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  1. #1
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    Converting Lbs to Psi

    I feel I'm between springs on my fork and would like to understand something.

    I used to put 65psi in my fork! The air spring has been replaced by a coil and I can now either use a 45lbs or 50lbs spring.

    Both rates can match my weight (ready to ride) because they overlap.

    45lbs: super supple and erases all the small chatter, but dives way too much when you start attacking.

    50lbs: transmits all the small/medium impacts, square edges ... but doesnít dive when attacking.

    Thatís why I feel I'm between 2 rates.

    Is there a way to convert a spring rate to a psi equivalent? Just to understand the gap between a 45/50 spring. Is it like adding/removing 5psi or 25psi? I totally get my head around air pressures ... so for example a 5psi increase will add a bit more support and prevent diving a bit more but it will still be in the same ballpark comfort-wise.

    So how many Psi like pressure represents a swap between a 45lbs vs 50lbs?

    If you tell me itís like adding 20psi then I totally get why itís so different.

    Thanks for your help guys!

  2. #2
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    Sounds like you need the 45lbs spring with a little more high speed compression damping.

    To be cheeky, the two measurements have nothing to do with each other. lbs/in completely characterizes a coil spring, while the pressure of an air spring doesn't tell you much about its spring rate, which is affected by other factors as well (temperature, volume, +/- chamber ratio).

  3. #3
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    Thx. So thereís no way to translate a 5lbs increase in spring rate into Psi?!

    At least on the first 1/3 of the fork just to get rid of other variables.

  4. #4
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    Converting Lbs to Psi

    Would pushing the fork against a scale and measure the weight vs SAG help understand the difference between the 2 spring rates? (And do the same with an air spring to try to compare the 2)

  5. #5
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    OP "I feel I'm between springs"

    Years ago when coil was king I heard this statement more often than not. So to break down your two impressions :

    "45lbs: super supple and erases all the small chatter, but dives way too much when you start attacking."

    -You are too heavy & or aggressive for this spring rate-

    " 50lbs: transmits all the small/medium impacts, square edges ... but doesnít dive when attacking.

    - You are too light or not riding aggressive enough -

    Your options are to adjust dampning to blend the characteristics of the linear coil spring.

  6. #6
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    This is very easy to do. I'd first calibrate the load cell with a control mass of approximately 50kg. Open up Excel and chart some points, then do a best fit interpolation. For a hanging scale, you could put the fork upside down in a vice and hang the scale on the through axle, run a cable down through the steerer to a piece of all thread, or just a loop for your foot.

  7. #7
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    From a physics/engineering standpoint, a coil spring generally obeys Hooke's Law, which is the Force delivered by a spring is proportional to its displacement x, or

    F=kx, where k is the spring constant or rate (your 45 or 50 lbs, really lbs/inch) and x is the displacement or deflection of the spring.

    The air spring is more complex and probably not linear. The pressure in an air spring (your 65 psi) gives the overall assembly some sort of spring rate as in Hooke's Law, but it likely isn't a constant k, rather something that varies with the amount of displacement or deflection of the spring (in addition to other factors).

    So other than by empirical measurement (which would be difficult as the posts above allude to), there's no way to correlate the two.

  8. #8
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    OK! if thereís no way to guess the relation between the two I only have one option left. Test, measure and compare.

    I'll find a way to solve this but it would have been cool to back up it up with some numbers.

    Thanks everyone

  9. #9
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    Quote Originally Posted by TwiceHorn View Post
    ...The air spring is more complex and probably not linear...
    Probably?
    Do the math.

  10. #10
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    Maybe you need a 45lb progressive spring, so that it becomes stiffer further into travel yet remains softer as the travel begins.

    Do spring forks/shocks have a sag system to adjust.

    Motorcycles have a lock nut to tighten or loosen to set sag, that twists the spring to be just soft enough that once the rider (of appropriate weight) digs into the travel they are in the optimal range.

