cutting fork springs

[erik]

Ahh, work. . . Ohh look , home time!

Anyways Yams oft have soft forks. Haven't we had this discussion before? I’ve modified fork springs like this on everything from my MB50 to the KDX to the RF900. Just have to watch for coil bind (work out stroke & space between coils & make up a spacer to replace removed length. Nowadays I just cut the coils with a disc grinder & heat the top coil with a propane torch to flatten it.

Works on forks & shocks
click here for stiffer springs for like FREE

Quoting from jrandom's fzx thread...

Yup, I think you've mentioned that before. I often have to hear stuff a couple of times for it to sink through my skull...

I've just pulled the springs out of the forks and measured the fork travel, the spacer size, the spring size etc.

The springs seem to be roughly dual rate (the spacing of the coils increases slightly on the section with the wider gaps).

Does this mean that I really need to remove coils from both ends of the spring? Or should I just remove them from the wider-spaced end to increase the higher-end spring rate?

This is going to be interesting...

[Posh Tourer :P]

If you cut from the thin end, won't it just make them shorter with the same spring rate, = bottoming out earlier?

[erik]

If you cut from the thin end, won't it just make them shorter with the same spring rate, = bottoming out earlier?

which end is the thin end?

From http://www.partsmag.com/0409/Progres...Susp-0409.html

Dual-rate springs have a “step” in their compression. A 10-20 spring, for example, will require 10 pounds to compress it an inch until the light coils bind, and then 20 pounds per inch.

So I'm thinking it might be good to increase the rate for the light (close together) coils as well as the rate for the heavier (wider spaced) coils.
Although I guess if I'm mainly worried about bottoming out, I could just leave the light coils and cut the heavy ones.

Thinking more about this and how dual rate springs work makes progressive springs seem more desireable. But I think I'll give this a go first.

[Motu]

You want to stiffen up the soft part of your spring,so take it out of the close coils.

Springs follow sound scientific principals,but defy logic when you try to figure them out with only our little brains.Looking at my 2 shocks yesterday I was shocked to see the TT spring much shorter than the XT - shit,this is going to be too stiff I thought,but then counted the coils and found they were the same - the TT has closer pitch coils,this will be softer than the course pitch XT spring,they are probably close to the same rate,but won't know until I fit it.

[Posh Tourer :P]

Yeah gets me every time too.....

If you cut from the open coils, wont it give you less range of hard rate spring and therefore easier bottoming out?

[FROSTY:Me]

Erik--before ya go cutting anything--try fitting slightly longer spacers on the springs and maybee a slightly heavier rate oil.

[2_SL0]

I put a heavier weight oil in the FZR 250 forks, it did the trick for me. But I didnt really push the bike very hard.

[erik]

As I understand it, the close together coils soften the springs by increasing the overall number of coils in the spring.
But because they are close together, they bind against eachother far before the rest of the spring does. When they bind, it essentially shortens the spring as only the wide-spaced coils are active. Because only the wide-spaced coils are active, the spring is stiffer (fewer coils = stiffer spring).

So by cutting the wide-spaced coils, I'd be stiffening up the springs when they're under heavy loading, which should help prevent the forks from bottoming out.

If I went overboard and cut too much off, I could end up with the forks bottoming out due to the springs binding, so I'll have to be careful of that.

I guess I should just try longer spacers first. Gotta find myself some 30-32mm dia. exhaust tube.
I've found a couple of articles about suspension tuning that have helped me understand a bit better and given me some ideas of things to measure and what to aim for with the setup:

http://www.strappe.com/suspension.html
http://www.mi-hsta.org/Articles/Suspension%20Setup.htm

I kinda have to wait till I've got the bike on the road again so that I can make modifications and then test them. I've already put some 15W oil in the forks and it doesn't seem to have helped, maybe I'll try thicker oil at a later stage, but I think the springs are really the main issue at the moment.

Feck this is fun though

[FROSTY:Me]

Mate a lil secret re Yamaha suspension tuning--dont tell the other Kbers
Go out and get $4.00 worth of 20c pieces (may be 50c with your bike)
Glue 2 stacks of 5 coins together with a dab of superglue.
You can then simply unscrew ya fork cap -toss one stack of coins in each fork leg and presto fork springs are preloaded by aprox 12mm.
If it aint enough -sweet as -pull em out -glue another coin on the stack etc etc.
Saves shit loads of cutting and fucking around. Trust me It works-Ive done it.
Ohh and the bonus is if ya ever run outa gas and money --unscrew ya fork cap and ya got money for gas home.
same principle applies to thick penny washers btw but theyre usually harder to glue.

[avgas]

Mate a lil secret re Yamaha suspension tuning--dont tell the other Kbers
Go out and get $4.00 worth of 20c pieces (may be 50c with your bike)
Glue 2 stacks of 5 coins together with a dab of superglue.
You can then simply unscrew ya fork cap -toss one stack of coins in each fork leg and presto fork springs are preloaded by aprox 12mm.
If it aint enough -sweet as -pull em out -glue another coin on the stack etc etc.
Saves shit loads of cutting and fucking around. Trust me It works-Ive done it.
Ohh and the bonus is if ya ever run outa gas and money --unscrew ya fork cap and ya got money for gas home.
same principle applies to thick penny washers btw but theyre usually harder to glue.

