Springs - How important are they, really?

[Incubus]

Ok, not whether you have to have them, but which direction they go.

My son and I finished changing out the valve oil seals in the cylinder head meant for my '79 XS11 project last night. Thanks to RokJok, I finally have a manual. It came, however, halfway through the pain in the butt oil seal installation. That's when I saw that the head valve springs are progressive and manual says they must all be tight coil down. I didn't notice as we did the head that there was a tight side. It ain't too damn tight, that's for sure. Anyway, I have no idea which springs are tight side down. I have conjured up several reasons why this could not possibly be that important, and only one why I should do these springs all again - it's the right way to do this. Will I be wasting my time with this top end rebuild if I don't do this all again? Anyone know what may happen if I don't.

Thanks, y'all.

Patrick

[Incubus]

OK, never mind, I was venting. I knew I was going to change the springs, I just am not receptive to do-overs at 8:30 on Saturday morning. We already adjusted them and found only two valves were completely properly springed. Sprung?

Thanks,

Patrick

[prometheus578]

"Is it spring, yet?"

? Anyone know what may happen if I don't.

Ok, here's the deal, best as I can 'member it, which isn't too well.
Springs, like everything else, can vibrate and pick up harmonic frequency fluctuations. A standard spring will compress at a standard rate, but this can vary at different RPMs and frequencies, thus you may not get a consistant pressure.
Valves have two springs, the inner and the outer, of differing diameter and differing thicknesses. This is so that when one spring hits it's weak harmonic fluctuation, the other spring, with it's different frequency variable, will still function and carry past the first spring's lull.
Progressive springs: one end coiled tighter than the other.
Springs compress easily at a uniform rate along their length. This is all fine and dandy for most applications, but for valves, you want the spring's compression to get harder at the end of it's travel(when the valve is almost fully open) This extra pressure sort of slows the valve's movement, but also makes it able to rebound upward quicker, preventing the valve from floating. (Floating a valve is when the rpm is so high, that the valve never makes it back upward and closes fully before the cam hits it again and pushes it down. Over-rev the engine, and you run the risk of the valves floating, and having a piston come up and slap one silly)
Tight coil to the bottom: If pressed uniformly, a spring will compress uniformly. But in our application, the compressive force is applied from above. This means that ever so slightly, the top of the spring will compress first, before the rest of the spring wakes up and realizes what is going on. And since we want the spring to have it's hardest compression at the end of it's travel, the tighter coils must be placed at the bottom.
Hey Pal... I didn't sit through eigth grade twice fer nuttin'!
(All you engineers out there... am I close?)

[79XS11F]

I'm No engineer but it sounds good to me.

In any case the springs are different and the manual calls for them to be tight end down so that's the way to do it. If it were to make no difference they would not be made like that and or the manual would not point this item out.
Rob

[MAXIMAN]

Pro nailed it...in a very funny and informative way.

Standard springs compress at a constant rate. Thus they ride stiffer on top and bottom out easier.

A progressive spring is designed to compress easy up top and progressively get firmer the more it is compressed. This is to allow the small bumps to be absorbed readily and still have enough tension to take on the big bumps.

When I say "up top" I don't mean on top of the spring....but at the top or beggining of the compression cycle. Actually if you look at a progressively wound spring the coils are tight on the ends and less dense in the middle of the spring.

[Pat Kelly]

I would think the XStra mass of the tighter coils at the top would be the problem.
Probably all of the above, what Pro said

[Ken Talbot]

My vague memories from statics class back in engineering school tell me that it makes no difference which way a spring is installed. When you apply a force against the end of a spring, the other end of the spring exerts exactly the same force against its support.

The folks at Progressive also say in their installations that their springs will perform exactly the same regardless of which way they are installed.

[MAXIMAN]

Originally posted by Ken Talbot
My vague memories from statics class back in engineering school tell me that it makes no difference which way a spring is installed. When you apply a force against the end of a spring, the other end of the spring exerts exactly the same force against its support.

