Jump to content
alvinlwh

Shocks 101 questions

Recommended Posts

Well, not that many questions, just a few.

For the past 2 years, I had been building CVAs as per the instructions; piston at the bottom, top off oil, seal off. This will result in when the shaft is pushed in, it will get pushed out from the pressure. However, when watching some videos, I see that some build shocks with the piston pushed in completely, or half way. This resulted in the shaft being sucked in when pulled out, or both in the case of half way. Which is the correct way?

Another question, is a vacuum pump necessary? I noticed that when building some CVAs with yellow (#400) oil, even after leaving it to stand for hours, I still get air bubbles when I "pump" it. I know it takes longer for bubbles to clear for thicker oils, but yellow is considered soft so should clear rather quickly right?

Share this post


Link to post
Share on other sites

It is... maybe ok? the air is compressible and there for that purpose.  It is also certainly expandable to effectively the same degree.  I would say the red/green seal isn't made to stretch to accommodate the lower pressure caused by building the shocks with the piston all the way at the top.  Maybe some other shocks are? The CVA seal is barely held in place - compression wouldn't hurt this, but expansion might pull it loose.  I have built some with the piston at ~1/3rd compression, since that is close to where it sits statically, but don't honestly have enough experience or driving expertise to imply it made a difference.

I just got a pump a couple weeks ago (Plaza Japan ftw), so my only experience is from the ~800 Cst I put in a CVA recently.  I was surprised at how many microscopic bubbles expanded into something when creating a vacuum.  I also think I had a couples extra bubbles because I didn't load up the seal area with a lot of oil; just some to lubricate things.  I suspect some of my air bubbles came from there.  Anyway, no I don't think they are necessary.  I'll use it, mostly because I can. :) The last few bubbles you see in a vacuum are tiny when pressure returns, so I can't see them causing any significant aeration.

The plastics are porous enough to accept a dye; I wonder how much air is in the cracks. Obviously not a whole lot, and it might be captured enough not to come out under mild vacuum.

Share this post


Link to post
Share on other sites
5 hours ago, SlideWRX said:

It is... maybe ok? the air is compressible and there for that purpose.

I don't think air bubbles are supposed to be in there as it is impossible to control the amount? It leads to inconsistencies. I was actually building a pair of shocks and noticed that one is harder than the other leading me to investigate this and found lots of bubbles even after leaving for hours. When I said lots, I meant so much that the oil actually looks cloudy. And I did pump the piston when filling and before setting it down!

For CVAs, the compression, or rather displacement of the piston and shaft is taken care of by the diaphragm? Or at least that is what I had read. I had built shocks without the diaphragm before and have a fixed amount of oil to put in so that there is also a fixed amount of air at the top to allow for the displacement of the piston and shaft.

Share this post


Link to post
Share on other sites

If you're getting bubbles after leaving the shocks to settle then it could be that the lower seal isn't good and the pumping is sucking air in via the bottom of the shock.  Strip it down, oil the o-rings well and rebuild.

I find it sometimes takes a couple of rebuilds of the odd shock to get it just right and match the others.  I fill them with oil with the shaft all the way out, pump a few times to clear the bubbles as best as possible then leave until the oil is completely clear.  Usually around 15-30 minutes.  Nothing fancy, and as per the instructions, and it seems to work for me.

  • Like 1

Share this post


Link to post
Share on other sites
37 minutes ago, Twinfan said:

the lower seal isn't good and the pumping is sucking air in via the bottom of the shock

I did wonder about this. Ok will try again.

  • Like 2

Share this post


Link to post
Share on other sites
  1. Filling the cylinder with the piston at the bottom will create positive pressure upon compression, i.e. the shock will resist compression. Positive pressure promotes oil leaking outwards.
  2. Filling with the piston on top will create negative pressure upon extension, thus resisting extension. Negative pressure promotes air leaking inwards.

