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Four Link + Panhard Bar- Thoughts

74
93
Exp. Type
HPDE
Exp. Level
Under 3 Years
Michigan
Buying this Mustang and building the tube chassis for my Falcon has sent me down a rabbit hole of trying to understand suspension geometry, dynamics and kinematics. In trying to gain a better understanding of why the Mustang handles the way it does I made a quick sketch which shows the difference in roll center and roll axis with and without a Panhard bar. The MM site does little to help someone like me understand why...maybe because nobody else cares, haha. I've never been a 'just tell me what parts to buy' kind of guy, though, I know plenty of people who are, and they seem much happier 🤣.

And, as much as I want to share what I think I've figured out, I'm hoping someone might have some corrections to make to my understanding if it is indeed wrong.

Anyway, it seems most people's understanding or explanation of why and how the Panhard bar improves the handling is "it makes the car feel more precise" or "it keeps the axle centered under the car better". All fine and true, but I'm more curious than that. Why?

Using a plumb bob last year I plotted all the connection points of my rear suspension and put them into CAD to see if it would give me some clues. [I'll be doing this with the front suspension as well in the next couple of weeks]
IMG_6935.jpg

You can see without the Panhard bar, the roll center is around 18" high, and the roll axis of the rear axle is about 4.7º which would seem to be the cause of the roll steer, as the outside tire would move forward and the inside tire moves rearward...in addition to the axle drifting side to side. On my car, which is lowered, you can also see how the upward inclination of the RLCA's (body to axle) affects anti-squat and shortens the lift point/SVSA (side-view swing-arm).

If the roll center height defines the length of the moment arm, then I think of it like the amount of leverage that the center of gravity has to roll the body of the car around the axles. Without the Panhard bar, the rear roll center is 18" and the COG is approximately 20". So, aside from the rear suspension being a big ol' bag of BIND, the COG only has a ~2" lever to try and force it into roll compliance. Right? So, once the available leverage has squashed the springs and the rubber bushings as much as it can, it goes solid, and the rear stiffness goes to infinity. But the interesting thing is that it doesn't get loose like popular knowledge would suggest the tighter end of the car would do. And I suspect the stored energy in the springs and bound up rear suspension is part of the cause of the 'snap' oversteer.

At the same time, the front of the car, especially when it's lowered, has a much longer moment arm (more leverage) and a bit more weight on it.

Once the rear suspension is sufficiently bound up, the front suspension absorbs the weight transfer. Since the front springs are absorbing the weight transfer, a softer setup is employed to lessen understeer. Which leads to excessive body roll which leads to tons of static camber. The rear suspension really is the crux of the issue. The bandaid is to stiffen the rear sway bars to try and induce some oversteer to balance the car out. But ideally, taking grip from one end of the car to fix the other is not the best solution. I'd rather add grip to the front to balance the car.
IMG_6934.JPG

The roll center when a Panhard bar or Watts link is employed is the center of the bar or the center of the bell crank on the watts. So, add the Panhard bar, and the roll center is now ~9". The COG now has an 11" moment arm. Now the COG has a lot more leverage to roll the car around the rear axle. Now, the rear suspension takes up some of the roll moment of the car when you go into a corner reducing some of the work the front axle has to do and bingo bingo, understeer is reduced. And now you can start to stiffen things up, reduce body roll, and maintain better wheel alignment in cornering.

Here's the part I'm less sure of- On a three link, the roll center and the convergence point of the RLCA's defines the roll axis. I do not know for sure and cannot find a solid answer to whether this is true or not when the triangulated 4 link is still installed. If yes, then the rear axle's roll axis is greatly affected, causing it to go about -1º, which would cause a bit of roll counter-steer. Which would explain the reduced roll steer.

The last thing to mention is Roll Axis Inclination. Which is the imaginary line that intersects the front and rear roll centers and is the imaginary axis the the car rolls around. This photo represents a good starting point for roll center heights on a RWD, Non-Aero race car.
Screen Shot 2021-12-09 at 9.44.08 AM.png
[Image borrowed from https://suspensionsecrets.co.uk/roll-centre-and-roll-moment/]

So, if you consider the stock mustang has about a 90% rear roll center height and it's essentially below ground in the front once you lower it on the stock k-member...you can see how it's backwards of the recommended starting point and by a lot.

