Shock pistons and "pack"

[@invisiblespeedrc]

I also think your conclusion is the opposite to what a driver would do. On a smooth track many holes would be beneficial, because you can set the suspension to be very soft and plush for cornering, but the many smaller holes will have more aggressive pack for the jumps. This also works tracks with a lot of sharp bumps. Then the opposite for generally rough tracks, less and bigger holes for less pack as well as a setting with thick oil to slow the responsiveness of the car down.

[@invisiblespeedrc]

After thinking about this more, I think the key point is that with less larger holes, you have less pack high speed, and because of that you can set the car up with firmer suspension for high grip tracks. The lower pack will still work well for landing and high speed jumps. For many smaller holes, you can set the suspension softer, so it is more plush and giving the car more grip when cornering, accelerating or braking, and so it handles a rough surface well, yet because it has more high speed pack, it will still land well and absorb high sped bumps well. So fewer bigger holes for high grip, and more smaller holes for low grip, basically. As for bumps, that is not something that can be explained universally, as it will depend on car design, driving style, and type of bumps.

[@floridalife7877]

@@invisiblespeedrc I find that the less amount of holes and/or the smaller the holes, the more high speed pack I get jumping/landing AND the more low speed pack I get.....Less chassis slap on jumps and less roll in corners. More damping all around. Not sure what Mosfet is saying....

[@echidna1263]

Wow so informative. Thankyou so much for taking the time to explain this. Just going to shed to change out my 8 hole for 5 hole on my 8ight X's and Xe's. This will suit our track

[@byknphil]

Finished all your informative videos. SO. MUCH. INFORMATION! But buddy, so helpful. Saved the in favorites to refer back to during set up. Thanks.

[@bricmerc1961]

Hi Mosfet, I try 10x1.1 pistons in two big jumps and works very good !!! (1/8 buggy).
Most of the times the buggy pass trough the land bump and no problem ...

[@ikariku]

brilliant explanation

[@nastycop]

this is the video i have been searching for. Great explination of pistons. Im a noob and it all helps.

[@Slatanicmushroom]

Very good explained, thanks!

[@invisiblespeedrc]

Only one problem. 2 x 1.7 piston has less pack than 2 x 1.5 piston, but in your graph it has more. I think the idea you have is correct, but the graph is wrong because hole size is also important. The shear rate is higher in smaller holes, so that would need to be accounted for also somehow. More pack = more holes and smaller holes. Less pack = less holes and bigger holes. How to show that in a graph I do not know. What do you think?

[@doctormosfet]

Yes, this is a good point. What you're basically getting at is the boundary layer is a non-insignificant fraction of the hole radius and so that must be factored in. I agree completely, so I should clarify here that this video assumes the boundary layer is much smaller than the hole radius.

[@jamesmccabe8736]

TH-cam served me this video. I seen other videos of yours before so it must have known I was looking for information about my rc shocks. Your maths agrees with my perceptions on the track. So if my car is a slapn' and bouncing on a flat land. I need more holes not more viscosity or spring? So more holes=more friction to absorb the high speed energy? Fluid dynamics are creepy.

[@doctormosfet]

Well what I showed here assumes that the boundary layer is a small fraction of the hole opening (I should have pointed that out). If your car is slapping and bouncing, then really that's two things. The slapping could be because the springs are too soft and/or you don't have enough pack, while the bouncing is because you are underdamped, which is either too thin on the shock oil or...not enough pack. You can increase pack with smaller holes, either something like a 2x1.5 or a 5x1.2 piston. There difference between those two is the 5x1.2 will give you less low speed damping, helping the car roll more in corners (good for hard dirt).

[@glennmathias1805]

After review (multiple times), I think that the High Speed Dampening (Pack) on the curve should be less as the perimeter goes up.

[@jasonwills6694]

I thought similiar

[@natofan]

Agreed. To be more specific it was around 8:06.

[@qianjuntang3324]

great video, nice yz10 at the back by the way

[@spellward]

Excellent, thanks.

