high speed / low speed

10 replies to this topic
  • sirthumpalot

Posted October 08, 2001 - 01:14 PM


Can someone give me examples of what is considered "high speed" and what is considered "low speed" as far as damping goes? I would assume that a square edged bump would be more towards the high speed side of things and something like the whoop section or the face of a jump would be more low speed type stuff, but can anyone give me a technical explination of how the separate adjustments on the shock work for each and how much one adjustment affects the other?



Posted October 08, 2001 - 03:09 PM


High speed is considered braking bumps (very fast) and low speed would be like whoops.
It is fun to play with the clickers on the shock and it does make a difference, but got in 1/3 turn increments.

  • DaveJ

Posted October 09, 2001 - 07:55 AM


Sir Thump,

When you smack something hard, say an edge, it's a high-speed movement. Even tiny ripples in the dirt that move the wheel rapidly are high speed.

Low speed is mostly in effect when you're doing jumps, landings and whoops.

Soft dirt can make a big difference since it really removes the aggressive edge that these bikes are known for. Or, the pain we encounter when our hands are beat from high-speed compression.

On forks, you have to modify the actual valve stack itself to change the characteristic or balance between low and high speed. However, you can switch over to a valving unit that allows you external adjustments for both high and low speed. About $250.

The shocks on these bikes come with both high speed and low speed settings.

Each controls how much oil is flowed around or into the valve stack. In other words, the screw or knob (nut, whatever) is in sense, operates as a by-pass valve.

I can ramble on about how they work and how rate is measured, but I think your question is really when do you adjust one or the other.

In general, low speed is a setting that controls how much inertia in the forward movement of a bike is lost when encountering a series of bumps, (read that again until it makes sense).

Let's say you have 25 yards of distance in which there are 10 bumps. When you go over these 10 bumps, you are going to lose some forward movement. This is caused when the bike moves up and down. In short, each bump is attempting to send you back in the direction you came from. These bumps are conquered with power and forward inertia.

Believe it or not, when you stiffen a suspension, you are in effect lessening the amount of movement the bike will go through when it encounters the bumps. This is why a stiffer suspension will allow a bike to travel faster from point A to point B. The best example of this is when you see a pro take on a series of whoops. The wheels of the bike don’t’ actually following the terrain, but skip over them. Forward movement is maximized.

You can actually experiment with this by holding RPMs over a fixed section of track, or series of bumps, having a friend time you from point to A to B. Adjust the suspension to achieve the least amount of time. Simple as that.

This of course doesn’t mean that the bike with no suspension is the fastest one the track. At a certain point, you want the wheel to move when it hits a bump. This allows the wheel to stick to the ground for traction, and allows the least amount of the bike to move when a bump is hit. This “least amount” is often called sprung-weight: the mass of the bike that moves when you hit a bump. The less the amount of mass that moves when you hit a bump, the better your forward movement is maintained, and the more comfortable your ride.

This is when the nirvana between speed and comfort come into play. Which is usually a factor of high-speed compression setting.

If a bump is encountered and everything from the tip of wheel to the top of your head moves in unison, you won’t be riding that bike for 24 laps around the track. You want the bike to absorb a portion of the bump with wheel movement, with little or no movement in the body. When large bumps are encountered, you lose - and you either take it like a man, or jump over them.

Which really means that in most cases, your high-speed compression setting is a matter of comfort. In theory, you could use a curb as a test bed to measure against. Hit it at 5 or 10 MPH and note how much it moves the bike and how much it hurts you. Repeat the test with a variety of settings until you find a balance between quickest speed and comfort.

This why the factory bikes are so plush yet considered “firm”.

Let me know if you want more technical information on how to modify the forks.


[This message has been edited by DaveJ (edited October 09, 2001).]

  • Scott_F

Posted October 09, 2001 - 09:10 PM


Dave's Zen explanation was a little unclear about how the shock compression adjuster works. Modern shocks have always had high and low speed circuits in the adjuster, and now in recent years we have externally adjustable high speed. The low speed is a simple needle and orifice circuit, while the high speed circuit is a shim stack in the adjuster. All the adjuster nut does is put preload on a spring behind those shims. This in effect determines at what pressure the high speed shims begin to flow oil.

Scott F '02 YZ250F < '01 YZ250F < '00 YZ426F < '99 YZ400F <'98 YZ400F

  • Scott_F

Posted October 09, 2001 - 09:22 PM


High and low speed impacts are often confusing. It is better to think of shaft speed and internal hydraulic pressures. The highest shaft speed I can think of is a flat landing from a high jump. That impact forces the suspension to collapse at the highest possible rate. IOW, a lot of travel in a very short time. Other HS hits are slamming big curbs, pot holes and whoop faces.

