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CVA shocks. Is this correct?

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I have built the CVA shocks as the manual shows. They feel smooth BUT without springs fitted,when I push the rod fully in it will not stay in.They all return to fully extended.Is this correct,i know real ones don't do this.

I have taken apart because some oil was seeping out of the top cap and reassembled with less oil and the same happens!

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That is because of "hydro-lock" of a sort.  The red portion in below diagram is the cause.  If you build the shock with piston all the way down (like below diagram) when you push the rod in, the red part comes into the cylinder.  The air pocket wants to push the rod back out, and return to the original state.  

So... I usually build it with the piston pushed in about 1/3. (make sure all are pushed in equal length)  

This way, "1/3 pushed in" would be the natural state.  If you push the rod in, it wants to go back down to 1/3 position.  You pull the rod out, it wants to go in slightly (but the spring pushes it out anyway).  You can do 1/4 way in, or you can do 1/2.    

If your spring is weak, build it like Tamiya instruction tells you to.  It will be stiffer.  If your spring is stronger than you'd like, build it with piston in half the way, etc.  You can use this characteristic to add or subtract small amount to the spring rate.  

oM26QNk.jpg

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Thanks for the info, its really interesting to know. So even though this "locking" is not really correct it can be made to work to advantage. being a newb I have no idea about the spring power so will run as manual with the "lock"

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I don't like the "air pocket got squished" effect :) so I nix it by venting the cap.

I prefer my shocks to use their innards for damping not "air springing" :P as there's perfectly good springs outside to do that job. Putting the oil under pressure makes it more likely to seep thru the seals... after a while your oil volume inside reduces and shock action changes.

You should play with some clear body CVAs. I find them fascinating...

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Some people drill a small hole in the shock cap which vents pressure from the air pocket inside, therefore the diaphragm acts like a very soft spring until it is deformed hard against the shock cap or stretched to it's limit into the cylinder. Seems to be more popular to do this amongst off road RC racing. I don't recommend this, it puts undue stress on the diaphragm and will add 'slop' to the shock action as the diaphragm has to deform all the way from inverted to non-inverted before you actually start pumping oil past the piston and getting a damping action. Effectively your suspension will have a significant amount of undamped travel.

If you pierce the diaphragm, oil will leak past it into the cap area and air will be sucked into the cylinder. Effectively you now have an aeration shock. How it performs depends on how much air and oil volume is contained in the shock when it is built. If your cap does not have a secondary seal, the shock will leak if you do this. Not recommended. If you want an aeration shock fill the diaphragm/cap area with blutack and leave some air in the cylinder. Same effect but won't leak.

If you want the most responsive damping action you should build it like tamiya instructs. This makes the air spring as stiff as possible so more oil is pumped through the piston for a small displacement of the shaft as the air spring won't be compressing as much. This results in more immediate damping. The side effect is just that you get an air spring which acts in addition to the coil spring. This doesn't need to be an issue, just install a softer coil spring than you would otherwise.  

If oil leaks from the shock (which is inevitable), it will revert to having less preload - as if it were built with shaft pushed in. So if you want it to remain like how Tamiya instructs, you need to periodically open the shock and top up the oil.

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Yep pierce the cap to vent, not the diaphragm :wacko:

 

I drill a tiny hole (0.5-1.0mm bit) in centre of cap into the ballcup. Ball does fine job of keeping dirt out, whilst staying not airtight.

 

Some people like airlock/rebound as a tuning tool, to each his own. 

For the longer throw CVAs (vs supermini) there's a fair noticeable difference. I just like it vented more because oil seeps out less leading to dirt buildup on shaft which wears out the seals. 

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btw you can tune them further by inserting those red sponge pellets into the diaphragm's cavity... :) 

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Here are the pictures of what @WillyChang talked about.  

Back in the days, Tamiya yellow CVAs had holes.  Just one hole per cap.  But newer black CVAs don't have holes.  I suspect that Tamiya stopped doing this because of the reasons @nbTMM mentioned.  It becomes a strange 2-stage shock; easy moving until diaphragm collapses up, and then hard moving after that.  Also, all the stress of the shock absorbing goes to the diaphragm.  

fSXD6Vm.jpg

 

If you are observant, you see the same kind of weird 2-stage thing with many other shocks, if there is a gap between E-clips and the piston.  Easily move up about 0.5mm until E clip hits the piston, and then move slowly.  It does the same when coming down.  Because of that, I sometimes put a washer to remove that gap.  

But it blocked the holes halfway, making the 4-hole piston move like a 2-hole piston.  

AlMxxyf.jpg

So I drilled more holes.  

eb48pkN.jpg

This way, it works like 4 holes because the washer blocks half. (Things I do to make stuff work...)

 

The sponge pellets WillyChang mentioned is to relieve the stress on diaphragm.  But I found that those red pellets were too hard.  If the diaphragm does not move, the piston can't move either.  At least that's how it was in Yeah Racing shocks (they also don't have breather holes).  I had to take out the urethane pellets.  

Back in 80's, there were shocks which used tiny springs above the diaphragms.  But they were finicky.  

Because the car should sit with shocks compressed about 1/4 of the way, I build my shocks with pistons 1/3 up.  It moves up easily, it is less likely to make the car jump up for no reason (which is the whole reason why shocks are there in the first place).  I like responsive shocks like real Baja trucks.  I often find that Tamiya instructs us to build them too stiff. 

Like everybody else said, though, try everything.  See what you like.  

lfAOHZy.jpg

 

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Before I asked for help I took  apart a shock and took some oil out thinking it might help,i didn't. After reading the replies I decided to try again with the piston partially retracted. What I did notice was a couple of the shocks diaphrams were misshapen/dented. sounds like what as been said about using the foam insert.

Any how I redid the shocks with piston partially down and I think they feel better,the piston takes longer to return now,thanks for help.

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