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Suspension update and upgrades

shogun32

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Steeda adjustables are about the same price as the Bilsteins. Unless there is something really wrong with the Steeda's for the street (maybe thats the issue)
I have zero problems with the pro-Action adjustable *dampers* on the street. Just don't use Steeda's DR springs especially up front unless you really mean to.
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fatbillybob

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SP089 / SP088 spring combination might suit your anticipated use a little better. Rough numbers put the theoretical undamped flat ride speed around 65 mph.
Norm
What is term flat ride speed? Are you talking about the calculated front rear chassis frequency 10% split for anti-porpoise seen in street cars?
 

Nate_V8

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bilsteins are a little better at everything? Hmmm. I wonder if I would notice a difference. I've never had a mustang/car that could handle really well by any means and they were all SRA. First concern is street comfort and then superior handling to stock/removing that bounce out of the rear and dive out of the front
 

Norm Peterson

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What is term flat ride speed? Are you talking about the calculated front rear chassis frequency 10% split for anti-porpoise seen in street cars?
Anti-porpoise would be a good synonym.

"Flat ride" is usually defined by the front suspension hitting a bump and the rear suspension hitting the same bump, with the front and rear frequencies such that the front and rear suspensions each complete exactly one cycle at the same point in time. Keep in mind that the rear suspension gets its vibration started later than the front, which because it has less time to run its full cycle is the essence of the rear suspension needing to have a higher frequency. Hence, wheelbase and car speed are involved.

Percentage guidelines that only consider the suspension frequencies (like that 10%) are just that, rough guidelines. For the undamped case (think dead or disconnected shocks) the actual math is a little more involved (only a little) but can give you a better answer than the guideline. Better yet is if you include the effect of shock/strut damping, which introduces somewhat more complexity to the math but can reveal quite a bit more.

With any combination of front wheel rate, rear wheel rate, and car wheelbase the above description of flat ride will occur at exactly one speed. But don't get concerned that every other speed will give you a crazy amount of front to back head toss because deterioration in a car's pitch behavior as speeds vary from any theoretically ideal speed is pretty gradual.


Norm
 

Norm Peterson

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Sometimes pictures can help. These first two hopefully illustrate the difference between dampers that are pretty close to dead players and dampers that are in much more performance-capable condition. The theoretical flat ride speeds do vary a bit (54.9 mph for the 5% case, 49.1 mph for the 45% case), as the front and rear traces cross the horizontal axis at different times. But note the differences in the red and green traces - those are what you feel. Pitch is (roughly) the amount of head toss, pitch jerk is more about the difficulty in resisting that head toss.

Stock at 54.9 mph, 5% damping.jpg


Stock at 49.1 mph, 45% damping.jpg


And here's a pair of plots showing that flat ride characteristics don't instantly fall in the toilet as speeds vary over a halfway reasonable range. These are for a more ride comfort amount of damping (20%), taken at 53.9 mph (theoretical undamped flat ride speed) and again at 90 mph.



Srock at 53.9 mph, 20% damping.jpg



Srock at 90 mph, 20% damping.jpg



Norm
 

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Norm Peterson

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For the curious, those curves were for my '08 GT when it was on its OE springs, which are more softer than S550 springs than my S197 is lighter than a S550. IOW, S550 frequencies would be slightly higher.

One more plot. Look what happens when the rear shocks are on their way out (10%) and the front struts are still good (40%). At 80 mph.

Stock at 80 mph, 40% front & 10% rear damping.jpg



Norm
 

shogun32

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One more plot. Look what happens when the rear shocks are on their way out (10%) and the front struts are still good (40%). At 80 mph.
this is why adjustable shocks always, or if non- they'd better be damped *correctly*.

Just put eyes on a set of Steeda's adjustable front coil-overs. NICE piece of kit. And their spring caps are like the J&M engineered to take the spring load while the bearing takes the shock shaft load. Not sure why they don't use industry standard 2.5" springs but I'll be looking into what a retrofit would look like on that front.
 

fatbillybob

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Anti-porpoise would be a good synonym.

"Flat ride" is usually defined by the front suspension hitting a bump and the rear suspension hitting the same bump, with the front and rear frequencies such that the front and rear suspensions each complete exactly one cycle at the same point in time.

Percentage guidelines that only consider the suspension frequencies (like that 10%) are just that, rough guidelines. For the undamped case (think dead or disconnected shocks) the actual math is a little more involved (only a little)

Norm
Norm,

My math is not good. Long forgotten and dependent on calculator when I run out of fingers. I only know the rough math


wheel freq= 188 sq root (wheel rate/sprung weight) wheel rate= (coil rate/suspension leverage)


Can we see more in depth math?
 

