Spring rate balance questions...

[kcbrown]

The spring rate balance on the stock Mustang suspensions is such that there is more rate at the rear than the front, as many here know.

For instance, the Brembo brake package has 167 lbs/in rear springs and 131 lbs/in front springs, for a 79% front:rear ratio. The front sway bar is 34.6mm in diameter (and tubular), while the rear bar is 24mm in diameter (and solid).

This seems to result in a rather neutral-handling car from the accounts I've read (I've not tracked mine yet so I don't know from firsthand experience what the real balance is, but when Randy Pobst says that the car doesn't understeer, I'm inclined to believe him).

The Boss 302 has an 80% front:rear ratio, so there's a bit more rate in front relative to the rear than the GT, and it has a staggered tire setup, but it also has a 25mm sway bar in the rear, so I can see how that combination could again result in a neutral handling car.

The Laguna Seca has a 72% front:rear ratio, a 26mm sway bar, and a staggered tire setup. I would thus expect it to oversteer more than the standard Boss 302.


But every lowering kit I've seen, along with every coilover setup I've seen or read about, has greater rates in the front than in the rear. Coilover setups, in particular, often seem to have massively greater rates in the front than in the rear (e.g., 550 lb/in in the front and 350 lb/in in the rear, for a 157% front:rear ratio).

My question, thus, is this: how in the world can the neutral handling characteristics of the car possibly be retained with, e.g., the coilover setups that are often run, most especially when (as Rehagen Racing apparently does) the rear bar is deleted entirely? Wouldn't the result instead be a car which understeers massively?

Moreover, it seems to me that even if you compensate by using more rear bar relative to the front, you'd wind up with more instability in the rear when going over asymmetric bumps.


So: is anyone running a coilover setup with less spring rate in the front than in the rear? And why is the prevalent setup the one that you would expect to result in significant understeer?


Or am I missing something monumentally important and fundamental in all this?

 

[Whiskey11]

The reason the OEM does it is for flat ride theory... not a very interesting read but it supposedly makes the car ride better.

As for why that is reversed with handling oriented setups, it has a lot to do with how strut cars gain/lose camber and how live axles don't, and how relative stiffness of a live axle is to the front. Lots of factors play into it but strut based front suspensions have a nasty tendency to gain negative camber too slowly and thus it requires more negative camber to operate the tires in their happy place.

The easiest counter to this is stiffer spring rates and on the S197 chassis, you end up gaining more front end grip and reducing understeer by going to stiffer springs up till the 350lbs/in area then more rear spring rate is needed to keep things balanced. That is assuming you have the negative camber to keep the tires happy.

My car, at 440 front, 200 rear, 35mm front bar at full stiff, 25mm rear bar in the middle position and the rear roll center jacked way up is loose enough that dropping the rear bar down a notch will make the car significantly less tail happy. Ironically my spring rate factor, after you account for motion ratios, ends up being close to 70% front bias. I'm beginning to think that keeping the front rates at roughly twice the rear rates will provide the best balance with room to tune in the bars.

 

[barbaro]

Good question. Interesting answer.

 

[Rabee]

Boss

I am learning a lot

 

[kcbrown]

The reason the OEM does it is for flat ride theory... not a very interesting read but it supposedly makes the car ride better.

As for why that is reversed with handling oriented setups, it has a lot to do with how strut cars gain/lose camber and how live axles don't, and how relative stiffness of a live axle is to the front. Lots of factors play into it but strut based front suspensions have a nasty tendency to gain negative camber too slowly and thus it requires more negative camber to operate the tires in their happy place.

The easiest counter to this is stiffer spring rates and on the S197 chassis, you end up gaining more front end grip and reducing understeer by going to stiffer springs up till the 350lbs/in area then more rear spring rate is needed to keep things balanced. That is assuming you have the negative camber to keep the tires happy.

Hrm. What does the front rate versus traction curve look like? I mean, it sounds like if you want the car to oversteer more, all you need to do is add front spring rate (to a point), right?

My car, at 440 front, 200 rear, 35mm front bar at full stiff, 25mm rear bar in the middle position and the rear roll center jacked way up is loose enough that dropping the rear bar down a notch will make the car significantly less tail happy. Ironically my spring rate factor, after you account for motion ratios, ends up being close to 70% front bias. I'm beginning to think that keeping the front rates at roughly twice the rear rates will provide the best balance with room to tune in the bars.

