Independent Rear Suspension, OEM, aftermarket, stock configuration or heavily modified, all makes and models, everyone is welcome here!!!

You are not logged in. Would you like to login?

8/06/2016 1:07 pm  #1

Caster/anti-squat? layouts? Jag/T-bird, Klaus Arning Watts, SLA . .

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?    



8/10/2016 11:50 pm  #2

Re: Caster/anti-squat? layouts? Jag/T-bird, Klaus Arning Watts, SLA . .

From what I have been able to glean a Watts at the bottom of the upright serves as a trailing arm.
A watts at the top of the upright is an anti-hop device.

Someone will be along to correct me if I have this wrong.

At these prices I want my gas 100 octane, leaded, and my windshield washed!

8/13/2016 10:50 pm  #3

Re: Caster/anti-squat? layouts? Jag/T-bird, Klaus Arning Watts, SLA . .

I built a K/A suspension for a 67 mustang. I ran this set up for a little over a year. I didnt notice a lot of anti squat but I wasnt looking for it either. I had been using a Jag xj6 set up and had problems with wheel hop under acceleration. This problem was nonexistent with the K/A set up. However the K/A .suspension limits wheel width. Those leading and trailing links are in the way of wheel offset in the + direction. The engine I am running would overwhelm the 7" 245/45/16 rear tires with relative ease. With out flaring the rear fenders that was the max width I could fit in. The XJ6 uses about 4 degrees of pos caster to create anti squat, the draw back is this creates wheel hop unless you have very aggressive shock valving. I had the top of the differential solid mounted with no rubber what so ever.The bottom of the diff has links triangulated to the old front spring eye mounts and these mounts held everything rock solid. You cannot run a jag rear hard with out the center foundation rock solid mounted. Any movement will change the alignment with each turn and bump. Jag control arms use needle bearings in board and tapered roller bearings out board. When set up properly do not have any free play to speak of. I use AFCO racing shocks at all four corners of my car. They are rebuildable, revalveable and cheap on Ebay. The K/A system with its watts link was something I toyed with for a while. I extended the trailing arm mount on top of the bearing carrier until it was the same distance from the axle center line as the leading link at the bottom. I cannot say I noticed any change in antisquat. I did manage to get rid of nearly all of the toe in and out related to this design. There were other mods required as well but the final set up worked quite well. As long as you dont need wide rear tires.

Last edited by tyrellracing (8/13/2016 11:07 pm)


8/14/2016 11:54 pm  #4

Re: Caster/anti-squat? layouts? Jag/T-bird, Klaus Arning Watts, SLA . .

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 setups.  I get the feeling that even a plain vanilla twin-A-arm setup moving the hub in a straight line would be capable of almost identical results in the real world (street cars), if you threw in a few degrees of permanent caster like the Jag setup has.  

Any other thoughts on all this? 


     Thread Starter

8/18/2016 9:26 pm  #5

Re: Caster/anti-squat? layouts? Jag/T-bird, Klaus Arning Watts, SLA . .

The XJ6 has too much antisquat for anything more than about 250 hp in a 3500 lb vehicle. The lower control angle is is too steep. Under acceleration the rear wheel load path would literally lift the rear of the car until traction was broken then the rear would fall back down only to gain traction and start the cycle all over again. I have enough torque to make the rear tires leave the ground under hard acceleration. This was very violent and sounded like it was going to come apart. That was why I put together the modified KA set up. Eventually I resolved this problem by reducing the anti squat to nearly zero like the XKE. Then I revalved my rear shocks and have been running 335 / 35 / 19's with out any problems since. As for the link locations, I studied the KA design as he built it and discovered toe charistics I didnt care for. Equilizing the distance the leading and trailing links were from the axle centerline combined with seperating the wish bone lower control arm into two parrallel links eliminated nearly all of the toe in/out that comes with that design. When the suspension goes through its travel the leading and trailing links being equdistanl from the axel made the center of rotation on the axel centerline. With the lower control arms seperated there is no fore and aft movement of the centerline when viewed from above. This eliminates the change in toe to less than one degree depending on static camber setting. Again. My real goal was to get rid of wheel hop and it did. I also have an engine that makes over 550 ft/lb's of torque. This overwhelms the 9 inch wide tires I have now. You can guess what it did to the 7 inch wide tires I used with the afore mentioned suspension.

