Convertible vs. Coupe Springs on AAS Dampers
06-05-11, 05:56 PM
#1
Convertible vs. Coupe Springs on AAS Dampers
After a couple of years of putting up with a set of underdampened coilovers, I've come to realize that there isn't much thrill left in having the car bounce on every road undulation, habitually lowering my head when driving over pavement transitions to avoid hitting the sunroof panel, and dreading every railroad crossing. Besides, my other FC is a dedicated track car..... and I'm at the point where I'd like to enjoy a nice ride in the street FC, for a change. So the coilovers are going away, and softer suspension is going in.
The plan is to return the car to its 1989 suspension setup, using the best technology Mazda had to offer at the time: AAS with OEM springs. Think of this as a kind of restoration project; the rest of the car is already nearly stock at this point.
So, I found myself staring at this parts fiche for s5 front springs (see attached):
Note that the higher performance coupes (LS diff and AA susp) have the same part numbers. Their spring rates are probably slightly higher than those of the stock coupe (first line in the table). Note also that the left spring on the AAS coupe is the same as the right spring on the vert. I'm guessing Mazda chose this setup because the less loaded side of the heavier vert model (right) is stiff "enough" for the more loaded side of the lighter coupe models (left). Following this logic, the left, more loaded spring of the vert should be the stiffest in the table. Indeed, it gets its own part number. So, I chose the vert springs to go into my soon-to-be AAS coupe. Likewise, the rear corners will also get the vert springs.
Depending on your perspective, there may be more interesting options for springs. I don't wish to debate this, and there are more threads than I can count on the topic of suspension component performance vs. streetability.
As it stands, I have a set of four new Mazda vert springs, and a set of four new AAS dampers. Looking at the springs more closely, I see a potential problem. When compared to the stock GTU springs, the lengths of the vert springs are different.
Here is a table comparing the spring lengths and coil thicknesses, in their fully unloaded state, resting on a flat surface:
Length:
Coil Thickness:
Notes:
- The stock GTU springs used for comparison have 97K miles and about 19 years of weight bearing. I don't know whether that would cause enough wear to make them shorter when unloaded.
- The two vert springs I received differed in length by at least 1/8". Left and right rears are supposed to be the same. Assuming this is within acceptable manufacturing tolerances, a difference of 1/8" in length is not significant.
Differences:
Length:
Coil Thickness:
Because of these differences, I am concerned that my ride heights will change compared to a stock GTU setup.
Without knowing the spring rates (or force constants) of the springs and the corner loads on the car, I can't predict exactly how much the vert spring setup will differ from the GTU spring setup. With zero load (springs at rest), it looks like the front is sitting 3/4" higher than the rear (negative rake). With the weight of the car resting on the springs, the difference should be smaller.
Assuming all eight springs are made of the same alloy, increased coil thickness means greater spring rate. And a small difference in thickness can have significant impact on the rate. Other parameters being equal (number of active coils, diameter of the coils), spring rate scales as the fourth power of thickness.
Spring Rate Differences:
So, the vert's rear springs are much stiffer than the stock GTU parts, with the vert fronts just slightly stiffer. That should offset the degree to which they compress under load. So, after the swap, I may end up with:
- front slightly higher than stock GTU
- rear unknown (unloaded vert springs are shorter, but much less compressible than GTU parts).
My concern is that, while trying to "improve" upon the original Mazda AAS damper/spring setup, I may have actually introduced some undesirable characteristics with the stiffer vert springs. The rear might sit lower (maybe) but it will also definitely be stiffer. Looking back through some corner weight sheets, a difference of >1/4" height at one corner can appreciably affect weight distribution. And having a stiffer rear end compared to the front is generally undesirable for street driving (or even road racing). Nothing is installed on the car at this point, but I have these worries.
If I insist on using Mazda OEM springs, I basically have a choice between installing the used GTU or new vert springs. I'd like some constructive feedback / opinions on whether my concerns are warranted, given the difference in spring lengths and rates.
For reference: Part Numbers and color designations:
My goal was a streetable, comfortable, and sporty (if a little outdated by today's standards) ride setup. I hope I'm not about to introduce something undesirable or awkward into the driver experience.
.
