Front Roll Center Migration

[kcbrown]

As you can see there is a significant difference in sidewall height as well as contact patch. Much of this is due to the very large stock front ARB exerting upward forces on the outside wheel. I expect this would yield different LCA angles than are showing up in the existing model. I am unsure how to factor it in correctly...

I would expect that, as a first approximation, you might model it as a conversion of some fraction of the roll to compression.

The LCA angles are going to, of course, be determined by both of those things.

The more interesting question is, perhaps, the change in camber due to the cornering forces involved combined with the soft bushings in the front suspension. This could, I expect, be modeled by allowing some variation in the LCA attach point locations as well as some variation in the LCA lengths.

The thing is, though, I'd expect those changes to yield second-order level effects on the resulting angles rather than first-order effects. But if you're at the point where a half inch of change makes a substantial difference in the instant center location, then those second-order effects start to become important.

 

[frank s]

What do you see happening in this picture?

Chuckwalla Valley Raceway, October 2010.

The car is a 2009 GT/CS. GT500 wheels, 18x9.5; Goodyear tires same as OE GT500 rears, 285-40 all around, not the sticky ones (that's about 60 pounds a corner), 35 psi cold 39-41 hot; FRPP "P" springs -.6 front, -.75 rear; Koni Yellows, mid-stiff both ends; Steeda front anti-sway bar, mid-stiff, Steeda sway-bar bracket braces; Strano adjustable front bar links, 220-lb driver in seat; H&R 26mm rear anti-sway bar; both bars have those squeaky red bushings at major junctions; FRPP "Bullitt" intake with ProCal tune;everything else was stock, as far as I can remember.

Car was very predictable, turned in nicely, very mild push at mid-corner, got a little loose at exit and full power, easily controlled by foot feed. I got crossways to the course one time, trying to go too fast over a crest-with-a-turn, but caught it and was able to continue without downshift. I was able to creep up on a couple of Shelby GTs with superchargers, but I'm not convinced they didn't let me do that just to encourage me. Very pleasant day. Used more than half the depth of the rear brake pads (OE) because of failure to stay "off" by the traction thing.

In the photo it looks to me as if the front and rear wheels are pointing the same way, straight forward, although the car is clearly loaded in a turn.

Just fuel for discussion.

 

[ddd4114]

Yep, I agree with Mark here. We've lowered hundreds of S197 Mustangs by 2 and even 3" from stock ride heights, and we don't ever use bumpsteer correction kits. Why? Because they don't need it. They drive just fine with the stock tie rods - and don't tend to have the unusual failures that bumpsteer kit equipped cars do. Same goes for aftermarket front control arms and a whole bunch of other parts made for the S197.

Remember: a good percentage of parts made for these cars are simply part of the array of "stuff" for you to buy. Available because someone makes it. Because people will buy it. Bolt-on braces, shiny doo-dads, whistle tips, and needlessly adjustable things to mess with, break and fail. They lighten your wallet but don't necessarily do anything worthwhile. This segment of the aftermarket is full of these things. If you add in all of the "style" crap made for the S197 that percentage of worthless available parts is approaching 90%.

There is an internet mass hysteria surrounding certain voodoo modifications that simply cannot be explained with calculations, formulas or measurements. People spend countless hours debating roll center migration, roll couples and use terms like "kinematics" and other pseudo scientific jargon... but from what I've seen in the automotive forums for the past 2 decades, the more incomprehensible the terms used the more full of crap the writer usually is.

Look - certain geometries and constraints are just part of a given chassis, and changing them to some new ideal could take tens of thousands of dollars worth in custom fabrication and parts - and usually have some serious side effects or entail other significant changes. The S197 chassis is light years better than the Fox/SN95 chassis that proceeded it (so much so that I always BEG customers that come to us with Fox/SN95 race car wishes to look at the S197 first), but it still isn't perfect. You can make significant track performance enhancements in TIRES, spring rates, dampers, antiroll bars, a few bushings, and rear axle location devices. But at the end of the day it is a big, heavy McPherson strut car with a solid rear axle. At a certain point, after you hit the big stuff mentioned above, you are just polishing a turd.