Last edited by tyrellracing (8/19/2016 12:09 am)


8/19/2016 12:27 am  #6

Re: Caster/anti-squat? layouts? Jag/T-bird, Klaus Arning Watts, SLA . .

If you go through all the different types of rear suspensions you will find that all of them have a change in toe with travel. Some are much worse than others. The new 5 links used now by Chevy, Porsche,and a few others are the closest to a perfect suspension I have seen so far. Very nearly no change in toe and what it does have is in a beneficial direction. The KA suspension actually has toe change in the right directions but the change during compression is too dramatic and when extended is too little to help. Keep in mind that suspension was designed for narrow bias ply tires. When ever you design a suspension you start with the tires and wheels and work your way in. Not like most people do and go backwards.with the tire selection as an after thought.


8/22/2016 9:31 pm  #7

Re: Caster/anti-squat? layouts? Jag/T-bird, Klaus Arning Watts, SLA . .

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.



Last edited by thedude (8/22/2016 9:41 pm)

     Thread Starter

8/23/2016 12:37 am  #8

Re: Caster/anti-squat? layouts? Jag/T-bird, Klaus Arning Watts, SLA . .

Putting the lower control arm at the angle the xj6 uses works fine with a low hp/weight ratio. Add  more than twice the hp and reduce the weight by 1000 lbs and you will have wheel hop. I did not eliminate anti squat entirely. I reduced the angle from aprox 3 deg to about 1 degree. Anti squat/ lift under braking really is not that huge of a concern on a car that has excelent shocks and is balanced(sorted out). Those charistics are from the relm of crappy shocks and loosey goosey rubber mounted suspensions running bias ply tires. The kind of car that you can wag its tail by turning the steering wheel back and forth at speed. I am a machinist and the steel was free at work from the rem bin in the saw shed. While I was converting back to the Jag set up I went to larger vented 12 inch rear rotors and used helsey hays 4 piston calipers that came off the frount. This was the secondary reason for making new inner mounts. The pivot had to move down to clear the rotor. . I also made half shafts from 1 ton Chevy truck drive shafts getting rid of the heavier solid Jag units. Tube drive shafts are easier to cut to length than the old solid ones were. The set up is narrowed from the jag width. When I shortened the solid jag axles they kept cracking so they had to go. When ever you weld a torsion loaded solid shaft, cracks are inevitable. The solid Jag rear brakes were insufficient for stopping once I made 6 piston calipers and 14 inch rotors off an Escalade for the front. I had to increase the swept area as well as the distance from center at the rear to get the brake balance back up with the front. Confusing I know. Years ago when I was running a 9" rear I used under ride traction links that attached to 6 inch long lever arms welded to the bottom of the axel tubes. The other end attached to the front spring eye mount bolts. The resulting angle produced nearly 90% anti squat. The car hooked up great but the unsprung weight sucked. My 67 shelby is set up for road racing not drag racing so having a heavy rear axel weight really made the cornering suck on bumpy roads. .

Last edited by tyrellracing (8/23/2016 1:40 am)


8/23/2016 1:12 am  #9

Re: Caster/anti-squat? layouts? Jag/T-bird, Klaus Arning Watts, SLA . .

The conventional double wish bone IRS is the best way to go with one huge exception, the 5 link. The biggest problem with retrofitting a double wish bone IRS into a car originally equipped with a hotchkiss drive live axle (leaf sprung) is there is no room for the upper control arm. The ones I have seen have upper control arms shaped like a U to detour around the frame rail in the way. or they notch the frame. Either way is bad. This drastically weakens the arm or the unibody and is a compromise at best. As for bump steer, It seldom helps. The only correct bump steer at the rear will be toe out on the outer rear tire in a turn. This gives a mild drift sensation from the drivers seat and helps the rear hang in longer before it actually starts to drift. Of course if you dont drive the rig hard enough to actually put the vehicle in a four wheel drift steering with the throttle, you would never miss the toe out effect. LOL. If I cannot dial in correct toe I will tend to adjust all of it out or as much as I can. Its just a little easier to sort out the handling with out another red herring to screw with your brain..

Last edited by tyrellracing (8/23/2016 1:26 am)


8/23/2016 6:09 am  #10

Re: Caster/anti-squat? layouts? Jag/T-bird, Klaus Arning Watts, SLA . .

