Koni Sport dampers and Evolution driving school..

[sheizasosay]

So the only reason to swap the EPAS seems to be if you get the steering shake. Guess I will stay away from the front control arms and bushings. I miss the feel, but it isn't something that I can overcome.

I was told by a gentleman at Performance Autosports that you can replace the large hydra bushing and not get the shudder so long as you leave the smaller bushing OEM.

 

[kcbrown]

Well, to be fair the m3 also has pretty soft fronts.

Right. But nonetheless, it's easier to drive fast (or so some claim) than the Brembo Mustang. If it ain't the springs, and the dampers are replaced on the Mustang with something competent (e.g., Konis), then we're left with CG height (which I can't determine on the M3, so we don't yet actually know that the M3 is superior in that regard) and steering feel (which I know nothing about on the M3, but wouldn't be surprised in the slightest if it's superior since the E92 has hydraulic steering, but regardless it's something that can't be fixed on the Mustang without replacing the whole system with a hydrualic one).

Can't underestimate the benefit of steering feel on how easy it is to drive the car fast, though.

 

[kcbrown]

So the only reason to swap the EPAS seems to be if you get the steering shake. Guess I will stay away from the front control arms and bushings. I miss the feel, but it isn't something that I can overcome.

What model year is your Mustang? I was under the impression that 2013 and later models don't have the steering shudder issue.

 

[sailorrhett]

2011 with the Roush suspension. I am pretty sure EPAS issues may be ahead if I mess around up front.

 

[csamsh]

2011 with the Roush suspension. I am pretty sure EPAS issues may be ahead if I mess around up front.

Probably so

 

[todcp]

2011 with the Roush suspension. I am pretty sure EPAS issues may be ahead if I mess around up front.

Vorschlag has covered this in their awesome SCCA forum updates. Do a search; but here is some input.
Modifications that can cause EPAS Feedback Failure

Aftermarket Front Control Arms
Aftermarket Front Control Arm Bushings
Radical changes to front suspension geometry
Significant tire/wheel changes (especially R-compound tires or slicks)

Rehagen has the expertise on this issue as well and solutions.

 

[Norm Peterson]

Right. But nonetheless, it's easier to drive fast (or so some claim) than the Brembo Mustang. If it ain't the springs, and the dampers are replaced on the Mustang with something competent (e.g., Konis), then we're left with CG height (which I can't determine on the M3, so we don't yet actually know that the M3 is superior in that regard) and steering feel (which I know nothing about on the M3, but wouldn't be surprised in the slightest if it's superior since the E92 has hydraulic steering, but regardless it's something that can't be fixed on the Mustang without replacing the whole system with a hydrualic one).

Can't underestimate the benefit of steering feel on how easy it is to drive the car fast, though.

Differences in geo roll center heights and suspension roll couple distribution. Turn-in ought to feel different if the rear LLTD is more from the RC height on one car and more from elastic suspension stiffness on the other. Think in terms of time rather than quasi-statically evaluating the TLLTD at mid-corner.


Norm

 

[kcbrown]

Differences in geo roll center heights and suspension roll couple distribution. Turn-in ought to feel different if the rear LLTD is more from the RC height on one car and more from elastic suspension stiffness on the other. Think in terms of time rather than quasi-statically evaluating the TLLTD at mid-corner.

Those differences can result in one car being just as fast as the other but harder for a non-expert to drive as fast?

 

[ddd4114]

Those differences can result in one car being just as fast as the other but harder for a non-expert to drive as fast?

With a high roll center and soft springs, more of the cornering load is transferred through the control arms than the springs/shocks, so body movement is reduced, and the response of the car is relatively quick if the bushings are in decent shape. Conversely, a car with a lower roll center and stiffer springs can achieve the same steady-state balance, but more of the load will be transferred through the springs/shocks. Therefore, the transient response of the car will be much different, and depending on the shocks, there could also be a larger difference between the initial balance of the car (at turn-in) and the final (steady-state) balance of the car.

 

[sheizasosay]

With a high roll center and soft springs, more of the cornering load is transferred through the control arms than the springs/shocks, so body movement is reduced, and the response of the car is relatively quick if the bushings are in decent shape.

To get all the load transfer to avoid springs and shocks would you need the roll axis level with the CG?

 

[ddd4114]

To get all the load transfer to avoid springs and shocks would you need the roll axis level with the CG?

Yup. Of course, in practice, you'll never avoid all load transfer through the shocks, but the closer the roll axis is to the CG axis, the less the springs and shocks are used in cornering. However, that isn't necessarily a good thing...

 

[Norm Peterson]

To get all the load transfer to avoid springs and shocks would you need the roll axis level with the CG?