Don't get sucked into these internet arguments that have almost no real solutions. This chassis has been around for a while and a lot of smart people have raced in them - pretty much the major development aspects that matter have been tackled, until there is some new unforeseen technological breakthrough. Ask the faster drivers in the same cars what they are doing, and why. Not all of them will know why, either - there is a lot of "follow the leader" mentality in racing. But for the most part, you will see a pattern of the Parts That Matter.

Yes, think outside the box. Strive to learn, and read the Carrol Smith books on racing set-up, the ThinkFAST books and the like. But don't get bogged down in trying to make your car into something it is not. Don't try to reinvent the wheel, when the one you have is perfectly round. I see people fixate on these intangible or unrealistic goals, yet they tend to be are the ones that are still running around on skinny street tires or have some other massive handicap they refuse to address. Get that low hanging fruit first then worry about the minutia improvements later.

Honestly, if you want ideal suspension geometry, better street ride, and faster track performance... just go buy a C5/C6/C7 Corvette. That one "change" will elevate your goals far beyond you could ever achieve in an S197 chassis. Trust me, those GM engineers spent BILLIONS to develop those three Corvette chassis, and you ain't going to come up with a better suspension on a Mustang than that. Ever. Not trying to discourage you free thinkers and engineers, just keep that in mind - there is always a better chassis to start with. All of us here have Mustangs for various reasons, and I love this model, but it isn't a Corvette or a 911 or a Ferrari.

I love this chassis, and feel that it is the new car performance bargain for the past decade, bar none. Mostly due to the solid chassis and powerful 5.0L engine. It pains me to see people waste money on worthless parts trying to make these cars into something they are not. We could sell a LOT more "stuff" for these cars, and make more money doing it, but we won't do that. We talk people out of upgrades all the time, like when it doesn't meet the needs of their uses, or their experience/skill level. If you want honest advice on what works and what is a waste, just ask a shop you trust.

Anyway... that's just my two "grumpy old man" cents. Get off my lawn! I'm not trying to poke fun at anyone or any company,just stating my long held opinion that most aftermarket parts sold are worthless crap, and most internet arguments are a waste of bandwidth.

Cheers....

Don't worry, I don't think you offended anybody.

I completely understand your stance, and I was pretty much expecting to get responses like this - except written less tactfully than yours. For the most part, I agree. I think you hit the nail on the head with "polishing a turd", and that's exactly what we're all doing. As you suggested, we happen to be polishing rather capable (albeit large and heavy) turds, but these cars will never match the performance capability of a higher-end sports/race car. Until you're designing your own chassis, you will always be balancing compromises, and even then, there's no such thing as a free lunch.

I have no intention of buying parts simply because they're available, because they promise a performance gain, or because other people have them. That's what led me to this in the first place. I'm a strong proponent of "simple is best", and I'd like to try and accomplish my goals with as few parts/changes as possible.

However, I do have to beat a dead horse a little. I completely agree that there's a lot of "follow the leader" in racing, and honestly, that drives me nuts. What happens when the leader is a moron, and if he's not, what happens when he inevitably fucks up? We're all only human, and I'm sure that every one of us has tried something different that ended with us thinking to ourselves "well that was dumb!". I do think it's smart to see what go-fast parts the faster guys are using, but I think it should be taken with a grain of salt. Why does that guy have that part installed? Does it really make the car faster? Did he copy off somebody else that was faster than him? Is it just a test part that might not have been completely successful? Is it just one component of the total package and cannot stand by itself (almost always the case)? Did a sponsor simply tell him to put it on the car in return for something else? Is it really just a placebo, and he just installed it to throw off his competition (and yes - that does happen)? The list goes on.

