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PostPosted: November 11, 2018, 5:57 pm 
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Hey guys.

So my Emod Seven has seen five or six autoX events now. I'm still learning to drive it, but I'm fairly certain it could use some swaybars and other improvements to handling. Thing is I do not have enough experience chassis tuning to say what it needs, and my inconsistent driving/lines make it tough to sort things out. Also due to some health problems, my head is too fuzzy to focus well. I'm trying to figure out what the next step is, but I am struggling. So I am just going to spout some things out and hope for some help from you Gents.

Car is:

-SLA front, live axle 3link+panhard rear.
-1850lbs w/driver and fuel
-~50/50 weight bias
-no sway bars, fairly soft sprung (spring rate does feel about right).
-205/50r15 Rcomp tires
-250hp/lbsft (large displacement/weight engine which creates a LOT of engine braking)
-increased F shock stiffness to max, R to min = better

So there seems to be oversteer from the back. I would say most notable at mid corner. Only at higher loads and does not occur during wet events. Possible jacking effect? On throttle the car is very stable, even if oversteering I can just let go of steering wheel and stomp throttle to get settled. Engine braking/ lift off oversteer is very noticeable in this car. This is always going to be a thing in this car. I'm learning to be much smoother coming off throttle in corners. Car will understeer at times, but I don't know what conditions. I would guess I need some more camber up front to maximize grip. I have had a more skilled driver do a few runs in it and he felt the same dynamics so it's not just my driving.

So I guess my first question is: what's the best way to diagnose this? We don't have any test n tune autox days. Car is not stable enough for a serious track day, but I am thinking maybe I should do one, just take it easy and treat it as a test n tune? That is next year though, and I'd like to have something done over winter. Next question is: How do I decide what size swaybar(s) to install? With the 3link, the rear has basically no roll resistance. Body roll feels like it comes from the rear mostly.

I don't think I have any of my chassis measurements due to a harddrive failure. I'll have to remeasure to check if there is any jacking effect (right?).

Thanks for any help. Sorry I can't be more precise/learned in my post; I just can't remember my chassis theory well enough anymore.

Cheers,
Cory

Some videos here:
Probably shows body roll the best. Was focusing on controlling engine braking on this run. Grip was mostly too low to cause oversteer.
https://www.youtube.com/watch?v=7wm1YklFYVI

A lot of bad lines, and this course was punishing to oversteering cars due to location of crests. Even Miatas were getting loose.
https://www.youtube.com/watch?v=aET6285xONg

I don't think much to be figured from this one. Was on old tires and wet. Veeeery low grip.
https://www.youtube.com/watch?v=h17zHc9Q7zM

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PostPosted: November 11, 2018, 8:06 pm 
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Skid pad. Paint a circle and drive round it smoothly, faster and faster until you know if it understeers or oversteers in a steady state condition.
I'd try a swaybar before changing rates that work well for you. I doubt you want to change the rear susp front lcaps or ucap heights.
If oversteering, you can try a front swaybar, lower the rear susp, raise the front susp, reduce front neg camber, tire pressures, etc.

As far as bar size, the stock miata bar would work well but you need extensions from the frame to reach the bushing locations. Just above and behind the lca shoud fit best. If you want to make your own, that can be locost too. A 4140 rod already hardened is about $50 for 3 feet, shorten to the outside rail width plus 2 inches each side, have a machine shop drill the ends for a 7/16 bolt, fit nylon tube to act as bushings and 1x3x4 aluminum pillow blocks to drill for 3/8 bolts, split in half, then bore to fit the bush od. Fab the ends from tube and 1/8 strip at 45 degrees to the bar.

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PostPosted: November 13, 2018, 10:54 am 
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Great question, Cory. Whatever path you choose, I hope you post your results (good or bad) because I think a testing section is really needed here.

Cheers,

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Damn! That front slip angle is way too large and the Ackerman is just a muddle.

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PostPosted: November 15, 2018, 1:59 am 
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With the 3link, the rear has basically no roll resistance. Body roll feels like it comes from the rear mostly.


I think most solid rear axle cars have high rear roll centers. This is set by your panhard bar. The high rear roll center is like having a rear anti roll bar and a pretty stiff one at that.

A front anti roll bar might help you. What angle are your front upper arms at? Maybe 10ish degrees would raise your front roll center a bit to match the rear.

Practice might help you a lot so keep going. Things like tire pressure can do this to you too, so you should run a bit less than the door slammers.
:cheers:
:zoom:

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PostPosted: November 15, 2018, 9:37 am 
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Stepping back a bit and looking at the subject of this thread, is there any chance you are bottoming out the rear suspension on bumps? Do you have bump rubbers installed on your coilovers?

