Independent rear suspension system converstion ? - Page 3 - Vintage Mustang Forums

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post #31 of 52 (permalink) Old 04-14-2017, 03:06 PM
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My "plan" if I inherit Dad's matching 67' vert (but 6 cyl) is to buy a used/low mileage Lincoln LS (same engine and IRS as the Cobra ) and do this swap...
I don't know about the IRS, but a Lincoln LS does NOT have a Cobra engine in it. It's a 3.9L version of a Jaguar engine and is notorious for blowing coil packs on a frequent basis. Perhaps you're thinking Lincoln Mk VIII? That had the old late 90's DOHC naturally aspirated Cobra engine in it.
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post #32 of 52 (permalink) Old 04-14-2017, 03:13 PM
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I'm just lol'ing over here about Griggs and affordable being in the same sentence...

I'm unsure on the roll center issue, so take this with a grain of salt, but my understanding is that the RC of an IRS basically functions well in a range that is all lower than the range that a stick axle functions in.... going higher than a certain point causes jacking in the irs, thus you need to be stiffer sprung when using an irs since you can't raise it to the levels that would be considered "low" for a stick...

does that make sense?
Well, price a Roadster Shop IRS vs. a Griggs rear suspension and you'll see what I mean !

An IRS can be designed with a low or high roll center, Though a good design will be slightly higher than the front RCH.

Jacking forces apply to solid axles just as they do with IRS or IFS systems. This is getting past what can be explained in a short forum post, but in either design a high RCH increased the instantaneous or geometric load transfer, and total load transfer, which reduces the overall grip available at that end of the car. It also reduces the load transfer that is resisted by the springs and antiroll bars, making tuning with those components less effective. It is easier to move around the RCH with a stick axle, since you don't have to worry about things like camber gain. Grigg's is correct about the traction issues caused by camber gain, but not with regard to adding enough camber to a solid axle. To achieve the ideal rear camber with a solid axle requires expensive drive flanges or CV joints. Look up what Australian V8 supercars do.

'65 A-code coupe, T-10 4-speed, 8" 3.25 limited slip
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post #33 of 52 (permalink) Old 04-14-2017, 03:25 PM
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I agree, it's complex stuff.... i love pondering it though.

I am not sure i understand how a solid axle would have jacking unless it's using a sloped panhard... otherwise i think it'd be negligible. Either way though, i'm just saying that the irs has to be lower RC than the solid in order to not have the jacking. That "requirement" means you need to be stiffer sprung...

I personally wouldn't spend the money for a cambered stick axle either....

Everything is a trade off, obviously there are great performing versions of both and crappy performing versions of both... getting the right combo to do exactly what you want as a driver is the key.
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post #34 of 52 (permalink) Old 04-14-2017, 04:57 PM
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i'm just saying that the irs has to be lower RC than the solid in order to not have the jacking. That "requirement" means you need to be stiffer sprung...
Sorry, but no, that's not how it works. Google RCH calculation for some more detail and pictures, it's not completely intuitive. Weight "jacking" doesn't care if the car has IRS, IFS, or a live axle, the only thing that matters is the actual Roll Center Height. Any height above ground level will introduce a weight jacking effect. In fact, a height below ground level will have a negative jacking effect, transferring weight to the inside, not the outside of a corner. A panhard bar RCH is not determined by the angle of the bar, but by it's average height (for a symmetrical PHB) at the cars centerline.

So, either system can be designed with a low or high RCH, and with a low RCH either one will require stiffer springs or antiroll bars to achieve the same roll angle. It is true that an Independent suspension can tolerate less roll angle before the camber/geometry gets out of whack, but for a performance car you want to limit roll angle regardless of suspension design for responsive handling.

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post #35 of 52 (permalink) Old 04-14-2017, 05:48 PM
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Well... i'll read up on this more, but there's logically no way for an upward jacking force to be transmitted to the sprung mass due to lateral acceleration in a solid axle suspension EXCEPT for via any non level panhard type locator.

I understand how RC is found, and i understand how jacking forces are transmitted at the angle between the contact patch and the roll center. The variable here is NO linkage that allows those forces to raise the sprung mass unless the panhard is not parallel to the ground.

