Don’t forget frequency also comes into play. I’ll get the rates and frequency I’m at now. It’s almost perfect. We made 1 minor change and I gained 2 seconds.
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coil over spring rate help
- Thread starterAnviltester
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Do you mind sharing what the rates are and what that minor change that gains 2 seconds is ?
And they are ALL fighting oversteer.
That's your driving, not the car. Unless you had a flat tire, there is no change which gives you 2 seconds.
I've run a lot of different spring combos. Every canned spring package is just wrong. It seems like every aftermarket spring package is just 'make it loose'. None of them have enough front rate and they all have too much rear.
500-600 #/in on the front rate will get you in the window. I run 500's. The rear spring (divorced) is a little more salt-to-taste. In a low-grip situation, you might be as low as 650 #/in or so. A smooth, high grip track will be more like 900-1000 #/in. If you're on coil-overs in the rear, that will translate to about 300-500 #/in. Heresy, I know, but it let's you do fun stuff, like go to throttle.
Let's get a couple things out of the way early.
Stiffer springs do not necessarily make your car faster, so there is no need to try to turn a GT car into a go-kart. You'll just be skating around the track everywhere and be S-L-O-W.
The S550, in general, is a car that OVERsteers, particularly on corner entry. That early corner instability will transisition to an understeer, particularly if you over-charge the corner even a little bit. Everyone feels the understeer, but, for me, the entry instability is a much bigger deal. Once you get that calmed down, a good bit of the understeer will be addressed, because you can take a better entry arc and carry the brakes further into the corner.
The reason the DH racecars were loosing CV boots/joints last year was because everyone would run the rear ride height so low that they were getting into the exhaust. They were doing this to try to get the rear under control.
My car is probably a bit of a worst case scenario, because it's an Eco and a couple % of weight distribution is a big deal, but they all have the same basic characteristics. V8 cars will definitely understeer more, but they also tend to have to have more power to try to hook up. Mine is about the same torque as a stock Coyote, maybe a little more. I'm about 40 HP down on power and have 1000 fewer revs to play with (have to run taller rear gear). Regardless of the numbers, I think the 4-banger hooks up a little better just because of the turbo lag. The V8 gives a more immediate hit that is great if you can hook it up, but will cause traction issues if you can't.
GT cars in general, and high-power, front-engine cars in particular, make their time by getting good drives off slow corners. If we shoot for a neutral mid-corner balance, we won't have enough reserve capacity on the rear to get out of the corner. My general approach is to concentrate on getting the rear of the car stable on corner entry to be able to carry appropriate entry speed on the right line while trailing brakes. This is a primary concern. Nearly as important is good traction on corner exit, that's our next goal. Only if those two things are addressed do I try to reduce mid-corner understeer. Sometimes it just is what it is. I will add one caveat. IF you're in the situation where we get in a completely cross-hands mid-corner understeer, then that will feed into a nasty snap loose as we go to throttle. In that scenario, traction can be IMPROVED by reducing understeer. Having said that, if you're at the apex with crossed-hands, then there's probably something going on at corner entry which is causing it.
I think the Ohlins coil-overs are a really good option for street/track use. They come with 515# front and 800# (divorced) rear springs. I've been partial to Ohlins stuff for a long time, but it's because they consistently produce a good product. Regardless of whether or not you use their dampers, I think their spring selection is a pretty good starting point for just about anyone.
500-600 #/in on the front rate will get you in the window. I run 500's. The rear spring (divorced) is a little more salt-to-taste. In a low-grip situation, you might be as low as 650 #/in or so. A smooth, high grip track will be more like 900-1000 #/in. If you're on coil-overs in the rear, that will translate to about 300-500 #/in. Heresy, I know, but it let's you do fun stuff, like go to throttle.
Let's get a couple things out of the way early.
Stiffer springs do not necessarily make your car faster, so there is no need to try to turn a GT car into a go-kart. You'll just be skating around the track everywhere and be S-L-O-W.
The S550, in general, is a car that OVERsteers, particularly on corner entry. That early corner instability will transisition to an understeer, particularly if you over-charge the corner even a little bit. Everyone feels the understeer, but, for me, the entry instability is a much bigger deal. Once you get that calmed down, a good bit of the understeer will be addressed, because you can take a better entry arc and carry the brakes further into the corner.
The reason the DH racecars were loosing CV boots/joints last year was because everyone would run the rear ride height so low that they were getting into the exhaust. They were doing this to try to get the rear under control.
