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GP Suspension 25 mm Cartridge Kit Overview

She’s got new legs!

Figured you guys would appreciate some photos, so I took a few before cleaning and installing the parts. Here is the top cap and spring retainer. Note that the damping rod lock nut can be accessed through the spring retainer (no spring compressor needed to swap springs)!

Nicely machined aluminum spring spacers (with press fit flanges). Here is a shot of the internals (rebound leg on top, compression on bottom). Note the long top-out springs and extra bleed ports (the holes in the holders leading to/from the needles):

Close up shot of the compression piston:

And a close up of the rebound piston:

Check valve:

The rebound shim stack:

A view of the brass 7.5 degree needle on the compression leg:

A close up of the spring buffer (large OD black spacer on the left) and the bump stop (rubber sandwiched between the buffer and the bearing cap in this view):

A side by side shot of the bump stops:

One of the things to note here is the absence of a hydraulic lock out. On the stock cartridge, that rubber bump stop and the machined aluminum rod above it slide down into the top of the bearing cap. Naturally, the cavity is filled with oil. So, as the fork reaches the bottom of its stroke, the oil is displaced by the bump stop and aluminum rod. Given the small gap between the OD of the bump stop / lock out and the ID of the bearing cap, the damping force ramps up very quickly. Under high speed compression, this lock out feels like bottoming out. It’s possible to bottom the cartridge (bump stop to bearing cap), but you’d either need a long, slow input or a very fast/high force input. The GP 25 mm kit maintains a bump stop (to prevent metal/metal contact), but effectively does away with the hydraulic lock out. The intent here is to increase feel/compliance at the bottom of the fork travel. Think very heavy braking…

To put the fork travel in perspective, here is approximately how much slider you’ll see when the bump stops are contacting the cartridge bearing cap:

So the good news is that the kit installs easily enough and shouldn’t give you guys any trouble. The only question is on the performance. No one’s ridden this configuration yet, and it doesn’t look like that will change any time soon out here. Winter appears to have finally made itself at home. I hope it moves out. But fear not, we’re already working on getting some real-world results ASAP. You’ll hear details here first…

2 thoughts on “GP Suspension 25 mm Cartridge Kit Overview

  1. I was wondering if you thought you could make my forks work properly or would I be better off with cartridges. I only weight 125 without gear and use this for a track bike. The suspension is aweful. I am purchasing a rear shock soon but can not make my forks work well at all. Please advise.
    Thanks Shauna goodman.

    1. Absolutely! Give us a call.

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