Guide to suspension forks

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supersonic
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Guide to suspension forks

Postby supersonic » Tue Dec 02, 2008 12:39 pm

A lot of people lately have being asking about suspension forks and a few for a guide of terms, so I have written a short guide explaining the basics. It is not meant to be exhaustive, but aimed a little deeper than 'twiddle it so feels nice'.

SPRINGS

Coil - Wound metal wire used as a spring medium. Heavier than air, but usually makes a smoother fork as no tight seals are needed to keep the air in.

Air - The stuff we breathe! When compressed is used as a spring medium which is simply adjusted by changing pressure via a valve.

Preload - Load applied to the spring by compressing it, usually via a wind down adjuster on a coil fork. Used to set sag. Preload does not change the spring rate, but does change the force required to start the suspension moving (and hence how much the fork sags).

Positive spring(s) - Main spring(s) that supports your weight and extends the fork after it has been compressed.

Negative spring - Opposes the positive spring, and works against it. Prevents top out, and if adjustable (ie RS Dual Air) can be used to tune initial stroke of the fork and overcome seal stiction.

Spring rate - Force required to move the spring a set distance. Coil springs are usually graded soft, medium, firm etc (or are rated as force needed to compress the spring a given distance. EG. 500lbs inch) whilst air springs are altered by changing the air pressure.

Spring curve - How the rate changes through the fork travel. Coil springs are usually linear, while air springs may have a 'rising' rate (as the fork compresses the air volume decreases making the fork harder).

Sag - The amount the fork compresses under static load (ie with you sat or stood on the bike). Correct sag allows the fork to extend into hollows, increasing traction. Can be adjusted via changing spring rate (ie changing spring or air pressure), or preload. Usual values are between 15% and 40%, longer travel forks generally utilising more sag. With coil forks, preload should only be used to fine tune the sag - big changes should be made with a spring change.

Travel - Maximum positive movement of fork. XC forks usually have 80-100mm, general trail forks 100-140mm, AM forks 130-160mm while freeride and DH forks usually 160mm plus.

Bottom out - Clunk when fork reaches end of travel (compressed). Usually a bumper is there to prevent metal on metal damage. A bottom out once or twice a ride shows that the fork is using all of its travel effectively.

Top out - Clunk felt when fork is unweighted (extended).

Travel adjust - Mechanism to change travel of fork, ie TALAS, usually achieved by limiting extension. Often done with an adjuster dial atop a fork leg or internally via spacers.

DAMPERS

Damping - Regulates the movement of the fork and converts energy stored in the spring and of impacts to heat. Is achieved by a system a hydraulic fluid, valves, shims and gates. Sometimes these are adjustable, or internally tunable. Not all forks are damped.

Rebound damping - Controls how fast the fork rebounds (extends) after a hit. Without it the fork would recoil too quickly and not absorb the energy of the bump. Too much will cause the fork to pack down over successive bumps so adjustable damping is desirable.

Compression damping - Governs the forks behaviour as the fork compresses. With many forks this is fixed (or absent), but plenty are available with low speed adjusters, and a few (usually long travel) with high speed adjusters. Note speed refers to the speed of the damper shaft as the fork compresses, not the speed of the bike!

Low speed compression damping (LSC) - Regulates the movement of the fork under low shaft speed conditions ie from braking, pedal bobbing etc. Too little and the fork will dive through its travel when not needed, while too much will stifle small bump response and traction.

High speed compression damping (HSC) - Controls the movement of the fork under high speed shaft conditions ie big hits, drops and square edged impacts. Too little and the fork will blow through all of its travel, while to much will prevent the fork utilizing its travel on the hits you need it to and will reduce traction.

Lockout - Reduced or zero fork movement. Many have a little travel when locked to aid traction.

Lockout blowoff or threshold - Most forks have a hydraulic lockout which will 'blow off' under a big enough impact before returning to its locked state. The threshold of this lockout can be adjusted on some forks ie Floodgate.

Spike - Partial or full hydraulic lock when too much damping force is generated which cannot be dealt with by the damper. Often felt as sharp hit through the bars. Most damped forks now have shimmed dampers that allow more oil flow as the speed increases to prevent this happening.

A Tora 318 solo air:

Image
Last edited by supersonic on Fri Jul 31, 2009 17:52 pm, edited 6 times in total.

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dave_hill
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Re: Guide to suspension forks

Postby dave_hill » Tue Dec 02, 2008 13:07 pm

supersonic wrote:Negative spring - Opposes the positive spring, and tends to extend the fork. Prevents top out, and if adjustable (ie RS Dual Air) can be used to tune initial stroke of the fork and overcome seal stiction.


Can we clarify this please?

If the spring rate of the negative spring is less than that of the positive spring, the fork will extend, or remain at full travel.

However, if it is greater than that of the positive spring it will pull the fork into it's travel.
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supersonic
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Postby supersonic » Tue Dec 02, 2008 13:15 pm

Yes, with air forks it will tend to pull it into the positive travel if the pressure is greater than the positive pressure. With coil spring though, the length and rate has an effect and is pretty much how internal travel adjust systems work.

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BlackSpur
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Postby BlackSpur » Wed Dec 03, 2008 21:08 pm

Perhaps something on travel adjust - U-Turn, Talas etc.
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supersonic
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Postby supersonic » Wed Dec 03, 2008 21:13 pm

There is a line devoted to it lol. I might add manufacturers terminology soon.

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BlackSpur
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Postby BlackSpur » Wed Dec 03, 2008 21:29 pm

Whoops - must have missed it! I thought I'd double checked. I must be tired!
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Postby nicklouse » Wed Dec 03, 2008 21:32 pm

Sonic a bit toooooo RS specific

Might do some editing. :lol:

also most (many) coil spring forks also use the same seals as air ones and some use the air in the fork as a secondary spring ....
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supersonic
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Postby supersonic » Wed Dec 03, 2008 21:53 pm

Lol, like I said, not exhaustive but might make some tweaks.

IWith air forks it is more to do with the shaft rings rather than the main fork seals. They generally do offer a little more stiction.

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nicklouse
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Postby nicklouse » Wed Dec 03, 2008 21:55 pm

nope the same (even on RS) easy to swap the coil for air with no seal or bush changes.
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supersonic
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Postby supersonic » Wed Dec 03, 2008 22:01 pm

I know, I have done it! You have to replace the whole pushrod and shaft assembly to fit the specific spring, which with solo air for example uses a further tube in the upper leg. Friction is created as the rings rub on this tube - there was a noticable increase in small bump response when I changed to the Uturn assemply.

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Postby MTBNaomi. » Mon Dec 08, 2008 19:42 pm

Would anyone like to explain to me what all the "SL" etc lark is all about when it comes to sus forks?!?! Thank you!!

Naomi :D

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nicklouse
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Postby nicklouse » Mon Dec 08, 2008 19:46 pm

MTBNaomi.
in a word nothing. as it depends which mak you are looking at.

It id better to read rhe specs on the makers sites.
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Postby petercarm » Sat Dec 27, 2008 00:43 am

On negative spring, you currently have "tends to extend" the fork, which is the wrong way round.

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supersonic
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Postby supersonic » Sat Dec 27, 2008 00:59 am

Ooops yep, typo cheers!


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