Anyway, once I get the front suspension plotted and input into CAD I'll be able to show you guys what the roll axis inclination looks like without the Panhard bar, with the Panhard bar and with the corrected front roll center.

Brad
 
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Frank.JD.Perez

FJD Performance
119
138
Exp. Level
5-10 Years
Pleasanton/Hayward, CA
in all fairness, the fronts of our cars are pretty good. its the rear that needs all the help it can get. a good topic for us though would be mcpherson strut vs SLA
*grabs popcorn*
*throws out popcorn because i dont like popcorn*
*grabs candy instead*
 

ChrisM

Mostly harmless.
986
1,083
Exp. Type
HPDE
Exp. Level
3-5 Years
SoCal
in all fairness, the fronts of our cars are pretty good. its the rear that needs all the help it can get. a good topic for us though would be mcpherson strut vs SLA
*grabs popcorn*
*throws out popcorn because i dont like popcorn*
*grabs candy instead*
*Picks up popcorn because I'm cheap, settles in for the debate*
 

Grant 302

basic and well known psychic
I've never been a 'just tell me what parts to buy' kind of guy, though, I know plenty of people who are, and they seem much happier 🤣.

😆

I applaud anyone who actually does the work to figure things out.

So many just watch YouTube or read an ‘article’ or disguised advertising and believe all the crapola that they’ve been fed.
 

Ludachris

Chris
Staff member
Moderator
1,322
1,411
Exp. Type
HPDE
Exp. Level
5-10 Years
Newcastle, CA
So many just watch YouTube or read an ‘article’ or disguised advertising
Ever notice how many 'articles' out there are simply disguised advertising these days? I look up so many things trying to get information about products or how to do something and the majority of the time I come across sites disguised as content, but are really just built to collect affiliate link revenue, full of fluff articles that someone on the other side of the world wrote with a little bit of direction. Ughhh...
 
39
42
Exp. Type
HPDE
Exp. Level
10-20 Years
San Jose
Buying this Mustang and building the tube chassis for my Falcon has sent me down a rabbit hole of trying to understand suspension geometry, dynamics and kinematics. In trying to gain a better understanding of why the Mustang handles the way it does I made a quick sketch which shows the difference in roll center and roll axis with and without a Panhard bar. The MM site does little to help someone like me understand why...maybe because nobody else cares, haha. I've never been a 'just tell me what parts to buy' kind of guy, though, I know plenty of people who are, and they seem much happier 🤣.

And, as much as I want to share what I think I've figured out, I'm hoping someone might have some corrections to make to my understanding if it is indeed wrong.

Anyway, it seems most people's understanding or explanation of why and how the Panhard bar improves the handling is "it makes the car feel more precise" or "it keeps the axle centered under the car better". All fine and true, but I'm more curious than that. Why?

Using a plumb bob last year I plotted all the connection points of my rear suspension and put them into CAD to see if it would give me some clues. [I'll be doing this with the front suspension as well in the next couple of weeks]
View attachment 70795

You can see without the Panhard bar, the roll center is around 18" high, and the roll axis of the rear axle is about 4.7º which would seem to be the cause of the roll steer, as the outside tire would move forward and the inside tire moves rearward...in addition to the axle drifting side to side. On my car, which is lowered, you can also see how the upward inclination of the RLCA's (body to axle) affects anti-squat and shortens the lift point/SVSA (side-view swing-arm).

If the roll center height defines the length of the moment arm, then I think of it like the amount of leverage that the center of gravity has to roll the body of the car around the axles. Without the Panhard bar, the rear roll center is 18" and the COG is approximately 20". So, aside from the rear suspension being a big ol' bag of BIND, the COG only has a ~2" lever to try and force it into roll compliance. Right? So, once the available leverage has squashed the springs and the rubber bushings as much as it can, it goes solid, and the rear stiffness goes to infinity. But the interesting thing is that it doesn't get loose like popular knowledge would suggest the tighter end of the car would do. And I suspect the stored energy in the springs and bound up rear suspension is part of the cause of the 'snap' oversteer.

At the same time, the front of the car, especially when it's lowered, has a much longer moment arm (more leverage) and a bit more weight on it.

