Specs ay K100 has 185mm suspension travel. What exactly does that mean? Is it measured from forks fully extended to fully compressed? How does translate to me sitting on the bike, compressing the forks and riding down the road and hitting a bump? Do I now only have 185mm less the compression from the bike and my weight? The reason I ask is that I would like to make a leading link ish fork for my R90/6 using the triples, fork legs and front wheel from my now defunct K100. I would like to shorten the fork legs by shortening the spring spacer and fork tube a similar distance. I am willing to shorten the spring also, if required. For the sidecar rig I am shooting for four ish inches of travel, but that sounds like what I might actually have with the 185mm full travel. I would like to use the K100 fork legs as the shocks. I am going for standard looking L.L. main frame with swingarm, just using the K100 forks legs as shocks mounted similarly to the red bike. I got the idea from the CCM forks on the yellow bike. Note that the "shocks" on the red bike mount below the bottom tree, allowing me leave room between the triples for the standard headlight mounting (remember, R90/6).

Is it measured from forks fully extended to fully compressed.
The drawing is the correct way to build a leading link.
To be bluntly honest the fist picture is absolute garbage and the top mount would probably break off very quickly. The second is not much better but would probably survive on a light bike.

Second one is genuine Rick; it's a Demon 250 scrambler from somewhere in the mid-Sixties. Here goes the usual: 'a mate had one' but sure as hell it wasn't in as good condition as the one in the pic!

If you shorten the spring you will stiffen it. That might be advantageous for your application. Just so’s you know.
Also, the difference between a fully extended fork (or any suspension unit, for that matter) and it’s length on the bike is generally called ‘static sag’ if it’s subjected the weight of the bike, or ‘rider sag’ if the rider is aboard also. Sag is a very important requirement since it allows the wheel to retain contact with the road as you hit a dip, and there are just as many dips as bumps - suspension isn’t just for bumps!

Suzi Q wrote:If you shorten the spring you will stiffen it.

The spring rate is the same regardless of the length unless it is a progressive rate spring.

glennpm wrote:

Suzi Q wrote:If you shorten the spring you will stiffen it.

The spring rate is the same regardless of the length unless it is a progressive rate spring.

The strength of a spring, leaf or coil is a function of the cube of the steel used. Keeping with the subject of your question, coil springs, the diameter of the wire and the length of the wire will give us the amount of steel used. For this whole discussion we will be talking about springs with the same wire diameter and the same inside diameter. The only thing that will change will be the length of the wire used to wind the spring. The longer the wire is the lower the spring rate. As the wire get shorter, such as when cutting the coil, the spring rate increases.

If you shorten a spring by cutting it in half, then it will only compress half the amount it did prior to being shortened, when subjected to the same load. Its rate; mm per kg or whatever, will have halved. Or if you rate it kg per mm, it will have doubled. Either way it will be twice as stiff.
Shortening a spring makes it stiffer.

Look at it this way; take a lorry spring, maybe if you stood on it it would compress half a millimetre. Get ten thousand of those springs and stack them end on end, stand on top of that and it will compress five metres.

And you'd be able to piss on the moon 

Another common misunderstanding is 'preload'. 

Firstly you cannot stiffen a spring by preloading it. It follows that you cannot stiffen suspension by winding up those preload adjusters. 
As glenpm says, a spring keeps the same stiffness regardless of the how much it is compressed. If the first inch of travel takes a hundred pounds, then the next hundred pounds will compress it a further inch and so on.

Preload simply precompresses the loose spring. Lets say you preload it by compressing it an inch. The spring 'thinks' it's under a load of a hundred pounds, that's all. So when you take the bike off the center stand, and sit on it, and subject it to two hundred pounds, say (you've just had lunch okay) the spring only 'sees' a net one hundred pounds and compresses by a further inch, to the same length it would have been before you preloaded it.


Any further compression of the spring as you set off and ride, over bumps and so on, is at the same rate because it's the same spring. The preload that you added has no effect whatsoever on the spring's behaviour.


What you have done, by adding preload, is effectively lengthen the suspension unit by an inch so your bike rides higher. That is all.


