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Spring rate change by cutting springs??

Why is Sam B so gnarbar?


  • Total voters
    13
  • Poll closed .

WBC

Monkey
Aug 8, 2003
578
0
PNW
Does anyone here know how to calculate the change in spring rate that occurs when you chop a spring?

The reason why I ask is that I have a pike 454 dual air U-turn that has felt horrible since day 1, and after dealing with it for 3 years, I want to go coil. I have a couple boxxer springs that I intend to cut down since I don't think RockShox makes a spring stiff enough for me. The red boxxer spring is 50 in/lbs, right? Could I get 70 in/lbs out of it? Do I need to injinere an air/coil combo to get close to that? I am 220lbs, and am looking for no more than 20% sag. And I want to spend less than $10 on this.

Every helps would most be triumphantly appreciate
 

jonKranked

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Nov 10, 2005
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wow, i can't believe i gave a serious response to this kind of injinereing request. :facepalm:
 
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WBC

Monkey
Aug 8, 2003
578
0
PNW
Nah, spring rates increase when you cut the spring. I don't know why this is, as I got a BA not a BS, but I do know it's true. I also DO intend to preload it like crazy, and I believe that RockShox never made a pike spring stiff enough. I had a coil pike before this, and I tried "the stiffest spring [RockShox] make" and never got the fork to have less than 40% sag. I do not care about damage to the inside of the fork, and I intend to run stainless washers top and bottom to keep the spring from pushing through the bottom of spring seat.

If this fork last through the summer, I'll be ecstatic. All the ano is worn off the stanchions, the lowers have fat gouges and there is cracking around the maxle, and I have applied RTV to the MoCo cartridge to keep it from puking oil all over the bed of my truck when the bike is at an angle.
 

Shepherdwong

Monkey
Apr 19, 2005
131
0
I believe that although coiled, springs act like diving boards. Shorten them and it's like rolling the wheel up, they get springier.
 

WBC

Monkey
Aug 8, 2003
578
0
PNW
Ok, well this isn't a trolling thread. I'm going to do this since I can't see putting money into a clapped out pike. You must be the 1%, so watch out or I'll occupy your ass.

And I've called SRAM direct when I worked in shops and never got satisfaction for this issue. This is why I'm doing it on my own. Really, it's my fault for posting on a US forum and not a Kiwi forum where people believe in using creativity instead of cash to better their riding experience.

H8rs
 

jonKranked

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Nov 10, 2005
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there's a fine line between "clapped out" and broken.

If you've got cracks around the maxle as you've indicated, then your fork has clearly crossed the line into "broken" territory. Use at your own peril.
 

gemini2k

Turbo Monkey
Jul 31, 2005
3,526
115
San Francisco
Well as long as you put spacers so it doesn't rattle, bend the cut end so it doesn't gouge the fork, and you have enough spring travel for the damper so that the spring doesn't "bottom out" and bind up, it COULD work.
 

no skid marks

Monkey
Jan 15, 2006
2,514
26
ACT Australia
Have you thought of putting an elastomer inside a coil spring. Far from ideal or consistent, but probably easier. Or yes, air and coil is great and better than an elastomer. If you have cracks, it's all over, get a new used fork. You pretty much have no choice IMO.
Cutting a coil would also be fine, just getting the end tidy will be hard.
 

dilzy

Monkey
Sep 7, 2008
567
1
Stop riding mountainbikes until you have more than $10? Seriously, cutting springs is fine, BUT!!!!! you need to make sure it has the requisite travel between coils, then you have to heat and bend down the end coil flat, then grind it, then re-temper the whole spring.

In other words, not what you were going to do. ......have I just been trolled?
 

tacubaya

Monkey
Dec 19, 2009
702
61
Mexico City
You can read about it on chapter 10 of Shigley's Mechanical Engineering Design.

K (constant of a spring) is equal to (d^4*G)/(8*D^3*Na) where G is the shear modulus of the material, d is wire diameter, D is the mean spring coil diameter and Na is the number of active coils.

If the spring is squared and grounded at the ends, the Na = Nt -2, where Nt is the total number of coils.

Hope it helps.
 

4130biker

PM me about Tantrum Cycles!
May 24, 2007
3,900
444
Lizard Town
I've run a cut spring in my boxxer for a long time. Grind the end as flat as possible, then use something to stop it from fvcking up the inside of the fork. I don't think it's that big of a deal. Also I agree that the x firm pike coil is still under sprung- for me at 190, but I like my set ups stiff. I still had to run the fork with floodgate and compression almost all the way up for enough support. At that point an air fork would've been better... Or the right spring rate coil.
 