"Reinvesting the wheel" was a bit of an extreme analogy, but my point was that if we all do "what works" and copy each other, how will you gain an edge? I think we all understand that driver skill is the most effective performance enhancer followed closely by proper tire selection, and those two will get you 80% of the way there. However, that last 20% can be tricky. There are certainly combinations of go-fast parts that are proven to work, but I think there is always room to improve. Is manipulating your roll centers the key to winning? Probably not. However, after tires, your overall suspension setup has the largest potential for lowering your lap times - and keeping the car on the track while doing so. You mentioned springs, dampers, and anti-roll bars as important performance upgrades. How do you know what spring rates are reasonable? How about ARB stiffness? How do you combine them? What range of adjustability should you dampers have? What sort of alignment works best with your setup? Testing and experience is certainly the only way to hone a setup, but until you get close, there are many combinations that might work. Until you have a rough understanding of the system - either through brute-force testing or some silly math equations - you're sort of shooting in the dark. The concept of roll centers might make people roll their eyes, but it's a fundamental component of basic lateral load transfer distribution approximations. However, like anything, it's just a tool, and it needs to be used properly to be useful.

For what it's worth, I think a lot of my stance comes from being a test engineer. In my job, every wasted hour is very expensive, and in my opinion, it's no different here. A set of extended ball joints might cost under $100 and take a short afternoon to install, but the testing time needed to evaluate them is much more costly. Between event registration and consumables, I'm effectively paying ~$150 per 20-min session. I'm of course doing this stuff because it's fun, but I'd still like to minimize the time spent driving with a setup I don't like. Why would I want to spend a day worrying about an unstable setup that might put me into a wall? Sure, I could change my driving to make it work, but that's just a band-aid fix that doesn't eliminate the distraction. It also might take a lot of time dicking around with tire pressures and damper settings if you don't have replacement parts, and you might still be left with band-aid fixes.

I do realize that you've been developing performance-oriented cars for years, and based on what information you've posted, you seem to be quite successful. Please don't take my post as a back-handed way of saying "you're doing it wrong!", because that's not my intention. Any information you or many other experienced guys provide on this forum is valuable, but I don't think anybody should assume that you're always right and that they should take your advice without thinking about it themselves. For the same reason, I hope people take my posts with a grain of salt too. There are many ways to accomplish things like this, and there isn't necessarily a "best" approach. It all comes down to your goals, your experience, and your strengths, and your resources (namely time and money). Plus, you know what they say: opinions are like assholes...

I would expect that, as a first approximation, you might model it as a conversion of some fraction of the roll to compression.

The LCA angles are going to, of course, be determined by both of those things.

The more interesting question is, perhaps, the change in camber due to the cornering forces involved combined with the soft bushings in the front suspension. This could, I expect, be modeled by allowing some variation in the LCA attach point locations as well as some variation in the LCA lengths.

The thing is, though, I'd expect those changes to yield second-order level effects on the resulting angles rather than first-order effects. But if you're at the point where a half inch of change makes a substantial difference in the instant center location, then those second-order effects start to become important.

I think if you're getting to the point where bushing compliance is that significant of a factor (which might very well be the case), the analysis really isn't worth it at the amateur level. It might not even be worth it at the semi-professional level. Testing and/or actual suspension measurements would be needed at that point. I think even developed software like Adams/Car would struggle to produce useful data with compliance issues like that.

 

[kcbrown]

I think if you're getting to the point where bushing compliance is that significant of a factor (which might very well be the case), the analysis really isn't worth it at the amateur level. It might not even be worth it at the semi-professional level. Testing and/or actual suspension measurements would be needed at that point. I think even developed software like Adams/Car would struggle to produce useful data with compliance issues like that.

I dunno. At the very least, figuring the range of instant center locations and, from that, roll center locations, might give you some idea of what you're likely to be facing when you're cornering hard. Everything depends on the ranges involved.