Once the rear suspension is sufficiently bound up, the front suspension absorbs the weight transfer. Since the front springs are absorbing the weight transfer, a softer setup is employed to lessen understeer. Which leads to excessive body roll which leads to tons of static camber. The rear suspension really is the crux of the issue. The bandaid is to stiffen the rear sway bars to try and induce some oversteer to balance the car out. But ideally, taking grip from one end of the car to fix the other is not the best solution. I'd rather add grip to the front to balance the car.
View attachment 70794

The roll center when a Panhard bar or Watts link is employed is the center of the bar or the center of the bell crank on the watts. So, add the Panhard bar, and the roll center is now ~9". The COG now has an 11" moment arm. Now the COG has a lot more leverage to roll the car around the rear axle. Now, the rear suspension takes up some of the roll moment of the car when you go into a corner reducing some of the work the front axle has to do and bingo bingo, understeer is reduced. And now you can start to stiffen things up, reduce body roll, and maintain better wheel alignment in cornering.

Here's the part I'm less sure of- On a three link, the roll center and the convergence point of the RLCA's defines the roll axis. I do not know for sure and cannot find a solid answer to whether this is true or not when the triangulated 4 link is still installed. If yes, then the rear axle's roll axis is greatly affected, causing it to go about -1º, which would cause a bit of roll counter-steer. Which would explain the reduced roll steer.

The last thing to mention is Roll Axis Inclination. Which is the imaginary line that intersects the front and rear roll centers and is the imaginary axis the the car rolls around. This photo represents a good starting point for roll center heights on a RWD, Non-Aero race car.
View attachment 70796
[Image borrowed from https://suspensionsecrets.co.uk/roll-centre-and-roll-moment/]

So, if you consider the stock mustang has about a 90% rear roll center height and it's essentially below ground in the front once you lower it on the stock k-member...you can see how it's backwards of the recommended starting point and by a lot.

Anyway, once I get the front suspension plotted and input into CAD I'll be able to show you guys what the roll axis inclination looks like without the Panhard bar, with the Panhard bar and with the corrected front roll center.

Brad
I'm not sure about that "recommended" RC height as shown in that photo. Maybe for a heavy rear weight bias car like the Porsche, definitely not what you want for a front heavy Mustang. You will want a higher rear RC. In the 9" range sounds much closer to what you will see lots of "race cars" running.

The quadra bind has a lot of issues in itself, it may be hard to dial it down to one thing.. Rather a lot of compounding pieces to the problem. LOL
 

Dave_W

Cones - not just for ice cream
450
449
Exp. Type
Autocross
Exp. Level
20+ Years
Connecticut
For a triangulated 4-link, the lateral movement of the body relative to the axle is controlled by the geometry of the trailing arms. For a parallel 4-link, or 3-link, you need another element to control the lateral movement - panhard bar, Watts link, etc. If you add panhard, Watts, etc. to a triangulated 4-link, you have two different lateral-locating geometries potentially fighting each other, and are almost guaranteed suspension bind all the time. The "cure" for this can be to use bushings in the angled arms that are soft enough to eliminate their ability to control the lateral movement, leaving it up to the panhard, Watts, etc.

You say the body is rolling around the axle, but it's really "trying" to roll around the roll center. If the roll center is the same height as the center of gravity, all the lateral weight transfer (force) in a corner gets transferred from the sprung mass to the contact patch through the suspension links, and none of it goes through the springs, shocks, sway bars, etc. As the RC moves away from the CG, more of that weight/force/load transfer goes though the springs, shocks, sway bars, etc.

I'm still getting my head around this, but the springs, shocks, sway bars absorb some of the energy of the roll from the time it starts to when it reaches steady-state - shocks especially can be used to "time" the weight transfer in transient maneuvers.

Another concept to remember is that increased downward force (weight) on a tire will increase it's grip, but not in a linear relationship. (Pulling numbers from the air as an example) if you double the weight on a contact patch, it's grip may only increase 70%. So a car will have it's most total grip when it has no lateral or longitudinal forces acting on it. As soon as you accelerate, brake, or turn, you're transferring weight/load from some contact patches to others, but because of the non-linear relationship between weight and grip, the total grip of the car will be less. A great deal of suspension tuning deals with managing that loss in grip - where and how it happens.

So, (assuming your calculations are correct) in your first case with the rear RC very close to the CG, but the front RC a good distance away, the rear suspension transfers the force of lateral g-load acting on the sprung mass almost immediately to the tires, taking away downward force and grip from the inside tire and adding (less) grip to the ouside tire. Meanwhile, the weight transfer across the front wheels is being temporarily absorbed by the squishy parts of the front suspension, so the inside tire's contact patch keeps more grip until the sprung mass reaches steady-state roll. The net effect of this is snap-oversteer on turn-in.

