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pattonme

Bitubo JBH dissected, ridden and rated

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pattonme
Posted (edited)

In about 2 hours I will be installing a second JBH into a FZ07 and the both of us will be flogging it.. But first I had my personal example to dissect.

 

Background

 

 

The Good

  • Very simple install - no modifications needed
  • Looks good, if a little unconventional
  • Good price
  • Pressurized REBOUND
  • Light COMP damping in 'C' leg
  • Very nice piston in the abstract and reversible.
  •  

 

 

The er, not so Good

  • .Excessive preload - Bitubo apparently copied the OE's rediculous 20mm preload. Spin the spring 'anchor' till it's barely touching the spring and you will end up with 15mm. Take out the thick Delrin spacer and that'll get you another 4mm.
  • UN-pressurized COMPRESSION - Inexplicably there is no floating piston in the Comp leg. So they put air above the oil which aerates it so damping is compromised.
  • No spring under needles - The needles can get stuck until oil pressure breaks them free or similarly oil flow (REB) can suck the needle into the orifice and dramatically change damping from what was expected. Free-floating needles are ALWAYS supposed to be trapped between a spring and a plunger so it stays put.
  • Compression piston oriented to hamper flow - Uses the small (3.5mm) holes instead of the 3 slots.
  • Rebound circuit very stiff in Compression direction - Normally a rebound piston has very little resistance if you push on it - the pop-off valve activates and floats about 1mm above the piston to maximize flow.
  • 'Sticky' floating piston - The piston is sized a bit loose but uses a large O-ring for sealing duties. Interplay between tube diameter and O-ring make for erratic movement.  It has really tight spots and then suddenly lets go and jumps forward. When retracting REB rod the piston doesn't follow smoothly. This may be helped somewhat when there is air pressure under the piston. Unfortunately it appears the tube ID is not precision sized.
  • 22mm ID and 10mm rod => 20% differential. A 12mm rod (30%) would have been more correct?

 

Riding Impression

I ran it up and down the road behind my house which is just frightful patchwork of potholes (2 inch deep sections just missing) and patches which makes for quite the punishing test. In a word, it was a "lively" experience. The REBOUND circuit is too tight as delivered. Needle full-open had trouble keeping up with bumps. Compression under braking stability was acceptable but bump absorption at full open was much worse than it should have been - not nearly as catastrophic as the Matris F15K straight out of the box, but it clearly needs significant work.

 

I don know if Mr. L will add his personal take but it's not good when the OE damper rods, 0.9kg/mm Sonic springs and Matt's elixir of fork oil (around 45cSt@40) tracks significantly better.. The XF11 shock did a decent enough job but it too suffers from too much rebound damping; only full soft or a couple clicks in were usable.

 

Matt's Fixes (so far)

  • slice 10-13mm from he Delrin preload tube - standard 1" PVC also works
  • Replace backside valving with pop-off
  • Rebuild REBOUND leg to reduce shim stack and stroke resistance. I started with 60mm of air under the piston and upon filling with oil and capping the other end, arrived at 55mm. Resistance in the compression direction was dramatically reduced. May try 70mm next time.
  • Add floating piston to COMP leg and replace floating piston in REB leg with my own creation with Teflon sliding bushing and close-fit O or square ring(s) 
  • Ream tube to consistent size or just replace with precision, polished Aluminum tubing in broad use by all other suspension providers.
  • Add spring under needle
  • Measure remaining air volume and compute correct oil height (not 170ml) so air-spring is configured as traditional, instead of trying to rely on cartridge internal pressure.

 

more edits coming.

Edited by pattonme
  • Like 2

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DewMan

Looking forward to your take on the JBHs.

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JanM

Have been waiting for this - thanks for doing the evaluation, Matt.

 

So the comp leg is also closed but uses an air pocket to handle rod displacement? Combined with a single rod piston with the small ports active, I would think the best part of this kit is the rebound and that the compression side needs work similar to Ohlins NIX and Andreani, similar but also different...

 

Based on the various kits available, I still think that the JBH rebound by design is the way to go, but I prefer the open cartridge rod displacement solution for compression side, when used on a roadgoing motorcycle.

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JanM

BTW, there is a good reason to not use a spring under the needle in a pressurized system - you can push down on the inner rod and see if the system is still pressurized. The earliest Ohlins FGRR systems were like this. But, as there still is a risk of a "hard seating" of the needle, the spring under the needle is a better system overall and you just need to look at the rod to make sure the system is still pressurized.

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pattonme

in-place edits don't notify, so here's a bump for much more text added. 

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FZ07R WaNaB

So I was the one who had the JBH cartridges installed by Matt today. As I told Matt several times, the following has nothing to do with Matt's wrenching as he is a fine mechanic/suspension guru.

 

That said, I wasn't happy with the performance of the cartridges to say the least. I decided that I'm going to hold off on any detailed thoughts/expletives until after I talk with the Bitubo folks. If my cartridges are a fluke, and the situation is made right, that will be great. I recognize that occasionally stuff happens during the manutacturing process, and I hope that is the case in this situation.

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pattonme

My new, replacement piston. Machined to 21.97mm instead of xx.

 

2018-05-14 01.22.42.jpg

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topazsparrow
5 minutes ago, pattonme said:

My new, replacement piston. Machined to 21.97mm instead of xx.

 

2018-05-14 01.22.42.jpg

What's the goal of this new part?

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pattonme

to sit straight in the tube and slide smoothly, and not get stuck like the factory piece. When I post the factory item you'll see why.

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pattonme
Posted (edited)
On 5/12/2018 at 2:50 PM, JanM said:

BTW, there is a good reason to not use a spring under the needle in a pressurized system - you can push down on the inner rod and see if the system is still pressurized. 

all fine in theory until the piston hangs up and you get a vacuum. The needle gets sucked down and you suddenly go from some rebound to MAX rebound. Makes the frontend act totally nuts. It's already happened.

Edited by pattonme

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JanM

Yes, I fully agree that a spring under the needle is a better solution, and as your example show maybe a fixed needle is even better (eg. Showa).

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pattonme

I test-fitted my new piston in a piece of DoM aluminum tubing (1", 0.0625 wall). The smoothness was a thing of beauty. I think I'm just going to toss the Bitubo steel tube and use this instead. Plus it'll save 50 grams, which is even more important. (not)

 

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Kargaburun5

I own JBH cartridges too and overall i am not satisfied with them. I noticed the excessive rebound damping but partly i was blaming my -probably- worn bushing induced stiction.

 

I am interested in the fixes that a mechanically inclined person (i like to think that this is me) can do.

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pattonme
Posted (edited)

I'll probably have a kit at some point:

  • replacement tube
  • sealing piston
  • top-hat and hair spring
  • different valving stack
  • under-needle spring

 

Edited by pattonme

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topazsparrow
13 minutes ago, pattonme said:

I'll probably have a kit at some point:

  • replacement tube
  • sealing piston
  • top-hat and hair spring
  • different valving stack
  • under-needle spring

 

Estimated cost?

 

They're still miles better than stock, but after reading this post I feel like I dun goofed.

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