20mm versus 25mm diameter fork cartridges

[Robert Taylor]

The size of the cartridge is just one of many factors involved in achieving a desirable rate of compression and extension.

Spring rate, air gap, oil viscosity, damper rod size, the size/ number/ position of the ports, the valving on the ports, the bleed size etc all contribute to the overall action of the forks. Taking all of that into account it's possible to have very bad 25mm cartridges as well as very good 20mm cartridges.

The fact that the new Ohlins cartridges provide instant solutions to the action of the previously popular manufacturers product is not solely down to it being 25mm.

Well..... I think I can fairly say that in NZ I have personally had way more experience than anyone here re the whole cartridge thing. Years back we were simply respringing and reoiling, then we started fitting high flow preloading piston kits to the predominantly 20mm cartridges we still see in oem cartridges today. Sourced from a road race specific supplier in the States I was told I would never need to change the settings. It was in effect intimated that the settings were perfect, if I can be a little impolite that turned out to be a load of old bollocks and we laboriously improved the settings to match our tracks.

Bumps are indeed bumps, its just that we have a higher percentage of bumpy roads and tracks and therefore a lower percentage of smooth groomed roads and tracks.

Then we sourced from the same supplier full 20mm cartridge kits, sold quite a few and won a few national champs with them. Once the settings were optimised they worked pretty good but to be frank not night and day better than a rekitted and optimised oem cartridge. And we went through exactly the same grief of them blowing through their stroke as JD intimated in another thread.

Ohlins released their 25mm cartridge kits and all of a sudden the game has been lifted. I hear what JD is saying that the advantage is not solely down to it being both 25mm and sweeping more volume for any given distance of stroke. That is so. Its easier to better tolerance a cartridge when the size is 25mm and it is clear that the performance of the larger size shims is both more responsive and stable. Its no accident that in the world of MX the cartridge i.d size has grown over the years from 20 to 25mm and now beyond 30mm. In ordinary everyday road bikes the size has remained at 20mm because of 3 or more predominant reasons, cost, cost and cost. A few years back the TL1000 S / R and first GSXR1300s had 25mm cartridges, a much better base to improve from. But clearly making such stuff in low volumes when 90% of everything else remained at 20mm didnt rest well with the accountants who call the shots in these large factories. Less volume and more material cost....

JD mentioned pressure balance and how true it is that you cannot get too aggressive with the 20mm cartridges otherwise you end up with cavitation. That is one reason a checkplate mid valve system is largely employed in oem and aftermarket 20mm cartridges. Bending shim stack midvalves have appeared in some oem cartridges but the setting cannot be firmed up as much as desired for a road race application without introducing the very bad effects of cavitation.

The human mind is always afraid of change and I was no less than that when we started embracing the bending shim stack midvalve in Ohlins 25mm cartridges. (This has only largely occured this race season.) But having now intensively worked with it the advantages are very very obvious, 3 to 4 riders under the lap record at every round of the national champs this year in Superbike, in part because they can go in deeper on the brakes. More sidegrip, better feel. And in a roadbike installation ( albeit with a slightly softer setting ) the rider is over the moon with the performance and ride compliance. And this same guy has shared the journey that I have been through with respec kits, then 20mm aftermarket cartridge kits and now 25mm.

Apples for apples a well sorted 25mm cartridge kit with good components will outperform a well sorted 20mm cartridge kit with good components. That in essence is what the top riders in the National champs are telling me and proving on track. It is always possible for a good rider to win a national level road race with 20mm stuff but in doing so being right on the edge of his comfort zone.

In this application ( notwithstanding the overall mix of excellent well developed components ) bigger is better.

[JD Racing]

There are many racers, road riders and track day riders who just don't have the budget for a full cartridge kit, does that mean they are facing imminent death on the road or have the grim reaper riding pillion for every lap of a race, absolutely not, does it mean they can't set lap records or win races and championships absolutely not, 20mm cartridges still have their place and can still be made to offer awesome performance.

[Robert Taylor]

There are many racers, road riders and track day riders who just don't have the budget for a full cartridge kit, does that mean they are facing imminent death on the road or have the grim reaper riding pillion for every lap of a race, absolutely not, does it mean they can't set lap records or win races and championships absolutely not, 20mm cartridges still have their place and can still be made to offer awesome performance.

