Spring ratio

[Performance nut]

Why is the Mustang significantly stiffer in the back than in the front? While researching different suspension upgrades, I came across some modifications that actually puts stiffer springs in the front that in the back.

Aside from inducing oversteer/understeer, is it because of launching vs. braking the car (stiffer rear translates to less squat vs stiffer front translates to less dive)? Seems like they were gunning more for drag racing than autocrossing...

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[Road Dog]

[MENTION=13927]Performance nut[/MENTION] The inboard position of the rear spring means the spring is fighting a considerable leverage effect. That requires the springs to be much stronger than if they were positioned closer to the wheel.

 

[Whiskey11]

Why is the Mustang significantly stiffer in the back than in the front? While researching different suspension upgrades, I came across some modifications that actually puts stiffer springs in the front that in the back.

Aside from inducing oversteer/understeer, is it because of launching vs. braking the car (stiffer rear translates to less squat vs stiffer front translates to less dive)? Seems like they were gunning more for drag racing than autocrossing...

Has to do with motion ratios. Motion ratios are the ratio of tire movement relative to spring movement. To achieve a certain handling bias, spring rates are compared to each other ONLY by their wheel rate, not by the raw spring rate. Strut suspension have a near 1:1 ratio (usually .95 to .98) when the spring is mounted on the strut.

With double wishbone suspensions and suspensions similar to it, like our rear suspension, the spring and shocks are mounted inboard of the wheel hub. This means the ratio will always be less than 1... in the case of the S550's rear springs it's something like .48, or basically .5 (for simplicity).

To go into detail, OEM's utilize a spring setup called "flat ride". Flat ride is a theory that when you go over a bump, the tires will respond at more or less the same time at a certain speed... so if the car is set up to have flat ride at 65 mph, the car will glide over bumps with little head toss in the passenger compartment... in theory. To achieve flat ride you need a rear wheel rate that is higher than the front.

To calculate wheel rate, it is: (Spring rate) * (Motion ratio)^2

The front of the car will essentially (for conceptual purposes) see the spring rate as wheel rate... the rear will see a QUARTER of the spring rate as wheel rate... so for 165lbs/in front springs, about 160lbs/in at the wheel (using a .98 Motion Ratio)... the rears 728lbs/in springs will only give a rear wheel rate of 182lbs/in. IE: Really soft at the wheel. I imagine this sets up flat ride frequencies around 65mph for a Performance Package car but I'm too lazy to do the actual maths! ;)

Wheel rate is what controls the final body roll angles and is far more productive to talk about than raw spring rates. It's important to note that spring manufacturers will always talk about the rate of the spring, not the wheel rate. This is done because that's what the spring actually takes to compress... it's just that leverage (motion ratios) get in the way of that when mounted to the car.

In reality, the rear of these cars is excessively soft and the increase in rear grip while cornering is why most folks in FS run the rear bar setup... personally, my co-driver and I are driving better with a slightly stiffer front bar than with the rear bar setups. Reality is the whole care is far too soft for any serious autocross duty. BMR's handling springs are a step in the right direction, but given my experience with my previous Mustang (2009), the front springs are still about 200lbs/in shy of the rates I'd like to run for autocrossing on street tires and probably about 300-400lbs/in shy of running on Hoosiers.

 

[Performance nut]

Has to do with motion ratios. Motion ratios are the ratio of tire movement relative to spring movement. To achieve a certain handling bias, spring rates are compared to each other ONLY by their wheel rate, not by the raw spring rate. Strut suspension have a near 1:1 ratio (usually .95 to .98) when the spring is mounted on the strut.

With double wishbone suspensions and suspensions similar to it, like our rear suspension, the spring and shocks are mounted inboard of the wheel hub. This means the ratio will always be less than 1... in the case of the S550's rear springs it's something like .48, or basically .5 (for simplicity).

