Low Speed & High Speed Compression

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Low Speed & High Speed Compression

Postby Deluxe » Tue Mar 22, 2011 8:39 pm

Does anyone here with a Fox Fork understand the significance of Low Speed Compression and High Speed Compression settings, how they relate to the rider and each other, or even the rebound setting? The manual talks about these adjustments affecting things like "G-outs" and "square-edged hits." WTF??!!
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Re: Low Speed & High Speed Compression

Postby Intense1 » Tue Mar 22, 2011 9:22 pm

Low speed compression controls the first half of the travel and small bump sensitivity. High speed compression controls the last half of travel and will be for large bumps (g-outs). Rebound controls how fast or slow the fork comes back to normal (rebounds) this adjustment is usually made last as it is effected by the others.
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Re: Low Speed & High Speed Compression

Postby Deluxe » Wed Mar 23, 2011 10:04 am

Thanks Intense. So on a trail like North Ridge where you build up speed in smooth sections then encounter a few 4-6 inch consecutive hits at high speed (without hoping them), do you want to increase (+) or decrease (-) lo speed compression? Would you increase (+) or decrease (-) hi speed compression? I presume you would want to increase rebound?
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Re: Low Speed & High Speed Compression

Postby Intense1 » Wed Mar 23, 2011 12:27 pm

well, generally speaking, If those 4-6 inch bumps are an example of what you would expect to bottom out your suspension then it would be the high speed compression. You can attach a plastic wire tie to the stanchion, slide it down to the seals and go for a ride. Wherever the wire tie ends up will show you how much travel you're using. Your fork may also have an o-ring or something already attached for doing just this. Increasing high speed compression should make the fork stiffer and use up less of the travel and vice versa. The low speed compression adjustment would be used more for light bumps that are using the first half of the total travel. Think of it as being able to have a stiff fork for large hits and a soft fork for small hits... best of both worlds. The rebound adjustment controls how fast or slow the fork returns to normal. Too fast and it won't track well in corners and too slow it will "stack-up" or in other words; when you hit a series of quick bumps the fork won't have time to return to it's travel and basically give you a rougher ride. If this is a new fork, I recommend going with baseline settings (should be something in the manual) and ride it a few times. The fork will "break-in" and change slightly, so there's no sense in putting too much effort into getting the perfect settings now.
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Re: Low Speed & High Speed Compression

Postby Intense1 » Wed Mar 23, 2011 3:08 pm

I should have first mentioned that the low/high compression adjustments are only for fine tuning. The proper spring rate ( whether coil or air, whichever the case may be) needs to be established first. You could experiment with the extremes: highest high speed and lowest low speed and vice versa. Once you learn how the bike feels then just find a middle ground that you like best. For instance, if the bike nose dives when you apply the brakes, you could increase the low speed compression.
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Re: Low Speed & High Speed Compression

Postby Deluxe » Wed Mar 23, 2011 3:17 pm

Awesome explanation. Where did you learn this shit?
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Re: Low Speed & High Speed Compression

Postby Intense1 » Wed Mar 23, 2011 3:55 pm

I just guessed :lol:
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Re: Low Speed & High Speed Compression

Postby sjc115 » Wed Mar 23, 2011 5:05 pm

I figure he just stayed at a Holiday Inn Express last night...
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Re: Low Speed & High Speed Compression

Postby Intense1 » Wed Mar 23, 2011 5:33 pm

Ha! I was gonna say that but didn't think anyone else would get it.
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Re: Low Speed & High Speed Compression

Postby Deluxe » Wed Mar 23, 2011 6:57 pm

?
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Re: Low Speed & High Speed Compression

Postby Jason » Wed Mar 23, 2011 7:28 pm

Intense1 wrote:I just guessed :lol:


and it was wrong... close but no cigar.

These adjusters affect the fork damping (not dampening, we are not making it wet) throughout the travel and do not have a predetermined zone where they control the damping.

Speed in low speed and high speed have nothing to do with the speed at which the wheel are traveling down the trail but the speed at which the fork is compressed.

A low speed bump example would be you pushing on the pedals up a hill, a long fast turn where a lot of weight is on the fork for a relatively long period of time, or a big roller coaster type g-out. A high speed bump would be a regular bump/root/rock/curb you encounter on the trail, it is more square edged and when you hit it it will make the fork travel very quickly.

The low speed damping will try and keep the fork higher up in the travel during low speed events so that there is fork travel left if you encounter a high speed bump. In motorcycle road racing it is possible to bottom the suspension in a turn because you are turning so hard, if the suspension is bottomed in that turn and you hit a bump you will bounce and probaly crash. It is a similar concept off road.