The first option seems more beneficial for the shock's intended function. Given the pressures involved (very small and not very contrasting), I would think that the effect of air compressibility is small. Perhaps the biggest effect of having a little residual air in the shock is getting that air mixed into the oil, creating an emulsion that changes its bulk viscosity. I "quick & dirty" tested this once where I had two CVAs with 400 oil, one with a few small bubbles and the other without and, aside from the squirting sound of the one with bubbles upon compression (which disappeared after it mixed), both felt indistinguishable when actuated (with or without spring). My gut feel is that going over-the-top to get bubble-free oil is unnecessary.

I also wonder if when pushing the threaded portion of the rod into the o-ring seals, the sharp threads can damage the o-rings. I have not investigated this but generally speaking, sharp edges and orings is not a good combo. What I do now preemptively, is put a bit of clear tape above the rod's threads before inserting them into the o-ring as it's easy to do. Maybe it's a placebo effect but I feel my CVAs leak less since I've been doing this.

Share this post


Link to post
Share on other sites
4 hours ago, alvinlwh said:

I don't think air bubbles are supposed to be in there as it is impossible to control the amount? It leads to inconsistencies. I was actually building a pair of shocks and noticed that one is harder than the other leading me to investigate this and found lots of bubbles even after leaving for hours. When I said lots, I meant so much that the oil actually looks cloudy. And I did pump the piston when filling and before setting it down!

For CVAs, the compression, or rather displacement of the piston and shaft is taken care of by the diaphragm? Or at least that is what I had read. I had built shocks without the diaphragm before and have a fixed amount of oil to put in so that there is also a fixed amount of air at the top to allow for the displacement of the piston and shaft.

I mean for the shock to operate there has to be some air in there to compress to take up the volume of the shock shaft.  Aeration shocks leave the air mixed in with the oil - your shocks built without the diaphragm - nothing separates the two and stroking the shock will cause the air that mostly settled on top to mix with the oil, possibly turning cloudy while running, and clearing up when allowed to settle.  CVAs have a red/green barrier/diaphragm to separate the two. The diaphragm itself isn't there for springy-ness, other than to return to its original shape when it can. It just keeps the oil & air separate.  There is some springy-ness to it, but I think if they made it thinner to be less springy then durability would be a concern, so there are trade-offs. I think some on there have drilled a hole in the cap of their CVAs to allow the air to flow in/out as the shock strokes, and that should eliminate most of the shaft pushing out. Seems like a good experiment to try.  With the right diameter o-ring to seal the cap instead of the diaphragm, or a big hole in the diaphragm you could run CVAs as Aeration dampers.

If the shock is cloudy after being pumped & assembled, something is up. leaky or damaged o-rings possibly, as was mentioned.

Share this post


Link to post
Share on other sites
2 hours ago, OoALEJOoO said:

I also wonder if when pushing the threaded portion of the rod into the o-ring seals, the sharp threads can damage the o-rings. I have not investigated this but generally speaking, sharp edges and orings is not a good combo. What I do now preemptively, is put a bit of clear tape above the rod's threads before inserting them into the o-ring as it's easy to do. Maybe it's a placebo effect but I feel my CVAs leak less since I've been doing this.

You're right. :) 

Share this post


Link to post
Share on other sites
19 minutes ago, SlideWRX said:

Seems like a good experiment to try.  With the right diameter o-ring to seal the cap instead of the diaphragm, or a big hole in the diaphragm you could run CVAs as Aeration dampers.

Instead of messing around with my CVAs, I just ordered some aeration dampers to experiment with. 😁

  • Like 2

Share this post


Link to post
Share on other sites
16 hours ago, alvinlwh said:

Well, not that many questions, just a few.

For the past 2 years, I had been building CVAs as per the instructions; piston at the bottom, top off oil, seal off. This will result in when the shaft is pushed in, it will get pushed out from the pressure. However, when watching some videos, I see that some build shocks with the piston pushed in completely, or half way. This resulted in the shaft being sucked in when pulled out, or both in the case of half way. Which is the correct way?