To go into detail, OEM's utilize a spring setup called "flat ride". Flat ride is a theory that when you go over a bump, the tires will respond at more or less the same time at a certain speed... so if the car is set up to have flat ride at 65 mph, the car will glide over bumps with little head toss in the passenger compartment... in theory. To achieve flat ride you need a rear wheel rate that is higher than the front.

To calculate wheel rate, it is: (Spring rate) * (Motion ratio)^2

The front of the car will essentially (for conceptual purposes) see the spring rate as wheel rate... the rear will see a QUARTER of the spring rate as wheel rate... so for 165lbs/in front springs, about 160lbs/in at the wheel (using a .98 Motion Ratio)... the rears 728lbs/in springs will only give a rear wheel rate of 182lbs/in. IE: Really soft at the wheel. I imagine this sets up flat ride frequencies around 65mph for a Performance Package car but I'm too lazy to do the actual maths! ;)

Wheel rate is what controls the final body roll angles and is far more productive to talk about than raw spring rates. It's important to note that spring manufacturers will always talk about the rate of the spring, not the wheel rate. This is done because that's what the spring actually takes to compress... it's just that leverage (motion ratios) get in the way of that when mounted to the car.

In reality, the rear of these cars is excessively soft and the increase in rear grip while cornering is why most folks in FS run the rear bar setup... personally, my co-driver and I are driving better with a slightly stiffer front bar than with the rear bar setups. Reality is the whole care is far too soft for any serious autocross duty. BMR's handling springs are a step in the right direction, but given my experience with my previous Mustang (2009), the front springs are still about 200lbs/in shy of the rates I'd like to run for autocrossing on street tires and probably about 300-400lbs/in shy of running on Hoosiers.

Extremely informative write up. This has given me much to think about (and research) as I now have more questions than what I originally came in with

If you are so inclined:

• You mentioned the motion ratio is significantly lower in the back than in the front. You also mentioned that strut mounted suspensions were much closer to one. So I take it a coilover suspension could get away with a significantly softer spring in the rear than what we have now?
• I understand how to change the spring rate, what about the wheel rate? This seems to be directly related to the spring rate in that spring rate will determine how far a wheel rises under full load (though it isn't the only consideration). I guess my question is based on motio ratio... can you change both sides of the ratio or is pretty much spring rate your only way to modify motion ratio without completely redesigning the rear suspension?
• You mentioned comfort in this but also spoke about autocrossing and suspension being too soft. Does the motion ratio relate to racing as well or is it more of a measure of "will you retain your fillings in your teeth when you go over a bump"? Wondering if there is math involved with finding the ideal suspension for me (just my cup of tea).

Again, thank you very much!

 

[Whiskey11]

Extremely informative write up. This has given me much to think about (and research) as I now have more questions than what I originally came in with

If you are so inclined:

• You mentioned the motion ratio is significantly lower in the back than in the front. You also mentioned that strut mounted suspensions were much closer to one. So I take it a coilover suspension could get away with a significantly softer spring in the rear than what we have now?
• I understand how to change the spring rate, what about the wheel rate? This seems to be directly related to the spring rate in that spring rate will determine how far a wheel rises under full load (though it isn't the only consideration). I guess my question is based on motio ratio... can you change both sides of the ratio or is pretty much spring rate your only way to modify motion ratio without completely redesigning the rear suspension?
• You mentioned comfort in this but also spoke about autocrossing and suspension being too soft. Does the motion ratio relate to racing as well or is it more of a measure of "will you retain your fillings in your teeth when you go over a bump"? Wondering if there is math involved with finding the ideal suspension for me (just my cup of tea).

Again, thank you very much!

A true coilover setup in the rear will help some with the motion ratio. The new motion ratio will be that of the rear shock... somewhere around .7:1. You'd be able to run a softer spring but it'll never be the near 1:1 of the front suspension just by virtue of there not being enough room there.

Changing your spring rate will change your wheel rate... wheel rate = spring rate * motion ratio squared. You can change the motion ratio as well by going to coilovers in the rear, but spring rates is by far the easiest way to change wheel rate.

