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Shock pack relative to the number of holes in a piston

Shock pack relative to the number of holes in a piston


Old 04-03-2016, 03:59 PM
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Default Shock pack relative to the number of holes in a piston

I'm trying to better understand the direction to turn in terms of pistons when I need more pack. I understand the basics, such as when comparing two pistons with the same number of holes using the same cst oil, but differing hole diameter that the smaller hole piston will generate more pack. I also grasp that calculating the lack of surface area by multiplying the number of holes by the hole diameter is only going to provide static damping. I've also read up on the difference between laminar vs. turbulent oil movement.

What I'm not gleaning from the available information is how the number of holes in a piston affects pack. For example, lets take an 8x1.5 piston and compare it to a 10x1.2 piston both using the same CST oil. Both have the same cumulative hole diameter of 12, but which one of them is going to generate more pack? Is it the piston with the least amount of holes? Or the piston with the smallest diameter holes? and why?
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Old 04-03-2016, 11:56 PM
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Pi r squared
8 x (3.14 x .75 squared) = 14.13 square mm of space for oil to flow through
10 x (3.14 x .6 squared) = 11.304 square mm
Shouldn't the 10 x 1.2 pistons give more static damping?
But when you figure in turbulent oil movement during high speed damping, does this change?
I'm going to guess that as the viscosity of the oil increases, the size of the holes will be more important than the number of holes during high speed damping.
and I'm going to guess that as the viscosity of the oil decreases, the number of holes will be more important than the size of the holes during high speed damping.
In other words, heavy oil...fewer but bigger holes (even if the area of the holes is the same) pack will be less than more, smaller holes.
With lighter oil...more but smaller holes...pack will be less than fewer, bigger holes.
I don't have a fluid dynamics degree...it's just me and PBR thinking out loud lol

Last edited by bubbaslash; 04-04-2016 at 01:36 AM.
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Old 04-04-2016, 12:15 AM
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And not all of us use single-stage pistons...so rebound damping is another issue. I love this hobby!
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Old 04-04-2016, 07:15 AM
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This is from JQ and not from me. Hope this helps.

First the very basics of what happens inside the shock and why different pistons change how the car goes round the track.

The basic idea is that the car always feels the same when feeling it on the pit table, but then when changing the pistons it somehow magically changes on the track. The reason is that when moving the shocks slowly on the table small holes and thin oil, or big holes and thick oil will feel close to the same, but on the track, when the piston moves fast, there is a big difference. To understand this one has to understand some basic points of fluid dynamics. Simply put, a fluid flows in two ways, laminar or turbulent. When the flow is laminar the particles flow parallel to each other, in the same direction. Think of a river with water flowing calmly down it. When the flow is turbulent, the particles move randomly which creates eddies, and the friction between the particles increases. Think that the previous river hits some rocks and becomes a rapid. Flow in a shock is laminar when speeds are low, and when the speed increases enough, it becomes turbulent. When the car hits a bump at speed, the piston will move up in the shock, and oil will pass through the piston holes. Some will flow past the piston but let’s think of that as constant. If the piston moves fast enough, the oil flowing through the piston will cause turbulence, which increases the friction, and it will seem like the shock locks up. This is called “pack”. With small holes this happens more often, at lower piston speeds, so the car will bounce a lot in bumps, and with larger holes it will happen less often, at higher piston speeds, so the damping will absorb more of the bumps, and not bounce so much. If we think that the same amount of oil passes the piston in both cases, in the case of larger holes, the speed of the oil will be lower, than when compared to a piston with small holes. Furthermore, with thin oil (used with small holes) turbulence occurs earlier, and with thick oil, it occurs later, which makes the difference more pronounced. This is the basics, and all you need to know in order to understand how the pistons work in the shocks.

As far as how pistons affect the car’s handling, I will try to explain:

Larger holes gives the car more traction, it goes through bumps smoother, specially through ones you hit fast, and the small bumps in the surface of the track, the wheel follows the surface better. The downside is that the car doesn’t jump as well, and specially doesn’t land as well. When the holes are too big, the car bottoms out when landing off jumps, and will want to flip over. It will also feel unresponsive and slow.

Smaller holes reduces traction, it jumps and lands better, but it is generally worse in bumps. When the holes are too small, the car will feel like it gets unsettled by even the smallest bumps and it “stutters” over an uneven track surface, even with thin oil.

Increasing the amount of holes has an interesting effect. If you compare a 21.5mm piston, and a 41.4mm, assuming that the 21.5mm is good, you would be excused to think that the 41.4mm piston would have too many holes and the car would bottom out all over the place. However, this is not the case. When increasing the amount of holes, it’s possible to have more overall hole-area, than with 2 holes, before the negative points like bottoming out become a problem. This way, with many holes, it’s possible to set up the suspension to be soft, and plush, so it soaks up the bumps, yet it won’t bottom out too much on the jumps. Apparently the oil flow becomes turbulent more violently with more holes in the piston.

By using different size holes, for example 21.3 and 21.5 on the same piston, a similar result can be achieved. The car will have traction and be good in bumps, but will still land well, due to the flow becoming turbulent more aggressively than with all holes being the same size.

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