Piston Chart

[Arn0]

After the Spring Chart thread (http://www.oople.com/forums/showthread.php?t=7571), this one is for a Piston Chart!

The following document (see below) summurize the charateristic of various damper / shock piston. It is orientated off road as some drivers look for equivalency and compatibility between different dampers / shocks brand. You will find the piston reference, the number of holes, the hole diamater as well as piston diameter and flow area oil get to pass throu the piston. Concider this document as a version 1, some information will be added.

Shock Piston Chart (http://www.petitrc.com/setup/OffRoadShockPistonChart.pdf)

I'm npw looking for the missing data about Kyosho piston and I think adding Schuey and Yokomo (if different from AE) will give a pretty complete document.

Let me know what you think about it, what would you change and so on.

[Ferret]

Great idea!

If you (or someone else) know to add to the chart a cross referance between the hole area and shock fluid weight to get the static damping of all the combinations it would be very helpful too.

Avner.

[Schjimmy]

Finally! I've been looking for a piston chart for a long time :woot:

[RogerM]

Remember that the difference in diameter of the piston OD to shock body ID will have a fairly significant impact if using non manufacturer pistons. Also hole and piston OD edge conditions.

If you want to do the maths then good on you ..... I looked at starting a simulink model for the popular combinations a couple of years back .... then went to Kyosho and lost interest as I now NEVER change pistons in either RB5 or ZX5-SP

[IceMike]

thanks for that chart will come in helpful, now its given me some questions.

1. Wot do you do then to calculate the spring rate of the car with oil and spring??
(oil / hole diamiter ) * spring = spring rate
2. Do you want a higher spring rate for dry tarmac, than you do for wet offroad??

thanks

mike

[Arn0]

Hi,

Sorry Mike for this late answsers, guessing the questions are for me!

My aim is to provide some information for those who want to compare a brand of shocks with another one, like x-factory drivers who can choose between AE, Losi or others.

Calculating spring rate is not that simple. I did look in some books to remind me some stuff and have to do it again but you need more than just oil and spring, you need the mass also.

I did push forward some investigation to provide a useful, trying to define with a figure how fast can move the piston depending the combinaison oil-piston used. You will find below a draft of the file, there is some information missing. The data on Kyosho piston are wrong, just a guess to start getting figure. So piston diameter and rod diameter are wrong. Do someone can provide the adequate data? With I'll complete the page and provide some details about haw to use it and maybe some others details.

http://www.petitrc.com/setup/ShockPistonComparison.pdf - Draft document, may disappear.

[IceMike]

my main question i guess would be could u make a shock filled with light oil and a big piston but with a really hard spring have near to or the same compression and spring bac rate as say a shock with hard oil and a soft spring. and how u would calculate this

[Welshy40]

What about traxxas pistons, from the TRX1 as an example. If your doing Kyosho the Traxxas will be just a fraction bigger than that.

[Arn0]

Hello Gents,

I first have to apologize for the delay, being pretty busy with several thing, including doing my homework for this chart,to provide answers on a clear manner.

my main question i guess would be could u make a shock filled with light oil and a big piston but with a really hard spring have near to or the same compression and spring bac rate as say a shock with hard oil and a soft spring. and how u would calculate this
Mike: As you know, the couple oil-piston works tougher to slow down the suspension movement, the spring on the other side want to return everything in the initial position. So, with light oil-small piston you get a certain effect and with heavy oil-large piston you get another effect. You may know this point, there is some tricky thing in fluid mechanics, the same fluid going thru the same surface can behave differently depending the shape of surface shape (round, square, oval) and the surface edges. For the spring, a soft (low rate) spring will need more travel (compression) to give the same amount of force as a hard (high rate) spring. To calculate everything tougher, you have to use some physics/mechanics, solving some equation. I won't go for the whole theory and dynamical equation but this theory ends with the following: damping coefficient (m for mechanical as it came form a mechanical equation) Cm = (2 x mass x spring rate)^0.5. This damping coefficient Cm is the one set by your mass and spring and it have to counter balance on some way the couple oil & piston which give a damping coefficient Ch (let name it so as it is fluid mechanics / hydraulic). Depending what you want to achieve, the type of car you are running basically, you want the damping ratio equal to a certain value. The damping ratio is Ch / Cm and, for passenger car, it is around 0.3 as for racing car such F1, it's around 1. That only for 1 side of the car, you now need to take in count the other and on the car as it is a whole mechanical system. If you use stiffer spring on one end, you get more mass on the other end. And now you entering in the process of testing and play with the car, in out case of RC car, rather than trying to do some math as math are not such straight forward to this point. I hope I'm clear.

What about traxxas pistons, from the TRX1 as an example. If your doing Kyosho the Traxxas will be just a fraction bigger than that.
Welshy: I can do the math for what ever piston someone provide data, Traxxas included.

As said, I did my homework as best as I can do. The following link (see below) provide much better data than the first document submitted (even if the link still the same :D). Those data are from off road cars, I'm waiting for some additional information, italic number are an approximation to do the math on first instance, reason the document still in "draft mode". If you want to see some more data, you need to provide the following:

Piston diameter
Piston holes diameter
Number of holes
Rod diameter
Shock bore

A good vernier/caliper and tools handling can do the job, otherwise, you can use some more professional-industrial tools
Depending what data are provided, I might put tougher the document on the first post and this one or leave them separated.

http://www.petitrc.com/setup/ShockPistonComparison.pdf

Hope that clear and helpful, just let me know!

[sosidge]

Arno, I'm trying to get my head around the values you have in the table below the measurements.

There is almost no difference in the values, suggesting that different pistons are almost meaningless compared to the gap between the piston and the damper body.