Sorry if that sounds like I am banging on, but I blame years of comic book bike magazine road tests where those big-balls road testers talked about 'stiffening up the rear' with the preload adjuster.

im not sure its as simple as that and are the fork springs rising rate as standard (been a while since i had mine apart) How To Calculate Spring Rate–and How to Understand Cutting Coils | Articles | Grassroots Motorsports
A rising-rate spring can be created by manufacturing the spring so that spacing between the coil layers varies along the length of the spring. As the spring is progressively compressed, this difference in spacing causes coil binding to occur first between the coils that are closely spaced. This initial coil binding reduces the number of actin coils (N), thereby causing the spring rate to rise. This type of rising-rate spring is very common in motorcycle suspensions.

Wrong Suzi

Suzi is wrong

Spring rate is force per unit length.
Cutting the spring just reduces it’s effective length. It does not change the spring rate

So those coil bound coils are no longer doing anything, which makes the overall spring stiffer.

You would achieve the same stiffer rate if you cut those coils off (which would be shortening the spring )

A rising rate/progressive spring is simply a spring that loses coils - and gets stiffer - as it compresses.

Shortening a spring makes it stiffer. 

And no, we're not talking about shortening it by compressing it  , we're talking about shortening it by cutting coils off - see the original post.

glennpm wrote:Suzuki is correct.

Spring rate is force per unit length.
Cutting the spring just reduces it’s effective length. It does not change the spring rate

All I can say to that is no.

Sorry, Suzi you are wrong.

Okay, you get the beers in, come back, and we'll have another go.

Lets say the spring is ten coils. Each coil compresses by a tiny amount when the whole spring is under a specific load of 1kg, say 1mm.

That means the entire spring will compress by 10 x 1.0mm = 10mm.

Cut five coils off.

Recalculate how much the spring has compressed under the 1kg load.

Has it got stiffer?

Has a sip of beer, nice pint that, cheers.

No, you have just reduced its travel.

Imagine getting ten sets of bathroom scales and stacking them on top of each other. You stand on the top. How much do you 'weigh' on each set of scales?

Twelve stone?

That doesn't mean you weigh one hundred and twenty stones in total.

(This is Britain, we mix metric and imperial like the rests of the world mixes cocktails)

glennpm wrote:No, you have just reduced its travel.

That means it must be stiffer......

I was trying to find an avatar of an interested party eating popcorn so I could display that I am having fun watching who will say what next. Instead I'm amused that no one (yet) has called damping dampening. Not to throw a wet blanket over a conversation about suspension.

It is amusing, however, to see adults continue to use it's when they mean its, and sometimes in its variations in the same sentence. 

"It's on its way," would hold up in proper grammar court. If the judge is a heavyweight, should we cut the springs beneath his high chair?

If the 'it's/its' offender is me, then I 100% blame auto-correct. There are many things I can't do (including explaining spring rates) but spellink isn't really one of them.

Enjoy the popcorn 

Suzi Q wrote:If the 'it's/its' offender is me, then I 100% blame auto-correct. There are many things I can't do (including explaining spring rates) but spellink isn't really one of them.

Enjoy the popcorn 

 

No one in particular, SQ. I can recall long part numbers, credit card numbers down to the CCV code, a girlfiend's SS number from the mid-'90s, my first telephone number in Tasmania in the '60s, but if I have to add, multiply, divide or subtract, out comes the calculator. It's all about how you shuffle the files in the headspace, and I'm not very good at that bit.

Ah, the nice bit where the beer kicks in, we're no longer too bothered about the topic that was all important a few minutes ago, we've happily and comfortably drifted into the philosophical...

Glenpm is 'right': he knows what 'stiffness' means to him, and he's probs been working with springs and stuff all his life. What he's saying, is right for him. It's not right for me, but that's my business.

To the entire cast of this thread,BRAVO! BRAVO! ENCORE, if you please

One of the things I enjoy about this forum is that the initial subject often gets hi-jacked or takes on a life of its own, leading the interested reader, and participants, down a multitude of garden paths (rabbit holes?). Another thing I like is, although there are very many different levels of understanding of technical aspects of the things we love, and are here for, for a variety of reasons, in the end we are mainly decent with one another about it if there's disagreement. Anything else would just be ego and drunkenness at the internet bar we're all leaning against.