To cure the snap-oversteer, you need to get the lateral load transfer timing better matched front-rear. But since the vast majority of the lateral load transfer in the rear is happening directly through the suspension links, tuning with the squishy bits has little effect. You need to get the RC further from the CG so you actually have some reasonable amount of the transfer going through the the parts you can tune.
 
74
93
Exp. Type
HPDE
Exp. Level
Under 3 Years
Michigan
in all fairness, the fronts of our cars are pretty good. its the rear that needs all the help it can get. a good topic for us though would be mcpherson strut vs SLA
*grabs popcorn*
*throws out popcorn because i dont like popcorn*
*grabs candy instead*
I appreciate your comment because it forced me to get out to the barn with the plumb bob tonight and take some measurements and plug them into the Racing Aspirations Calculator. Again, not an expert. I just took an interest in this and have spent the last two years, on and off, reading about this stuff and I'm starting to put some things together...I think.

So, for the stock suspension- I guess it depends. Lowered? Stock K member and pickup locations? In that scenario the roll center is below ground and you're maxed out at like 6º of caster. You're pretty much left to running tons of static camber to keep from going positive while cornering. The MM stuff helps. Raises the connection points on the k member and moves the ball joint forward so you can run more caster and improves Ackerman.

Theres also the lateral force going through the LCA's to consider. Whether that LCA is pointed upward or downward will have a compression effect or a jacking effect. So, especially in the case of the lowered car where the LCA's are already pointing downward to the chassis before you dive into a corner, now they really are. And geometrically, the force going through the LCA on the outside wheel is now pressing downward on the chassis, inducing more roll.

There are plenty of reasons to go SLA. More adjustability. Probably a better (possibly adjustable) KPI in the aftermarket uprights. To run wider wheels and tires on our cars you have to run deep dish wheels which moves the brakes further from the outside of the wheel. So you have the opportunity to increase track width without burying the brakes and ending up with too much scrub. Better selection of performance shocks and springs? Wheel rates? Ability to adjust/alter anti-dive. Just thinking out loud here. It just depends. I'd probably have to say a well designed SLA would be the "better" suspension for a serious racer for all the reasons mentioned...but its a matter of $$. I don't personally have the budget for that nor am I running in a competitive series that would justify it. Not to mention the constant testing and tuning for different combos, different tracks and different setups. Though, Matt James (@Regularassmustang) is killing with his SLA Cobra. But also look how many guys are way faster in their CMC car than I would be in Matt's Mustang, ya know?

Run whatcha brung! I was just theorizing on why the Panhard bar improves this rear suspension when logically it seems like it should put it further into bind, haha!

Now, this is on my car, which has the MM k member and LCA's in the upper holes and a tall enough ball joint to still have a 2º angle from car to spindle. But here is the rundown...

We start with -3º static camber.
Screen Shot 2021-12-09 at 8.14.51 PM.png

2" straight compression: -4.6º camber.
Screen Shot 2021-12-09 at 7.59.52 PM.jpg
Roll. With 3.6º of body roll and 2" compression on the outside tire your camber on that wheel is now -1.5º. The thing with this is, it doesn't take caster into account. So, you might have 10º of steering input here, and that would equate to some negative camber gain.
Screen Shot 2021-12-09 at 8.00.19 PM.jpg

The model is neat but I'm not sure how helpful it really is considering there's caster involved in the real world. And again, I'm just figuring this stuff out. I've done zero testing to find out if all the 'theory' equates to faster cars. I can see how the physics make sense, though.

But I certainly am appreciating why real race cars don't have much suspension travel! The transient handling characteristics can become a mess. Those King of Hammer vehicles are all the more impressive!


I'm not sure about that "recommended" RC height as shown in that photo. Maybe for a heavy rear weight bias car like the Porsche, definitely not what you want for a front heavy Mustang. You will want a higher rear RC. In the 9" range sounds much closer to what you will see lots of "race cars" running.

The quadra bind has a lot of issues in itself, it may be hard to dial it down to one thing.. Rather a lot of compounding pieces to the problem. LOL

Yea, and that RRC couldn't be achieved in a stick axle car. Would have to be IRS. So, in the real world of muscle cars, most of them being solid axle, I'm sure they are mostly around 9" as that's about as low as you can go with the Panhard bar.