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 about the only big departure from OEM style.  


Last edited by thedude (8/23/2016 6:11 am)

     Thread Starter

8/23/2016 5:53 pm  #11

Re: Caster/anti-squat? layouts? Jag/T-bird, Klaus Arning Watts, SLA . .

The five link has a favorable toe charistic built in plus it has anti squat in its geometry. I wish I had the link but there is a aftermarket 5 link being sold and the engineer that designed the kit explained all the benefits and the guy knew his "I used a word I shouldn't have". As for my car... It doesnt launch like it did with the 9". However it doesnt do too poorly all things considered. The Tires I am running are not drag slicks, there low profile P zeros and are fairly soft. I cannot get any where near full throttle in first or second but it will launch as well as most 16 mustangs and camaros. More importantly it will do circles around them cornering.  I have a Fontana aluminum block 351 Cleveland with a 4.125 bore and a 4.000 stroke. That comes out to aprox 427. The same as a LS7. bore and stroke wise only I built it 10 years before the ls7 . I have the C3 roush nascar heads and valve trane. They were cheap at 2K on Ebay.  The crank is steel, the rods are H beam eagles and they are attached to forged JE pistons with floating pins. It has a hybrid dry/wet sump. CONFUSED? The Grigs racing front suspension would not clear the front sump of the pan with the 10 inch engine set back. This puts all of the engine except h2o pump behind with the front axle center line and provides a near 50/50 weight split. Sooo I have a dry sump pump and a rear sump pan with a roving pick up and no front hump, lump or anything that would interfere with the cross member related to the oil pan. I have a 50mm roller bearing type roller cam from cam dynamics.I had no choice, thats what the block was bored for. and a stock type would not work no way no how. It has 645 lift on E and I on 108 lobe centers. Its a little nasty sounding and performs very well. The Dana 44 in the jag rear has its work cut out for it.

Last edited by tyrellracing (8/23/2016 6:45 pm)


8/23/2016 6:13 pm  #12

Re: Caster/anti-squat? layouts? Jag/T-bird, Klaus Arning Watts, SLA . .

I have set up cars for road racing for years. In my experience it is best to go soft on the springs and get the best shocks money can buy. All sway should be controlled with bars not the springs. I worked for a spring company that bent all of the sway bars for Quickor. I made gazillions for my self too. Then I discovered torsion type sway bars and never used a bent to fit bar again. I have made a few for my self over the years. Cutting splines on the high carbon steel is a "I used a word I shouldn't have" but worth the effort. The torsion type bars I like are gun drilled. This makes them much lighter and stronger for their size. a win/ win deal.
As for roll center heights. I have seen sooooo many people really screw up cars by moving the roll center where they think it should be before they ever drove the car. I believe in leaving things like that alone until I drive the wheels off the dam thing and analize exactly what is needed. The only exception to this is during set up then I look the suspension as a whole to figure out the roll cuple. Or the line that goes through front and rear roll centers. This is critical and must be done with knowledge of how roll centers affect the suspensions f/r then as a whole. I dont mean to get all Zenn on you. but thats how I do it. If I am changing a existing design I make only one or two alterations at a time so if something goes terribly wrong I can easily track it down. That becomes impossible after you have redesigned the wheel and have to look at the car as a whole to figure out the problem it never had before.

Last edited by tyrellracing (8/23/2016 6:28 pm)


8/23/2016 7:32 pm  #13

Re: Caster/anti-squat? layouts? Jag/T-bird, Klaus Arning Watts, SLA . .

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?  


     Thread Starter

8/24/2016 1:38 am  #14

Re: Caster/anti-squat? layouts? Jag/T-bird, Klaus Arning Watts, SLA . .

I have the entire set up still. It would be easy to get those lengths for you. Its 1 am so it will have to wait until tomorrow. The upper mount on the bearing carrier has 1/2 inch holes on 1 inch centers. All of my links used heim joints so they all had one inch of adjustment on each end.The trailing link had to be replaced due to length issues but I saved the two original links. They are all stainless and looked pretty sharp. I made threaded ends and welded them in at work where I could use their stainless wire. Then I turned them full length so all of my work was invisible. I agree with your assessment of the design in theory but the actual way the first iteration performed I couldnt really feel the dramatic  anti squat I was expecting. However it did deliver far better grip than the jag xj6 ever did with out wheel hop. In all fairness I beat that K/A set up pretty damn hard. I also used two adjustable coil overs per side. The engine could overwhelm the tires so easily in my opinion the anti squat would have been more pronounced with better tires and alot more traction. As I recal the original K/A design used non adjustable leading and trailing links. The inner mount of the LCA was all the adjustment that design had. For exact numbers I will have to find my original drawings with all the pivot points layed out at 1/2 size. What platform are you putting this under?

