Quiz: Why Not Equal Spring Rates?

[Dan Baldwin]

I still don't believe it should be a problem for 90% of all track-driven Zcars, though. Motions from track irregularities should be damped out in short order on our cars, and shouldn't lead to any excessive pitch oscillations. Cars with high downforce (on the order of car weight or more at speed) might have to worry about this as they seem to be underdamped due to having to run very stiff springs.

 

Picking up tires seems to be pretty normal for cars with high grip, high center of gravity, and narrow track. I've seen pics of my competition's Shelby Omni picking up BOTH inside tires at Turn 3 at NHIS! I have pics of mine picking up the inside front (not much) there as well. Wheee! I know it's best to have all four on the ground, but given the above conditions, it is possible that tires will leave the ground no matter what the damping is. (Not saying mine is optimized, far from it I'm sure)

 

Anyway, again I don't think there's anything magical or special about the spring rates specifically, though I'd agree that in some cases having differing natural frequencies of the front and rear spring/mass/damper systems may be a good idea. Then again in other cases they may want to be close to the same. I run slightly stiffer springs and damper settings in the back of my car, don't know what the natural frequency difference is front/rear, but they're probably not all that far apart (stiffer spring => higher freq., stiffer damper => lower freq.).

[260DET]

Originally posted by Dan Baldwin:

............ I'm currently at 160F/200R spring rates, 1" front bar, and no rear bar (disconnected). The faster ITS guys run more like 350F/300R (something like that, anyway), big front bar, and no rear bar.................

Interesting. My mate's RB30ET engined car runs 300F/250R progressives, with a large front bar and NO rear bar. Ridden in it but not driven it, firm but comfortable on the road and it seems ok on a circuit track, not much body roll looking from the rear.

[johnc]

Here in the US there are easy coast and west coast schools of thought regarding ITS Z spring rates and anti-roll bars. Easy Coast used higher spring rates in front, a big front anti-roll bar, and no rear anti-roll bar. West Coast reverses the spring rates and uses a small front anti-roll bar and a medium or big rear anti-roll bar.

 

One day I would like to arrange a test at a neutral track somewhere in the midwest, invite Chet Whittle and Erik Messley, bring a well powered ITS 240Z, and a bunch of springs and anti-roll bars to do hours of back-to-back testing.

[TimZ]

Guys - You are focusing on the wrong thing - close but not quite...

 

The thing that you are trying to tune is the natural frequency of the suspension - that is, at what frequency will each corner of the car oscillate (move up and down) when it is perturbed. The spring rates are what you use to tune this parameter, BUT the natural frequency is dependent on the mass that the spring is supporting (heavier mass = lower frequency). They can be used interchangeably in this case, since John mentioned a 50/50 weight distribution, so the mass is assumed to be the same (or nearly the same) at each corner. You also have to assume equal install ratios, as somebody already pointed out.

 

In a camaro, for instance, the front might be heaver, so a stiffer spring would be required to keep the natural frequencies equal.

 

Saying what the spring rates are front to rear without knowing the weight distribution does not tell you enough information to do anything useful.

[Dan Baldwin]

Tim, you're leaving out DAMPING. It is just as much a determining factor of natural frequency as the spring and mass, and you can get dampers that are externally adjustable. Repeat after me: spring, mass, DAMPER. With a forcing function (da road acting on the mass and polar moment of da car).

 

And again, Camaros have WAY stiffer front springs because they have a lot of leverage acting on them. The wheel rates are a LOT closer to the same front/rear than the 600 front/140-160 rear spring rates would SEEM to indicate. F/R weight distribution is ~55/45. Wheel rates are probably on the order of 60/40.

 

As far as "oscillating", none of our cars' suspensions SHOULD oscillate. If they do, new dampers are in order. Leave the oscillating to yer great uncle Fred's '76 Caddy.

 

John,

I don't get the East Coast/West Coast setup discrepancy, but I'd bet money on the East Coast team. Unfortunately they've taken the year off from Zs. I think Chet's a hired gun driving a Miata now. Anyway, I arrived at my current setup somewhat independently, sort of a street/track compromise with aspects of East and West coast race setups. Works well enough for me!