Finally some good ol'e school logic, that my dad taught me.
This works a charm (ive done it on my other bikes). Good on ya frosty

[Gixxer 4 ever]

I put a heavier weight oil in the FZR 250 forks, it did the trick for me. But I didnt really push the bike very hard.

Yep this is good advice. You will get a good result from this and it can be set back should you not be happy. The thick oil will help no end. I would not cut when you can pack the top with coins ( or washers ) as Frosty said. This really is the solution.

[F5 Dave]

Eric, you are on the right track.

To the others with your thick oil & preload ideas; respectfully
–Wash your brains out with soap!

The thick oil will increase compression damping which will make the bike less prone to dive under braking, but this will also affect rebound damping. Worse these simple forks are orifice limited so High speed compression damping (like stutter bumps) will cause the forks to lock up.

More oil will decrease the air gap which will help slightly, but go too far (ie not very) & air space will run out & hydraulic/take the fork seals out.

Use springs as springs

Preload: If you look at a graph of a spring the line as it compresses say ‘x’ being kilos & ‘y’ being centimetres, then the steepness of the graph will be the spring rate. (ignore progressive or 2 rate springs for the moment). Adding preload will not alter this ‘curve’ (straight line) but simply increase it’s position up the ‘x’ axis (at the intersection 0 cm with some preload to start with).

In the big picture lots of preload & a soft spring tends to make the initial hits hard & the spring rate comparitively flat, so it would be harsh at the start & travel full stroke moderately easily.

Preload certainly makes it feel stiffer when you push on the forks, but should only be used to get the right amount of sag positioning the ride height correctly, not to compensate (poorly) for too soft of a spring.

Note: Too much preload will cause coil bind


Coil bind: Yes you must make an educated guess here. If you think it is going to get close you have to measure the spaces between the springs, add them up. Take away the amount the spring is compressed when you screw the cap down, then take away the full fork stroke & come up with some breathing space.

I’ve usually taken from the closely wound side as it means the wire is easier to bend flat again & usually the bike responds well to this as it will be riding higher on the spring & the total spring rate WILL be higher so prevent bottoming. I use thick wall PVC pipe (plumbing) which is hella strong in compression & has been used by zillions of racers as it is cheap & comes in convenient sizes.

[scroter]

Quoting from jrandom's fzx thread...

Yup, I think you've mentioned that before. I often have to hear stuff a couple of times for it to sink through my skull...

I've just pulled the springs out of the forks and measured the fork travel, the spacer size, the spring size etc.

The springs seem to be roughly dual rate (the spacing of the coils increases slightly on the section with the wider gaps).

Does this mean that I really need to remove coils from both ends of the spring? Or should I just remove them from the wider-spaced end to increase the higher-end spring rate?

This is going to be interesting...

i dont know about cutting them but if you do youll have to increase the size of the spacer to take up the space obviously. i wouldnt cut them at all. i dont know what kind of damping youve got on that. if its got high speed and low speed then dont change the oil weight. if its just damping then increase it to heavier weight. another thing you can do is add more fork oil, all it does is make the air pocket smaller making your shocks feel harder at the bottom of the stroke(just like a progressive rate spring, which is what your springs are). i dont know how much your willing to spend on it but new springs are usually under $300. losing the progressive rate springs for linear ones and adjusting your fork oil level would be a start.

[scroter]

Eric, you are on the right track.

To the others with your thick oil & preload ideas; respectfully
–Wash your brains out with soap!

The thick oil will increase compression damping which will make the bike less prone to dive under braking, but this will also affect rebound damping. Worse these simple forks are orifice limited so High speed compression damping (like stutter bumps) will cause the forks to lock up.

More oil will decrease the air gap which will help slightly, but go too far (ie not very) & air space will run out & hydraulic/take the fork seals out.

Use springs as springs

Preload: If you look at a graph of a spring the line as it compresses say ‘x’ being kilos & ‘y’ being centimetres, then the steepness of the graph will be the spring rate. (ignore progressive or 2 rate springs for the moment). Adding preload will not alter this ‘curve’ (straight line) but simply increase it’s position up the ‘x’ axis (at the intersection 0 cm with some preload to start with).

In the big picture lots of preload & a soft spring tends to make the initial hits hard & the spring rate comparitively flat, so it would be harsh at the start & travel full stroke moderately easily.

Preload certainly makes it feel stiffer when you push on the forks, but should only be used to get the right amount of sag positioning the ride height correctly, not to compensate (poorly) for too soft of a spring.

Note: Too much preload will cause coil bind


Coil bind: Yes you must make an educated guess here. If you think it is going to get close you have to measure the spaces between the springs, add them up. Take away the amount the spring is compressed when you screw the cap down, then take away the full fork stroke & come up with some breathing space.

I’ve usually taken from the closely wound side as it means the wire is easier to bend flat again & usually the bike responds well to this as it will be riding higher on the spring & the total spring rate WILL be higher so prevent bottoming. I use thick wall PVC pipe (plumbing) which is hella strong in compression & has been used by zillions of racers as it is cheap & comes in convenient sizes.

sorry i didnt read this before my comment. but i agree with it.

[2_SL0]

All Iwas wanting to achieve with mine was to lesson the dive factor when heavy breaking, the heavier oil did this perfectly without causing any side effects. Othere than that I would start looking at a set of aftermarket springs, and to top it off a set of Gold valves. But that would cost money.