[k2t2]

After Installing a XXZ31 rear shock, I found the front cartridges particularly weak on small bump compliance.
They also get very harsh on high compression values.
Several forum members found good improvement of front forks with lower oil level than stock and different grades (5wt, 7.5wt).

Example Oil level curves on damping force from Ohlins NIX22 cartridge for R3:

I wanted to try the same but I need to get new fork springs and a new cartridge with springs is not that far away price wise.
The stock springs are progressive, starting at 0.82 kg/mm and ending at 0.89 kg/mm creating too much static sag for me (~170 lbs)

Among the front fork cartridge options I looked, most of them are standard open cartridge style (similar to stock) but with different materials, piston diameters/designs and separated fork functions (comp & rebound)
e.g. The stock is 20mm. Andreani is ~22mm, Ohlins has NIX22/NIX30 etc..

Typical standard twin-tube fork cartridge design according to Lee Parks Suspension Bible:

One big problem with open cartridges is cavitation (esp. on cartridges with both comp and rebound)
A very good demo about it:

The other design used heavily in racing/Motocross is the Spring/Gas pressurized closed cartridge system.
which is supposed to deal with cavitation much better and give consistent damping.
The biggest problem with this style of cartridges is you need to take them to a service center for rebuild if it's gas based.

Showa's BFF fork used in GSXR-1000R/ZX-10R, Ohlins NPX in R1M, CBR1000RR-R/V4S/V4R are the OEM pressurized cartridge options I can think of.

There are very limited aftermarket pressurized cartridges on the market with Ohlins FKR, Bitubo JBH/EBH/ECH, Ktech DDS, Andreani pressurized, WP Apex pro 7500 being the options I could find. Unfortunately all these 30mm pressurized kits are very expensive, racing only and require maintenance frequently as suggested by the manufacturer (at around an year/10k miles or 10 hours of track).

Bitubo JBH (previous model ABB) seems to be an exception given it's a simpler and smaller (22mm) design has higher service intervals 20k miles.

I found a service manual for it and looks like the cartridges are closed style, with 22mm pistons and pressurized with nitrogen at 8bar.
The gas and the oil are separated by a floating piston with two rubber o-rings, similar to decarbon style rear shock.

Bitubo Service Manual.pdf - Motorcycle Insight

Bitubo Service Manual.pdf - Motorcycle Read more about cartridge, piston, bitubo, insert, tube and wrench.

www.yumpu.com

So, I've ordered a set from an Italian retailer with the correct set of springs for me at 0.9 kg/mm which came out to be ~550$ shipped.

PRESSURIZED FORK CARTRIDGES BITUBO JBH01V1 PER SUZUKI KATANA 1000 2019/2020

Bitubo pressurized cartridge, adjustable in compression, rebound and spring preload. It replaces the original cartridge, without any modification.

motomaniashop.com

I just received them. Will install and report the details after the ride.

 

[k2t2]

I am waiting for delivery of cartridge bottom bolt copper washers. Will install this weekend.

 

[k2t2]

I have finally managed to install the cartridges and it was mostly painless except for compressing the spring to fit the fork caps in the end.
I removed the stock fork internals as per the manual with Traxxion dynamics fork cartridge toolkit and a Biltek KYB cartridge holding tool.

Initially I was worried if the fork bottoms have to be removed, but I was happy to find that the cartridges are direct drop-in.
Cartridge bottom:

Fork Inside after stock cartridge removal. (I backed out the stock compression screw)

Filled with Silkolene 02 synthetic fork oil (not provided with kit) up to 150mm as recommended.

After riding my favorite twisty, JBH with the 0.9kg/mm springs gave me a huge improvement over stock on small bump compliance.
I've tried higher compression settings as well and they don't seem to make the ride considerably harsh unlike the stock cartridge which was unusable beyond 1.75 round out of compression for me. The nitrogen gas must be helping a lot with small bumps.

I have started with recommended settings comp: 12, rebound : 12, preload: 4mm
After some testing ended up with 9 clicks out on comp, rebound and 10mm of preload which helped reduce brake dive and improved front end feel.
Lower oil level makes a lot of difference in linearity of stroke. I was able to get lower into the stroke without any harshness!

Finally the bike's front and rear suspension are at similar level and the damping feel at the front is now the same at the start of the ride (vs) after 1-2 hours of hard riding unlike the stock which would drop off after hard riding.

