Reshimming For A .90kg/mm Spring?

[flyguyeddy]

i will be using VTR1000 fork innards and fork lowers on my 91.

i want to increase the spring rate to a .9 and i will probably have to reshim it.

any ideas on where to start with a shim stack? i believe that these cartridges hacve HMAS valves that i will be porting and facing.

i have shims from my stock forks for parts if i need them.

[thx1139]

i will be using VTR1000 fork innards and fork lowers on my 91.

i want to increase the spring rate to a .9 and i will probably have to reshim it.

any ideas on where to start with a shim stack? i believe that these cartridges hacve HMAS valves that i will be porting and facing.

Heavier spring rate generally calls for stiffer shims on the rebound valve. Racetech usually shims their rebound GoldValves with a straight stack of 17mm shims; like 6 of them or so. .15mm thick, IIRC. I used to have a chart somewhere...

That said, I used VTR guts in my 4th gen. The VTR fork had already been revalved by Racetech, with.90 springs and GoldValves. They left the HMAS rebound valve stock; including the shim stack.

I don't think I would port the HMAS valves. The compression bleed is effected by a bleed shim at the base of the stack. Porting the valve might undesirably alter the low-speed characteristic.

I'm going to try low speed compression through a #58 drilled bleed hole on my next go-around.

-if I can find someone who can drill me some #58 holes.

[HispanicSlammer]

i will be using VTR1000 fork innards and fork lowers on my 91.

i want to increase the spring rate to a .9 and i will probably have to reshim it.

any ideas on where to start with a shim stack? i believe that these cartridges hacve HMAS valves that i will be porting and facing.

Heavier spring rate generally calls for stiffer shims on the rebound valve. Racetech usually shims their rebound GoldValves with a straight stack of 17mm shims; like 6 of them or so. .15mm thick, IIRC. I used to have a chart somewhere...

That said, I used VTR guts in my 4th gen. The VTR fork had already been revalved by Racetech, with.90 springs and GoldValves. They left the HMAS rebound valve stock; including the shim stack.

I don't think I would port the HMAS valves. The compression bleed is effected by a bleed shim at the base of the stack. Porting the valve might undesirerably alter the low-speed charachteristic.

I'm going to try low speed compression through a #58 drilled bleed hole on my next go-around.

-if I can find someone who can drill me some #58 holes.

You should be able to find a #58 drill at a hobby shop, the kind that has model trains for less than a buck! That is where I found the #52 drill bit for my ractech gold vavles. I would increase the shims on the rebound stack if you plan go using 5 w oil as this will need more rebound damping vs the stock 10 w.

[thx1139]

You should be able to find a #58 drill at a hobby shop, the kind that has model trains for less than a buck! That is where I found the #52 drill bit for my ractech gold vavles.

I have the drills, but I don't trust my wobbly ancient drill press!

Also I'm using stock Showa 3 port valves, and I want to drill the holes at an angle like Showa does their drilled bleeds. There's so little flat area on them it's hard to find a perpendicular surface to drill into. The material looks like some kind of brittle powdered metal too, not nice soft brass like the Racetechs.

[flyguyeddy]

i guess i should replace "porting" with chamfering the port holes for turbulence reduction. everyone is doing it lol

i wish i could get ahold of some used racetech valves. i dont wanna shell out the cash for them right now

[thx1139]

i guess i should replace "porting" with chamfering the port holes for turbulence reduction. everyone is doing it lol

i wish i could get ahold of some used racetech valves. i dont wanna shell out the cash for them right now

Your VFR fork probably has 3 port hi-flow compression valves already. Just swap those into the VTR cartridges. 3 port Showas more similar to the Racetech GV than many realize.

VFR Showa 3 port valves have shallow cuts in the valve sealing surface instead of using a bleed shim. (both the compression and rebound valves.)