But on that- it seems to make sense to me that if the center of mass is more toward the front of the car, the lower you'd need the rear roll center for it to be affected equally by the roll moment, especially because there's also a pitch moment as the car transfers weight to the outside front tire. It's sort of counter-intuitive in that way considering the stiffer, lighter axle is generally looser. But in this case, the inability for the rear suspension to absorb any of the roll moment transfers it all to the front axle.

So I sort of see what you're saying. But, the rear engine car also wouldn't technically "need" a lower roll center as the center of mass would be able to more easily effect the rear of the car by virtue of it's location over the rear axle. I'll do some digging and see what kind of roll center heights are to be found on front and rear engine door cars. Wonder what the Australian Super Car's are set up like?

My point really, or my theory, is not the specific height of the roll center, but the effect it has. And that specifically on a suspension that is required to compress rubber bushings in order to articulate, some additional leverage comes in handy and possibly hits the 'bind' point later because of it. Therefore rendering the rear suspension slightly more useful. Bushing compliance as a factor of spring rate is hardly ideal. But it's just my theory of why the same suspension might handle better with the addition of a Panhard bar and lowered rear roll center.
 
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74
93
Exp. Type
HPDE
Exp. Level
Under 3 Years
Michigan
For a triangulated 4-link, the lateral movement of the body relative to the axle is controlled by the geometry of the trailing arms. For a parallel 4-link, or 3-link, you need another element to control the lateral movement - panhard bar, Watts link, etc. If you add panhard, Watts, etc. to a triangulated 4-link, you have two different lateral-locating geometries potentially fighting each other, and are almost guaranteed suspension bind all the time. The "cure" for this can be to use bushings in the angled arms that are soft enough to eliminate their ability to control the lateral movement, leaving it up to the panhard, Watts, etc.
Thanks, Dave. That's some good info and thank you for taking the time to share it!

That's exactly what I was curious about, since the triangulated 4 link and the RLCA's + Panhard bar are now both attempting to define the roll axis. Which is what's confusing. A lot of people run that setup. Like, all CMC Mustangs. The squishy rubber upper bushings are a must in this suspension with or without Panhard, also. So, without PHB, the axle drifts side to side like crazy but without the compliance in the bushings there would be little to no roll.

You say the body is rolling around the axle, but it's really "trying" to roll around the roll center. If the roll center is the same height as the center of gravity, all the lateral weight transfer (force) in a corner gets transferred from the sprung mass to the contact patch through the suspension links, and none of it goes through the springs, shocks, sway bars, etc. As the RC moves away from the CG, more of that weight/force/load transfer goes though the springs, shocks, sway bars, etc.

What I meant when I said the body was rolling around the axle is that the axle stays planted to the ground while the body rolls. Not necessarily that the axle defines the path of roll. But, I think what you're saying is exactly what I was theorizing. By moving the RC further from the CG the moment arm between RC and CG is increased and will have more rotating force in the roll axis
Another concept to remember is that increased downward force (weight) on a tire will increase it's grip, but not in a linear relationship. (Pulling numbers from the air as an example) if you double the weight on a contact patch, it's grip may only increase 70%. So a car will have it's most total grip when it has no lateral or longitudinal forces acting on it. As soon as you accelerate, brake, or turn, you're transferring weight/load from some contact patches to others, but because of the non-linear relationship between weight and grip, the total grip of the car will be less. A great deal of suspension tuning deals with managing that loss in grip - where and how it happens.

So, (assuming your calculations are correct) in your first case with the rear RC very close to the CG, but the front RC a good distance away, the rear suspension transfers the force of lateral g-load acting on the sprung mass almost immediately to the tires, taking away downward force and grip from the inside tire and adding (less) grip to the ouside tire. Meanwhile, the weight transfer across the front wheels is being temporarily absorbed by the squishy parts of the front suspension, so the inside tire's contact patch keeps more grip until the sprung mass reaches steady-state roll. The net effect of this is snap-oversteer on turn-in.

To cure the snap-oversteer, you need to get the lateral load transfer timing better matched front-rear. But since the vast majority of the lateral load transfer in the rear is happening directly through the suspension links, tuning with the squishy bits has little effect. You need to get the RC further from the CG so you actually have some reasonable amount of the transfer going through the the parts you can tune.

