|   help! compression damping | iridebritney
Oct 20, 2002 10:19 AM | | can any one tell me, does compression damping make a shock firmer under all conditions (if so what is the difference between this and increasing spring rate) or does it ramp up the firmness under big hits? i have a fox rc with a 400 spring. it's a little soft for me so can this be compensated for by running a higher preload and compression damping, without losing small bump performance?
and before anyone is smart enough to point out that i should be riding and finding out for myself, i'm out for the season with an injury so i won't get to ride my first full sus dh bike till next year. |
| re: help! compression damping | sim0n
Oct 20, 2002 11:53 PM | | here's my bit of confusion for the day:
The purpose of damping is really to damp out unwanted movements. Compression damping performs two functions, one is to reduce movements of the "chassis" ie bouncing from pedalling, the other is to reduce movements of the wheel ie to stop it leaving the ground when you go over a bump. These two functions can be thought of as "low speed" and "high speed" damping" The real difference between springing and damping is that the damping varies according to the speed you are travelling at. Compression damping is also useful to help to stop the suspension bottoming out, since any damping adds to the force needed to compress the suspension. Generally on bikes the knobs on the shock control the low speed damping and these have only a small effect on the high speed. High speed damping is normally set internally. Increasing the high speed damping would make the shock firmer on big hits, but i think on the fox rc the adjusters only control the low speed damping so you will end up with a slightly harsher ride. It is better to get the correct spring and preload it to the right sag. If this is wrong then no amount of fiddling with the damping is going to help.
simon |
| re: help! compression damping | iridebritney
Oct 21, 2002 7:20 AM | | so is the puropse of increasing the (low speed)compression damping adjustment on my fox shock to reduce pedaling bob at the expense of suspension performance over the rough? |
| yep | sim0n
Oct 21, 2002 12:17 PM | | but it won't change too much the performance over the really fast rough stuff
simon |
| re: help! compression damping | fh
Oct 21, 2002 4:29 AM | | exactly what Simon said, if you run excessive preload and compression damping to patch up the spring rate your small bump sensitivity will be compromised, if not completely thrown out the window. Get your springrate correct the first time and make tuning the fork easier and it can be done properly with no nasty side effects that can't be dialed out.
Spring rate affects the flow through the fork's damping unit, and therefore the both combine to affect the overall rate of compression (and rebound for that matter). But don't confuse this with the opposite situation of damping affecting the spring rate, it's not the same. You really do need to set the spring rate independently of everything else, so maybe you need a new spring. |
| The difference between spring rate and damping | OldSchool
Oct 21, 2002 10:21 AM | | Assume a shock with a spring and damper. Assume this shock experiences a sudden step-change in force.
(a) Spring Rate.
Before the step-change, the displacement at equilibrium was and X. After the step-change, displacement is at Y. Ergo, the change in travel = Y-X. If you change the spring rate, the change in travel will be different (more or less depending on whether the new spring is softer or firmer).
(b) Damping.
If you change the damping, but not the spring rate, the final equilibrium displacement will not change. It will still be Y-X. However, the time to reach equilibrium will change. If the shock is frictionless AND undamped, it will vibrate forever. If it is over-damped, it will take too long to run through its travel from X to Y.
Tuning the damping is a compromise between excessive oscillation and too "dead" of a feeling.
Does this help? Or, am I just being too much of an engineer.
Tim |
| The difference between spring rate and damping | iridebritney
Oct 22, 2002 6:30 AM | | i'm an engineering student too and understand the principals of how true damping works but found this to be quite far from the way in which my shock performed. thanks for your help though! having read and pondered all feedback i have come to the conclusion that the term compression damping is used losely in respect to a fox shock. but maybe that's the engineer in me. |
| re: help! compression damping | ridebackcountry
Oct 22, 2002 12:09 AM | | first of all: everyone has one good point or another, but we need to combine minds or something cause no-one's got the full picture. PLEASE if someone dissagrees w/ what i say hit me back. i dont know your fork, but if you run a dh settup you must have a coil w/ hydralic damp.. spring rate & damp. are seperate entities which work together to help you control your bike. spring rate is the amount of pressure reqired to move your fork through a set distance (i.e. 1 inch). your spring should be set to fall ~ 25-33% into your forks stroke w/ you sitting on your bike STATIONARY (around 33% for dh). of coarse this is a thing fine tuned by riding style & personal preference. the spring is to absorb the energy of an impact; rebound damp. is to control the rate at which that energy is released back into the frame. basic comp. (not things such as the 5th element shock which uses low-speed comp. to cancel out the unwanted low-frequency inputs of pedal strokes)is used to ramp up your suspetion on larger hits; primarily the larger the hit the more the increase in this ramping effect. some systems are a bit more complicated than this, ie manitous' tpc+, but this is the basic overview: your oil is sealed in a chamber w/ a valved plunger at one end. that plungers valving (size of hole in plunger) is decreased or increased when you increase or decrease your damp., respectively. comp. damp. ramps up your sus. by increasing press. in your oil. when your fork is comp. quikly on a big hit the plunger pushes through the oil quikly. only so much oil can move through the valve at a time. as the plunger pushes through the oil, the oil cant displace itself past the valve fast enough to compensate for the movement of the plunger. this causes the pressure to increase w/in the oil chamber, thus slowing the rate of cmpression as the fork progresses into its travel. depending on the viscosity of your oil (weight) this progression will be faster or slower. this is the aplication of high speed damp.. you asked 4 questions: 1) comp. damp. only makes your suspention firmer when compressed quikly (high speed comp.), but not when compressed slowly over a period of time (ie pedal stroke, or a 400lb dude sits down on your bars slowly)
