Ride Height
- Thread starter dmoody
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74ProII
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I just did alot of discussing front springs with Chassis engineering. Our coil springs are 14" long as required to achieve the nearly zero angle for the ball joints. That is the biggest problem with most of the street MII coils. They are built for streetrod geometry, thus are only 12" long. By cutting the coils, you are doing the same thing to the front geometry. I would suggest the new coils the first time around.
Look at the compressed spring. The compressed coil spacing remains the same after shortening. For example, if you have 1-1/2" (I just pulled a number out of my arse) from coil-to-coil, when you cut a single coil you shorten the compressed spring 1-1/2". But since the spring is not riding on the ball joint, that's not how much you lower the car. Measure the distance from the lower control arm pivot to the ball joint (13.5"?). Then measure the distance from the lower control arm pivot to the center of the lower control arm spring perch. The ratio of these two numbers (between 1.5 and 2?) is also the ratio of the change in ride height to the change in compressed spring length. For giggles, let's say it's 2. If we cut 1 coil off our example spring, we change the compressed spring length 1-1/2" and drop the car 3".
Working backwards, if we want to drop the car 1", we divide 1" by 2 and find we should shorten the compressed spring by 1/2". How many compressed coils makes 1/2"? 1/2" divided by 1-1/2" per coil or 0.5" divided by 1.5" per coil = 0.33 coils or 1/3 of a coil.
Use your own numbers, but I think it's clear that cutting a whole coil is too much. This approach ignores nonlinear effects due to changes in geometry, but it's pretty close. When in doubt, cut a little less now and a little more later, cuz like my mama used to say, "Boy, you can always cut if off, but you can't put it back."
I'm still not a fan of cutting stock springs because they're not stiff enough. If you cut 10% off of a weak spring, you make it 11% stiffer, but that's still weak. The magnitude of suspension motions is almost the same and you're closer to the ground. You're more likely to shear off that #1 Hedmann header pipe.
Working backwards, if we want to drop the car 1", we divide 1" by 2 and find we should shorten the compressed spring by 1/2". How many compressed coils makes 1/2"? 1/2" divided by 1-1/2" per coil or 0.5" divided by 1.5" per coil = 0.33 coils or 1/3 of a coil.
Use your own numbers, but I think it's clear that cutting a whole coil is too much. This approach ignores nonlinear effects due to changes in geometry, but it's pretty close. When in doubt, cut a little less now and a little more later, cuz like my mama used to say, "Boy, you can always cut if off, but you can't put it back."
I'm still not a fan of cutting stock springs because they're not stiff enough. If you cut 10% off of a weak spring, you make it 11% stiffer, but that's still weak. The magnitude of suspension motions is almost the same and you're closer to the ground. You're more likely to shear off that #1 Hedmann header pipe.
dmoody said:
The advantage to cutting a spring is that you can select a spring by size, designed load capacity, rate and trim as required, without being "stuck" to the springs cataloged for a particular application. I usually try to select one close to the OE length and load with the desired rate to limit the changes that the spring will undergo after modifying.
On some springs, you will have to cut in full increments (those with full wire open {FWO} ends top and bottom), however a II uses a spring that is FWO on the bottom, and a taper wound closed at the top. The TWC top makes the seat surface flat, so you can cut as required and rotate the coil to seat properly in the lower pocket.
Another superior II design element
I thought my '78 sat too high in the front, so I cut 1 full coil off the top so it would fit back into the same pocket positions it was in. It dropped it at least 1" and looks much better. I did this about a week before the Nashville 40th, then drove it from Michigan and back. No changes to alignment, drove great, NO tire wear. I do have to be more careful on driveway approaches with the spoiler and on big bad bumps it does bottom out on the snubbers easier, but overall, I am pleased. Its not a daily driver, but I do drive it whenever I can on the weekends in nice weather. One thing to consider is that I am running the stock wheel and tires. Attached are 2 pics, maybe you can tell the difference. One is with the other Mustangs before the coil cut, the lone shot is after the cut.
Mike
Mike
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Dano78
Founding Member
IIRC I took 1 1/4 coil off the V6 springs on the 78 V6 CBII I had and I'm trying to remember if I took 1 1/2 or 2 full coils off the blue Fastback (tunnel ram car) All i know is the blue fastback, I cut too much off and I had to twist in those aluminum spacers between the coils to make up for it. Even now as low as it sits, I can't get a floor jack under the main x-member without phsycally lifting up on the car and scooting the floor jack under with my foot... really irritatiing. 
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