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RC Shock Dyno Test Results

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RC Shock Dyno Test Results

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Old 03-02-2015, 09:14 PM
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this is awesome, thanks much for doing this, it takes a bit of the voodoo out of it...I would too like to see 2 stage pistons thrown into the mix. I bet even mip and rc shox might be willing to donate to get the info to us because if their products are as good as they claim they might make some more business

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Old 03-05-2015, 06:09 PM
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Excellent information. Great presentation. Kudos! I have a BS in Mech engineering so I'm generally critical of people's analysis of how car setup affects handling and performance. This was well planned and clearly presented. You have definitely increased my understanding of shock science.

One idea for general information is looking at the relationship between spring stiffness and viscous damping. For instance how wide is the sweet spot for oil viscosity for a particular stiffness spring. And also, if you increase/decrease spring stiffness which general way should you adjust oil viscosity. Or maybe oil viscosity is more dependent on temperature. Or where ever the data leads you. Damn I wish I had a lab.
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Old 03-05-2015, 06:35 PM
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Thanks keep up the good work. much appreciated!
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Old 03-05-2015, 07:59 PM
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Originally Posted by David A View Post
Excellent information. Great presentation. Kudos! I have a BS in Mech engineering so I'm generally critical of people's analysis of how car setup affects handling and performance. This was well planned and clearly presented. You have definitely increased my understanding of shock science.

One idea for general information is looking at the relationship between spring stiffness and viscous damping. For instance how wide is the sweet spot for oil viscosity for a particular stiffness spring. And also, if you increase/decrease spring stiffness which general way should you adjust oil viscosity. Or maybe oil viscosity is more dependent on temperature. Or where ever the data leads you. Damn I wish I had a lab.
Once you get to try things, and start seeing data, yes, the imagination runs wild with possibilities. I'm going to look into many things as I get the time to do them.

As for your sweet spot of viscosity question, I believe this really is a function of the Damping Ratio... and this has to do with all the other aspects of your car and suspension setup. fredswain's setup tips he describes in a detailed thread explains this damping sweetspot. First, you balance the springs, then you match the oil. Basically, this is precisely what the Damping Ratio is.

The Critical Damping is determined by your car's ride frequency (set with springs, shock motion ratio, mass of components). The Damping Ratio is a percentage of Critical Damping that your dampers provide (Damping Coefficient). Many guidelines give a range of damping ratios that usually work for typical applications. That's your "sweet spot"

I'm of the belief that the results of this work I'm doing will help drivers find this damping ratio sweet spot easier, so you don't need to rely on trial and error. To help you with finding your actual ride frequency, RC Crew Chief is an amazing modeling program where you measure your car, enter values, and it calculates your ride frequency for you. Then, with that information, you will be able to determine your damping ratio if you know what your damping coefficient is. Or, you can use the ride frequency to know what the critical damping is, then, based on some estimates for damping ratio, you can predict what "should" be the optimum viscosity oil to use with your setup based on the damping coefficient of your shock. I have been providing Bob with my damper data, and he has been developing predictive damping coefficient calculations. This means, you will be able to enter the dimensions of your shock, what piston you are using, and the viscosity of the oil, and it will tell you what your damping coefficient is. I'll tell you that his predictions are very very good at this point.
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Old 03-06-2015, 06:12 PM
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+1 for the big versus small bore shock comparison.

I'm thinking most of the testing you did for the hole size and viscosity comparison testing was with emulsion shocks, correct? You mentioned the spring effect several times through there, so I assumed emulsion was the base setup.

I spent quite a bit of time trying to calculate some sort of damping factor for various pistons, viscosities, etc. I run emulsion shocks, and that caused an extra unknown I didn't have an answer for. I had the calculation set up to work with different shock positions, spring setups, sprung weight at each corner, etc. for my SC10. Really geeky stuff, but I'm an engineer, too, so the geeky stuff sort of just happens.

I'd be interested in some testing comparing front and rear shocks with emulsion setups. Comparing the two is where my calculation bogged down due to the different amounts of air in the emulsion to compensate for the different shaft lenghts.

Higher piston speeds would be interesting, too. Assuming the shocks follow an orifice flow model or possibly a short pipe flow with entrance and exit losses included, the damping would be exponential with speed, wouldn't it?

Again, this is awesome stuff you are providing. I'm looking forward to more test results!
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Old 03-06-2015, 07:00 PM
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Thought I would share some very preliminary results comparing one of Scott's Shock Dyno tests to the Shock Simulation I'm working on for RC Crew Chief. This is the AE big bore shock, 2x1.5 piston, 350cst oil. Graphs are the Force vs Displacement (F-D) and Force vs Velocity (F-V) that are the typical shock dyno outputs Scott has presented.

So far it's a pretty decent correlation. While these shocks are fairly simple there is a lot happening so it will be difficult to reproduce every bump and bulge in the curves. As has already been mentioned the key parameter is the slope of the F-V graph. That is what determines the damping coefficient.

We all need to say a big thanks to Scott for sharing this data. Without it none of this would be possible.
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Old 03-06-2015, 07:30 PM
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Originally Posted by Riv2SC10 View Post
I spent quite a bit of time trying to calculate some sort of damping factor for various pistons, viscosities, etc. I run emulsion shocks, and that caused an extra unknown I didn't have an answer for.
Yes, they were with emulsion shocks. The air is a big unknown, but it's not completely impossible to account for. The question is how much air. An air/oil mixture is simply two springs in series. The lighter spring squishes easier than the stiffer spring, so the sum of the two will always be less than the lighter one. The spring property of air and oil is bulk modulus. If you know each of them, you can calculate the spring effect. Oil IS compressible, it's just very small displacement or volume change. Air is highly compressible, so it dominates. If you know the % Air in the oil, you can calculate it with this formula (Effective spring rate, Keff = 1/(1/Koil)+%air*(1/Kair)) K = bulk modulus. Air is simply the pressure of the air or 101kPa at atmospheric pressure. Oil is approximately 850,000kPa for 300 cSt silicone oil at 25C. The difficult part is the %air.

