Piston Chart

[Arn0]

After the Spring Chart thread, 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

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

[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.

Originally Posted by 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

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.

Originally Posted by 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.

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 ). 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!