Rear Shackle Geometry - how are you adjusting it?

[M_Body_Coupe]

Folks!

Alright...so my Coupe is no drag racing machine, however proper setup should never be lost on anyone, so I've started to look at the leaf-spring geometry seeing that last time out at the track I was showing dismal 60' times and the video of car launching showed barely any chassis movement with a good amount of wheel spin (275-65R15 Nitto NT555Rs, so there is plenty of tire there).

NOTE: I suppose I shouldn't be surprised by this seeing as the car is much more focused (setup wise) on street handling: big sway bars, tight shocks, etc.

...but the angle the spring rear shackle makes has always bothered me. I don't have a picture handy at the moment, but the lower part of the shackle is ever so slightly leaning forward (at the spring eye), which is telling me that when launching the spring doesn't really have the "room" to flex and cause the body to rise - which normally is what you want to do b/c that body rise is actually what plants the tires against the pavement.

So off I went measuring my sping front and rear segments: 24.5" on the front and 34" on the rear, which is per spec (allow for +/- 0.5" here seeing as this isn't precisely off of the center bolt to the center of the front/ rear spring eye).

I am using a 5 leaf pack, and the rear of the car has always been somewhat high. I feel like it would be better for it to be about 1" lower.

All of this begs the question: so when our F/M/J-body cars needs such geomtery adjustments, what are you guys doing here?

Our rear suspension is different from the earlier Mopars where you could shim the shackle mounting bracket and that would in turn move the shackle pivot forward thus allowing the lower part of the shackle to lean/point back.

It would seem that the only way on our rides to accomplish the same thing is to re-locate the shackle mounting bracket forward on the frame rail and well, given that this is all boxed in I honestly can't quite fathom doing this w/o cutting things up, etc.

I did consider making a plate that would itself mount into the current as-is shackle bracket mounting points, but which would effectively relocate the NEW mounting point forward...that seems doable, at worst it requires some clearancing holes to be drilled in the framerail and of course the plate itself needs the nuts welded on so that the shackle mounting bracket can attach to it.

....but even that approach seems like it's very R&D (LOL)...so is there an easier way that I'm simply overlooking here?

Maybe a spring slider like these Calvert pieces (SPRING SLIDERS for Calvert Springs (SL-300CR ) – Calvert Racing, Inc. ...but even these would still require some creative way to mount them onto the rear frame rails!

As always, thanks for ideas and suggestions!!!

 

[Oldiron440]

Body lift is more than likely controlled by the leaf springs, I use SS springs with the appropriate hangers but the rear hangers have always leaned slightly forward. I get four or five inches of body lift from the rear springs with the car capable of running 1.3x 60 foot times. Do you have you have the clamps removed on the rear half of the springs? Also when I used stock rear springs I also used traction bars, the only down fall was the wheels up launching, the long slapper bars tended to lift the front end also.

 

[Infiniti Dave]

I hate to sound dismissive , but it sounds like you may be "over thinking " this. Sounds like the suspension is just way too stiff for what your hoping to do and you've already mentioned that the rear body sits higher than you want. Sounds like a simple spring replacement , (maybe 4 leaf) would help the situation with much less work and expense. A taller/adjustable pinion snubber wouldn't hurt either.

 

[M_Body_Coupe]

...I use SS springs with the appropriate hangers but the rear hangers have always leaned slightly forward. I get four or five inches of body lift from the rear springs with the car capable of running 1.3x 60 foot times....

As it happens I have the required front hangers as well to install the shorter (front segment) SS springs, but honestly was hoping that would not be necessary. My best 60' time was 1.870 and most of them are over 2.0.

...Do you have you have the clamps removed on the rear half of the springs?...

Nope, I have the clamps placed at all the factory locations.

 

[M_Body_Coupe]

I hate to sound dismissive , but it sounds like you may be "over thinking " this. Sounds like the suspension is just way too stiff for what your hoping to do and you've already mentioned that the rear body sits higher than you want. ..

No, you do not sound dismissive at all. I am well aware of the limitations of my setup right now, or to be more correct the limitations of the adjustments I can make given my primary criteria (that being road handling), but I figured I'd run this by the group here to see what others may have done in the past.

...Sounds like a simple spring replacement , (maybe 4 leaf) would help the situation with much less work and expense. A taller/adjustable pinion snubber wouldn't hurt either...