The cartridge damping feel is quite linear (similar to stock) unlike the XXZ31 rear shock which is clearly digressive feel with a solid piston TTx style design.
I guess the shim stack is tapered and not preloaded.

A lot of suspension experts seem to prefer the linear feel but I like the initial firmness from a digressive damper.
Showa Damper evolution seems to be going the small piston linear feel route: e.g. Stock GSXR1000 - BPF forks - ~40 mm piston - feels digressive, GSXR1000R - BFF forks - ~25mm solid piston - digressive feel still (but less than the bpf).

Linear damping rate is good for predictability but feels a tiny bit mushy initially during turn-in.
With a digressive rate, you can feel it when you get on the bike and move it with your feet.
The digressive firmness makes it feel like you have less parasitic spring movement energy losses.

I guess the bigger pistons in (~30mm) Bitubo EBH / Ohlins FKR cartridges would give me the firm digressive damping character I was looking for as they move more fluid similar to Showa BPF/BFF forks, but they are too much $$$$

The cartridge upgrade did not completely blow me away like the stock rear shock upgrade to Bitubo XXZ31. Shows you how bad the stock rear shock was.
Overall the upgrade is fantastic for ride quality and consistency of damping. Maybe I would have picked 0.95 kg/mm springs if I were to order again.

I would highly recommend this to anyone that wants a quality (20-22mm class) cartridge kit for sub 600$

 

[k2t2]

I would highly recommend picking up Lee Parks Suspension bible book. Helped me open up a whole new world!

 

[k2t2]

That's a good deal!

 

[k2t2]

Oops, I meant 'Biltek'. It's a cartridge holding tool.
Mind you, the length of this tool could run short, if you haven't separated out the inner/outer fork tubes while replacing the cartridge bottom bolts.
The eyelets of this tool would be inside by around an inch with the outer tube completely dropped to it's lowest position on the inner tube.
I've used a screw driver on one of the eyelets as a leverage and an Allen key at the bottom to remove the lower cartridge bolt.

This Biltek cartridge holder won't work on the Bitubo cartridge and requires a special tool
I didn't need this as the cartridge bottom bolts tightened up good and for removal I could use an impact wrench.

I've used ratchet straps for compressing the OEM stock springs.
Reused the copper washers of the bottom bolts as they still looked thick and good.

As I've said, compressing the Bitubo springs is tricky.
I've inserted a wrench on to the cartridge lock nut via the spring gap, twisted the spring till the lock nut comes out at the top.

The silkolene 02 synthetic fork oil comes in a 1 liter bottle. With both cartridges filled to ~150mm, I still have 300ml left.
You can use their Pro RSF 5w oil as well or any other one you like. The oil weight here doesn't matter as it doesn't do any damping.

Tool links:
Biltek damper rod tool: https://www.amazon.com/dp/B08MV6T4Y7
TD fork service tool kit: https://www.amazon.com/dp/B08V6Z3JT2
Bottom bolt copper gaskets: Suzuki 51148-41310 GASKET - RevZilla
Bitubo 00397 cartridge holding tool: Bitubo Fixing Cartridge Tool - Universal parts & accessories - Storm Motor
Silkolene 02 syntetic fork oil: https://www.amazon.com/dp/B09ZQ3T7YY
Silkoleve Pro RSF 5W oil: Amazon.com: Silkolene 601450181 Pro-RSF Shock Fluid - 5W - 1L. : Automotive

 

[k2t2]

Yes. I left the old compression adjusters in place. They don't obstruct anything.
I guess, you won't even need the Biltek tool to remove the OEM cartridge as a lot of folks had success with impact wrench.

 

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This would be a typical flow most people would recommend and there are multiple youtube videos explaining every step.
It's a bit daunting for beginners but getting a feel for these setting changes on the same road is the only way. Experiment. A lot.

1. Set your suspension sag correct for your weight.
Adjust preload only. Aim for ~35mm for street. ~30mm for spirited riding. ~25mm for track. This makes a big difference than you think.
If you can't get these recommended values with reasonable preload, time to get new springs.

Follow a consistent method in measuring sag that accounts for sliding friction.

2. Set rebound.
Start with default setting. Do a bounce test and set it to a baseline value where the bike bounces back and settles after overshoot quickly.
Then start experimenting and fine tune it. Rebound determines your dynamic ride height.
Tightening rebound will get a good road surface feel till a certain point before becoming harsh on the forks. High rebound on the rear doesn't give you any 'feel'
Loose rebound will feel good at leisurely pace but the bike wallows a lot when the pace picks up. Forks will jump up before corner exit. Shock will pogo and jump during braking.
From my experience, the rear doesn't need as much rebound damping as the front as we need rear up faster than the front for better geometry during corner exit.