Since I'm not a suspension engineer (or any sort of real engineer -- I just have an engineering mindset and an interest in the subject), I can't say what the implications are of the instant center location problem you're having here. Even so, for all I know, it may be that the geometric roll center can still be somewhat useful here. For instance, the instant centers might wind up being out at or near infinity (at least, many multiples of the car's track width) in terms of distance, but if their angular locations relative to their respective contact patches are constrained, then that should, I'd think, be reflected in a somewhat constrained roll center. And there may be ranges of geometric instant center locations that are simply invalid on some basis.


Regardless, to the degree the model itself produces valid results, I think it can be useful. If, for instance, the roll center really does drop more quickly than the CG as you lower the car, then that has some useful implications: it means that lowering the car may make it more prone to roll if you are keeping everything else (spring rates, especially) constant. For someone like myself who is interested in maintaining good street manners, such a thing is valuable to know. Someone who doesn't care about street manners will just jack up the spring rates and be done with it.

 

[ddd4114]

On a side note, any way you can see what happens when the LCAs are lowered 3/4" on the wheel side after the car is lowered 2"? This is the equivalent of using the boss 302s/r extended ball joint.

I had a little bit of time tonight to see what happens. Here's the results:

If I drop it by 2", the same problem happens; it's basically impossible to locate the roll center after ~1 deg of roll.

With the exception of slightly more RC height change vs. roll angle, it actually looks pretty reasonable compared to the original result. Of course, that's comparing apples to apples (or maybe rotten apples to rotten apples?).

What's also interesting is that there is just about no change in the camber curves when you change the ball joint location. The curves are almost entirely dictated by the shock angle... which makes sense when you think about it. The only other way to effectively alter them is by changing the LCA length, and that's a lot less practical. This could be one reason why csamsh, Terry, and others don't worry about roll centers since you can easily design around them. Ultimately, what matters for grip is wheel loads and dynamic camber. If you lower the CG, you reduce the total weight transfer, and that's always better for grip. If you also increase roll stiffness enough to keep dynamic camber in check, then the roll centers don't really matter. The only real downside is less grip on rougher surfaces since the suspension will be less compliant. However, unless you're racing at Nelson Ledges (or driving in Michigan), that's usually not a huge concern for a race car.

 

[NoTicket]

I must be confused... Aren't extended ball joints supposed to put the LCA more level with the ground when the car is lowered? Shouldn't this result in the cg being raised?

 

[kcbrown]

I thought the amount of roll (a reflection of transferred weight, if I'm not mistaken, which directly impacts dynamic camber change, again if I'm not mistaken) you had to worry about was a function of the distance between the CG (of the sprung mass) and the roll center, so if lowering the car lowers both the CG and the roll center, then how much of a gain (or loss!) you get depends on the change of that distance, right?

Since the CG drop and the ride height drop are essentially the same here, then what matters is the rate the RC drops relative to the CG, yes? And since for this chassis, it appears that the RC drops faster than the CG, it follows that the distance from CG to roll center increases as you lower the CG, which means that the total weight transfer has to increase unless you control it through, e.g., stiffer springs.

What am I missing here?

 

[ddd4114]

I must be confused... Aren't extended ball joints supposed to put the LCA more level with the ground when the car is lowered? Shouldn't this result in the cg being raised?

No, it just lowers the outboard point. You're still using the same wheels and struts, so the CG height is the same.

I thought the amount of roll (a reflection of transferred weight, if I'm not mistaken, which directly impacts dynamic camber change, again if I'm not mistaken) you had to worry about was a function of the distance between the CG (of the sprung mass) and the roll center, so if lowering the car lowers both the CG and the roll center, then how much of a gain (or loss!) you get depends on the change of that distance, right?

