Jump to content
Sign in to follow this  

lighten hs without sacrificing ls forks

Recommended Posts

How would you go about absorbing hs hits like rocks and roots with out sacrificing ls damping. I would think that a high speed shim stack is actually two stacks, the low speed and the high speed. I also am inclined to think that by increasing float, you will compromise the ls damping. Also by using a smaller diameter clamp, it will affect both stacks, so how does one go about doing it?

Share this post


Link to post
Share on other sites

How would you go about absorbing hs hits like rocks and roots with out sacrificing ls damping. I would think that a high speed shim stack is actually two stacks, the low speed and the high speed. I also am inclined to think that by increasing float, you will compromise the ls damping. Also by using a smaller diameter clamp, it will affect both stacks, so how does one go about doing it?

 

This is a complex topic, and there are lots of ways to achieve what you're after.  First consider the basics of shim stack design.  A two-stage stack does not really function as two separate valving stacks.  The crossover shim softens the low speed damping until the face shims contact the secondary "high speed" portion of the stack.  Thus the result is softer low speed relative to high speed - the opposite of what you are looking for.  A single stage stack has a higher ratio of low speed to high speed damping.  A preloaded shim stack has even more low speed relative to high speed.

 

Port size also needs to be considered.  Larger ports will facilitate a higher ratio of low speed/high speed since the port itself will not contribute to flow restriction at high speeds.

 

Hydraulic bleed is the lowest of the low speed range, so should be minimal.

 

Regarding midvalve float - you are correct that increased float values reduce low speed damping.  A high ratio of low speed/high speed damping will require a soft shim stack, large midvalve ports, and low float.

 

The final option is a high speed blow-off or bypass device.   This allows you to tune the shim stack independent of the bypass circuit.   Designs of this type are only limited by your imagination - King's bypass type automotive shocks are one example.  I'm currently testing a WP 4CS fork fitted with a high pressure bypass.  This kind of setup has a lot of potential but in my experience requires a ton of testing to get it right.

Share this post


Link to post
Share on other sites

By high speed blow off, I take it you mean that you have reduced float down to zero or close to it, and the stack will be displaced on a high speed pressure spike by overcoming a stiff spring. I have wp cc forks. I have thought about that concept, but I have not found a solution to preventing cavitation with this approach. What techniques have you employed the facilitate this?

Share this post


Link to post
Share on other sites

Ok very little float. This would cause cavitation, unless increasing ics rate(lower or no ics float) or in my case bladder pressure. Yes this greatly effects the base valve rebound but also assists with the mid valve as well. By increasing ics rate or bladder pressure it is counter productive because it essentially works the same as a high pressure tire (rough ride). So by the sounds of it, you have some float with a stiffer midvalve spring, and a softer mid valve stack. Does any of this sound correct or am I missing something or misguided?

Share this post


Link to post
Share on other sites

The thing with degressive design is that they need some kind of "float-substitute" aka bleed.

Otherwise these setups will feel very stiff on the small stuff.

Share this post


Link to post
Share on other sites

How about the cavitation issues in discussion above.....? Are my thoughts and questions on this topic sound or relevant?

Share this post


Link to post
Share on other sites

rjg, you are making some incorrect assumptions about what I mean by the term "bypass."  Zero or low-float at the midvalve has nothing to do with a bypass circuit.  For example, your compression and rebound adjusters are bypass circuits.  The terms just identifies a secondary means of flow control which circumvents the primary valve.   You seem to have some specific design in mind...

In any case, I would suggest turning your attention to what can be accomplished with changes to valving and pistons.  Design solutions (like a bypass) are seriously complicated and should only be necessary when the more straightforward tuning approaches are not effective in delivering a specific result.

Share this post


Link to post
Share on other sites

Sorry, I guess I got fixated on a certain design. I am riding off road/mx. Problem is I have to compromise for both since I only have one set of forks. I like the approach in discussion above because it seems like the best compromise.

