20mm versus 25mm diameter fork cartridges

[Robert Taylor]

Bugger & here I was thinking that now I have Ohlins forks that ya didn't need to do anything to them .

If you are happy with them fine but even good gear can be better.

[JD Racing]

Thats what I figured and I keep going back to the Ohlins 20mm road race pistons ( small ports ) as they do work very well. But this whole checkplate mid valve thing sets everything in motion, ''the horse bolts'' and then the damping has to play catch up. I know I described that not very well but it seems to make more sense that there is instant control on the midvalve rather than instant oil dump?...........
With the 2 port piston the shadowing shim for road race application will I imagine leave a thin ''sliver'' of port on each side.

Problem is the thin sliver is still a large area relative to the flow volume so it's hard to control, as one old sage once told me, "it's like trying to let 2psi out of your back tyre with a bead breaker".
With such a huge port acting on opposite sides of the piston the bending dynamics of the shim are in one plane so you need a thick shim to stop it folding real easy, so you need thick shims to control the area and the folding and end up with something that when you hit the brakes feels like you hit a corrugated dirt road at 200k's.

[Robert Taylor]

Problem is the thin sliver is still a large area relative to the flow volume so it's hard to control, as one old sage once told me, "it's like trying to let 2psi out of your back tyre with a bead breaker".
With such a huge port acting on opposite sides of the piston the bending dynamics of the shim are in one plane so you need a thick shim to stop it folding real easy, so you need thick shims to control the area and the folding and end up with something that when you hit the brakes feels like you hit a corrugated dirt road at 200k's.

Thanks for that and I guess we will draw our own conclusions when we get to test in varying combinations. You know, that Ohlins 20mm piston has stood the test of time very well and is an ''old faithful''.

[Shaun]

Bigger or smaller holes in the piston, does it really matter?

Is it not possible to control the extra quick dump of the fluid with the bigger port piston with a more fuller comp stack of shims, ie 10 of the chosen size, versus 6 of a chosen size for the smaller piston?

As the holes in the piston only determine the amount of fluid flowing, and the way that fluid flows due to design of the holes in the piston, surely this is all finally controlled by the stack?

Kind of like changing just the bleed jet in a shock, ie a smaller or bigger bleed jet= MORE OR LESS OIL FLOW-

To me, I would choose as much fluid flow as possible, and then turn around and design the shim stack to work with this controlled flow, by directing it to control the area in the fork stroke, that I was not happy with, ie, mid stroke, top stroke etc

[Robert Taylor]

Bigger or smaller holes in the piston, does it really matter?

Is it not possible to control the extra quick dump of the fluid with the bigger port piston with a more fuller comp stack of shims, ie 10 of the chosen size, versus 6 of a chosen size for the smaller piston?

As the holes in the piston only determine the amount of fluid flowing, and the way that fluid flows due to design of the holes in the piston, surely this is all finally controlled by the stack?

Kind of like changing just the bleed jet in a shock, ie a smaller or bigger bleed jet= MORE OR LESS OIL FLOW-

To me, I would choose as much fluid flow as possible, and then turn around and design the shim stack to work with this controlled flow, by directing it to control the area in the fork stroke, that I was not happy with, ie, mid stroke, top stroke etc

Actually yes it does matter, a whole lot.

To once again recite the phrase ''the best youve ridden is the best you know'' Well, you havent ridden on the very latest generation of Ohlins FGK cartridges with bending shim stack mid valves and enjoyed set up specific to you. Time and progress doesnt stand still and if anyone anywhere says they have the perfect set up and it doesnt need changing then they are in denial.

Being thankfully blessed with a very sturdy memory I can well recall that on a number of occassions right through until last years nationals that you said a set of forks you had in a GSXR750 ( around 2000 ) were the best youd ever ridden on. I remember putting those forks together with Ohlins small port pistons, the ''old faithfuls '' that I eluded to in another post.

I will try and explain the whole big port thing to you in terms that you as a rider will understand, without launching into too much technical jargon. Big port pistons will dump a lot of oil when the shim stack opens, but therein lies the first issue of control. Someone said to me ''youll never have to revalve these, ever'' Fine if you want a pleasant afternoon ride on a bumpy road as opposed to braking as late as you can get away with on a racetrack. To stop the forks blowing through the stroke with such pistons you do indeed need stiff stacks, but then the initial opening pressure becomes so high ( knee on a dyno curve ) the forks become harsh and sidegrip etc is compromised.
I also recall a set of forks that Chris Perris was sent out from the US with as I had to fix them for you, the opening pressure of the shim stack was so high that they were impossibly harsh. Bumps are bumps, everywhere.
During Sam Smiths 600 season last year we changed the compression pistons 4 times and each time the front end got better. We started with Traxxion 20mm pistons, followed by Yoshimura pistons, better ( a rip off of the Ohlins 20mm preloading pistons but with bigger ports ) Then the Ohlins preloading pistons, small port and better again. Finally the ''old faithful'' linear face Ohlins 20mm pistons, best of all. Just ask Sam... This was all with old style checkplate midvalve but we have now moved on to something clearly better.
JD Racing has in fact broadly answered your questions anyway when it comes to controlling big port pistons. He has also correctly stated the limitations of 20mm cartridges in trying to get them to work with bending shim stack midvalves, there are pressure balance issues which means the forks will drop their lunch with cavitation.
Have a look at rebound pistons in Traxxion stuff, they are big port and need a preloading shim to give some control. Not only that the big black anodised back up washer immediately next to the clamping shim acts as a shim stack travel restrictor to stop the shim stack opening too far and letting the forks ''drop their lunch'' on rebound.