You are probably right in that a 4 leaf setup would drop the ride height and be the right move for drag racing performance focus...it would also go the opposite way I want it to go re: road handling though.

Yes, I have an adjustable pinion snubber already installed, 1" from the floor and honestly I doubt it actually manages to travel up that far. But but...that brings up a good point: I'm going to set it up dead against the floor, aka manual trans style to see what that might do...worth a try!

 

[Duke5A]

If you want your cake and eat it too then you need to start with double adjustable shocks on all four corners.

 

[Oldiron440]

Something else I forgot to mention is that I use shocks to that are the length of the shocks on the rear of a half ton pickup, 2” longer than the stock shocks.

 

[Fresh Air Inspector]

Hello M_Body_Coupe,

Interesting rear shackle mounts. I wonder how much noise these generate as the rear axle moves up and down? It would be interesting to experiment with them to see the affect on launch. The price isn't too bad and who knows it might prove beneficial.

From what I read in your posts the missing piece of the puzzle is, 'what is happening to the suspension during that 60'!

For the experimenting I'm suggesting below you can hopefully find an infrequently used stretch of road or abandoned parking lot to do 60' test launches?

If I may suggest; try mounting two cameras on the underside to show what happens to each rear leaf spring as well as two cameras showing what the left and right front suspension does during the 60'. This would allow you to see how each leaf spring deforms as well as understanding what happens up front.

Apologies if you already know this; because Chrysler engineers placed the rear axle forward of the leaf spring centres the spring responds differently under hard acceleration from leaf sprung rear axles found under other manufacturers cars. To improve Mopar rear axle traction the consensus of opinion is to remove the spring clamps on the rear portion and keep them on the front. Traction bars have always been a HUGE topic of discussion in Mopar circles. Some swear by them others say they are useless. Personally I'm not convinced one way or another, although I would like to experiment at some point in the future.

Some drag racers have found that removing the front sway bar has benefits with respect to weight transfer. On an M body it would be easy to disconnect the sway bar ends and run a few tests. I recognize this will affect the street handling.

The video of the read springs may highlight that one tire is seeing more weight transfer (sidewall deflection), than the other. This would indicate more 'squat' on that side. It would be interesting to check this with both sway bars connected, then the front disconnected / rear connected, followed by front connected / rear disconnected. I can't recall from your chassis pictures if you have front and / or rear sway bars?

The old standard of 'front drag shocks' that allow the front of the car lift higher than normal at launch transferring more weight to the rear could be an possible solution.

Hope it helps.

 

[Fresh Air Inspector]

Hello M_Body_Coupe,

Forgot to include the link below, it is just over 10 years old but the info is still solid, good pictures too. If memory serves you or your Dad has a Calvert traction bar set up.

https://www.hotrod.com/how-to/mopar-rear-leaf-springs-suspension-get-hooked-up/photos

Enjoy the read, if you haven't read it already.

 

[Oldiron440]

I have ran leaf sprung cars at the track and on the street sense the mid seventies. With properly setup springs there is no need for a pinion snubber they are a last resort for weak springs.

 

[M_Body_Coupe]

If you want your cake and eat it too then you need to start with double adjustable shocks on all four corners.

...but Mark, you know I can't do that!!!

 

[M_Body_Coupe]

...Forgot to include the link below, it is just over 10 years old but the info is still solid, good pictures too. If memory serves you or your Dad has a Calvert traction bar set up....

You are thinking of @Duke5A.

 

[M_Body_Coupe]

Interesting rear shackle mounts. I wonder how much noise these generate as the rear axle moves up and down? It would be interesting to experiment with them to see the affect on launch. The price isn't too bad and who knows it might prove beneficial.

There are some videos floating out there that show these on a Mopar drag car. Amazing to see just how much the spring eye rides up (towards the front of the car) as the car launches and the pinion tries to climb the ring gear thus causing the axle to twist and the chassis itself to rise. And well, for all that movement to happen something has to give, that being the slider which allows the the rear section of the spring to flex.

...From what I read in your posts the missing piece of the puzzle is, 'what is happening to the suspension during that 60'!

For the experimenting I'm suggesting below you can hopefully find an infrequently used stretch of road or abandoned parking lot to do 60' test launches?

