Cutting Fork Springs?

I have a question regarding suspension setup, I could not find an answer with a quick search, hope it hasn't been asked before.

A friend of mine is getting his first gen sv set up for track days, and in order to increase spring rate, he mentioned he was thinking of cutting springs, and replacing the lost length with increased spacer length. This seemed counter intuitive to me, I believe the first gen sv has non progressive springs, and even if it did this seems odd, as the rate would change according to how much the spring was compressed, not how much of it was there.

Also, if indeed springs do somehow gain spring rate through being cut, I would think that any gains would be offset by the decreased fork travel. When I questioned him, he referenced this article:

http://www.pegmonkey.com/node/566

Just hoping someone smarter than me with suspension can give a heads up. Thanks in advance.

Mike

 

That does not compute but not for reasons that I may or may not fully undrestand.

I was always under the impression that wire diameter and number of coils are what affected spring rate.

 

Yes, cutting a spring will increase the spring rate. Think of a spring as a coiled up torsion bar. A shorter bar will have a higher rate than a longer one with the same cross section. Compression of a spring does not affect the rate unless you start binding the coils.

The question for your friend is why bother with cutting a spring? Fork springs are cheap and easy to replace on an SV, plus they come with the ends already ground and a rate more dependable than you would get by cutting it.

 

I don't believe cutting the spring will increase its rate. All of the higher rate springs I see use a thicker wire.

 

Using the torsion rod theory then it would take more force to twist a 1/2" diameter rod that's 2 feet long than it would to twist 1/2" rod that's 10 feet long.

That's doesn't seem right to me but then again I'm not an M.E.

 

From the friend kildagin spoke for

Possibly this URL sheds some light on our topic:

http://forums.studentdoctor.net/archive/index.php/t-415792.html

Seems valid. But the original poster's observation - that the increased spring rate comes at a cost of suspension travel - seems to dwarf any interest in cutting the springs...

 

Reading the above post I'm thinking that since five coils will all compress together they are all sharing in absorbing the force which means that each coil will compress slightly less than would a single coil absorbing the same force so in effect it's still the same rate of compression just being distributed between more coils and making it appear that a single coil is stiffer but in actuallity a given load will compress 5 coils the same distance as 1. Maybe this is beyond my knowledge of springs but I believe a straight weight spring is just that. It retains the same rate all through it's compression. Cutting off coils will just leave you with a shorter springs of the same rate. I suspect that the guy writing the article in the link thought that his spring work made the difference but it was actually the fact that he used a higher weight suspension fluid when putting it all back together again along with possibly a different oil height. All the improvements he mentioned had more to do with better damping than just a higher spring rate. Also putting too long of a spacer in there can limit your fork travel as the spring compresses and runs out of room. Remember a spring compresses, a spacer doesn't. Basically, just buy some stiffer springs.

"In my opinion the stock springs are worthless to start with, so I decided to go for it. Suspension damping is a bit tougher to figure out. I knew that the damping with 15 weight fork oil was still too soft, so I opted to try some 20 weight. I decided to run the fork oil at 140mm from the top of the tubes to start with and then see how that worked."

 

Back in the bad old days, we used to cut the springs to stiffen them as part of normal track prep of a street bike.
But that was way before stiffer springs were readily available.....

 

So, lets say that i have a 12" long fork spring. If i cut it down 1" & install 1" of spacer to substitute, it now takes more weight to compress the front end of my bike. That's a fact. So, how do you figure that by cutting the spring, you don't increase the rate?
I'm all ears.

 

Hello!
Anyone there?
The NUMBER OF COILS. Ding Ding!

REDUCE the number of coils in a spring increases the rate of that spring.

 

thicker wires is way to increase rate without changing length.
cutting length will do same.

 

Well I now have no choice to but to agree. Still doesn't click though so I suspect its only of those things I just need to see the physical demo done to click.

so if I cut my .95 springs down to one coil and put a massive spacer in there...it'd be the same as if I completely compress my spring to almost coil bind. interesting.

 

No.

 

But where does the cutting stop making it stiffer?

 

it does not. preloading it does not make it any stiffer

 

It's probably easier just to show you, but it's true.

No, because the rate will be different.

 

I'm a little slow. So if I have a 1/2" diameter rod coiled at 10 coils per foot. That's a softer spring than say a 1/2" rod coiled at 5 coils per foot.

If I cut the 10 coil per foot spring down to 5 coils that doesn't seem to me like it would give me the same rate as the 5 coil per foot spring.

This seems to me as a way to only reduce the amount of effective travel...not spring rate. But then I guess I have to factor in the effective lever length over the distance of the spring. The tighter the coil effectively the longer the lever acting on the rod to induce twisting forces.

 

The torsion bar analogy is good and about the easiest way to understand. Note that theta and L are directly proportional so less L= less deflection, i.e. stiffer rate assuming all other things equal. Also note that diameter is to the 4th power and thus has a very strong influence on the rate.

From http://www.engineeringtoolbox.com/torsion-shafts-d_947.html

Torsional Deflection of Shaft

The angular deflection of a torsion solid shaft can be expressed as

θ = 584 L T / (G D4)

where
θ = angular shaft deflection (degrees)
L = length of shaft (mm, in)
G = modulus of rigidity (Mpa, psi)

 

Obviously this is the limit at which a spring can be cut to make it stiffer. You can only cut and add a spacer to the point that it doesn't affect the travel of the fork by the spring binding.

If you're suspension is sprung for a 95# rider and you weigh in at 300#, you're probably going to have to buy new springs! There's no way you can make that work by just cutting the spring and adding a spacer.

Your spring rate is correct when you're sag numbers are correct. (rider and static) So, you could cut your springs / add spacers, adj pre-load and check to see if you got the spring rate right by measuring your sag.

 

No..your spring rate is correct when it allows you to ride the bike as fast as possible. Sometimes the spring rate isn't what you think it should be.