Flat Front Shock

[murmur]

This thread (forum/threads/silvio-air-shock-to-retain-pressure.5533/) almost touched on the problem I have with my S30's front shock: It goes flat. As in the boot looks completely compressed at least most of the time when I'm riding. BUT! When the ride's overa and I put the pump gauge on it, it has 180 psi in it (and it's always been extended back to the neutral position at that point, too.) So far I've always pumped it up to 185 before I ride.

So the only thing I can think of is that the fork is sticking in the compressed position. I weigh 190 lb, not enough to compress the shock flat, so it must be compressing when I hit a bump and then sticking there.

Does this sound familiar to anyone?

Dave

 

[Jeremy S]

Very familiar. Eventually I inflated to 200 psi and it stopped happening. I tried greasing under the shock boot and exercising the shock, but that never helped. I also never found signs of a leak.

 

[BrianA]

Can't say I've ever noticed any problem with the front shock on my S30 (which I pump up to 150psi). However I do note that the instructions for the front shock state that the pressure must not exceed 185psi

 

[marshall2389]

Does your pump gauge read 180 psi as soon as you screw it in far enough that it opens the valve to your shock bladder? Or do you give a few pumps and then it reads 180? I ask because it's possible the bladder is very low but every time you screw the pump the valve isn't opening. Without the valve opening a few pumps will pressurize the pump tube and cause the gauge to read high pressure without putting any air into the shock's bladder.

Another possibility: you have the bolt just above the head tube too tight. I'm talking about the bolt that compresses the clamp on your boom just above the head tube. If this is too tight then the boom can't pivot at that location. When your suspension goes up and down the boom pivots at this point.

 

[RAR]

Every individual gauge will have it's own reading. Unless you have had the gauge certified ,it's just an estimate. Plus it's a really small air chamber, one little psst could be a lot of PSI.

 

[marshall2389]

Oh yea. Procedure is important when using the provided pump. Make sure to pump up your shock and then unscrew the flexible tube without letting the black piece that's in contact with the pump rotate. This allows the pump's tooth to back out, the valve can close, and then you unscrew the black piece. If you unscrew the pump using the black piece only then seal created by the black pump head will break before the shock valve has closed and the air in your shock's bladder will drain.

 

[Jeremy S]

the instructions for the front shock state that the pressure must not exceed 185psi

The Kind Shock manual says this, but the Cruzbike assembly instructions say "adjust to 150psi, working range is 40 to 200psi".

Another possibility: you have the bolt just above the head tube too tight. I'm talking about the bolt that compresses the clamp on your boom just above the head tube. If this is too tight then the boom can't pivot at that location. When your suspension goes up and down the boom pivots at this point.

The carbon fiber chainstay flexes. I do not think any of the joints in the Silvio front triangle are designed to pivot.

 

[super slim]

Are you following these procedures carefully?, especially what Elliot has said about unscrewing the pump while the black valve part is still sealed to the valve stem, as per fig 3 of the instruction manual.

http://cruzbike.com/DOWNLOADS/KindShockPumpOperation.pdf

 

[brokemyback]

I'll send you a new fork, and cover shipping (if you're in US).
I'll come clean. I'm a dumba$$. When I got my new Silvio last January I didn't operate the pump correctly. I thought my shock was leaking. Robert PROMPTLY sent me a replacement fork. So I have the new fork removed from my new Silvio...ready to ship to you.

 

[marshall2389]

The carbon fiber chainstay flexes. I do not think any of the joints in the Silvio front triangle are designed to pivot.

It's true that the chainstay flexes, but it is also true that the boom pivots about a horizonal axis running left to right at the top of the head tube. You can see that the design allows for this rotation by telescoping the boom (as one would do when adjusting for BB-to-seatpan distance) and noticing the angle of the boom w.r.t. the ground changes.

 

[murmur]

wow thanks, brokemyback. I do want to make sure I have a real malfunction before I take you up on your generous offer. I'm going to look at some of the possibilities that people have raised here. I do not have the Kind pump that was supposed to come with the bike (I'm the second owner). But when I first screw the pump chuck onto the valve I see 180 psi, so the pressure can't be the problem unless the gauge (it's on a Planet Bike "shockmate" pump) is way off. I'll try going to 200 psi as Jeremy does, to see if that solves things. Surely the gauge couldn't read more than 50 psi high, right?

If it's reading 50 psi low, then I'll proably hear something go bang. Maybe I'll wear ear protection.

EDIT: I'll take a look at the boom-bolt that Elliot is talking about before I try anything else.

Dave

 

[marshall2389]

You can test to make sure the bolt is loose enough by:

• Remove front skewer and wheel
• Unhook the lowest portion of cable housing that leads from the fork to the rear derailleur.
• Pop chainstay off of fork.
• At this point you should be able to rotate the boom about the aforementioned bolt smoothly. This will lower your handlebars while raising the BB and vice versa.

