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. ...

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 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.