What the title says.  Speaking about an IRS trying to put the power down in a straight line (drag race, etc).  I mean the grand total amount of negative camber relative to the road surface.  It's increasing as the body rears back and compresses the rear suspension. 

When does a (typical street radial) tire's straight-line grip really start suffering?  2 degrees?  4 degrees?  Does the problem steadily get worse as the camber steadily increases?  Or does it start worsening more rapidly beyond some tipping point?  

I guess I'm asking, how bad is bad for this issue?    

(Whoa, I just now saw your last comment from weeks ago)

A single curb or pothole can cause a very expensive day when there is nothing but metal between the wheel rim and the car's frame.  That issue alone is enough to make me want some cushioning.  It's less of a desire for comfort and more like a plain old durability thing from my POV.

I totally get why nobody likes bushings on the track.  Different situation from public streets.  


The OEMs seem to mainly adjust bushing hardness by bushing size.  Bigger = softer.  They seem to design more softness for wheel movement in the front/rear direction.  For lateral movement they are less generous with the rubber.  I'm following that plan. 

Between 2nd-gen Camaro bushings and Mustang-2 stuff, I think I've got the bushing thing worked out now.  Fingers firmly crossed.  And toes.  

 

The front T-bars are archaic in the sense of weight and a bit more LCA complexity.  On the other hand they move a lot of loadbearing structure down low & towards the center of the chassis.  Mopars don't need the kind of huge shock tower bracing that early Mustangs did.  They also have more room for wide motors.  

The T-bar's chassis mount location being farther inwards from the wheels (particularly in comparison to the widely-placed rear leaf spring mounts) also helped create a natural understeering bias.  That natural bias in the corner springs alone probably matters more when bean counters aren't allowing swaybars on most cars.

The biggest high-speed handling drawback of the T-bar setup is probably the bumpsteer issues caused by the T-bars occupying the space where the center steering link probably needs to be.  

For racing purposes the T-bar setup's biggest PITA is that it's just difficult to tune & modify.  For a set-it-and-forget-it street car it's fine.  But you can't move the LCA chassis mounts around, you can't swap wheel springs very quickly, they don't make a whole lot of spring rates for it, the under-chassis T-bar anchoring point limits how low you can set the chassis ride height, etc.    


The UCA's major anti-dive angle probably contributes to the bumpsteer problem too.  The upper BJ is moving all over the place between droop and compression.  Hotchkiss (the aftermarket company, not the suspension design) sells an UCA conversion dealie that basically flattens out the UCA's chassis mounts to remove the anti-dive.  


As for the camber geometry?  You're right, the camber gain is not ideal for modern tires.  It isn't ideal on half the modern sporty cars out there today either.  The old Mopar front end was designed during the Eisenhower administration and it was subject to Detroit bean-counter approval at the time.  IMO it has aged pretty darn well in light of all that.    



Wasn't the Jag IRS rubber chassis mounting an original pa

Thanks.  

The platform does have a lot to like about it in general.  Mopar unibodies are surprisingly rigid for that era.  And durable as heck.  If you can get one that isn't either rusted apart or too rare & valuable to drive then it's a great car to fling around.  

The B/RB motors are hard to dislike too.  They sound awesome even in moderate tune.  Their torque outputs are unreal for the displacement, especially the 383s.  They are heavy MoFos though.  

I'd love to run an aluminum block but it costs something like $4500 and weighs about 80 lbs less than stock.  That's hard to justify.  The math would look better if I was forced into some kind of aftermarket block by sheer power levels.  The aftermarket iron blocks cost like $2500.  And they are so heavily reinforced that they weigh more like 170 lbs above the aluminum blocks.  But I'm not building a hot enough motor to break stock blocks so I'm sticking with them for now.  


There is one issue I see with bracing the diff to the front leaf spring eye area: running a diagonal brace across the muffler location makes it hard to put a muffler there.  I'm unsure how the KA Mustang setup was originally going to deal with that problem when it was being developed for mass production.  Aside from that, the KA setup looks like it uses the available space under a leafs/axle chassis extremely well.   



 

The car is a classic Dodge Charger.  Unibody chassis with the typical Hotchkiss axle/leaf setup from the factory.  

The length of the watts links on the Arning/Carling T5 setup appears to have been governed by the early Mustang chassis.  The front link re-used the mount for the stock leaf springs and I imagine that largely determined the length of both links.  My own experimenting leads me to think the two watts links need to be pretty close to equal length, be it ideal or not.  Significant mismatching causes more harm than good even when it allows one link to be closer to ideal. 


 

        
The only possible benefit that I see for a 5-link (instead of double A-arm) is that you can get more creative with the wheel's movement path in the side view.  That's what I started this thread in attempt to hash out; what the ideal side-view movement path is.



21st-century sporty cars all have big grippy tires.  The body motions are all pinned down with heavy swaybars & shock valving.  It works, but it's a different kind of chassis from what I'm putting together in general. 

I AM wanting to build a softly-sprung car with small tires.  15x8 wheels & street radials.  Rubber bushings & ball joints.  Ride height is on the tall side for a sporty car.  

The front end of the car has strong "stabilizing" geometry from the factory:  The front UCA has a big anti-dive angle.  The front roll center is about 6" off the pavement (which seems kinda high to me).  I've been thinking that continuing this general outlook on the rear end would produce a well-sorted & predictable street car.  Use heavy stabilizing geometries in my IRS design.  