    I have a spring rate too stiff for my weight, I can loosen preload all the way and not meet sag requirements. I need a lighter spring rate. However, for general riding I am okay, but I've had to adjust rebound properly to not launch me upward.

    As mentioned above, you probably need to change the damping.

    It is similar to your vehicle. If you remove shocks from your vehicle you really aren't able to control compression or rebound. Installing a shock that is meant for the vehicle weight allows proper damping.

  11. #11
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    Does it really matter what the exact equivalent air pressure would be if 45 and 50 lbs/in are your only options anyway? Just pick the coil feels better to you on the trail. I would try running less compression damping with the 50 lb/in spring and more compression damping with the 45 lb/in spring to see how that feels. I assume you're adjusting your rebound to account for the change in spring rate, but if not, you should definitely do that too. You should also try raising your bars a bit with the lighter spring to offset the increased sag and that diving you're feeling (https://www.youtube.com/watch?v=Inrf...youtu.be&t=511).

  12. #12
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    Here's the kicker: Springs vary wildly per spec. 7% both ways being the range many suppliers operate in. (IOW, if you buy another 45#, it may be perfect. Or even further off than the one you have currently) If you havent measured your springs, you dont know the rate. That's the other fun part of coil

  13. #13
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    There is no straight-forward way. Even when you do all the work to map out an air-spring, they behave differently when compressed at different speeds and when the overall rate matches the mid-stroke is a bit weak.

    It sounds like you don't need more spring-rate. You need LSC damping that works properly without making the fork harsh.

    What is the fork and setup?
    Owner of www.shockcraft.co.nz, Mech Engineer, Tuner, Manitou, Motorex, Vorsprung EPTC, SKF, Enduro
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  14. #14
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    Coil spring rates and air spring rates don't match up well at all and they behave so differently its hard to see a correlation. BUT to make a comparison using sag, a 170lb rider changing from a 45 to 50lb spring should see about 4mm change in sag. Theoretically (with a capital T) the same rider would need a roughly 5psi adjustment to see the same change in a 36.

    This isn't a comment on whether sag is important or not, and any 2 springs at the same sag (let alone coil or air) will not feel the same anyway, just a way to compare the size of adjustments.

    5lb is about the smallest increment you will see a fork spring come in, it is generally accepted that you need at least a 10% change for anything to make a noticeable or worthwhile difference, so since 45 is 10% less than 50 you are already as close as you can get between rates

    Have you tried raising handlebars?
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  15. #15
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    See why we went to air?

    Go coil for the consistency and zero maintenance. It's awesome for that. It's lacks in tuning overall, you just pick the rate with the least compromise.

  16. #16
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    I'd try a ~5mm puck between the top cap and 45# spring. Still too harsh? Shave it down to 3.5mm and try again.

  17. #17
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    Converting Lbs to Psi

    Thank you all for the brainstorming! it helped ...

    - I kept the 45lbs spring but used another preload adjustment spacer (2mm). So I now use 3 instead of the 2 max recommended by Push
    - Added a 5mm spacer under the stem
    - At the same time I dropped the spring rate on my shock (from 450 to 400) ... I had a doubt since the beginning about the spring rate and when Dougal released his ęadvanced spring rate calculatorĽ he did a few for free ... he told me 45lbs FR and 400lbs RR ... and to be honest I was skeptical about going so low ... he nailed it! Thanks man

    I'm blown away by the result

    1. Because itís now perfectly balanced front and back. No more diving, still compliant (both at any speed)
    2. Because to make it RIGHT, it was just a matter of adding a few mm here and there. I really wouldnít have thought it would make such a difference.

    The lesson to take away here I guess is, if you feel that you are in between springs ...

    Try these:

    - go for the softer one but add more preload
    - add a spacer under the stem to raise your bars to put less weight on the fork.
    - (maybe) add a bit more SAG to the shock so it will shift some weight away from the front

    Those changes donít need to be dramatic, a few mm can put you in the right spot.

