Pack

[emzy]

Originally Posted by pidge

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

haha, i was just thinking of that!

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

[Northy]

Originally Posted by jimmy

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

[Dunc]

Originally Posted by 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

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 , 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?

[Col]

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

[usagi]

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

[rich_cree]

Originally Posted by Dunc

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 , 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?

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?

[Northy]

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

[Dunc]

Originally Posted by Northy

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!

Originally Posted by rich_cree

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 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.

Originally Posted by usagi

Bloody hell Dunc.....do you have a degree in fluid dynamics?...it's beyond me

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

[elvo]

Originally Posted by jimmy

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

Bound to be a small article

[BenG]

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



What are xx4 shocks like?


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

[elvo]

Originally Posted by www.tyrc.co.uk

lol.



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

jumping and landing.

[Dunc]

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

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.....

[SCOTT DICKINSON]

How (your going to think im mental ) 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

[SCOTT DICKINSON]

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

[SCOTT DICKINSON]

Must be! all has gone quiet

[Dunc]

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....

[Northy]

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

G