If i'm wrong, please tell me how, maybe i'm not seeing something correctly.... it's been a while since i read about this stuff lol
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post #36 of 52 (permalink) Old 04-14-2017, 08:42 PM
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Like I said, it is counter-intuitive

You said it yourself, the line of force is from the contact patch to the roll center. If your panhard is level at 12" from the ground, then your RCH is 12". This imaginary point is where the cornering force from the tires is applied to the chassis. This vector is obviously inclined upwards, and has a lateral component equal to 1/2 the track width, and a vertical component of 12". It's been a long time since college, but IIRC this vector (total cornering force) is the sum of the X (1/2 track width) and Y (RCH) components. So the cornering force will be divided between the X and Y components in proportion to their lengths. So if X=12" and Y=36" then the force applied in the X axis is 1/4 of the cornering force. X is your weight jacking component, and clearly not zero.

I don't know if that really helped, but I'll see if I can find something better on the web tomorrow !
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post #37 of 52 (permalink) Old 04-14-2017, 09:25 PM
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Yeah.... OK, so we agree there's a force in play.... but do you agree it doesn't affect sprung mass? the only thing for it to push against is the springs, which is (in theory) significantly different than lifting up the body/chassis and removing spring tension and changing weight transfer like what would happen in an IRS.... I think

I think either way the inside tire will lose some grip, but my feeling is that this is more "stable" than what happens with an IRS.
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post #38 of 52 (permalink) Old 04-15-2017, 07:41 AM
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Yeah.... OK, so we agree there's a force in play.... but do you agree it doesn't affect sprung mass?
No, it does. The rules of weight transfer are exactly the same regardless of suspension design. There is 3 types of weight transfer, unsprung, elastic and inelastic. Unsprung is weight not supported by the suspension, tires, brakes, solid axle, etc. Elastic weight transfer is that portion of the sprung mass which is resisted with compliant components like shocks, spring, and antiroll bars. Inelastic or geometric weight transfer is what we've been discussing, and is weight transfer applied directly to the Roll Center and sprung mass through the suspension links (Panhard bar or A-arms). It is not resisted by the springs and shocks, and does not cause body roll. It's such a powerful tuning aid because it responds instantly to cornering forces, where elastic transfer is spread out over time ( "taking a set" as the car rolls over on the springs ). Only RCH determines how cornering forces are divided between elastic/inelastic WT, not whether it has a Live Axle or IRS. The Panhard bar doesn't push on the springs, but directly on the chassis mass. It is effectively a "pole vault" running from the contact patch to the Roll Center, that is how if lifts or jacks the car up.

One racing truism to remember when comparing IRS to a Live Axle: An imperfect design that is Fully Developed will always beat the perfect design that is Not fully developed.

'65 A-code coupe, T-10 4-speed, 8" 3.25 limited slip

Last edited by stephen_wilson; 04-15-2017 at 07:45 AM.
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post #39 of 52 (permalink) Old 04-15-2017, 08:55 AM
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The Panhard bar doesn't push on the springs, but directly on the chassis mass. It is effectively a "pole vault" running from the contact patch to the Roll Center, that is how if lifts or jacks the car up.
THIS is exactly the point I was trying to make.... my stated assumption was that the bar stayed level... which i understand doesn't exactly happen in real life... BUT when the bar is level, there can't be an upwards force on the body.

Let's replace the panhard with a watts since it can do what my hypothetical panhard cannot...

Still think there's inelastic jacking happening with a watts?
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post #40 of 52 (permalink) Old 04-15-2017, 11:22 AM
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Yes. There are still jacking forces with a Horizontal Panhard bar or a Watts linkage. This is really the hard way to try and learn, I would suggest some Google searches on the subject, and basic suspension design. Really the best way is to buy a book on the subject.

Look at it this way, for there to be zero jacking, the Panhard would have to push on the chassis at ground level. That clearly can't happen, since obviously there is no chassis structure at ground level

'65 A-code coupe, T-10 4-speed, 8" 3.25 limited slip

Last edited by stephen_wilson; 04-15-2017 at 11:32 AM.
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post #41 of 52 (permalink) Old 04-15-2017, 12:24 PM
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OK. I've read quite a bit over the years, and Milliken says that the horizontal/vertical coupling on a stick axle is ONLY via the lateral locating device used. He seems to contradict himself a bit by saying there is ZERO couple when the bar is horizontal but also says that a panhard and watts always have some coupling but that it can be minimized. My assumption on that contradiction was because of the elastic component thru the springs, but not really sure.