[@glennmathias3723]

Doctormosfet,
Are you sure that the pack (high speed dampening) is not inverse? The more circumference, the less pack. For example, 2x1.5mm pistons should have more pack than 2x1.6mm pistons. But in your explanation, you state otherwise. Other than this detail, I think it is the best explanation on the internet about shock pistons and pack. Period.
Thank you,
Glenn

[@doctormosfet]

Hello Glenn,
Right, so I was isolating my description of pack as what happens only along the piston hole perimeter, assuming that as you change the perimeter diameter, there is no resulting overlap of the spatial shear stress distribution of the shock oil. If you have 2x1.6 and 2x1.5 pistons, there should ideally exist more pack in the 2x1.6 pistons, and more low speed damping in the 2x1.5 pistons; however, if that piston diameter is small enough to start resulting in overlapping shear stress "fields," then 2x1.5 will have both more low speed damping and more pack.
Thanks for the nice comments. I'm glad you like the video ;)

[@glennmathias3723]

Nice.

[@jimjimaco4217]

sorry its me again....if i may ask one more question please???i am an asphalt guy,so i suppose things are easier for us in terms of suspension because there no jumps...but there are some bumps ofcourse,but the main issue is the turning,so the question is in a track with no bumps you suggest less holes and thicker oil,and the opposite in the case of bumps???and how the heck can i understand if i have issues with turning,which thing should i change first????holes???silicone???? now its more than one question....sorry again,but sometimes when you are reaching a level you can not just rely on anyone opinion..thanx...

[@doctormosfet]

If you have issues with turning, that's probably the most complex. This is because a host of things will impact turning. Ackerman, caster, roll center, shock angle, rear toe, etc. My suggestion is to first figure out what part of the turning you want to improve (corner entry, corner exit, mid-corner) and under what conditions (high speed, low speed). From there, you can down select which change/s to make.

[@jeffbryner5355]

What would you suggest on piston size and oil for a indoor dirt oval track? Its a well maintained track

[@floridalife7877]

You are saying that a 2 hole piston with larger holes has more high speed pack than a 2 hole piston with smaller holes? From what I have found it’s the opposite. If my car is bottoming out on large jumps, I decrease the hole size and it prevents bottoming out and feels like it has more pack...

[@doctormosfet]

I should have clarified that my analysis assumes the boundary layer thickness is an insignificant fraction of the hole radius.

[@floridalife7877]

@@doctormosfet great videos

[@toad2039]

What are your thoughts about drifters who run oiless shocks???

[@doctormosfet]

Well I don't do any drifting myself, but if I had to guess, taking the oil out would help speed up weight transfer and perhaps get the car unsettled more easily so it drifts. I can't imagine it would be very stable, though.

[@jtrace72]

Hi Doctormosfet, can you put oil viscosity in the equesion?

[@aslanhaikal4509]

Why more piston holes have more pack? isnt they gives way more flow?

[@doctormosfet]

High speed vs. low speed damping.

[@jimjimaco4217]

hi there its me again...trying to understand as much as i can...so by saying low speed damping is getting lighter when we use less holes,u mean that if for example there are no jumps and we only have to do with corners and changing directions from left to right,the action is faster with less holes?or is the opposite?

[@doctormosfet]

Fewer holes will increase low speed damping. You'll note in the plot I show in the video that LSD drops as total piston hole area increases. So if we have fewer holes, that would trend toward less total piston hole area and therefore more low speed damping, which in turn would slow down the shocks (all else being equal).
Of course, the picture gets more complex than this as we change the shock oil viscosity, as you can achieve the same LSD with fewer holes + lighter oil and more holes + heavier oil. The difference between the two will be pack, with the latter option having much more pack.
There is also a second order effect in pack that I didn't really cover here, wherein if the piston hole is small enough, then the radius becomes comparable to the thickness of the boundary layer, which in turn will result in an increase in pack. I'm assuming in this video that we are not in that regime, but it is possible with thick enough shock oil and/or small enough piston holes.

[@jimjimaco4217]

@@doctormosfet so by increasing the low speed damping means that the car will be more responsive?

[@doctormosfet]

@@jimjimaco4217 sort of ;) The springs play more of a role in responsiveness than damping. If you increase the damping, it makes it harder for the spring to rebound the shock, so the car will feel less responsive on rebound. However, it will feel more responsive on compression, as higher LSD will make it harder to compress the shock in the first place.

[@jimjimaco4217]

@@doctormosfet thanks a lot

[@fondfarewell2]

learned a lot from this than you man.