Pistons 6 hole, 4 hole, 3 hole etc?

[mattr]

Originally Posted by Chris Doughty

All of this is how I understand the physics to work

Bingo. Few tweaks, but in laymans terms, that's about it.

[bigred5765]

so basically exactly what i said in the first place smaller or less holes more pack

[TonyM]

Originally Posted by bigred5765

so basically exactly what i said in the first place smaller or less holes more pack

I think Carl the potential confusion comes from whether you are considering total equal hole area or not. If you are considering the same area and compare say a 4 hole piston to a 6 hole piston the 6 hole piston will give more pack. If however you are not considering equal area but comparing say 4x1.2 holes with 6x1.2 holes then your statement is correct on both counts.

[bigred5765]

Originally Posted by TonyM

I think Carl the potential confusion comes from whether you are considering total equal hole area or not. If you are considering the same area and compare say a 4 hole piston to a 6 hole piston the 6 hole piston will give more pack. If however you are not considering equal area but comparing say 4x1.2 holes with 6x1.2 holes then your statement is correct on both counts.

tony read my post carefully
i said SMALLER OR LESS HOLES MEANING EXACTLY THAT
so running 6 holes 1.4 coming down to 5 1.4 are less no matter how you look at it or 6 hole 1.4 and 6 hole 1.2 are still smaller or less holes

[mattr]

Ahhhh, the written word, so easy to misinterpret.

Your first post was correct, but not very clear (neither of you initially mentioned that surface/hole area must remain the same) that's quite important

Your second post was even less clear. The oil starts moving very fast, very quickly, and gets very turbulent (through the small holes), so you then get pack (i think the technical term would be stalling). You must have the initial movement (however small) to get the pack.

[TonyM]

Originally Posted by bigred5765

tony read my post carefully
i said SMALLER OR LESS HOLES MEANING EXACTLY THAT
so running 6 holes 1.4 coming down to 5 1.4 are less no matter how you look at it or 6 hole 1.4 and 6 hole 1.2 are still smaller or less holes

It still depends upon if you are comparing equal surface area or not.

[chet]

All this is info about pack is great. But it would help every one if we wrote what kind of track we run on? Like is it high bite or low and does it have a lot of jumps?


For me I have a local track that is pretty low bite, looks like the same carpet as the last Euro series. Its not as big though and I like 1.3 4 hole fronts there with losi 50wt and brown springs. Rears I like 1.3 4 holes and 27.5 with blacks. This is with a shorty pack

On a larger track I race big races on its high bite and really big. I run 1.3 4 hole with 60wt losi and dark greens fronts. Rears I run 1.2 4 hole with 27.5 losi and dark green springs. I run the 1.2 on this track because I run a full size pack. This track has some big air time.

So please guys lets start a post on what you run on for carpet and your piston, oil and spring!

[av4625]

Originally Posted by chet

On a larger track I race big races on its high bite and really big. I run 1.3 4 hole with 60wt losi and dark greens fronts. Rears I run 1.2 4 hole with 27.5 losi and dark green springs.

u run dark greens front and rear the front must be sooo under sprung and take for ages to come up again with that oil/pistons!

[ConceptRacing]

This is a article from the RC car handling site, hope it helps, i kind of understand it.
Heres a link to the main site and articles: http://users.telenet.be/elvo/1/1.html


2.2 Damping

Damping is needed to absorb the energy associated with suspension travel. That suspension travel can be induced by bumps, or lateral or longitudinal acceleration. Without damping, the magnitude of the suspension movement would never stop increasing, leading to a very humorous situation. In terms of energy, damping absorbs most of the energy the car receives as it moves, unlike springs, who store the energy, and release it again. Imagine a car with no damping driving on a bumpy road. The subsequent impacts from the bumps on the tires would make the suspension bounce very intensely, which is not a good thing. Dampers absorb all the excess energy, and allow the tires to stay in contact with the ground as much as possible. This also indicates that the damping should always be matched to the spring ratio: never run a very stiff spring with very soft damping or a very soft spring with very stiff damping. Small changes however can give interesting results. Damping that's a bit on the heavy side will make the car more stable; it will slow down both the vehicle's pitch and roll motions, making it feel less twitchy. Note that damping only alters the speed at which the rolling and pitching motions occur, it does not alter their extent. So if you want your vehicle to roll less, adjust the anti-roll bars, or the springs, but not the dampers.

Something you can adjust with the damping rate is the speed at which the suspension rebounds: if a car with soft springs but hard dampers is pushed down, it will rebound very slowly, and a car with stiff springs and light damping will rebound very quickly. The same situation occurs when exiting corners: in the corner, the weight is transferred, and the chassis has rolled and/or dived, but when the steering is straightened out, and the cornering force disappears, the chassis comes back to its original position. The speed at which this happens is controlled by the damping rate. So the car with the soft springs and hard damping will tend to want to continue turning when the steering is straightened. It will also tend to continue running straight when steering is first applied; it will feel generally unresponsive, yet very smooth. The car with firm springs and soft damping will be very responsive: it will follow the driver's commands very quickly and aggressively.

You may not always be able to use the spring and damping rates you'd like, because of bumps. Small, high-frequency bumps require soft settings for both damping and springs. You can't use such soft settings for big, harsh bumps, because the car would bottom out a lot, so you'll need to set your car a little stiffer. On very smooth tracks you can use very stiff settings for both springs and damping.

But it's not quite as simple as that: even in the simple dampers used in R/C cars, there is a difference between high-speed and low-speed damping. Maybe I should point out that the speed which is being referred to is the speed of the shaft in relation to the housing, not the speed of the car. In most full-scale cars, the difference is implicated by means of an array of spring-operated valves in the piston. In less sophisticated damper units, as used in R/C, the difference is an effect of the inherent properties of the fluid being used.