Motion Ratios are just an easy way to think about the lever working the spring when it goes over a bump. Your wheel rate is going to control the overall handling and ride comfort of the car going down the road.

Is there math out there to find the "perfect" setup? There is math out there, but the perfect setup isn't going to be perfect all the time. There is a lot of "theory" that goes into suspension design but at the end of the day even an engineered car needs to be tested in the real world and real world results work better than playing around with engineering formula's and all that.

 

[Norm Peterson]

. . . mentioned comfort in this but also spoke about autocrossing and suspension being too soft. Does the motion ratio relate to racing as well or is it more of a measure of "will you retain your fillings in your teeth when you go over a bump"? Wondering if there is math involved with finding the ideal suspension for me (just my cup of tea).

The "flat ride" approach to choosing relative front to rear springing isn't as applicable to the very hard running that exists in autocrossing, HPDE/tracking (and perhaps hard canyon running) as it is for normal street driving. The reason it isn't is because if you're going about it anywhere near right you'll be working with shocks and struts that provide higher amounts of damping during roll, squat, and dive body motions. Which has the side effect of damping out body motions going over bumps as well.

In more normal driving with shocks and struts tuned more toward ride comfort (something like 1/3 to 1/2 as much damping as might be ideal for cornering grip), the suspension is going to overshoot a bit, and what you don't want is the front of the car going up while the rear is still going down (or vice-versa).


Some years ago I put together a spreadsheet plot for flat ride, including the effects of shock damping. "Pitch jerk" is a measure of how fast pitch acceleration changes; in terms of how you might sense this, it's about the difficulty you might have in keeping your head in the same position against the head toss (which would require you to vary the amount of neck muscle control in step with the car motions). It's possible that some people have seen similar plots before.

It may be easier to see . . . the first plot is more or less what normal street shocks provide - with the front damping run down really low for exaggeration (like they were almost completely dead players). The third plot is with front and rear being new street shocks. The second represents something more or less like a track day setup (same car, same weight, same everything except for the shocks). It's going to be a lot more composed at least over heaves and bumps that aren't too sudden or sharp (like railroad tracks, potholes, and washboard roads).


Norm

 

[Simmons-Racing]

For an example......I ran 650lb front springs and 1,650lb rear springs. BMR front bar on full stiff, and stock PP rear bar. The car handled amazing and because of the JRZ shocks it rode really nice on the street as well.


Simmons

 

[wildcatgoal]

^Sheeezus with those spring rates!

 

[Simmons-Racing]

It's still a lower wheel rate then I run on my other cars.

Simmons

 

[DickR]

It's still a lower wheel rate then I run on my other cars.

Simmons

FYI for formula cars without wings, etc. it is common for the wheel rates to be equal to 2 times the corner weight of each corner of the car.

 

[Performance nut]

For an example......I ran 650lb front springs and 1,650lb rear springs. BMR front bar on full stiff, and stock PP rear bar. The car handled amazing and because of the JRZ shocks it rode really nice on the street as well.


Simmons

Could you define rode really nice please? As in "so long as the road was perfectly flat, it rode really nice on the street".

I'm also curious why it is past tense...

 

[BMR Tech]

We would make springs even stiffer than what we offer, but there are other factors when designing an OEM style/replacement spring. One primary factor is fitment and free height requirement.

 

[Simmons-Racing]

Could you define rode really nice please? As in "so long as the road was perfectly flat, it rode really nice on the street".

I'm also curious why it is past tense...

I feel it rode as good as stock on any road. The past tense is because my engine blew up at 12k miles and Ford didn't cover it. That's why I am selling all the parts I had on the car.

Simmons

 

[Performance nut]

I feel it rode as good as stock on any road. The past tense is because my engine blew up at 12k miles and Ford didn't cover it. That's why I am selling all the parts I had on the car.

Simmons

I'm sorry to hear that man. I had a car that I ran a really stiff suspension on and it rode well enough, though not as good as stock (comfort wise).

Where did you get those springs at?

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