kg/mm - lb/inch

[Papakeith]

Me being a newbie to the wonderful world of suspensions, I was pondering a couple of things, and wanted to post here to see if I had my thinking straight.

1.) If I have a spring that is rated at .54 kg/mm multiplying .54 by 2.2 would give me lbs/mm. Then by multiplying that product by 25.4 should give me lbs/inch.
Is this correct?

2.)When a spring (lets use .54 again) is compressed it takes .54 kg of force to compress it 1 mm. If I compress it another mm(2) does it take a total of 1.08kg of force to complete the task? Or, am I missing some piece of the puzzle?

3.) I'm trying to fully understand the difference between having a spring at one rate with a little bit of preload, and running a lighter spring with more preload. So, I kinda need to understand #1 and !2 before I can finish mulling over #3.
thanks

 

[Jeremy Wilkey]

PK,

LBS/INCH to kg/mm

lbs/inch/55.8=kg/mm

 

[Papakeith]

Thanks Jeremy.

Ok, I was on the mark with #1. Even with my longer process ;)
What about #2? If it takes 1.1 kg of force to compress a spring 1 mm Will it take 2.2kg of force to compress the spring 2 mm from a resting state. Or is there some exponential function happening?

 

[Anssi]

Originally posted by Papakeith
If it takes 1.1 kg of force to compress a spring 1 mm Will it take 2.2kg of force to compress the spring 2 mm from a resting state. Or is there some exponential function happening?

On a so-called straight-rate spring, yes (approximately). On progressive springs there is a non-linear thing happening.

 

[yzboy]

F= KX
where
F = Force (kg)
K = Spring rate (kg/mm)
X = Distance traveled (mm)

K (spring rate is constant)
This is a linear function!

 

[marcusgunby]

3 -you will not find a great deal of difference between the 2 models,
A stiffer spring with less preload will hang lower in the travel when riding.

A more preloaded softer spring will hold up in the travel and then not be able to control the force upon it ,but at this point the airspring is playing a major roll anyway.I like softish springs with a tad more preload than a hard spring with little preload.I feel a softer spring is more active -the harder spring may give a more dead feel

 

[Papakeith]

I wanted to see how two different rate springs compared to one another. So I created this chart. It shows a .54 spring with 5mm of preload, and a .52 spring with 20mm of preload.

 

[marcusgunby]

So you can see the softer spring could be used and it will hold the front up better, and suffer no disadvantage as it goes deeper into the stroke.

 

[Papakeith]

Alright, but I also wonder how much you can compress a spring before the space between the coil is non-existant. Or is there more than enough even with the extra preload?
Granted 20mm is a fair amount of preload, but I wanted a comparison.
:think:Maybe a chart with the same spring with different preloads to compare function would be helpful. BRB

 

[Papakeith]

This one shows a much better comparison between the two springs .52 and .54 with varying preloads from 5mm-20mm

 

[marcusgunby]

Whos a clever boy then! :worship:
If you measure the space between one coil and the next and mutlipy it by the no of gaps you get the approx travel of the spring without coil bind-then minus the preload.Then compare this with the travel on the forks-approx 300mm.Generally it takes alot to get coild bind.

 

[bclapham]

Marcus:

1) can you explain the terminology of hanging up or down in the stroke? thus, if the fork is hanging 'low' in the stroke is the wheel closer or or further from the mudguard than it should be? i think it means further (ie suspension fully extended)

2) to what extent can we get away from the "suggested" spring rates by running too soft springs with slightly more preload?

3) can we get around running these slightly off spring rates by valving and clicker settings, or do we quickly end up in a whole load of trouble, especially in the rebound department?

i think we went through number 1 when i was figuring my oil levels, but i never really grasped it so please go slow.....for example with the fork you can offset bottoming because of a weak spring/weak compression by adding more oil and letting the airspring take over. conversly, if your fork spring/ compression are too stiff we can hopefully run less oil to get more useful travel from the fork and rely less on the airspring....which one of these senarios is the worst case, or are they both as bad as each other? and i also assume one of these extreames is what causes this "mid stroke harshness" we always hear about.

sorry for my drawn out questions, maybe you guys should write a book and then i can leave you all alone! :aj:

 

[Papakeith]

I think I can answer #1. Hanging low in the stroke would be if the -in this case fork- was compressed somewhat in a static state.

#2 is kinda what I was trying to figure out with my charts.

#3. I'm starting to believe that nothing is constant. That you can be off the "standard" by a bit on the spring/preload/comp and the results will still be ok.

 

[marcusgunby]

bruce hanging down in the stroke refers to when the fork is using alot of travel already and has little left for bumps.

2 i would say you can go one or maybe 2 rates different from the charts-then again most charts vary more than this so who knows :moon:

3 no one has a perfect setting so trial and error is the only way.

when ever you have a issue you need to focus on the part of the problem-hanging down in the stoke will be fork spring stiffness/preload and LSC as the airgap only comes in late in the travel.

midstroke harshness is a good topic and one i think of alot-most of my settings dont have much midstoke harshness, but my settings may hang to lower in the stoke and not have enough bottoming resistance when the bike is heavy with mud.Its a balancing act.Most modern suspension is set up for SX and has alot of midstoke harshness.I wish i could get suspension to soak up landings into huge holes and yet still soak up ripple bumps on acceleration.SO far i ahvnt found the magic pill but im determined to.Im thinking of trying some enzo tanks on the forks.

 

[russ17]

Marcus! I was just talking about those sub tanks over with John.I thought that they would work just great for a woods application.Basically
trying pretty much to get the same effects that you mentioned. I figured you would be able to run lite valving and then high oil levels for bottoming.or maybe even some sort of air check valve that would blow on fast hard hits.

 

[bclapham]

i see, so hanging up in the stoke occurs when either the valving or spring is too stiff and the wheel deflects rather than compressing over the bump.

is hanging down is a closely related phenomenom to packing? for example hanging down is caused by too little compresion damping whearas packing is casued by too much rebound, but they have a similar nasty effect?

yes or no's will do here!;)

 

[marcusgunby]

yes yes yes:))

 

[Papakeith]

bump

Just because I was thinking about this stuff again. :)

 

[Pete Payne]

Yep --- good thread!