I also see where FSAE people are saying FBD shows zero jacking with a stick axle and watts.... I've never done a FBD so no clue if that's a true picture of what really happens or not.

We can drop it though, it was fun to talk about either way, i know you have a background in racing so I was hoping for a little more than "go read a book" to prove how the forces can create vertical lift on the sprung mass when they travel along a horizontal plane.

Have a good weekend!
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post #42 of 52 (permalink) Old 04-15-2017, 07:12 PM
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I didn't mean to brush you off, but we're at the point now where I need to dig up some reference material. It's been some time since I last read them, and I was more focused on SLA suspensions than solid axles. I just don't have the time right now, I'm trying to get my race car together before I miss too much of this season ! If I get a chance to dig up more on the subject I'll be sure to post an update.

Have you read Milliken's Race car Vehicle Dynamics ? I found much of it a difficult read, and a bit math intensive. I've been out of school too long !

'65 A-code coupe, T-10 4-speed, 8" 3.25 limited slip

Last edited by stephen_wilson; 04-15-2017 at 07:16 PM.
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post #43 of 52 (permalink) Old 04-16-2017, 08:54 AM
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Not cover to cover. Just used as a resource when i have specific questions.

I still have to assemble my engine for this year... I may end up throwing some decent tires on my '14 if i get too lazy to put the 68 back together lol

Good luck with your car!
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post #44 of 52 (permalink) Old 04-16-2017, 09:48 AM
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He seems to contradict himself a bit by saying there is ZERO couple when the bar is horizontal but also says that a panhard and watts always have some coupling but that it can be minimized.
Do you remember where you found this comment? I haven't dug into Milliken yet, but I did check a few other books this morning.

Herb Adam's comment applies to the OP's question: "a well-designed and well-developed live axle will beat a poorly designed independent rear suspension, even on rough roads. On smooth roads, is is usually difficult to see any advantage for an IRS." He also advocates a low rear roll center, which admittedly can't be as low as a SLA design.

After reading Herb Adams, Alan Stainforth, and Carroll Smith, none directly address our question. Carrol Smith actively hates solid axles! The common theme: None go into detail about an angled PHD, (I assume because it's just poor design) other than to say it increases the asymmetry inherent to a PHB.

My take: I think jacking effects are always attributed to Independent suspensions because they can be more severe and progressive, think swing axle. They also allow you to place the RCH almost anywhere, including very high, while it's hard to more a solid axle RCH outside a limited range.

Why I think they have weight jacking: We both agree a SA has a RCH that can be varied. The definition for RCH does not change for a front or rear suspension, so the function is the same. Smith: " ... is that point about which the sprung mass will roll under the influence of centrifugal force... the point through which the lateral forces from the tire's contact patches act upon the chassis. " Dynamic Load Transfer: " load transferred ... due to moments about the CofG or it's Roll Centers. " He then talks about weight jacking in the IFS section, which every author does, I think creating the common perception that it's only an IFS/IRS problem. Remember, authors are not perfect, contradicting themselves, especially if we take every sentence a gospel. " If the roll center is above ground, then the line of action between the tire contact patch and the Roll Center will be inclined upwards toward the vehicle centerline. ...the side force developed will have a vertical component which will lift or "jack" the unsprung mass."

So, a solid axle has a Roll Center, and it is above ground, and Physics don't change with suspension design, so it must have a vertical component. To prove this assumption, changing the RCH of a PHB changes the handling balance and roll force distribution. The only way it can do this if it takes load OFF of the springs. How does it transfer this load? From the chassis connection to the contact patch. Connect the dots and you have your inclined force line and jacking. Think of it as a J-bar, it doesn't matter that it's curved, the force is still transferred in a direct line from the two end connections. This is analogous to the contact patch and the PHB chassis mount. The actual load path may be up through the tire/wheel, to the axle/PHB axle mount, PHB to chassis, but the force vector is still a straight line. In your assumption with a flat PCH, where does the load go, it can't end at the axle side, because is has no connection to the ground. It has to pass down through the wheel and tire.

There, you officially got my longest post on any site, by a long shot !

Happy Easter, I'm going to Brunch !
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post #45 of 52 (permalink) Old 05-13-2017, 05:30 PM
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I really want a cobra IRS


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