Spring lengths with same spring rate - is there is a difference?

[ninekrpm]

I am looking for some technical feedback on spring lengths that are same in rates (lb/in)...

For example 6" 900# vs 5" 900#

Making a few adjustments on the car and likely going to a 900# rear, I have a 5" on hand...and have never used a 5" in the back (coilover).

Always ran 6" or 7"...assuming I can make the 5" work on the damper, the question is does length matter when the rate is the same? Would there be a performance difference? Would one respond different due to number of coils?

[M3SQRD]

Only difference is spring travel before it coil binds. Coil diameter (not spring diameter) and spacing are different to achieve the same rate for different length springs.

For Hyperco springs:

900 lbf/in, 2.25”-dia, 5” free length has 2.794” of travel/deflection

900 lbf/in, 2.25”-dia, 6” free length has 3.257” of travel/deflection

[jfritz27]

Originally Posted by M3SQRD

Only difference is spring travel before it coil binds. Coil diameter (not spring diameter) and spacing is different to achieve the same rate for different length springs.

... but to piggyback on the preload discussion on the other thread, for a given ride height and all other things equal, a longer spring will result in greater preload than a shorter spring, yes?

[M3SQRD]

Originally Posted by jfritz27

... but to piggyback on the preload discussion on the other thread, for a given ride height and all other things equal, a longer spring will result in greater preload than a shorter spring, yes?

To maintain the same ride height, you’ll have to lower the spring perch (decrease preload) so it’ll depend on how the delta lengths of the compressed length to the free length changes. However, the longer spring of the same rate will likely be compressed more for the same ride height and, therefore, have a higher preload.

[M3SQRD]

Originally Posted by jfritz27

... but to piggyback on the preload discussion on the other thread, for a given ride height and all other things equal, a longer spring will result in greater preload than a shorter spring, yes?

Speaking of spring length, you can install a spring rubber, which makes a pair of coils in the spring “rigid” and fixes their relative length, and, therefore, increase the stiffness of the spring without having to swap springs. This allows you to quickly test a stiffer spring setup before committing to a spring change.

[M3SQRD]

Originally Posted by jfritz27

... but to piggyback on the preload discussion on the other thread, for a given ride height and all other things equal, a longer spring will result in greater preload than a shorter spring, yes?

Speaking of travel, you can add bump stop shims to limit damper travel by engaging the bump stop sooner. So many variables to play with

[FaRKle!]

Originally Posted by jfritz27

... but to piggyback on the preload discussion on the other thread, for a given ride height and all other things equal, a longer spring will result in greater preload than a shorter spring, yes?

Originally Posted by M3SQRD

To maintain the same ride height, you’ll have to lower the spring perch (decrease preload) so it’ll depend on how the delta lengths of the compressed length to the free length changes. However, the longer spring of the same rate will likely be compressed more for the same ride height and, therefore, have a higher preload.

The preload could be different if you're trying to keep your springs in a certain working range (% of free length). For example if a spring mfg says "the rate is linear up to 95% of free length." then you'd want at least a 5% preload, and 5% of the longer spring will be a greater compression distance than 5% of the shorter spring.

Once you're in your linear rate region though, there isn't going to be any difference in additional compression for the same ride height since the rates are the same.

[robbo mcs]

It will make a difference, for all the reasons listed above.

Assuming you changed nothing else with the coilover, then for a shorter spring vs a longer spring of the same rate :

The car will sit lower
The shock piston will be more compressed at rest
There will be less useful suspension travel under load / weighting before the spring binds and/or the shock runs out of compression range
There will be more available shock extension / droop on unweighting
There is more potential for the spring to be loose on unweighting / droop

You can compensate to some degree for those by adjusting the perch height, preload, tender springs etc.

Also worth noting, if you put a stiffer spring on, exactly the same length as the current spring, and you adjust nothing else on the shock, then exactly the reverse is true, the car will sit higher, etc, etc. However, the effect will be much less dramatic, as the increase in height will be much less than an inch

It also worth noting that when you put lowering springs on the standard shocks, the car sits lower because they are shorter, and much of the above applies

[jfritz27]

So let's say you have an MCS-style shock and a linear spring for sake of argument... then you sequentially make adjustments such that the spring becomes increasingly compressed with each adjustment (the static ride height will change as this is done). We do this just up to the point of (but not attaining) coil bind.

