Swift Spec-R Springs - Trim or cut the bump stops?
07-27-2018, 11:37 AM
#11
One thing that jumped out at me reading this thread, is the difference between wheel travel and shock travel.
The car is lowered 1.2". That's effectively at the wheel. At the shock its roughly half that (based on Motion Ratio). This what B and some others have been saying.
So basically if you cut half the cars lowered distance off the stops, or .6", you would have about the same amount of shock travel before you hit the stops. But as B points out, you'd be altering the progressive nature of the stop. Similar to cutting a spring, you'd make it a lot stiffer.
So in addition to what B said about the effects, when you did hit the stop the effect would be more instant and dramatic. Instead of a gradually increasing spring rate, you'd have a sudden large increase, then increasing rate from there on, as more load to that corner was transferred.
So just like cutting a spring, proceed with caution, and make sure you know what you're doing. Its kind of a pain to fix it if you screw it up.
The car is lowered 1.2". That's effectively at the wheel. At the shock its roughly half that (based on Motion Ratio). This what B and some others have been saying.
So basically if you cut half the cars lowered distance off the stops, or .6", you would have about the same amount of shock travel before you hit the stops. But as B points out, you'd be altering the progressive nature of the stop. Similar to cutting a spring, you'd make it a lot stiffer.
So in addition to what B said about the effects, when you did hit the stop the effect would be more instant and dramatic. Instead of a gradually increasing spring rate, you'd have a sudden large increase, then increasing rate from there on, as more load to that corner was transferred.
So just like cutting a spring, proceed with caution, and make sure you know what you're doing. Its kind of a pain to fix it if you screw it up.
Well...cutting a linear spring doesn't alter its rate.
But cutting a progressive spring like a bumpstop will alter the rate only because you'll either be left with the soft part or the hard part. It also changes the rate GAIN. By that, I mean...the rate at which the spring rate changes.
07-27-2018, 11:42 AM
#12
07-30-2018, 12:58 PM
#14
That happens because the spring stays the same rate but it shorter...and the car is riding the bumpstops.
If rates rose after cutting springs....then lowering springs would be free. Lowering springs are shorter springs with more rate.
A linear rate spring is always the same rate. If you cut a 500LB/IN spring in half...it still takes 500LB to compress each severed half by 1".
07-30-2018, 02:45 PM
#15
^Lol nope - cut your 500-lb spring in half and you'll have two very short 1000-lb springs. Moreover, if you put one 500-lb spring on top of another, you'll have one very long 250-lb spring.
Try it and see!
Try it and see!
Last edited by twohoos; 07-30-2018 at 02:48 PM.
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ZDan (07-31-2018)
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twohoos (07-31-2018)
07-31-2018, 10:50 AM
#17
^It's certainly not obvious when you first start to think about it, right? A helpful intuition (for me) is thinking of every coil as "an opportunity for more deflection". This is because the applied force transmits to and through the entire spring, so that *every* coil of the spring feels the *entire* force. Therefore every coil deflects according to that force, regardless of how many other coils there may be. So, the more coils you have, the more total deflection.
Mathematically, the formula for coil spring rate S (see my paper, first link in sig) is S = G*d^4/(8*N*D^3), where S is the spring rate, G is a constant, d is the wire diameter, D is the coil diameter, and N is the number of coils. Since N is in the denominator, you can see the spring rate is inversely proportional to the number of coils. When you cut a spring in half, everything else in the formula stays the same, but now N is only half as big, so the spring rate doubles.
Since we're talking about Swift's springs: the way progressive springs work is by limiting the spacing of some of the coils. As the force increases, the closely-spaced coils gradually become fully-seated against each other, so they can no longer deflect. This means there are fewer coils remaining to deflect, and hence less "opportunity for more deflection" as force is added. Mathematically, N is effectively a function of how much total deflection has occurred; as the load increases, the close-spaced coils pack down, so N gets smaller and smaller, and S gets bigger and bigger.
Mathematically, the formula for coil spring rate S (see my paper, first link in sig) is S = G*d^4/(8*N*D^3), where S is the spring rate, G is a constant, d is the wire diameter, D is the coil diameter, and N is the number of coils. Since N is in the denominator, you can see the spring rate is inversely proportional to the number of coils. When you cut a spring in half, everything else in the formula stays the same, but now N is only half as big, so the spring rate doubles.
Since we're talking about Swift's springs: the way progressive springs work is by limiting the spacing of some of the coils. As the force increases, the closely-spaced coils gradually become fully-seated against each other, so they can no longer deflect. This means there are fewer coils remaining to deflect, and hence less "opportunity for more deflection" as force is added. Mathematically, N is effectively a function of how much total deflection has occurred; as the load increases, the close-spaced coils pack down, so N gets smaller and smaller, and S gets bigger and bigger.
Last edited by twohoos; 07-31-2018 at 10:53 AM.
07-31-2018, 04:01 PM
#18
Yep. I understand both mathematically and practically. Just had to think about it lol.
Simply put, each half of a 500LB spring deflects 1/2" under a 500LB load. So...each half is 1000LB/in.
And yes...each coil brings more opportunity for deflection.
Simply put, each half of a 500LB spring deflects 1/2" under a 500LB load. So...each half is 1000LB/in.
And yes...each coil brings more opportunity for deflection.
07-31-2018, 05:21 PM
#19
Registered User
Thread Starter
Thanks for the input fellas!
I decided to leave the bump stops alone, partly since they were already beginning to disintegrate and I feared trimming them would cause them to disintegrate more rapidly.
Prior to installing the Swift Spec-R springs I was running stock ap2 springs on the lowest perch. I haven't spend much time driving with the Spec-Rs, but they seem to ride very similar to the ap2 springs on the lowest perch. I'd be hard pressed to tell much of a difference in ride quality, truth be told. However, the understeer of the ap2 springs has been rectified, thankfully.
I'm bringing the car to Deals Gap this week for the annual Miatas at the Gap event so I'll be able to push the car and make good use of these new springs!
I decided to leave the bump stops alone, partly since they were already beginning to disintegrate and I feared trimming them would cause them to disintegrate more rapidly.
Prior to installing the Swift Spec-R springs I was running stock ap2 springs on the lowest perch. I haven't spend much time driving with the Spec-Rs, but they seem to ride very similar to the ap2 springs on the lowest perch. I'd be hard pressed to tell much of a difference in ride quality, truth be told. However, the understeer of the ap2 springs has been rectified, thankfully.
I'm bringing the car to Deals Gap this week for the annual Miatas at the Gap event so I'll be able to push the car and make good use of these new springs!
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