fork tuning for enduros

no problems I think its just too stiff MS/HS with bigger than 10mm pivot and perhaps too soft at LS with the 22 bleed

the smaller the pivot and the larger the OD of the face shims the more shims you need and the easier it is to tune a stack.

06 YZ250 2stroke forks and up to 08 KX250 KYBs use a BV with 4 of 2.5 mm holes and OD of teh face shims is 22mm the ID is 8mm so the smallest pivot is practically minimum 10mm it makes it hard to tune, verry sensitve to 1 shim in and one shim out.

stock pivot in the 250 was 12 I changed it to 10 and put more shims in the stack. its much nicer progression and easier to tune.

the new yz forks are the same except the BV is bigger and the pivot can be 11 and the OD of the shims is 31mm they have a lot more shims.

so the nicest WP BV may be an 8mm pivot with twice as many shims.

when you increased the pivot diameter its probable that the MS and HS is a lot stiffer than the LS which may be extreemely soft with the 2 bleed shims. it also changes the way the shims interact with the 18mm backer.. less deflection before the shims contact it. maybe take it out?

so maybe its got very little initial harshness from soft LS but wallows as a result and works too far into the stroke engaging the air spring sooner than ideal and then being perhaps a tad stiff now for HS and MS its maybe got a lumpy progression?

so.. very soft at LS (lots of bleed from the 2x22 so not much initial hashness) then suddenly thump its ramping up the damping for MS and HS

increasing the pivot makes the valve less digressive which can be helpful if the ports are too big, problem is it gets difficukt to tune. some early Goldvalves ive seen use this setup.

so you could use prelaod with some bleed holes and a "normal" pivot for a smoother progression. prelaoding should increase the LS damping (stiffer) without increasing the HS as much as if you were just increasing the pivot diameter.

just adding bleed without doing anyhting else makes LS softer which reduces initial harshness. but a softer LS is perhaps not ideal so moving damping rather than just adding or softening damping can sometimes help a lot to BOTH reduce initial harshness and make the forks firmer and smoother. so doing both at once drill holes and add shims or use a bleed shim and prelaod the stack.

the point of using bleed IMHO is to increase initial stack lift as the "extra" oil flow flows over the stack reducing initial harshness. ideally the BV clciker could be closed off and the bleed flow redirected to go over teh satck and increase lift.

Its hard to imagine how a 0.5 degree prelaod makes any difference. and yes it makes it a bit harsher on its own but then add some bleed holes or a bleed shim and its both plusher and firmer

so the stack is ready to go all the time and the dribble is controlled better by the bleed

its a similar idea to using a stiffer stack in conjunction with bleed holes only the "dribble" is controlled better.

usually prelaoding is used on compression where you want to increase the LS and MS without increasing the HS as much. so changing the balance of LS/MS/HS to be stiffer overall and relatively more digressive.

used on the compression side of things at the BV in conjunction with bleed it can help to make the forks more responsive, plusher and firmer.

a page or so back in this thread I explained how to prelaod dish a piston by draw filing, its not hard and if you don't like it just reface the valve flat.

I use prelaod on all the rebound stacks I do in forks, tried getting similar results without it its just not the same. have done one KTM shock with a prelaoded reboud stack and a bleed hole in the piston, works good.

might have to start prelaoding BV stacks in other bikes too it does work on my bike, the new Bvs in jap TC forks seem to have a very slight prelaod dishes in them from the factory.

yeah thanks for that bushy.
yes i do now understand your reasoning.
for now i will try a preloaded stack on rebound only,by dishing the valve face & the rebound stack you recomended.if it packs.. drill a 1mm hole in the side of the valve rebound side of course....right?
for the compression side of things ,next ride i will see if i can notice a lump in progression,it might take me a few rides over different terrain to show up any faults.
whats the jappers got in the way of rebound & compression.have they got 2 compression & 1 rebound valves like wp?
..weed..

the good jappers have a closed cartridge just like a shock that slips in the telescoping forks , same as ours but its hydraulically sealed from the outer chamber oil ..... mostly

the oil displaced by the rod goes out through the BV and into a small pressure chamber, its forced back in to the cartridge on rebound by the ICS spring or bladder. just like a shock. otherwise they're the same.

instead of a bladder (like a shock) they use a piston with a spring, zokes, WP TCs and ohlins use bladders and MX-techs element 7 stuff for showas is a WP bladder in place if the ICS piston.