If a coilover bottoms out that raises the spring rate a great deal. I guess it sets the sprig rate to the flexibility of your chassis and tires.

Bottoming out a rear coilover would explain mid corner oversteer on dry pavement, you don't bottom out on wet pavement. It probably happens on a little bump in the turn so feels unpredictable.

A high rear roll center might be noticed on corner entry because the effects of a roll center appear as soon as the car accelerates laterally. A sway bar will not introduce it's effect until the car rolls.

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PostPosted: November 15, 2018, 9:56 am 
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I was just looking at the videos you posted again. So, is your basic question about body roll? It does seem from the first video that the front rolls much less than the rear. How about measuring the body roll front and rear through repeated runs through the same course? That could be on the street too, couldn't it?

I was thinking of something cheap and simple like using a small camera to capture the movement of an inclinometer or angle gauge. You could find a home for it front and back, have someone level it with you in the car, and then make a movie of several runs through the same course. Comparing front and back would give something to go on, at least.
Attachment:
File comment: Simple, cheap, mechanical inclinometer.
Mechanical inclinometer.jpg
Mechanical inclinometer.jpg [ 10.1 KiB | Viewed 2951 times ]


If you find it is the rear rolls too much, look at what you can do to stiffen it up.

I'm not a suspension guru, but I do know having an anti-roll bar at rear without having one at front is pretty unusual. Maybe there is a basic geometry issue relating rear to front that needs to be corrected? Do you know the approximate height of your front and rear roll centers? Perhaps calculating the inclination of the roll axis would give a clue?

Cheers,

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Damn! That front slip angle is way too large and the Ackerman is just a muddle.

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PostPosted: November 15, 2018, 11:24 am 
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if oversteering I can just let go of steering wheel and stomp throttle to get settled. Engine braking/ lift off oversteer is very noticeable in this car


It kinda sounds like you may have a bit too much "Anti-Squat" built in to your 3-link rear. The "A-S" that plants the tires so well on acceleration could very well be lifting them under braking. Just a thought!

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PostPosted: November 16, 2018, 12:15 am 
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Thanks guys.

I think either way I will make some ARBs for the F&R (or at the least, the mounts etc) over winter so I can at least have something done to tune with. Once the car is insured again, I will do a skidpad test and probably a trackday and just treat it as a test n tune. The fact that stiffening the front shocks and softening the rear probably means it's in need of a front ARB, but I expect the rear will need one too. I also realize one of the guys I race with is hugely experienced in chassis building/tuning so maybe will hire him for some diag time (though he probably won't fit in the car.....).

MiataV8: Skidpads are hard to come by but maybe low speed circle will be enough to test with. You're absolutely right in that I don't want to be changing pickup points. Even adding swaybars is tough because "Yesterday Cory" was a jerk and decided "Tomorrow Cory" could figure out F ARB mounts. So there aren't any sturdy nodes for the ARB to frame mounts where I need them. oops.

Marcus: Not sure what angle my A-arms are at. I think my camber gain was fairly tame and I'd guess I need more static camber up front. Panhard bar height is at the bottom of axle tube (as low as viable) and is level. So probably higher than ideal. Tire pressures are already at 20psi F&R which is the min (according to Nitto anyhow). I'd guess I'm still a pound or two high. My large rear bumpstops show some contact, but not hard. It doesn't feel abrupt enough to be bottoming out, I will keep that in mind though. I do have a lot of wheel travel for a Seven btw. 3" bump, 2" droop IIRC. Rear may have more droop. It's pretty tough to tell when the roll starts. I'd guess it's further into the corner when the body is rolling, but I may also not be catching it soon enough. From what you describe, I'd guess probably a combination of high roll center and not enough front bar with the front bar being more important.

Lonnie: The inclinometer probably won't help as I don't know what values I should shoot for (and I think the G force would hold the ball to the side?). The keys on my dash do a good job of showing the G-forces, but I'm tempted to make a little plumb for the dash :mrgreen: I do have a data recorder now that will show that, but mechanical stuff is so much more fun. I'll have to measure my roll centers and go from there. My rear is probably higher than ideal but not much to be done there.

Mike: It is possible that I have too much antisquat. I remember being fairly aggressive, though I am currently at my lowest setting. I have no brake hop, and excess antisquat should show more on corner exit I think?

Thanks again for the help guys. Always appreciated.

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PostPosted: November 16, 2018, 11:50 am 
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My panhard is at the same height as the bottom of the chassis (roughly 4" off the ground). It works well to minimize jacking, but I did need to add a very small 1/2" sway bar at the back. I had already added a 5/8" sway bar up front. I am of the (relatively) soft spring & fat bar camp on suspension.

