Recent fork spring cutting feedback needed - can't find post

[GaryM]

To achieve a nice stiffer fork spring by cutting the stock 40mm fork spring, someone with a engineering know how posted how much he cut and about what the rate came to after cut.

Don't what to go to a 44kg spring , but want to get in at maybe 40 area up from what i believe is a standard 37kg. Sorry i just seen this here and my searching did not find it .

How much should one cut ?

 

[Darkside]

I've never heard of cutting a spring to increase spring rate

 

[jack topper]

To achieve a nice stiffer fork spring by cutting the stock 40mm fork spring, someone with a engineering know how posted how much he cut and about what the rate came to after cut.

Don't what to go to a 44kg spring , but want to get in at maybe 40 area up from what i believe is a standard 37kg. Sorry i just seen this here and my searching did not find it .

How much should one cut ?

Gary,

Wish I could help you more but there's not enough info in your post for me to even guess. The only reason I'm chiming in here is because I had to have one custom made about a year ago for the internals of my silencer. (It cost me a small fortune) While researching the project I came across a number of online compression spring calculator systems such as the one at www.acxesspring.com. There are several others as well. Going through the design process once or twice will help you understand your needs better. You might even be able to plug in your actual values and come up with a true solution.

One thing I do know is that having fewer coils WILL stiffen the spring, assuming the materials and the free length remain the same. All other things being equal, more coils means a softer spring and vice versa. Having said all that, I don't know how you could cut off a coil or coils and then stretch it back to its original free length (and re-form the end to closed and ground) without applying heat with a torch. Putting enough heat on it to stretch and/or bend it may well affect the "torsional rigidity" (my term) of the spring wire. Then again, maybe adding a spacer to make up for what you remove will get you where you need to go...☺

Hope there's someone else on CH with better info for you...
JT

 

[Picklito]

Hi GaryM,
I cut the old Husky springs all the time. Many of the 40mm springs are so long that they stick way up out of the chrome tubes and, therefore, install with about 3/4" of preload once you screw the caps on... that's way too much for good action. So, spring too soft and too much preload = harsh on the littles and bottoms on the bigs.

You are correct that they are ~.37 stock. Since they are so long to begin with, there's plenty of room to cut off some coils, get a rate increase, get the preload right, and NOT have coil bind (coil bind and the limits of their metallurgy are critical points here). It's the best-kept secret for the 40's.

If you simply cut about 10% of the active coils, the rate goes up roughly 10% which will get you right near .40. I use a cutoff wheel. Use mild heat and refinish the ends flat, then use a simple PVC spacer to set the preload where you want it. With these cuts I like 3-5mm of installed preload, so cut a spacer to achieve that. I think you'll really like the change, in both the forks and the behavior of the whole package. It makes a difference.

Oh... If you use a cutoff wheel, make sure you don't nick any nearby coils... it will break quickly in use. Ask me how I know.

 

[silverstreakNZ]

As above . Active coils are doing the work and setting the rate . Work it out as a %

A coil spring is a tortion spring re packaged to a more convenient shape . So shortening a tortion spring makes it harder . Easier to understand than a coil

 

[ruwfo]

"One thing I do know is that having fewer coils WILL stiffen the spring", Yes this is 100% correct, so start slow, and don't take too
much off, it's a measure twice cut once mind set, once u cut the material u can't put it back, except with PVC spacers.

And your a big boy, so your going to have to ride the bike a bit to test what you've done,
take you time & you'll get it right...

 

[markt2]

To figure out the maximum that can be cut what I do is measure the spaces between all the coils with the spring out and laying on the bench.

Example, fork travel = 8 inches plus estimated preload of one inch = 9 inches. (these are just round numbers for the example!)

After cutting, for this example you need to have more than 9 inches of total space between the coils or the spring will coil bind (completely collapse solid) before the fork is collapsed completely... that would be bad. So before cutting where I think I want to cut, I would measure and add up all the free spaces between the coils up to the point I plan to cut and make sure it's more the 9 inches needed to avoid coil bind.

 

[PEZBerq]

I have cut my 40mm fork springs down on many of my Huskys. Made a good improvement to stiffness over original long soft spring rate.

Use PVC tube to fill in the missing space in the fork tube. Apply very little preload - say 2- 3 mm only. I cut about 5 turns out from memory. Simply cut the spring wire using a small power file or grinder. Didn’t try to flatten the end of the spring. Simply cut the PVC tube on an angle to match spring helix. Do the calcs to check for no coil binding. I will post some photos of the off cut piece of spring.

 

[GaryM]

Thank you for the feedback !

Humm do you know Race Tech Says the standard rate is only 32KG other says 37

 

[Picklito]

Ya. I’ve seen lots of claims in the .30’s and just know they’re too soft! What PEZBerq shows is about what I do. RT also sells a nice .44 (which I’ve used) and I think one firmer around .48 or so (have not used).

 

[GaryM]

I am working on three bikes that riders will be in the 160 to 180 range. I run the .44 which I found best for me on my bikes. Tried all other for race tech 44 is best you can trade out springs with them after trying. Others above .44 will not dive in corners and turn as well.

 

[Darkside]

I learn something new every day

 

[Picklito]

Good thread.

 

[Bigbill]

I have put pvc pipe in as spacerson other brands of bikes but never cut the springs. Interesting.

 

[silverstreakNZ]

There was 58 active coils in my cr500 .
Cut 10 off . Feels way better