Intermission is over!

"How do Springs Work?
Springs operate based on a principle known as Hooke’s law, which is attributed to the British physicist Robert Hooke who published his ideas on springs in 1678. Hooke’s law states that the force exerted by a spring is proportional to the displacement from its initial or equilibrium position. This relationship can be expressed mathematically as:



where (F) represents the force generated by the spring, (Δx) represents the displacement or the amount of deformation from the spring’s relaxed or neutral position, and (k) is a parameter that is known as the spring constant.

https://eeuroparts.com/blog/lesjofors-coils-springs-explained/

"Different Types of Coil Springs

Although they come across as reasonably simple, coil springs are much more than just thick piece of iron that’s been bent into a helical shape. There’s far more to them than that. For starters, springs are created to match specific vehicles’ weight and performance.
The way springs are coiled can tell a lot about their performance. Some are tuned to be linear in the way they respond to the road surface. On the other hand, progressive springs offer two very different performance profiles in a single spring.

Spring Rates

The whole progressive spring design vs. linear spring design leads us straight into spring rates. Spring rate tells us how stiff the spring is. In most cases, this rate is expressed in kilograms per mm. Here’s a simple example. Say your spring rate is 5kg/mm. That means that you’d have to place 5 kilograms of weight on a spring to compress it one millimeter.
There’s also a matter of sprung and unsprung weight, but we won’t go into that right now. These terms and many others will be covered in a more advanced coil spring guide!

Lowering Ride Height Using Different Springs

Spring rate is a factor when you decide to lower your car. As we all know, a vehicle with a lower center of gravity achieves a better grip. That’s suspension tuning 101. But, there’s a catch. You can’t just go and trim your springs to lower the car. Doing so would change your ride height but not your spring rate. As a result, you’d be left with a vehicle that pitches forward and backward far too much for its new height, thus running the risk of bottoming out.
The only way to do this properly is to fine-tune the spring rate to match the ride height. Understanding this simple concept will put you far ahead of many drivers who are just looking to slam their ride."


https://www.motorcyclistonline.com/making-sense-motorcycle-suspension-springs-tech-tips-mc-garage-technology/



Glenn

That post is wrong.
Many other spring-related articles are misleading too.

Spring rate is deflection vs load. It can be load vs deflection if you want, but it's the same thing, inverted.
So, if an engineer talks about spring rate, or 'stiffness', they're talking about how many millimetres it compresses for a given load, typically mm per kg (or Newton) or, they're talking about how many kg per millimetre deflection. 

'Stiffness' to an engineer, doesn't mean anything else. It means how far the spring will travel when you put a load on it. That's it. It doesn't mean how thick the wire is, or how many coils it has (or hasn't) or what it looks like, or what you've done to it, or anything else.

A 'stiff' spring will travel less for an applied load of, say, 1 kg, than a less 'stiff' spring.

A spring that travels less for an applied load of, say 1kg will be described as 'stiffer' one that travels further. 

There is no more to it than that.

If a ten coil spring travels 10mm for an applied load of 1kg, then that's each coil travelling 1.0mm. That's how springs work.

If you cut 5 coils off it. It will only travel 5 x 1.0mm = 5mm.

It will have travelled less for the same applied load.

It will be a 'stiffer' spring.

It will be stiffer, and the only reason can be, is that you have cut some coils (five) off.

It is much more simple than you think.

We're on the philosophical here.
A spring has, if you like, two qualities: it can be seen as providing resistance, or it can be seen as providing compliance. That may seem obvious but if we think about it, we're actually saying that it provides two completely opposite qualities. That is a defect in semantics, a conflict between two opposing concepts. How can language possibly work after that?

Let's think of pillows or cushions. They have only one real quality, which is softness. But they are springs too. They compress under an applied load, and they return after removal of that load. If they didn't, then the next sleepyhead would find a hard surface instead of a compliant one. Yet the key here is the purpose to which pillows are put: it is compliance only. We do not ordinarily use pillows for their quality of resistance and so we do not inflict ourselves with the dichotomy of semantics, as we do with springs. We simply deploy pillows where we want compliance.