Another question, is a vacuum pump necessary? I noticed that when building some CVAs with yellow (#400) oil, even after leaving it to stand for hours, I still get air bubbles when I "pump" it. I know it takes longer for bubbles to clear for thicker oils, but yellow is considered soft so should clear rather quickly right?

CVA's have no air in them when build properly; there is however air trapped (on purpose) between the bladder and the top cap. This volume of air is a compensation chamber which as the name indicates compensates for the tiny volume of the shock shaft as it enters and exits the shock body. Without it, no movement would be possible, pressure would build up as you compress the shock and it would leak. So the movement you see when the shaft "springs back" is perfectly normal and is the result of the air inside the compensation chamber compressing. That the rebound is caused by trapped air bubbles is a common misconception which is also maintained by our friends The Youtubers Of Doom :D

The position of the shaft at the moment you close the damper is a tuning option. If you build them with the shaft fully extended you will get plenty of rebound. If you want even more rebound, you can actually add a pink urethan busing (53577) inside the compensation chamber! Conversely if you build them with the shaft all the way in you will not get any rebound or even negative rebound, meaning the shaft wants to get sucked in and only the spring is keeping the shock extended. The track conditions will dictate how to best build them but as often, if you are simply bashing (nothing negative about it, far from it) you would not see any difference at all. If you are keen on experimenting, lower traction would call for higher rebound.

The vacuum pump is not necessary but saves a lot of time. Without it you have to wait 20min at least for the micro bubbles to very slowly rise to the surface before closing the shocks. I cycle the shaft, see more bubbles, wait some more and so on. Better to do it in parallel with another maintenance task! And it gets worse with higher the oil viscosity. With the vacuum pump I add the oil, vacuum the shocks and the bubbles become large and rise quicker. I still have to wait at least 5min. Sometimes I need another cycle. Now before you spend 40$ on a Tamiya vacuum pump, you might have seen some of my posts where I showed how I made a vacuum pump for free with an empty jam jar, a Unibit and a wine vacuum pump we already had in the kitchen (not sure why, we are not exactly wine drinkers :D).

Hope this helps.

  • Thanks 2

Share this post


Link to post
Share on other sites
26 minutes ago, Pylon80 said:

Now before you spend 40$ on a Tamiya vacuum pump, you might have seen some of my posts where I showed how I made a vacuum pump for free with an empty jam jar, a Unibit and a wine vacuum pump 

Thanks, very detailed explanation. For a while I thought I had been building shocks wrongly.

Yes, it is your post that made me purchased a vacuum food box, but it turned out to be broken on transit!

  • Like 1

Share this post


Link to post
Share on other sites
7 hours ago, OoALEJOoO said:

threaded portion of the rod into the o-ring seals, the sharp threads can damage the o-rings

Funny you mentioned this. I am rebuilding some shocks right now, cannot even remember their brand, and noticed that they have a much better setup than Tamiya's.

VrafMEJ.jpg

The treaded portion is much smaller than the shaft portion, meaning there is zero interference with the o rings. A much better design I must say.

  • Like 1

Share this post


Link to post
Share on other sites

Rebound, is a good tuning tool for shocks, and it’s important to set/match both sides of a pair.

Set the pair of pistons to the same length, body to eyelet, and then you can use a combination of spring cups to set various rebound rates by setting how far the piston is into the body when you build. 
 

I like more rebound on the front than the rear, allowed a little extra tuning for understeer iirc.

Adam Drake’s video is very good on this subject

This vid is the reason I ran the Aeration buggy shocks with bladders and foam inserts and the option for vented caps.

Share this post


Link to post
Share on other sites
1 hour ago, matisse said:

This vid is the reason I ran the Aeration buggy shocks with bladders and foam inserts and the option for vented caps.

I just ordered some aeration shocks and also internal sprung shocks. Get ready for questions 102...😁

  • Like 1

Share this post


Link to post
Share on other sites

The main issue most folks encounter that I've seen were either they don't remove the bubbles entirely or they overfill the damper housing where the shaft pushes out so fast after compression that they appear they are sprung.  It should push-out a little bit (from the diaphragm expanding as @Pylon80 mentioned above) but it shouldn't push out all the way or push out quickly. 