If the lower trailing arms at the rear are pointing up towards the front (for real "drag racing" anti squat), you are going to get roll steer which will make the car loose. The lower arms should be parallel to the ground so as to tame the rear on corners.

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PostPosted: November 17, 2018, 10:02 am 
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A relative simple way to measure body roll (not stiffness) is to mount the video camera on a flat, level surface, like a roll bar(?). Take a run with a good view of the horizon. The ones in your 1st video are good. Import a snapshot from the video to Autocad to measure the angle difference between the horizon sitting still and the horizon in a hard corner. Or the angle between the horizon and some horizontal part of the car in view. I did that for JD (I think it was him) some years back. I can do that again if you'd like. You could also print the screen shot out on a piece of paper and measure with a compass.

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PostPosted: November 17, 2018, 2:52 pm 
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I'm pretty sure you are way low on tire pressure. At 20 that's getting into wide slicks/formula car territory. When I ran a SHowrrom Stock Fiesta in the 70's I ran around 40 lbs. on 165 tires and it was an 1800 lb. car too. I don't remember what I ran my 185/60's at but would be surprised if it was under about 30 lbs.

So I think your oversteer is rolling over the edge of the tire?

You can figure that stuff out by learning to look at your tires. I have spent hours doing this. What can I say, I'm a little weird. They will tell you a lot, but it takes a lot of time to learn to see what they are showing you.

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PostPosted: November 18, 2018, 8:59 am 
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You can figure that stuff out by learning to look at your tires.
Buy some liquid shoe polish, white. Put a few stripes that run across the sidewall to the edge of the tread, like spokes. After a run, look at how far down it's "scuffed" and that'll tell you if the tire is rolling over. If you don't have a pyrometer, at least put your hands on the tread and try to tell if the the inside, outside or center of the tread is the hottest. Cold center is under-inflated. Hot center, vice-versa. Inside hot usually means too much camber. Outside hot in front usually is understeer. And so on... It's always a bit of a guessing game. For me, at least!

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I did that for JD (I think it was him) some years back.
Yeah you did, Chuck. I remember that, it was kinda cool!

:cheers:
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PostPosted: November 20, 2018, 1:19 am 
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Thanks guys.

Greg: If a 4" RC is what I need, I'm boned. Mine is near the center of the axle so 10-11". I'll have to look but I don't think I'll be able to lower it much w/o replacing everything from bulkhead back, or having a very short panhard. Lower arms are level iirc, upper arm is slight down for anti-squat.

Question to anyone: If high rear RC is the problem but is not viable to adjust, what can I do to compensate? I've been reading conflicting information on this.

Chuck: Yeah, when I do the test n tune track day I'll see if I can convince my Dad to photo/film the car with that in mind. Though the question remains: What is an acceptable amount of roll for a Seven? My Yaris has too much since it has no camber gain to compensate and uses the outside 1/3 of the tire. I guess I need to measure and find out what my Seven's camber gain is, then ensure that the body roll stays less than that through the entire travel?

Marcus: Looking at tire wear I am probably still a pound or two high, but close. I think I will gain from more neg camber F and lower press a little more. Rear is hard to tell from the sliding/wheelspin. I'll do better about testing next time.

JD: Reading tires is the one thing I do know :mrgreen: Thanks for the advice though. I use grease pens as they are less messy.

Thanks again for the help. So the plan is: measure my geometry, see if lowering panhard is viable, check camber gain and adjust static camber to suit. Hoping to get to shop next weekend; health permitting.

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PostPosted: November 20, 2018, 9:42 am 
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In order to see if what I'm going to say is relevant, I'd have to go back and look at your build log to see how your Panhard rod attachments are located on both the chassis side and axle side. I'm too lazy to do that this morning. So, at the risk of embarrassing myself, here goes.

If lowering the rear roll center is the issue then lower the chassis-side and axle-side Panhard rod mounts. The roll center for a Panhard rod is where the rod crosses the centerline of the car. Drop it low! If you're only using the car for autocross, you can consider running the Panhard rod under the bottom chassis rail for length, if necessary. That would be a problem on a normal street car subject to the real-world road, but OK on an autocross car that's only going to see smooth surfaces like an asphalt parking lot, no bumps, and very little chassis roll.

Cheers,

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Damn! That front slip angle is way too large and the Ackerman is just a muddle.

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PostPosted: November 20, 2018, 10:35 am 
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Here are some examples of measuring roll from (ahem) 10 years ago.

viewtopic.php?f=5&t=4461&hilit=roll+autocad&start=42

And some later work for JD:

viewtopic.php?f=35&t=8526&start=2113


JD's measurement was initially flawed. It required a flat (driving straight or sitting still) reference view to zero out the angle of the camera.

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