The plan is to return the car to its 1989 suspension setup, using the best technology Mazda had to offer at the time: AAS with OEM springs. Think of this as a kind of restoration project; the rest of the car is already nearly stock at this point.
So, I found myself staring at this parts fiche for s5 front springs (see attached):
Note that the higher performance coupes (LS diff and AA susp) have the same part numbers. Their spring rates are probably slightly higher than those of the stock coupe (first line in the table). Note also that the left spring on the AAS coupe is the same as the right spring on the vert. I'm guessing Mazda chose this setup because the less loaded side of the heavier vert model (right) is stiff "enough" for the more loaded side of the lighter coupe models (left). Following this logic, the left, more loaded spring of the vert should be the stiffest in the table. Indeed, it gets its own part number. So, I chose the vert springs to go into my soon-to-be AAS coupe. Likewise, the rear corners will also get the vert springs.
Depending on your perspective, there may be more interesting options for springs. I don't wish to debate this, and there are more threads than I can count on the topic of suspension component performance vs. streetability.
As it stands, I have a set of four new Mazda vert springs, and a set of four new AAS dampers. Looking at the springs more closely, I see a potential problem. When compared to the stock GTU springs, the lengths of the vert springs are different.
Here is a table comparing the spring lengths and coil thicknesses, in their fully unloaded state, resting on a flat surface:
Length:
Code:
rear LF RF vert 14 5/8" 14" 13 3/4" GTU 15 1/4" 13 7/8" 13 1/2"
Code:
rear LF RF vert 11.1 mm 12.6 mm 12.4 mm GTU 10.4 mm 12.1 mm 12.1 mm
- The stock GTU springs used for comparison have 97K miles and about 19 years of weight bearing. I don't know whether that would cause enough wear to make them shorter when unloaded.
- The two vert springs I received differed in length by at least 1/8". Left and right rears are supposed to be the same. Assuming this is within acceptable manufacturing tolerances, a difference of 1/8" in length is not significant.
Differences:
Length:
Code:
rear LF RF vert -5/8" +1/8" +1/4" GTU --- --- ---
Code:
rear LF RF vert +0.7 mm (7%) +0.5 mm (4%) +0.3 mm (2%) GTU --- --- ---
Because of these differences, I am concerned that my ride heights will change compared to a stock GTU setup.
Without knowing the spring rates (or force constants) of the springs and the corner loads on the car, I can't predict exactly how much the vert spring setup will differ from the GTU spring setup. With zero load (springs at rest), it looks like the front is sitting 3/4" higher than the rear (negative rake). With the weight of the car resting on the springs, the difference should be smaller.
Assuming all eight springs are made of the same alloy, increased coil thickness means greater spring rate. And a small difference in thickness can have significant impact on the rate. Other parameters being equal (number of active coils, diameter of the coils), spring rate scales as the fourth power of thickness.
Spring Rate Differences:
Code:
rear LF RF vert +30% +18% +10% GTU --- --- ---
- front slightly higher than stock GTU
- rear unknown (unloaded vert springs are shorter, but much less compressible than GTU parts).
My concern is that, while trying to "improve" upon the original Mazda AAS damper/spring setup, I may have actually introduced some undesirable characteristics with the stiffer vert springs. The rear might sit lower (maybe) but it will also definitely be stiffer. Looking back through some corner weight sheets, a difference of >1/4" height at one corner can appreciably affect weight distribution. And having a stiffer rear end compared to the front is generally undesirable for street driving (or even road racing). Nothing is installed on the car at this point, but I have these worries.
If I insist on using Mazda OEM springs, I basically have a choice between installing the used GTU or new vert springs. I'd like some constructive feedback / opinions on whether my concerns are warranted, given the difference in spring lengths and rates.
For reference: Part Numbers and color designations:
Code:
vert LF: FB67-34-021, two orange dots vert RF: FB67-34-011, two green dots vert rear: FC33-28-011, one orange dot GTU LF: FB01-34-011, two red dots GTU RF: FC01-34-021, two yellow dots GTU rear: FC01-28-011, two purple dots
My goal was a streetable, comfortable, and sporty (if a little outdated by today's standards) ride setup. I hope I'm not about to introduce something undesirable or awkward into the driver experience.
.