My car is probably a bit of a worst case scenario, because it's an Eco and a couple % of weight distribution is a big deal, but they all have the same basic characteristics. V8 cars will definitely understeer more, but they also tend to have to have more power to try to hook up. Mine is about the same torque as a stock Coyote, maybe a little more. I'm about 40 HP down on power and have 1000 fewer revs to play with (have to run taller rear gear). Regardless of the numbers, I think the 4-banger hooks up a little better just because of the turbo lag. The V8 gives a more immediate hit that is great if you can hook it up, but will cause traction issues if you can't.
GT cars in general, and high-power, front-engine cars in particular, make their time by getting good drives off slow corners. If we shoot for a neutral mid-corner balance, we won't have enough reserve capacity on the rear to get out of the corner. My general approach is to concentrate on getting the rear of the car stable on corner entry to be able to carry appropriate entry speed on the right line while trailing brakes. This is a primary concern. Nearly as important is good traction on corner exit, that's our next goal. Only if those two things are addressed do I try to reduce mid-corner understeer. Sometimes it just is what it is. I will add one caveat. IF you're in the situation where we get in a completely cross-hands mid-corner understeer, then that will feed into a nasty snap loose as we go to throttle. In that scenario, traction can be IMPROVED by reducing understeer. Having said that, if you're at the apex with crossed-hands, then there's probably something going on at corner entry which is causing it.
I think the Ohlins coil-overs are a really good option for street/track use. They come with 515# front and 800# (divorced) rear springs. I've been partial to Ohlins stuff for a long time, but it's because they consistently produce a good product. Regardless of whether or not you use their dampers, I think their spring selection is a pretty good starting point for just about anyone.
Ride frequency is a good way to be able to characterize dissimilar cars, but since we're all talking about the same car, it kind of comes out in the wash. None of use will really use that information for anything other than sticking a finger in the wind.
- Thread starter
- #47
so im good talking about anything divorced spring - I have mcs triple adjustable coilovers on all 4 corners so spring rates im asking for are specific to that. Also I have great turn in as its setup now. My mid corner and final exit are my issues as the rear steps out and I can't get the power down soon enough. also feels a little unstable in high speed sweepers - I am used to feeling a 4 gentle wheel slide that always catches itself on a full race slick and a slightly muted version of that on a dot slick. As setup the back wants to step out and no real 4 wheel slide (if that makes any sense lol). I relate the "4 wheel slide" to the tire slippage in angle more so than complete loss of traction.
A 4-wheel drift is too loose and you're looser than that. Reduce the rear bar and rear spring rates. Lower the rear ride height. Both will help. Make a big step and go farther than you think you need go. Get the car to consistently push, then gradually free it up until you get the balance you want.
Dave_W
Cones - not just for ice cream
Jacques Villeneuve talked about this exact scenario during the Sky/ESPN F1 Practice 1 coverage today. As you pass the apex, the front tires have too much slip angle for maximum grip. Then as you apply throttle and unwind the wheel, the slip angle on the fronts is reduced, they "grip up", and the steering angle you still have shoots the nose to the inside. You think it's power-on oversteer, but it's actually a secondary effect of steady-state understeer. It's exagerated on tires that have narrow envelopes of optimum slip angle.
Well... when I can switch the DSC on the fly and do two laps at 151.xx and switch it back and do two laps at 149.xx and do it again and again I would have the say the data is the proof.
When I get under the car i'll get the rates again. Ive changed them a bit as of lately. We softened the front end. I use DSC tracktive suspension so what I tell you wont mean much but we softened the front dampers 5%. No knobs or clicks, its really useless for most everyone here. I will say the DSC tractive is a learning curve and really great stuff when you figure it out. I dont know how to use it but have a data engineer from the FD airforce team and a few other guys help set my stuff up. I just drive for now.
OK, then that's not a minor change. A minor change to me would be a tire pressure, ride height or anti-roll bar adjustment. Yes, they'll have an effect, but don't expect any more than a tenth or two for equivalent laps.
I don’t t know your experience with DSC tractive suspension, but it’s insane. No air pressure change or sway bar. Just a change with the controller on a laptop. This set up is so crazy and fun to play with. Hopefully we can connect and BW and I’ll show you how it works .
Тhe issue is that if the rear is stable for corner exits aka press the gas pedal whatever you feel like it. You will be fighting understeer in slow stuff. I run the Ohlins as you know there is a lot to be desired of that setup especially in terms of adjustment for smooth tracks you run this things as stiff on rebound as they will allow and still it's not enough.
And the springs actually make the rear too stable so in slow corners aka hairpins I'm fighting understeer you can see it on some of the videos from fast laps on Serres in turn 5. Also my rear have a tendency to step out in fast corners with this rates Serres Turn 13 or at the newly rebuild Dracon on the fast corner going to the longest straight.