In doing so, the preload increases with each successive adjustment? If so, does it do this approximately linearly? And therefore with each successive adjustment, the driver would perceive that end of the car to behave with increasing stiffness on compression (similar to if more bump was dialed in on the shock)? (Because the more preload, the more force is needed to be overcome before the piston starts to move, as M3SQRD explained in the other thread?)

Always been a bit confused about preload, trying to get this straight...

[M3SQRD]

jfritz27

The spring has a long linear range with nonlinearities at the beginning and end of travel/stroke. The nonlinear stiffness region is a small % of total spring length. This % varies with spring stiffness and length and spring manufacturer. You’ll either get a % of stroke or a useable displacement vs. a max displacement to define the linear range of the spring load-displacement curve. The higher the preload, the higher the applied force required before piston relative motion begins and, therefore, the larger the bumps the tires will skip over without compressing (i.e., no damping) rather than keeping the tire in contact with the track surface (i.e., maintain grip/contact between the tire and surface). Basically, preload acts like a bump size filter that the damper doesn’t respond to. This would definitely be perceived as a stiffer and harsher response.

If you have a home/portable N2 tank setup to set reservoir pressures, you can run the min recommended pressure on the street to improve ride quality and harshness by minimizing the reservoir pressure lifting force and, thus, getting the dampers to respond to smaller bumps, undulations, etc.

[jfritz27]

Thanks for this.

To further clarify, once the spring is compressed sufficiently to overcome that initial, small % that comprises its non-linear portion, and is now fully on its linear part of the curve, then preload ceases to increase with further static compression? Or it continues to increase (just not as much)?

I'm basically trying to wrap my head around whether it's true that effective compression "damping" is altered on some continuum throughout a spring's static compression range, indicating that ride height is a dynamic determinant of compression characteristics across the entire compression range of the spring (something that I was not aware of before) -- but to my point above, is this true only for that small % of a spring's non-linear range?

[M3SQRD]

Originally Posted by jfritz27

Thanks for this.

To further clarify, once the spring is compressed sufficiently to overcome that initial, small % that comprises its non-linear portion, and is now fully on its linear part of the curve, then preload ceases to increase with further static compression? Or it continues to increase (just not as much)?

I'm basically trying to figure out whether it's true that effective compression "damping" is altered on some continuum throughout a spring's static compression range, indicating that ride height is a dynamic determinant of compression characteristics (something that I was not aware of before) -- but to my point above, is this true only for that small % of a spring's non-linear range?

Preload still increases with changes to static ride height even after the initial nonlinear load-deflection relationship.

The damper generates force based on the relative vel of the piston rod and damper body. It’s only on the lack of initial relative displacement and velocity due to the preload preventing the piston rod from moving. Once it starts moving, the damper force varies linearly with piston rod relative velocity and spring force varies linearly with piston rod relative displacement. At the end of spring travel the nonlinear spring load-displacement relationship (if it exists, not all springs exhibit this behavior and instead remain linear until they reach their block height) does not alter/increase the initial spring preload and no effect on damper response because its relationship is still linear with relative velocity. There will be a change in spring force and rate because its response now varies nonlinearly with relative displacement. The frequency of the sprung mass also changes (proportional to the square root of nonlinear stiffness/sprung mass). However, we’re talking about the last ~0.25” of travel but, more importantly, you’ll likely reach the block height of the spring (coil bind). Unless you’re Lewis Hamilton or Max Verstappen, I doubt you’d notice the changes in stiffness/frequency as you approach the end of spring travel. You absolutely will feel the coil bind condition and the sudden loss of grip.

So preload only affects initial motion. Once preload is exceeded, it doesn’t change/increase with increasing relative displacement. Now in the linear range of spring travel, the response of the spring force/rate and damper force/coefficient will respond linearly with relative displacement and velocity, respectively. Spring end of travel motion, practically speaking, will have a negligible effect on spring force and sprung mass frequency as well as no effect on damping force. Coil bind will have a very noticeable instantaneously effect on spring rate, spring force, sprung mass frequency and an abrupt loss of grip, as well as no effective change in damping force.

I hope the above makes sense

[jfritz27]

Originally Posted by M3SQRD

Preload still increases with changes to static ride height even after the initial nonlinear load-deflection relationship.

