fork tuning for enduros

Taffy
fork is aprox. 5mm taller now its same length as KTM
if you stand 04 and 07 side by side footpeg to footpeg the 07 stands taller and front
axle is 15- 20mm forward of the 04
smorgy
I newer checked the swingarm mounting pos.
these changes is what the factory did for the works bikes Joakim talked about his frame
extended 15mm behind steering head and bjorne was 25mm
so long VIKING

Did the two Ø5mm holes to the sides at 170mm mod in the lower leg, as seen in Taffys "The Doc". This mod could be more of a spell than anything really useful. The gap between the upper leg and the lower leg is 0.5mm on the 2000 Ø43mm fork legs, as the upper leg bore is about Ø44mm. In theory the pressure/suction created by the lower leg movement would be depending of relative speed and of viscosity. How extended the fork leg is also a factor and the gap width would probably be a very strongly influential factor. If the fork leg is nearly totally extended the two mod holes would vent any pressure/suction very well. But when collapsed, the unvented bit is quite long and the two hole mod would be less effective. The thicker the oil, the more pressure/suction. I don't know wether the 3 bushings forks would benefit more from the 2 holes mod than the older Ø43's with 2 bushings? Since people do these two holes mods I have done it too, more as a matter of course than that I understand what actual benefit it would ever offer.
After beveling the edges of the holes, they were polished with a little cratex abrasive rubber wheel in the dremel and then fine polished with a little felt wheel and some flitz metal polish. They came out very nicely and with little effort and may with some luck not damage the seal. Maybe, when by mistake you are exposing the two holes outside the seal when working with the fork legs, they will it cause lots of oil spilling on your garage floor and soaking your pants too (fork oil, hopefully!) and cause a lot of foul language. Regards.

Taffy, I was thinking of your two MV shims getting all the beating. Going a little bit overly philosophical here as well as lengthy, but please take a deep breath and dive in because I value your input. I've stuck to the subject anyway.
I like the idea of the MV stack acting like a gail-wrung umbrella, sparing tube, rim and yourself at the sharpest hits. But after having taken apart the CP of my 2000 OEM 43 mm fork and looked at the shims I'm concerned about the piston facing shim getting sucked out of shape at rebound.
If I get it right you are concerned about what the compression hits does to the shims.

The shims getting sucked through the piston ports.

Looking at the deformed shim from the OEM stack and holding it against the piston, you realize that it just can't seal until the pressure is high enough to force the shim to adapt to the port edges. The port suction causes a sealing problem as it deform the shim, even if it doesn't totally wreck the shim. Is this BTW the reason why some people prefer the stronger MV spring?, to iron out the port facing shim by spring force rather than letting the port suction do the job. Even if you don't know what is happening exactly, you might still feel the difference to form a preference?

There are only 3 ways that I can see.
Use a thicker shim facing the piston ports.
Use more and smaller ports in the piston to give better support at rebound.
Replace the fatigued MV shimming at every oil change, at least the port facing one.

If it true that the shim bending resistance is proportional to the cube of the thickness, then the equivalent of your 2 x 0.1 mm shims would be one single 0.126mm. There is hope to find such stock since it is very near.005"! It should be only 2% stiffer. 0.15 mm shims are then 3.4 times stiffer and could perhaps replace a 4 x 0.1 stack. Japanese 0.11 mm shims would then be 33% times stiffer, if you can find the same size that is, for instance you'd use 3 off of them to replace a 4 x 0.1 mm stack.
Of course a thicker shim would not take as much bending at sharp hit compression without damage. That is the back side of it.

It would be possible to add a wagon wheel "spoke" or two in every MV port and that would offer a lot better support for the poor MV face shim. One way would be to braze sheet metal spokes in there and then file off excess material and grind the whole thing flat again so that everything is at the same level. (Flat like a smorgasbord...) Additional spooks would increase oil flow resistance at sharp hit compression, but it may be that you would not notice the difference since there are other places of resistance that possibly are worse, or perhaps not?.

Considering how much is taken apart anyway and cleaned, it may not be such a chore nor expensive to replace the MV piston facing shim at every oil change. I have no idea how quickly the MV face shim deforms though. If it deforms at the first bump then the replacement might be in vain, at least regarding the sealing properties of the shim.


The strain of the shims at compression.