Terry hay suggested I could make my OC forks into TC forks by making an ICS piston run between the ID of the fork lower leg and the OD of teh cartridge it would probably be very good.

can go to the ohlins page to find a pic of the TTX inserts to give some idea.

the bleed hole ah drill it in one of the webs or flutes that run from the centre of the valve out to the outer circumference a bit like the holes in this pic. or anywhere that will let oil bypass the rebound shims. perhaps as close to the centre as possible to encourage shim lift. the other spot used is at 5:30 on the OD. easier to drill but perhaps not as good.

i can hear KTMlew bellowin at you now weed: the edges of the shim are going to shoot sttraight through the shim - they just don't have any support and will collapse soon.

that's why i use 23s.

regards

Taffy

i'm gather you are talking about on the bv taff, going from 3x24's back to a 20 with no 22 in between?
don't worry i haven't finished tinkering yet.i have a few minor changes coming.
bushy,did that rebound tap came out of your wreak 550?
did you drill bigger bleed holes or was it like that?
picture shows a bit more than half a fly poo in dish,(that is 1 degree)
what cha recon
..weed..

that rebound tap I sent came out of a dead 550 2006 model. my 08 taps have the big hole too

my homemade taps have a 2mm bleed circuit to make the stem stronger and so the "nut" portion doesn't have to be as thick. I really don't think its that important the size of the bleed when the main restriction in the bleed circuit is supposedly at the needle.

I'd just put it in as is and see howsit goes.

smorgys dished rebound was 0.2mm I did 0.15 and wished i'd done less lile 0.05 first up I reckon a fly would have no trouble passing a 0.05 stool

I do 0.5 degrees first, test and then go to 1 or 2 degrees if i need to.

Pics look good. howsit go ?

Nice to see people working on their rebounds.
Also fun to see that people are concerned of wether a fly can pass a 0.05 stool or not, as a step on the way towards a better suspension.
Yes I made my rebound preload concave 0.2 mm deep, had to start somewhere, didn't even think of the capabilities of a fly.
That's 2 degrees at the shim edge and 1 degree halfway in toward the center. The dish was made spherical since it was made in a tool grinder that could do that and the radius is 360mm of my dishings. Half of the depth would have likely been enough. Conical preload at a fixed angle should be easier to do in a lathe and should even be better to preload the 2nd stage ( which is at a smaller diameter where my preload is weaker than at the edges )
In spite of it maybe not being totally optimized, this is possibly the best mod I have made to the bike and I am still very happy with it.
Maybe, if anything, I would try a slightly heavier 2nd stage of the rebound. I would have gotten it automatically if I had used a conical preload instead of spherical.

The thing some people sometimes have a hard time getting their head around is that preload and free bleed are used simultaneously.
Dont they cancel each other out?, yes of course they do, but since free bleed and shim bleed have very different characteristics there will be a change of the slow speed progression that is very nice and the rebound characteristics feels a lot more controlled once the telescopic speed goes up. Very nice when hitting rock gardens hard.

Taffy said:

... the edges of the shim are going to shoot sttraight through the shim - they just don't have any support and will collapse soon...

Click to expand...

This is the BV bleeds, right?, the pressure is on the other side, the intake check plate will open to relieve any pressure at rebound, at compression the shims will not be in contact with the piston port edges as the oil pushes them away from there.
So, no worries mate.
I like it, it redistributes the bleed to a spatially wider area and the clickers can be shut a little to compensate. Nice and firm stack too to maintain cartridge pressure at high speed compression.