Since the CG drop and the ride height drop are essentially the same here, then what matters is the rate the RC drops relative to the CG, yes? And since for this chassis, it appears that the RC drops faster than the CG, it follows that the distance from CG to roll center increases as you lower the CG, which means that the total weight transfer has to increase unless you control it through, e.g., stiffer springs.

What am I missing here?

The amount of body roll, spring rates, and location of the roll centers have no impact on total weight transfer. The roll center locations will change the distribution of weight transfer, but that's it.

Here is a free-body diagram of the front view of a car that I found on Google:

The only thing it's missing is the reaction force that the ground exerts on the tires, and then all of the forces will be balanced. The force vector for the tires will be collinear with the ground plane (obviously), and it will be pointing to the right.

If you balance the moments about the CG, you'll find that (lateral load transfer = may * h / t). You can see that the only factors that affect your total weight transfer are your CG height and track width, and your tire grip will determine the maximum total weight transfer.

As the body rolls, the CG will move a little and change the total weight transfer, but it's a negligible effect.

 

[Houstonnw]

The amount of body roll, spring rates, and location of the roll centers have no impact on total weight transfer.

I think that is a key point, and one this is often misunderstood.

Roll does not directly cause loss of grip. However it can indirectly cause less grip because of the effect on tire camber.

 

[NoTicket]

No, it just lowers the outboard point. You're still using the same wheels and struts, so the CG height is the same.

Sorry that was a typo. I meant RC height. Your chart shows a lowered roll center to go along with extended ball joints. That is, from my understanding, the opposite of what the real world effect is.

 

[Norm Peterson]

I expected the front roll center to move around a bit, but I didn't realize how significant it was. With my car at its current (stock) height, the roll center is about 3" off the ground, but if I lower it by only 1", it goes right down to ground level.

Entirely likely - struts suck as far as stability of the geometric roll center vs ride height is concerned. As a rough estimate, plan on the geo RC dropping somewhere around twice as fast as the car is lowered.

Your static geo RC heights are about what I got when I measured my car.

I'm beginning to wonder if there's an error in the model here.

For a strut suspension, Puhn seems to locate the roll center at the intersection between the horizontal center of the vehicle and the line drawn between the instant center and the tire contact patch. His discussion of roll center location as it varies with roll seems to omit the possible case when the two swing arm lines are parallel.

You need to look at the figure that shows how the geo RC migrates in roll for a SLA suspension (it's on pg 37 in my edition). Lateral migration of the geo RC for a strut suspension is similar - in fact if you were to develop your own strut RC model you'd use a virtual upper arm that attaches at the strut top and remains perpendicular to the strut axis (keeping in mind that the strut axis also varies slightly).

I think you need to use a force-based RC model to avoid or at least get around the geometric difficulties with parallel virtual swing arms.

Also, suppose the roll center really is at infinity, or at least an order of magnitude or so more distant from the car's center of gravity than is the ground. Would that not basically mean that the car cannot roll any more than it has at that point? Transition through infinity is an impossibility, no?

Is this about lateral or vertical migration? If laterally, perhaps it means that you get roll plus a little heave instead of pure roll.

Milliken's book also describes a force-based roll center solution, but I haven't read much about it. If I have time this weekend, I'll compare the two.

The way I understand it, a force-based RC ends up needing to know the individual inside and outside tire loadings (and the lateral forces generated, which strictly speaking are not quite proportional). At least one approach assumes a 75/25 distribution of outside to inside vertical load ("weight") distribution. But that's a simplified assumption at max lat-g, and it may matter how you get there as much as what it is when you do.


Norm

 

[Vorshlag-Fair]

I do realize that you've been developing performance-oriented cars for years, and based on what information you've posted, you seem to be quite successful. Please don't take my post as a back-handed way of saying "you're doing it wrong!", because that's not my intention. Any information you or many other experienced guys provide on this forum is valuable, but I don't think anybody should assume that you're always right and that they should take your advice without thinking about it themselves. For the same reason, I hope people take my posts with a grain of salt too. There are many ways to accomplish things like this, and there isn't necessarily a "best" approach. It all comes down to your goals, your experience, and your strengths, and your resources (namely time and money). Plus, you know what they say: opinions are like assholes...