Last edited by tyrellracing (8/24/2016 1:47 am)


8/24/2016 8:56 am  #15

Re: Caster/anti-squat? layouts? Jag/T-bird, Klaus Arning Watts, SLA . .

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. 


Last edited by thedude (8/24/2016 9:00 am)

     Thread Starter

8/25/2016 6:58 pm  #16

Re: Caster/anti-squat? layouts? Jag/T-bird, Klaus Arning Watts, SLA . .

Dont those use a four bolt mounted front spring eye hangers? Seems to me they did. If so, you couldn't have chosen a better candidate for the conversion. The obvious leading links mount there and the lower differential stabilizers mount there too. Having removable mounts would mean the custom mounts would be removable and the conversion could be reversed back to stock if the cars value was an issue for resale. You said you were going to use a big block, B or RB? I have built some sh@t kicking 440's and 383" when I was a younger man. Back when I was in college a friend of mine had a 440 4 spd cuda. I did the majority of the rebuild on his engine after we blew it up drag racing. When it was done it could do 10's all day long. That isnt that big of a deal now but in 83 it rocked.


8/25/2016 11:51 pm  #17

Re: Caster/anti-squat? layouts? Jag/T-bird, Klaus Arning Watts, SLA . .

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.  

Last edited by thedude (8/26/2016 12:01 am)

     Thread Starter

8/26/2016 7:19 am  #18

Re: Caster/anti-squat? layouts? Jag/T-bird, Klaus Arning Watts, SLA . .

The 400 is a rendition of the B block with a bigger bore than the 383. Those 400 make fantastic foundations for strokers. Smaller than the 440 with the potential to be bigger than the 440 internally. All I was getting at was the springs can be removed with the spring eye hanger leaving this reinforced pad for building a new custom bracket for supporting the leading link as well as the triangulated lower differential support mount. That is how the K/A was set up but on the mustang you run into trouble getting the diff support hitting the frame rail preventing it from fitting into the front spring eye mount. A custom bracket must be made to attach the diff support to. I extended the bolt through the frame so the diff mound could be held at the correct location sandwiching the frame. This was no easy task because the rocker is in the way of a long enough bolt to fit in.  You will not have this problem. I am aware that your platform has other short comings but they are sum what minor. That said. It sounds like a fun project !

Last edited by tyrellracing (8/26/2016 7:25 am)


8/26/2016 12:51 pm  #19

Re: Caster/anti-squat? layouts? Jag/T-bird, Klaus Arning Watts, SLA . .


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.   


Last edited by thedude (8/26/2016 12:59 pm)

     Thread Starter

8/26/2016 5:18 pm  #20

Re: Caster/anti-squat? layouts? Jag/T-bird, Klaus Arning Watts, SLA . .

I agree with all your mopar thoughts. The older 60's=70's mopars are very ridgid for any car of that era. When any one talks about big block mopars they immediately go to the HEMI.  The hemi is good but their too big too heavy and no one has any spare parts for them. If you have one and want to rebuild it, you will spend as much as you would building three 440's.   The 413's 426 wedge, 440's and 383,400's are the un known step children that were forgot in the shadow of their hemi relative. However, any one that knows engine architecture knows there is a mountain of hp lurking in them engines just waiting to get out. There are few special tricks to make these engines scream. It just takes common sense and machining know how. Too bad they did not get the ink and popularity they deserve..


8/27/2016 7:23 am  #21

Re: Caster/anti-squat? layouts? Jag/T-bird, Klaus Arning Watts, SLA . .

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.    


Last edited by thedude (8/27/2016 8:28 am)

     Thread Starter

8/27/2016 8:50 am  #22

Re: Caster/anti-squat? layouts? Jag/T-bird, Klaus Arning Watts, SLA . .