[Mudge]

Not only that, but the rear wheels are your DRIVE WHEELS, you want traction, overly stiff rear will reduce traction. Most of the time you are PUSHING the front wheels also, you need to ensure that you have good grip there as well, but you usually run a stiffer spring in the front for these reasons.

 

The 4th gen Camaro is about 57/43 - 58/42 depending on options.

[Mudge]

Originally posted by johnc:

Here in the US there are easy coast and west coast schools of thought regarding ITS Z spring rates and anti-roll bars. Easy Coast used higher spring rates in front, a big front anti-roll bar, and no rear anti-roll bar. West Coast reverses the spring rates and uses a small front anti-roll bar and a medium or big rear anti-roll bar.

The generic rule of thumb that I've been tought, big sways for AutoX, small springs.

 

Med-Big springs for Road Race, and smaller sways.

 

Open Road, small sways, big springs, and compression adjustable shocks, and a zero camber/toe or nearly zero alignment.

[Mudge]

Originally posted by Dan Baldwin:

Picking up tires seems to be pretty normal for cars with high grip, high center of gravity, and narrow track. I've seen pics of my competition's Shelby Omni picking up BOTH inside tires at Turn 3 at NHIS! I have pics of mine picking up the inside front (not much) there as well. Wheee! I know it's best to have all four on the ground, but given the above conditions, it is possible that tires will leave the ground no matter what the damping is. (Not saying mine is optimized, far from it I'm sure)

Sounds like too much sway bar to me. That is why moderate sway bars are used in high speed situations.

[TimZ]

Originally posted by Dan Baldwin:

Tim, you're leaving out DAMPING. It is just as much a determining factor of natural frequency as the spring and mass, and you can get dampers that are externally adjustable. Repeat after me: spring, mass, DAMPER. With a forcing function (da road acting on the mass and polar moment of da car).

 

Okay, now I'm going to have to get the physics book out to be sure - unfortunately, I don't have time right now so I'll post a quick response...

 

As I recall, damping is not strongly coupled to the natural frequency (if at all - that's what I don't remember) - it primarily determines how closely the suspension follows the forcing function.

 

If the system is underdamped, the car will oscillate for a period of time after perterbation.

 

If it's critically damped, the car will follow the perterbation without oscillating.

 

If it's overdamped, the car will not oscillate, but will lag behind the perterbation.

 

As far as "oscillating", none of our cars' suspensions SHOULD oscillate. If they do, new dampers are in order. Leave the oscillating to yer great uncle Fred's '76 Caddy.

Hopefully you were just pulling my chain here, but the oscillation reference was to illustrate the point. Regardless of damping, there is an underlying frequency that the suspension wants to oscillate at.

 

The point that I was trying to make was that when you are talking about spring rates front and rear, what you are really trying to talk about is the balance of the natural frequency front to rear, and you can't get there without knowing the mass being supported (and a few other things, like the installed leverage ratio.

 

The reason that I was trying to make the point in the first place was that questions like "how come the Camaros can run these spring rates when we can't" were cropping up. Remember the original discussion was related to preventing bucking.

[johnc]

What TimZ said...

[Mudge]

Originally posted by Dan Baldwin:

As far as "oscillating", none of our cars' suspensions SHOULD oscillate. If they do, new dampers are in order. Leave the oscillating to yer great uncle Fred's '76 Caddy.

Yeah, that post is a bit of a goof. Spring/shock oscillation should be 1 oscillation and no more, that is how they work. If they dont move, then your driving a suspensionless Go-Kart, not a street or race car.

[Dan Baldwin]

Damping DOES have a large effect on resonance frequency (not natural frequency, my goof (I think)). Only if the system is significantly underdamped is the effect of damping on resonance frequency negligible. The resonance frequency is what we're concerned with (I'm pretty sure).

 

I wasn't exactly chain-pulling with the caddy reference, that was sorta MY point. That in some systems that are underdamped, the "underlying" natural frequency is pretty much the resonance frequency of the system, but for a 240Z in track tune, you should be pretty near critical damping, I'd guess. Anyway, enough damping for it to have a profound effect on the resonance frequency.

 

As far as trying to "calculate" appropriate spring and damping rates, as I said before I don't think any of us is qualified to do a good enough job, taking into account the distributed masses of the car and suspension components, tire stiffness and damping, chassis stiffness and damping, bushings, etc (highly nonlinear and complex interacting systems). Time and effort better spent at the track anyway.