When I fill the dampers I fill to like 1.5- 2 mm below the top and drop the diaphragm in.  If the diaphragm touches the oil enough that it floats just a tad, then that is plenty of oil in there.  The instructions where it shows to wipe off excess oil pouring out from the side of the threads is using too much oil.

As for the damper vacuum pump, it's good to have, but not necessary.  

  • Like 2

Share this post


Link to post
Share on other sites

After building a few CVA’s I’ve found that most of the air gets trapped under the piston when it’s at the bottom of the shock body. So  now I half fill with oil and slowly just pump the piston up and down a bit, without exposing the piston out the oil, until I can’t see anymore bubbles (twisting the shaft while doing so also helps) The bubbles don’t have so far to come out then.

I then fill almost to the top, drop the seal on and let it settle (as mentioned by @Willy iine) and screw the top on slowly with the shaft 1/3rd the way in.

I haven’t tried it myself but I believe some people drill a 1mm hole in the top cap to rid any unwanted rebound. I have a feeling it’s called for in the new BBX rear shock instructions? Just seem to remember that on a video I saw. 

  • Like 2

Share this post


Link to post
Share on other sites
1 hour ago, Re-Bugged said:

So  now I half fill with oil and slowly just pump the piston up and down a bit, without exposing the piston out the oil, until I can’t see anymore bubbles (twisting the shaft while doing so also helps) The bubbles don’t have so far to come out then.

I did exactly that but after letting them sit for hours for any bubbles to rise up, I got back to pumping them, more bubbles appears. As someone pointed out, it could be air being drawn in from the seals, I will get around to rebuilding them with more Green Slime.

  • Like 1

Share this post


Link to post
Share on other sites
On 10/2/2023 at 1:56 PM, alvinlwh said:

Funny you mentioned this. I am rebuilding some shocks right now, cannot even remember their brand, and noticed that they have a much better setup than Tamiya's.

VrafMEJ.jpg

The treaded portion is much smaller than the shaft portion, meaning there is zero interference with the o rings. A much better design I must say.

I've found CVAs to be just "good enough", ideal for bashing, noticeably better with TRF pistons, rings, and TiN rods. But the aftermarket is usually better if you're picky about handling.

A little tip if you ever need more shock lube, either head to motorcycle or car shop and pick up some shock seal grease. It's the same thing as "Green Stuff", same price, but you get much more. "Green Stuff" is more or less just Noleen SF3 in little canisters.

  • Like 2

Share this post


Link to post
Share on other sites

Since we are sharing shock tips, I wanted to show what my shock oil looks like after two runs of 30 min of postal driving. It's supposed to be yellow. We have had a lot of wind lately and the basketball court I'm using is covered with thin desert dust.

Screenshot_20231004-120211.thumb.png.e523aff73d497f8976ad344071db4a57.png

I was wondering how often people are rebuilding their shocks? I have ruined shafts in the past by running the car with dirty o-rings eating through the metal.

Share this post


Link to post
Share on other sites
13 hours ago, Pylon80 said:

I was wondering how often people are rebuilding their shocks? I have ruined shafts in the past by running the car with dirty o-rings eating through the metal.

I kind of asked this question before and the answer is really "it depends".

Share this post


Link to post
Share on other sites
7 hours ago, alvinlwh said:

I kind of asked this question before and the answer is really "it depends".

 

I would never reuse oil since it's full of gritty sand particles after only an hour. But it's true that most of the damage to the damper shaft is done by the dirt that accumulate on the o-rings - actually, on the bottom o-ring. I usually open, clean and change the oil every 5h (this week I was experimenting with internal spacers on that particular car). I was wondering if people tend to rebuild shocks all the time or as a once a year thing 🧐

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now

×
×
  • Create New...