Which means that setting up a Mustang for low speed tracks that have a few key fast corners is a real challenge with the Ohlins.
Personally for 200TW tires I think a bit stiffer front aka 650 lbs with a bit stiffer rear aka probably 1200 lbs (divorced) should be overall a better setup but the Ohlins highest spring is 914 lbs which mean that whatever springs you have you will never get the rear in the window you need for it to be even remotely playful. And the problem with remotely playful is that tradeoff that in fast corners the car will not be stable enough aka oversteer.
The beauty of the Ohlins is that they could be great on the street making them a great street setup though because of the 500 lbs front springs a bit on the hard side of things.
steveespo
@21CarProductions @NJS58 Racing @NJS58 Eurosport
Soft and high.
with all the talk about spring rates and their behavior, I assume sway bars would be a factor also. 800lb rear spring would be different in turns with a 18mm bar vs a 22mm bar i would imagine. if it doesnt then ignore me and ill continue to let more knowledgeable people post.
Dave_W
Cones - not just for ice cream
Yes, they are. However, they contribute differently to the wheel rate depending on whether you're looking at roll, one-wheel bump, or two-wheel bump. Generally, they don't contribute to wheel rate at all in 2-wheel bump because the bar just swivels in the bushings. That's if they are set up correctly - I've seen several cars with aftermarket urethane sway bar bushings that were so tight they "bound up" the bar to where it was nearly impossible to move by hand. After some sandpaper work, and/or shimming the bracket out, I could rotate the bar easily with one finger after the bracket was torqued down. Note that the "bonded bushings" used in recent Mustangs do act like a slightly 'bound up" bar on purpose, though the amount of force is engineered from the start by the size & durometer of the bushing.
steveespo
@21CarProductions @NJS58 Racing @NJS58 Eurosport
how about no rear bar?
Fair point.
It's taken me a week to write this brain dump. I hope it's worthwhile.
This is going to get long winded, but I’ll explain how I approach Spring and ARB selection.
TL;DR: Springs primarily control pitch and heave motion. Bars primarily control overall roll angle and roll-couple distribution. Clearly define the problem before choosing the tool to do it.
I feel like I need to preface this a little bit. Without getting into particulars, this is something I do _a lot_ and I’ve spent about 30 years figuring out a method. This is not “THE” method, because there’s no such thing as a single way to choose springs and bars for a car. Everyone has their own approach; this is a Cliff’s Notes version of mine.
There are three main car motions we’re concerned: Heave, Pitch and Roll. Heave for our cars is confined to the crests and compressions of the track. If you start to add some aero into the mix, then you’ll need to support that as well. Pitch is the motion which has the biggest influence on the handling of the car. Generally speaking, as you lower front ride and/or raise rear ride height (known as increasing chassis ‘rake’.) you will make the tend towards oversteer, particularly early in the corner. As the rear squats (reducing rake) the car will tend to understeer. Controlling rake angle is very important. Roll is the motion we get to play with the most and it’s the one that will ultimately determine our roll-stiffness distribution front to rear. Springs determine base roll couple, but the bars produce anywhere from 10-50+% of the total roll stiffness and can allow you to heavily influence the handling balance. These are our 3 primary modes of chassis motion.
Springs give a vertical stiffness to each corner. They will deflect and resist the car chassis motion as the inertial loads of the car shift due to driving. Springs are active in all chassis modes. Anti-Roll bars are springs which are only active in the roll mode. Bonded anti-roll bar bushings are a trick OEM’s use to produce low friction motion. The spring rate of bonded bushings can be added to the spring on that corner for it’s influence. It’s not ‘binding’. They’re intentionally using it to stiffen ride.
When I’m taking a first cut at springs, My initial goal is to choose a front spring which will support the front end adequately during heavy braking. If the car has a lot of travel on the front while braking, that limits how quickly the driver can ramp up brake pressure and, in general, reduces the driver’s confidence by allowing excess brake dive (positive chassis rake) which can cause rear instability. (With stock spring rates of 170#/in, I had a lot of brake dive and had to catch the rear of the car all the way to the apexes and it was tough to get the car to act consistently.)
The rear spring is chosen to allow as much vertical travel as possible, which will improve our traction and corner exits) without the driver complaining of excess chassis motion (squat or reduced rake) on throttle or a general lack of responsiveness in direction change even with appropriate rear roll stiffness. The rear will end up moving about 15-30%% more than the front even though the G-load in braking is 50% or higher than our G-load in acceleration. Cars are always more sensitive to ride height on the front than they are the rear. Allowing the rear a bit of extra compliance help with traction almost always pays off.
