How do i build so called "zero rebound" into tamiya CVA shocks?

[ThunderDragonCy 7881]

More on road things i don't yet understand! It was recommended to me to build my shocks on thr TA06 with "zero rebound" which as i understand it is when you compress the damper all the way with no spring and it does not extend at all. I tried doing this today with my 55mm black body cva dampers trying the piston in various positions and none worked. It seems to me that with a CVA bladder damper it's impossible to buold a zero rebound option because the diaphragm is pressurised by screwing the cap down. That is effrctively an air spring, and on these dampers with the rod fully compressed even a flat TRF diaphragm won't push doen onto the top of the body because the piston is in the way. 

Building them the traditional Tamiya manual way, fully extended means no air in them, but they will extend almost completely back out again when compressed with no spring. They are at least consistent built this way. 

Am I missing something? 

[Juls1 1661]

Build the shocks up with no air in the oil best you can, now push the piston to the top, but not quite out of the oil. Put the bladder down against the top of the piston, pushing out excess oil, the bladder should be prevented from sealing at this point because the piston and shaft is too high. 

Now slowly pull the shaft down while allowing the bladder to centre and seal. Pull the shaft to the bottom, now hold the shaft at the bottom it should be trying to recompress and screw the cap on tight. At this point when you let go of the shaft it should try to return into the body slightly. And you should have no rebound now from fully compressed. 

Juls

[ThunderDragonCy 7881]

@Juls1 Perfect description. Thank you. I will try that tonight. 

[markbt73 5316]

I have also seen people drill a very tiny hole in the cap near the mounting boss, to let the air out between the cap and diaphragm. Some other brands of shocks even come with a hole in the cap for that purpose.

[ThunderDragonCy 7881]

37 minutes ago, markbt73 said:

I have also seen people drill a very tiny hole in the cap near the mounting boss, to let the air out between the cap and diaphragm. Some other brands of shocks even come with a hole in the cap for that purpose.

I did wonder about drilling them as being a solution. Will try building them 'properly' first

[Collin 1956]

You can just play around with the piston position when you close the oil damper. Depending on the position you have more or less rebound. Keep the volume of the damper rod in your mind. If the rod is pulled down you can put more oil inside the damper. This gives you the rebound. Can you follow? Hard for me to explain in english.

[Juggular 4964]

Diaphragm is a spring of a sort.  Because of this, I usually build shocks with piston right at halfway point.  If the buggy hits the air, the diaphragms would want to pull in the wheels.  If the buggy lands hard, the diaphragms want to push.  

But it's really not the diaphragm.  Just as @Collin said, it's the volume of the oil.  

If the cap has a hole, the diaphragm moves more freely because you have eliminated the air spring.  But it still doesn't change the fact that oil (and the diaphragm) want to maintain the same volume.  (I wish on-road diaphragm was thinner to allow easier movement. But it has to withstand a lot of force, which is why it's kinda thick.)  Below shows what would happen if you build a shock with piston at the bottom.  

Even with the hole in the cap, the oil cannot take more volume than the cylinder allows.  At that point, it gets that "hydrolock."  Hydrolock can even bend piston rods of an internal combustion engine.  

On the other hand, if you build it from the dead top, the oil loses volume when the piston is down.  The diaphragm has to stay ballooned.  

So, what to do? 

I build them with the piston set---right at the center point of where the shock would move the most.  I build on-road shocks at bottom 1/3 or 1/4, just like the right-most picture above.  When it drives, the springs get compressed only about 1/3 to 1/2, most of the times.  So why not allow the piston to move Zero-Rebound right in that area?  (I measure how many mm it gets pushed in when it gets built, so I can build the shocks the same way)  What @Juls1 told you is the least rebound. The springs would push back the least because the diaphragm would pull in the most.  On the other hand, "normal" buggy shock building Tamiya shows you has the most rebound. 

Depending on where the piston was when you cap the shock, it can add or remove 1-2mm of ride height.  I just prefer that pistons be half way point when I cap it.  But you can set it anywhere.  Of course, the oil viscosity is just as important as minimizing hydro-rebound (for lack of better words).  

 

[DK308 219]

Put oil in them as normal. Put the diaphragm on top as normal. Hold the diaphragm, and slowly push up the piston so that oil leaks out. Before screwing the top cap on, fill the top "bowl" of the diaphragm and shock cap with high weight silicone oil(500k or more). Put the shock cap on.

This is how I get them as close as I can to zero rebound, but without sticking. Works well.

[ThunderDragonCy 7881]

Thanks for those explanations. I spent quite a lot of time with this and have come to the conclusion that it is almost impossible to build zero rebound cva shocks because of the diaphragm is an air spring (effectively). However, i did manage to build them with only a few mm of rebound after discovering a couple of things. Firstly, i put the spring collar back on restricting the fully compressed movement to what is actually is. This allowed me to push the diaphragm on without it hitting the piston even fully compressed. Secondly, wiping the oil as much as possible from the threads of the cap and body once its overflowed lets air escape as you thread the cap on. If you don't the oil seals the threads about halfway down, pressurizing the diaphragm and giving loads of rebound. Ths takes abit of rinse and repeat, but i got there in the end and got about 3mm of rebound out of all the shocks. 

I tried @juls method and i just couldn't stop getting loads of rebound. Don't know why. 

Anyway, i have much calmer shocks. Thanks all. 

[Juls1 1661]

Generally when I build that method the damper will compress itself slightly after assembly, I'm wondering what your definition of "rebound" is?

I assumed your talking about when you compress the shaft to bottom out it rebounds back out of the body.  

Personally I like to have a little bit of both with a diaphram shock, because when the car is settled on the shelf (meaning sagged to it's static ride heigh), it means there is no load on the diaphram so it won't try to suck in air through the seals, secondly when the shock reaches top out the slight compression force helps reduce harsh top out from the weight of the wheels and helps reduce bottom out/severe body roll by having a little more end stroke resistance. I generally build my emulsion shocks the same way. 

You can always just build them semi emulsion if you want no internal pressure. CVA's rarely stay air free for very long anyway, especially the buggy ones.

Most types of dampers in the real world have some form of internal pressurization and usually "rebound" to full length without the spring. (can't really do that with RC Type shocks though)

Those that don't have a much more complicated displacement relief system than a rubber diaphram or just air in the oil. 

Juls

 

 

[TwistedxSlayer 1290]

I missed the part of.... why do you want or need zero rebound shocks?