Granted, the post was about the relative technical and performance benefits which are very real. 20mm cartridges have their place because we are stuck with them but not struck with them. Improvement is a term of relativity and given the often abysmal components and settings in the oem 20mm stuff very real improvements can be effected. And to that end we personally do a lot of work with rekitting 20mm systems because as you say cartridge kits are unaffordable to many.
The Japanese manufacturers are geared up to produce 20mm in volume, that they have found even cheaper ways to produce 20mm cartridges eg 07 / 08 ZX6 is testament that accountants rather than engineers control their world.
We have a top level road race customer with a ZX6, the lad weighs in at over 100kg and is tall. He tried to get away with just fitting 11 newton springs in combo with his oem cartridges and heavier oil, it still blew through its stroke under braking. I have no doubt that we could have improved the stock cartridges ( once we figure a way we could stitch the cheap and nasty construction back together ) but only to a certain level. When we fitted appropriately valved 25mm Ohlins cartridges in combo with the same 11 newton springs the problem was fixed and he was instantly competitive.
I imagine that other manufacturers will soon follow suit with this very nasty and cheap cartridge construction. It may work okay on the road ( excepting square edge bump compliance! ) but it is a piece of shite on the track, no question.

[JD Racing]

Its easier to better tolerance a cartridge when the size is 25mm and it is clear that the performance of the larger size shims is both more responsive and stable.

So those FGR700 forks I've just been servicing with a 20mm cartridge, I need to call the guy up and tell him sorry mate, you've just wasted a whole bunch of dollars on a poorly toleranced product. Applying the same logic that small parts are badly toleranced I would then have to question the angles on all Ohlins 1.5mm, 2mm etc bleed needles, the 20mm piston kit that Ohlins supply for R6's, should I also question the tolerances on the TTX body-that's a lot smaller than the PRX????

[Shaun]

The ballance of thought and communication that you 2 are sharing here, is fantastic for people to read
Very funny ( Normall I guess) How 2 obvious experts can differ on opinions of a product so much! But that is another good thing about having you both on here, I don't think any thing is perfect, Just different


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

Cheers guy's keep it up

[Robert Taylor]

Its easier to better tolerance a cartridge when the size is 25mm and it is clear that the performance of the larger size shims is both more responsive and stable.

So those FGR700 forks I've just been servicing with a 20mm cartridge, I need to call the guy up and tell him sorry mate, you've just wasted a whole bunch of dollars on a poorly toleranced product. Applying the same logic that small parts are badly toleranced I would then have to question the angles on all Ohlins 1.5mm, 2mm etc bleed needles, the 20mm piston kit that Ohlins supply for R6's, should I also question the tolerances on the TTX body-that's a lot smaller than the PRX????

As you well know the FGR is based on TTX principle and is not so dependent on shaft displacement to build damping. As we also both well know mass volume 20mm oem cartridges and the small bit part aftermarket manufacturers of same struggle to build to the exacting tolerances and quality that companies such as Ohlins and WP achieve. Albeit front fork components or shock absorber components. I have seen the Ohlins quality control department a number of times, the investment they have in this alone would blow many companies out of the water, and I would guess WP is very similiar.
In the end event I think that you know exactly what I am saying without any need to split hairs.

[Robert Taylor]

As a further adjunct to the whole cartridge deal. JD has quite correctly pointed out that not everyone can afford a 25mm cartridge kit....or indeed an aftermarket 20mm cartridge kit.
Race tech have recently released a 20mm two port preloading type piston. The basic function of a preloading type piston is that when it is combined with ample port flow area it delivers good ride height control combined with excellent abrupt bump compliance. There is always a compromise though with huge port area as there can be a tendency to blow through stroke too readily, especially under brakes.

The Race Tech G2R valve is designed and machined in such a way that it in effect introduces another 2 tuning parameters. Where a preload packing shim is normally installed if it is made of large enough diameter that it can tune the size of the ports. Therefore you can enjoy the benefit of good brake dive control that you get with a small port piston. BUT ALSO, if that ''shadowing shim'' is made thin enough it too will deflect at higher velocities exposing more port flow area when riding over abrupt bumps.
Maybe someone has done this before, I really dont know. For those familiar with damping curves such a set up will give a first ''knee'' in the curve as the bleed bypass chokes off and the shim stack opens. The size of the shadowing shim will determine the steepness of the slope. And then dependent on the thickness of that shadowing shim there will be a second more subtle knee further up the curve. I.e ''abrupt bump valve lift''
Over the winter we will be developing base settings for both road and road race application, using both our dyno and then road and track tests. The result I confidently expect will perform better than aftermarket 20mm cartridge kits at a fraction of the cost. While it wont approach the very real advantages of a top level 25mm kit the bang for buck will be very very good.

[Tony.OK]

Any chance of showing the cost differences?Including the Race Tech?

[johnsv650]

great reading and we can all learn alot from this,
thanks for your knowledge sharing...

[JD Racing]

As we also both well know mass volume 20mm oem cartridges and the small bit part aftermarket manufacturers of same struggle to build to the exacting tolerances and quality that companies such as Ohlins and WP achieve. Albeit front fork components or shock absorber components. I have seen the Ohlins quality control department a number of times, the investment they have in this alone would blow many companies out of the water, and I would guess WP is very similiar.
In the end event I think that you know exactly what I am saying without any need to split hairs.

Sorry I don't understand what you are saying, I suspect your suggesting that the tolerances accepted by other aftermarket manufacturers other than Ohlins and WP are of a dubious standard, you would have to have some serious measuring equipment to come to that conclusion.

