SPRING RATES & DROPS: ALL IN ONE THREAD!!

BmacIL

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BmacIL

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Added revised BMR SP084 front springs to the Ride Frequency chart and tweaked the MRs to match test data.
 

Stuntman

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Updated 8/4/17

I've compiled a list of all the linear spring options and combinations that I could see being used together (based on if they're sold in pairs or not) and uploaded it to google. Please put this one on the first page [MENTION=13598]Gibbo205[/MENTION]!

Wheel Rate:
Wheel rate is the effective spring rate the tire sees, based on the spring and the suspension's motion ratio. The motion ratio is the ratio of spring travel to wheel travel. On the Mustang, the front is ~1 because it's a strut, and the rear is 0.48, meaning for every 1" the tire moves up/down, the spring is compressed/released by approximately 0.5".

Wheel Rate = Spring Rate * (MR)^2

Ride Frequency:
The ride frequency is the natural frequency of the body, with no damping considered. Higher = Stiffer.
Empirically tested values by application are as follows:

RF = 1/(2*pi) * sqrt(K/Msprung)

0.5 - 1.2 Hz - Street-only/Passenger Cars
1.2 - 1.5 Hz - Sporty Cars, Lower end of track-focused street cars
1.5 - 2.0 Hz - Track-focused street cars to Low-downforce racecars
2.0+ Hz - Racecars, HPDE/Auto-X-only street cars

There's no cutoff of where you should be, but for the enthusiast who daily drives and occasionally tracks, 1.4-1.6 Hz is a good ballpark.

One important aspect about ride frequency is the relative frequencies between the front and the rear. General rule of thumb is to achieve the best ride response, you want your rear ride frequency to be ~10% higher than your front. This is dependent on weight distribution, of course, and the more front-heavy, the closer to 1:1 you want. A 50/50 car would want 10% for optimum results. For our cars anywhere from 1-10% will achieve good ride response characteristics. The way to think of it is this: when you hit a bump, the body of the car hasn't settled fully before the rear hits it, so you need an appropriately high rate to manage that well.

Ride is far from the only concern, so you may give up some of the ideal response characteristics for improved response/handling. See GT350R, for example.

The reason I put all this up is that in going about deciding on spring selection, there are a few important variables to consider: Rates relative to chosen dampers, ride height drop front and rear, and relative rates if choosing something other than a set of 4 springs.

Assumptions:
Mass of car: 3736 lb (from what things weigh thread)
Sprung mass: 3188 lb (from what things weigh thread)
Weight Dist. 53% F/47% R

https://docs.google.com/spreadsheets/d/1qExrjc7BqL7y8j1XQGotlHd9L3hT41aTpKySDTfhvCE/edit?usp=sharing
"~1.0 MR" is quite vague and from your chart it appears you used .99 for the front MR.

Where did you get 0.48 for the rear MR, or how did you go about measuring it?

FWIW, I measured an S197 at 0.95 for the front MR and BMR measured rear Motion Ratios of 0.492 for the spring and 0.744 for the shock.
 

BmacIL

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"~1.0 MR" is quite vague and from your chart it appears you used .99 for the front MR.

Where did you get 0.48 for the rear MR, or how did you go about measuring it?

FWIW, I measured an S197 at 0.95 for the front MR and BMR measured rear Motion Ratios of 0.492 for the spring and 0.744 for the shock.
The chart was corrected a couple weeks ago after getting the exact rear MR value from Kelly (0.492). The 0.98 front was a measured swag. It may be slightly different. Maybe [MENTION=9985]BMR Tech[/MENTION] can correct me if it's wrong.

The ~1.0 comment was mainly to help a broader audience understand the concept.
 

BmacIL

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wildcatgoal

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So is anyone going to change their spring selection now that they have all kinds of maths to do? Or just debate whether BmacIL's information is correct to the nth degree?

Keep it simple, folks... you aren't racing in the Continental Tire Series.
- the front... MR is 1.
- the rear.... MR is .5.

In having conversations with Steeda before and after I made my suspension selections (given my budget), they setup their damper valving the way they did using this kind of snazzy math and came up with their most recent Dual Rate Comp springs the same way. Of course then they slapped them on a few different cars including Ecoboosts and actually tested it... and made changes, even against the math before coming to market. That's what should happen.

In cordial paddock discussions, I have had people question my setup. Maybe they don't like Steeda or they don't like progressive/dual rate springs or they don't like Koni-based shocks. Numerous times they got their coilovers and perfect spring rate selections passed by a driver with coming up on a year of HPDE experience that makes an insane amount of mistakes, especially braking. I'm not out there competing and if something feels like I race, I'm out - I can't afford it. But man did I get some incredible value with what I have! Things could be better, sure, but honestly... the car needs to lose weight more than anything. I've also had a what felt like "too hard of a time" pulling away from an S550 with just Eibach Sportlines and slightly better tires... oh, and a race car driver for a cousin showing him the ropes that day vs. my arrogance that I didn't need an instructor... so. Spring rates be damned! Haha.