After having bent the shims several times over the zp3 like mod, it feels like the stack wants to Pringle crisp or saddle shape over the "zp3" rather than umbrella. Of course i could be wrong, but I don't think so and it's tempting to connect the piston to a compressed air hose somehow to verify. It feels emphatically that they will saddle and they have a lot of freedom and willingness to take that particular shape on the tap. Maybe at extremely hard pressure they would take the wrung umbrella shape but the risk is that it will get a lot of sharp bends if it does, perhaps not as sharply pleated as a traditional Scotsman's kilt, but you might know what I mean when you think of that gail-wrung umbrella. If you could control the radius of the saddle shape you could also use the shim material to its full potential.
Suppose that you used that single 0.127 mm shim as your MV. It could not bend to the same small radius as the two 0.1 mm shims that you use, not without breaking it. You'd have to restrict the bending radius of it, in the same way the reeds in 2-stroke intake are limited and for the same reason. It is not impossible that on the zp3 the radius will be on the safe side, but it would feel safer using a saddle shaped former to guarantee that the bending does not get too sharp anywhere on the shim. A former that the shims can bend around in a more controlled manner. It could either be the tap machined with the radius or, maybe easier to make, a washer or very thick shim carefully bent to radius and added to the MV stack and upon which the spring could act as a return spring for the whole package.
The stress on shims of different thickness calculated at two different bend angles and radius of former, it's just all theoretical of course but could serve as an indicator of what to expect:
Modulus of elasticity = 210000N/mm2. Length of curvature = 24 mm (shim diameter).
30 deg bend from mid to edge. Lift = 3.1 mm. Radius of former = 22.9 mm. End-end = 22.9 mm.
Thickness mm / material stress would not exceed
0.1 mm / 458.149 N/mm2
0.11 mm / 503.964 N/mm2
0.127 mm / 581.849 N/mm2
0.15 mm / 687.223 N/mm2

45 deg bend from mid to edge. Lift = 4.5 mm. Radius of former = 15.3 mm. End-end = 21.6 mm.
Thickness mm / material stress would not exceed
0.1 mm / 687.223 N/mm2
0.11 mm / 755.946 N/mm2
0.127 mm / 872.774 N/mm2
0.15 mm / 1030.84 N/mm2


60 deg bend from mid to edge. Lift = 5.7 mm. Radius of former = 11.5 mm. End-end = 19.9 mm.
Thickness mm / material stress would not exceed
0.1 mm / 916.298 N/mm2
0.11 mm / 1007.9278 N/mm2
0.127 mm / 1163.6985 N/mm2
0.15 mm / 1374.4470 N/mm2

Then how much strain can the shim material take? I have not a clue!

Regards

Hi smorgasbord
I warned Taffy about MV shims getting sucked through piston port a thicker shim would
fix that but will hurt performance, solder a support bridge would fix that problem
solder in cast iron is tough and can fail
in 04 I had custom mid valve/rebound assembly which had a custom made piston to
address the problem with shim support knowing what know 12mm cartridge rod is
much better than the old 14mm rod in terms of performance
so long VIKING

Interesting things that you say, VIKING. If for instance you compared the 4 x 0.1 to the
3 x 0.11 CP stack, if the cube theory was true with these two CPs equally stiff, then there is more to it than shim stiffness when you use thicker shims. Could it be that thicker and fewer shims more easily are getting permanently deformed and that people actually can feel a difference in performance because of it? In that case the shim former would cure it, or is there even more to it that we don't understand? Yes, there must be.

It does not seem to be cast iron, gray iron would be more easily "carved" with a knife than this material seem to be. It could be some sort of investment cast steel or something. I think it will braze well, but the idea of something coming lose in there in case I'm wrong puts me off.

Taffy, I'm following your progress closely and with great interest.

Regards

smorgasbord
here is the skinny on shims a .11 shim is almost as fragile .10 the difference is negligible
you have to go up to a .15 to get durability problem is that .15 shim is stiffer than
two .10
problem that can be experience in mod piston port with bridge is that you are reducing
surface area of port which alters low speed and break away
I have to wipe the egg off my face I stated earlier in this post that 07 frame was extended well I measured last night 04 and 07 are identical from swingarm pivot to
fork axle and swingarm length the 07 frame is taller in order to fit the longer fork thats
the only difference that I could see
so long VIKING

Another free mod. Base valve check plate nuts drilled to help the weak conical spring shut the check valve faster by avoiding any suction cup effect. And to open full wide faster by reducing oil cushioning in nut head. Used a Ø2.5 mm drill for the holes. (Ø .1")

.15 shims are, if the qube theory is true, 3.4 times stiffer than .1 shims.
And the .11 ones are 33% stiffer than the .1, hardly negligible. The fragibility might be about the same though. I do not know, but if the .1 doesn't shatter but get out of shape, then the .11 wouldn't shatter either but might get less deformed thanks to the greater stiffness. Or is it the other way around? .125 might be the way to go, being twice as stiff as the .1 but just moderately thicker.
One weakness in using fewer but stiffer shims would be that any unflatness of a shim would be very dominant in the stack, while if there are many thin shims in the stack, the shim unflatness would average out to a great extent. The Ø12 rod BVs with their necessary thick stacks with lots of shims must be very good for stack flatness.
Then again if the MV shim facing the piston is a .1 doughnuting heavily, then a just slightly thicker face shim might be less bad even if it is not perfectly flat to start with either.

Regards

Yes, indeed, we'll see how it works out. Regards.

Merry Xmas Taffy
I think that 03-06 chassi puts more weight on the front wheel that would explain your
woes raise the oil level and add some shims in the middle of the stack and run the
chain adjustment short and go out there and test the setup

have jolly Xmas Per

Taffy
there is not enough holding power even with a 15mm shim you need at least 22or 20
to get some edge control on 24 shims that makes better mid speed control
the fork angle is the same one other thought on the weight on the front wheel the new
tank holds the fuel further back on the frame compared to the old tank that has to account for some of lighter feel of the 07

HAPPY NEW YEAR Per

Drop the float to .70...