Pack

[Elliott Hopkins]

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.

[DCM]

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.

[dave g]

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.

[Richard Lowe]

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.

[BenG]

Originally Posted by dave g

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

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?

[dave g]

Originally Posted by www.tyrc.co.uk

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.

[jimmy]

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

[Lee Martin]

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

[dave g]

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.

[jimmy]

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

[dave g]

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

[jimmy]

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

[dave g]

you havent seen my sportwerks then jimmy

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

[jimmy]

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

[dave g]

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

[DCM]

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!!!

[dave g]

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

[Elliott Hopkins]

ilovethisplace.

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

I think I was a bit right.

Thankyou for the information.

Elliott.

[Northy]

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

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

[jimmy]

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