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Independent Rear Suspension Forum » How much neg camber starts costing a lot of straight-line grip? » 1/14/2018 3:10 pm

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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?    

Independent Rear Suspension Forum » seeking (rear) roll center ideas . . . . » 10/11/2016 1:42 pm

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What the title says.  I intend to make it adjustable, but what about a broad range?  

I think I understand roll centers adequately.   The front & rear RC's form a "roll axis" line upon which the body rolls.  The distance from the CG to the RC is the leverage that the body's weight has on the springs/shocks when it leans.  The roll axis line is better off sloping downwards towards the front of the car to some degree, so that it oversteers more in transient moves and less in steady-state cornering.  Solid axles don't have the kind of jacking problem that IRS's do when the RC is high. Jacking gets worse (on IRS's) as the angle gets steeper from the tire contact patch going up to the RC.  

The vehicle - 

It's a 100% street car.  Mid-size old musclecar chassis.  Street radial tires and the chassis isn't very stiff or low in the big picture.  Tires are perhaps 255/60/15.  

Front RC  - about 3" above ground.  
Rearend track width - 59" (center of tread).  
Tires - 27" tall.  
Wheelbase - 117" (kinda long for a sports car.  That would make the overall roll axis flatter for a given difference in F&R roll center heights. )

It was a solid axle & leafs car before the IRS conversion, with the factory rear RC probably 15" off the ground.  Of course that is too high for an IRS but I don't want to go too low with it now.  I seem to prefer it on the high side.  

How high is too high for this?  6"?  8"?  10"?   Any suggestions?   


Independent Rear Suspension Forum » Caster/anti-squat? layouts? Jag/T-bird, Klaus Arning Watts, SLA . . » 9/18/2016 12:03 am

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(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.  


Independent Rear Suspension Forum » Caster/anti-squat? layouts? Jag/T-bird, Klaus Arning Watts, SLA . . » 8/27/2016 9:10 pm

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Yes I'm aware of rubber's consistent hardness.  

The OEMs do a lot of different stuff with bushings.  Putting strategic holes in the rubber, using thicker or thinner layers of rubber on the same diameter of bolt, etc.

At first I was trying to figure out what THE bushing application is that everyone uses.  (Mustang-II? C5 vette? etc.)  But everybody is trying to eliminate the bushing compliance, not keep it.  Nobody has any advice on picking rubber bushings.

I've been thinking of using 73-81 Camaro front end bushings.  The dirt oval racing world loves 2nd-gen Camaros so they should be on the aftermarket's radar.  The bushings are big enough for the job and simple enough to incorporate into custom-fabbed parts.  The leading & trailing end of the front LCA used two different thicknesses of the rubber layer (allowing two options for stiffness).  

But I got a few of these Camaro bushings from a local parts store and they don't seem very well-made.  The sleeve hole for the bolt isn't perfectly round.  I dunno if this should bother me or not.  

I'm also thinking about Fox Mustang bushings.  A classic Charger is a much heavier car than a Fox but it might work for places where I need a smaller bushing.    

If you have any other suggestions I'm listening.


Independent Rear Suspension Forum » Caster/anti-squat? layouts? Jag/T-bird, Klaus Arning Watts, SLA . . » 8/27/2016 11:28 am

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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

Independent Rear Suspension Forum » Caster/anti-squat? layouts? Jag/T-bird, Klaus Arning Watts, SLA . . » 8/27/2016 7:23 am

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The Mopar wedges are pretty well-respected among anyone who races them.  Or races against them.  Only the mainstream "that thing got a Hemi?" world underestimates them. 

I'm really trying to make my car feel like it's a factory-built independent rearend.  Rubber bushings on the control arms, lots of ground clearance & wheel travel, etc.  I'm not looking to give it a track car MoJo.  If Mopar had done an IRS fifty years ago I'm sure they would have given it good geometry and durability.  They built everything strong enough to be police cars & taxicabs in those days.  

