# Cutting fork springs?

10 replies to this topic
• # Polishhammer

Posted November 06, 2012 - 02:44 PM

I one were to cut one coil length off, how much stiffer would the spring become? Anyone know what he rate change might be?

• # mog

Posted November 06, 2012 - 03:22 PM

10% shorter is 10% STIFFER

• # kan3

Posted November 06, 2012 - 05:03 PM

http://www.proshocks.../coilsprate.htm

this is pretty close

Posted November 06, 2012 - 05:23 PM

Rate change depends on the current rate, number of active coils, winding diameter, etc etc. Kan3's calculator works well if you know the inputs.

If all you know if the number of active coils, you can figure it out this way :
• New rate = Old rate x (original number of active coils / new number of active coils)
• Say you have 0.44 kg/mm rate springs with 25 active coils. If you cut 1 off you end up with 24 active coils, and the rate goes up to
• X = 0.44 x (25/24)
• X = 0.4583 kg/mm

Don't forget that you'll
1. Need to make a spacer to make up for the spring you cut, unless this is part of lowering a bike.
2. Need to make sure with a spacer that the spring doesn't run into coil-bind issues near bottom out.

If a coil bottoms out on itself when 11" tall and the fork design compresses it to 12" tall all is fine and dandy. But if you cut it down and then add a 1.5" spacer you are going to bottom the spring on itself before the fork is fully compressed. No bueno.

Edited by adam728, November 06, 2012 - 05:23 PM.

• # Polishhammer

Posted November 06, 2012 - 05:55 PM

Have a 07 yz250. Has .44s in it. I cut a coil off an 2010 crf450 stock Kyb spring thinking it would be about a .45-.46 rate with the Yz .44 in one and modded crf Kyb in other leg. Bike didn't feel right when riding. I'm supecting some binding issues from modded spring and too much of an increase in spring rate like a .47 combined rate? Shock has a 5.6 in and I weigh 210. The guy I bought bike for said forks are from an 07 250f. Pro action valved.
Guess I'll just have to buy a set of .46 s

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• # kan3

Posted November 06, 2012 - 06:35 PM

If you cut one spring down and didn't adjust with a spacer like noted above then you have one fork that is shorter than the other. If you lined up the forks in the triple clamps without taking the difference into account than you'll get binding.

• # Polishhammer

Posted November 06, 2012 - 07:00 PM

I cut the crf spring because they are slightly longer by about one coil length compared to the sprains for the Yamaha kybs. They are equal in length now.

• # EnglertRacing

Posted November 06, 2012 - 09:13 PM

10% shorter is 10% STIFFER

Yes I've tested it.
And 10 percent longer. As in adding a piece of another spring to make it 110% is 10% softer.

• # Pete Payne

Posted November 07, 2012 - 05:12 AM

The real trick to it all is bending the fresh cut coil down and grinding it square to the spring so it does not break .

• # YHGEORGE

Posted November 07, 2012 - 03:14 PM

Have a 07 yz250. Has .44s in it. I cut a coil off an 2010 crf450 stock Kyb spring thinking it would be about a .45-.46 rate with the Yz .44 in one and modded crf Kyb in other leg. Bike didn't feel right when riding. I'm supecting some binding issues from modded spring and too much of an increase in spring rate like a .47 combined rate? Shock has a 5.6 in and I weigh 210. The guy I bought bike for said forks are from an 07 250f. Pro action valved.
Guess I'll just have to buy a set of .46 s

Stock spring from that CRF was a .47.

• # Pete Payne

Posted November 08, 2012 - 07:50 AM

Yep . YHGeorge is right. .47 or .48 stock and if you cut it, it will be stiffer.

Edited by Pete Payne, November 08, 2012 - 07:51 AM.

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