You can swap the 3 port rebound valves for the HMAS 4 ports as well, but you must surface the valve to get rid of the bleed notches, since the rebound bleed will be taken care of by the VTR rebound adjusters.

[flyguyeddy]

i guess i should replace "porting" with chamfering the port holes for turbulence reduction. everyone is doing it lol

i wish i could get ahold of some used racetech valves. i dont wanna shell out the cash for them right now

Your VFR fork probably has 3 port hi-flow compression valves already. Just swap those into the VTR cartridges. 3 port Showas more similar to the Racetech GV than many realize.

VFR Showa 3 port valves have shallow cuts in the valve sealing surface instead of using a bleed shim. (both the compression and rebound valves.)

You can swap the 3 port rebound valves for the HMAS 4 ports as well, but you must surface the valve to get rid of the bleed notches, since the rebound bleed will be taken care of by the VTR rebound adjusters.

i guess that isnt that hard of a task to do.

so if i took the 3 port showas and put them in, i could run a shim stack similar to that of a gold valve?

[thx1139]

i guess that isnt that hard of a task to do.

so if i took the 3 port showas and put them in, i could run a shim stack similar to that of a gold valve?

No more difficult than putting in Gold Valves.

Buying all the shims in a GV kit can add up $$$ quickly though.

Traxxion used Gold Valves with a preloaded shim set before they started making their own valves.

...but that's another story.

[flyguyeddy]

what if i happened to have shims already?

i took apart one compression valve and one rebound valve today from my stock forks and the shims were all the same size with the exception of the small ones at the end of the stack. the rebound valve had 10, if memory serves me right, and the compression had 5. of course there were the check valve ones with the wave washer on the other side of the valve.

i am getting a set of vtr1000 stuff that i can rob shims from. should i stick to the same size shim like is in it now?

[HispanicSlammer]

I must admit I never believed in preloaded shims in terms of that it actually does anything. There is the same amount of metal in a prebent shim vs a strait one, the spring rate is exactly the same. All your doing by bending it (preloading it) is stressing the shim for premature failure. The rate of flex is not going to change by bending it, I mean you flatten it anyways when you clamp it down with the screw? Its a waste of time in my opionion and it shortens the life of the shim.

To me a more practical approch to getting better low speed damping is to do a dirtbike shim shack, with a small diameter shim after the first shim which allows more flex till it binds on the shims directly after the small diameter shim.

[thx1139]

Preloading the shims is done to maintain or increase the digressive tendency of the valve.

Practically all dampers that use deflected disc valving are digressive. "Digressive" meaning the shock/fork cartridge provides proportionally diminishing resistance with higher shock shaft/fork tube velocities.

The point is to provide good "feel" and resistance to pitch or dive under braking through a low-speed circuit; but a plush/compliant ride over sharp-edged bumps as the shims blow open to activate the high-speed circuit.

Stock setups are often compromised for a soft ride, with a light stack that opens early. The trade-off is increased brake dive and less feedback. (I'd count the 3rd/4th gen VFR in this category.)

A firmer shim stack will hold the shock/fork in the low speed circuit longer. On a shock dyno, we would see this as the "knee", -the transition between steep low-speed and shallower high-speed portions of the resistance curve- ...moving upward on the curve.

More resistance at low shaft velocities = more feedback and reduced brake dive.

However a stronger stack also steepens the high-speed part of the slope. The result may be unnecessary harshness.

By pre-bending the shims, a similar raising of the "knee" is achieved. Shock/fork are held in the progressive low speed circuit longer, as more pressure is required to blow off the preloaded shims. However, once the shims open they don't require any more force to bend than the same non-preloaded stack; so the shallower high-speed slope of a softer stack is maintained.

That is the theory anyway.

In practice there are many other factors that affect the knee and slopes of the curve; it's an educated guess at best without a shock dyno.

Pre-bending the shims is actually a popular method among the shock manufacturers. Ohlins uses a stepped ring/centering shim pair to preload the shims above it in the stack. Penske uses dished sealing surfaces on their valves/pistons for the same effect.

