Mirage - Still lifting a front tire

[Loren]

Well, dang it!

Since we put wider/grippier front tires on the car, now it's back to significantly lifting the inside front tire. The car doesn't FEEL tippy or unstable at all, but this is a situation that I'd want to be very much the exception rather than the rule. And Philip has photos of both me and Dan doing it yesterday in several places on the same course. Gonna have to think about this some more, I guess.

What's been done:
150# of weight reduction
15x7 wheels, 205/50 front, 195/50 rear
Coilover kit, lowered approx 2" with 4k/3k spring rates
Rear swaybar (torsion beam stiffener)
Custom lower control arms, 1/4" longer
Custom lower ball joint, 1/2" taller
Custom caster plates, added +2 degrees caster
Custom tie rod ends for bump steer correction

What can be done to further improved the situation:
Lower CG - I could lower the car a little more, but that would actually do more harm to the front suspension geometry than it would help. Not a lot more can be done here.
Raise Roll Center - Bringing the roll center closer to the CG height would help. More of the cars roll energy would be directed straight sideways rather than into "body roll". This is what made the biggest change before. WOW, it transformed the car!

How do we raise the roll center?
Taller ball joints - My initial desire was to go 1" taller, and I had to settle for 1/2" because that's all that would fit within the 15" wheel diameter. To go another 1/2", I'd have to run larger diameter wheels! I'm not sure I'm ready to do that... but, it's an option. I'd likely have to do a further adjustment to the tie-rod ends (or move the steering rack) to fix the bump steer again.
Wheel spacers or offset - Pushing the wheels further out has an effect on roll center. It's not as strong of an effect as the ball joints, but it's something. And it's fairly easy to do. And I do have some clearance on the front to move the wheel out some. So, that's an option.
Raising the inner pivot of the lower control arm - I've not looked much into this one, as it wasn't something I really wanted to do. But, I'm going to have to start taking a closer look at it, see how hard it would be to do, and how much benefit there would be. This is more or less the same thing as fitting a taller ball joint (which is the same as raising the spindle height on the hub), it's correcting the angle of the lower control arm. I could lower it by either doing major surgery on the front subframe (which is a simple bolt-on part), or maybe by fitting offset bushings. Again, not something I really wanted to do, so I haven't looked at it much.

Other things that could help:
Maybe stiffer front springs. I'm not sure this would rectify the problem, it would just make it harder to get into.
Same with front shock adjustment. Stiffer COMPRESSION damping could do somewhat the same thing as stiffer springs and resist the compression of the loaded front corner. But, my shocks are probably only adjusting REBOUND, which wouldn't help at all.

I don't think doing anything to the rear suspension would really help. I guess maybe reducing rear tire grip (less tire, more or less tire pressure) could induce oversteer earlier in the process, allowing the car to slide before getting to the point that it lifts the inside front tire. But, that seems counterintuitive to me. I don't want to just give up some grip. I want to continue to maximize grip. The car is nicely balanced now, and making the most of the grip on both of the outside tires. It would be good to just stabilize that... keep the inside front on the ground to maybe provide more front grip (which would induce oversteer... but at a much higher threshold) and less possibility of tipping.

I'm sure I'll spend more than a few nights pondering this stuff...

[aw614]

what is the offset of the current 15x7 wheels? Im inclined to try the spacer option or trying a lower offset 15x7

What about a custom smaller front sway bar?

[Loren]

I think they're 38 offset. Ideal for the front would have been 32-35. Ideal for the rear would have been 40. I split the difference... and rolled the rear fenders.

Front swaybar is on a shelf in my garage. Can't get smaller than that!

[aw614]

Forgot you removed the front sway bar.

I have some 3mm 4x100 spacers with a 56.1 center bore if you want to try them out. But 3mm seems like it wouldn't make that much of a difference vs say a 10 to 15mm spacer.

[Loren]

I have a set of 1/4" spacers, but I don't think I have enough lug stud to safely use them.

I also have a set of bolt-on 10mm spacers, but I don't like how they're made. The bore is so deep that there's not a lot of material left under the head of the lug nut. Seems too fragile for something that's intended to hold a wheel. They would also require that the lug studs be TRIMMED because the ends of them would interfere with the back of the wheel. Can't win with those.

So, best option for spacers is probably to get some longer lug studs.

But, before I do any of that, I'm going to play with that suspension design calculator that I used before and figure out which of these changes will have the greater effect, and what it's going to take to make enough of a difference to matter.

If I end up having to go with the taller ball joint, that means 16 (or 17) inch wheels. So, when I buy those, I can just get the correct offset and skip the spacers.

I'm in planning mode.

[CaptainSquirts]

Did you remove your helper springs in the front?