I'm thinking of making the rear roll center around 9-10" high.  Yeah, I know that sounds like skyscraper height.  But the RC on the stock Hotchkiss leaf setup is around 14-15" so it's still a big step down.  The car has a longish wheelbase too.  If I set the rear RC below 9-10" then the overall front/rear roll axis line is virtually parallel with the ground.


The motor is a big-block V8.  There is no hope of hooking it up very well and I'm okay with that.  I just want to lead-foot around on the street comfortably.  The rear end preferably doesn't squat into the ground upon throttle inputs.  And no big wheel-hopping.  Ideally, when you give it gas, the car either launches smoothly forward or else the rear tires spin.  The goal is a factory-style IRS for this kind of muscle car during about 1975-1995.  I want to use modern hub bearings & CV's instead of Corvette/Jag half shafts, but that's abou

Thanks for the responses guys.


Tyrellracing - 

Does the Jag setup have the wheelhop problem because the upper end of the spindle/carrier is not tied down well enough (no UCA)?  That seems to be the case with the simple upper arm on the T-bird version too.


About your K/A watts link addition, 

If you mounted the upper pivot point equally far from the wheel hub as the LCA mounts, then wouldn't you basically be eliminating the signature K/A parts of the deal?  Taken together with the existing forward longitudinal link, the wheel's up/down path with would come out as roughly a straight line when viewed from the side.  (Maybe not straight vertical, but a straight line.)  

I'm not trying to criticize what you did, I'm just trying to understand it.



I am in the process of designing & building my own IRS right now.   It's more of a DIY than a modified version of any single OEM setup.  I really like the watts-type setups that move the wheel backward during squat & twist the hub for anti-dive during braking.  But I am struggling to come up with a layout of hub carrier & links that do very much of that.  

Basically, I can't put much curving & rotating in the hub & carrier's movements because of the packaging limitations of the whole thing.  The squat/dive action depends on the upper longitudinal link being mounted right near the hub, and the lower link being far below it.  The hub & wheel rim dimensions don't allow for very much of either. 

I've picked apart some online photos of Duane Carling's KA setup.  Even there, I only see perhaps 11" of total spacing between the upper & lower link, and the center of the hub is perhaps 3" below the upper link.  When I map out those movements on paper it isn't very impressive.  Is this really what all the fuss is about?   The hub & carrier's travel paths are still hardly curving & rotating at all in the big scheme of things. I wonder how it could be worth the trouble compared to more traditional common IRS setu

Hi all.  Longtime lurker.  Occasional poster. 

I'm kicking around designs for a muscle car IRS retrofit. Not any specific existing IRS being transplanted (Jag, T-bird, Cobra, etc.)  More of a clean-slate job, probably using Ford 8.8/Cobra drivetrain parts.

Right now I'm struggling with ground clearance & wheel travel.  As best I can figure, this issue is a crappy compromise with pretty much any IRS compared to a solid axle.  


A (real street) vehicle gets designed by the factory to remain safe all the time.  An OEM suspension can be compressed VERY hard without the undercarriage hitting the ground.  Suspension arms hitting the bumpstops, tire sidewalls mashed flat, etc.  But this is only easy with modern wheels (rims).  

With old muscle car wheel dimensions, this becomes a problem.  The short (diameter) rims allow the chassis to fall very low when it's bottomed out & tires compressed.  That calls for the differential to be mounted higher up in the chassis for clearance.  But the wide offset (low backspace) of old wheels makes the half-shafts shorter, which calls for a lower diff location.  

Therefore, when it comes to using muscle car wheel sizes with an IRS  .  .  .  . 

--  good wheel travel
--  mild CV angles  
--  the differential cannot whack the pavement

Pick any 2 of the 3.  


The only fix I can think of is using CV half-shafts capable of larger angles.  The standard ones must be kept less than 10-12* ride height and maybe 20* in brief extreme swings.  The inner "tripod" plunging end is always the hangup.

But after a couple days of internet surfing I have not found any hint of a solution here.  The half-shaft customizing activity all falls into two groups: #1 stronger versions of the stock shafts, and #2 higher-angle shafts that eliminate the telescoping feature (with the whole suspension being redesigned for that).

Do I have an accurate picture of the situation here?  Any feedback?  

Thanks.  

  

Hey all.  First post.

I am coming off years of experience with RWD muscle cars, mainly old 60s/70s stuff.  I am considering trying to custom-fab an IRS for a car of this era.  But I am wondering about how it changes the feel of the car.  The only RWD IRS cars I have had were very modern.  They are not a very viable comparison because there is way too much else about the car that was too far removed from an old musclecar. 

I really love the old-school feel of the 60s/70s stuff. Very mechanical, you feel the torque . . . they don't feel like "the car is an extension of the driver" at all, they feel more like you are riding a bull.  Modern cars are so fast but they manage to bore you while they do it. 

I guess I am wondering how much of that feeling is related to the SLA setup, between the torque twist upon hitting the throttle, to the heavy unsprung weight in the back, etc.  I wonder if I will kill too much of what I like about the old car to update it to an IRS. 

Who has done this kind of conversion and lived to tell about it?  Did it "modernize" the feel of the old car dramatically?  Can you describe it? 


I am picturing an IRS that feels factory built, if that makes any sense.  Rubber bushings on the arms, no super lightweight fragile components, ball joints rather than heims, lots of ground clearance and wheel-travel like the stock SLA had, etc.  When I say "converting to IRS" I don't mean taking the whole chassis & car in a different direction while I'm at it.

Thanks for any feedback.