The folks at Progressive also say in their installations that their springs will perform exactly the same regardless of which way they are installed.

Exactly Ken. The weakest part of the spring will compress first whether on top or bottom.

The progressives I bought were progressively wound from the ends to the center.

[Incubus]

That really is what I was thinking, Ken. A couple weeks ago I put new fork springs in another of my vintage bikes and the instructions with the springs said you can put them in tight coils up or down, as long as both were oriented the same. I figured whichever way I put the springs in the soft end of the springs would compress first and the harder end would kick in later. I decided to redo the springs primarily because I was concerned that if I put the big spring in with the tight coils down and the smaller spring with the tight coils up it could potentially lead to binding between the springs as opposite ends of the springs were contracting first. Reasonable supposition? I don't know. I'm not an engineer. I write for a living.

Perhaps if the directions would have said the springs could go in with the tight coiled end either up or down, but that both springs in each valve had to be oriented the same it would have made more sense to me. Then again, it is much easily and simpler to instruct folks just to put the tight end down - that saves a lot of words and explanation.

Thanks for all the responses guys. It was an interesting and instructive discussion.

Patrick

[prometheus578]

"Cry havoc, and let slip the dogs of war!"

Again... I'm not a learned, nor certified, engineer by any means. But that doesn't mean that I'm afraid to challenge one once in a while.
I arrive at conclusions based upon logical thought, after applying the fundimental principals that I know. My errors are caused by not factoring in the principles of which I'm unaware that may also play a part. I rely on experts in the field to explain things to me and set me straight.
Ken... prepare to do battle!

My vague memories from statics class back in engineering school tell me that it makes no difference which way a spring is installed. When you apply a force against the end of a spring, the other end of the spring exerts exactly the same force against its support

On the whole, that is a true statement. If one has a spring compressed with ten pounds of force, it will exert a counterforce of five pounds in each opposing direction. I feel that we can agree on that.
What's in question here, I believe, concerns the movement of the spring, it's rate of compression, and the differences between a uniform spring, and one of variable pitch, that is, progressive.
(I can string together technical phrases to impress the masses... but Ken is neither amused, nor impressed, by this amateur's ploy)

What I believe we agree upon: A progressively wound spring will compress at different rates over it's length of travel. (The theory is [as I see it]that the weaker area will compress first, till it matches the pitch of the tighter wound coils, then the whole structure will compress in unison)
Is this correct?
While agreeing that a compressed spring exerts an equal force in both directions, I disagree that when compressing a spring, an equal force is applied uniformily throughout it's length, but state that more force is applied to the end from which the force is coming from. Hence... that end of the spring will begin to compress first... which brings us to the question of "Which Way to Install Progressively Wound Valve Springs", and hopefully to gain an understanding of why.
Overcoming inertia, the force required to get a mass moving comes into play. As well as, from which direction that force is applied.
Compressing a "non-progressive" valve spring: As the bottom of the spring is immoveable against the head, it is the top of the spring which moves and receives the initial compressive force. The spring may seem to compress uniformily, but it doesn't. Inertia, the spring's resistance to movement(in this case, compression) must first be overcome. As it is the bucket's movement downward that is actually applying the force, the force for movement is coming from that direction. This force, as it hits the spring, then has to travel through it's whole length. Due to the springs mechanical structure, crystaline nature of steel and other such terms I may not fully understand, this all takes time. And since it take time to transmit this force down the whole length of the spring, it's only natural that the end of the spring that's actually receiving the force, will have it's inertia overcome and start to compress first.
Someone please correct me if I'm wrong.
As I have no outside input on this at this time... I shall move forward based on these unverified conclusions.