[Loren]

Yeah, I never got that setup correct. Still running fixed rate 4 kg.

[Loren]

Still thinking. But, here's some stuff from the chassis simulator. Some of this is a little counter-intuitive. And I haven't made any decisions yet, just exploring ideas.

Control Arm Length:
If I rework the lower control arms for an extra half-inch of length, it raises the roll center by 0.3" (again, we WANT to raise the roll center, the closer it is to the CG height, the less of our weight transfer goes into body roll... it goes straight sideways). So, that would be an improvement with little or not negative effects. It would give me more negative camber, by about 1.2 degrees... which I might have to adjust out, but not really. I want to say my camber was only about -2.5 the last time we checked it, and it was maxed out. So that would be good all around.

Ball Joint Height:
I can easily fit a taller ball joint to my control arm without modification. It would just require a larger wheel to accommodate it. A half-inch taller ball joint (readily available) would raise the roll center by 1.1".

I'm not going to try to wrap my head around the interaction, but with the taller ball joint, the control arm length makes less of a difference... only about .2". So, if I'm going to do the taller ball joint, the added benefit of modding the LCA to make it longer wouldn't be worth it. (though, it is an easy way to get some more camber)

Tire Diameter:
I didn't consider this before. Just changing my front tire size from 195 to 205 changed my roll center. It lowered it by 0.3". So, the reason it's lifting the front wheel now isn't JUST that it has more front grip. It's also got slightly more "roll couple". (the distance between the roll center and the CG) Interestingly, it's the change in tire OD that caused that. It raised the car without changing any of the suspension geometry. Yeah, I haven't wrapped my head around all of those dynamics, either. But, it's interesting. So, my current RC height is about 1.7" rather than the 2.0" that I thought it was. And where I *wanted* to be when I initially did the control arm & ball joint mod was 3-3.5".

If I go to a larger wheel, it pretty much requires an even larger tire diameter... which raises the roll couple even higher... The gains that come from the taller ball joint (which requires a larger wheel) are countered by the height that comes with the larger wheel. Fuckin' compromises.

Wheel Offset:
Makes stunningly little difference! A 1/4" spacer would raise the roll center by like 0.04". There are other benefits to wider track, of course. It just doesn't have a huge effect on roll center.

Ride Height:
This is a weird one. It's the nature of CG Height vs. Roll Center Height. As you lower the car, the roll center lowers by like twice as much as the CG. That why I had to correct the roll center in the first place. So, the front of my car is pretty low now. About 2-2.5" lower than stock. I could easily raise it up by an inch, and still be kinda low. Raising the front Ride Height by 1" brings the Roll Center up by 2.3". That's pretty significant. And weird. For all the goodness that lowering a car does... RAISING my car would actually result in LESS body roll due to bringing the roll center significantly closer to the CG. I've studied this before. It still makes my brain shudder just a little bit.

Control Arm Inner Pivot:
Raising the control arm inner pickup point has a similar effect on lower control arm angle as raising the car, or fitting a taller ball joint. Raising that pivot point by just a half inch would raise the roll center by 1.3". But, it wouldn't be easy to do. The front bushing on the LCA is pretty typical with a longitudinal bolt securing it. It wouldn't be TOO difficult to move that one. Maybe. (I haven't actually looked at it) But, the rear bushing mounts the other way, it has a through bolt going straight up through the bottom. I guess it would be about the same difficulty as the other one. Just different. If I was trying to do it with offset bushings, only the front one could be done that way. The rear one... maybe could get a little bit of height by switching to a heim joint and putting spacers under it to bump it up. I don't think I'd get more than 1/4 inch out of offset bushings, though. And it's getting into modifying the subframe, which is technically legit in M4... but, it's getting into something that's a lot more difficult to return to stock. (maybe not, the subframe doesn't support the engine... probably comes out easily enough) Anyway... with all of the other options, I'm not likely to do this one.

[Loren]

Soooo... playing with compromises on some of the easier things to do:

If I were to raise the ride height by 1", go with a 205/50-16 tire on a 35mm offset 16x7.5" wheel, and fit a 0.5" taller ball joint... my roll center would go from 1.7" to 4.1". No cutting or fabrication required. Just a pair of wheels and tires, and a pair of ball joint studs.

The ball joints and the wheel/tire change would have to be done as a unit. Wheels are $189 x2, tires $140 x2, ball joints about $30 x2. Total a bit over $700. Ouch. But, if I hold off until it's time for another pair of front tires (4 months?), it's a little more palatable. I'm also not considering the fact that I could sell my old wheels to offset some of that cost.