Agreed - I always tell people to question everything they hear. Even from me.

It is just when I see people with stock wheels/tires/bushings/suspension parts looking at these significant geometry re-engineering tasks as their first modifications, I want to remind them to COVER THE BASICS first and don't ever lose sight of what this car is and always will be - a heavy, stick axle McStrut car.

 

[kcbrown]

Agreed - I always tell people to question everything they hear. Even from me.

It is just when I see people with stock wheels/tires/bushings/suspension parts looking at these significant geometry re-engineering tasks as their first modifications, I want to remind them to COVER THE BASICS first and don't ever lose sight of what this car is and always will be - a heavy, stick axle McStrut car.

The problem is that lowering the car, and thus changing the geometry, almost always happens automatically as a result of changing the dampers/springs in order to improve the handling of the car, and dampers/springs are regarded as part of "the basics", are they not?

So aside from the approach I intend to take (coilovers built to my specifications) how does one cover the basics first so as to avoid changing the geometry? I suspect I'm the only one here who plans on going to coilovers without substantially lowering the car. Even a mere half an inch drop in front, an amount far smaller than what most people would do, will result in a 1.5 inch drop in the roll center height.

I've already got the other basics covered, namely wheels and tires. Springs and dampers will be next, but it'll still be a while as I first have to discover the shortcomings of the stock suspension.

 

[csamsh]

The problem is that lowering the car, and thus changing the geometry, almost always happens automatically as a result of changing the dampers/springs in order to improve the handling of the car, and dampers/springs are regarded as part of "the basics", are they not?

So aside from the approach I intend to take (coilovers built to my specifications) how does one cover the basics first so as to avoid changing the geometry? I suspect I'm the only one here who plans on going to coilovers without substantially lowering the car. Even a mere half an inch drop in front, an amount far smaller than what most people would do, will result in a 1.5 inch drop in the roll center height.

I've already got the other basics covered, namely wheels and tires. Springs and dampers will be next, but it'll still be a while as I first have to discover the shortcomings of the stock suspension.

I'm going to suggest a different approach, which you may not like:

Lower it, go to the track, and see if the geometry is holding you back/noticeable. If it isn't, quit worrying about it.

Incidentally, what is your wheel/tire setup?

 

[SoundGuyDave]

Add in the "supporting mod" theory: Drop the car, and if the geometry pisses you off, then fix it. You can get 3/4" at the ball joint studs to recover some roll center location (need to add a bumpsteer kit, though), and IIRC Steeda makes an arm relocation kit as well, moving the inboard mounting points upwards. Between the two, you could probably get the RC somewhere near the orbit of Venus, if you wanted to.

FWIW, and I normally HATE subjective data, but my car is dropped, not an insane amount, and has the ball joints in, and the universal comment made by other drivers (in the last year: 5 with comp licenses, 3 with TT licenses) is that it's just a very easy car to drive. There's no issue with the car doing unpredictable things, there's no issue with being able to place the car EXACTLY where you want it, it's not a huge fight to make turn, and when it gets out of shape, it's predictable and easy to recover. Oh, and it has/had multiple track records to back up those opinions, so it is reasonably fast, as well.

The bottom line is that for just about any evil you could care to mention in the suspension world, there IS a fix. Some easier than others, but the fix exists, none the less. If you REALLY want to obsess about RC location, camber curve, and CG, call Griggs or A47, they both have SLA kits... That will fix the evil strut problem once and for all.

 

[ddd4114]

I think that is a key point, and one this is often misunderstood.

Roll does not directly cause loss of grip. However it can indirectly cause less grip because of the effect on tire camber.