Regards

this is a stock 09 KX 250 F mid, Showa Twin chamber

1.2mm bleed hole in piston ( in addition to the clciker bleed)

FORK REB
3-30X11
20X9
10X11
10X13
10X15
10X16
10X17
2-10X18
2-10X19
2-10X20
PISTON
MID
10X17 BLEED
6-10X20
10X17
2.0X8.0 COLLAR
15X11
20X11
SPRING

the format is weird its off thumpertalk.

they have a hefty rebound stack with no X over

yeh that kx250 rebound stack looks a bit much,suppose they must have a large clicker bleed hole to counter act that a bit?
Took her for a burl yesterday, and man is this thing starting to feel good. Didn"t have any real big stuff to test it on but all the same i gave it a fair work out.
rebound, i ended up with this.
24d
24d
16
24d
24d
20
18
14
10 .3

I only have the concave valve face in one leg, I'll do the otha when the nut arrives.
Still like the compression side of things, could be a tad on the soft side when i get to the big stuff. That later model rebound tap gave it another .4mm float, so i was expecting that to soften it up a bit,might have to fine tune that a bit yet.
I'm out to 130mm air gap with 5wt oil.forks are about 10mm up past the top of the triple clamps.
As anyone tried different spring preload in ea.fork leg? Probably not the done thing i suppose?
I had 2mm in one side and 9mm in the other side, thought it might bring the springs in a bit progressively. Seems to work,still , it is the best that its been.
gone are the days of the .44 springs with 90mm air gap.
i also saved my coin & bought an omero alloy tank & a under seat fuel cell, appart from the slim feel,it shifts the weight back more,which feels grouse,less front wheel loading.
i love it
..weed..
[/b]

yeh that kx250 rebound stack looks a bit much,suppose they must have a large clicker bleed hole to counter act that a bit?

Click to expand...

ahm no its a tiny clicker bleed

this is on the compression side

MID
10X17 BLEED
6-10X20

Click to expand...

I guess this is why they go for such a beefy rebound stack, because of teh bleed just like the setup thats in the 09s Ive not ridden a kxf250 proper fast yet ( I kept slowing down for corners and it wouldn't get going again ) so I can't say really what it needs but it ill be surprised if it has a consistent rebound action with that bleed setup on the comp.

all they need to do is

20
12
14
5x20
pivot

no float, but first shim moves 0.2 easilly and with a face shim the rebound can be tuned properly.

sounds good weed, its easy to decide which way to go with rebound by experimenting with the clciker

if its tricky to decide which rebound speed needs to be faster or slower you could use a cable tie to see how much travel your using at the time, more travel used means more spring energy to deal with so its operating at a faster speed. more compliocated than taht obviously with the degree of wheel contact to the ground and forward bike speed to consider.

another thing i did sometimes in the beginning wais push hard into some freshly raked breaking bumps and go check the tracks, it show up the degree of contact with the ground and whether its packing or comming out too quick.

its better obviously to be able to feel what to do but if its not working gotta try summink different.

shifts the weight back more,which feels grouse,less front wheel loading.
i love it

Click to expand...

me too, hey weve only gotta move the steering head 160mm forward and it will feel like an ultra stable 09 only with a lower COG better fuel economy and sh&*6t loads more grunt the silly shock angle needs upgrading to 09 spec too.

with head extended 50mm bike now has similar geometry to jap bikes except footpegs still gotta go back 20- 30mm. then 0.48s might be stiff enough for sand MX and whoops

yeh, well lad...if anyone could redesign this thing & make it work..i'd say that would be you.
if you are the bushmechanic....then that would make me the wallum scrub mechanic.
with the weight futher back it seems to get more traction.out of corners the front wheel use to stay planted & it would just broadslide everywhere,now it bites in doesn't fish tail as much but picks the front wheel up,feels a bit like me mates crf450.
certainly the 2 best things i have was suspension tinker & get these 2 tanks.
still cheaper than an unproven 09 model &... my babys got balls....650 balls
..weed..

New age BV.

I've tried to think up the ultimate BV but every time it ends up with what is practically the Öhlins fork BV.
Here are some pictures of some intermediate version that got very complicated.
No tap or spring shown.

Refill intake view. The hole on the right is where the tap goes.




Wirbelzimmer view. Checkvalve plate, but no springs shown.




The shims, cone and high speed laminar shear section.



The complicated wholeness. Many ways of making this simpler.



Wirbelzimmer.
The Wirbelzimmer is probably another Öhlin thing, to compensate for temperature variations in the very low speed domain, or something... Here with small spin up jets.
The thinner the oil the more freely it spins, the thicker the oil the less freely it spins in the wirbelzimmer.

All of this is just conceptual.

Regards.

