Calculating ride frequency for the Mustang

[Radiation Joe]

OP didn't say why he wanted to calculate the ride frequency, so lets not get too sidetracked as to which spring is 'best', mmk? Ride frequency is just a data point.

True.
I haven't posted in a while and guess I had some pent up meaningless chatter to share. There is really good info in this thread on how to calculate wheel rates and ride frequencies. I also use Brian's spreadsheet for calculating both.

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[Brian@BMVK]

For an example of what I believe is a proper track rat set-up (you've got to pay to play...) is the Vorshlag MCS spring shock combo.

• Front Spring Package includes a pair of Hyperco 60mm ID Coilover Springs. These can work with the included MCS upper spring perches or the optional Vorshlag camber/caster plates and radial bearing upper spring perches

• Rear Spring Package, coilover: includes a pair of Hyperco 2.25" ID Coilover Springs and MCS upper spring perches, and Vorshlag upper shock mounts.

• Vorshlag's GT Spring Package is 450 #/in Front / 550 #/in Rear
• Vorshlag's GTS Spring Package is 600 #/in Front / 750 #/in Rear
• Vorshlag's GTR Spring Package is 800 #/in Front / 1100 #/in Rear

Ground Control doesn't list spring rates on their web store but if the BMW rates I'm familiar with are indicative they'd probably be recommending spring rates similar to Vorshlag for competitive track use.

I guess my point is that 250 lb/in rates are a compromise based upon shock and strut geometries. Truly effective spring combinations for track use with sticky tires are generally twice as high.

Those Vorshlag rear numbers are also with a true coilover rear with a different motion ratio than stock.

 

[Järn]

Thank you all so much for your help with this project.

OP didn't say why he wanted to calculate the ride frequency, so lets not get too sidetracked as to which spring is 'best', mmk? Ride frequency is just a data point.

My purpose in making the spreadsheet is to help people (the average Mustang owner like me) relate a spring rate to a level of performance that they would want out of a "suspension upgrade" project.

There are so many people asking the question in this forum "What springs should I buy?"

For example, spring rates of 200/800 calculate to a ride frequency of about 1.5 which is right at the dividing line of Sporty to Very Sporty. (I think this is where I personally want to be.)

Sporty Road Car ("ST- Versions"): 1.3 Hz - 1.5 Hz
Very Sporty Road Car ("RS-Versions"): 1.5 Hz - 1.8 Hz
Super Sports Car (Ferrari, Lambo etc): 1.7 Hz - 2.0 Hz

I also want to get data on the GT350, GT350R, and the GT500, for comparison reasons.

 

[shogun32]

I also want to get data on the GT350, GT350R, and the GT500, for comparison reasons.

Brian's sheet has some of that covered. Do post up when you can fill in the other blanks.

 

[Bluemustang]

Those Vorshlag rear numbers are also with a true coilover rear with a different motion ratio than stock.

Right. The 750 or 1100 lb. Rear true coilover spring rate would look huge next to the equivalent rate for stock MR.

 

[Brian@BMVK]

Right. The 750 or 1100 lb. Rear true coilover spring rate would look huge next to the equivalent rate for stock MR.

Yes. 750 at the coilover location is approximately like 1600 at the stock location, and 1100 like 2400 (far too stiff for anything but a max downforce S550-based racecar).

 

[Brian@BMVK]

It's not quite that cut and dry. When I work with customers I explain how to find optimum selection for their application.

 

[Norm Peterson]

This is true. It's a pretty stout dual purpose setup though.

This ^^^

I think as you go much past about 250 lb/in that you're into diminishing returns. Right now, my S197 is running wheel rates close to those of the GT350/350R. Meaning we can skip right over those IRS vs not-IRS and motion ratio matters and conclude that yes, Kenneth, the frequencies are about the same.

It rolls about 2.5°/g, about half a degree per g of which is due to tire vertical stiffness effects. If I was to run roughly 50% stiffer springs, I'd only be cutting about a third of a degree/g roll from that (still about 88% as much roll from 1.5 times as much spring). That might be an easy case to make for a competitive CAM-C or occasional but semi-serious time trial car, not so easy to justify for DD/performance street and the occasional HPDE.


Norm
(apologies to REM)

 

[Norm Peterson]

I disagree. With stock geometry struts we're limited to 250 lbs/inch or they'll fall out. Shortened struts and shocks are necessary to run acceptably effective springs for track use. With porky cars such as ours that means 400 to 600 lbs/in depending on how sticky of tires we're running to stay off the bump stops under late braking maneuvers and curb hopping. Same applies to the rear suspension. That's why you see so many setups with wheel rates in front 50 percent higher than in the rear ... weight transfer under braking is much more significant than under acceleration.

With that much of a "mis-match" in undamped frequencies, your %critical damping had better be up in a firmer range at both ends, else the ride is likely to be a lot "pitchier" than necessary.


Norm

 

[Flyhalf]

What a great thread here.

I've been working with Brian for a long time.
We went through the first setup that allowed me to do well at MuscleCup.
That was FB Struts and BMR handling springs. (The stiffer springs available for OEM strut shape)
As I been taught,
Car is a system. Not a bunch of single separated pieces.
so of course in the equation of choosing the right spring rate we must include sway bars. And of course considering factors like tires and AERO.
I'm now on coilovers Cortex JRi DA.
Moved from 550F/650R to 650F/800R
Massive changes in the sway bars too. (To avoid the oversteering)
I'm in a phase now where i can "feel" and regulate compression and reboud to help the system working well. But it is a process
I. E. Soften the compression in the rear to trasfer more weight in corner exit to increase grip and reduce oversteering.

I find the graph you are making a great thing.
Alessandro

 

[shogun32]

Moved from 550F/650R to 650F/800R

do you have potentiometers on the shocks to measure speed/amplitude of displacement?

 

[Flyhalf]

do you have potentiometers on the shocks to measure speed/amplitude of displacement?

I don't. We went to this stiffee set up to accomodate the more aero DF mainly and support the usage of A7 HOOSIER.

 

[Radiation Joe]

With that much of a "mis-match" in undamped frequencies, your %critical damping had better be up in a firmer range at both ends, else the ride is likely to be a lot "pitchier" than necessary.

Norm

^^^ This has been my experience in the past. In my dual purpose car I'd actually reduce rebound damping at the track to improve performance. I'd increase the rebound damping front and rear to improve the ride for the trip home. Ride frequencies were 2.4 front and 1.7 rear.

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