I’m servicing my rockshox reba motion control at the moment and I wonder if someone could explain how the damping circuit works..?

a very good link in a thread the other day:

http://www.singletrackworld.com/forum/topic/motion-control-nicely-explained-with-pics-link

Great – thanks!

Ok, I’ve read that, and sorry for being thick, but I don’t quite get what’s going on with the oil in the damper?

If anyone else isn’t down the pub, could they explain it over a virtual pint (my round)?!

its moving through the hole in the bottom plate. Oil is more viscous than air so it moves through the hole relatively slowly (or at different speeds depending on the setting – i.e. diameter of the hole)

The damper is otherwise sealed so the whole thing can only moves as fast the oil moves through the hole.

I took the MoCo out of my Rebas, and have done it with my SIDs. I can’t tell any difference, luddite that I am. I never used the lockout, and fail to see any other performance difference. What am I actually missing!?

depending on your weight, how you ride and the trails you ride, possibly nothing. But also, possibly quite a bit of control/grip that you could have. It wont kill you though.

It wont kill you though.

That kind of depends on when you discover you don’t have

quite a bit of control/grip that you could have

doesn’t it

But what does it actually do, why would it provide more grip? Does it just stop it blowing through it’s travel? Tim Flooks berated me for doing it on my Rebas, but I never really got a reason why it was so bad. I can understand if I used the lockout, and used it as a ‘platform’, with the Flood Gate. But in an unlocked fork, what does it do?

I’ve never noticed any detrimental performance without the MoCo damper, fork still works fine, just weighs 100g less!

On the compression stroke, does the oil go through the ports on the piston AND the port in the mo-co.

Otherwise I can’t see how the piston would move at all.

Is there any air in the equation?

njee

have you got any damping at all now, or just a 100mm air sprung fork without any damping?

Got rebound, no compression damping. Like I say, no one’s ever really managed to tell me what it should do! How would it give me more grip?

The fork doesn’t appear to blow through it’s travel any more than before, it’s only 100mm after all! Suspect it’d be much more noticeable on a longer travel fork.

I thought the ports on the piston did the rebound damping and the port on the mo-co did the compression damping.

Seems to be a separate rebound damper… Twiddling the lever still has a marked effect on rebound speed anyway!

I always leave my Revelations fully open on the compression damping. I find if I run any on it, then they feel like they spike too much. So you may have a point on a 100mm Reba. Personally I much prefer a very active fork, rather than an overdamped one, and I never use lockout either.

But I’m not especially heavy (12 stone), maybe I should be running slightly lighter oil than standard to get the desired effect in my forks perhaps?

I’m 10.5 stone, with 50% less travel!

mboy – Member
I always leave my Revelations fully open on the compression damping. I find if I run any on it, then they feel like they spike too much. So you may have a point on a 100mm Reba. Personally I much prefer a very active fork, rather than an overdamped one, and I never use lockout either.

But I’m not especially heavy (12 stone), maybe I should be running slightly lighter oil than standard to get the desired effect in my forks perhaps?

Open the floodgate a bit.

The description Stoner has posted misses out a key point, without tuning the floodgate to a fairly light setting, you are essentially running a port orifice fork (hence the spiking).
I have my LSC (blue dial) set to about 3/4 closed, and floodgate about 4 clicks from open. This gives minimal dive, but fully active on bumps

wheeliejim – Member
On the compression stroke, does the oil go through the ports on the piston AND the port in the mo-co.

Otherwise I can’t see how the piston would move at all.

Is there any air in the equation?

Yes, the bottom part (rebound damper) has a check valve which allows oil to flow freely on compression, but though the limited size rebound port on rebound.
When the rebound pushrod enters the stanchion, it displaces some oil, this is what the swiss-cheese part acts on.

When the rebound pushrod enters the stanchion, it displaces some oil, this is what the swiss-cheese part acts on.

Eurika!! Now I see!! Thank you!
So it follows the volume of oil transferred through the mo-co port = the volume of the piston rod.

So to allow this to happen, I guess the air sitting in the top of the mo-co either compresses or gets pushed out and sucked back in again.

I read somewhere that apparently a lot of motion control units “leak air” (and oil)

To me, it sounds like mine is farting air out the top (for want of a better way of putting it 🙂 )

Is this supposed to be happening, or should that little bit of air be contained/compressed?

wheeliejim – Member
Eurika!! Now I see!! Thank you!
So it follows the volume of oil transferred through the mo-co port = the volume of the piston rod.

So to allow this to happen, I guess the air sitting in the top of the mo-co either compresses or gets pushed out and sucked back in again.

I read somewhere that apparently a lot of motion control units “leak air” (and oil)

To me, it sounds like mine is farting air out the top (for want of a better way of putting it )

Is this supposed to be happening, or should that little bit of air be contained/compressed?

🙂

I’m not sure about the air to be honest. I did wonder if it acts a bit like an IFP on a rear shock, helping to prevent cavitation in the oil.
I do know that you’re not supposed to change the oil height as you could in a Marzocchi fork, maybe too much internal pressure would bust the little skirt seal at the top or the o-ring at the seal head ❓