Exactly. Actually, that was one of the main reasons I wanted to measure my suspension points in the first place. Estimating the roll center locations is important for selecting spring/bar rates, but ultimately, I wanted to see my current roll gradient and camber curve. From there, I could get an idea of what I wanted instead.

Sorry that was a typo. I meant RC height. Your chart shows a lowered roll center to go along with extended ball joints. That is, from my understanding, the opposite of what the real world effect is.

I just realized that my first graph is very ambiguous, but I think you're misreading my second one.

In the first graph, I forgot to say that I had lowered the car 1" at its default (0) position. If you want to see the effect at the OEM ride height, the dotted line needs to be translated up by 2" (making the RC higher than OEM).

In the second graph, the solid line is with no lowering, and the dotted line is with both a 1" drop and the extended ball joints. The ball joints add 2" to the RC height after lowering.

Your static geo RC heights are about what I got when I measured my car.

...

The way I understand it, a force-based RC ends up needing to know the individual inside and outside tire loadings (and the lateral forces generated, which strictly speaking are not quite proportional). At least one approach assumes a 75/25 distribution of outside to inside vertical load ("weight") distribution. But that's a simplified assumption at max lat-g, and it may matter how you get there as much as what it is when you do.

Awesome, thanks for the feedback. That means I'm probably in the ballpark.

That was my understanding of force-based roll centers too. Basically, you would need to balance the forces transferred through the LCA's and the struts to find the RC. I'm guessing you would need to find a location at which the moments about that point are balanced? I read through Milliken's book, and it didn't go into much detail. Since I already have a good idea of the TLLTD, I should be able to approximate the loads going through each member. Unfortunately, the sway bar further complicates things especially because the rate is nonlinear. You're right that the lateral forces developed by each tire will be difficult to estimate because I don't have tire data, and there is a HUGE difference between the inside and outside wheel loads at max cornering. At this point, it probably isn't worth worrying about finding the roll centers more accurately. I know the front will be low as hell (almost literally), and I know the rear will be high but fairly stable. That should be enough for my purposes.

Another interesting bit to take away from this is that due to the height of the rear roll center, the jacking forces on the sprung mass will be fairly significant. As a result, the chassis will actually rise a little in hard cornering and transfer more load to the outside wheels (thus further reducing grip). Stiffer rear springs would definitely help prevent the chassis from rising so much, but they won't change the jacking forces. A stiffer rear ARB will not help at all in this case. On the other hand, the front roll center is near the ground, and even though it moves around a lot, the jacking forces there are negligible.

 

[kcbrown]

I'm going to suggest a different approach, which you may not like:

Lower it, go to the track, and see if the geometry is holding you back/noticeable. If it isn't, quit worrying about it.

Well, I'm actually going to take an even more incremental approach than that:

1. Figure out how the stock suspension behaves.
2. Get coilovers (and camber plates, of course) with spring rates that I expect will take care of whatever I feel is lacking. Leave the ride height nearly untouched (1/2" lower front and rear is probably what I'll do). Test and tweak to improve as much as possible (this may involve the use of adjustable sway bars as well).
3. If the car still doesn't behave to my liking, try lowering the car. Test and tweak again.
4. Lather, rinse, repeat with different options along the way, as necessary.

Each change I make will be to one variable only, with the sole exception of the initial 1/2" ride height change, and that change is primarily for aesthetics.

Since I'll be getting coilovers, adjusting the ride height won't be a problem. What is different about my approach is that lowering the car will be a separate step, an independent variable of the equation.

Incidentally, what is your wheel/tire setup?

Bridgestone S-04 Pole position tires, 285/35-19 on Forgestar F14, 19x10 wheels.

Slicks are not in the cards, and neither are track-only tires. Remember, the whole point of this is maximum grins, not maximum performance. It may be that slip-sliding around the track proves to be more fun than simply going around it fast!

 

[Norm Peterson]

That was my understanding of force-based roll centers too. Basically, you would need to balance the forces transferred through the LCA's and the struts to find the RC.