If you have a Penske shock it probably has preloaded shims in it.

Stock KYB dampers that Yamaha used on many R6/R1s ran preloaded shims on both compression and rebound from the factory, IIRC.

Usually the amount of preload is very small. One could get carried away and build an overstressed stack prone to fatigue and breakage though. -but that is also possible with zero preload.

I tried preloaded stacks on my '96 fork and it worked well. You can feel the blow-off point. On rough pavement you see bumps as you approach them, and because of the increased feel you're anticipating a jolt that turns out a fraction of what's expected. I find myself intentionally driving over pavement seams and manhole covers.

All is not perfect however; 4th gen fork has too much bleed on the compression valves. -even with heavier oil. That's why I'm going to a smaller drilled bleed next.

For us laymen with no shock dynos, the optimum setup is either preceded by lots of trial-and-error, or dumb luck.

[thx1139]

i took apart one compression valve and one rebound valve today from my stock forks and the shims were all the same size with the exception of the small ones at the end of the stack. the rebound valve had 10, if memory serves me right, and the compression had 5. of course there were the check valve ones with the wave washer on the other side of the valve.

If you've got 10 shims on a VFR rebound valve, somebody has been in there already.

You really need to mic the shims to know what you have. A .15mm thickness shim is about 3.5x stiffer than a .10 shim of the same diameter, IIRC

i am getting a set of vtr1000 stuff that i can rob shims from. should i stick to the same size shim like is in it now?

Your best bet is sticking with a known setup, something being run successfully by others. Not many tuners are in a hurry to share that info though. the Racetech charts are a good place to start. (wish I still had one handy. ) Some of the folks on the Independent Bike Suspension Forum have copied late model GSXR stacks with good results.

Rules of thumb:

Thinner shims go on top of thicker ones, not underneath.

No more than a 2mm reduction in diameter at a time for tapered stacks, excluding the clamp shim. (except in a two-stage MX stack like HS described.)

edit: wait, thats backwards; thicker shims of same diameter go on top of thinner ones.

[flyguyeddy]

i'll mic those shims today at lunch (i have all my fork parts at work).

[thx1139]

i'll mic those shims today at lunch (i have all my fork parts at work).

What ever happened to the guy who had some Lindemann Engineering forks on the workbench. Said he was gonna grab some specs and post; -then he disappeared. It's not nice to tease!

Read the thread here.

Slo97VFR, ...where are you???

[flyguyeddy]

i'll mic those shims today at lunch (i have all my fork parts at work).

What ever happened to the guy who had some Lindemann Engineering forks on the workbench. Said he was gonna grab some specs and post; -then he disappeared. It's not nice to tease!

Read the thread here.

Slo97VFR, ...where are you???

ok i just mic'd my stuff. this is off of one side of the forks

rebound

17x.1 (10)

8.5x.2 (1)

compression

17x.1 (5)

8.5x.2 (1)

[thx1139]

ok i just mic'd my stuff. this is off of one side of the forks

rebound

17x.1 (10)

8.5x.2 (1)

compression

17x.1 (5)

8.5x.2 (1)

My stock '96 was valved much lighter. I can't find my notes tho so can't say exactly.

Your stack does look similar to one Honda used on NC30 VFR 400s:

3 port Showa

reb:

17x.1 (10)

8.5x.2 clamp (1)

comp:

17x.1 (6)

8.5x.2 clamp (1)

[HispanicSlammer]

Sorry I am not convinced, a bent shim still bends at the same rate as a strait one. The only factor that can change that is using a thicker shim or a wider shim or physically stamp ridges into the shim as this effectivly turns the shim into two smaller shims. Its the rate that does not change. When you bend somthing you limit the amount of travel before it binds or breaks, but the rate of flex remains the same, its just physics. The amount of force to flex the shim a given distance is the same if the shim is bent or not, however the shim will either bind or break sooner.