And with that, my Steeda Dual Rate Comp springs, which have the HIGHEST ride frequency of all the springs in the spreadsheet drive exceptionally well on the street with Steeda's Pro-Action adjustable dampers, especially when the rear is set to 2 turns and the front is set to 1.5 turns. They are a dual rate spring (or, maybe we can say "rapidly transitioning progressive" since it's one spring, not two separate coils) and at curb weight the linear rate applies. When weight is OFF at wheel (inside of a turn), the lighter progressive rate comes into play helping the car stay flat. I run my sway bars softer than I used to with Swift Spec R springs and get better drive confidence (less roll-feeling). Now, I am aware that Koni-based shocks are not as precise as Penskes. I didn't pay for that level of precision. Anyway, point being... with even this, let's call it, entry-level improved dampening, I am vastly impressed that my car now seems to fit squarely within the exact ride frequency category that I use my it for (HPDE) and one would therefore assume the car would be overly stiff and harsh on the street (or even the track, for that matter). And yet somehow my car isn't - it is not brash, harsh, or unacceptably stiff to drive on the street. Concrete expansion joint bumps will expose stiff springs no matter how good your dampers are, but no complaints still. Just feels like a sports car. A lot of this, I think, is because I've only dropped the car basically .75" - I have wheel travel, still. Dang, another variable to consider... wheel travel. Oh, then theirs weight balance after the drop (i.e., Steeda Ultralite Linears bias weight more toward a 50/50 distribution than stock, and you can sort of see that with the lesser wheel gap in the rear vs. front; BMR springs tend to balance toward the front with a more raked drop profile. Yes, it makes some difference.).

Anywho... babbling. Just... cool to understand all this stuff but understand what you're going to do (and CAN do) with that information, too. Otherwise... just drive it, dude.
 

VertMustang98GT

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Installing my BMR Performance: 170/740 (1.2" F / 0.5" R) Springs this week!

Can't wait.
 

Stuntman

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So is anyone going to change their spring selection now that they have all kinds of maths to do? Or just debate whether BmacIL's information is correct to the nth degree?

Keep it simple, folks... you aren't racing in the Continental Tire Series.
- the front... MR is 1.
- the rear.... MR is .5.
Well the difference between a .98 and .95 front motion ratio puts a 240lb GT350R front spring at a 230.5lbs & 216.6lbs respectively. That's a 6% difference.

The difference between a .48 and .492 rear motion ratio puts a GT350R's 890lb spring at 205.1lb & 215.4lbs respectively. That's a 5% difference.

If you're looking at front to rear wheel rate %s, there's a pretty big difference between going with a .98 & .48 motion ratios vs .95 & .492 motion ratios - 230lb/205lb vs 217lb/215lb.

If you care to nitpick between the rates of a GT vs GTPP rates or a GT350 vs GT350R, vs. BMR/Steeda/etc... then using the right MR numbers is kind of important.


0.02
 

BmacIL

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Well said ^^^. Those differences will not be night and day, but to someone who drives it every day, it would be noticeable from both ride and handling.

You may notice that I am constantly updating and correcting the chart with any new data. It doesn't upset me that I either had a wrong assumption or typo or what have you. I learn and thus we all do.
 

GTP

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I used the spreadsheet from this thread, and [MENTION=10281]BmacIL[/MENTION] 's chart in making my final selection.
Ride height and spring stiffness were important to me. DD to track car.

GT350 springs
GT350 front swaybar
GT350R rear swaybar
Steeda camber plates
Steeda adjustable dampers all round
Steeda rear shock mounts
BMR jacking rails
BMR cradle lockout
Steeda alignment sleeves

Took me all weekend to install everything. :doh:
 

Since-64'

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I used the spreadsheet from this thread, and [MENTION=10281]BmacIL[/MENTION] 's chart in making my final selection.
Ride height and spring stiffness were important to me. DD to track car.

GT350 springs
GT350 front swaybar
GT350R rear swaybar
Steeda camber plates
Steeda adjustable dampers all round
Steeda rear shock mounts
BMR jacking rails
BMR cradle lockout
Steeda alignment sleeves

Took me all weekend to install everything. :doh:
How much of a drop do the GT350 springs offer on a GT?
 

BmacIL

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How much of a drop do the GT350 springs offer on a GT?
Should be 0.3" all around. In [MENTION=16078]GTP[/MENTION]'s case his car went up about 0.2", so there is something wrong :headbonk::lol:

My contact at Ford vehicle dynamics stated 9 mm lower than a GT.
 

GTP

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Should be 0.3" all around. In [MENTION=16078]GTP[/MENTION]'s case his car went up about 0.2", so there is something wrong :headbonk::lol:

My contact at Ford vehicle dynamics stated 9 mm lower than a GT.
I must have installed them upside down. :headbonk:
 

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Steeda Linear Sport Springs Height Measurements

Front middle splitter: 6 1/4"
Front right/left quarter in front of tire: 6 1/4"
Front right/left quarter tire corner under 5.0 emblem: 5"
Rear right/left quarter in front of tire: 6 1/4"
Rear right/left quarter in back of tire: 7.5"
Rear brake light from bottom of painted plastic lip: 11"

**Measurements were taken from the exact middle front/rear, and then where the frame/body ends on the wheel openings.
**Notable that the car is completely even at all measuring points after a year and a half of settling in.
 
 
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