 

[BrianA]

Jeremy S - You are right, the assembly instructions does say that the working pressure is 40 - 200 psi - which contradicts other information about how to use the shock pump provided by the fork's manufacturer which states maximum pressure is 185 psi. It doesn't help owners when such contradictions exist in information provided on the Cruzbike site. I will stick to the maximum pressure advised by the manufacturer of the fork as I think there is less chance of damage to the seals.

 

[Jeremy S]

You can see that the design allows for this rotation by telescoping the boom (as one would do when adjusting for BB-to-seatpan distance) and noticing the angle of the boom w.r.t. the ground changes.

The instructions say to loosen the bolts (and quick release) before adjusting like this. I do not think it's designed to pivot when tightened.

 

[marshall2389]

The instructions say to loosen the bolts (and quick release) before adjusting like this. I do not think it's designed to pivot when tightened.

It becomes clear with an S30 in front of you that compressing the suspension puts a torque on the boom clamp pivot on the top of the headtube (it's hard to explain/visualize this, much easier to see when you can play with it). When I first got my S30 I didn't realize that the boom must pivot here for best suspension performance. I tightened the bolt to the point that the boom did not pivot smoothly. As I rode and the suspension went up and down it did so with jolts and accompanying creaks. Sometime the suspension would compress and get stuck. It wouldn't extend back out. The bolt was so tight that the clamp grabbed onto the piece at the top of the headtube (the stem?). I looked closely at this juncture and experimented with different tensions in my living room. Adjusting the bolt is now easy. When tightening the bolt the resistances gradually increases until the clamp narrows and contacts the stem. At this point the torque required to further tighten the bolt shoots up dramatically. Tightening much beyond this point leads to large friction and non-smooth rotation of the boom clamp.

 

[mzweili]

I think that there is no need to make the boom pivot above the stem. The fork is the element in the front triangle that changes it's length when the suspension compresses. The boom remains rigid and the carbon fiber chainstay flexes to compensate for the retractation of the fork.

 

[marshall2389]

Thought experiment: let's say you've got an S30 in front of you. Now make the section of the fork from the bottom of the headtube down to the dropouts disappear (all of the fork below the headtube). Keep the BB and boom angle w.r.t. the frame fixed. Now apply a vertical force to the dropouts of the carbon fiber chainstay (carboyoke). The dropouts of the carboyoke will follow an arc as the carboyoke flexes. If that arc is not an arc but a line and that line is parallel with the axis of the headtube (the suspension) then when the dropouts rise, the shock compresses, and the BB can stay where it is w.r.t. the frame. I think it is more likely that the flexed carboyoke will cause the dropouts to trace a curve and space. Therefore, with the bottom half of the fork magically back in place, the BB has to move w.r.t. the frame.

 

[super slim]

wow thanks, brokemyback. I do want to make sure I have a real malfunction before I take you up on your generous offer. I'm going to look at some of the possibilities that people have raised here. I do not have the Kind pump that was supposed to come with the bike (I'm the second owner). But when I first screw the pump chuck onto the valve I see 180 psi, so the pressure can't be the problem unless the gauge (it's on a Planet Bike "shockmate" pump) is way off. I'll try going to 200 psi as Jeremy does, to see if that solves things. Surely the gauge couldn't read more than 50 psi high, right?

If it's reading 50 psi low, then I'll proably hear something go bang. Maybe I'll wear ear protection.

EDIT: I'll take a look at the boom-bolt that Elliot is talking about before I try anything else.

Dave

There is a piston in the front suspension NOT an airbag, so nothing will go bang if the pressure is too high.

 

[mzweili]

Elliot,
I don't say your are wrong, but I believe you are splitting hairs. It would be difficult to simulate that even with a good CAD software, because the carbon fiber chainstay can, to a certain extend, pivot on both ends and the flex of it is difficult to estimate.
On my Silvio 2.1 all bolts are tight and the suspension works fine without leaking for over two years now.
If a shock is leaking I would clean or change the valve.

 

[super slim]

Elliot,
I don't say your are wrong, but I believe you are splitting hairs. It would be difficult to simulate that even with a good CAD software, because the carbon fiber chainstay can, to a certain extend, pivot on both ends and the flex of it is difficult to estimate.
On my Silvio 2.1 all bolts are tight and the suspension works fine without leaking for over two years now.
If a shock is leaking I would clean or change the valve.

The carbon fibre + titanium front stay is VERY stiff sideways, but very flexible vertically with only 10 kg of force to bend 40 mm at the hub end, bending is a S shape. (See the photo below for the S shape of a 0.47 mm SS top spring for a rear Air Bag suspension.
This is an EXCELLENT design as there are no rotating pivots to wear out.