# i measured front spring rate of my 230

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short answer : stock springs are dual stage 0.32-0.50 kg/mm,  after cutting 10 coils (from the side with less spaced coils) stock springs became 0.38-0.50kg/mm. these numbers come from math formula and *are verified by my test at the scale*  .  imho they are very accurate (error could be +/- 0.01kg/mm)

stock springs are: 64 total coils, 30mm outside diameter, 4.3mm wire diameter

long answer:  i used a scale with continuous refresh, a nut , a washer, a theaded 20mm rod.  then i measured the necessary weight to compress (by hand) the spring for a given distance , reading the scale when  the rod was touching the scale. easy and fast, i tested 25-50-75-100-125 mm of compression.

the statement "short spaced coils compress first" it's completely false. in any point of the spring the distance between coils changes in the same way untill short spaced coils bind. at that point only large spaced coils remain active, causing an increased K (less coils =harder spring).

change between "soft" and "hard" spring rate happens when compression is equal to (distance between short spaced coils) x (number of active coils). in a stock springs it happens at about 2.5mm x 62 active coils = 155mm of compression

i wasn't able to compress the spring to that point but i verified that compressing  my shortened spring (52 actice coils , 0.38-0.50kg/mm,  change of spring rate predicted at 2.5mmx52=130mm) coils were almost binding during the 125mm compression test.

if you put the spring infos into an online calculator like this:

you will see that  you cannot shorten the stock spring too much , in fact the recommended max travel is lower than the total space between coils (20-25% lower).  i do not know the math behind the 25% reduction of travel for safety reasons. probably the limit is there to allow a high number of cycles at 75%. going up to 80-85% reduces life of springs for sure so it should not be considered a final mod but rather a try before buying new springs with proper K.

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How well do you think these will work on the bike?

What do you think static and race sag will be?

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I have an Excell spreadsheet that also does the calculations based on the formula:

Formula for in/lbs is: =(11250000 * POWER(wiredia,4)) / (8 * activecoils * POWER((springOD-wiredia),3))

Multiply by  0.01846 for kg/mm

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How well do you think these will work on the bike?

What do you think static and race sag will be?

0.38-0.50 springs with 15mm of preload give me about 35mm of static and 60mm of race sag. i weight 145lbs+gear.

to get the same race sag with 5mm of preload i would need 0.44 springs

Edited by 30x26
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0.38-0.50 springs with 15mm of preload give me about 35mm of static and 60mm of race sag. i weight 145lbs+gear.

to get the same race sag with 5mm of preload i would need 0.44 springs

What springs would be needed for a 175lbs+gear rider to get 45mm of race sag with 5mm of preload?

What is keeping you from procuring a set of the right springs?

Has anybody ever seen or might have a pic of a set of Bruce Triplett modified fork springs?

I don't wanna steal his secrets, I just wonder how much of the spring he modifies.

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What springs would be needed for a 175lbs+gear rider to get 45mm of race sag with 5mm of preload?

What is keeping you from procuring a set of the right springs?

Has anybody ever seen or might have a pic of a set of Bruce Triplett modified fork springs?

I don't wanna steal his secrets, I just wonder how much of the spring he modifies.

you would need an unrideable  spring rate , 0.57 or something like that!

45mm are 20% of total travel, many other tuners suggest 25% or 58mm. to get 58mm a 0.46 spring should be enough, it could give reasonable sag without being ridicously harsh

i live in italy, we have no aftermarket springs available at low cost. i wanted to know the exact spring rate of my modified springs, in order to choose the right spring at first attempt (in the future i'll probaly order a custom made pair).

there is no secret about a simple spring. if you remove wire the spring rates increases.  the problem is that you cannot remove a lot of wire from OE spring. if you do it the life of the spring will be shorter  because coils will flex to much

Edited by 30x26

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I've been thinking more about spring rate and how a progressively wound spring or running two different springs in our forks will ramp up the spring pressure the more it compresses.