Interesting. I guess the next logical question I have is what the consequences to the handling balance would be of maintaining (roughly) the spring rate balance of front:rear while you increase the front spring rate.

In my case, I'm not thinking of increasing it a whole lot. I have two goals for the modifications I expect I'll eventually make to my 2014 GT track pack:

1. Improve the stance of the car without lowering it too much. The GT500's stance is, in my opinion, perfect, so I'm looking to lower the car by at most 3/4 of an inch at both ends.
2. Improve the handling dynamics of the car to the degree possible, while maintaining or exceeding the stock ride quality (which includes the ability to absorb relatively large bumps gracefully, hence maintaining near-stock bump travel is important) and maintaining the handling balance (assuming, as is highly likely, I find the handling balance of the car in stock form to my liking).

My initial thoughts are to go with the Boss 302 springs on the front and the 2012 GT500 SVT performance package springs on the rear. The front rate would thus be 148 lbs/in and the rear rate 194 lbs/in, for a front:rear ratio of 76%, which is reasonably close to the 79% of my car (it theoretically gets me a little more oversteer at the limit).

But since the time gap between the front and rear tires when hitting a bump at a given speed is a constant, while the period from the ride frequency is inversely proportional to the square root of the spring rate, keeping the spring rate ratio constant won't maintain the "flat ride" effect. Instead, you'd have to use an even higher rear rate in order to have the two sinusoids converge at 3/4 of their ride wavelength. I'm currently working out the formula for determining the desired rear spring rate if given the desired front spring rate, current front spring rate, and current rear spring rate.

 

[Whiskey11]

Hrm. What does the front rate versus traction curve look like? I mean, it sounds like if you want the car to oversteer more, all you need to do is add front spring rate (to a point), right?

This is pretty hard to generalize with because suspension is a bit more complex. I don't have a graph because I've never looked into the math but conceptually you will gain front end grip by keeping the tires in the happy portion of the camber curve. IE: Less roll = less static negative camber necessary to keep them there but you will eventually, at some point start to trend heavily back into the understeer portion of that curve if you keep the rear spring rate the same.

That is one area where the "stiffen the front = more understeer, stiffen the rear = more oversteer" generalization fails the test and strut cars happen to be perfect examples of why that isn't always true. It certainly is true if the car is already stiff enough to keep the tires in the happy portion of the camber curve. Sadly that too is not an exact amount since every tire is different.

Interesting. I guess the next logical question I have is what the consequences to the handling balance would be of maintaining (roughly) the spring rate balance of front:rear while you increase the front spring rate.

In my case, I'm not thinking of increasing it a whole lot. I have two goals for the modifications I expect I'll eventually make to my 2014 GT track pack:

1. Improve the stance of the car without lowering it too much. The GT500's stance is, in my opinion, perfect, so I'm looking to lower the car by at most 3/4 of an inch at both ends.
2. Improve the handling dynamics of the car to the degree possible, while maintaining or exceeding the stock ride quality (which includes the ability to absorb relatively large bumps gracefully, hence maintaining near-stock bump travel is important) and maintaining the handling balance (assuming, as is highly likely, I find the handling balance of the car in stock form to my liking).

My initial thoughts are to go with the Boss 302 springs on the front and the 2012 GT500 SVT performance package springs on the rear. The front rate would thus be 148 lbs/in and the rear rate 194 lbs/in, for a front:rear ratio of 76%, which is reasonably close to the 79% of my car (it theoretically gets me a little more oversteer at the limit).

But since the time gap between the front and rear tires when hitting a bump at a given speed is a constant, while the period from the ride frequency is inversely proportional to the square root of the spring rate, keeping the spring rate ratio constant won't maintain the "flat ride" effect. Instead, you'd have to use an even higher rear rate in order to have the two sinusoids converge at 3/4 of their ride wavelength. I'm currently working out the formula for determining the desired rear spring rate if given the desired front spring rate, current front spring rate, and current rear spring rate.

Well Maximum Motorsports has a set of springs out that uses the flat ride theory for their springs (rear spring rates are some absurdly high rate compared to the front) and I spoke with Jack Hidley on FB about the springs after I made a comment about that being an insane amount of rear spring rate. Their recommendation is a heavy front bar and NO rear bar.

