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Xtrainer Suspension - BPS-K9 installation and review

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I finally got around to installing the K9 kit.  Here are some pictures, my install notes and my feedback.  I'm not a great rider or a suspension guru by any means, I just wanted to share my experiences with the group. 

What's included:  The cartridge, stickers, instructions, fork oil. 

Installation Process: (I didn't replace the spring so I just did the cartridge side):  Remove left fork, remove fork cap, drain fluid, remove bottom cartridge bolt / cartridge (drain more fluid), replace with upgraded cartridge, torque to spec, fill with 550ml oil, done.  It is a very easy job, if you can change a tire you can do this job.  While I was there I pulled the shim stacks apart to see what the updates were.  

Initial Review:  The Cartridge is exactly the same as stock except for the re-machined base and the base bolt that allows for bleed / adjustable compression. The Cartridge is all stock except for the shim stacks and base, the valves and basic design are the same.  The quality seems OK.  The needle for compression adjustment is crude by modern suspension standards, not a finally machined multi-stage adjustment needle, but it appears to do the job of restricting flow.

My kit came with 2 x rebound adjustment rods that were way too short, I ended up using the stock rod.  (How 2 x short rods make up for one proper length rod is beyond me).  

The k9 kit came with Motul 7 wt fork oil.  I used 10 wt oil based on internet reviews of the kit and what I had been running with my previously updated valving.  

This kit could easily have just included the updated shims and base for a big cost reduction.  I would expect to pay less than $150 for just the parts that are upgraded.  Review of this upgrade really begs the question why they didn't just put this on the stock bike, the extra cost beyond making the stock components is minimal.

The design of the cartridge and dampening fork makes working on this setup very easy, much easier than a more traditional fork with a spring in each leg.  I can pull the fork, cartridge and update shims and back on within 1/2 hour after some practice.  It's really enjoyable to work on.  

Would I buy it again?  I need to test it a bit first.  Right now it doesn't seem like good value compared to a used set of WP OC forks for a little bit more.  The main redeeming points are the ease of installation and the ease of working on the single-leg fork.  

 

Valving: 

This is very interesting to me, the design of the base and rebound valves is unconventional compared to other designs (WP, Showa, KYB, etc).  The shims however are 8mm(ID) and the valve stacks respond to tuning. 

I'm going for a cross between the original xtrainer soft playful suspension and an enduro suspension.  I would like more of a progressive feel with higher speed capability for the faster sections I do while retaining some slow speed plushness for rocks/roots etc. 

Items of note: 

- The check plates have altered bleed holes with the K9 kit, I'm not entirely sure how these small bleed holes or the differences from stock impact the overall suspension besides the obvious (more bleed).   It would be interesting to experiment with these but I have no idea where / how to get different configurations of them.  

- The stock and K9 rebound stack is the same, the Hat is different (larger diameter on the K9 kit), this means that if you have altered the rebound stack for your stock bike you will want to transfer that stack over. I swapped over my rebound stack (which I had previously upgraded).  I tried the stock rebound stack first but it seemed too soft and I didn't like how far I had to adjust the clickers in to slow it down. 

- The stock and K9 compression stack is drastically different.  I assume due to the bleed / adjuster and also to stiffen it up.  They used very few, very THICK shims for the K9 stack.  By the "Cube Rule" for suspension shims stiffness is proportional to the thickness cubed...i.e. the 19mm x .30mm shim is equivalent to 8 * .20mm shims or 27 * .10 shims of the same diameter...  Because the one cartridge performs all the dampening it makes sense this needs to be thicker.  What is strange is that due to warping / deflection (so I've read) it is better to use more thinner shims than fewer thicker shims.  I guess we will see how this holds up in the long term.  I don't want to say they took the cheap way out but....

Followup:  After assembling everything the 2nd time the fork responds well to the rebound adjuster while bouncing in the Garage, turning the rebound adjuster close to fully-in makes it too stiff, but leaving it all the way open makes it too quick, so as far as crude tuning goes I think I found something I can work with.  The Compression dampening shims feel too soft, to get stiffer progressive dampening I need to turn the clickers down to 2 or 3 turns from closed.  At that range each turn makes a large difference.  I think I will be adding more compression shims in the future, I want to test it first. 