I have to say that personally I feel that the effect of piston holes is over-emphasised by a lot of people, I have never managed to notice a significant change in the behaviour of a car just by changing piston (maybe a big hole piston feels a bit plusher over the bumps, but I am talking about either end of the piston range, not one step along), but these tiny differences are surprising to me!

[Arn0]

Sosidge, the value on the chart are approximation as the italic figures are, let say, guessed value so all values will change slightly but you got the rough idea, the range of value. That is the reason of the draft version.

I did some math and the theorical "leak" between piston and body is around or smaller that one smallest hole. So one third or less of the oil by-pass the piston oil, bigger the hole, smallest the rate.

I forget to mention the model is valid for laminar flow and so give the damping coefficient at "low" pistion motion, you need to use Reynolds number here so the suspension geometry to sort ot the Reynolds and so on you enter something with various parameters. So, as Ferret ask for, it can be considered as static damping. As already mentioned, fluid behave differently depending speed, temperature... so the damping coefficient will be different at high speed suspension movment.

[Northy]

Just so you guys know, I hope to have piston drills in stock in the next day or two ;)

G

[sosidge]

Sosidge, the value on the chart are approximation as the italic figures are, let say, guessed value so all values will change slightly but you got the rough idea, the range of value. That is the reason of the draft version.

I did some math and the theorical "leak" between piston and body is around or smaller that one smallest hole. So one third or less of the oil by-pass the piston oil, bigger the hole, smallest the rate.

I forget to mention the model is valid for laminar flow and so give the damping coefficient at "low" pistion motion, you need to use Reynolds number here so the suspension geometry to sort ot the Reynolds and so on you enter something with various parameters. So, as Ferret ask for, it can be considered as static damping. As already mentioned, fluid behave differently depending speed, temperature... so the damping coefficient will be different at high speed suspension movment.

Since I am not a graduate engineer (although I wish I was!) this goes a little over my head...

I did do a little calculation myself, purely on the surface areas, and it seems to me that a difference in diameter of 0.13mm between piston and body (as you measured on the Kyosho) gives a surface area roughly equivalent to 2x 1.1mm piston holes.

I assume that the calculation for laminar flow is a lot more complex than simply surface area where oil can pass?

So, to ask in complete laymans terms...

Do you think that the pistons you have measured make any significant difference to low-speed damping?

[Lee]

If 2 pistons at a slow shaft speed have the same surface areas then they will feel the same, it's when the shaft speeds up and the flow goes from laminar to turbulent that the number of holes and the shape of the pistons edges come into play.

You will find that a losi piston with 3 holes will not pack as abruptly as an AE piston with 2 holes, an AE pistons sharper edges also make it pack more abruptly as the rounder edge of the losi lets the oil go down the outside of the piston with less resistance.

[Arn0]

You got it right Sosidge, it is 2 holes and not one. I don't know in what I was thinking of but it is two and not one hole as mentioned. Sorry for that!

The calculation for laminate is more that just calculate surface area even surface area are part of the mathematical model.

I did not measure any piston, the gentlemen mentioned at the bottom of the PDF did - thanks again! I can't do so as I run pan car.

Lee explain the main idea, plus details in fact. Fluid behave differently to each condition. The hole edges have an influence as the number of holes and the distribution.
The edges get a influence on the shape of the flow through the piston, the round edge make the fluid kind of sticking to the round as the square edge create more turbulence. I might try to get more details about this point.

So, those calculations are theorical but, as probably already said, there is a good amount of testing for whatever the vehicle is; car, bike, truck, both racing or street legal one, the same apply to RC.

The chart is made for off-road now, but it can be easly be made for on-road. The point of this chart is to try to provide some reference point for drivers who swap the original shock for other brand or to get some reference point when you try to copy-paste a setup and you do not use the same shocks as the one mentioned on the setup sheet.

[Arn0]

Gentlemen,

I bring this thread back to indicate an update on the Piston Chart (http://www.petitrc.com/setup/OffRoadShockPistonChart.pdf) to correct the Kyosho piston and adding the Tamiya ones

[SlowOne]

Since I am not a graduate engineer (although I wish I was!) this goes a little over my head...

I did do a little calculation myself, purely on the surface areas, and it seems to me that a difference in diameter of 0.13mm between piston and body (as you measured on the Kyosho) gives a surface area roughly equivalent to 2x 1.1mm piston holes.

I assume that the calculation for laminar flow is a lot more complex than simply surface area where oil can pass?

So, to ask in complete laymans terms...

Do you think that the pistons you have measured make any significant difference to low-speed damping?Sosidge, in layman's terms, I think Arn0 is saying this...

This is all totally theoretical based on some simple maths. It cannot be specific since the actual behaviour of the piston/oil combination depends on many factors like hole size, piston size, hole shape, piston shape, etc.

In order to be specific, you also need to consider the entire behaviour of the car chassis as the mount of weight in the car, the rate of the springs and the suspension layout will change the damping 'value' you get. For example, if you have a lighter spring then it will travel more for the same force input than a heavy spring. If you have more travel you get more damping - the specific value of the damping changes.

This is a well-researched piece of work which if used as Arn0 intended...

The point of this chart is to try to provide some reference point for drivers who swap the original shock for other brand or to get some reference point when you try to copy-paste a setup and you do not use the same shocks as the one mentioned on the setup sheet....will allow you to swap from car to car, or damper unit to damper unit, with a little bit more knowledge than you might optherwise have.

Any attempt to use this as a definitive basis for creating the perfect set-up is, of course doomed to failure!! :D As any cursory check will show you, the tolerances on piston diameter, hole diameter, any piston edge radius and the cylinder bore itself means that a definitive basis is impossible.

As Roger points out above, the effect of the clearance between piston and cylinder will dwarf any change in hole by a few numbers of drill bit. Great work, Arn0, use with care everyone!! :)