short answer : stock springs are dual stage 0.32-0.50 kg/mm,  after cutting 10 coils (from the side with less spaced coils) stock springs became 0.38-0.50kg/mm. these numbers come from math formula and *are verified by my test at the scale*  .  imho they are very accurate (error could be +/- 0.01kg/mm)

stock springs are: 64 total coils, 30mm outside diameter, 4.3mm wire diameter

long answer:  i used a scale with continuous refresh, a nut , a washer, a theaded 20mm rod.  then i measured the necessary weight to compress (by hand) the spring for a given distance , reading the scale when  the rod was touching the scale. easy and fast, i tested 25-50-75-100-125 mm of compression.

the statement "short spaced coils compress first" it's completely false. in any point of the spring the distance between coils changes in the same way untill short spaced coils bind. at that point only large spaced coils remain active, causing an increased K (less coils =harder spring).

change between "soft" and "hard" spring rate happens when compression is equal to (distance between short spaced coils) x (number of active coils). in a stock springs it happens at about 2.5mm x 62 active coils = 155mm of compression

i wasn't able to compress the spring to that point but i verified that compressing  my shortened spring (52 actice coils , 0.38-0.50kg/mm,  change of spring rate predicted at 2.5mmx52=130mm) coils were almost binding during the 125mm compression test.

It sounds like the the softer short spaced coils do collapse and bind first.

Ah! But only because the space between them is less, so they bind first.

I have an Excell spreadsheet that also does the calculations based on the formula:

Formula for in/lbs is: =(11250000 * POWER(wiredia,4)) / (8 * activecoils * POWER((springOD-wiredia),3))

Multiply by  0.01846 for kg/mm

Has anyone entered spring rates into an Excel spreadsheet or maybe PowerPoint and then graphed the result?

Two of the same rate spring should result in a straight line angled upwards as it extends from the left to the right on a graph.

Wouldn't a mixed pair or a compound/progressively wound spring produce a curved line?

VortecCPI is running one OE spring and one BBR spring.  The two springs rates add.

So at 6mm of fork spring preload, his front end has 5.46kg/mm of spring force with the forks fully extended.

At Race Sag, 57mm + 6mm, his front end has 57.33kg/mm of spring force.

My bike at Race Sag 57mm + 5mm, has 57.04kg/mm of fork spring force. (A pair of 0.46kg/mm springs, 5mm preload)

At 4" of fork spring compression,

Vortec has 97.9kg/mm of spring force.

I have 98.07kg/mm.

After the softer coils collapse and bind at say 6" of fork spring compression, don't we have 3 different rates to consider?

0.38kg/mm for the first 6".

0.50kg/mm for the next 3.5"

0.53kg/mm straight rate for the BBR spring.

At 7" of fork spring compression,

Vortec has 170kg/mm of spring force.

I have 168kg/mm of spring force.

At 9" of fork spring compression,

Vortec has 222kg/mm of spring force.

I have 215kg/mm of spring force.

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Has anyone entered spring rates into an Excel spreadsheet or maybe PowerPoint and then graphed the result?

I have a bunch of spreadsheets I started working on years ago but the unknowns with respect to the stock factory springs made the efforts futile.

I can work on them again using 30x26 data as well as my own data and see what happens...

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VortecCPI is running one OE spring and one BBR spring.  The two springs rates add.

Just to be sure I think you meant to say "the two spring rates average".  For example one 40 kg/mm spring and one 50 kg/mm fork spring results in a combined spring rate of 45 kg/mm.  If we have:

• 21 kg/mm and 53 kg/mm the starting rate is 37.0 kg/mm each (published)
• 32 kg/mm and 53 kg/mm the starting rate is 42.5 kg/mm each (30x26 measured)
• 38 kg/mm and 53 kg/mm the starting rate is 45.5 kg/mm each (My measurement)

Sorry.  Never mind.  I see you are speaking of TOTAL rate for two fork springs:

• 21 kg/mm and 53 kg/mm the starting rate is 74 kg/mm TOTAL (published)
• 32 kg/mm and 53 kg/mm the starting rate is 85 kg/mm TOTAL (30x26 measured)
• 38 kg/mm and 53 kg/mm the starting rate is 91 kg/mm TOTAL (My measurement)

The first seems too low given my Free Sag and Race Sag measurements.  The second seems right on given my Free Sag and Race Sag measurements. The third seems too high given my Free Sag and Race Sag measurements.