Once I’ve selected springs, I use the anti-roll bars to set my overall roll stiffness and roll-couple distribution. The springs will have their roll stiffness contribution and the anti-roll bars will have theirs. If you calculate the roll stiffness of the front and the roll stiffness of the rear, you can come up with a Mechanical Balance ratio of the front roll stiffness to the total. The slang is ‘Mech Bal’. If we’re running a 60% Mech Bal, then 60% of our total roll stiffness is taken on the front axle. This number will vary somewhat due to the track and conditions, but if you hold the mech bal consistent, you can do a global stiffen or soften and not change the mid-corner balance of the car all that much. It’s a cool party trick.
Here’s something that’s a little less intuitive. If we were going to put our car in some sort of simulation program to optimize lap time using all the variables that can change on the car, it wouldn’t give us one setup. It would give us a bunch of setups that all run the same basic lap time within a fractions of a tenth. The spread in mech bal would probably be at least 15% from setup with the lowest to the one with the highest mech bal. It’s not like there’s a single specific value to shoot for. There are any number of tuning variables we can use to tune the balance of a car. They all have compromises. If the car is doing what I want it to do in terms of heave and pitch motion, then I’ll try to leave the springs alone and tune the balance in a different way. If you’re racing, a lot of times you’re in the position where either you can’t change the springs due to time, you don’t have the spring you really want, you’re not allowed to change the springs or whatever. Regardless, you still have to figure out how to make the car turn.
If someone is making the argument that a stiffer rear spring will reduce understeer, I’ll completely agree. Having said that, it’s often not the most efficient way to do it. By that, I mean the compromise of increasing spring rate (decreased compliance) can be a greater negative than the compromises we might have by achieving that same balance with a different choice.
Some ideas to reduce understeer other than increasing rear spring:
Lower front ride height
Higher rear ride height
Softer FARB
Stiffer RARB
Softer front compression damping
Stiffer front rebound damping
Stiffer rear compression damping
Softer rear rebound damping
Reduce rear camber
Reduce rear toe-in
Optimize front camber (It’s not always a gain to increase)
Optimize front toe (In or Out is equally valid. It varies by car and tire.)
Increase caster
Increase rear weight distribution
Optimize tire pressures
I know that a fair number of people calculate their ride frequencies and talk about their suspension stiffness in those terms, which is perfectly fine. I’d have to get sprung and unsprung weights to do the calculations personally. I’ve promised myself I wouldn’t, but that might change. It’s a good way to compare the suspension stiffness of dis-similar cars. Where people get into trouble is when they start using those ride frequency calculations to apply Flat-Ride criteria to a performance car. That’s a bad idea. In very general terms, I find my best cars end up with having the rear ride frequency about 85-90% of the front. There’s nothing magic about this value. It doesn’t influence car motion in any specific way. It’s just a pattern I’ve found over the years. Once I’ve got the rear ride freq up to 93% or so of the front, I know I’m getting to the point of diminishing returns. I need to look to other variables to influence balance. If I can be at 83% and still have a positive front end without carrying a ton of RARB (which will eventually hurt traction), I’m stoked. Generally speaking, though, you have to carry a little more rear spring and/or bar than you’d want to get a balance everyone is happy with.
It's also important to discuss the target balance. That will depend what you’re running. If you’re doing an AutoX, then you need a ton of front end and you just have to deal with the tail wagging around. Having said that, traction will be important, too, so it can’t be a complete sacrifice. If you’re on a smaller racetrack with 2-3 gear corners, then you’re going to be challenging the rear of the car with traction, especially if the track is bumpy, and you’ll soften the car to dial in a little more understeer. If a track gets really fast, especially if there’s a lot of rubber down and banking, then you might have to increase the rear spring and free the car up a little, again, because you don’t have to deal with traction issues as much.
Another thing to consider is how long you want to drive the car in any one go. If you’re doing TT stuff, you can use the rear tires pretty hard, because they only need to live one lap at a time. If you’re doing a 20 minute session, then you can’t use them as hard. If you’re running a double race stint, you have to genuinely protect them by starting the car with an intentional understeer knowing the balance shift will favor the front as the tires deg.
I have found that more experienced drivers will almost always prefer a car which has *more* understeer than a novice will want. The reason is fairly simple, the novice does pretty much everything wrong and can’t load the front of the car correctly at the entry of the corner, so they either under-shoot or over-shoot the corner and induce a fair bit of the understeer they experience. A better driver will brake later, load the front-end better on the corner entry and still not over-drive the turn-in point. He’ll be able to carry more rake in the chassis all the way to the apex with the brake and have the greater share of the turning done in the corner at the apex. He won’t induce the understeer the novice does, so he’ll *want* more rear grip. To a guy like this, more rear grip means he can load the front harder. At some point, the driver will reach the point where he can’t load the front any more. That’s where we find the limit of adding rear grip. The goal is always driving straight off the corner. You don’t want to be balancing the car on the throttle all the way to the exit curb.