The dynamic of rebound from fully closed to fully extended is that you initially have a lot of spring force to control requiring a lot of ( high speed ) rebound damping. As the fork ( or shock ) re-extends the spring force progressively lessens so the damping needs to be progressively weaker to be in unison. That function is best served by a small port piston ( i.e Ohlins, WP, Race Tech and a lot of oem stuff ) In combination with a less agressive shim stack. A stiff stack on a big port piston will give too much low speed damping, if you are searching for grip rebound is often where its at.

Same for compression, within sensible limits and varied applications ( road compliance, road race control ) the ports need to be smaller with a less aggressive stack. The opening pressure is less so you are not risking hurting side grip / tyre edge grip so much and the slope of the curve ( which often is used to catch too sudden movement ) is not too determined by the shim stack!

The bending shim stack midvalves that our riders are so delighted with are a whole new area. Much less emphasis is placed on the traditional base valve main compression pistons to ''catch the runaway train'' because the checkplate system provides a whole lot less initial control, the door is opened right away. The mid valve becomes the main piston and beleive me playing with the shimming on that piston yields great results once you understand it.

[JD Racing]

Thanks for that and I guess we will draw our own conclusions when we get to test in varying combinations. You know, that Ohlins 20mm piston has stood the test of time very well and is an ''old faithful''.

Ha haa, absolutely, you can spend a whole lot of time playing around but if there's one thing you can always fall back on it's those.

Don't rule out the bending shim midvalve in a 20mm cartridge it can be done with great results and no cavitation, checkplates can also be successfull, Honda HMAS pistons share the same dimensions as an Ohlins kit but with a significant difference, use that with a good checkplate build and it really adds dive control, some KYB rebound pistons can be used to achieve the same thing.


Shaun, get a piece of paper, lay a rule across the middle and hold the rule down, pick up the outside edge of the paper, the paper will fold at the edge of the rule, stack up 50 sheets and try the same thing, the paper no longer folds at the outside edge of the rule, apply this to shims and it means that what started out as a small increase of seat pressure becomes something very stiff once the shims start deflecting.

[JD Racing]

Bigger or smaller holes in the piston, does it really matter?
To me, I would choose as much fluid flow as possible, and then turn around and design the shim stack to work with this controlled flow, by directing it to control the area in the fork stroke, that I was not happy with, ie, mid stroke, top stroke etc

Unfortunatley damping is speed sensitive not position sensitive.

[Shaun]

Unfortunatley damping is speed sensitive not position sensitive.

Understand but, if I wanted my mid stroke to be furmer, surely we would consider altering the shim stack at that stage, ( To change any other parts, would have a big effect over the entire range) that is assuming all other of the fork is fine?

[Robert Taylor]

Understand but, if I wanted my mid stroke to be furmer, surely we would consider altering the shim stack at that stage, ( To change any other parts, would have a big effect over the entire range) that is assuming all other of the fork is fine?

Midstroke is a contradiction in terms because it is all about velocity as has been correctly stated. Changing it as you understand it will invariably affect low speed as well, that is not always desirable. That is exactly why a big port piston which places too much emphasis on the shim stack for control is so hard to get right.

And digressive curves are not the holy grail as invariably there is not enough slope on the curve to ''catch'' a sudden stroke movement.

No fork or shock will work perfectly for every scenario with just a spring and clicker change, FACT.

[Shaun]

Midstroke is a contradiction in terms because it is all about velocity as has been correctly stated. Changing it as you understand it will invariably affect low speed as well, that is not always desirable. That is exactly why a big port piston which places too much emphasis on the shim stack for control is so hard to get right.

And digressive curves are not the holy grail as invariably there is not enough slope on the curve to ''catch'' a sudden stroke movement.

No fork or shock will work perfectly for every scenario with just a spring and clicker change, FACT.

Cheers, so then, if Midstroke is a contradiction in terms then, and it is all about velocity, how do you go about determing in what area to make the change then?

Because with all the years we worked together on suspension Robert, WE always refered to area on the fork that a change was required in, not the Velocity range?



Midstroke is a contradiction in terms because it is all about velocity as has been correctly stated. Changing it as you understand it will invariably affect low speed as well, that is not always desirable. That is exactly why a big port piston which places too much emphasis on the shim stack for control is so hard to get right.




