Boomerang Rear Shocks Stiff

[TrueBlue 45]

So as the title says, kind of, my Boomerang has very stiff rear shocks. I've made countless checks to make sure they are build correctly... which they are. Trimmed many tiny pieces of excess plastic off to make sure they fit sung... which they do.

If you press down on the Boomerang, the shocks seem to compress just fine, but they're so stiff that they can't even lift the car back up. They barely move a millimetre after you let go. I've spent several hours on these shocks so far, but the results are always the same. They are just so stiff. The front shock however, is a completely story. They're just as they're supposed to be, and can easily lift the car back up again when compressed. All three shocks are built 100% identically, with the same parts and the same amount of Soft damper oil.

I'm just so confused. I'm guessing this is clearly not normal and not how they're supposed to act. I also have no idea what could be wrong with them. Off the tops of your heads... does ANYONE have any thoughts or ideas about this??

[nowinaminute 2815]

Have you got preload collars on the rear shocks? Did you lubricate the o rings? Did you use the piston discs it says to in the manual?

Is it a re-release or vintage?

It's fine to not have complete rebound, preferable even. But they shouldn't fail to recover at all.

[TrueBlue 45]

Hi, thanks for replying!

To answer your questions: Yes, yes and also yes. And it's a re-release model.

Yeah, no recovery at all in the shocks. It's like the shocks are stuck or something, but it's just very tight.

[Saito2 6686]

You might have to pull them apart and start from square one. With the shock in pieces, take just the assembled piston and insert it into the shock body with no O-rings, O-ring spacers or even the lower shock body caps. Run it up and down in the shock body and make sure its free. Tamiya CVAs are not terribly tight or precise, so the piston should go up and down with ease. If that's good, try sliding on just the o-rings and spacers. The spacers should have no drag and the o-rings should not be tight when they aren't compressed into the lower shock body yet. Check the lower shock cap separately as well for drag.

On occasion, of the dozens of CVA shocks I've come across over the years, I've seen the hole in the lower shock cap molded off-center. Usually it isn't problem. My Fire Dragon rerelease had this issue. You should be able to see if its way off by eye. To check it, assemble the shock without oil or O-rings and spacers. The piston should move completely freely, falling up and down as you invert the partially assembled shock in your hands. 

Something could be out of tolerance and hopefully doing the above will isolate it. BTW, if you fill the shock with oil and screw the top cap on with the piston fully up in the shock body, the piston will have a tendency to "suck" back in when its extended. This reaction should not be enough to overcome the springs though. Cross contamination with differing oils could cause O-ring swell, but that's not the issue here. O-rings grossly out-of-spec could "crush" too much when the lower cap is screwed on, gripping the shock shaft too much. If the buggy is upside down on the bench and you compress the rear shocks by hand individually, do the still stick in the compressed state?

[Wooders28 4723]

I'm sure you have, but have to tried the shocks out of the car?

I've had wishbones stiffen up once all the screws are tight.

[TrueBlue 45]

Saito2 - Dang. Oh well, maybe fifth time's the charm. I'll test all the O-rings and spacers etc once I get around to taking them apart and trying again. I'm pretty sure that nothing is off or bent or anything like that. I've tried to check them all and everything looks straight. What's still odd is how the front shock is working perfectly yet the two rears, built the same way, are having problems.

Wooders28 - I have indeed tried the shocks out of the car. All joints and screw points are nice and loose and move freely on their own.

[markbt73 5316]

1 hour ago, Saito2 said:

On occasion, of the dozens of CVA shocks I've come across over the years, I've seen the hole in the lower shock cap molded off-center.

I've seen this too, even back in the "good old days." Sometimes the hole is a little too small as well. It's weird; sometimes they'll work fine with the lower cap almost tight, but as soon as you snug it up, it binds on the shaft. The fix is usually to just ream out the hole in the bottom cap a tiny bit (and I do mean a tiny bit) to free up the movement.

One more little note: a small amount of "stiction" is normal for these shocks, even when they're perfect. They'll feel like they're stuck in place after the car has been sitting for a while, but as soon as you work them up and down a couple of times, they'll free up.

[TrueBlue 45]

So... using the shocks WITHOUT the damper oil actually makes them work properly. They're obviously a little softer without the oil but they do spring back up almost the entire length of the damper. So the oil is seemingly preventing them from lifting the car back up again, even though I know that shouldn't be the case. Should I run the car without oil in the shocks, or might it cause damage from the car bouncing too much?

[Wooders28 4723]

It might be worth a try,

Push the shock up half way, and then fill with oil, bleed and fit the cup etc (all with the shock half way)

If the right amount of oil isn't expelled when built (shock too full) it can hydrolock once the shock shaft starts to take up room inside the shock.

The rubber cup is designed to deform and stop that though,but if the shocks too full.

[TrueBlue 45]

Well, I seem to have found the issue, or at least part of it. My father, who built the Boomerang (it's his car) used the wrong discs in the shocks, the parts with the wee holes where the oil comes through. He attached the one with the single, tiny hole on it rather than the one with the two larger holes.

While the shocks are better than they were.. they now spring up... they don't spring up a lot. But I think this was the problem all along. I didn't even notice it until a few days ago.

Also, new topic about my new Hot Shot (Re-re):