Funny you'd mention the tire traction as it just came up in another thread about off-throttle understeer. I happened to have watched a good youtube video called Physics Of Racing where they highlight the bell curve of traction to load in tires. A tire may increase grip to a certain load point but after that it will start to lose grip. Which is what I believe is the case with the understeer in this body style in spite of the soft setup, or at least a contributor.

I've got some more processing to do, haha! Thanks.
 
3,946
4,006
Well FWIW, I'm not all that swift, I kind of have to see things work, so I've built tons of models of suspensions using balsa wood and popsicle sticks. We didn't have all this trick computer stuff growing up, so that's how I did it, only in scale. You know sometimes, simple is the best, if you can't use the PM3L, then I would try and use a 3 link, back in the day, someone actually made one for an SN95, but they are no longer in production. So, instead of redesigning the whole suspension system, and possibly it's attachment points or, you don't want to use a torque arm, I'd pursue figuring out how to build the one upper link, make it adjustable, and leave the star wars stuff for somebody else.
 
3,946
4,006
I've looked around to try and find some pics, to no avail, but as I recall, the bracket bolted to the upper link holes in the diff, and had a bracket that ran down the sides and actually bolted to one of the cast in cooling fins on each side of the diff. IT then had a heim joint and adjustable bar that ran forward, and in this case, into the passenger compartment to the roll cage. A couple of points, here, many sanctioning bodies don't allow suspension points to enter the driver compartment because of safety, if broken, and also to keep fire out, but this guy actually ran it into the compartment so that it would be the same length as the lower arms, and it also ran parallel to them. He then built a containment box around it so the unit had to be serviced from underneath, but the bolt holding it all together was accessible from the top, it was a pretty cool setup.
 
74
93
Exp. Type
HPDE
Exp. Level
Under 3 Years
Michigan
Thanks, @blacksheep-1 , This was a bolt-in three link system that was made by Evolution Motorsports. Someone shared these with me last year. I like the design as it would leave the rear seat pan intact, but there isn't any adjustability in it. Which is sort of my hang-up with the torque-arm as well. It's not that I could out-drive either of these systems at this point, but that I enjoy building things and have spent the last couple of years studying the information available so that I can.

There is another system by Lateral Dynamics, that I think you're referring to. It's an offset three link. I actually spoke with Mark by email and he answered some of my questions while I was designing the rear suspension for my Falcon. Their link runs into the vehicle and the third link pocket is boxed in, like you said. And they have an adjustable and non-adjustable version.

My Christmas gift to myself is Milliken's Race Car Vehicle Dynamics. I don't think it's quite as mystical as it's made out to be. Fancy modeling or spreadsheets definitely help but theres going to be some trial and error, tuning and figuring out what works. I'm like you, and will probably make scale models of this stuff as I go with my designs. Or at least mock it up full scale with cheaper materials before I build the final product as I'm not great with 3D design. It takes me longer to draw it in 3D than to mock it up. And a lot of these parameters are laid out in 2D, anyway. Designing in 3D definitely helps with packaging, though.

E0C59FF6-D184-4EAE-8D0E-F40162F5988E.JPG


07A63DBB-69F4-43B6-89C6-ECEF8C4FD87F.JPG2B8D6A06-3698-4101-840F-2B581EA1F561.JPG

The original point of this post was to throw my theory against the wall that lowering the rear roll center helps to force the 'quad-bind' into articulating. Which, is still my hypothesis as to why adding another constraint to an already over-constrained system actually improves it. I think it gives the CoM more leverage to roll the body of the car, which seems to check out considering that in any car, lowering the roll center softens that end of the car. The Panhard bar also creates a lateral force application point through the rear axle and into the chassis that is more or less parallel to the ground and is also very low.

One day, I will probably fabricate the offset three link for the rear with lots of adjustability. Or, maybe fabricate a light-weight tube-frame IRS. Just taking it a step at a time and there are some items above that on the laundry list. Ultimately, like you said, it will be determined by the rule book of whatever I decide to run in, if I ever decide to go W2W with this car. But I'm leaning toward something like Formula Ford for my sanctioned racing endeavors.

Thanks again, Rob!
 