But, the ride height change is free. I can do that before the next autocross. An inch of height = 2.3" of roll center. So, doing that would bring the roll center up from 1.7" to 4.0". It's raising the CG by an inch, as well... but it's still a 1.3" improvement (decrease) in roll couple. Seems like a no-brainer thing to try. Counter-intuitive as hell... but, it's a pretty significant change in roll center and roll couple... should be easy to tell if it's a positive change or not.

I'm all about trying free stuff.

[twistedwankel]

So, best option for spacers is probably to get some longer lug studs.

Dan is all ready longer than you and he's still slower. So much for long studs.

[Loren]

It's all in how you use it.

[mymomswagon]

Soooo... playing with compromises on some of the easier things to do:

If I were to raise the ride height by 1", go with a 205/50-16 tire on a 35mm offset 16x7.5" wheel, and fit a 0.5" taller ball joint... my roll center would go from 1.7" to 4.1". No cutting or fabrication required. Just a pair of wheels and tires, and a pair of ball joint studs.

The ball joints and the wheel/tire change would have to be done as a unit. Wheels are $189 x2, tires $140 x2, ball joints about $30 x2. Total a bit over $700. Ouch. But, if I hold off until it's time for another pair of front tires (4 months?), it's a little more palatable. I'm also not considering the fact that I could sell my old wheels to offset some of that cost.

But, the ride height change is free. I can do that before the next autocross. An inch of height = 2.3" of roll center. So, doing that would bring the roll center up from 1.7" to 4.0". It's raising the CG by an inch, as well... but it's still a 1.3" improvement (decrease) in roll couple. Seems like a no-brainer thing to try. Counter-intuitive as hell... but, it's a pretty significant change in roll center and roll couple... should be easy to tell if it's a positive change or not.

I'm all about trying free stuff.

In for hearing more about the ride height change.

[Loren]

This week, I learned that I have a lot less free time when my wife is on vacation.

[mymomswagon]

Thanks for posting the detail. it motivates me to finally take a crack at measuring things out and modeling.

it doesn't sound like you'll do it, but I've kicked around the offset bushing idea and I wonder if it really does help with roll center. Does modeling allow one to capture it?

It might increase the downward slope of the LCA and therefore help with camber curve(?) but the pivot point at the subframe doesn't change.

There are products for other cars that similarly, shim the ball joint and increase the downward slope of the LCA (but the ball joint pivot is basically at the same height.

[Loren]

If you're putting an offset bushing at the inner end of the control arm and positioning it to where the offset is UP or DOWN (rather than IN or OUT), you ARE changing the effective pickup point. The new pickup point becomes (more or less) the new center of the bushing... which is not where the bolt is.

You can model for that. It's not going to make a huge difference, though. Ball joint or spindle height makes a much bigger difference. It's easy to play with all the parameters in the simulator, though, once you figure out what's what.

[Loren]

After 9 days of procrastination, I finally got out to the garage and raised the front ride height. I didn't mess with the spring perch height, I think I have that set in a good place. It's got enough compression travel to not bottom out constantly, and as much droop travel as I can get away with. (the shocks only have a 4.5" stroke with 1-5/8" very stiff bump stops, so there's not much more I can do there... pretty much 1.5" up and 1.5" down) So, I just extended the shock length by 3/4" to make the ride height even front/rear.

Ride Height:
This is a weird one. It's the nature of CG Height vs. Roll Center Height. As you lower the car, the roll center lowers by like twice as much as the CG. That why I had to correct the roll center in the first place. So, the front of my car is pretty low now. About 2-2.5" lower than stock. I could easily raise it up by an inch, and still be kinda low. Raising the front Ride Height by 1" brings the Roll Center up by 2.3". That's pretty significant. And weird. For all the goodness that lowering a car does... RAISING my car would actually result in LESS body roll due to bringing the roll center significantly closer to the CG. I've studied this before. It still makes my brain shudder just a little bit.

This still strikes me as weird and counter-intuitive. Making the car more low is supposed to be more better, right? Not necessarily.

I had the front ride height (lowest point of the front subframe) at 6-3/8". This had my front roll center at about 2.7", and my lower control arm angles slightly downward at the outer end. Control arm angle was nearly dead level at 2.5 degrees of body roll. By 4 degrees, the roll center had moved outside the tire footprint, and below ground.

Now, 3/4" taller, I'm at 7-1/8". Roll center is 4.5", so nearly 2" higher, and the downward angle is steeper at static height and zero roll. Control arm angle doesn't go fully horizontal until almost 5 degrees of roll. Roll center doesn't hit the contact patch until 6 degrees of roll.

