BC shocks: Why Sponges ?

[JV]

The shocks supplied with the BC & BC-SP include small doughnut-shaped sponges which fit inside the base of the damper units.

Anyone know what what they are for ?

I've looked at instructions for a couple of other buggies (B4, XXX), and their shocks don't seem to include these sponges.
The purpose seems quite subtle, since if the sponges were saturated with oil, I wouldn't expect them to do much.

The instructions with the diaphram shock caps say to omit the sponges.

Is this about air entering the bottom of the shocks on compression ?

[sosidge]

It's about volume compensation.

As the shock compresses, the shaft takes up more space inside the body, but the oil has nowhere to go.

The sponge is supposed to give it somewhere to go - either air within the sponge does the job of absorbing the extra pressure, or the foam itself is designed to compress, I'm not quite sure which way it's designed.

[Merciless]

sponge compresses.No air in the shocks.

I feel there better than the diaphragm ones too ;)

[Gaz_Stanton]

As above.

Associated shocks are of the emulsion type so are therefore designed to operate with some air within the oil. (not too much though!)

The Yokomo shocks are not of the emulsion type which is why the foam is used and you are required to bleed them.

The shocks on the BC Special '03 (threaded, gold shafts, blue bits etc) are probably the smoothest i've ever built. (and the best looking too! :D)

[shane.tidley]

well said, i ve built the yokomo shocks and they are with out question the best for built and feel ;D ,the Associated shocks have more air in them than a air line !!!!!!!!!!! ::)

[BC Rog]

All this is true, and the Yoko shocks are amazing.

So why do people change them???


It's to do with the relative lengths of body and shaft and what this does to the shocks as the piston moves through the oil (due to the position of the piston in the shock at normal ride heights.

Sufice it to say that Yokomo got their sums slightly wrong. I will soon be letting you know how to get the best from the Yokomo shocks but It will probably require a change of rear shock tower (fronts are ok, so long as you use the longer shock bottom).

I will let you know when I have something.

Trust me, if I was going to choose a shock design to carry forward onto another car then it would be the Yokomo ones ............

;) ;) ;)

Rog

[BC Rog]

Forgot to mention ..... pistons could do with a bit of a skim to let some more oil bypass them ... when I have the definative size I'll let you all know.

Rog

[AzNDRAGster]

But don't go crazy with sanding the pistons, just enough to knock off a 1MM or so.

[JV]

Sorry for not replying earlier. For me, understanding what's going on in the shocks means having at least a crude quantititive model.

Without some sort of quantitive model I don't feel I have any understanding of what is going on.

My fluid mechanics mostly comes from an applied maths course a few years ago, where the fluid mechanics was mostly a vehicle for the maths, and I have almost no practical experience. I can drive Maple & Mathematica though.

Sosidge writes:
> It's about volume compensation.

> As the shock compresses,
> the shaft takes up more space inside the body, but the oil has nowhere to go.
>
>The sponge is supposed to give it somewhere to go - either air within the sponge
> does the job of absorbing the extra pressure, or the foam itself is designed to compress,
> I'm not quite sure which way it's designed.

Rapid compression seems to be the interesting case, with the possibility of quite big pressures, compared to the expansion/recovery case, which is slow, with only the spring acting to extend the damper - unless the car is landing upside down...

As the shock compresses, the pressure above the piston is going to go up, and the pressure below is going to go down. So the oil isn't going to go into the sponge on compression.

Air trapped in the sponge is going to expand. The medium of the sponge itself is going to be incompressible for practical purposes (any gas is going to compress/expand first).

Or is the sponge about rebound ??

Merciless writes:
> sponge compresses. No air in the shocks.

Well there will be air in the shocks, at the top. A diaphram maybe limits how much air can mix with the oil, but either way there's something springy at the top.

Wibble Wibble.

Anyone care to share a damper model ?

[BC Rog]

Sanding pistons?????

No, no, no ..... you'll never get them round or prep' to the shaft.

The only real way is to put them on a shaft and spin it against a cutting (not grinding / sanding) tool. If you have a lathe great, if not a high speed drill mounted in a vice (use a good fitting bolt to hold the pistons and don't clamp them up too tight on it) will do. You would be supprised how effective a knife endged chisel is as a cutting tool if handled correctly!!

I think 1mm is a bit much. I'll have to go over the calcs again but I think it's in the order of 0.15 radial (ie 0.3mm of the diameter).

The aim is to get a bit more 'controlled' flow around the outside of the piston not put the majority of the flow around the outside.

The reason for doing this is the way in which the fluid behaves as it flows around the edge of the piston and between the piston and the shock body. It's too complicated to go through here but there are a lot of good books about fluid dynamics available if your interested.