4130biker

PM me about Tantrum Cycles!
May 24, 2007
3,900
444
Lizard Town
Ps why not guess and check? It's a stupid spare boxxer spring. I do agree with others that a different old fork would be safer...
 

no skid marks

Monkey
Jan 15, 2006
2,514
26
ACT Australia
If you could find a metal cover for the end of the spring that's fit in the stanchion you could get a away with less finishing. Or get something machined up with your $10 to fit in the end of the cut spring to locate it and step out to cover the top of the spring, Just make sure cut end is the stationary end.
 

IH8Rice

I'm Mr. Negative! I Fail!
Aug 2, 2008
24,550
490
Im over here now
You can read about it on chapter 10 of Shigley's Mechanical Engineering Design.

K (constant of a spring) is equal to (d^4*G)/(8*D^3*Na) where G is the shear modulus of the material, d is wire diameter, D is the mean spring coil diameter and Na is the number of active coils.

If the spring is squared and grounded at the ends, the Na = Nt -2, where Nt is the total number of coils.

Hope it helps.
totally
 

WBC

Monkey
Aug 8, 2003
578
0
PNW
I've had a number of bikes disintegrate under me, including a Sunday that sheared at WFO. I've ridden bikes for 15 years, I've been a wrench for eight, I've taken university level engineering courses. A few stress fractures aren't of too much concern, especially around the dropout. JonKranked, did I mention I build the airplanes your family flies on?

Thanks to those that helped, especially tacubaya (sp?), I was looking for a starting amount so that I could cut the preload spacer and spring about right and then fine tune with washers. I'll use that formula from mi primo.

And both answers on the poll are correct, I just couldn't get the second photo to load.
 

jonKranked

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Nov 10, 2005
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A few stress fractures aren't of too much concern, especially around the dropout.
"its just a flesh wound"




I've had a number of bikes disintegrate under me, including a Sunday that sheared at WFO. I've ridden bikes for 15 years, I've been a wrench for eight, I've taken university level engineering courses. A few stress fractures aren't of too much concern, especially around the dropout. JonKranked, did I mention I build the airplanes your family flies on?
tumblr_kudj1dFTrj1qzvqipo1_400.png

come back and try to impress us when you start designing airplanes
 

Tmeyer

Monkey
Mar 26, 2005
586
1
SLC
Can't believe I just read this entire thread.... Must be Friday.

Props for the Monty Python pic!
 

DirtyMike

Turbo Fluffer
Aug 8, 2005
14,317
903
My own world inside my head
Ummmmmm.... Really? College level engieering class's?

You should understand that the spring in your fork was engineered for a specific length with the coils it has so it works properly in that fork.

Before cutting the spring maybe it would be better to do some research and find out more about it. Maybe find out if it is proggesivly wound or anything like that. I have no idea, but I work on cars and suspension all day long, went to college and everything, and I know that cuting springs does nothing but cause problems, as does too much or too little preload.

But hey, its your bike, your life, your safety....... Do the math, figure out how much you want to cut and go for it if your so inclined. I jsut cannot belive ht if I can find springs that are strong enough for me with a pike, that your going to be undersprung with their extra heavy spring....... Just throwing this out there, but maybe its a compression control issue, and not a weak spring?
 

MinorThreat

Turbo Monkey
Nov 15, 2005
1,632
41
Nine Mile Falls, WA
I got educated much from mtg and others in my Stupid Ti Spring Question thread. After that, I read a lot of other threads in other disciplines (cars, dirt bikes, etc,) that confirmed: shortening the spring increases the rate proportionately.

As DirtyMike says, the rate is dependent on (in my crude, non-engineer-speak way) all of the coils being there to lend their amount of 'give.' Whatever percent you shorten a spring by, you stiffen it by that percent.

In my case, I was looking at shortening a 400# 7.5" spring to 6.5" - - taking an inch out - - and grinding a new flat end. BUT, it would have made it a 465# spring then. Now, if I got a 350# 7.5" spring and removed an inch, it would make it a 402.5# spring; right where I wanted to be. (Decided against it because I was also advised against cold-working ti and I didn't want to dump $100+ on an experiment to save money)
 
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