RE: New age BV.

from anodized 7 series aluminum bolt on bling brustwartze to fully functional wirbelzimmers

It looks great smorgy I can't think of much to improve conceptually, time for a test!

Im trying to get 330mm of travel out of my forks that should level the playing feild then we'll see if an OC can compete with a TC for LS bottoming resistance

the chrome tubes can do 330 the oem 4.4 springs can do 340 and the catridges have 330mm of wear surface, its all there, just has to be tweaked.

The stroke, that is why this New Age BV is no good, it's too tall and interferes with the mid valve piston nut. And it is too complicated. Öhlins made their high speed laminar shear section by just drilling a number of long holes, same thing with the rebound intakes, just drilled holes, a lot simpler and the piston will not have to be fabricated but can be made from 1 piece.

I think that maybe by shortening the plastic spring guide (=bottoming piston) a bit and by either making a new series 7 blue anodized bottoming cone that is deeper to allow the top out spring to enter deeper into it, or to make an extension of the cartridge tube with the same effect using the stock bottoming cone. Maybe as simple as by using spacers?

regards

spacers yes I have a 4mm one but the threads start to become a little too few after that I think,

I can machine the BV stem down a little and make the BV or the MV thinner and shorten the spring guide till the dust wiper hits the axle lugs but I think I need to extend the bottoming cone end of the cartridge by 10-15mm might take some welding.

I got a question on this so will try to explain a bit;

look at it like this, I like 0.44s for intial plushness etc but want 0.51s for bottoming resistance, increase travel and use soft springs ..... get both. or say you like 10mm prelaod for bottoming but 2mm for cornering and initial feel, increase travel by 10mm and use 2mm prelaod .. got best of both worlds.

of course the geomtry changes but thats easy fixed.

im at 300mm now up from 285 and its arguably the best improvement ive made so far to the forks definately the cheapest and the easiest to understand.

it also incraeses the difference in air volume from fully extended to fully compresssed a bit like adding sub tanks. say you like 130mm air chamber but need 120 to stop bottoming, just increase travel by 10mm and its done.

again jap TCs have 315 to 320 its not for show

it would be about $340 AUD plus shims to update the 12/28mm forks to 12/23. (2007-2008 spec to 2009)

need new cart tubes, bottoming cones, MV pistons and piston bands.

cart tube, $63, MV$48, band$20, bottoming cone $40

http://www.mototech.com.au/index.php?pa ... ct=viewCat

cheaper than changing 14mm rods to 12mm, dunno if anyone actually did that

hey lads
done a little fork modification, based on bushy's idea of increasing travel in the front forks.
"simple as" really,& if i don't like it,i can just put back in the original set up.
fork travel has gone from 295mm std to 310-315mm
remember if the same is done for the later model forks the gain is more 25-30mm extra travel because of the extra long top outs.
basically,the idea like bushy says(once it is valved to suit it) is to have the same bottoming resistance but use the extra travel in plushness.
that is the way i intend to valve it.
tried it out on the weekend,& had the front wheel off the ground many times & the short top outs were fine,no clanging or banging.
what cha recon?
..weed..

I reckon its all good, 320mm travel on these 12mm rod OC succkers with backfill and you can keep expensive jap tcs and save $200 on new fork springs that you don't need either

forks.... finished, steering damper and chassis fix next

spent some time on an 09 honda last wend... PHark me it felt like a berg we measuerd it up and the front wheel is moved 17mm closer to crank and rear 18mm on average further back from crank from 08 ...s0.. . it feels a lot like an ultra light berg with a link... the shock sucks and the forks are soft the weight transfer to the front makes it more pronounced ... needs some work too..

here is another pic, if anyones interested.
the 2mm thick washer goes behind the nut.it stops the rubber top out from crawling its way over the nut & also makes the rod go further down the cartridge,without it you couldn't have a 10mm top out cush(shorter the top out the more travel),cause the piston/valve ring would come up too far & get caught at the lip at the end of the cartridge bore.
the 2 small washers were filed down & placed over the end of the rebound plunger, so i have the same rebound adjustment as before.
not rocket science but works.
i originally planned to go the whole hog but got lazy.325mm-330mm is possible,but it is a much bigger job & i haven't got the time.
..weed..