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4,006
Well, one thing that I have picked up from suspension designs is the A. all the links should be parallel at normal ride height and B. they should all be the same length. I auto crossed an 84 Camaro, with the torque arm, and I really liked it, it was like tuning a piano with a sledge hammer, but it did its' job. You gassed it, the car shot forward, you jumped out of the gas, it planted the nose. Most of the stuff I've been around is all production type cars with mandated factory attachment points, I've always tried to stay away from re creating what some engineer with a cad cam created, although, I will admit, I've had to try and work around some of their screwups. (like the PM3L workaround)
BTW the SN95 roll center at the front is also below ground level, which is why everyone runs the 2x Steeda ball joints. When I was doing this we were so limited at what we could do on the rear of the car, that we concentrated more on the front, it was really magic, those ball joints, a bump steer kit, all the camber the car could stand, and this is a big deal: all the caster you could add to the car because after 4 degrees of caster those cars picked up camber gain during turn in
 
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Thanks, @blacksheep-1 , This was a bolt-in three link system that was made by Evolution Motorsports. Someone shared these with me last year. I like the design as it would leave the rear seat pan intact, but there isn't any adjustability in it. Which is sort of my hang-up with the torque-arm as well. It's not that I could out-drive either of these systems at this point, but that I enjoy building things and have spent the last couple of years studying the information available so that I can.

There is another system by Lateral Dynamics, that I think you're referring to. It's an offset three link. I actually spoke with Mark by email and he answered some of my questions while I was designing the rear suspension for my Falcon. Their link runs into the vehicle and the third link pocket is boxed in, like you said. And they have an adjustable and non-adjustable version.

My Christmas gift to myself is Milliken's Race Car Vehicle Dynamics. I don't think it's quite as mystical as it's made out to be. Fancy modeling or spreadsheets definitely help but theres going to be some trial and error, tuning and figuring out what works. I'm like you, and will probably make scale models of this stuff as I go with my designs. Or at least mock it up full scale with cheaper materials before I build the final product as I'm not great with 3D design. It takes me longer to draw it in 3D than to mock it up. And a lot of these parameters are laid out in 2D, anyway. Designing in 3D definitely helps with packaging, though.

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The original point of this post was to throw my theory against the wall that lowering the rear roll center helps to force the 'quad-bind' into articulating. Which, is still my hypothesis as to why adding another constraint to an already over-constrained system actually improves it. I think it gives the CoM more leverage to roll the body of the car, which seems to check out considering that in any car, lowering the roll center softens that end of the car. The Panhard bar also creates a lateral force application point through the rear axle and into the chassis that is more or less parallel to the ground and is also very low.

One day, I will probably fabricate the offset three link for the rear with lots of adjustability. Or, maybe fabricate a light-weight tube-frame IRS. Just taking it a step at a time and there are some items above that on the laundry list. Ultimately, like you said, it will be determined by the rule book of whatever I decide to run in, if I ever decide to go W2W with this car. But I'm leaning toward something like Formula Ford for my sanctioned racing endeavors.

Thanks again, Rob!
I've run the Fox 4 link with a panhard bar, I have first hand experience how screwed up this is. With the car setting level, you can literally jack up one side of the car by cranking on the PHB, it doesn't move the diff, it moves the right side of the body up. For this reason, I never ran a PHB with the cheesy Fox 4 link, in fact, in order to cancel out some of the snap oversteer as the upper links try to force the diff to center, I used to soak the uppers in kerosene to soften them up so they would kind of pull, rather than yank, the diff to center. (we had to run stock stuff) I also used to pound nails into the lower bushings to make them "harder" so they would do more of the work, offsetting the uppers (the nails would disappear into the bushing, you couldn't tell they were there) Like I said above, fixing engineers' mistakes.

BTW, if you don't have these, you need them in your library..