We can use your 46 kg/mm fork springs as a sanity check.  What are your Free Sag and Race Sag dimensions?  If I recall correctly your Race Sag is closer to ~2 inches or less.  If the third rate (45.5 kg/mm) was correct I would have less Race Sag than you as I only weigh 150 pounds.

Also worth mentioning again is my Free Sag and Race Sag measurements (1.125" / 2.250") are the exact same with:

• Two stock fork springs with 3/4" spacers at the top
• One stock fork spring and one BBR fork spring

• Data set 1 - Stock fork springs no spacers
• Data set 2 - Stock fork springs w/3/4" spacers
• Data set 3 - 1 stock fork spring / 1 BBR fork spring
• Data set 4 - Two BBR fork springs

It takes ~107 pounds of force to compress the forks to get my Race Sag of 2.25" with either:

• Stock fork springs w/3/4" spacers
• 1 stock fork spring / 1 BBR fork spring

This exactly matches my findings.  Also note two stock fork springs without spacers results in 3" of Race Sag with a ~107 pound load, which also exactly matches my findings.

Also note the amount of force it takes to fully compress the forks:

• 453 pounds for stock springs with 3/4" spacers
• 487 pounds for 1 stock and 1 BBR spring

It is interesting to see two stock forks springs with 3/4" spacers actually offer close to the same rate at the bottom of the stroke as one stock and one BBR fork spring.  This is because we started with an extra 3/4" of preload so the fork springs have been compressed 10.25" as opposed to only 9.5".  This is why we can not defeat bottoming issues with slightly-heavier fork springs.  We can only do so by changing damping (i.e.,fluid and orifices) and by changing the air spring.  I am working on an Air Spring spreadsheet right now.

It is also interesting to see two stock forks springs with 3/4" spacers are stiffer at the beginning of the stroke than one stock and one BBR fork spring.  Again this is because we started with an extra 3/4" of preload.  This explains why I had all that harshness years ago with the stock fork springs and 3/4" spacers.  Both John and Bruce told me the initial spring rate would be too high and they were exactly right.

Assumptions:

• I am not considering any opposing force due to seal stiction
• All data sets in the spreadsheet show resulting force for a set of two (2) fork springs
• I am using data acquired from the work done by 30x26 (i.e., 0.32 kg/mm for first 155 mm and final rate of 0.50 kg/mm)
• There is an air spring at work here and I have not taken it into consideration.  The forces shown here would only be accurate if we drilled a hole in the top of each fork cap to defeat the air spring.  The additional opposing force created by the air spring is significant.
• I am not considering any preload developed by threading of the fork caps.  All things being equal the data would simply move up one row.  As I mentioned elsewhere my fork springs have taken a set and are shorter than new stock springs.  One of the stock fork springs is 1 mm shorter than the other and the BBR fork spring is somewhere in the middle.  In other words they are ALL less than the stock length of 597 mm.  This could result in an approximate maximum error of 2.5% in the calculations.

Edited by VortecCPI

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Just to be sure I think you meant to say "the two spring rates average".  For example one 40 kg/mm spring and one 50 kg/mm fork spring results in a combined spring rate of 45 kg/mm.  If we have:

• 21 kg/mm and 53 kg/mm the starting rate is 37.0 kg/mm (published)
• 32 kg/mm and 53 kg/mm the starting rate is 42.5 kg/mm (30x26 measured)
• 38 kg/mm and 53 kg/mm the starting rate is 45.5 kg/mm (My measurement)

The first seems too low given my Free Sag and Race Sag measurements.  The second seems right on given my Free Sag and Race Sag measurements. The third seems too high given my Free Sag and Race Sag measurements.

We can use your 46 kg/mm fork springs as a sanity check.  What are your Free Sag and Race Sag dimensions?  If I recall correctly your Race Sag is closer to ~2 inches or less.  If the third rate (45.5 kg/mm) was correct I would have less Race Sag than you as I only weigh 150 pounds.