    It works brilliantly, so happy

    Thanks to everyone who contributed to this thread, even if the initial post turned into a problem solving one.
    (Kinda glad it did)

    FYI: https://www.shockcraft.co.nz/technic...te-calculators

  18. #18
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    Only just got around to reading this thread, bit late to the party, but my method seems unique.

    If you are half way between rates, mix spring rates. I bought some die springs from leespring and worked out exactly what I wanted.

    I even went fancy pants to get a progressive coil by combining very differing spring rates. Not hard maths.

    Die springs tend to be a bit heavier than fork springs, but a lot stronger. I've broken a few fork springs, never a die.

  19. #19
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    Converting Lbs to Psi

    You know what? I thought about it ... combining 2 springs with 2 different rates and make it however you want.

    Ex: 45lbs for the first 50% then 50lbs ... to make it "kind of" progressive

    I thought about cutting 2 springs in half but how do you join/reconnect them afterwards?! Thatís was the issue.

    MRP does it with shock springs. So you get for example a 350lbs at 25% SAG then it becomes a 375lbs at 90% SAG. I think itís brilliant even if the bike's kinematic should normally take care of that ramp up. But on forks with a 1:1 ratio we are screwed ... so it would really be nice to have the same option. I only know one guy (Seanan O'Riordan) from raceonlysprings.com who makes some for the boxxer but thatís it ...

    https://mrpbike.com/products/enduro-...e-coil-springs

    If anyone has any idea about were to source some or even better ... how to make some (by mixing them for example), itís still interesting to know.
    Last edited by digev; 10-07-2019 at 10:50 AM.

  20. #20
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    Did you manage to test the springs to see what the actual resulting progression was?

    Off road vehicles use a floating collar on a short spring with very soft rate that gets blocked at a certain point in the travel so you get a plush feeling at the top then your normal rate for the rest of the stroke. The transition point is adjustable as well. Potentially would be too noisy on an mtb, and packaging it in to a fork would would be especially hard but itís a nice idea

  21. #21
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    Quote Originally Posted by digev View Post
    Thank you all for the brainstorming! it helped ...

    - I kept the 45lbs spring but used another preload adjustment spacer (2mm). So I now use 3 instead of the 2 max recommended by Push
    - Added a 5mm spacer under the stem
    - At the same time I dropped the spring rate on my shock (from 450 to 400) ... I had a doubt since the beginning about the spring rate and when Dougal released his ęadvanced spring rate calculatorĽ he did a few for free ... he told me 45lbs FR and 400lbs RR ... and to be honest I was skeptical about going so low ... he nailed it! Thanks man

    I'm blown away by the result

    1. Because itís now perfectly balanced front and back. No more diving, still compliant (both at any speed)
    2. Because to make it RIGHT, it was just a matter of adding a few mm here and there. I really wouldnít have thought it would make such a difference.

    The lesson to take away here I guess is, if you feel that you are in between springs ...

    Try these:

    - go for the softer one but add more preload
    - add a spacer under the stem to raise your bars to put less weight on the fork.
    - (maybe) add a bit more SAG to the shock so it will shift some weight away from the front

    Those changes donít need to be dramatic, a few mm can put you in the right spot.

    It works brilliantly, so happy

    Thanks to everyone who contributed to this thread, even if the initial post turned into a problem solving one.
    (Kinda glad it did)

    FYI: https://www.shockcraft.co.nz/technic...te-calculators
    Great to know the system is working! We've had almost 200 people using our on-line spring calculators in the short time they've been up.
    Owner of www.shockcraft.co.nz, Mech Engineer, Tuner, Manitou, Motorex, Vorsprung EPTC, SKF, Enduro
    www.dougal.co.nz

  22. #22
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    Quote Originally Posted by Dougal View Post
    Great to know the system is working! We've had almost 200 people using our on-line spring calculators in the short time they've been up.
    Yes it does, thatís really cool. Thanks again. Push was telling me to get the 50lbs to fix my diving issues and I did and it was horribly harsh and the spring suggestion for the back (using the CC calculator) was also off. 450 instead of 400! A good calculator should at least take the front and back into account to get the bike balanced ... for me thatís the most important. Good job

  23. #23
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    Oh I almost forgot

    I paid j-techsuspension.co.uk for a ęspring calculation requestĽ, months ago. And they were recommending a 500lbs!