____________________________________________________________
S550 specific info:
Some of the numbers I throw out are a little polluted for everyone, because I have an Eco. I will just naturally have less mass to control on the front and less understeer overall. Regardless, these are numbers I’m certain that anyone could put on their car and have a hell of a good day a the track running.
I’m presently running a 500# front spring. I think that’s a reasonable spot for an AutoX/Street car. 400 #/in would be my ‘Street-only’ choice. If I were going to the track with 200tw tires, I might bump it up to 600’s. If I were running slicks, I’d definitely bump it up. I can see running in the range of a 750# front spring, easily. I don’t see any need to go softer than 400 #/in if you have Magneride. If you’re on some sort of stock-ish damper, you might have to go a little softer, but that’s more a damper issue than anything.
I never thought the stock rear spring felt all that bad in terms of stiffness, I just thought it felt too high of a ride height. Regardless, I followed what everyone was saying (at least to some extent) and put a stiffer (950 #/in) rear spring in than the stock (720 #/in) one. I was told to go to 1100. I could lower the rear ride now but now I was feeling the rear skate on bumps and it felt like the rear of the car was bouncing a lot on the tire. Traction was not very good on rough pavement and on smoother pavement, I was getting a lot of drifting on throttle. After that, I got a 20mm rear anti-roll bar (my car came with a 24mm rear bar). It improved mid-corner balance, but it still wasn’t good on bumps. I’ve dropped rates to 850, then 750. Both felt better than the last. At that point, I thought, well, how far *can* I drop? I’m presently on 500 #/in rear springs with a 20mm rear anti-roll bar. I’m not going to lie, this combo is pushy, even for me. Having said that, it rides great and traction is ridiculously good, even on bad pavement. I haven’t driven this in AutoX or on a track, and I don’t necessarily recommend anyone else to run it, but this proves we can. I think somewhere in the 700-900 #/in range is a good track rear spring rate. I have a friend with a 700+ HP S550 who loves the 700 rear spring, because he can finally get a little traction. I can definitely see going stiffer depending on the track, tires, driver, etc. My recommendations are what someone can try as a good starting point, not what will magically put them on pole of the next race.
A stiffer front bar will increase the load on the front tires in roll, which will increase understeer steady state. Stiffening the front bar will also increase turn-in responsiveness, so there are aspect of it that help the front. By taking more cornering load, increasing the front bar stiffness can help traction (if it doesn’t induce understeer). On a front-engine GT car, I intentionally try to run a bit of front bar, because carrying around a V8 between the front tires really sucks for getting the damned thing to change direction. If you can balance it, increasing front roll stiffness can really help in point-and-squirt type corners. Having said all this, in long-duration corners, specifically if they’re banked, having a stiffer front bar will create excessive understeer. There’s just no way around it. Front bar stiffness is very track specific. Run as much as you can, but no more than you need.
My approach to rear anti-roll bar stiffness is very salt-to-taste. I’m happy to run it disconnected if traction is really a problem. The rear does roll around a lot if you do that, though. I was surprised at how little time I actually lost in an AutoX by disconnecting the rear bar. It was only a few tenths quicker hooked up. Directional response with it unhooked is crap, but you can get filthy with the throttle and the rear will still stick. I’ll stiffen the rear bar without much hesitation as well, until it starts to cause problems, which is usually wheelspin/traction. Sometimes people are a little scared by the rear bar. My advice would be to don’t hesitate to make fairly large tuning changes. You’ll likely be able to move the balance a bit one way or the other, but don’t expect huge chunks of lap time. I have 2 bars, 20 & 24mm. I’ll run either depending on conditions. Sam Strano makes an adjustable bar that covers a really good working range.
In terms of overall roll stiffness, particularly with an extended splitter, it’s easy to get in the situation of bottoming the splitter edge when combining braking and cornering, especially if there’s a compression in the track. If you want to reduce the overall roll in the car without influencing balance, then you have to figure out how to go up on both evenly. Remember while their primary influence will be with respect to the end of the car the anti-roll bar is located on, it’s a total roll stiffness we’re working with. Increasing the front bar will slightly reduce rear roll and increasing rear bar will slightly reduce roll in the front, both as a secondary effect. Going a set up all the way around on bar stiffness is a good way to improve response and driver feedback. If you go too far, the car will start to feel 'skatey' and 'on top of the road.' The tire will be loaded too quickly and breaks free early in the corner before the car has reached peak lateral load.