Also worth mentioning again is my Free Sag and Race Sag measurements (1.125" / 2.250") are the exact same with:

• Two stock fork springs with 3/4" spacers at the top
• One stock fork spring and one BBR fork spring

When I said "add", I meant the force of both springs, not their rate.

If I have 2 fork springs then I have 2 sources of force.

If a 0.46kg/mm spring is compressed 10mm it should be exerting 4.6kg of force.  Two springs so 9.2kg of force.

As far as rates go and how they average, I'm not sure how they measure exactly.

Someone measured the close together coils to have a 0.21kg/mm rate and the other coils to be 0.53kg/mm.

30x26 measured the spring rate to start off at 0.32kg/mm and then increase after 6" of compression when the softer coils bound.

I may have mixed up the rates 30x26 posted.  0.32kg/mm for OE uncut springs.  0.38kg/mm with 10 coils removed.

We do need to devise a tool for actually measuring fork spring rate through it's compression stroke.

Then graph all of the spring rates on a standardized graph so we can compare and experiment with mixed springs etc.

I'm also wondering if my math is accurate to how a straight rate spring develops force when being compressed.

Will a 0.46kg/mm spring increase in force through it's entire compression stroke?

From 0 to 75mm is the increase a consistent 0.46kg for every millimeter?

What about from 75 to 200mm?

Maybe there is a known deviation or a known increase/decrease in rate.

I'm also starting to wonder why fork spring preload has the effect it does.

Preload just increases the force the spring is exerting by compressing it some.

What is the difference between a 0.46kg/mm spring with 5mm of preload, 2.3kg of force.

....and a 0.23kg/mm spring with 10mm of preload, still 2.3kg of force.

I may need to look at that a different way though.  What it takes to make the same race sag will result in alot more spring preload and lots more spring force when the fork is compressed less than at race sag which is probably where it is going over the small stuff.

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When I said "add", I meant the force of both springs, not their rate.

If I have 2 fork springs then I have 2 sources of force.

If a 0.46kg/mm spring is compressed 10mm it should be exerting 4.6kg of force.  Two springs so 9.2kg of force.

Sorry.  I did just catch that and corrected it.  Thank you.

I'm also wondering if my math is accurate to how a straight rate spring develops force when being compressed.

Will a 0.46kg/mm spring increase in force through it's entire compression stroke?

From 0 to 75mm is the increase a consistent 0.46kg for every millimeter?

What about from 75 to 200mm?

Maybe there is a known deviation or a known increase/decrease in rate.

Your math is correct.  The force required for a given distance is the rate multiplied by the distance.  For a linear spring it is a linear relationship.

I'm also starting to wonder why fork spring preload has the effect it does.

Preload just increases the force the spring is exerting by compressing it some.

What is the difference between a 0.46kg/mm spring with 5mm of preload, 2.3kg of force.

....and a 0.23kg/mm spring with 10mm of preload, still 2.3kg of force.

This is easily calculated and Race Tech's Motorcycle Suspension Bible has a great explanation (p.20).  I wish I could post the graph in the book but I will not do so.  Adding 3/4" of preload, for example, means the initial force required to compress the spring is the same as the force required to get the spring to 3/4".  In my case the initial force required to compress a set of stock fork springs with 3/4" preload is >26.9 pounds instead of >0 pounds.  This is why they become so darn harsh.  The book talks about the impact of spring preload and mentions some great things I would not have considered, such as the wheel's inability to extend back into the terrain with too much preload.  This is the exact OPPOSITE of what one might assume or conclude.  There are actually several sections that discuss spring preload, its purpose, and its impact.

I will post this from the book:

"The stiffer spring builds force at a faster rate (it is stiffer) but starts off at a lower initial force.  It will actually "feel" more progressive than the softer spring with more preload."

For springs in parallel, like our forks, the equivalent rate is the sum of all the rates.  For springs in series the equivalent rate is the reciprocal (or inverse) of the sum of the reciprocal (or inverse) of each individual spring rate.  If it helps, it is exactly the opposite of resistors in series and parallel.