In my opinion, when you start talking about marginal increases in spring rate over stock, you aren't going to see a whole lot in "gains". Larger changes like going from stock to say the Steeda Sports (200/175) produced better gains. I'm just not sure you can achieve what you want with OEM spring rates and swaybars without some Roll Center raising (or CG lowering) to help keep roll down. These cars are kind of boats at stock spring rates, even with nicer shocks/struts.

 

[SoundGuyDave]

There are a LOT of factors involved in spring rate selection, but for the handling end, here are a few of the essential concepts...

1) You do want to run as soft a spring as you can, while still keeping the car off the bump-stops during "normal" track activities.

2) You want to tailor the spring rates to maximize the contact patches (dynamically!) of the tires you have elected to run.

3) Generally speaking, McStrut front suspensions LOSE negative camber in bump travel, so it's a good idea to find the sweet spot between static negative camber and spring rate to get to the tires desired camber range under high dynamic load conditions.

4) From a PURE handling standpoint, you want to select spring rates that take maximum advantage of load transfer without undue chassis motion. Think squat/lift during accel and decel, but think about them at each end, as well!

5) With the S197 specifically, there is a MASSIVE compromise in the rear with regards to motion ratio. The wheel rates (which are what really matter) are radically different in pitch and roll conditions. This is due to the spring locations, nothing more. Again, the raw spring rate really means nothing, it's all about how that rate relates to wheel motion that counts.

6) Moving roll center, AS%, AD%, and swaybar rates around should be the final icing on the cake, but may indeed affect the needed spring rate as they can affect wheel rate.

In the end, again from a pure handling standpoint, it's all about the rubber you run. The goal of a suspension designer/tuner is to keep the maximum amount of grip possible under dynamic conditions, without sacrificing too much in other areas.

The two basic schools of thought:

Soft springs/heavy bars: Roll is controlled via the swaybars, keeping the pitch and flat-ride component soft, to handle surface irregularities. Downside is that in longitudinal transfer conditions (accel/braking), the car tends to move. A lot. Under braking, it will face-plant, as the load transferred creates a LOT of suspension travel. You can play with the "anti" suspension angles to offset this a bit, but then the ride begins to suffer on bumpier track surfaces. Good dampers go a long way to offsetting the issues by controlling the RATE at which the forces transfer.

Hard springs/soft bars: Overall, suspension motion is reduced in longitudinal loading, with roughly the same motion in lateral load, as the wheel rates remain the same. The downside is that the ride will be a lot harsher over irregularities, and good dampers again are extremely important, as they are now required to control the suspension properly with (relatively) little suspension motion to work with.

My personal opinion is that the soft/big setup isn't a comfortable one to drive hard. At bigger, high-speed tracks, with long aggressive braking zones, the "face plant" that you get under braking effectively unloads the rear end of the car to the point where it starts to wag, which at 100+MPH is NOT a comfortable feeling. OTOH, if you DD the car, a hard/small setup could leave you peeing blood after a trip to the corner store...

Oh, and if you're running aero, you have to revisit spring rates all over again!

Please note that all of my comments are coming from a pure handling perspective on road courses, not factoring in "ride quality" or "NVH" concerns at all...

Didn't we have this discussion before? Yes, we did!

 

[Sky Render]

S197s are not neutral-handling from the factory. They understeer like drunken Subarus on the factory suspension.

 

[sheizasosay]

All cars from the factory understeer, with a possible exception to exotics. Bottom line- Car manufacturers do not want you going off the road tail first.

 

[Norm Peterson]

That's true, but at -1.8° the S197 doesn't understeer as badly as people seem to think either. Push hard enough and it'll still show up, but you do have to be trying harder than you ever should on public roads. Take a little off the cornering lateral g's and you can throttle-steer your way to more rotation instead, at least on street tires.

It's difficult to isolate the degree of understeer to only the front spring rate unless you're making large changes. The front sta-bar in particular is quite effective in minimizing the change in the car's overall roll rate and in the front proportion of LLT.


Norm

 

[barbaro]

Yes, we did![/URL]

The wagging tail end is precisely what happens during high speed braking and I have felt that before. The other possibility not mentioned is to run a torque arm watts link set up and then you can run the soft springs without the faceplant on high speed braking. The torque arm solves your dilemma. Definitively. But I believe we have had this discussion as well. For a non coilover set up the torque arm is the best handling modification you can make and still maintain your stock spring rates or close to it.