When a high speed bump is encountered there are small metal disks that will deflect and blow off the low speed compression and then the high speed takes over. High speed is there to prevent (for example) a 4" travel fork from bottoming on a 4" bump. The reason you don't want this to happen is usually there are more bumps behind that and you will need some suspension left over for the next bumps. The high speed compression helps keep some suspension left over for succesive bumps.

Rebound damping pays a huge role in how a fork feels. If your rebound damping is too slow the fork cannot recover to full travel between bumps and it will get packed down into it's travel. You may incorrectly blame your spring as being too soft or your high speed damping not being high enough. I would say it is safe to say that 95% of the forks I work on have their rebound damping set too high. You want to run as little rebound damping as you can. It difficult to set it perfectly by yourself but you can get close. If you have a partner you will want then to assist in compressing the suspension while you are on the bike. Then release the suspension and watch it closely and note what looks like, if it bounces a couple times like a pogo stick, then increase rebound, if it just comes back to the to and stops, decrease until you get a pogo, then increase a click or two.

The very first thing you need to establish is your spring rate. Most suspension works best with a 20-30% sag. Setting sag can be nearly impossible without a partner. First: Think about what riding position you want your suspension to work correctly. For me: I descend standing and slightly over the front of the bike, I climb standing, slightly over the front of the bike. The only time I am sitting is when the terrain is smooth and rolling or I am getting tired. So I set my suspension up while I am standing in a aggressive position, the result of this is a much higher spring rate in the front and a softer spring rate in the rear. Be sure to include a camel back if you wear one. I see most people setup suspension seated, this results in a overly stiff rear suspension and a soft front.

I went through this description backward. Like Intense1 said above, setting the proper spring rate is the first thing to do, without that you might as well throw everything out the window. Next work on the rebound and get that set. Lastly set the compression adjusters in the middle and go ride. Ideally you will have a shot loop with varying terrain and keep doing laps. Only change one adjuster at a time and maybe only a couple clicks at a time. You want to adjust one direction until the suspension starts to feel bad then go the other way until the suspension feels bad. Then you know your ideal setting is in the middle. Take notes the whole time and so that six months from now you will remember what worked and what did not.

With compression damping manufacturers do not give us much rope to hang ourselves on. You may not notice much difference from full open to full closed. Especially rear shocks, if you look at a rear Fox shock you will see "velocity tune" on the shock next to a bar graph. That is the programed compression damping. There is nothing you can do about that short of sending it back to Fox or to Push. The smaller the bar graph, the lighter the damping.

That is all I have for now, I am sure I will think of more later.
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Re: Low Speed & High Speed Compression

Postby Deluxe » Wed Mar 23, 2011 8:44 pm

You both have been very helpful. So Jason uh where can I go to get like some hands on help? I have been riding Fox Forx since 05. Never quite sure if I have them dialed in correctly. I know its personal preference to a degree but so is an M-16A2 and they didn't hand those out without some training first.
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Re: Low Speed & High Speed Compression

Postby Intense1 » Thu Mar 24, 2011 7:54 am

I need to stop trying to read the Chinese section of the manual... :P
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Re: Low Speed & High Speed Compression

Postby Jason » Thu Mar 24, 2011 9:11 am

I am not aware of any shops that do suspension setup in full. Mainly because to do it correctly it would take many hours. I currently work in the service department at Dirty Harrys and have worked in may other shops in my nearly 20 year work career. The sales guys at DH setup the suspension pretty much the same as I have seen at many other shops (although I have seen a few shops that don't do any setup). A good shop will set the sag on the suspension and then they set the adjusters in a pretty neutral position. They will then go over what the adjusters do, kinda a brief version of what I typed above. With the sag set (the most important part) and the adjusters in a neutral position the suspension is going to work pretty well.

I feel that I am still studying suspension and I feel that there is more than can be done to get it tuned properly, therefore at this time I don't want to get into advanced tuning on other peoples suspension. A lot of suspension today is air sprung, I believe there is a lot that can be done with tuning air volumes. I like Fox suspension but the Rock Shox dual air setup is really nice because how tunable it is. Being able to tune the negative spring is a pretty nice feature. Fox uses a coil spring as their negative spring, it is not a bad way to go but to tune it you will have to change the spring. On the flip side, Fox dampers are very advanced compared to Rock Shox. Both damping systems work very well but Fox is more tunable, it is very similar to a motorcycle damping system. It uses shim stacks that can be tuned.
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