For example, two 0.46 kg/mm fork springs:

• In parallel = 0.46 kg/mm + 0.46 kg/mm= 0.92 kg/mm
• In series = 1/0.46 kg/mm + 1/0.46 kg/mm = 1/(2/0.46 kg/mm)  = 0.32 kg/mm
Edited by VortecCPI

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I sure would appreciate it if somebody would please verify my work in the attached spreadsheets.

I used data from here:  http://rogercortesi.com/ideas/public/gasspring.html#83

Edited by VortecCPI

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i cannot see the updated spreadsheet. the first one is wrong. excluding "2 bbr springs" force increases too much going from 6 to 6.25 inches

Edited by 30x26

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i cannot see the updated spreadsheet. the first one is wrong. excluding "2 bbr springs" force increases too much going from 6 to 6.25 inches

In your initial post you stated:

"change between "soft" and "hard" spring rate happens when compression is equal to (distance between short spaced coils) x (number of active coils). in a stock springs it happens at about 2.5mm x 62 active coils = 155mm of compression"

Therefore the close coils collapse at 155mm, which is 6.1 inches.  This should be the point when the stock spring goes from 0.32 kg/mm to 0.50 kg/mm.  The stock fork springs are not progressive springs -- They are dual-rate springs.

Which part of your post am I misinterpreting?  If you give me the location at which the coils bind and the rate changes I will update the spreadsheet.

Edited by VortecCPI

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look at "2 stock springs"

5.75 = 93.5kg

6 =97.5kg

6.25 = 158.8kg !!!  instead of 97.5 + (0.50kg x 6.35mm  x 2 springs) = 103.85

Edited by 30x26

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look at "2 stock springs"

5.75 = 93.5kg

6 =97.5kg

6.25 = 158.8kg !!!  instead of 97.5 + (0.50kg x 6.35mm  x 2 springs) = 103.85

Okay.  Now I see.  Thank you for the clarification.  Changes are on the way...

Great catch.  I revised the spreadsheet and uploaded a new version.  Please have a look when time permits.

Edited by VortecCPI

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i've read the second spreadsheet. i assume you are predicting air pressure.

it's not easy, you must consider volume of cap and *spring*.

another variable: if your oil height is 150mm (compressed without spring) it will be 450mm extended without spring (i measured it in the past). difference  is not equal to max travel of the fork.....

max travel is 230mm metal to metal, in the real world you will never see more than 225mm, even after a catastrophic landing

Edited by 30x26

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i've read the second spreadsheet. i assume you are predicting air pressure.

it's not easy, you must consider volume of cap and *spring*.

another variable: if your oil height is 150mm (compressed without spring) it will be 450mm extended without spring (i measured it in the past). difference  is not equal to max travel of the fork.....

max travel is 230mm metal to metal, in the real world you will never see more than 225mm, even after a catastrophic landing

Also excellent points to consider.  I can calculate the spring volume and reduce the cylinder volume accordingly.

The spreadsheet does consider cylinder length based upon fork oil level but it does not match your findings.

I thought the numbers were way too low.

Edited by VortecCPI

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......

another variable: if your oil height is 150mm (compressed without spring) it will be 450mm extended without spring (i measured it in the past). difference  is not equal to max travel of the fork.....

...........................

Agree, that is because the slider ID is greater the the stanchion ID so the oil volume from the slider moving up one inch is greater than  one inch of stanchion volume.

So I think the best way to determine the compression ratio of the air spring is to measure the oil height at full compression, and then at full extension. then factor in the fork cap, etc. Adding in the volume of the spring wire might be more difficult as it is only partially in the oil at full compression.

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Agree, that is because the slider ID is greater the the stanchion ID so the oil volume from the slider moving up one inch is greater than  one inch of stanchion volume.

So I think the best way to determine the compression ratio of the air spring is to measure the oil height at full compression, and then at full extension. then factor in the fork cap, etc. Adding in the volume of the spring wire might be more difficult as it is only partially in the oil at full compression.

Yes, sir.  This becomes complicated very quickly but the impact if the air spring is big.  I removed the air bleed screws on the XR250 and did a push test and the forks felt significantly different.  For reference it has a .38 kg/mm fork spring and a .46 kg/mm fork spring.