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Low speed and High speed adjustment technicalities

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9.7K views 9 replies 4 participants last post by  Just1fix  
#1 ·
I have a very important question about suspension that no matter how much people talk about suspension they never discuss this one very important factor.
It is my understanding that the low speed compression adjustment is always being used when suspension is traveling and the high speed only comes into play when the pressures increases, meaning hitting a quick bump that is too much for just the low speed to relieve.
Another way to explain it is that it is my understanding that the low speed is a week relief spring and the high speed is a stronger relief spring. Just like pressure relief valves on hydraulics systems, sort of speak, right?
if that is the case, then during a high speed heavy hit, both high and low will be relieving at that point and during a lesser extreme hit, only the low speed will be relieving.

I’m truly trying to understand the low and high speed adjuster internals. Just telling people it’s a low speed and a high speed adjustment is too vague and leaves people uneducated.

Any answers would be greatly appreciated. I’ve never taken a shock apart and had the ability to dissect the facts to my question.

it is also my understanding that the low speed and high speed adjustments will only relieve that of which doesn’t bleed through the shaft valving (shim stacks) and that of what it can push passed the inside reservoirs internal pressure psi.

I believe that once I get my head around all the physics involved, I can better help others with a much better understanding.

Silimilar to why most people can’t understand why their jeeps suspension is too damn stiff. It’s because they need to use a longer spring with a lesser spring rate, instead of a shorter spring with a higher spring rate (most typical applications). However towing ability (tongue weight) will require an additional fix to that problem. Something so simple that most people get left in the dark about.
 
#2 ·
Best advise—> Search out the setup thread by Poundsand.....

get ride height correct, 16” ish.... crossover ring gaps set. Then read setup info from several people and try recommendations...

for dunes; where I ride. Ride height 16-17”, crossover gaps, (no front sway bar) 1” front, 2” rear. High speed full open front and rear. Slow speed full closed, then open 1/2 turn for front and rear. Rebound 8 clicks open for rear, and 14 clicks open for the front...

if if you want a full shock tune, there are several good tuners out now that do great work on the X3 shocks...

also, the stock upper springs on the cars suck and keep sagging.. replace them, or the whole set with eibach etc...
 
#4 ·
Although Ed's post is quite lengthy, he covers this topic pretty well, along with many others you never thought you needed to know. I suggest you set aside some time to read that through and ask a specific question of the forum if his posts haven't satisfied you desire for education. Starting another one may not be the best option at this point.
 
#5 ·
So is the low speed adjustment always being used and the high speed only comes in play when hitting hard bumps, or jumps, or g outs or whatever? Meaning when you hit something hard and fast, that both low and high are relieving? But only the low is relieving on soft hits?
 
#7 · (Edited)
So is the low speed adjustment always being used and the high speed only comes in play when hitting hard bumps, or jumps, or g outs or whatever?
You have the general idea. One could write volumes about how a shock works, but I will keep it brief.
As stated, low speed and high speed refer to shaft speed, not ground speed.

Oil flows through the adjusters due to the shock shaft displacing oil. That oil has to go through the adjusters and into the reservoir.

Low speed adjusters are simply a needle and seat design. Think of a conical needle and orifice.
It is not spring loaded, it is a fixed opening set by your clicker position.
The adjuster setting determines how far the needle can unseat from the orifice.
The low speed adjuster can be fully closed and the shock will still work because the oil can flow through the high speed circuit.
From fully closed to half open, the change in flow is significant. From half open to full open the increase in flow is less.
In other words, it is not a linear adjustment.

High speed adjusters consist of a small piston and small shim stack.
The adjustment changes the preload on the shims, which determines how much pressure it takes to open the shim stack.
The high speed shims are tune-able and can be "revalved" just like the shock piston shim stacks.

There are many different designs for these adjusters, but they all do the same thing.

Here is a cutaway view of a KX450 adjuster:

Image
 
#6 ·
Low speed adjustments is low shaft speed, rolling terrain, used all the time, and the last bit of damping before you bottom out, since the shaft has slowed down as you bottom out or G out. Keeping this one open will let you bottom out bad...

high speed is high shaft speed. Ed describes this as square edge impacts, Sharp fast hits. The shock shafts are moving fast... the stock valving is harsh, that’s why you can leave it open for the most part.

Lots of small sharp impacts at slow speed is where the stock setup sucks. I’ve installed 2 of the Walker links on the rear bar, this helps a lot with cross chop where you would get tossed around a bit... I’m hoping to save up enough spare change to get my shocks done this summer......
 
#8 ·
Scott F, thank you for that explanation. That is exactly the answer I was looking for. Granted what you are saying is completely accurate for my XRS rear fox shocks. I greatly appreciate the time you took to help me understand this. This I can confidently say has summed up a huge unknown thing amongst most people.
It sounds like the high speed shim stacks will only flex or bleed by only when they are hit quickly and abruptly and depending how much pre tension adjustment I allow via the high speed adjuster.
which again in other words, means the low speed is always being used and the high speed only depending on the level of heavy oil flow, per fast shaft movement, for the most part.
Right?
If that is the case, it sounds like I will need to set my crossover ring adjustments just right to be the better non bottom out protection, if I also want to make my stock shocks and Eibach spring combo to work the plushest.
 
#9 ·
The high speed shims will open when two things happen, the flow rate of the low speed orifice is exceeded, and the pressure is sufficient to crack open the high speed shims.

The low speed orifice is tiny compared to the piston ports. It only has to flow the volume of the shock shaft displacement.
On our Fox bypass shocks, the bypass zone is primarily responsible for bottoming resistance.
 
#10 ·
Thanks again Scott. I am very happy to have this finally explained. Not many people understand what you just explained. I completely understand what you mean by shaft displacement.
And yes, I get how the shock size, reservoir psi, spring rates, compression adjustments, rebound adjustments, crossover rings, valving, oil types, internal by-passes, and preloads all play quite the suspension science.
That’s what makes it such an addictive fun challenge.
Thanks again.