I'm not sure how the air affects the viscosity and flow effects though, that's something we'll be working on. I did show air influence in the presentation, but it doesn't seem convincing enough for me, (even though I repeated the test a few times getting the same results each time). I need to come up with a better, more convincing way. Otherwise, if the data holds up, the air doesn't influence the damping coefficient much, only the hysteresis.

As for your calculations, you should be on the right path, as long as you have the right damping coefficient, or at least understanding how it changes depending on your damper configuration (piston, viscosity, etc)
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Old 03-06-2015, 07:47 PM
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I would be curious to see how piston hole area to hole quantity plays out. Is it only the area that affects high impact dampening or does the wall area from a difference in number of holes also play a role. I've generally just stayed simple in this regards and always run the same number of holes front and rear but only changed hole size. And oil too obviously.

I don't believe that there is any one perfect dampening ratio but I do believe there to be a fairly narrow range to stay within. This again is based on hunch from observation rather than scientific method.
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Old 03-06-2015, 08:22 PM
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I would also be interested in the hole area vs. hole quantity analysis. I have tinkered with it on my car and can't really "feel" the difference and lap times have not been significantly different.

Amazing thread! I've also been tinkering much more with vented bladder shocks on my vehicles. Theoretically the vented bladder build should give a much more consistent shock and so far I have had pretty good success with them. I have however had a more challenging time getting them built identically from shock to shock.
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Old 03-06-2015, 08:24 PM
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Thanks Ice...

The time and attention given is appreciated.

When testing pistons please try to include 2+2's, 2 1.5mm & 2 1.1mm and 2 1.6mm & 2 1.1mm. I am very curious to see the difference from 2 1.6mm and 2 1.7mm.

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Old 03-08-2015, 11:08 AM
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Brilliant tests ! Thank you very much, it is very useful. I've read it and it's top notch.
Most helpful. Glad I finally have something to show to people who didn't believe me when I said I didn't see how a tapered piston would change anything.

One thing I'd like to remark as I couldn't find anything on it is the tolerance of the piston/cylinder itself.
On my shocks the cylinder bore is 12.04mm and the piston itself 11,87mm.
That's a 0.075mm gap all around which is not bad but will influence of "hole" area.
So cylinder bore area = 6.02x3.14= 113,795mm
Piston surface area = 5.935x3.14= 110.604mm
Gap around the piston = 113.795-110.604= 3.191mm
Which is not negligible I think.
Plus the inner piston hole on the rod which will also "leak" a little but this is possibly negligible.

Again many thanks for such precious info ! Looking forward to more tests .

Jonathan
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Old 03-08-2015, 12:12 PM
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Very cool data..

Now all you need is someone to help you put this into a mobile app so you can:

Choose Temp : XXX F/C
Surface Type: Smooth/bumpy/outdoor
Scale: 1/10th 1/8th
Jump 1: Small/Medium/Large
Jump 2: Small/Medium/Large
Jump 3: Etc...
Whoops section(s): yes/no

Output would be:
350 CST 1.2 mm 8 hole flat, Spring 2.32 lb/in 41.08 gf/mm Front
400 CST 1.3 mm 8 hole flat, Spring 4.00 lb/in 71.44 gf/mm Rear


Anyone? I got my cash ready =)
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Old 03-08-2015, 12:13 PM
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Originally Posted by shannow View Post
Brilliant tests ! Thank you very much, it is very useful. I've read it and it's top notch.
Most helpful. Glad I finally have something to show to people who didn't believe me when I said I didn't see how a tapered piston would change anything.

One thing I'd like to remark as I couldn't find anything on it is the tolerance of the piston/cylinder itself.
On my shocks the cylinder bore is 12.04mm and the piston itself 11,87mm.
That's a 0.075mm gap all around which is not bad but will influence of "hole" area.
So cylinder bore area = 6.02x3.14= 113,795mm
Piston surface area = 5.935x3.14= 110.604mm
Gap around the piston = 113.795-110.604= 3.191mm
Which is not negligible I think.
Plus the inner piston hole on the rod which will also "leak" a little but this is possibly negligible.

Again many thanks for such precious info ! Looking forward to more tests .

Jonathan
I did not mention the dimensions of the shock at this point. I did measure everything, and yes, they would be important. When I do the tests with all sorts of different piston hole # and size configurations, I'll be sharing actual dimensions as well. Because I cannot make my own pistons, I'm at the mercy of what is available for clearance tolerances.

If people have pistons of different brands (for 10mm, 12mm, and 16mm shocks), please let me know of some alternative options to try different clearance option pistons. (Measure with a good calibrator to X.xx mm) I'll need to know brand and part number. Also, I'm in need of blank pistons for the 12mm, if anyone has a suggestion, I'm struggling to find them.
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Old 03-08-2015, 01:08 PM
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Pity I don't live next-door (France) I could machine you come blank pistons in PTFE.
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Old 03-08-2015, 01:31 PM
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Excellent thread! Gives me something R/C related to do while I'm stuck at college with none of my cars...

Very interested to see the results from the piston tests as well, especially for # of holes vs hole size.
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