Blah, blah, blah, numbers. Quick test drive. YES, you can feel the difference! There's a little less body roll. The steering feels a little LESS responsive (because our primary feedback to steering input is body roll). Hanging a corner feels LESS dramatic. Time and some photos from Philip will tell if it's going to life a front tire less or not. But, I feel like it has to be an improvement. If it DOES still lift a tire, I suspect it's going to do it at a higher G-level... I might be okay with that. As long as it feels stable, and doesn't LOOK like it's going to turn turtle.

[Carracer]

Blah, blah, blah, numbers. Quick test drive. YES, you can feel the difference! There's a little less body roll. The steering feels a little LESS responsive (because our primary feedback to steering input is body roll). Hanging a corner feels LESS dramatic. Time and some photos from Philip will tell if it's going to life a front tire less or not. But, I feel like it has to be an improvement. If it DOES still lift a tire, I suspect it's going to do it at a higher G-level... I might be okay with that. As long as it feels stable, and doesn't LOOK like it's going to turn turtle.

I'll see if I can get some video.

[Loren]

Poking around in the suspension simulator, I realized that it actually gives me some of the angles I was guessing at. Plus a few more dimensions that I was really wondering about.

Where I was... 6.375" ride height.

Static control arm angle = 1.76 degrees down
Static shock length approx = 23.5"

At 4 degrees roll, it goes to:
2.26 degrees up on the loaded control arm
Outside shock 21.5", Inside shock = 25.5" (2" compression, 2" extension... and that's absolute max extension for the strut)

Beyond 4 degrees roll, it's gonna lift. And it did.

Where I am... 7.125" ride height.

Static control arm angle = 5.0 degrees down
Static shock length approx = 24.25"

At 4.4 degrees roll, it goes to:
0.35 degrees up on the loaded side
Outside shock 22.08", Inside shock = 26.26"

So, it allows for about 10% more roll before running out of droop on the inside. And I suspect that it's going to take a higher G load to get that much roll.

I'm going to play with going a little bit stiffer on the front shocks, too. It's hard to find reliable info on these Godspeed coilover shocks. But, I know they are monotube, and found a couple references that claim the adjustment is both rebound and compression. More compression damping would be good. Too much rebound damping would actually cause wheel lift to happen quicker. Everything is a compromise.

I may also go back and revisit spring rates. Just a tiny bit more spring rate would resist compression on the loaded corner when the inside rear tire lifts... and that would be a good thing.

I don't want to many any major changes, though. I really like how the car feels on the street, and it handles quite well at autocross, too. (for what it is) I don't want to make it uncomfortable on the street. Subtle changes.

[twistedwankel]

So it is equipped with a gopro/camera and flys. It must be a Drone and they aren't allowed at any airport Best be keeping all those tires on the pavement then.

[Loren]

Well... while the latest change to ride height DID improve the suspension geometry, reduced the amount of body roll relative to steering input, and made the car feel pretty great in general... it did NOT solve the wheel lift problem. In fact, it either made it worse, or just made the car feel so confident and planted that we're driving it harder. Hard to say for sure.

But, I saw the car up at about a 30-40 degree angle when Dan made an aggressive right turn yesterday. Not good.

Re-evaluating the suspension mods to date, they've all been effective at making the car feel better, but it seems that the car always has some degree of front wheel lift. I think the problem is that I've been trying very hard to address the issue while maintaining full "comfortable streetability". Read that as "I don't want to slam the car to the ground, make it uncomfortable, and scrape over speed bumps." So, I've done just about everything BUT that.

I'm getting desperate now. On the verge of giving up on this grand experiment of "autocross Mirage". But, since I have a practice event next weekend, I want to try something quick and easy. I'm gonna go ahead and lower it another 3 inches from where it is. It's probably 1.5" lower than stock right now. The coilovers would probably let me put it another 4 inches lower. 3 inches is no problem.

That puts my ground clearance at an awful 3". But, I've lived with that before with a lowered Miata. Just have to be mindful of it.

It lowers the CG by another 3", which is fundamentally good. I don't know if it's enough to solve the problem or not, but it's got to help.

Unfortunately, it lowers the roll center from about 4.5" to 3.25" below ground. Body roll will be more prevalent, and the steering won't feel as crisp and confident. But, if it keeps the inside front tire on the ground... I can live with it.

That will have to be the next phase.

From there, if I want to fix control arm angles, I could consider modifying the front subframe to bring the lower control arm pickup points up to improve that control arm angle and get the roll center back up to at least ground level. Raising it by 1.25" would do the trick. Will probably be easier to accomplish that by using heim joints on the lower control arm... which means rebuilding the LCA, or custom-fabricating a new set. Not impossible, or even "difficult" in the grander scheme of things. But, I don't really want to do it if I don't have to.

But, let's start with going back to basics. Slam it to the ground. It might not feel right, it might be somewhat lower than what I consider "practical", but... maybe it will be less tippy?