of travel.  We are using a similar rear end set up this year and are currently trying to decide how to mount the rear shocks
 
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Post Options Post Options   Thanks (0) Thanks(0)   Quote OctoberSky Quote  Post ReplyReply Direct Link To This Post Posted: Dec/06/2009 at 7:16pm
if you are using solidworks (even graph paper would work), all you have to do is a simple sketch representing your arm and shocks, and by adjusting the mounting position of your shocks, you can figure out wheel travel.  I attached a version of the sketch below...same idea works for just about any suspension setup.   Play around a bit and you start to realize why certain teams can get such crazy suspension travel
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Dax Quote  Post ReplyReply Direct Link To This Post Posted: Dec/06/2009 at 7:51pm
The easy way for a quick and dirty check is just doing it on the chalk board with a calculator. Or you could use CAD (cardboard - assisted - design)
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Post Options Post Options   Thanks (0) Thanks(0)   Quote paasch Quote  Post ReplyReply Direct Link To This Post Posted: Dec/08/2009 at 12:02pm
As good suspension program like OptimumK or Mitchell will help a lot with kinematics.  For example, Octobersky's solidworks drawing above shows a motion ratio of 2.  In fact, the motion ratio is changing continuously as the suspension moves through its range.  You need to know how it is changing (progressive, digressive, or both) to set your spring rates and damping.

We used air shocks on OSU cars, but moved back to coil-overs in 2006 to get better control of preload and spring rates.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote OctoberSky Quote  Post ReplyReply Direct Link To This Post Posted: Dec/08/2009 at 2:33pm
Yes, Paasch is right.  That motion ratio really isn't 2:1 to start with, the shock is just mounted halfway up the arm.  you can try to figure out the actual motion ratio and progression using a drawing, but it's a pretty painful process, and I too would recommend using an actual suspsension design program if you can afford it.
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