If there's anything a racing enthusiast needs to know about fluid dynamics, it's that there are two basic ways for a fluid to flow; laminar and turbulent. A flow is said to be laminar if the particles move parallel to each other, creating flow lines that never intersect. Laminar flow occurs when the velocity is low, the fluid has a high viscosity, and the surface is smooth and well-rounded. A flow is said to be turbulent if the particles move randomly, creating eddies. Situations where the velocity is high, the fluid is thin and the surface is rough favor turbulence. In case of turbulence, a lot more energy is required(or wasted, depends how you look at it) because there is a lot more friction between the particles. Also, for a laminar flow the pressure (resistance, in case of a damper) is proportional to the velocity of the fluid whereas in case of turbulence, it's proportional to the velocity squared. There is no strict distinction between the two types; there's a big gray area in between.
To predict whether or not a flow is turbulent, the Reynolds number is used. It's defined as Re = D * V /n . D is the diameter, V is the velocity of the fluid, and n is its viscosity. If Re is smaller than 2000, the flow is most likely to be laminar, if it's in between 2000 and 4000 it's something in between, and if it's greater than 4000, the flow is most likely turbulent.

Now consider a typical R/C damper unit: you have oil of a certain viscosity passing through orifices of a certain diameter at a certain speed. Some oil flows around the outside of the piston, this is almost always laminar, since the gap between the piston and the housing is so narrow, so it creates a lot of drag. For the oil flowing through the holes in the piston however, it's hard to predict. When the shaft speed is very low it will be laminar, and when it's high it will be turbulent. Exactly when the transition will happen is hard to predict, but easy to feel: because the resistance of the shock is proportional to the shaft speed when the flow is still laminar, and proportional to the shaft speed squared the very next moment, when the flow has turned turbulent, it feels like a kind of hydraulic lock has occurred because the difference in resistance is usually quite substantial. The transition is sometimes also described as 'pack'; it feels as if the shock 'packs up'.

This effect can both be useful and unwanted: it can prevent your car from slapping the ground when landing from a jump, but it can also make your car bounce very badly over sharp ruts or bumps taken at high speed. So it's pretty important to get this adjustment right.

The way to achieve this is to select the right piston and shock oil: both the combination of a piston with small holes and a low viscosity oil and the combination of a piston with large holes and a high viscosity oil will yield the same static damping; it will feel the same when you bump your car by hand. It will also make the car handle the same in low-speed transitions, such as smooth cornering and low-frequency bumps. But the real difference is in the high-speed damping: the first combination will pack up very rapidly because of the low viscosity fluid and the increased fluid velocity. (the same amount of oil has to pass through smaller holes in the same amount of time, so its speed must be higher) The second combination will have a relatively high resistance to turbulence, because of the very thick fluid which flows at a much lower speed. Hence, turbulence will occur at much higher shaft speeds, or it may not occur at all.

So selecting the right piston and oil depends largely on the track layout. Killer jumps or chassis-wrecking bumps require pistons with small holes to prevent the chassis from slapping the ground and usually making the car very unstable. On the other hand, if the track has lots of bumps or is very rutted, any packing up of the shocks would make the car bounce and thus very unstable. In that case you should try pistons with large holes.

Note that judging if the holes in the pistons are too small or too large isn't as straightforward as you'd like it to be; because the shock absorbers aren't in direct contact with the ground, there is some elasticity to the whole suspension system. Suspension arms aren't infinitely rigid and neither are rims so expect a little flex, and hence also a little bounce from them. Then there there's some more elasticity in the tires, although this is a far less 'bouncy' form of elasticity. These effects are most apparent when your car lands off a big jump, and it bounces up a little, without the chassis having touched the ground. It means the pistons are way too small, which makes the shocks lock up too fast, so the impact has to be taken up by the elasticity in the suspension arms and the rims.

[chet]

Originally Posted by av4625

u run dark greens front and rear the front must be sooo under sprung and take for ages to come up again with that oil/pistons!

It's dark green one stiffer then lite brown front and rear is one stiffer then black?I dont have chart handy

[OneKiwi]

Originally Posted by chet

It's dark green one stiffer then lite brown front and rear is one stiffer then black?I dont have chart handy

Thats olive green for the fronts.
Its pretty similar to what we run
Im running:
2x1.3 32.5 light brown
4x1.3 27.5 and dark red. I had 25 and just refilled them, they feel softer that the 25... strange

Tomorrow I may try:
Olive and maybe 4x1.3 50-60
4x1.2 25/27.5

[av4625]

oh cause dark green is lighter than dark blue

[chet]

Originally Posted by OneKiwi

Thats olive green for the fronts.
Its pretty similar to what we run
Im running:
2x1.3 32.5 light brown
4x1.3 27.5 and dark red. I had 25 and just refilled them, they feel softer that the 25... strange

Tomorrow I may try:
Olive and maybe 4x1.3 50-60
4x1.2 25/27.5

Thanks for the info! How do you like 2 hole pistons in the front vs 4 hole feel. Does it make the streering less twitchy? What kind of carpet do you run on. High bite or low?

Wish more people would put the piston, oil, spring combo up it would be great to see!

[chet]

I've tried 2 hole pistons on really high bite carpet and I find you can carry more speed on sweepers. Car is not as tippy too

I just copied the AE and Losi U.S pro's there all going 2 hole 1.6 front and back.

When in dout copy what the Pro's from AE and Losi are doing