You'd at least need to have the "roll axis" run through the CG of the sprung mass . . . and the chassis would need to be more or less uniformly rigid torsionally along its length. But you'd throw away the capability to tune the handling via springs, bars, and shocks (there would be no need for bars at all if there isn't any cornering roll for them to work with, they'd just become poorly located ballast that would reduce mechanical grip on one wheel bumps for no advantage at any other time, truly a lose-lose solution)

A car with a less level "roll axis" and roll stiffness that's more front-biased (to balance that at steady state) tends to become progressively more understeerish during the time it takes for roll to develop, at least from these two effects. All else held constant. This doesn't necessarily imply heavily understeerish, just that the amount of understeer from these two related effects changes a little more than with a more level "roll axis" and more balanced roll stiffness. Driving technique(s) matter.


Norm

 

[sheizasosay]

However, that isn't necessarily a good thing...

Agreed.

But you'd throw away the capability to tune the handling via springs, bars, and shocks (there would be no need for bars at all if there isn't any cornering roll for them to work with, they'd just become poorly located ballast that would reduce mechanical grip on one wheel bumps for no advantage at any other time, truly a lose-lose solution)

I'm not suggesting that it was a goal or profitable. Just discussing. I do, however, like the idea of using the rear RC in a higher-than-stock position to provide a legitimate reason to remove rear sway bar so as to move away from the negative effects that are inherent in the function of a sway bar.

A car with a less level "roll axis" and roll stiffness that's more front-biased (to balance that at steady state) tends to become progressively more understeerish during the time it takes for roll to develop, at least from these two effects. All else held constant. This doesn't necessarily imply heavily understeerish, just that the amount of understeer from these two related effects changes a little more than with a more level "roll axis" and more balanced roll stiffness. Driving technique(s) matter.


Norm

I don't want to assume but, does your "less level" equate to: high in the rear to low in the front with respect to RC heights? Or are you saying that is irrelevant? Or just plain retarded any other way since it's not a likely scenario of having the front RC higher than the rear with an S197?

 

[Norm Peterson]

I don't want to assume but, does your "less level" equate to: high in the rear to low in the front with respect to RC heights? Or are you saying that is irrelevant? Or just plain retarded any other way since it's not a likely scenario of having the front RC higher than the rear with an S197?

In boldface ↑↑↑. "Less level" would be something like 3" front RC height with 12" rear RCH (not too far off the S197's case) . . . compared to 3" with 6" which is more like what you'll find for RC heights in IRS cars.

It's rare you'll find any car with a front RC located higher than its rear RC, though over the years I think there have been a few.


I have a feeling that raising the rear RC enough to eliminate the rear bar entirely risks putting it too high.


Norm

 

[sheizasosay]

It's rare you'll find any car with a front RC located higher than its rear RC, though over the years I think there have been a few.

I thought you were referring to the "norm", but wasn't sure. I have read some rear engine cars are.

I have a feeling that raising the rear RC enough to eliminate the rear bar entirely risks putting it too high.


Norm

My "enough"= dead center of the diff with 570/340 in lb rates and a stock front bar.

 

[Pentalab]

Well, to be fair the m3 also has pretty soft fronts.

No, the steering rack doesn't do anything. The fact that we have EPAS and not electro hydraulic steering is the main culprit in the lack of feel dept. I'm perfectly happy to trade steering feel for having to mess with an additional hydraulic system though. It's also nice to ditch the #1 fire starter that is power steering fluid.

My 2010 doesn't have epas. I have driven a few 2011 cars over the years..and noticed that 10-20% more effort was required to turn the steering wheel on the 2011's. The 2011's had 245 wide tires on all 4 corner's (19" diam rims). My 2010 has 275-40-18's on the fronts...on 9" wide rims. I notice no difference in steering wheel effort, when I swap the 275 fronts to the 235's I use for winter. My 2011 fusion is at the other extreme, you can turn the steering wheel with one finger, stopped, or moving at any speed...ok for a DD but you can't 'feel' the road. It feels like you are driving a servo.

 

[kcbrown]

With a high roll center and soft springs, more of the cornering load is transferred through the control arms than the springs/shocks, so body movement is reduced, and the response of the car is relatively quick if the bushings are in decent shape. Conversely, a car with a lower roll center and stiffer springs can achieve the same steady-state balance, but more of the load will be transferred through the springs/shocks. Therefore, the transient response of the car will be much different, and depending on the shocks, there could also be a larger difference between the initial balance of the car (at turn-in) and the final (steady-state) balance of the car.

Makes sense. Which configuration (high roll center and soft springs, versus low roll center and stiff springs) is easier for the novice to drive fast, assuming the dampers are good on both? And note that the M3's wheel rates are about the same as the Mustang's (somewhat stiffer, but not dramatically so), but I haven't a clue where its roll centers are located.