Yes it will , but then you can re adjust externall settings on the dampner clicker, or fit a softer spring, or lighter oil, to work on the low speed issue

[cowpoos]

Ya man Cowpoos, is loving it, I have even see some big cleaver words from him

Cheers guy's keep it up

Lol...yeah your right...I do have alot of interest in this subject.

But...the problem with learning in depth about suspension dynamics is how it relates to feedback on a race track/road... understanding what is been discussed and how the best comprimises are found..why changes in piston design,differnt valving,mid piston stacks,check plates,etc are done..are all good..but it would be nice to know the difference's in how a bike reacts..

From a bike setup point of view I have learned alot from Robert and JD...by picking bits of imformation out of the various threads...more so when a circumsatance has been explained and as a rider relating it to something I have felt or close to what I think I may have felt...as in my mind it helps greatly between distinguishing what maybe a rider input fault and a bike suspension adjustment thats needed..and having these guys here on KB is easily I feel cutting years of trial and error learning..which In my personal situation is fanfuckingtastic...as I know I have a very sensitive feel to changes on a motorbike..be it road,race track or dirtbike..and pre these guys and TDC making a appearence here I was rather indecisive about making changes as I didn't know the extent of the pro's and con's..

And because of...my perception on testing has changed...and what is required...and has made me far more egar to getting a new machine [once I decide what class to run in] and starting it all from scratch..and learning the development more as I believe its a key to running up near the front..as much as riding ability helps aswell of course.

[JD Racing]

You can't tune for midstroke with damping, if your hard on the brakes loading the forks at maximum velocity you want lots of highspeed damping. If your mid turn the forks loaded by g force you want lots of low speed for feel and little highspeed to soak up the ripples.

[Robert Taylor]

Cheers, so then, if Midstroke is a contradiction in terms then, and it is all about velocity, how do you go about determing in what area to make the change then?

Because with all the years we worked together on suspension Robert, WE always refered to area on the fork that a change was required in, not the Velocity range?



Midstroke is a contradiction in terms because it is all about velocity as has been correctly stated. Changing it as you understand it will invariably affect low speed as well, that is not always desirable. That is exactly why a big port piston which places too much emphasis on the shim stack for control is so hard to get right.




Yes it will , but then you can re adjust externall settings on the dampner clicker, or fit a softer spring, or lighter oil, to work on the low speed issue

Lots of riders use lots of different ways of describing things and as you well realise I work with lots of riders at all levels. Midstroke clearly is an unfortunate term that should be expunged from your own vocabulary and frankly my understanding of the whole suspension thing has gone ahead in leaps and bounds since the time I was valving suspension units for you.

JD racing has answered your question perfectly. Also lighter oil is only a tuning option I would use in an old damper rod fork to tune rebound speed! You really must get up to speed Shaun!

[Shaun]

Lots of riders use lots of different ways of describing things and as you well realise I work with lots of riders at all levels. Midstroke clearly is an unfortunate term that should be expunged from your own vocabulary and frankly my understanding of the whole suspension thing has gone ahead in leaps and bounds since the time I was valving suspension units for you.

JD racing has answered your question perfectly. Also lighter oil is only a tuning option I would use in an old damper rod fork to tune rebound speed! You really must get up to speed Shaun!

Thanks for the reply Robert.

I know your knoledge has grown in the last few years yet again Robert, that is the best part about having you around here these days. ( Midstroke has been expunged mate,Don't let me here you use that term again) Nah, don't want to get up to speed to quickly mate, it all takes time to learn

[Shaun]

Lol...yeah your right...I do have alot of interest in this subject.

But...the problem with learning in depth about suspension dynamics is how it relates to feedback on a race track/road... understanding what is been discussed and how the best comprimises are found..why changes in piston design,differnt valving,mid piston stacks,check plates,etc are done..are all good..but it would be nice to know the difference's in how a bike reacts..

From a bike setup point of view I have learned alot from Robert and JD...by picking bits of imformation out of the various threads...more so when a circumsatance has been explained and as a rider relating it to something I have felt or close to what I think I may have felt...as in my mind it helps greatly between distinguishing what maybe a rider input fault and a bike suspension adjustment thats needed..and having these guys here on KB is easily I feel cutting years of trial and error learning..which In my personal situation is fanfuckingtastic...as I know I have a very sensitive feel to changes on a motorbike..be it road,race track or dirtbike..and pre these guys and TDC making a appearence here I was rather indecisive about making changes as I didn't know the extent of the pro's and con's..

And because of...my perception on testing has changed...and what is required...and has made me far more egar to getting a new machine [once I decide what class to run in] and starting it all from scratch..and learning the development more as I believe its a key to running up near the front..as much as riding ability helps aswell of course.

You are obviously learning a lot mate, my post about you using big words, was actually meant as a compliment! you have learned a lot since I first met you a couple of years ago