suspension and spring interactions
Discussion
brain teaser (maybe) for the weekend.
take two linear springs with a rate of 300lb/inch and measuring 8 inches long. squeeze the springs horizontally between two vertical gate posts placing a block in the middle (creating an H shape with post,spring,block,spring,post).
both springs are compressed 3 inches.
how much force do you have to apply to the block in the middle to compress the spring on the left by one more inch?
take two linear springs with a rate of 300lb/inch and measuring 8 inches long. squeeze the springs horizontally between two vertical gate posts placing a block in the middle (creating an H shape with post,spring,block,spring,post).
both springs are compressed 3 inches.
how much force do you have to apply to the block in the middle to compress the spring on the left by one more inch?
Edited by fioran0 on Saturday 13th April 03:58
jacks got it, 600lbs.
the spring is rated at 300lbs/inch meaning it should need only 300lbs to compress it an inch further however it actually takes 600lbs. the spring that is uncompressing as you push the block has the effect of changing the effective rate of the spring being compressed from 300lbs/inch to 600lbs/inch.
to compress it 3 inches from starting point would require 1800lbs.
once the spring on the other side is fully extended however it no longer plays a part and the effective rate instantly halves. you only need half as much effort (ie 300lbs) for each inch of movement now compared to the previous inches. in the above example it would take 1800lbs to move it 3 inches but 2100lbs to move it 4 inches and 2400lbs to move it 5 inches.
its quite interesting in itself to realise that a spring thats uncompressing can impact the movement resistance of another spring. it gets even more interesting when you realise that your cars weight is the wooden block.
this is some of the reason that tender springs are used to create a dual spring setup and why anyone fitting aftermarket suspension or changing their setup and springs need to pay careful attention.
if your car reaches a point during a corner where the unloaded side fully extends, the resistance to roll from the loaded side instantly halves.
the spring is rated at 300lbs/inch meaning it should need only 300lbs to compress it an inch further however it actually takes 600lbs. the spring that is uncompressing as you push the block has the effect of changing the effective rate of the spring being compressed from 300lbs/inch to 600lbs/inch.
to compress it 3 inches from starting point would require 1800lbs.
once the spring on the other side is fully extended however it no longer plays a part and the effective rate instantly halves. you only need half as much effort (ie 300lbs) for each inch of movement now compared to the previous inches. in the above example it would take 1800lbs to move it 3 inches but 2100lbs to move it 4 inches and 2400lbs to move it 5 inches.
its quite interesting in itself to realise that a spring thats uncompressing can impact the movement resistance of another spring. it gets even more interesting when you realise that your cars weight is the wooden block.
this is some of the reason that tender springs are used to create a dual spring setup and why anyone fitting aftermarket suspension or changing their setup and springs need to pay careful attention.
if your car reaches a point during a corner where the unloaded side fully extends, the resistance to roll from the loaded side instantly halves.
Edited by fioran0 on Saturday 13th April 18:32
Interesting point well made.
If the block is 1" further to the left the LH spring now is compressed 4" abs has a load on the block from the LH spring of 1200 whereas the RH block is released 1" is now compressed 2" and has a load applied to the block by the RH spring of 600 so a balancing load would have to be the difference of 600 additional force to the left to keep the block there. I have answered your question (but as I expected I look at it a different way - something I cannot help) and you have said how easy it is to set up a 996 (I think you did anyway).
If so - here's one for you - that also can demonstrate what you know in a practical rather than theoretical way - what spring rates should be applied to the front and rear of a 996 with 40% 60% weight distribution front to rear) and a Boxster (with 45% 55% weight distribution) and what proportion of the total roll stiffness front and rear should the ARB have comnpared to the springs at the front and rear of the same 996 weighing 1405 kilos compared to the Boxster weighing 1310 kilos?.
Answer's probably available on the fastest 996 at Brands Hatch 1st week in May!
Baz
If the block is 1" further to the left the LH spring now is compressed 4" abs has a load on the block from the LH spring of 1200 whereas the RH block is released 1" is now compressed 2" and has a load applied to the block by the RH spring of 600 so a balancing load would have to be the difference of 600 additional force to the left to keep the block there. I have answered your question (but as I expected I look at it a different way - something I cannot help) and you have said how easy it is to set up a 996 (I think you did anyway).
If so - here's one for you - that also can demonstrate what you know in a practical rather than theoretical way - what spring rates should be applied to the front and rear of a 996 with 40% 60% weight distribution front to rear) and a Boxster (with 45% 55% weight distribution) and what proportion of the total roll stiffness front and rear should the ARB have comnpared to the springs at the front and rear of the same 996 weighing 1405 kilos compared to the Boxster weighing 1310 kilos?.
Answer's probably available on the fastest 996 at Brands Hatch 1st week in May!
Baz
the point of this thread was simply to show folks how the side that is uncompressing has an effect on the spring rate of the side that is compressing. and why this matters not only for calculating things, but is hugely critical for when a shock fully extends.
having your rate suddenly half is not great so your springs and set up need careful consideration.
like many things, no one cared
having your rate suddenly half is not great so your springs and set up need careful consideration.
like many things, no one cared
Edited by fioran0 on Monday 15th April 12:28
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