If you add a pound of weight to a fork spring it will compress so far, by adding 10000 pounds it will compress much more perhaps so much so that it binds and is probably gets damaged in the process - bent. When you take off the wieght it could be shorter in length when fully uncompressed. however if you put the pound weight back on it will compress the same distance as before - the same rate. That is why you measure the length of fork springs to see if they have been stressed shorter, the rate is the same but now your bike sits lower cause the spring is shorter - it is bent.

Stock valves do use smaller less firm shim stacks but also limit the size of the port holes quite a bit, as with Honda Valves compared to Race Tech for example. Where a great deal of damping is done via the port holes. Seems to me that honda builds compromise into its forks by underspringing and overdamping them, to accomidate a wider range of rider weights. The diagression you talk about is smoothed out by very small port holes and small shim stacks, heavy oil, and medium springs. A shock that can handle a good deal of different loads somwhat well but is not ideal.

I agree the purpose of a shim stack is to provide a digressive resistance to high and low speed bumps, (Thats the speed of the fork travel and not the bike). Most compression valves have varing sizes of shims to accomplish this in a tailered way, while the rebound valve is just a stack of same sized shims because that is a function of the rate of the fork spring. When you build a shim shack you push it up against the valve body and clamp it down with a screw so its flat, so bending the shims before hand accomplishes NOTHING in my opionion, prebending the shim cannot in any way change the ability of the shim to flex any less/more under load than when it was not bent. The only way you could possibly do that is to bent it with creases in the shim, of course it cant sit flat that way - like a ruffles potato chip.

Springs rates dont change - Test it by taking a paperclip and clamping it to the side of a table, hang a weight on it and see how far it bends, then slightly bend it and test it again by hanging the same wieght on. It will bend the same distance but just at different a different start point, in the shim stack its pushed flat against the valve, it will flex the same rate preloaded or not.

Unless I am missing an important point here such as stamping ridges into the shims? I dont see how that can work with the way valves are designed to be flat? Never seen a shim shack with ridged shims before? I have seen a top hat with a bent shim instead of a coiled spring but thats just the blow back port for the rebound stroke. Not the actual damping stack.

[Baileyrock]

Just what in the fork are you talking about??? :beer:

How do you really feel about pre-loaded shims! :beer:

[flyguyeddy]

should i stick with what i have in the front?

ill just transfer all the parts from my VFR stuff to the VTR if thats a good idea.

and face the valves, of course. i do have to remove those notches, right? rebound adjustable forks here....

[flyguyeddy]

here are the pics of the valves.

[thx1139]

Sorry I am not convinced, a bent shim still bends at the same rate as a strait one.

HS; yes, you are 100% correct about the spring rate of prebent shims. A preloaded shim has exactly the same rate as it did with zero preload. Nobody is saying otherwise.

Preloading a shim, like any spring, cannot change its rate; -it only applies a static load. A load which must be overcome before that spring may be deflected further.

Suspension springs are a good analogy. Consider this:

Suppose we have a fork with the preload adjusters wound all the way out. Let's say, for the sake of argument, when our imaginary bike is off the ground and fork fully extended, we have just enough spacer length to fill the gap but put zero preload on the spring.

Next we apply a force, say the combined weight of the bike and rider. We measure the deflection and find our fork has compressed 30mm under the load.

If we doubled the force applied, the spring would compress another 30mm.

What if we cranked our preload adjusters in 10mm. We now measure deflection again with the same load we applied before. This time of course, the fork compresses only 20mm.

The spring rate didn't change; but with the addition of preload the fork compresses less.

Now if we wanted to compress the preloaded fork the full 30mm from before, we would have to add more weight; or find some way to apply additional force against the spring.

The total force required for the same amount of travel is higher.

Let's assume we could screw the preload adjusters in even further, a full 30mm in. We then apply the same force we used the first time. Result = zero deflection. The fork does not compress.