Norm Peterson

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I've always found it easier to keep frequencies in terms of Hz, which uses 3.133 x sqrt( [WheelRate] / [SprungCornerWeight] )
where [WheelRate] = [SpringRate] x [MotionRatio]^2
and [MotionRatio] is the ratio of spring movement to wheel movement. I consider three components of motion ratio; spring location relative to the lower arm, spring angle, and a front view swing arm correction from ball joint to wheel center.

Do that for both ends of the car.

When you introduce damping, the wheel frequencies slow down, and the correction for damping looks like SQRT( 1 - [%CriticalDamping]^2 ). Strictly speaking, bump damping and rebound damping are generally different, but for most car enthusiast purposes using some sort of average should be good enough. At 15% critical damping, the frequency would be 97.75% of the undamped frequency calculated above. At 45% critical damping, it would only be 79.75%.
Flat Ride is when one full cycle of the front suspension and one full cycle of the rear suspension are completed at the same time, where the rear suspension frequency gets started later than the front suspension by the time it takes the car to travel a distance equal to its wheelbase. You can use the formula for this time delay as being equal to 0.0568 x [Wheelbase(inches)] / [CarSpeed(mph)], or if you're using Excel let Excel convert mph to inches/second and divide the wheelbase by the speed in in/sec.


Norm
 

fatbillybob

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Some think you can completely ignore flat ride when designing for lowest laptime. What is your or brian's opinion on that?
 

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Norm Peterson

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Some think you can completely ignore flat ride when designing for lowest laptime. What is your or brian's opinion on that?
I'd put flat ride of lower priority than most other considerations but I don't think you'd want to ignore it completely. For a pure street car, it makes some sense to target some flat ride speed choosing springs from within a ride comfort oriented range of spring stiffnesses, and fix the handling balance with sta-bars.

Since I started introducing damping to the analysis, I've come to the conclusion that focusing on any specific flat ride speed is not time spent wisely, but that awareness of the concept and more or less where your tuning is going to put you still has value. With lots of damping, like you're probably going to be using when chasing lap times, the magnitude of pitch isn't going to be all that high after the first half cycle even if you're traveling at three or even four times the car's theoretical flat ride speed. And no matter what you do, the first half cycle at either end is kind of a bust anyway; there just isn't much you can do about it.

There's a philosophical difference between choosing to ignore something out of hand and determining that it may in fact be less important under the specific driving conditions that you're designing for than it might be for other driving conditions that you're not.


Norm
 

TeeLew

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Some think you can completely ignore flat ride when designing for lowest laptime. What is your or brian's opinion on that?
Complete opinion here, but as long as it's been mentioned, I'll chime in.

In practice, race cars that tend to run in the range of 'flat-ride' spring combinations are relatively low power formula cars or sports racers that don't have much trouble with power down/wheelspin, but must run very 'free' while cornering. Higher HP cars tend to run lower rear spring rates to improve drive off low(er) speed corners. Flat-ride spring packages are generally too loose for the best lap times and especially the best tire longevity.

Having said this, very fast drivers tend to challenge the rear of the car much more than recreational drivers. In my experience, the faster a driver is, the more understeer they will request/require. It's counterintuitive to think that an amateur would want a 'loose' car and an pro would want it to 'push', but amateurs tend to induce understeer with their driving.
 

TeeLew

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Cars with light understeer are much easier to drive at the limit and in particular, on corner exit with lots of power. There are multiple ways to get your understeer-biased roll stiffness.
Keep in mind, it's much more than just easier to drive; it's faster. Your last sentence is 100% true.
 

fatbillybob

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the faster a driver is, the more understeer they will request/require.
That's only to balance the throttle steering because the faster driver is more abusive to the rear of the car. I don't really think that car understeers but neutral or oversteers to point the car around the corner and down the track. The slower driver would feel that the car understeers because his rearend is planted.
 

fatbillybob

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There's a philosophical difference between choosing to ignore something out of hand and determining that it may in fact be less important under the specific driving conditions that you're designing for than it might be for other driving conditions that you're not.
Norm
Fixed budgets and conversations like these really make for difficult chassis development. It is amazing the go to is do what others before you have done but those things are actually often wrong in the sense that they don't reflect your compromises. The real racers will let you go down the headfake path because they want to beat you. When racing vettes we had a unique situation where about 10 of us racing the same chassis kind of co-developed together and what we discovered was fast was not the conventional wisdom. Right or wrong my tendency is to go my own way but development is slow and painful. There are so many factors to control just to test one thing. There is a reason the high dollar teams win. 90% is being able to afford it. Many say that racing is expensive sighting registration fees, tires, brake pads but the real costs are development and testing. The race is cheap.
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