The difference with those curly metal things we call springs is that, in another universe where semantics rule, we would have two different words for them: one were we deployed them for their ability to provide comfort, and one where we deployed them to provide resistance.

Why do I say this? Well, we're still on the semantics. If we have more of something, then ordinarily we expect to have more of its dependent qualities.

So, if we have more 'spring', then we should expect to have more of its dependent quality. But what quality is that? Is is more 'comfort', or is it more 'resistance'.

That is why it can be counter-intuitive to think that cutting a spring -less spring- can equal more stiffness.


Mine's a pint. Thanks.

Sorry, Hooke is correct you are wrong. so no pint from me :-)
I'm done ...

Glenn

Of course Hooke is correct.
But Hooke's law doesn't mention cutting springs....?
Hooke just says that the a rate of A PARTICULAR spring is constant, which it is.
If you cut some coils off that spring.....it's a DIFFERENT SPRING. If it weren't then A) it would defy nature and B) you wouldn't need a box with different sized springs.

What do you think happens to a spring if you cut some coils off - something must happen - what is it?

Sorry, you have already answered that question in #17

"...you have reduced its travel"


Which means (it can only mean?) that it compresses less for a given load. Which means it is......stiffer?????

What else does stiffer mean?

Okay.
You're going bungee jumping>


First go, hundred foot cord. Nice long bounce, yippee, really enjoyed that.


Next go, same cord....same cord....but we've cut it down to two foot long. 

How was that, did it feel....stiffer?

No you probably wouldn't describe it as 'stiffer'. 'Shorter' maybe, or 'rubbish'. Or anything you want, it doesn't matter.

But, the fact would be that you'd deflected a whole lot less for that same load (i.e. you) than the first jump. If you wanted to use the correct term for the super-short-but-rubbish bungee you'd call it stiffer. But, as a normal human being, you'd probably not think of it as such and say it was 'less bouncy'. Because that's got more meaning. 

Believe me, there's semantics, and there's science. The two live on opposite sides of the street.

....I don't want to get off track with this tennis match.

I just think it might be useful to establish that, if you want a spring to be a bit stiffer then you can, if other factors permit, cut a few coils off it.

'Course, you need to be clear about what you mean by 'stiffer' 

So the stiffest spring is the one that has no coils left.  Makes sense in a strange kind of way.

Whenever I have read about "spring rate", it never occurred to me that number of coils had anything to do with it; probably because I try not to get obsessed with physics beyond "fire burn and cauldron bubble" and " what goes up must come down." When it comes to shock absorbers, following adjustment instructions to get what I wanted from it is as close to obsession as I get. 

Reading this exchange compelled me to look for a definition of coil spring. As it turns out, the number of coils in a spring is essential in defining it as a spring with a certain rate. A 10-coil spring and a 5-coil spring perform differently because their total lengths vary despite being made of the same material, coiled in the same diameter with identical coil spacing. I came across a formula that plainly supports Suzi's assertion. I don't recall ever reading "number of coils" and "spring rate" discussed together. Perhaps if those articles had somehow gotten pillows involved, I would have taken more notice.

Regardless, I'd be interested in glennpm's review of this formula.

Yep, and yep again.

I think It's just dawned on me why glen is off-kilter.

In post 11 he said that 'spring rate is force per unit length'.

Which almost sounds correct, and certainly got under my radar. But spring rate is actually force per millimetre (or whatever) of overall spring deflection. I think Glen is taking 'force per unit length' to mean 'force-to-deflect-the-spring-a-specific-amount' per 'metre-of-spring' which, if that was how spring rate was measured (it isn't) then he would be correct that the 'spring rate' refers to the 'springiness' of each millimetre, or metre or whatever, of spring length (it isn't). If that were the case, then Glen would be right in thinking that the 'spring rate' or 'spring constant' was independent of spring length. 

Glen, Hooke's Spring Constant is the force required to produce a given deflection for a given spring. Not 'per metre of spring' as I think you might be thinking. Obviously, if the 'given spring' undergoes a change (i.e. somebody goes at it with a disc cutter), then the Spring Constant/Spring Rate, is allowed to change too. Even by Hooke himself.