Their front suspension thinking was quite modern.  Stiff unibody, the front roll center height was tolerable, and the wheels saw negative camber gain in bump.  About the only serious problem with their IFS geometry was the static alignment settings & spring rates being set for bias-ply tires.  That, and the lack of caster angle which was probably a compromise for building manual-steered cars on the same chassis.  

I'm finding out that designing a ground-up suspension like this, with rubber bushings, seems like almost uncharted territory.  At least in the amateur enthusiast world.  Everyone focuses on removing all the compliance from a suspension setup.  I'm trying to keep at least some of the compliance without making a floppy mess.  That means the chassis-end control arm pivots are not static points but more like slightly dynamic factors (engineering buzzwords alert!)  I don't understand it all well enough.  I'm mostly just studying existing OEM setups and trying to mentally back-engineer their design choices.

All that mental back-engineering of OEM suspensions has certainly said one thing:  For real street cars, the pivots have to be rubber bushings & enclosed ball joints ONLY.  There are few exceptions.  Even fewer with decent reputations for long term durability.    


Independent Rear Suspension Forum » Caster/anti-squat? layouts? Jag/T-bird, Klaus Arning Watts, SLA . . » 8/26/2016 12:51 pm

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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.   


Independent Rear Suspension Forum » Caster/anti-squat? layouts? Jag/T-bird, Klaus Arning Watts, SLA . . » 8/25/2016 11:51 pm

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Running 10's in 1983?  Wow, that WAS a big deal.  That's more like running 8's now.  

Mine is getting a decent 440.  The smog-era 400 blocks can actually be a better starting point with all the Chinese stroker cranks/etc out there now.  But the difference is not important unless you intend to really max everything out on the motor (or you are cramming the engine between the shock towers of a compact little Dart or Duster).  

The leaf springs' front eye mounts do indeed attach with 4 bolts.  However, that's about the only good news.  The shock absorber crossmember isn't really strong enough to carry big weight/loads.  And those infamous Mopar 6-leaf springs extend a good 3 feet behind the axle, so the shackle mounts are located too far back to incorporate them into the IRS.  

My intention is to make the IRS mostly bolt-on.  It should be functionally reversible with wrenches.  But there will have to be some welding & drilling holes.  No way around it.  

It was a rusty car though.  Lots of sheetmetal is replaced back there already.  And the car isn't valuable for VIN numbers.  I don't mind doing some metal work on it.   There are no disposable 68-70 Chargers but some are safer to customize than others.  

Independent Rear Suspension Forum » Caster/anti-squat? layouts? Jag/T-bird, Klaus Arning Watts, SLA . . » 8/24/2016 8:56 am

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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. 


Independent Rear Suspension Forum » Caster/anti-squat? layouts? Jag/T-bird, Klaus Arning Watts, SLA . . » 8/23/2016 7:32 pm

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Adjustments -

I've got no illusions about getting everything right the first time.  Nobody ever does.  But I gotta start somewhere.  It would be nice to start with things close enough to fiddle with some built-in adjustments instead of having to cut & weld major things.  I say "9-10 inch rear roll center" because I'm pretty sure it will end up more than 5 inches and less than 15.  

BTW, when you did the Klaus-style Watts link setup, how long were the upper & lower links?  Do you know how long they are on the KA factory T5 thing that Duane Carling remade?  Even very rough ballpark figures would be appreciated.  It's easy for me to study online pics and guess at the sizing of the hub carrier but the links are not shown as clearly.  

When I study the design of the whole thing, I see a pretty direct relationship there:  shorten the watts links = increase the anti-squat & anti-dive.  And vice versa.  

I'm also curious about the height of the KA rear watts link chassis mount, in relation to the wheel hub height (at static ride height) - Was the watts link higher, same, or lower?  