If I may suggest; try mounting two cameras on the underside to show what happens to each rear leaf spring as well as two cameras showing what the left and right front suspension does during the 60'. This would allow you to see how each leaf spring deforms as well as understanding what happens up front...

So here is the thing: I am literally just waiting for the weather to get nicer so that I can do precisely what you suggest. I have my GoPro, a little older than today's "latest & greatest" but plenty functional for this purpose.

...Some drag racers have found that removing the front sway bar has benefits with respect to weight transfer. On an M body it would be easy to disconnect the sway bar ends and run a few tests. I recognize this will affect the street handling....

You know, I have often wondered about this. You are right, the typical/standard recommendation is to either not run a sway bar, or disconnect one if it's installed. But assuming of course that the car itself goes straight, the bar should have no impact on what's going on.

I suppose given the engine rotation and the fact that in higher output engines it is the RIGHT side that will get that "hit" first, you would NOT want a bar there as that would tend to un-load the opposite side's wheel. I think Mopar designed the SuperSprings with the beefier right side for this very reason: to manage that hard hit and keep the car launching straight.

All great ideas, this season I suspect will be spent studying this very thing and experimenting with a much of stuff. I think even with a road-handling configured car (which my Coupe is: 1-1/8" front bar and 1" rear bar) you can still do more to account for how the suspension itself works when drag racing such a vehicle.

 

[M_Body_Coupe]

I have ran leaf sprung cars at the track and on the street sense the mid seventies. With properly setup springs there is no need for a pinion snubber they are a last resort for weak springs.

Genuinely I have no idea if the snubber is doing anything. I have never taking the trouble to see how much movement is taking place there during launch, although I suspect it is very little given the remarks I got from folks that watched the car launch at the track.

Again, good option to toss that GoPro at it and see what happens.

 

[Oldiron440]

Put a dab of grease on top of the subber make a hit and see if there’s a mark on the sheet metal.

 

[Duke5A]

...but Mark, you know I can't do that!!!

But you know you want to!

 

[Oldiron440]

Geez I must have been doing something wrong with my Volare only running 1.3x sixty foot times with hydraulic rear shocks for a pickup and none adjustable gas front shocks. Carrying all that extra money I had in my pocket slowed me down.

 

[M_Body_Coupe]

Geez I must have been doing something wrong with my Volare only running 1.3x sixty foot times with hydraulic rear shocks for a pickup and none adjustable gas front shocks...

OK, c'mon, spill the beans: so what did you find worked? Or is this a story of "all I have here is untouched factory suspension"??? lol

Re: the other tech stuff...hmm, my thinking goes along the lines of 'if there is another way to a better result I am willing to look at it, even if that means changing some hardware'. What @Duke5A shared is pretty good stuff, I know this works for him very well. It is not the sort of stuff I could use b/c of my crazy ass insistence ontrying to max out that factory setup with barely any other changes (exception being replacing rubber with poly bushings).

 

[Oldiron440]

I already have, the brackets for SS springs and Bbody springs with the rear clamp removed, off set shackles and hydraulic shocks ment for a seventies Dodge pickup. It’s that simple and right out of the mopar chassis book.

I will add that the front spring hangers are long so I did tie them to my rollcage for reinforcement.

You will see an improvement if you simply removed the clamps on the rear of your spring's. A stiff front half is perfect and works like a ladder bar which is how the SS springs work but they need the back half to be able to separate. This too is in the chassis book.

 

[M_Body_Coupe]

I already have, the brackets for SS springs and Bbody springs with the rear clamp removed, off set shackles and hydraulic shocks ment for a seventies Dodge pickup. It’s that simple and right out of the mopar chassis book...

OK, duly noted, and you are absolutely right, you did already say this...I suppose I was anticipating some kind of additional adjustment/work for this stuff to be effective in our chassis, given your remark:

...but the rear hangers have always leaned slightly forward...

perhaps that shackle angle is not such a huge contributor after all?

I will go back to my MP Chassis book to re-read this stuff. While the firm hook giving you great 60' times is a great goal I haven't quite decided to what extent I want to sign up for changes that require a move to different spring setup, etc. I really was focusing on correcting the shackle geometry and assessing the difference this makes.

Need to prototype some stuff...more to come...