Pack
 

What is an oil shocks ability to pack specifically? Jimmy and DCM were talking about this in another thread and it's had me thinking for a couple of days.

How is it not just the damping ability of a shock? Is it the shocks resistance to small shock movements? Or is it a shocks inability to bounce or rebound i.e. would a shock with more air in it have less pack and more rebound.

Answer my question and allow my brain to think about something else. :)

Thanks.

Elliott.

pack, if I remember rightly, is resistance to flow by constriction.

You can make a shock 'feel' the same with a large hole and 70wt oil or a small hole and 35wt, the fact is, the lighter the oil, the easier it is to flow, but you have to have a small hole to restrict it.

Easiest way to sample pack, is build a set of shocks up, with a large hole and normal oil, stick them on the car and watch what happens when you drop the car, then go to the pistons you should have, the results are pretty big.

you need air in a shock for it work correctly..but the air must be kept seperate(in the shock cap above the diaphragm).

air can compress,a liquid cannot so if you filled totally with oil the piston rod would lock solid as you tried to move it into the shock body,hence why you need the air.

pack is when the oil is asked to travel thru the hole in the piston faster than it physically can..so you can pump the shock nice and slow and its fine,try it fast and it feels like the shock has locked up momentarily(in effect you trying to squash the oil faster than it can escape..and since oil cannot compress you get a hydrolock effect)

if you take a shock and remove the spring,stand it on the desk holding it by the plastic rod end and push the shock up and down it will move freely,now give the top of it a sharp hit with the flat of your hand and you will feel the pack effect if the piston hole is small enough.

Pack is the point at which the shock becomes very stiff, as the oil is unable to flow through the holes in the piston quickly enough. Increasing pack (smaller hole pistons, lighter oil) reduces the piston speed that is needed for the shock to stiffen up. Decreasing pack (larger hole pistons, heavier oil) does exactly the opposite, it takes a higher piston speed to make the shock go stiff.

Generally on a rough track you would reduce pack to allow the suspention to work on smaller, higher frequency bumps. Reducing the pack too far will make the car bottom out when landing from bigger jumps and can sometines also make the car harder to drive. I always try to run with as much as I can get away with but keeping the car stable though the rougher sections of the track.

AE shocks have no bladders;)

In these shocks its the moving of the oil from above the piston to below it that creates its damping action;)

Bu t is there some air in these? Does that mean that the oil will get cloudy?

all shocks(for our purposes) get their damping by moving the piston thru the oil...so dont get why you think associated are different?

without a bladder you can get the air and oil mixed up,this can create an inconsistent damper,hence why most shocks have the bladder.

You need a correct ammount of air in there also- it was pretty obvious to watch on sunday Richard and Woody racing around - over the big jump richards car always seemed to land smoothly, but woodys (mine) would usually bounce slightly - pack being pretty much spot on, but there being very little air inside the shocks, so on full compression there was no slowing of the shaft from pressure inside the shocks - so only on very hard landings the shaft would bottom out hard and unsettle it.

The reason I built the shocks like that was because of the bleed screws - it made it easy to get a perfectly filled shock- but easy to take too much air out and leave it without any pressure..

The AE shocks say in the manual how far the shaft should push back out on its own - and thats probably a good guide. If it doesn't come back out on its own (no spring) then its not right

lol........i fried woodys brain with this talk at ozwastree

what happens is that if you dont have enough air in the top of the shock,the damper will hydrolock because there isnt enough air to compress as the piston rod takes up volume in the shockbody..the air shouldnt be used to slow the damping down,thats what the piston is for.

having too much oil in the shock and not enough air will blow seals on hard landings and cause excessive pack thru hydrolocking of the piston rod not the piston itself.

i always build the shocks so that piston rod only rebounds out around 5mm max,if you go anymore,it can become hard to see wether pack is occuring thru piston hole size or because thees too much oil in the shock.

Nah, I think you have me wrong - What I mean about lack of air is with the piston FULLY pushed in. Pull the shaft out and it will suck back in to some degree. No problem at all with full compression - thats the lack of air I am talking about (vacum).

5mm of extension on full compression is more than none - so you are getting an effect (no matter how small) - more so than no effect at all.
The slowing of the piston at the end (more than the 'pack can acheive') would only come into play on the heaviest of landings - so would have no effect on the general handling of the car. *as I understand it :)

yes see what you mean now,its always a fine balance on compression push and vacuum pull,and im talking 5mm on rallycross..its very rare you would end up with the shock in such a compressed state on 1/8th.

the 1/8th ones i have,have a .5mm open bleed hole in the top so you dont get the vacuum effect

Those rallycross things handle like a brick no matter HOW you tune the shocks tho dave! :D;)

you havent seen my sportwerks then jimmy :)

floats thru the bumps hehe..should be fun at the grp this weekend.

Have to swapped from the Xray or just testing this out?

yes running sportwerks now,cracking buggy,best ive had so far and soooo much steering its unreal.

tell you what, after building the TRF shocks recently, and playing with them, with a bladder on top, they just make the AE shocks a right royal pain in the butt....

Tamiya - Fill shock, pump shock shaft to releive air from underside of the piston, top up, place bladder on, allow excess oil out, fit top, jobs a good-un.

AE shocks - fill shock, pump shock shaft, top up oil, put some in cap, put both together, the fiddle with it for 5 mins to get the rebound right.... then build next one and try to get it the same....aaaarrrgghhhh!!!

yep they are steve,thats why i use to use the yokomo caps on mine as they had a bleed screw in them.

ilovethisplace.