I wonder if there's a way to ignore the RC completely and work with "anti-roll" geometry, suspension compression/extension, and the stabilizer bar effect directly.


Norm

 

[ddd4114]

I wonder if there's a way to ignore the RC completely and work with "anti-roll" geometry, suspension compression/extension, and the stabilizer bar effect directly.


Norm

I've thought about that too. If you do that, you're essentially taking the same approach needed to find the force-based roll center, except you're not actually using it (and therefore don't need to explicitly find it).

The problem I see is that you need the wheel loads to figure out the suspension movement at each corner, and to figure out the wheel loads, you need to know LLT and TLLTD. LLT is easy, but traditionally, you need roll centers to calculate TLLTD.

All of the forces acting on the sprung mass have to be transferred through the suspension linkages, so if you could figure out how much force acts on the springs and bars, you might be able to approximate body roll. It sounds possible to me, but it certainly sounds like a pain in the ass. It probably needs to be an iterative approach that requires Matlab or something similar.

 

[Houstonnw]

I understand that ddd4114 and kcbrown are very interested in the engineering side of handling. I actually quit talking to Sam Strano when he tried to tell me that trial braking wasn't useful to keep more weight on the front tires and improve grip at turn in.

However I continue to feel obligated to tell new people to the sport that what ddd4114 and kcbrown are discussing is at the highest level and may or may not have any appreciable difference on the track, much less the street.

Terry Fair was dismissed slightly above as being "old school racer" and not open to the science or engineering of designing a proper suspension. Well Jason at Vorshlag can talk numbers with any of you, the difference is that they know what matters and what doesn't.

ddd4114, you are a test engineer. Just curious, if a particular spec is +/- 0.10, do you worry about 0.001? I think that is what Terry is trying to tell the both of you.

For example, here are the points required by NASA Time Trial rules for various modifications:

UTQG treadwear rating of 120-200: +2
UTQG treadwear rating of 50 to 130: +7
UTQG treadwear rating of 40 or less: +10
Hoosier A6: +13

Here are the points for tire width increase equal to or greater than:

10mm +1, 20mm +4, 30mm +7, 40mm +10, 50mm +13, 60mm +16, 70mm +19, 80mm +22, 90mm +25, 100mm +28, 110mm +31

Here are the two modifications that seem related to this thread:

Bump steer kits or shimming of the steering rack +2
Alteration of ball joints/dive angles +2

Why are the points for tires so much higher than for improving front suspension geometry?

So, just curious, do you think that NASA Time Trial drivers will accept an uncontrollable car to get their one fast lap, especially since if they go four off or spin their time does not count for that session?

If I haven't made even a little bit of a point yet, here is some CMC undercar video from TWS:

http://vimeo.com/23297940

I have read all of the Carroll Smith "to win" books, and even back in 1978 he gave the tire designers credit for overcoming a lot of race car geometry issues.

Again, for the new guys, having your roll center underground doesn't mean that it will catch and flip the car like a driveshaft! Seriously, that is what this thread seems to imply!

Having your roll center underground simply means that the car may or may not roll more, depending on the springs and resulting spring rate that were used to lower the car. And that may or may not affect the camber of the most important outside front tire. And that is easily compensated with more static camber with a slight loss of braking force. And none if that is probably even noticeable by a driver with less than 10, 20, even 30 track days under their belt!

I don't pretend to be an expert, but I did 50 track days in 2007-2008, a year of NASA Time Trials in 2009, and drove a ridiculously loose 1998 Camaro in NASA CMC (till one of our fast guys drove it and said he wouldn't drive it in traffic!)

So to the OP, I understand your interest, but IMO the roll center just doesn't matter. BTW, I did my first 14 track days with a stock 2007 Mustang GT and the 17" Pirelli P Zero Neros.

Now if anyone has suggestions for my TTD build in the thread below, let me know!