I have recently taken my torque arm off and the car does not do anything as well as it did before. Taking it off is even more instructive than putting it on. I hate stiff non compliant rides and driveway scraping. Modern cars should not drive like that. if you want a streetcar that handles but that you can also take your girlfriend out in then get the torque arm and the Boss Springs and be done with it. Mixing and matching the Boss fronts with the GT 500 rears doesn't make much sense to me.

If you are at the track then there will be no substitute for spring rate but on the street who wants to piss blood after a Sunday drive? Not me.

 

[sheizasosay]

you do have to be trying harder than you ever should on public roads.

Norm

That's not entirely accurate. If you are on a 55mph road doing 55 and approach a 4 way intersection with a green light and the speed limit of the road you are turning onto is 55mph......totally legal...

 

[kcbrown]

I have recently taken my torque arm off and the car does not do anything as well as it did before. Taking it off is even more instructive than putting it on. I hate stiff non compliant rides and driveway scraping. Modern cars should not drive like that. if you want a streetcar that handles but that you can also take your girlfriend out in then get the torque arm and the Boss Springs and be done with it. Mixing and matching the Boss fronts with the GT 500 rears doesn't make much sense to me.

The only reason I'm looking at the GT500 rears is for the stance. The Brembo stance in the rear is just too high for my tastes, and the Boss 302 rear springs lower that by only 1mm, i.e. not enough to make a difference there. So this is only for cosmetic reasons, and not anything else.

 

[csamsh]

So...not to digress into this conversation again, but I had yet another passenger this weekend ride in my 550# spring AST shock car this weekend.

His surprised conclusion...as is always the conclusion- "this rides better than stock!!"

 

[sheizasosay]

Well Maximum Motorsports has a set of springs out that uses the flat ride theory for their springs (rear spring rates are some absurdly high rate compared to the front) and I spoke with Jack Hidley on FB about the springs after I made a comment about that being an insane amount of rear spring rate. Their recommendation is a heavy front bar and NO rear bar.

What were the rates?

 

[NoTicket]

If you are interested in the springs: Here they are on MM's website

 

[kcbrown]

There are a LOT of factors involved in spring rate selection, but for the handling end, here are a few of the essential concepts...

1) You do want to run as soft a spring as you can, while still keeping the car off the bump-stops during "normal" track activities.

2) You want to tailor the spring rates to maximize the contact patches (dynamically!) of the tires you have elected to run.

3) Generally speaking, McStrut front suspensions LOSE negative camber in bump travel, so it's a good idea to find the sweet spot between static negative camber and spring rate to get to the tires desired camber range under high dynamic load conditions.

Yep, and (per the previous discussion we had) the S197 is apparently quite a bit better (if Sam Strano's right) about this than a lot of McStrut platforms, hence the need to go with much higher front rates in order to maximize grip is significantly reduced. Between that and the front bar, it's sounding like the only real reason for significantly stiffening the ride is high speed controllability under heavy braking.

4) From a PURE handling standpoint, you want to select spring rates that take maximum advantage of load transfer without undue chassis motion. Think squat/lift during accel and decel, but think about them at each end, as well!

5) With the S197 specifically, there is a MASSIVE compromise in the rear with regards to motion ratio. The wheel rates (which are what really matter) are radically different in pitch and roll conditions. This is due to the spring locations, nothing more. Again, the raw spring rate really means nothing, it's all about how that rate relates to wheel motion that counts.

I was thinking about this a bit. Seems to me that the wheel rate at the rear isn't a simple constant like it would be with an independent suspension, but is instead a variable, and (thanks to the fact that the springs are located somewhat inboard) the wheel rate at one end is not just due to the spring at that end, but due in part to the spring at the other end, as well. Which is to say, if you keep one end of the rear suspension anchored and move the other end vertically, the resulting wheel rate is a function of both springs.

6) Moving roll center, AS%, AD%, and swaybar rates around should be the final icing on the cake, but may indeed affect the needed spring rate as they can affect wheel rate.

How does AS% and AD% affect the wheel rates in steady state cornering? I can obviously see how they would affect it during braking and acceleration.

In the end, again from a pure handling standpoint, it's all about the rubber you run. The goal of a suspension designer/tuner is to keep the maximum amount of grip possible under dynamic conditions, without sacrificing too much in other areas.

Okay, well, in my case, the rubber is going to be more or less a constant: 285/35-19 street tires. I'm not in this for competition, I'm in it for fun, so getting minimum lap times isn't what this is about for me.