 
74
93
Exp. Type
HPDE
Exp. Level
Under 3 Years
Michigan
Well, one thing that I have picked up from suspension designs is the A. all the links should be parallel at normal ride height and B. they should all be the same length. I auto crossed an 84 Camaro, with the torque arm, and I really liked it, it was like tuning a piano with a sledge hammer, but it did its' job. You gassed it, the car shot forward, you jumped out of the gas, it planted the nose. Most of the stuff I've been around is all production type cars with mandated factory attachment points, I've always tried to stay away from re creating what some engineer with a cad cam created, although, I will admit, I've had to try and work around some of their screwups. (like the PM3L workaround)
BTW the SN95 roll center at the front is also below ground level, which is why everyone runs the 2x Steeda ball joints. When I was doing this we were so limited at what we could do on the rear of the car, that we concentrated more on the front, it was really magic, those ball joints, a bump steer kit, all the camber the car could stand, and this is a big deal: all the caster you could add to the car because after 4 degrees of caster those cars picked up camber gain during turn in

I've run the Fox 4 link with a panhard bar, I have first hand experience how screwed up this is. With the car setting level, you can literally jack up one side of the car by cranking on the PHB, it doesn't move the diff, it moves the right side of the body up. For this reason, I never ran a PHB with the cheesy Fox 4 link, in fact, in order to cancel out some of the snap oversteer as the upper links try to force the diff to center, I used to soak the uppers in kerosene to soften them up so they would kind of pull, rather than yank, the diff to center. (we had to run stock stuff) I also used to pound nails into the lower bushings to make them "harder" so they would do more of the work, offsetting the uppers (the nails would disappear into the bushing, you couldn't tell they were there) Like I said above, fixing engineers' mistakes.

BTW, if you don't have these, you need them in your library..


Yikes! That's wild! That's a cool trick with the kerosene and nails.

I've decided to just give Maximum Motorsports all my money for this car and save my energy for the Falcon. Hard to do as a fabricator...but, theres no sense in attempting to reinvent something for this car until, I as a driver, warrant it. Just really easy to read too much and go overboard.

The MM stuff is very fairly priced and really good. I don't have any plans to run this car in anything that wouldn't allow the torque arm. I'm coming to terms that this will not be my race car. Rather, a fun track day toy. I'm now looking seriously at formula cars or maybe a radical for some sanctioned racing. I'd rather keep the Mustang streetable and not cut it up too much as to fabricate a 3 link or whatever. Thanks for helping me think it through.

And thank you for the book recommendation. I have 'Drive To Win' and 'Tune To Win' sitting in the Amazon cart already. I'll add this one, too!
 
This is a great thread.

I run CMC with an SN.

I don't know if you care, but I'll give some real life experience with the triangulated 4 link and a PHB.

When I got the car it had aftermarket uppers and lowers, and the rear end was about all I could say was good about the car. After a few races I destroyed one of the uppers in qualifying, after looking it over I figured I could run it and the car was nearly undriveable. At 7/10s it was fine but anywhere near the limit the back end was everywhere. Turns out both uppers were bad, and a visual inspection of the lowers wasn't enough, those bushing went too.

After a new set of soft uppers (MM), and a fairly new set of lowers, the car still drove like crap. Someone mentioned to pull the PHB because "if they aren't exactly right, they do more harm than good."

Once I pulled it, the car was confident again but it lacks grip pretty bad. It feels like it lifts a tire (it felt the same way with the PHB), but it's a very controlled loss of grip. The car oversteers badly coming out of a turn.

I spoke with BS1 on another forum about this, he still recommends a PM3L. An old timer in our group tried out the PM3L some years back and absolutely hated it. He still has the parts for it and offered them up to me. I may give it a shot.
 
74
93
Exp. Type
HPDE
Exp. Level
Under 3 Years
Michigan
This is a great thread.

I run CMC with an SN.

I don't know if you care, but I'll give some real life experience with the triangulated 4 link and a PHB.

When I got the car it had aftermarket uppers and lowers, and the rear end was about all I could say was good about the car. After a few races I destroyed one of the uppers in qualifying, after looking it over I figured I could run it and the car was nearly undriveable. At 7/10s it was fine but anywhere near the limit the back end was everywhere. Turns out both uppers were bad, and a visual inspection of the lowers wasn't enough, those bushing went too.

After a new set of soft uppers (MM), and a fairly new set of lowers, the car still drove like crap. Someone mentioned to pull the PHB because "if they aren't exactly right, they do more harm than good."

Once I pulled it, the car was confident again but it lacks grip pretty bad. It feels like it lifts a tire (it felt the same way with the PHB), but it's a very controlled loss of grip. The car oversteers badly coming out of a turn.

I spoke with BS1 on another forum about this, he still recommends a PM3L. An old timer in our group tried out the PM3L some years back and absolutely hated it. He still has the parts for it and offered them up to me. I may give it a shot.

Thanks for the info and good real life experience. Very helpful. Thanks
 

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