 

Regarding the "how come the Camaros can run these spring rates when we can't" question, for the THIRD time, the front suspensions on those cars are HIGHLY leveraged, and the wheel rate is nowhere NEAR the 600 lb/in spring rate mentioned previously. Their front and rear WHEEL rates are much closer than their front and rear SPRING rates.

 

Mudge,

I don't *think* I have too much front sway bar, I'm getting too much roll as it is, and the car handles pretty neutral. I might experiment with softening it at Lime Rock in a couple of weeks, though.

[johnc]

For a street car you're fine and a lot of this doesn't really apply until you're pushing the edge on a race track.

[TimZ]

Originally posted by Dan Baldwin:

Damping DOES have a large effect on resonance frequency (not natural frequency, my goof (I think)). Only if the system is significantly underdamped is the effect of damping on resonance frequency negligible. The resonance frequency is what we're concerned with (I'm pretty sure).

 

Yes, you are correct - the damping will effect the resonant frequency. My bad. I still can't find the equations for it, though - left my Gillespie book at work.

 

I was pretty sure that the resonant frequency usually ends up somewhere in the same ballpark as the natural frequency, though (could be wrong). In other words, I was viewing damping as being used to fine tune the response, once the spring rates and mass have put you in the ballpark.

 

Regarding the "how come the Camaros can run these spring rates when we can't" question, for the THIRD time, the front suspensions on those cars are HIGHLY leveraged, and the wheel rate is nowhere NEAR the 600 lb/in spring rate mentioned previously. Their front and rear WHEEL rates are much closer than their front and rear SPRING rates.

I acknowleged this in my second post. After the first time you said it.

 

And for the THIRD time, it doesn't do any good to talk about spring rates if you don't know what mass they are supporting.

[Guest Anonymous]

Dan Baldwin: So with a V8 in front with a strret driven 48-49 to 51-52 weight distribution my equal weight 185 # coilovers should be ok? /or with heavier dampening strut cartridges in the rear?

[Dan Baldwin]

Tomahawk, I wouldn't have any reservations about running 185 #/in springs all around with the same dampers.

 

Tim, if you acknowledged the much greater leverage ratio issue regarding Camaro front suspension, I guess I missed it.

Originally posted by TimZ:

And for the THIRD time, it doesn't do any good to talk about spring rates if you don't know what mass they are supporting.

Knowing the spring rate and the mass supported isn't enough to calculate the natural frequency. You need to know the equations of motion of the wheel related to spring compression. Or just the instantaneous leverage ratio at static ride height, which should give sufficient accuracy for us. Anyway, the leverage ratio should be pretty close to the same front and rear on our Zs, and the weight distribution is known to be pretty near 50/50. So, assuming a basic knowledge of Z suspension, just talking spring rates is adequate for this discussion.

[BayAreaZT]

Doesn't natural frequancy = resonant frequancy.

natural freq. = sqrt of k/m

 

I think damping does not change the natural frequancy, it changes the system response.

[SleeperZ]

Originally posted by BayAreaZ8:

Doesn't natural frequancy = resonant frequancy.

natural freq. = sqrt of k/m

 

I think damping does not change the natural frequancy, it changes the system response.

I'm pretty sure you are correct on this one. The spring rate and the flexing of the suspension and chassis should determine the natural (resonant) frequency. All the shock will determine is how much damping will occur. Critical damping is what we want in that case - the fastest response to a surface change without rebound.

[Mudge]

Can someone please explain why the Z cars use such a high rate in the rear so favorably?

 

Is it due to IRS being different than a solid axle? Is it spring location? Or...?

 

With weight transfer to the front on hard breaking, and the rear being the drive wheels, I am still not making a connection here.

[260DET]

Originally posted by Mudge:

Can someone please explain why the Z cars use such a high rate in the rear so favorably?

 

Is it due to IRS being different than a solid axle? Is it spring location? Or...?

 

...................

My bet is on the IRS factor. Which may be another reason why no rear sway bar can be made to work well ie that setup allows each rear wheel to work fully independantly irrespective of the spring rates, whereas a sway bar ties them together to some degree.