The only reason most American cars of the 60's didnt come with an IRS is because live axles are cheap and can be made to hook up better. Drag racing has always been more popular than all other kinds of racing because it doesnt require driver skill until you advance well above stock vehicles. If you can stomp on a throttle, you can drag race. As for rubber, I agree and disagree. The rubber makes the suspension too compliant. In the front it allows the tires to go out of alignment in corners. At the rear it allows the vehicle chassis to shift off center causing excessive body roll. This is all for the quality of ride by nullifying harshness. The effect rubber has on the handling of most vehicles could be summed up as handling like a row boat in heavy sea's. Todays vehicles have a more harsh of a ride because its safer than a row boat like an old caddy. Being able to drive around a problem is safer that being big enough to survive the impact. You cannot put antilock brakes on a suspension that is too compliant. It just creates problems that the brakes cannot overcome. The Jag rear was designed to be entirely solid mounted. If you look at how Jag took a good Dana built and shipped design and turned it to mush by mounting the carriage in rubber. Speaks for its self. Jag compromised handling for the elimination of ride harshness.The early mopars used torsion springs. The only way these can work is by placing them at the pivot point. This puts them at a mechanical disadvantage. A coil spring properly placed as close to the wheel as possible will support the same weight at 1/50 th the spring weight and size. The early geometry is archaic. The caster/camber curves were based on bias ply tires and will not perform optimally with modern radials. Wide radial tires do not require the camber gain that the old bias ply tires did. You cannot expect good results by just slapping modern tires on to an ancient suspension with its bias ply geometry. Entirely new camber curves are required to make these modern tires work the way they were designed to. 

Last edited by tyrellracing (8/27/2016 9:04 am)


8/27/2016 11:28 am  #23

Re: Caster/anti-squat? layouts? Jag/T-bird, Klaus Arning Watts, SLA . .

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 part of the design?  I though that was relevant for the lower (longitudinal) trailing arms being able to work without too much binding.  I don't know the Jag IRS setup very well.  

I agree that OEM bushing cushioning is generally too soft.  But I still want some rubber bushings in my setup.  Different strokes for different folks.  I'm trying to choose rubber bushings on the thin side.  I'm also going for big long control arms and a tallish spindle.  An overall "bigger" suspension should see less geometry deflection for the amount of cushioning.  (Recall how the upper BJs are located above the wheel on many modern cars.)

That's interesting about the anti-lock brake problem with overly soft bushing compliance.  Makes sense though.  Braking-induced wheelhop must confuse the living heck out of the computer.  

Last edited by thedude (8/27/2016 11:48 am)

     Thread Starter

8/27/2016 3:14 pm  #24

Re: Caster/anti-squat? layouts? Jag/T-bird, Klaus Arning Watts, SLA . .

The bump steer issue is not a t bar issue but rather across the board. All rear steer cars had some bump steer. Mustangs were terrible. Some of this had to do with Ackerman angle. They made the tires turn different angles so when moving slow there would be less scrub. All the manufacturers built in under steer. They knew the tires available would get people killed if they made cars that steered neutral. The AC cobra, some Ferrari's,lotus and very few cars sold in the sixties didnt have bump steer.  Basically all rear steer with recirculating ball steering boxes have some bump steer. It wasnt until NASCAR started using full tube frame cars did any one actually make a good rear steer suspension. Shortly after that they all went to front steer. The stock mustang suspension is a prime example of a real pile of excrement. The coils over the top of the upper control arm over loaded the unit body in a weak area of the front structure because the springs sat at the center of the A arm the spring seats were heavily loaded.. Ford put braces on the towers but that wasnt enough. Shelby put an export brace and a monty carlo bar on the towers to trianglate the support but that was a bandade at best. To support 250 lbs a 600 lbs spring was required.. Then the UCA's inner pivot was in the wrong place and induced under steer. Because of this pivot location was too high the inner bushings took a serious beating and failed frequently tearing huge holes in the strut towers. The Ackerman was never quite right so the more you turned the tires the more exaggerated the toe out got. This was to the magnitude of 8 degrees toe out at full lock. The least amount of bump steer I was able to obtain was 4 degrees toe from 6" bump. That is pathetic.  The manufacturers really didnt try to make good suspensions back in the 60's They all did crap like this. The car buying public didnt know any better so it was the status quoe. I worked with the Mustang suspension until I could not make the bad design work any better. I had raced with this handicap for many years and knew I could do better. Thats when I discovered Griggs racing suspensions. I scaled their GR350 racing suspension from images on their web page, made an exact copy and have been happy ever since. Front steer power rack and pinion fully adjustable upper and lower control arms, Very nearly zero bump steer. Tuneable camber curve. Coil over shocks attached to the LCA at the lower ball joint. Adjustable track width (3") each way. Pluss I made my own 3" drop spindles and 6 piston calipers. It out performs the old mustang suspension like you cannot believe with out going for a ride. Google Griggs racing and you will find the suspension I plagerized. All of my numbers were from my racing notes. I own a hunter alignment computer. Mine predates their lazer machine but still produces good numbers. I have owned it fifteen years now and am competant of its proper use.