    Iím surprised that youíre only getting 17.5mm sag with a 450 lb/in spring fitted, that is the exact amount that our software indicates you should have with a 500lb/in spring fitted and thatís the figure we would recommend, in terms of sag. Iíve re-check the calculation and found no fault
    When I asked them to re-run the test ... letís just say it didnít go too well and they nicely told me to f*ck off! Great service

    And now, months later ... I eventually find out what was needed (I knew/felt I needed less but I didnít know by how much) ...

    I thought Iíd share that story if it can help anyone save countless hours of testing, frustration, ruined rides and of course $$$

    Anyway, now letís find a way to make some progressive springs

  24. #24
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    Iím surprised theyíre expecting the real sag to be the same as calculated sag🤔 between different weight distribution, friction and gas pressure thereís going to be huge discrepancy!

  25. #25
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    Quote Originally Posted by digev View Post
    You know what? I thought about it ... combining 2 springs with 2 different rates and make it however you want.

    Ex: 45lbs for the first 50% then 50lbs ... to make it "kind of" progressive

    I thought about cutting 2 springs in half but how do you join/reconnect them afterwards?! Thatís was the issue.
    Unfortunately, that's not how coils work. If you managed to cut two coils in half and stack them inside the fork, they would both compress simultaneously resulting in a linear spring rate that's roughly half way between the two springs rates you combine (~47.5 lbs in this case).

    If you wanted a true dual rate (aka "progressive") coil, you need one of the two coils to stop compressing mid-travel. This is typically accomplished either by using a crossover ring that essentially bottoms out one of the coils before the shock bottoms out, or by using a more tightly wound coil that partially binds before the shock bottoms out.

    Quote Originally Posted by digev View Post
    MRP does it with shock springs. So you get for example a 350lbs at 25% SAG then it becomes a 375lbs at 90% SAG. I think itís brilliant even if the bike's kinematic should normally take care of that ramp up. But on forks with a 1:1 ratio we are screwed ... so it would really be nice to have the same option. I only know one guy (Seanan O'Riordan) from raceonlysprings.com who makes some for the boxxer but thatís it ...

    https://mrpbike.com/products/enduro-...e-coil-springs

    If anyone has any idea about were to source some or even better ... how to make some (by mixing them for example), itís still interesting to know.
    If you have a look at the MRP progressive coils, you can see that they employ the latter technique. The more tightly wound portion of the coil is designed to bind mid-travel, which basically reduces the length of coil that can be compressed and results in a firmer spring rate.

  26. #26
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    Arf... donít get me started. I was telling them I needed a lower spring rate but needed to find out how many lbs I should drop ... and they got back to me with a firmer one. Go figure! Thatís the problem with people being focused mainly on SAG ... and they spread that info to the community, thatís BS. Your suspensions can suck, if the SAG is right ... too bad, not my problem ... LOL!

  27. #27
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    Quote Originally Posted by dlxah View Post
    Unfortunately, that's not how coils work. If you managed to cut two coils in half and stack them inside the fork, they would both compress simultaneously resulting in a linear spring rate that's roughly half way between the two springs rates you combine (~47.5 lbs in this case).

    If you wanted a true dual rate (aka "progressive") coil, you need one of the two coils to stop compressing mid-travel. This is typically accomplished either by using a crossover ring that essentially bottoms out one of the coils before the shock bottoms out, or by using a more tightly wound coil that partially binds before the shock bottoms out.