It's a completely valid approach to run no bars at all, but we don’t have the conditions with this car that would suggest doing it as standard practice. If I were going for a rain session, I would disconnect bars front and rear.
Potential S550 Mustang 200TW/square fitting tire starting setup
500 #/in front spring (450 #/in if you’re using AJ Hartman’s Magneride kit)
750 #/in rear spring
Front Bar BMR 35mm 2/3 adjustment both sides.
Sam Stano’s rear bar, mid setting
We can discuss dampers. Basically, you get what you pay for.
-3.0* front camber / 0.5mm (at the wheel) front toe-in
-1.7* rear camber / 2mm (at the wheel) rear toe-in
~15mm lower front ride height than stock
~35mm lower rear ride height than stock
There are entirely too many variables on any car to list them all, but if you put that combination on the car and take it to the track, it won’t suck.
I won’t argue that I’m coming up with any of this in the correct manner, I’m just sharing how I do it. If anyone makes it this far, thanks for reading.
This is going to get long winded, but I’ll explain how I approach Spring and ARB selection.
TL;DR: Springs primarily control pitch and heave motion. Bars primarily control overall roll angle and roll-couple distribution. Clearly define the problem before choosing the tool to do it.
I feel like I need to preface this a little bit. Without getting into particulars, this is something I do _a lot_ and I’ve spent about 30 years figuring out a method. This is not “THE” method, because there’s no such thing as a single way to choose springs and bars for a car. Everyone has their own approach; this is a Cliff’s Notes version of mine.
There are three main car motions we’re concerned: Heave, Pitch and Roll. Heave for our cars is confined to the crests and compressions of the track. If you start to add some aero into the mix, then you’ll need to support that as well. Pitch is the motion which has the biggest influence on the handling of the car. Generally speaking, as you lower front ride and/or raise rear ride height (known as increasing chassis ‘rake’.) you will make the tend towards oversteer, particularly early in the corner. As the rear squats (reducing rake) the car will tend to understeer. Controlling rake angle is very important. Roll is the motion we get to play with the most and it’s the one that will ultimately determine our roll-stiffness distribution front to rear. Springs determine base roll couple, but the bars produce anywhere from 10-50+% of the total roll stiffness and can allow you to heavily influence the handling balance. These are our 3 primary modes of chassis motion.
Springs give a vertical stiffness to each corner. They will deflect and resist the car chassis motion as the inertial loads of the car shift due to driving. Springs are active in all chassis modes. Anti-Roll bars are springs which are only active in the roll mode. Bonded anti-roll bar bushings are a trick OEM’s use to produce low friction motion. The spring rate of bonded bushings can be added to the spring on that corner for it’s influence. It’s not ‘binding’. They’re intentionally using it to stiffen ride.
When I’m taking a first cut at springs, My initial goal is to choose a front spring which will support the front end adequately during heavy braking. If the car has a lot of travel on the front while braking, that limits how quickly the driver can ramp up brake pressure and, in general, reduces the driver’s confidence by allowing excess brake dive (positive chassis rake) which can cause rear instability. (With stock spring rates of 170#/in, I had a lot of brake dive and had to catch the rear of the car all the way to the apexes and it was tough to get the car to act consistently.)
The rear spring is chosen to allow as much vertical travel as possible, which will improve our traction and corner exits) without the driver complaining of excess chassis motion (squat or reduced rake) on throttle or a general lack of responsiveness in direction change even with appropriate rear roll stiffness. The rear will end up moving about 15-30%% more than the front even though the G-load in braking is 50% or higher than our G-load in acceleration. Cars are always more sensitive to ride height on the front than they are the rear. Allowing the rear a bit of extra compliance help with traction almost always pays off.
Once I’ve selected springs, I use the anti-roll bars to set my overall roll stiffness and roll-couple distribution. The springs will have their roll stiffness contribution and the anti-roll bars will have theirs. If you calculate the roll stiffness of the front and the roll stiffness of the rear, you can come up with a Mechanical Balance ratio of the front roll stiffness to the total. The slang is ‘Mech Bal’. If we’re running a 60% Mech Bal, then 60% of our total roll stiffness is taken on the front axle. This number will vary somewhat due to the track and conditions, but if you hold the mech bal consistent, you can do a global stiffen or soften and not change the mid-corner balance of the car all that much. It’s a cool party trick.