Last edited by tyrellracing (8/27/2016 4:24 pm)


8/27/2016 4:06 pm  #25

Re: Caster/anti-squat? layouts? Jag/T-bird, Klaus Arning Watts, SLA . .

Rubber bushings are not all bad. Its the durometer of the bushings that make the difference. What most people dont know is that pretty much all rubber is equally soft. The difference is how much rubber they cram into your pivot bushings. Basically they will cram a 6" in dia rubber bushing tube into a 1.5" diameter sleeve with a press and a steel funnel. Thats how they adjust the durometer for different applications.  The higher durometer required, the more rubber they cram into the bushing sleeve. If you get the oppertunity, cut open a new or lighty used rubber spring eye bushing. You will be shocked just how much rubber they can get in there. Old ones will not expand as much due to a collapsed structure at the molectular level.  A typical 2" bushing will yield a 4" rubber bushing un compressed.

Last edited by tyrellracing (8/27/2016 4:16 pm)


8/27/2016 9:10 pm  #26

Re: Caster/anti-squat? layouts? Jag/T-bird, Klaus Arning Watts, SLA . .

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.


Last edited by thedude (8/27/2016 9:31 pm)

     Thread Starter

9/03/2016 4:46 pm  #27

Re: Caster/anti-squat? layouts? Jag/T-bird, Klaus Arning Watts, SLA . .

Now I understand.  My personal opinion is there is little harshness removed with rubber bushings. There is a boat load of compliance that comes from using rubber bushings. So I dont use them. Urethane bushings are better but squeak real bad and wear quickly if not lubricated regularly. In my opinion  if you want to find a rubber bushing by its durrometer you will have to hunt and peck till you find what you like. Rubber bushings are usually for stock applications and therefore no one will have them described by there durrometer.. That will make your task far more difficult but not impossible.


9/18/2016 12:03 am  #28

Re: Caster/anti-squat? layouts? Jag/T-bird, Klaus Arning Watts, SLA . .

(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.  


Last edited by thedude (9/18/2016 12:06 am)

     Thread Starter

12/08/2016 6:59 am  #29

Re: Caster/anti-squat? layouts? Jag/T-bird, Klaus Arning Watts, SLA . .

It sounds like you have already covered a lot of the ground I am going over and really know your stuff so if you don't mind, can you give me some feedback?
I am building 65 Mustang powered by Coyote motor.   I have settled on a Griggs front end and the first parts should arrive any day.
For the rear end,  I have been looking very hard at Duane Carlings K/A setup.  My biggest concern is one you pointed out,  the lack of back spacing due to the leading and trailing arms.  I have done a slight home brewed minitub and moved the inner wheel well edges inward about 2 inches, giving me room for up to a 295 tire.  I am concerned that I won't be able to take advantage of this added width with the K/A setup.  Duane claims that there is more backspacing than stock rear end but looking at the pictures,  I think I will run into issues.  Do you see any workarounds?
Griggs is adamant that his solid rear setup will outperform an IRS and he may be right but I would give up a slight edge to get the ride comfort.  The car is not going to see the track except maybe an occasional fun day.  And in researching the subject,  it seems that the K/A design overcomes many of Bruce Griggs objections to IRS.
What say 'ye?


8/19/2018 1:27 pm  #30

Re: Caster/anti-squat? layouts? Jag/T-bird, Klaus Arning Watts, SLA . .

I'd really like to hear about your KA suspension project. I'd like to do the same thing for a 65 Rcode tribute project. What hub carrier did you use? What center section? Did you make your own LCAs or did you find some that would work?


Board footera


Powered by Boardhost. Create a Free Forum