Why the engineers wanted the tighter end of a progressively wound valve spring at the bottom: With a progressively wound spring, the looser end compresses first. As I have already stated (though possably incorrectly), the top of a valve spring is the first part to compress. The engine designers have already determined that they want the top of the valve spring to compress first, hence the directional orientation of the progressive spring. Sure... with an upside down progressive spring, the 'now bottom' weaker coils would compress first anyway, but from an engineering standpoint, why waste the extra boost given when overcoming the inertial forces during the initial phases of compression?
If anyone can supply a better reason, or hopefully the correct reason, for having the tighter coils at the bottom, please feel free to correct me.

The folks at Progressive also say in their installations that their springs will perform exactly the same regardless of which way they are installed.

Ken, I imagine you're referring to fork springs, but the same principle should apply. As I haven't installed any "Progressives"for quite a while, I see no reason to doubt you, but that may only apply to that model spring, or to only certain applications of bikes. This I do not know, but I cannot feel that ALL progressive springs can be installed in this manner for the following reasons.
I ran down to the garage and thumbed through a few manuals of different breeds of bike concerning fork springs.
"Install tighter coils down", "install tighter coils down", "install tighter coils down"... seemed pretty consistant, till I ran into a few "install tighter coils at the top"! (some people may not be aware, but some forks actually have two springs... a long one, and a tighter wound shorter one. Gives the same progressive compression effect)
All boils down to what was the design engineer's intent on how he wanted the front end to behave. Newer race bikes have their forks inverted, but their handling characteristics are designed around that set-up.
Some of the Victory cruisers have inverted forks, others have them still oriented as God wanted forks to be.
I understand variable rate springs... but am just looking to satisfy myself and answer the question posed by Succubus regarding putting the springs in upside down and whether he should correct them.

Anyone know what may happen if I don't.

All "new" comments regarding springs are welcome...
("Prom... you're likely to bust a spring, you're already off balance" has already been stated many times before, though I do seem to be getting "progressively" worse)

[Pat Kelly]

Hoyt reincarnated

[TomKat]

Whew.....!

[Ken Talbot]

Hmm, it appears some form of gauntlet has been tossed down. But, is it a gauntlet, or is it one of those fingers-cut-off shorty gloves?

Let's say you take a progressively-wound spring and stand it on a precise scale with the strong coils on the bottom and the weak coils on the top. Press down on the spring with, say, 5 pounds of force. The weaker coils compress a bit, and the scale registers 5 pounds. Press harder, say with enough force to overcome the weaker coils and start compressing the stronger coils. The scale will now register whatever new force you're applying. Sound okay so far?

Now, let's do it again with the coil the other way around, i.e. with the stronger coils on the top and the weaker coils on the bottom. Press on it again with the initial 5 pounds. What does the scale register - it has to be 5 pounds. Again, the weaker coils are compressing a bit, but this time it is because of the 5 pounds of force being applied from the scale. Press harder and the same thing happens - the scale will always register whatever force you're applying on the other end.

Hang the spring horzontally on a fine balance wire between two precise scales and see what happens when you start moving the scales towards each other, i.e. loading the spring. The scales will always read exactly the same amount. It doesn't matter which way the progressivly spring is wound or which end which coils are at, the two scales will always be the same. I believe some guy named Newton talked about this.

OTOH, it might only work this way here in Canada with metric springs. And I won't even try to think what might happen in Australia where it is all upside down to start with.

[Pat Kelly]

On a cylinder head the bottom of the spring is stationary and the top moves up and down with the movement of the cam/valves.
Would it be more efficient having the coils with more mass (tighters coils) or less mass (more space between coils) as the end that is moving?
PSI is still the same on either end.

[mro]

Spring in head can exceed 140 compression cycles per second.

More at work here than equal/opposing force.

Part of reason why there's two different springs on each valve.

Now for something totally different.........While I've had XS valve springs in hand, never bothered to inspect ends to see if "top" and "bottom" are identical and if there's any difference in the bottom of bucket or area the bottom of spring sits on head. If not same could cause "wear" issues...................



mro