    If you have a look at the MRP progressive coils, you can see that they employ the latter technique. The more tightly wound portion of the coil is designed to bind mid-travel, which basically reduces the length of coil that can be compressed and results in a firmer spring rate.
    Thanks!

    yes thatís what they say here about the rate of springs in series.

    https://www.acxesspring.com/calculat...in-series.html

    Keq = (k1)(k2) / (k1 + k2)

    Keq = Equivalent Rate
    k1 = Rate of spring 1
    k2 = Rate of spring 2
    Keq = (20)(40) / (20 + 40)
    Keq = 800 / 60

    Keq = 13.333 lbf/in (pounds of force per inch)
    Damn, It would have been too easy

    I created another thread called ęProgressive Fork SpringsĽ to see if anyone has info about what could be done or whatís already on the market. Itís frustrating to see them available for motorbikes and not for MTBs.

  28. #28
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    The Push ACS is basically a progressive spring

    I received my backordered MRP spring this week and it went straight on the dyno, you can see it in action and how it works here (yes you need an instagram account) -

    https://www.instagram.com/stories/hi...6642683/?hl=en
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    Servicing in Rotorua, NZ/Vorsprung Elite Tuning Centre/DVO service centre/Insta @thesuspensionlab

  29. #29
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    Quote Originally Posted by JohnnyC7 View Post
    The Push ACS is basically a progressive spring
    Well, yes and no!

    Yes, because they have a ramp up feature (more a bottom out safety net)
    No, because they rely on other parts to do the job of a "progressive" spring.

    Why do that instead of making a progressive spring? I donít get it.

    Letís watch those dyno videos

  30. #30
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    Quote Originally Posted by digev View Post
    ...parts to do the job of a "progressive" spring.

    Why do that instead of making a progressive spring? I donít get it.
    Srsly? This is not a Car/Truck/Mo'cycle where there's static-weight. System-weight (rider + bicycle) is a constant-variable to anyone designing suspension, why there are 25lb increment Shock coil springs.

    What riding is needing the entire range of tuning altered around your fancy, Fancy??
    I like Sand - I don't like Witches


  31. #31
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    Just back to the original question, % spring rate is probably similar to % air pressure.

    50lb->45lb = 10% difference
    65psi->58.5psi = 10% difference

    If you just wanted a SWAG.

  32. #32
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    Quote Originally Posted by digev View Post
    Well, yes and no!

    Yes, because they have a ramp up feature (more a bottom out safety net)
    No, because they rely on other parts to do the job of a "progressive" spring.

    Why do that instead of making a progressive spring? I donít get it.
    It is essentially a progressive spring. It's a linear coil spring for the first 2/3 of travel, and then it's a coil spring and an air spring working in parallel to create a progressive spring rate in the last 1/3 of your travel.

    The air bump stop allows for more adjustibility and keeps the cost down. You can choose whatever coil spring rate you want for the 2/3 of travel, and then dial in your bottom out support by adding or removing air pressure from the bump stop. If Push wanted to offer a similar level of adjustibility with a progressive coil, they would have to design, manufacture, and stock a huge number of different coils for every possible combination of initial spring rate + amount of bottom out support you might want.
    Last edited by dlxah; 10-11-2019 at 11:06 AM.

  33. #33
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    Quote Originally Posted by digev View Post
    Yes it does, thatís really cool. Thanks again. Push was telling me to get the 50lbs to fix my diving issues and I did and it was horribly harsh and the spring suggestion for the back (using the CC calculator) was also off. 450 instead of 400! A good calculator should at least take the front and back into account to get the bike balanced ... for me thatís the most important. Good job
    I had the same issues sizing turbochargers. I was taking a different approach and getting massively different results to everyone else. But I followed my own calcs and loved the results. Slowly others started trying and trusting my figures and I've got hundreds worldwide running my recommendations.

    I'm doing spring rate calculations in a completely different way to everyone else and getting significantly different results. But they work extremely well for me.

    They work so well I can hang with riders who I know are far better and faster than me.
    Owner of www.shockcraft.co.nz, Mech Engineer, Tuner, Manitou, Motorex, Vorsprung EPTC, SKF, Enduro
    www.dougal.co.nz

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