It is true that little ripple bumps, chop and washboard terrain can be high speed impacts at high speeds. But due to suspension friction and too much low speed damping, the high speed shims don't come into play. Then the suspension hardly moves, and your teeth chatter.

Typical slow speed hits are gradual takeoff ramps, smooth jump landings on downslopes, and mild G-outs.

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  • DaveJ

Posted October 09, 2001 - 10:26 AM



First, thanks for clarifying what I left out.

Secondly, I may disagree with the rod speed of a flat landing verses the curb strike,
(If I understand what you said correctly). Seems like a flat landing would produce a pro-longed high-speed movement, but would not produce a rod speed as fast as a curb strike.


And I agree with the ripples not effecting the high-speed stack. My complaint was more along the lines of the ripples or bumps common to hard-pack trail downhills. Perhaps they are larger than just a "ripple". But in these cases, I was able to dial out the hand damage effect via high-speed.


  • Scott_F

Posted October 09, 2001 - 10:54 AM


A curb hit will use 3-6" of travel in a very short time. Landing flat can use 12" of travel even faster. It really depends on the particular impact, gravity and the vehicle speed.

  • sirthumpalot

Posted October 09, 2001 - 11:26 AM


Thanks all for the information. Scott F I think you answered my question with exactly what I was looking for. If the low speed screw controls a needle valve and the high speed controls the point at which the needle valve is bypassed and oil is sent instead through a separate stack of shims then this tells me exactly what the difference is. It also tells me that they should have called it "slow speed" and "slow speed threshold" or "slow speed pressure releif" or something like this. Would have been much clearer. Posted Image

I can definitely see landing from a jump being high speed, especially if your slow speed setting is set on the stiff side (small needle valve opening). Think about coming up short on a double. If all of the oil in the shock cannot squeeze through the needle valve in time and the pressure is great enough to start bypassing it then that would definitely push oil through the high speed oil path. I'm sure the same is true of landing flat from a higher jump. For example, say you come up short on a big table top and fall flat from a peak altitude of 12ft. Do a bit of math and that comes out to landing at 27.7 ft/sec in the verticle direction. If I were a gambling person I would gamble that this would be enough to use the high speed circuits.

Now that I know what I'm twiddling with I'll have to start playing with the settings to see what the results are. Thanks guys!!

  • Scott_F

Posted October 10, 2001 - 04:31 AM


Sir, you still don't have it quite right. The high and low speed paths in the compression adjuster are separate, and don't affect each other. The low speed bleed (needle) is always open. The high speed circuit opens when the spring preload (nut) is overcome. That's when the shims come into play. You have the right idea about pressure though.

  • sirthumpalot

Posted October 10, 2001 - 05:59 AM


Scott F, first thanks for the conversation. This is very educational for me so far. Posted Image

I do see your point but here is why I believe that one will affect the other. Lets use the half way point as a reference, so begin with the needle valve exactly half open. Also assume that the high speed adjustment is not adjusted during this example. If the needle valve is then open further than this (say max open) then given equal pressure more oil can flow through for any given time slice. This means that the shaft must move that much faster before enough pressure is built up to start operating the high speed path. This effectively changes the speed at which the high speed path begins to be used. Now close up the needle valve almost all of the way. Now very little oil can get through and it will take a relatively slower shaft speed to build enough pressure to force the high speed side open. This will effectively lower the speed at which the high speed path begins to operate.

Also when the high speed side is operating, assuming it doesn't shut the slow speed path off, then the total oil flow will be the high speed path plus the low speed path. For a small needle valve opening the difference may be relatively insignificant, and perhaps having the needle valve open very far may make a significant difference (or maybe it's still insignificant?). In any event, if both paths operate at the same time then I believe that they would affect each other.

I could still have this all wrong and please let me know if I do, but this makes sense to me logically. Posted Image

[This message has been edited by sirthumpalot (edited October 10, 2001).]

  • Scott_F

Posted October 11, 2001 - 04:44 AM


Sir, that was a very nice and accurate explanation of how it works. When I said they don't affect each other, I meant they are separate circuits, and adjusting one does not adjust the other. But you are right that the low speed setting would affect the opening of the high speed circuit.

I believe that when the high speed circuit is fully open, it would flow way more oil than a fully open low speed bleed needle.

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