With how easy it is to work on this fork there's no reason not to test multiple stacks and oil weights. I'd like to get to a thinner oil, maybe 7 wt.  Next step is to try a 2-stage compression stack, maybe 2 x 19mm x .20mm and a 13mm x .20mm crossover and drop the oil to 7.  There is also the tuning of the clamping shim, note that both the stock and k9 base valves have a clamping "shim" cut into the base itself, a raised step, so this is also something to work with.  The valves themselves can be opened up more (drilled).  Based on my math, the area between the hat and the 27mm cartridge ID is larger than the combined inner orifice size through the valves, so there is room there. 

Stock base valve

Hat: 25.9mm

5 hole 25.75mm x .30 check plate (others the same except for bleed hole quantity)
13 x 1.73
19 x .10
19 x .10
16 x .10
13 x.20


stock rebound valve
Hat: 25.45mm

2 hole
13 x 1.73
19 x .10
19 x .10
13 x.15

k9 base valve

Hat: 25.9mm

3 hole
13 x 1.73
19 x .30
16 x .20
13 x .30

k9 rebound valve
Hat: 25.9mm

1 hole
13 x 1.73
19 x .10
19 x .10
13 x.15

K9 reb Mod
1 hole
13 x 1.73
20 x .10
20 x .10
19 x .10
19 x .10
13 x.15
 

K9 Base Mod
3 hole
? :  I want to test first, but I expect to update to a stiffer stack.  

 

Sorry I didn't take many good pics, I grabbed some video but I have no time to edit it.  

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IMG_20180121_133407411 (Large).jpg

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IMG_20180121_134358211 (Large).jpg

 

 

IMG_20180126_154706738 (Large).jpg

Edited by bikesandcars
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Great write up!

What I find interesting is that my kit, installed about 4-5 months ago, came with a large unmarked bottle of 15 wt fork oil.  My first ride after installing the kit with that oil revealed the action to be way over-damped, subsequently I went back to the 10 wt oil I was using before.  With the 10 wt, I'm nearly full out with the compression adjustment (least) but the rebound roughly centered seems right on for most off-road situations.  I wonder if the  US distributor(s) add the oil themselves which might explain the variation?

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Great write up!
What I find interesting is that my kit, installed about 4-5 months ago, came with a large unmarked bottle of 15 wt fork oil.  My first ride after installing the kit with that oil revealed the action to be way over-damped, subsequently I went back to the 10 wt oil I was using before.  With the 10 wt, I'm nearly full out with the compression adjustment (least) but the rebound roughly centered seems right on for most off-road situations.  I wonder if the  US distributor(s) add the oil themselves which might explain the variation?
It seems there is some variation in kit accessories, did you get 2 x wrong rebound rods?

I should add some personal info:. I have 3 up over stock front spring and 2 up over stock Racetech rear spring. Stock shock no mods. I weigh 175 nekked, about 200/205 trail ready in summer. I'm riding rocky PA singletrack and dual track /haul roads inbetween and do occasional dual sport rides (want to do more). I don't hare scramble or race yet.

I still need to test on-trail, so I'll definitely post my post ride observations and keep updating the latest shims / mods.

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Yep, I got the rods too.  I couldn't figure out what they were for...LOL. They didn't fit anywhere. 

I ride mostly very tight woods with worn trails, lots of stutter bumps, square-edged holes and so-on, occasional logs, roots etc.  I'm about 150 naked and the stock springs are perfect for me - most likely that's why the damping was overboard with the 15 wt. I read that overseas the kit comes with a heavier fork spring in addition to the damping cartridge so maybe the 15 wt oil was designed to be a part of that setup?  I may end up reducing the compression stack as some of the trials-like obstacles I come across are more difficult to handle with this setup...but it is a compromise, as the K9 definitely stabilizes the bike on faster trails and works very well overall, IMO.

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