There is some inconsistency in your argument when you then go on to state that your soon to offer mass produced brass pistons from another bit part manufacturer.

Good luck with those, I read a statement by Jim Lindeman that he hated working with the 2 port, now there's a guy who's been around longer than all of us, so if he's struggling I'll steer clear. Ohlins did have a 2 port rebound piston in 2000, it didn't last long.

[Robert Taylor]

Sorry I don't understand what you are saying, I suspect your suggesting that the tolerances accepted by other aftermarket manufacturers other than Ohlins and WP are of a dubious standard, you would have to have some serious measuring equipment to come to that conclusion.

There is some inconsistency in your argument when you then go on to state that your soon to offer mass produced brass pistons from another bit part manufacturer.
Good luck with those, I read a statement by Jim Lindeman that he hated working with the 2 port, now there's a guy who's been around longer than all of us, so if he's struggling I'll steer clear. Ohlins did have a 2 port rebound piston in 2000, it didn't last long.

You never fail to dissappoint!
Frankly I think Im being realistic and ''sharing'' of my own experience. Improvement is relative and what I am intimating at is a cost effective mod, if people are on the other hand able and willing to pay the big bucks for the ultimate then they have the choice. Bit part kits that offer a sizable improvement at a cost that more can afford....but at the very same time being very candid that these will not offer the same level of outright improvement as my beloved Swedish product.

Lindemann is a respected name but I would wonder if there is a slight whiff of politics? Anybodies guess. Traxxion rubbished the Ohlins cartridges in the US ( look at US forums ) but service there can be patchy. Here in NZ the Ohlins stuff is working so well it has contributed significantly to lap records being smashed.
I will keep all that are interested posted on our progress with these pistons, perhaps Jim Lindemann is now too advanced in years and too ''tired and blase'' to spend the time to work with new stuff ( or variations on a theme) ?
BTW, a year ago I adapted a TTX40 to fit into a GSXR600, Mats Larsson from Ohlins said it wouldnt work so well in a bike. Well we got it working really well and that bike won races, a couple emphatically so on a tight, technical track. NOT afraid of hard work!

[JD Racing]

I'm 3 years ahead of you on the stack within a stack design and still working on it, to stop it collapsing in a 20mm cartridge your going to need smaller ports, by a huge margin.

[Sensei]

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

[Pussy]

My first GSX-R to be modded in the front was my K1 1000. The pistons in the stock cartridges had extremely small transfer holes, which led to a "lock" situation on me one day on a rough corner. With only an average riding ability I found a front end slide a bit too character building. RT suggested to me to give the Traxxion piston/valving mod a go, and I had them fitted with a road-bias setting. The flavour of the day was also a short top out spring... which I bloody hated!(I will elaborate on this some time).This valving was transfered over to my K3 GSX-R1000. One problem that showed up pretty early in the piece was stiction, so RT finally got sick of me bitching about it, stripped the forks, and we put a dial guage on them. The damper rod wasn't concentric with the cartridge body. Good old KYB! Robert then designed and had made some floating top bushings for the KYB cartridge. Worked a treat! When the cartridges were to bits I had the stock LONGER top out springs refitted. Again, better for my preference.
I sold my K3 and got a K4 GSX-R1000, and RT had removed the modded cartridges, they were sold to a mate with a K3.
In May 2005 I had a set of AK-20s fitted to the K4, with a road-bias compression valving... we left the rebound "as delivered". The AK-20s had VERY weak higher speed rebound, so had to come out to get beefed up a bit in the valving. These cartridges were smooth in action, but we seemed to have quite a few issues getting them to perform.. RT suspected some leakage at the top of the cartridge. Quite honestly, I found the modded stock cartridges with the floating top bush a better bet, and wished I hadn't bloody sold them, especially after riding the bike they went in.
After several decent earning months, I decided to bite the bullet and get myself a set of the recently released Ohlins cartridges. With 20/20 hindsight, this is what I would have done in the first place. After an initial bit of pissing about with the clickers I found "My" settings, and left them alone after that. I have only ridden one bike with full Ohlins forks (an Aprilia) since, and reckon, IMHO, that the gixxer forks with Ohlins guts are about as close as you will get to them. Thankfully, the cartridges were a straight drop in for my K6 GSX-R 1000 when I got it. No worries
Then I buy a K6 GSX-R750... the cartridges WON'T fit that. Never mind, RT takes them off my hands, and I end up trading them for a set of FGK 132 cartridges for the 750. These have the bending shim mid-valve that the 1000 ones didn't.
And this now takes me to the present... and my questions on the theory behind why these things work so very nicely. Keep up the input, peoples....

[Robert Taylor]

I'm 3 years ahead of you on the stack within a stack design and still working on it, to stop it collapsing in a 20mm cartridge your going to need smaller ports, by a huge margin.

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.