Tapered versus Flat Shock Pistons – What is the difference?

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Hopefully you have read my previous article about what the Losi Shock Matching Tool can do for you. If you have yet to read it just click here. It is filled with useful information and some of the items that I discovered during assembly while performing my first test.

Going back to almost the dark ages I remember shock pistons being primarily flat and only have 2 holes in them if they were Team Associated. Losi pistons had 4 holes in them along with different size of holes too. Now it seems like everyone offers their own version of shock pistons; 2, 3, 4, 5, 6 or make your own holes in the pistons.  Some have even taken it a step further and offer variable dampening. Even the aftermarket companies like M.I.P. have their own version of shock pistons for almost every car on the market. Yes there are other companies making pistons but I want to talk about different styles, flat and tapered.

I think the flat piston name is pretty obviously as both sides are the same, flat. The tapered piston is different. One side is flat and the other side is thicker in the middle, by the shock shaft, and is not as thick. So as you go from the inside of the shock piston to the outer edge you are going down-hill.

Ok, now for the test. I had a theory; I believe that the tapered pistons did not provide the same dampening or rebound of flat pistons provided they both had the same amount and size of holes and you used the same weight of shock oil. Since I wanted to compare apples to apples, other than the design, Team Associated offered what I was looking for. I picked up a set of ASC91352 Flat Shock Piston Set (3×1.4mm) and ASC91349 Tapered Shock Piston Set (3×1.4mm) and 25 weight shock oil.

For my first test, I installed the tapered piston with the taper side up. ]If my theory was correct, the dampening for the tapered piston would have less force than the flat piston when the shock is compressed. As I slowly compressed the shocks I could see, based on the arrow, the shock body with the flat piston marked with an “F” had more force on the pivot than the other shock body.

For the second test I decided to check the rebound. I inverted the tapered shock piston on the shaft and rebuilt both shocks. Based on the previous test, I believed that the shock with the tapered piston would rebound faster than the flat piston shock. This idea would be confirmed if the arrow on the shock matching tool would point towards the shock body with the “F” on it. When you watch the video you will see that my idea was confirmed.

So what does all of this mean? Well, as long as you are testing apples and apples as opposed to apples to oranges you will find that tapered pistons will provide less compression or rebound force than regular pistons. Now if you wanted to go crazy I could check multiple pistons with different size holes to see if I could match up a tapered piston. The only problem with that thinking though is that the force it takes to move the piston up and down in the shock body would be the same. Whereas, the amount of force to move a tapered piston would be different based on the direction the tapered piston is installed. The next step is to test my theory out on the track to see if the work I did on my bench is valid. After all, when racing against the clock, that is where it really matters.

 

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6 Comments

  1. Scott Fredrickson on

    The taper does nothing for slow speed travel. This side by side difference is an illusion because of the assumption that only the # and diameter of holes needs to be the same. The tapered piston has less resistance (damping) overall than the flat piston because the length of the holes are thinner, so it ends up exactly the same as running a lighter oil. Having the taper up or down will make no difference in a directional sense, so your compression and rebound will be the same. I have tested many different types of pistons on my shock dyno, and this conclusion has been consistent every time.

    I started a thread in the Electric Off Road section titled “RC Shock Dyno Results”. On that thread, in my signature I have a link to my website where you can download my results. (free).

    I still have not tested tapered pistons in “pack” conditions (impact testing, such as jumps), so I believe if there is any difference with flat pistons, it will be shown there. Impact test is slowly a work in progress to get consistent and repeatable results.

  2. Kraig O'Brien on

    Hi Scott, thanks for commenting. I have read your article and I took the poor man’s way of testing my theory. When I read yours, unless I read the wrong chart (Piston types 2015.pdf), your choice of tapered piston was not the same as the flat piston in your comparison. What I mean by that is the hole size and the number of holes were different. That was the part I was trying to show because most racers would grab the same piston hole sizes with a different design and try them out. I could be wrong though.

    Based on what I read, I hope you do get some time you will be able to test same piston holes and size with the differences between tapered and flat.

    For those of you who want to read Scott’s work here is the link to the thread on RC Tech. It is definitely worth the time to read.
    http://www.rctech.net/forum/electric-off-road/865246-rc-shock-dyno-test-results.html

  3. Scott Fredrickson on

    I understand your logic, you are correct with your assumptions, as they were the same as mine initially. I should clarify my comment a little better.

    A piston with the same # of holes of the same diameter (e.g. 4 x 1.3mm) will not provide the same amount of damping if one piston is “tapered” and the other is “flat”. The tapered will provide less because, assuming the same thickness (center portion of piston), the taper will reduce the length of the holes, thus decreasing the damping drag. This is what you experienced with your test. It’s true, and accurate what you did, and your results show that.

    What I was more focused on comparing was how the “tapered” piston is marketed as a piston that provides a different rate of damping in one direction vs the other (compression vs rebound). I never saw any evidence that shows there is a difference…. just like the flat pistons show no difference between compression or rebound damping.

    As I showed in the report, there are pistons available that do provide different compression vs rebound damping. But the tapered PISTON does not provide this (tapered holes, and valved pistons do.

    Time… does anyone have a coupon code to buy more of that?

  4. Hello,

    Thanks for this comparison test and discussion. I was curious to know how the needle acted when:
    1st video you – ‘rebounded’ the shock? was the rebound rate the same, since the rebound sides of both pistons were flat;
    Similarly in the 2nd video – what happened when you compressed the shocks?

    According to my thinking, in the above mentioned situations (which were not portrayed in the videos), the shocks should have been equal? If they were not equal, I then see Scott’s observations regarding hole depth is probably more accurate..

    Thanks in advance for your response to my queries..
    Cham 😉

  5. Kraig O'Brien on

    Hello Crixman, sorry for the late response as I was on vacation.

    What I found was hardly any difference between compression and rebound. The difference I did see I would have to chalk it up to the tolerance values in the tool since it really isn’t a precision instrument like Scott used during his testing.

  6. Guillaume Chane on

    Hello Scott,
    Impressive works 🙂

    For your study about the piston hole size and the number of holes (page 25 of RC ShockdynoTest), my personal study points out also on the piston thickness.
    Thin piston works as a diaphragm and thick piston gives more pressure loss through hole and more impact speed damping. In reality when your piston thickness is 2.5mm against a hole diameter of 1.2mm, that’s not negligible and the piston thickness must be take in account for the study.

    I also calculated theoretical reynolds number, for different pistons with multiple holes but the same flow area.
    Increasing the number of holes while maintaining the same flow area (ie multiple of small holes VS two big holes) while give you turbulent mode (reynolds number >2000) at smaller speed, giving you more pack.
    For exemple with the same flow area :
    low speed damping (3 holes piston 1.1mm ; 1.1mm ; 1.2mm) = low speed damping (2 holes piston 1.4mm ; 1.4mm) (note that holes don’t need to be all at the same diameter)
    but
    impact speed dampening(3 holes piston 1.1mm ; 1.1mm ; 1.2mm) > impact speed dampening(2 holes piston 1.4mm ; 1.4mm)

    Hole diameter plays a role as it’s use to calculate the hydraulic diameter for calculating the pressure loss

    To sum up :

    Impact speed depends on the thickness and the hole diameter
    Low speed dampening depend on the flow aera (ie hole diameter and number of hole)

    I made my theoretical study with Excel, and verify it in practice 🙂

    my two cent

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