Share this post


Link to post
Share on other sites

Yes, it is his design type that would be closest to what I have in mind. I want to attempt to make the changes to my forks myself. The kings auto shock is the style of design I have in mind, similar to what he is known for. But I will attempt to do it on a budget. I am not going for an additional piston in my forks, but using what I have currently available to achieve the compromise for mx/off-road. Currently have a stock 12 Ktm 250 sx. The rear shock is ok at this point,I am concentrating on the forks since my upper body is tiring so fast from root and rock defection. I do not want to give up much low speed compression (many enduro bikes do). I think it is possible to achieve a happy compromise for me to do both off-road and mx with this type of fork set up.

Share this post


Link to post
Share on other sites

I get the result of less HS and same LS by using a higher float 0.25 - 0.3 on Showa and kyb, weak mid valve stack and allow a large deflection there. To remain the low speed and prevent caviation you need a quite stiff basevalve stack. I used this on a lot of bikes with always good results.

Share this post


Link to post
Share on other sites

Nine-thirtysix, so far I have not adjusted float, but I might.  I have adjusted the mid-valve, took off 1 face shims (only has three I believe) and tightened up the rebound clicker. So far this has helped, but I will consider the float issue.

Share this post


Link to post
Share on other sites

I ride offroad only and I've lost track of how many times I've pulled the forks apart and tested changes.  For many of these revalves, I was using high float amounts because that is what most seem to recommend for offroad riding.

 

The high float definitely helps with rocks, roots and ruts, but I could never get them working they way I wanted.  If I made the mid-valve soft, then the forks where just too soft everywhere and had a mushy feeling.  If I firmed up the mid-valve, then the forks got harsh due to the forks compressing quickly due to the high float and then hitting a stiff mid-valve.

 

I finally tried a low float and soft mid-valve combination and knew instantly that was the approach I was looking for. The forks are still compliant over the small stuff, but have enough damping to keep me from bottoming on anything I would encounter in the woods.

 

I started messing around with aftermarket spring seats to see if I could soften the forks up a little more but use the seats to help with bottoming.  Some other projects have come up and I haven't been riding in a very long time to finish testing.

  • Like 1

Share this post


Link to post
Share on other sites

Hmm, sounds like you have walked this road before me. I will take your advice.

Share this post


Link to post
Share on other sites

I tried the heavy LS shims and lightened the HS shims as a starting point a few months ago... Let me say that Vietze is right.. When I tried a more "digressive" approach to the valving the bike beat me up on the small to medium hits.  

 

I had to ride fast and hit things hard on purpose to get the suspension to move.   It did handle reasonably well, I think that is a product of being stiff initially.  

 

Since then I have learned that it is a combination of the entire valve stack which makes the damping stiffer or softer.    I have been experimenting with single stage stacks, although I have realized that a crossover shim in the rebound is necessary to eat up the small chop, yet be controlled on the return from large hits. 

Share this post


Link to post
Share on other sites

Despite the fact that we talk about LS shims and HS shims there is no separate function of these two...

If you lighten up the hs stack, you will also lose ls damping.

If you stiffen up the ls stack, you will add hs damping too.

 

You MAY can divide a stack more into hs and ls if you use a crossover. However, this is also not entirely true since it depends on what kind of XO you use (position, diameter in relation to clamp, xo shim arrangement, thickness, diameter and so on) and of course... its still the whole stack the creates damping.

 

 

But my first post was directed to "real degressive" setups like the DDT or similar stuff that is around nowadays.

Share this post


Link to post
Share on other sites

Great thread. I have been thru total misery trying to get my MX forks working in the woods and  am currently where Bailey28 describes. Forks seem rigid at lower speeds, work pretty well when going hard in the woods then stop moving about 2.5 inches before normal bottoming. Strange but actually better than past valving.

Share this post


Link to post
Share on other sites

H-B-R what you mean for low float and soft mid-valve ???? float around 0.20 and the mid ???

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
Sign in to follow this  

×