The two basic schools of thought:

Soft springs/heavy bars: Roll is controlled via the swaybars, keeping the pitch and flat-ride component soft, to handle surface irregularities. Downside is that in longitudinal transfer conditions (accel/braking), the car tends to move. A lot. Under braking, it will face-plant, as the load transferred creates a LOT of suspension travel. You can play with the "anti" suspension angles to offset this a bit, but then the ride begins to suffer on bumpier track surfaces. Good dampers go a long way to offsetting the issues by controlling the RATE at which the forces transfer.

From what I've seen of pictures of these cars in stock form under heavy braking, it looks to me like the bulk of the motion under heavy braking is actually in the rear due to the suspension "jacking up" back there. How will changing AD% affect the ride on bumpier surfaces? Will it result in more "skipping" over irregularities under heavy braking or something?

Hard springs/soft bars: Overall, suspension motion is reduced in longitudinal loading, with roughly the same motion in lateral load, as the wheel rates remain the same. The downside is that the ride will be a lot harsher over irregularities, and good dampers again are extremely important, as they are now required to control the suspension properly with (relatively) little suspension motion to work with.

My personal opinion is that the soft/big setup isn't a comfortable one to drive hard. At bigger, high-speed tracks, with long aggressive braking zones, the "face plant" that you get under braking effectively unloads the rear end of the car to the point where it starts to wag, which at 100+MPH is NOT a comfortable feeling. OTOH, if you DD the car, a hard/small setup could leave you peeing blood after a trip to the corner store...

And since mine is a DD, there's going to be a limit as to how hard I can make the spring rates without compromising the ride. I have driven a car with KW v3 coilovers and the ride was surprisingly good (equal to or better than stock over most bumps that did not exceed the suspension travel capability of the car with the given springs), so damper quality seems to be capable of making up for a lot. But there's a caveat here: the KW v3 springs are apparently progressive, and I don't know what rates they have at/near ride height compression.

Oh, and if you're running aero, you have to revisit spring rates all over again!

Please note that all of my comments are coming from a pure handling perspective on road courses, not factoring in "ride quality" or "NVH" concerns at all...

And when you factor those in, I expect the "win" goes primarily towards the "soft springs/heavy bars" school, right?

Didn't we have this discussion before? Yes, we did!

We did, but nobody answered my latest questions there, so for anything new, that thread seems to be dead. Part of that is probably due to the "noise" from me that resulted from some of my misunderstanding of how ride height can be changed with coilovers without affecting spring compression. Sorry about that, guys.

The requirement for a drop no more than 3/4" from stock ride height is a hard requirement for me, because I have an annoyingly steep driveway to contend with and I'd like to retain most of the stock suspension travel in order to accommodate the large bumps I encounter on my drive to work. And thus far, nobody has been able to tell me if a coilover system (like the AST 4150s) would be able to get me such a small drop, or how that would be achieved. Ride height adjustment on the S197 coilovers I've thus far seen seems to be achieved by changing the location of the spring perch, which means the length of the strut body itself (in particular, the distance between the suspension attach point and the base of the shaft) remains fixed, and that means that the higher you set the ride height, the greater the available bump travel and the less the available jounce travel. Since the coilover struts are fixed length and shorter in order to accommodate lowering the car while keeping the shaft length (roughly) centered at the new, lower ride height, it follows that raising the ride height to the degree I want may easily result in a strut that is extended more than it should be at ride height.

And if I can't achieve what I want with coilovers, then I must do so using standard lowering springs and struts/shocks.

I was very impressed with how well, in the video you posted, your coilover setup handled the irregularities of that road course compared with the BMR springs and Tokicos. The car just seemed completely stable with the coilovers. Between that and my experience with the KW v3 coilovers, they are a very tempting route for me. But given my hard requirements, I don't (yet) see how they can be made to work for me. Help!

 

[Sky Render]

I think you should go race your car and find out what YOU feel needs to be addressed based on YOUR driving style instead of bench-racing and asking everyone else's opinions.

After a point, "correct" car setup comes down to how the driver wants the car to feel.

 

[neema]

So...not to digress into this conversation again, but I had yet another passenger this weekend ride in my 550# spring AST shock car this weekend.

His surprised conclusion...as is always the conclusion- "this rides better than stock!!"

What are your rear spring rates? I think those are the main culprits of a harsher ride.

 

[Norm Peterson]

For rear ride harshness, I think I'd blame (too much) high speed bump damping and LCA inclination/SVIC location a whole lot more.


Norm