A lot of folks typically add more rebound damping than required. But it's good to have a bit less than what feels good.

3. Set Compression damping
I feel compression is a little tricky to setup. Start with the default setting.
Experiment to see and add (a little less than) as much as you can based on your suspension stroke length usage and feeling of control.

Few things to keep in mind:
- Your tire pressures play a huge role. At typical stock 36/42, your ride will be rough.
I would recommend ~33/33 to start with for street/canyon riding which reduces a lot of small bump harshness.
This can vary based on tire carcass type and style of riding

- Suspension fluid fill level.
As stated in the first post, the stock forks have way too much fluid.
This would make it very difficult to use more than half the stroke length.
Tie a zip-tie to fork tubes to check for stroke length usage.
In ideal conditions, you should be able to use most of the fork stroke except the bottom ~1.5 inches.
Try reducing the fluid level.

- Tire wear patterns.
Keep checking periodically how your tires look. Wear patterns can tell you a lot!

https://lifeatlean.com/motorcycle-tyre-wear-guide/

Keep in mind that street riding wear will typically look like low rebound damping wear. (leading edges of tire blocks taller than trailing)

- Quality/Architecture of suspension components
Some forks/shock adjusters only work on a limited range of their provided adjusters.
Stock fork is pretty decent in this regard. Stock shock, utter garbage.
Solid piston TTx style shocks/forks due to their architecture typically ride much better.

- Top out spring strength.
The stock top out spring is a bit weaker than I like.
This might cause more than required force for the whole fork to start working at the top of the stroke

- Geometry/ride height
No amount of suspension can fix handling woes caused by geometry.
GSXS inherently has higher rake angle and longer wheelbase than a GSXR which makes it much more stable but lazy to turn.
Make sure the front/rear are balanced when you adjust compression/rebound.
Front low, rear high dynamic ride height typically gives you the best turn-in/corner exit. So take it easy on rear rebound. If possible, raise the rear shock height.

Troubleshooting:

 

[k2t2]

You can start first with either the top caps or the bottom cartridge bolts, but draining out the fluid is much easier with the top cap first.
If you have bottom cartridge bolt removed as well, yes, the whole cartridge can come out.

Loosening the fork top caps while they are already attached to the bike would make your life easier later.
I did not do that and had to stick the fork tubes in a vise for loosening the top caps.
Make sure you have the correct socket ready. For Bitubo JBH, I guess it's 28 or 29 mm socket.

 

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Hi @gpounce, Did you finish your install? Interested in your upgrade impressions.

 

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I think they should fit. Older fork tubes has compression screw that clicked between settings. Other members can confirm

 

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Make sure you turn out all the clickers on the top cap and install it on the cartridge threaded rod. The jam nut is then locked to the top cap.
Yes, both the cartridges are gas filled. Hence even without spring, the piston rods need some force to pull/push.

 

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Thanks for your impressions gpounce!
If you are having more fork dive than you like even after preload/compression adjustment, the spring might be softer for you.
As a quick fix you can raise the oil level by ~10-20 mm in the fork legs.

My Zip-tie still shows ~25-30mm fork travel left which is inline with champ school guys recommendation.
Forks should be compressed by no more than 100mm for perfect cornering geometry
.
Bitubo sells springs in the range of 0.8N/mm-1.05N/mm for JBH. I plan to get MX82(1.0N/mm) this summer if I can improve my riding. Stock is 0.9N/mm

FG Gubellini stealth : KIT CARTUCCE COMPLETE - FG Gubellini
Andreani Misano Evo : Andreani Misano Adjustable Hydraulic Cartridges for Kayaba 41 for Suzuki Gsxs1000 F 2016 2019 - 105

Both of these are traditional twin tube architecture cartridges just like the OEM KYB, perhaps with different valving.
A lot of people found Andreani valving to be stiff. Not sure about FG.

Bitubo EBH is similar to Ohlins FKR and other pressurized cartridges in ~30mm piston class. These track class cartridges provide more damping and are $$$$

 

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Yeah. The preload adjuster is not very easy to deal with. I can't seem to find a replacement in the official catalogue.
Maybe some one can: CATALOGO BITUBO 2022