My motivation for doing this is to try and allow the use of thicker oils to stabalise and control the piston action better without getting huge levels of pack. I think this is one area that the Associateed shocks have over the Yokomo ones, although in every other way the Yokomo shocks are far superior.

Rog

[BC Rog]

JV,

what you say abou the the pressures is almost correct.

When the psiton is moving there will be a slight inbalance of pressure above and below but the pressures will never be great. There is enough however to influence the way the fluid flows around and through the piston.

The 'volume compensation' aspect of the foam is an interesting thing to try and explain properly. What is actually happening is that the foams are not 'incompressable' as the material itself is elastic. When the extra volume, due to the shaft, needs to be compensated the pores in the foam allow some oil to escape and disipate around the shock. This makes the 'semi-solid' volume of the saturated foam a little smaller thus allowing the shaft to enter the shock without the whole system going 'solid'.

The oposite is true as the shaft extrends. The foams pores accept more oil and the 'semi-solid' volume increases preventing the shaft being pulled back in.

All this relies upon the fact that the oil in the foam is saturated ....... of course we all soak our foams in oil overnight before building shocks don't we ... ;) ;)

Rog.

P.S. NOTHING is incompresable!!!! Trust me at the pressures I work with (upwards of 4000 Bar) even steel is compressable!!

P.P.S. There is no need for a model ..... get you slide rule on jotter out you lazzy boy ... lol (youngest old git on the planet!!)

[sosidge]

Are the Yokomo buggy shock sealed by a single O-ring, like the tourer versions?

[JV]

sosidge writes:
> Are the Yokomo buggy shock sealed by a single O-ring, like the tourer versions?

The note that comes with the shocks in the SP kit suggests using one for on-road and two for off-road.

[JV]

[In a hurry]

Rog -

From the instructions, it seemed like they only wanted to put a bit of oil on the outside of the foam, rather than saturating them - but that's just what I made of the instructions - possibly nonsense.

My comment about compressability was based on the foam being mostly full of air.

Are you trying to add a lot of area with small linear dimension (low Re),
while keeping the big holes (higher Re) ?

Presision seems important, since (from a dimensional point of view) a lot seems to depend on the fourth power of the linear dimension of your gaps/holes.

FWIW, the Rayspeed pistons are a much closer fit: I measure 10.00mm vs. 9.72mm for the Yokomo moulded parts. The Rayspeed pistons are radiused on the edges, and arround the holes. No flash anywhere.

I've hunted arround the web for an analysis of flow in a short pipe, but didn't find what I was looking for. But a short pipe flow is going to be quite sensitive to the shape of the entry/exit. My Batchelor & Tritton books don't really cover this kind of situation, at least not in a nice canned way.

Regards,

John.

[JV]

JV wrote:
>Are you trying to add a lot of area with small linear dimension (low Re),
> while keeping the big holes (higher Re) ?

In which case, maybe drilling a lot of small holes would work too.

John.

[cobra81li200]

Decreasing the diameter of the piston would be more prone to decrease the chances of turbulent flow (in the second part of the body) than a lot of small holes.

[BC Rog]

JV, not a bad book but not really coving this sort of stuff.

The profile of the piston is important yes, as is the entry and exit of each hole.

The easiest (note esiest, not best) profile to useis a sharp edge as it is repeatable to make.

Short pipe flow is not as important as treatig it to a bit of basic orifice theory. The conditions in the shock are farto turbulent for you to be able to work pipetheory succesfully, it sort of misses the point as well.

What i am trying to achieve is a situation where changes to the hole size are the major player rather than external flow around the piston. This is just so that it's easier to predict what the effect of a given change will be. There is limited time to spend tunning at a track so it's best to make things easier even if it does forsake a bit of acuracy.

Close fitting pistons actually make things more difficult as the effects due to the boundry layers on each surface become major players and as I'm sure you are aware they are more or less unpredictable in this applications.

As for models and the like i think thisone is a bit beyond me, even if i had the time as there are some many unknows.

In a nutshell what i am trying to achieve is a certain, consistant, flow around the piston. That is best achieved, assuming that you will be using several pistons, by keeping the piston to body gap large.

The hole with then control the fine performance of the damper.

Finally this opens up heavier oil weights which in turn make the hole performance more sensitive to small changes.

So if it all comes together I intend to be in the situation where i can make small changes to a sensitive system whithout introducing large changes in pack.

Piston dia. seems to be in the 9.5 to 9.6mm range.

You have to remember that i am dealing with off-road shocks which are subjected to greater and more violent movements. Hi pack is more desirable on tarmac so i can understand why the Rayspeed pistons have gone in the oposite sirection.

Rog.

P.S. Build both on and off-road shocks with 2 o-rings each.