Here’s something that’s a little less intuitive. If we were going to put our car in some sort of simulation program to optimize lap time using all the variables that can change on the car, it wouldn’t give us one setup. It would give us a bunch of setups that all run the same basic lap time within a fractions of a tenth. The spread in mech bal would probably be at least 15% from setup with the lowest to the one with the highest mech bal. It’s not like there’s a single specific value to shoot for. There are any number of tuning variables we can use to tune the balance of a car. They all have compromises. If the car is doing what I want it to do in terms of heave and pitch motion, then I’ll try to leave the springs alone and tune the balance in a different way. If you’re racing, a lot of times you’re in the position where either you can’t change the springs due to time, you don’t have the spring you really want, you’re not allowed to change the springs or whatever. Regardless, you still have to figure out how to make the car turn.
If someone is making the argument that a stiffer rear spring will reduce understeer, I’ll completely agree. Having said that, it’s often not the most efficient way to do it. By that, I mean the compromise of increasing spring rate (decreased compliance) can be a greater negative than the compromises we might have by achieving that same balance with a different choice.
Some ideas to reduce understeer other than increasing rear spring:
Lower front ride height
Higher rear ride height
Softer FARB
Stiffer RARB
Softer front compression damping
Stiffer front rebound damping
Stiffer rear compression damping
Softer rear rebound damping
Reduce rear camber
Reduce rear toe-in
Optimize front camber (It’s not always a gain to increase)
Optimize front toe (In or Out is equally valid. It varies by car and tire.)
Increase caster
Increase rear weight distribution
Optimize tire pressures
I know that a fair number of people calculate their ride frequencies and talk about their suspension stiffness in those terms, which is perfectly fine. I’d have to get sprung and unsprung weights to do the calculations personally. I’ve promised myself I wouldn’t, but that might change. It’s a good way to compare the suspension stiffness of dis-similar cars. Where people get into trouble is when they start using those ride frequency calculations to apply Flat-Ride criteria to a performance car. That’s a bad idea. In very general terms, I find my best cars end up with having the rear ride frequency about 85-90% of the front. There’s nothing magic about this value. It doesn’t influence car motion in any specific way. It’s just a pattern I’ve found over the years. Once I’ve got the rear ride freq up to 93% or so of the front, I know I’m getting to the point of diminishing returns. I need to look to other variables to influence balance. If I can be at 83% and still have a positive front end without carrying a ton of RARB (which will eventually hurt traction), I’m stoked. Generally speaking, though, you have to carry a little more rear spring and/or bar than you’d want to get a balance everyone is happy with.
It's also important to discuss the target balance. That will depend what you’re running. If you’re doing an AutoX, then you need a ton of front end and you just have to deal with the tail wagging around. Having said that, traction will be important, too, so it can’t be a complete sacrifice. If you’re on a smaller racetrack with 2-3 gear corners, then you’re going to be challenging the rear of the car with traction, especially if the track is bumpy, and you’ll soften the car to dial in a little more understeer. If a track gets really fast, especially if there’s a lot of rubber down and banking, then you might have to increase the rear spring and free the car up a little, again, because you don’t have to deal with traction issues as much.
Another thing to consider is how long you want to drive the car in any one go. If you’re doing TT stuff, you can use the rear tires pretty hard, because they only need to live one lap at a time. If you’re doing a 20 minute session, then you can’t use them as hard. If you’re running a double race stint, you have to genuinely protect them by starting the car with an intentional understeer knowing the balance shift will favor the front as the tires deg.
I have found that more experienced drivers will almost always prefer a car which has *more* understeer than a novice will want. The reason is fairly simple, the novice does pretty much everything wrong and can’t load the front of the car correctly at the entry of the corner, so they either under-shoot or over-shoot the corner and induce a fair bit of the understeer they experience. A better driver will brake later, load the front-end better on the corner entry and still not over-drive the turn-in point. He’ll be able to carry more rake in the chassis all the way to the apex with the brake and have the greater share of the turning done in the corner at the apex. He won’t induce the understeer the novice does, so he’ll *want* more rear grip. To a guy like this, more rear grip means he can load the front harder. At some point, the driver will reach the point where he can’t load the front any more. That’s where we find the limit of adding rear grip. The goal is always driving straight off the corner. You don’t want to be balancing the car on the throttle all the way to the exit curb.
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S550 specific info:
Some of the numbers I throw out are a little polluted for everyone, because I have an Eco. I will just naturally have less mass to control on the front and less understeer overall. Regardless, these are numbers I’m certain that anyone could put on their car and have a hell of a good day a the track running.
I’m presently running a 500# front spring. I think that’s a reasonable spot for an AutoX/Street car. 400 #/in would be my ‘Street-only’ choice. If I were going to the track with 200tw tires, I might bump it up to 600’s. If I were running slicks, I’d definitely bump it up. I can see running in the range of a 750# front spring, easily. I don’t see any need to go softer than 400 #/in if you have Magneride. If you’re on some sort of stock-ish damper, you might have to go a little softer, but that’s more a damper issue than anything.
