Prevent bottoming out Z2 BAMs

[hoojammyflip]

Just had a brilliant day of riding on the Monkey Trail in Cannock. Seriously underestimated 22km in 30 degree heat, and took a wrong turn adding 2k to the route.

Several times I managed to properly bottom out the Z2's. I think there was a section called the Devil's staircase, and I went totally through the travel and heard a metallic thud as the forks bottomed out.

What can I do to prevent bottoming out the fork travel, please? I've used Motul 7.5wt oil which looks to be right for the dampers. I've got the stock springs and not added any preload. I am about 165lbs at the moment, slightly tubby but within spec for the stock springs. Is this just something to expect with forks from the late 90's?

 

[novocaine]

Preload to get your sag right and 10wt. After that it's a spring change.

 

[hoojammyflip]

It strikes me that the bottoming out on a major bump may not due be to the spring, but instead is down to the compression damping being insufficient. I just wonder if you are doing a rough section on a hot day whether the oil can get too thin? I've never messed around with oil in the past, as I just had Indy SL's. Maybe swapping out the oil is the answer, but the Motul 7.5w that I have in there at the moment looks close to the original spec.

At 40 and 100, here are the respective ratings for the CST (viscosity):
"Marzocchi" from https://www.peterverdone.com/wiki/index.php?title=Suspension_Fluid: (26.1, 5.25)
Motul 7.5w: (24.6, 4.9)
Motul 10w: (36.3, 6.5)

 

[Dr-Zocchi]

... the compression damping being insufficient. ...

It is not about 'compression damping', this are 2 different tasks.

The compression force is taken by the spring. Some damping will be taken by the open oil bath, but there is no adjustable control like in later forks having both for compression and rebound. The Z2 has also that 20mm rebounds springs.
The early Z2 has only 1 rebound cartridge - thanks to @hoojammyflip remembering me in this detail.

I leave that pictures in the post, because I believe if one plays around with thicker oil you will stress the cartridges. You can easy try this yourself if you take a single cartridge in a glas with different oil types.

 

[hoojammyflip]

I thought the cartridge in the Z2 was just for rebound, and the compression is fixed and controlled by the square ports at the bottom of the stanchions, which force oil through them when the stanchions compress into the lowers. I'd be very surprised if that flimsy cartridge is used to control compression damping?!

Sorry, but there is a main error in this : 'compression damping' should never about cartridges taking the users force during riding (that happens if you use a wrong oil). Use correct springs for the compression force you need!

Damping is a parallel task in elastic/dynamic systems, to reduce swinging in spring systems only.

Means : you need to have a spring suitable for your+bikes+riding style weight and the the damping system will take care about swinging spring system.

I know it has been talked about a lot in changing oil to change damping, but that is a huge technical fail. Yes, the damping will change (in fact it will be eliminated) and yes, some force will go into the cartridge, but that's a bad idea, because it eliminates the correct spring/oil/cartridge system, means the tire will not stay on the ground and cartridge will be over powered. That's the reason such thinny aluminium cartridges will blow up.

 

[hoojammyflip]

I'm going to revise my guess that its bottoming out. Copilot suggests topping out on the rebound. Makes sense to me as the air in the top of the stanchions must ramp the spring rate up.

 

[Dr-Zocchi]

I thought the cartridge in the Z2 was just for rebound, and the compression is fixed and controlled by the square ports at the bottom of the stanchions, which force oil through them when the stanchions compress into the lowers. I'd be very surprised if that flimsy cartridge is used to control compression damping?!

Correct. Z1, Z2 and other forks from the 1997+ have only rebound cartridges. Z1 has 2, Z2 has only 1.
Marzocchi startet compression cartridges with 2000 - afaik. That where the ones with the black inner needle screw (center one).
I'll correct my post.

 

[hoojammyflip]

The rebound cartridge is screwed into the bottom of the fork, so when the pressure builds as the fork is compressed, it must be pressing against the sides of the rebound cartridge, so the cartridge walls must be pretty strong, as otherwise it would be possible to crush the rebound cartridge. How have folks managed to blow these cartridges up in your photos? I guess the wrong oil, like you say. I think the Motul 7.5w is really just about perfect.

Thinking about the air at the top of the legs, there is 4.5cm free space after filling up with oil, in the compressed position. Now when you stick the top cap in, I guess that there is maybe only 0.5cm left. In the extended position, there is 6.5cm of air, so that means the air goes from atmospheric pressure to something like 12 bar, or 180psi. So it seems pretty impossible to bottom the fork out. Instead, I think I just need to make sure the rebound damping is a bit higher, by adjusting the rebound damper! It also means that getting that oil height is pretty important.

In use, I think the reactive force when you compress is `F = -k.x -c.v`, where k is the spring constant, x is the spring compression, c is the damping factor and v is the speed at which the fork is being compressed. I imagine that whatever the oil, if the shock movement speed, v, is high enough, the damping component will dominate the reactive force. In contrast, when the motion of the fork is small, then the spring will take over.

Really, these forks are pretty incredible. Here is a photo from the weekend. Absolutely loving the Voodoo. I used to have a Kona CinderCone back in the day, so this is just like it. I need to do a build thread....I built the wheels myself, Maverick rims with offset spoke bed. 2mm spokes on the drive side. Apologies for the use of the blue seals, but I did use the Dr Zocchi tool for getting the old seals out.