2) say you take a 3' drop; your fork compresses 2", then takes another 3' drop before the fork has a chance to rebound. w/ a normal comp. damp. sys. (not press. sensitve such as manitous' TPC+) & stait rate spring the second drop will comp. the fork another 2" w/ the same plushness as the first drop. a rising spring rate is created by a spring becoming thicker throughout its' length; causing the thinner lengths to compress first and the thicker parts to compress later, progressively. so, on the first hit the fork may compress 1.9" (no noticable diff.), but on the 2ndhit it may only comp. 1.3", due to the now increased spring rate. this makes consistant, big hits harsher & harsher, yet makes single huge hits less painfull by not letting the fork bottom out. the only difference between comp. damp. & rising spring rate is comp. is dependant on speed of compressions, no matter the point at which the fork is at in its travel, while rising spring rate creates more resistance as you progress through the travel. you always want to rely on your spring to absorb the energy of a hit; so a strait spring rate is better for constant hits to provide a better small hit sensitivity, while a rising spring rate is much better suited for dirt jumping, freeriding, & overbuilt dh runs p.s. a) even a srait rate spring will have a slight rising rate (ie thats what preload is all about) b) air springs have a natural significant rising rate 3) i think i've already answerd this 4)set your preload to~1/3 of your forks travel (give or take a little for personal preference; less for big hit performance, more for sensitivity. use your comp. damp. to tune your ride to your preferences only after youve found your preload (this may not be easy; you may need to adjust each together or seperately, depening on how you feel your bike). all your final adjs. must be made by trial & error on the trail. recording what works for you & reading up a bit more on the technolegy available will help you more than anything. in a sport that demands you to be in perfect sync w/ your body & equipment complete understanding is absoluely priceless. i'm a bike freak and a thinking of starting a home buisiness. i have no formal training. i hope ive helped you and anyone who reads this. i would apritiate any feedback, good or bad, on this response |
| re: help! compression damping | iridebritney
Oct 22, 2002 6:46 AM | | i have little experience of riding sus with adjustable compression so i will not disagree with what you have said, which was very comprehensive, thanks for taking the time. although i follow what you say my limit experience of my fox rc rear seems to agree more with the first posting, ie, that the compression adjustment makes more of a difference to low speed hits. it feels noticable firmer under low speed hit (pedal forces) while not improving the performance under big hits substancially. though as i say i haven't been able to test thoroughly and wont be able to until next year thanks to my efforts at amature bike lumberjacking last weekend.
one other thing, what did you mean when you said preload made a shock more progressive? it was my understanding that applying a preload of say 100lbs would mean that the shock would react only to the ballance of the force applied above that 100lbs. after that 100lbs has been exceeded, does the shock not still follow the same spring rate as without the preload (asuming linear spring rate)? |
| re: help! compression damping | ridebackcountry
Oct 23, 2002 1:24 PM | | your right about the preload. was tired when i wrote that & i'm really not sure what i was saying there, so dissregard that. oh, and by the way i noticed i had said that tpc+ is pressure sensitive: it's not, it's possition sensitive. |
| So what would you recommend for this... | pmoon
Oct 22, 2002 10:44 AM | | Very nice explanation. So what would you do in this position?
I have a Manitou Black Elite 120 that came with the stock medium spring. Using that spring the sag seemed about right but on a small drop (2 - 3 feet) or loading up for a bunny hop I would bottom the fork. I replaced the spring with the firm spring from manitou. Preload still seems okay, (the spring actually seems to be dual rate spring) but now I am not getting nearly as plush a ride. Would you recommend putting a heavier oil in to increase the compression damping and going back to the softer spring? |
| So what would you recommend for this... | iridebritney
Oct 23, 2002 1:25 PM | | putting a heavier weight oil in to increase the compression damping would make the fork feel harsher, not more plush. if you're happy with the range of damping adjustment you have then you shouldn't change the weight. increasing the oil level in your fork will make it more progressive, so you could run the initially plush med spring without it bottoming as easy. but that only applies to poen bath psylo or marzocchi style forks, isn't the damping cartidge in your manitou sealed? in which case the oil in your fork is for lubrication not damping |
| So what would you recommend for this... | ridebackcountry
Oct 23, 2002 1:39 PM | | i've got the same fork, and had the same problem. with only 120mm of available travel to work w/ & no rising rate springs to be found (to the best of my searching abbilities) it becomes a lesser of two evils sittuation. manitou says no oil over 5wt.; i run 7 wt. i recomend caution when using over 5 wt.; as i've heard some horror stories of destroyed dampining units. the thicker oil helps a little. i weigh ~ 170. i run the medium spring for almost everything, especially when using the 100mm setting. if i plan on doing a day of hucking i switch to the heavy spring. on the trail though, i would gladly bottom out on the bigger stuff in trade for better response and control |
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