However, shots of the cars mid-corner indicate that the M3's body roll appears to be quite similar to that of the Mustang. Here's the Mustang:

And here's the M3:

That suggests to me that the distances of the roll centers up front to the CG are about the same, or the Mustang's might even be a bit shorter given the spring rate difference, but I wouldn't be surprised if the roll center in the rear is lower for the M3, which would give it more roll back there. Less roll in the rear on the Mustang, but about the same in the front, would (if I understand this stuff right) mean more weight transfer off the inside rear on the Mustang, and more onto the outside rear, which would give it more understeer.

That makes me wonder why Randy Pobst seemed to think the M3 had more understeer than the Mustang. Odd...

 

[B2B]

You are trying to draw a conclusion by comparing cars on two completely different track/turn with speed being an unknown. For the comparison to be valid, you really have to have the same driver driving the cars on the same track with full data logging.

 

[Norm Peterson]

However, shots of the cars mid-corner indicate that the M3's body roll appears to be quite similar to that of the Mustang.

Not really surprising.

Car and Driver has been running some of their cars through a K&C rig and publishing the results. I'm way behind on my reading so I've only scanned a couple of them, but the 2013 Mustang was a 2.4°/g car. For comparison a 2013 Honda Accord Sport was out there at 4.1°/g.

For the Mustang vs M3 and ignoring the different track/different corner matter completely, I'm sure that the M3 is closer to 2.5°/g than 4°/g, and you simply aren't going to see with your naked eye differences on the order of two or three tenths of a degree/g. Hell, you can barely pick up half degrees with a protractor on identical, perfectly head-on camera angles.

That suggests to me that the distances of the roll centers up front to the CG are about the same, or the Mustang's might even be a bit shorter given the spring rate difference, but I wouldn't be surprised if the roll center in the rear is lower for the M3, which would give it more roll moment back there. Less roll moment in the rear on the Mustang, but about the same in the front, would (if I understand this stuff right) mean more weight transfer due to the RC heights off the inside rear on the Mustang, and more onto the outside rear,

Is this what you're trying to say?

which would give it more understeer.

Don't you mean something like 'more initial oversteer' here, before roll displacement has occurred?


FWIW, until you get involved with chassis torsional stiffness the amount of roll rotation should be considered constant over the length of the car.

As far as which one is easier for a novice to drive fast, I suspect that a car that progressively adds the slightly greater amount of understeer is going to be more novice-friendly, and be a little more forgiving. It may also lead to a driver acquiring a habit of adding a little steering 'correction' partway through the turn even if the steering including such corrections is all done 'smoothly'.


Norm

 

[kcbrown]

Not really surprising.

Car and Driver has been running some of their cars through a K&C rig and publishing the results. I'm way behind on my reading so I've only scanned a couple of them, but the 2013 Mustang was a 2.4°/g car. For comparison a 2013 Honda Accord Sport was out there at 4.1°/g.

Is any of that online? If so, where?

Is this what you're trying to say?

Yes. For some reason the terminology was escaping me.

Don't you mean something like 'more initial oversteer' here, before roll displacement has occurred?

Where would the initial oversteer come from? The car has greater weight up front than in the rear, but the adhesion curve of the tires does not rise linearly with compressive force. Additionally, the front's camber is negative so the contact patch is smaller than it is at some point later.

That sounds to me like a recipe for initial understeer. Of course, you don't get either understeer or oversteer until the adhesion capability of the tires is exceeded somewhere, so you're well into the roll response of the car at that point. And for that, see below.

FWIW, until you get involved with chassis torsional stiffness the amount of roll rotation should be considered constant over the length of the car.

Understood. My thinking on it is this:

The front roll moment is greater than the rear roll moment. Therefore, at the point where the rear would be stabilized in roll due to the weight transfer in the rear, the front has not stabilized. The additional roll in the front is transferred to the rear via the chassis, which increases the amount of compression force on the outside rear tire while the amount of lateral force there remains the same, and therefore the rear gets greater compression force at the tire and thus greater adhesion. Greater adhesion in the rear than the front gets you understeer.

As far as which one is easier for a novice to drive fast, I suspect that a car that progressively adds the slightly greater amount of understeer is going to be more novice-friendly, and be a little more forgiving. It may also lead to a driver acquiring a habit of adding a little steering 'correction' partway through the turn even if the steering including such corrections is all done 'smoothly'.

The M3 might well behave like that.

What I've noticed from the Mustang's suspension is that if I go into a corner hard enough, I'll get a lot of initial understeer unless I go into it with some trail braking. Once the car is going around the corner, I can control the rear with the throttle. With the stock suspension without camber plates, I was able to make the car "tuck in" by lifting the throttle, but the rear wouldn't come out. With camber plates, I can make the rear come out simply by lifting off the throttle.

So it seems like the car's behavior is basically corner entry understeer (except when trail braking), and mid-corner neutrality (or darned close to it), and corner exit oversteer can be induced if you give it enough throttle on corner exit.

What changes can be made to the car to make corner entry neutral, but otherwise retain the characteristics it seems to have with camber plates?