Independent Rear Suspension Forum » Caster/anti-squat? layouts? Jag/T-bird, Klaus Arning Watts, SLA . . » 8/23/2016 6:09 am

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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

Independent Rear Suspension Forum » Caster/anti-squat? layouts? Jag/T-bird, Klaus Arning Watts, SLA . . » 8/22/2016 9:31 pm

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So you practically took out the anti-squat function, for the sake of controlling wheelhop and bumpsteer?  

How does it grip on launch now?  Wouldn't it be pretty terrible?  Does it squat like hell?  I'm kind of surprised that little or no AS was the best setup here.  

Solid-axle muscle cars use TONS of anti-squat and they love it.  Is it more effective on solids because their spring rates are generally much stiffer than IRS (due to the axle's huge unsprung weight)? 

Your story suggests that a more conventional twin A-arm setup would be better for an IRS than the longitudinal watts link (at least when the power levels are so far above the tires).  Give it a horizontally parallel UCA + the LCA angled up in front, for some anti-dive action during braking, and just forget about anti-squat entirely.  Control the bumpsteer with a well-placed toe link.  Does this sound about right?

My feeling on bumpsteer has always been to just get rid of it (to the extent that is possible).  We call the same thing a pure problem when it happens on the front end.  Whatever subtle benefits it might possibly bring in certain IRS situations is probably not worth the drawback any time it isn't helping.



Independent Rear Suspension Forum » Caster/anti-squat? layouts? Jag/T-bird, Klaus Arning Watts, SLA . . » 8/14/2016 11:54 pm

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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

Independent Rear Suspension Forum » Caster/anti-squat? layouts? Jag/T-bird, Klaus Arning Watts, SLA . . » 8/06/2016 1:07 pm

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I am under the impression that KA's longitudinal watts link layout results in the best anti-squat situation.  The amount of A/S starts multiplying as the rear of the chassis leans farther down/back.  The rear wheel is being pulled farther & farther backwards as it moves farther up into the wheelwell.  

The Jag/T-bird setup (with the LCA being an "H" shape) makes the rear wheel travel a straight line up/down.  The only anti-squat in that setup is caused by whatever amount of permanent caster angle is built into it.   

The conventional SLA (twin A-arms + toe link), when used in the rear suspension, is usually laid out with anti-dive in mind.  The A-arms are tilted to make the wheel start to arc forwards as it falls down out of the chassis.  The wheel goes near vertical as it moves above ride height & into the chassis, giving basically no anti-squat.  

Do I have all this right?  

Does anyone have any feedback on the degree of difference between these layouts, in the real world sense?  The KA watts seems ideal but the Jag/T-bird setup involves far less complexity.  How does this play out in high-powered cars on public roads?  Assume we're talking normal street radial tires and 300-600 hp.  

Does this stuff have specific implications for wheelhop?  I have always thought that IRS wheelhop is mostly about the diff housing thrashing around because of too much bushing play.  But are rubber control arm bushings liable to cause it too?    


Independent Rear Suspension Forum » Ground clearance, wheel travel, and CV angle - design question » 9/17/2015 1:31 am

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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?  



Independent Rear Suspension Forum » noob, seeking some general IRS opinions » 9/02/2013 12:33 am

Replies: 3

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Upgrading the existing rear end -

I'm considering an IRS conversion for the deeper things it fixes - reduce the unsprung axle weight and stop the rear end from breaking loose on mid-corner bumps.  So there aren't really any lesser steps worth taking before biting off the whole thing. 

I'm dealing with a muscle-era Mopar so the basic front suspension geometry wasn't bad to begin with.  The stock caster & camber settings are way off because the factory was designing everything to accomodate bias-ply tires and manual steering boxes.  But the more dynamic stuff (roll center, camber gain, anti-dive, etc) is already decent.   

That old Colin Chapman quote about "any suspension design will work, if you don't let it"  - my thinking is pointed totally the other direction.  I want something that moves freely, isn't too stiffly sprung, and is stable & predictable no matter where each wheel is or what they are rolling over.


Independent Rear Suspension Forum » noob, seeking some general IRS opinions » 8/24/2013 1:03 pm

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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.


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