I never thought the stock rear spring felt all that bad in terms of stiffness, I just thought it felt too high of a ride height. Regardless, I followed what everyone was saying (at least to some extent) and put a stiffer (950 #/in) rear spring in than the stock (720 #/in) one. I was told to go to 1100. I could lower the rear ride now but now I was feeling the rear skate on bumps and it felt like the rear of the car was bouncing a lot on the tire. Traction was not very good on rough pavement and on smoother pavement, I was getting a lot of drifting on throttle. After that, I got a 20mm rear anti-roll bar (my car came with a 24mm rear bar). It improved mid-corner balance, but it still wasn’t good on bumps. I’ve dropped rates to 850, then 750. Both felt better than the last. At that point, I thought, well, how far *can* I drop? I’m presently on 500 #/in rear springs with a 20mm rear anti-roll bar. I’m not going to lie, this combo is pushy, even for me. Having said that, it rides great and traction is ridiculously good, even on bad pavement. I haven’t driven this in AutoX or on a track, and I don’t necessarily recommend anyone else to run it, but this proves we can. I think somewhere in the 700-900 #/in range is a good track rear spring rate. I have a friend with a 700+ HP S550 who loves the 700 rear spring, because he can finally get a little traction. I can definitely see going stiffer depending on the track, tires, driver, etc. My recommendations are what someone can try as a good starting point, not what will magically put them on pole of the next race.
A stiffer front bar will increase the load on the front tires in roll, which will increase understeer steady state. Stiffening the front bar will also increase turn-in responsiveness, so there are aspect of it that help the front. By taking more cornering load, increasing the front bar stiffness can help traction (if it doesn’t induce understeer). On a front-engine GT car, I intentionally try to run a bit of front bar, because carrying around a V8 between the front tires really sucks for getting the damned thing to change direction. If you can balance it, increasing front roll stiffness can really help in point-and-squirt type corners. Having said all this, in long-duration corners, specifically if they’re banked, having a stiffer front bar will create excessive understeer. There’s just no way around it. Front bar stiffness is very track specific. Run as much as you can, but no more than you need.
My approach to rear anti-roll bar stiffness is very salt-to-taste. I’m happy to run it disconnected if traction is really a problem. The rear does roll around a lot if you do that, though. I was surprised at how little time I actually lost in an AutoX by disconnecting the rear bar. It was only a few tenths quicker hooked up. Directional response with it unhooked is crap, but you can get filthy with the throttle and the rear will still stick. I’ll stiffen the rear bar without much hesitation as well, until it starts to cause problems, which is usually wheelspin/traction. Sometimes people are a little scared by the rear bar. My advice would be to don’t hesitate to make fairly large tuning changes. You’ll likely be able to move the balance a bit one way or the other, but don’t expect huge chunks of lap time. I have 2 bars, 20 & 24mm. I’ll run either depending on conditions. Sam Strano makes an adjustable bar that covers a really good working range.
In terms of overall roll stiffness, particularly with an extended splitter, it’s easy to get in the situation of bottoming the splitter edge when combining braking and cornering, especially if there’s a compression in the track. If you want to reduce the overall roll in the car without influencing balance, then you have to figure out how to go up on both evenly. Remember while their primary influence will be with respect to the end of the car the anti-roll bar is located on, it’s a total roll stiffness we’re working with. Increasing the front bar will slightly reduce rear roll and increasing rear bar will slightly reduce roll in the front, both as a secondary effect. Going a set up all the way around on bar stiffness is a good way to improve response and driver feedback. If you go too far, the car will start to feel 'skatey' and 'on top of the road.' The tire will be loaded too quickly and breaks free early in the corner before the car has reached peak lateral load.
It's a completely valid approach to run no bars at all, but we don’t have the conditions with this car that would suggest doing it as standard practice. If I were going for a rain session, I would disconnect bars front and rear.
Potential S550 Mustang 200TW/square fitting tire starting setup
500 #/in front spring (450 #/in if you’re using AJ Hartman’s Magneride kit)
750 #/in rear spring
Front Bar BMR 35mm 2/3 adjustment both sides.
Sam Stano’s rear bar, mid setting
We can discuss dampers. Basically, you get what you pay for.
-3.0* front camber / 0.5mm (at the wheel) front toe-in
-1.7* rear camber / 2mm (at the wheel) rear toe-in
~15mm lower front ride height than stock
~35mm lower rear ride height than stock
There are entirely too many variables on any car to list them all, but if you put that combination on the car and take it to the track, it won’t suck.
I won’t argue that I’m coming up with any of this in the correct manner, I’m just sharing how I do it. If anyone makes it this far, thanks for reading.
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