Point-Seven-five wrote:So the stiffest spring is the one that has no coils left.  Makes sense in a strange kind of way.

It's the philosophy of it.
What you've just said equates to 'The hardest bed is the one with no pillows on it'. Doesn't sound so strange now does it?

If you're looking at your bike and thinking 'I need that suspension to cushion my ass', then more spring (cushion) is good.
If you're looking at your bike and thinking 'I need that suspension to stop pitching me off on long bends', then more spring (resistance) is good.

Either way you go into the shop and ask for 'more spring'.

Is it any wonder we sometimes struggle to communicate?

A long time ago I think I read that a coil spring is just a torsion spring coiled up to make it compact. Is that crap or is it right?

It's completely right.

Thinking of it that way helps to understand why

1: thicker wire makes a spring stiffer.

2: shortening the rod/torsion bar means that what's left, will rotate less for a given load (it'll be stiffer..... )

3: the diameter of the coils plays a part - bigger diameter coils give the same effect as increasing the length of the lever arm on a torsion bar. Bigger diameter coils make a spring softer.

Thanks Suzi,

That helps. Maybe one day it will be easier for me to wrap my head around springs, it doesn't come naturally to me.

Not your fault. There’s a counter-intuitive aspect to coil springs. They’re made of steel, which we associate with the quality of strength. Anything else made out of steel normally gets stronger the more steel you use. Think of a bracket, make it bigger, or thicker, and it’ll be stronger: more steel = more strength. That’s simple.
But, in a coil spring the steel is arranged to reveal an opposite quality - compliance. So, the more steel you have (in terms of how many coils there are) the more compliance you have or, if you like, the LESS strength you have.
‘Course, if you add more steel by increasing the thickness of the coils, the spring IS stronger. Just adds to contradiction.

So if you're riding along and the spring compresses after hitting bump, does one end of the coil (or both, or neither) twist against its support?

daveyson wrote:So if you're riding along and the spring compresses after hitting bump, does one end of the coil (or both, or neither) twist against its support?

A spring is just a coiled torsion bar, so, yes both ends twist, since neither end is fixed.

Goodo, thanks. I wondered about that all those years ago when I first read it. Hard to picture when it's lapped over itself at the ends, and funny that there's not much wear there, or that they don't really squeak there. I've never had a good look though, usually just replace them.

Wow them spring things are really difficult to nail down. However in the bottom of the fork is a little thing bolted to the bottom called a damper tube it tops out and bottoms out and the difference is called the travel. Shorten the length of it and you'll have shortened the travel. Then you'll want to read up on springs so as to know where to cut your springs to fit or chuck the spacers etc. Hopefully page two will be full of damping stuff.

K75cster wrote:Wow them spring things are really difficult to nail down.

In that regard, these springs are much easier but production using them seems rather slow.

Wow! Wooden springs, I woodn't know how to set the sag on them. But now I know why bamboo sheets are available. Wood you sleep well on them?

Suzi Q is right: shortening a spring makes it stiffer. It's the same phenomenon than when you're playing harp with a ruler at the angle of a table. The ruler is a flat coil spring.

Yet there's a way to stiffen temporarely a spring by adjusting preload: that's when coils are joining when compressed: then the spring response is the same as if it was shortened. This was common on off road bikes in the 70s, with long travel suspension: a short spring with coils very close was added to a stiffer spring: on low bumps, it was providing comfort, but on harder compression, the coils of the short spring were joining and only the stiffer spring was working.
Most of the fork springs use these joint coils so you can easily compress the spring to install it into the fork stanchion. It also has the function to always compress the spring and never let it free, it avoids noise and resonance phenomenon to appear.

The theory certainly states that the ends of a coil spring will rotate relative to each other as the spring compresses and extends. It's not hard to imagine that being the case, without torturing your brain with the mathematics of it. Thing is, as already mentioned, you never seem to notice evidence of this - wear at the end of the spring/bearing surface - on the bike, despite the gazillions of times they compress.
Otoh, some air rifle guys add bearings to the spring seats so's the springs are free to rotate, and they swear this makes the rifle smoother.
Can anyone think of any other applications where you find bearings incorporated into spring seats, cos' I just,,,dunno?