The damper generates force based on the relative vel of the piston rod and damper body. It’s only on the lack of initial relative displacement and velocity due to the preload preventing the piston rod from moving. Once it starts moving, the damper force varies linearly with piston rod relative velocity and spring force varies linearly with piston rod relative displacement. At the end of spring travel the nonlinear spring load-displacement relationship (if it exists, not all springs exhibit this behavior and instead remain linear until they reach their block height) does not alter/increase the initial spring preload and no effect on damper response because its relationship is still linear with relative velocity. There will be a change in spring force and rate because its response now varies nonlinearly with relative displacement. The frequency of the sprung mass also changes (proportional to the square root of nonlinear stiffness/sprung mass). However, we’re talking about the last ~0.25” of travel but, more importantly, you’ll likely reach the block height of the spring (coil bind). Unless you’re Lewis Hamilton or Max Verstappen, I doubt you’d notice the changes in stiffness/frequency as you approach the end of spring travel. You absolutely will feel the coil bind condition and the sudden loss of grip.

So preload only affects initial motion. Once preload is exceeded, it doesn’t change/increase with increasing relative displacement. Now in the linear range of spring travel, the response of the spring force/rate and damper force/coefficient will respond linearly with relative displacement and velocity, respectively. Spring end of travel motion, practically speaking, will have a negligible effect on spring force and sprung mass frequency as well as no effect on damping force. Coil bind will have a very noticeable instantaneously effect on spring rate, spring force, sprung mass frequency and an abrupt loss of grip, as well as no effective change in damping force.

I hope the above makes sense

I'm sure it does, but I'll need to chew on it for 1-2 weeks

Thanks for the detail

[M3SQRD]

Originally Posted by jfritz27

I'm sure it does, but I'll need to chew on it for 1-2 weeks

Thanks for the detail

lol.. we haven’t considered the manufacturing tolerance on spring rates yet and the effect it has on handling

+/-3% is typical. Hyperco is designed for +/-2% but actual measured values are in the +/-1% range. Hyperco used to etch the measured spring rate on the top flat mounting coil so you could easily match spring rates. I forgot to look for this on the last three MCS setups I’ve installed.

[FaRKle!]

Originally Posted by M3SQRD

lol.. we haven’t considered the manufacturing tolerance on spring rates yet and the effect it has on handling

+/-3% is typical. Hyperco is designed for +/-2% but actual measured values are in the +/-1% range

Or the effect of damper gas force (typically variable based on where shaft is in its travel, creates parallel spring to mainspring), and seal drag

A lot of aftermarket OE style springs have much worse tolerance, upwards of +/-5%. BMW's tolerance is +/-4%.

[M3SQRD]

Originally Posted by FaRKle!

Or the effect of damper gas force (typically variable based on where shaft is in its travel, creates parallel spring to mainspring), and seal drag

A lot of aftermarket OE style springs have much worse tolerance, upwards of +/-5%. BMW's tolerance is +/-4%.

Gas pressure force acting like a spring in parallel was discussed somewhere. Also briefly discussed the flat ride concept.

Did not know BMW was 4%. Surprised OE style lowering springs are as accurate as 5%.

Let’s use BMW (oops accidentally used 5%, not 4% in calcs below) as an example. If they were using a 600 lbf/in spring and wanted to switch to a 700 lbf/in spring, with tolerances it would look like this:

600 lbf/in +/-5% is 570-630 lbf/in
700 lbf/in +/-5% is 655-735 lbf/in
Which means worse case you’d go from 570 lbf/in to 735 lbf/in! Much different from the 100 lbf/in increase and then you can also see the range of variability between F & R L-R spring rates. Will definitely impact handling characteristics.

[ninekrpm]

Guys, so much great detail here...and like someone said, it will take 1 - 2 weeks to digest

For me...I think I am going to spend the $150 and get the 6" spring (rear) as opposed to using the 5" to ensure there is enough height adjustment and good travel.

Going to read again...and see how much more I can digest this time around.

Here is the last reported weight of the car (with driver), at that time running 900/800 springs. Did manage to take out another 70ish #'s...however, will be upgrading the springs to 1000/900.

[M3SQRD]

Originally Posted by ninekrpm

Guys, so much great detail here...and like someone said, it will take 1 - 2 weeks to digest

For me...I think I am going to spend the $150 and get the 6" spring (rear) as opposed to using the 5" to ensure there is enough height adjustment and good travel.

Going to read again...and see how much more I can digest this time around.

Here is the last reported weight of the car (with driver), at that time running 900/800 springs. Did manage to take out another 70ish #'s...however, will be upgrading the springs to 1000/900.

Nice weight reduction and weight distribution

With the scales, you’ll be able to get the ride heights and cross weights dialed in for the new spring rate and length changes.