Fantastic, I'm glad I've started a discussion on this.

I think I was a bit right.

Thankyou for the information.

Elliott.

There is a little more to generating pack, but that means we need to discuss fluid dynamics also :o

Anyway, if you have a 3 hole piston setup an 2 hole piston setup with the same static damping then the 3 hole setup will create more pack.

G

I see a 'northys knowlege' (tm) artlcle in the works

haha, i was just thinking of that!

i think i ran away as soon as i heard what you were all talking about... :D

:D :D :D :D :D :D

I'd be interested to hear more Northy.....

By static damping, I guess you mean the same viscosity oil (and stiffness of spring)? Also when you talk about 2 and 3 holes I presume they're all the same diamater?

Have to admit I can't stop thinking about this since I read your post :D , but my thoughts are:

Increased pack results from increased shear stress, with shear stress proportional to the velocity gradient and viscosity being the constant of proportionality. Each hole in the piston will have a velocity gradient associated with the fluid passing through it, so by adding more holes you increase the total shear stress and therefore the force required to move the piston.

An extra hole does present more area for the fluid to pass through, but I'm assuming in this instance where viscosity is relatively high and the areas (and change in area) are small, viscosity effects, and therefore shear, dominate.

Would you expect the same effect if you only increased the diameter of the existing hole(s)? I'm thinking not, as (for the same velocity) the velocity gradient, and hence shear stress, will be decreased.....

Am I close, or have I just wasted several hours of my life? :D :o

Bugger me :confused: my head's a mess now
Can't we please talk about something funny?;)

Bloody hell Dunc.....do you have a degree in fluid dynamics?...it's beyond me :o
If my shocks go up and down without leaking I'm happy :wtf:

Could it have something to do with the distance over which the velocity gradient spans and the non proportional nature of which area of a circle varies with radius?

I'm no expert, but I think you guys are looking a little deep into it.

I'll see what I can come up with.

G

Sorry, Northy. From your previous post I was under the impression you were holding out on us! :D

I think you're right, Richard. That is certainly what I was trying to imply. A linear velocity gradient is defined as a change in velocity over a given distance. Therefore if your velocity difference remains unchanged, but the distance over which it occurs increases, you will see a decreased gradient.

I'm not sure how the area of a circle varying disproportionately to its radius will affect things. That's getting quite deep! :)

I was simplifying things quite a bit by quoting a flat plate theory where the velocity gradient is constant between two plates (one moving and the other stationary). In this situation both 'plates', i.e. the hole edges, are effectively stationary. In addition you may see boundary layer effects, recirculation and no doubt a myriad of other phenomena playing their part. I hadn't thought of it before, but I think recirculation of the fluid behind the holes is significant (as it causes drag), and hence another reason why more holes would result in increased pack.

Indeed I do. Plenty more to learn though.....

Bound to be a small article ;)

lol.

I am getting my first buggy next week, after 5 years of snoring cars. You lot are doing a good job of worrying me about the shocks lol

:D:D:D

What are xx4 shocks like?
:confused:

Also, this may a silly question, but why do you need pack?


Also, heres an interesting read, at wikipedia

http://en.wikipedia.org/wiki/Fluid_dynamics

jumping and landing.

I would like to point out that I don't think about things in quite this depth when at the track :D

As most of us are merely in the business of use, and not design, I would suggest it's more important to have a good understanding of the cause and effect relationships rather than the why, i.e. have a general feel/understanding for how changing various settings will affect the car's handling.

Even what appears, on the surface, to be a simple problem can often become complex very quickly if you try to fully understand what is occurring. I posted my thoughts, which may well be way off the mark. Even if some of what I said is there or thereabouts I am sure there is still much more to it, assuming anybody understands it fully at all. After all, there are many things in this world that we are still unable to truly comprehend; plus the nature of engineering is to often describe real processes with an approximate equivalent that shows good correlation.....

How (your going to think im mental:wtf: ) do you think a 4wd with short shocks(front ones) all round would get on????

or
how could you get get the same amount of pack on the rear of a 4wd if it had 4 short shocks ( 2 on each w/bone ) to reduce cg/drag/silly looks)

could it be done??
not that im trying....but i have wondered
cheers

I know its a silly question...........:wtf:
but realy :confused: ...think about it:confused: ......is it realy that much of a silly idea?

Must be!:) all has gone quiet:wtf:

Remember the length of the shock absorber will determine other factors such as droop.

One reason for having longer shock absorbers at the rear of a car is because they are typically mounted further out on the wishbone relative to the front and hence must be able to cope with a larger arc/travel, whilst maintaining comparable droop and total travel. Additionally, mounting further out on the wishbone dictates a taller shock tower if you wish to maintain comparable shock angles front and rear, also leading to a longer shock absorber. Shorter rear shocks would inevitably need mounting further inboard on the wishbone, which may have an undesirable effect (depending on the overall suspension geometry??)

I guess you could try and do some simple spring/damper calculations to see what spring rates and damping coefficients would be required for comparable damping between 1 spring and damper versus 2 springs and dampers. This sort of thing is the basis of dynamic systems analysis and should be easy enough to research if you're that keen. Not sure how you could easily to determine an accurate damping coefficient though....

Well Scott, the answer is it's very difficult, almost impossible I would say. Not sure on the two shock idea.

G