Because with 30mm of preload, the spring is pushing the fork outward against its stops with the amount of force that results from compressing the spring 30mm.

How much force would be required to compress this preloaded fork the same 30mm distance as before? It would require exact opposite amount of force currently holding it at full extension, plus the amount required to compress the spring another 30mm. Or in other words, double the amount of force required to compress the zero-preloaded fork 30mm.

Same springs; the rate is unchanged. Yet preload has altered the amount of force required for a particular degree of fork deflection.

Preloading shims works the same way. The preload increases the the amount of pressure required to reach the valve opening point. Once open, it deflects at the same rate as with zero preload. -a shallow digressive high-speed slope may be maintined, because the spring rate of the shim remains the same.

I wish I was at my own computer now; I'd left it in Charlotte while I came home to attend an Uncle's funeral yesterday. On my PC I've got .pdf manuals from Ohlins that list some of their stacks in both preloaded and non preloaded configuration. Included are many dyno graphs of the different stacks for comparison. Very informative.

I'd setup some kind of download for anyone interested if only I had access to it right now.

Google for Ohlins' "X-stack" manual. Also the Ohlins T44 racing damper manual, and Penske 8900 series manual are out there on the web somewhere and excellent resources.

[thx1139]

ill just transfer all the parts from my VFR stuff to the VTR if thats a good idea.

and face the valves, of course. i do have to remove those notches, right? rebound adjustable forks here....

The valves in your pictures are Showa 3port valves. They are a high flow valve, like Racetech GV compression. The stack you described is much stiffer than what I found in my '96 VFR fork (also with Showa 3 port valves.) Overall, it looks like maybe a middle of the road stack among what's found in any stock fork.

Probably an ok starting point.

For comparison a typical Racetech stack may look like this:

rebound:

17 x .15 [8]

9 x .10 clamp

compression:

17 x .15 [5]

15 x .10

13 x .10

12 x .10

11 x .10

10 x .10

9 x .10 clamp

If you use the VFR valves with the SuperHawk rebound adjusters, you must remove the bleed notches from the VFR rebound valves.

The compression circuit is still non-adjustable, so either leave the VFR compression valve notches alone, or find a way to drill your own calibrated bleed hole in them to replace the notches.

[flyguyeddy]

have you ever heard of using carb jets in your lower bolt?

i read somewhere someone makes a lower cartridge bolt that has a spot in it to use a carb jet for the bleed, that way you can adjust it just by chnaging a jet and you dont have to buy a new valve if you go too big.

what are my options if i want to get rid of some of the bleed that the compression side has?

[thx1139]

have you ever heard of using carb jets in your lower bolt?

i read somewhere someone makes a lower cartridge bolt that has a spot in it to use a carb jet for the bleed,

At one time Max McAllister (Traxxion Dynamics) mentioned offering hollow cartridge bolts with different bleed orifices. I think that project was an outgrowth of a decade ago when CBR600f3s were a current racebike. Then fully adjustable F4s replaced them along with like products from the competition. Not much has been heard since.

Ducati has apparently offered compression-adjustable Showa forks with a similar scheme. The axle even had holes so the bolts could be accessed without removing the front wheel.

Adapting something like that would require more than a bolt replacement. The compression valve tower would also need to be modified with passages to direct the bleed oil. If anyone has a mod like this up and working I'd like to hear about it!

On "pre-bent" shims: It occurred to me that many people (including me) find that term misleading.

"pre-bent" just means they are partially deflected when in their installed position. They aren't permanently deformed. (not "bent".) They're flat before going on and flat when they come off. Maybe "pre-flexed" would have been a better term?

"Pre-loaded" is still the best adjective.

From now on I am banishing "pre-bent" from my suspension vocabulary...

[flyguyeddy]

damn.

so i guess what i have to do is get rid of the bleed notches and just drilla bleed hole and try it. should i start with a 1mm hole?