MP leaf springs too stiff?

[jeremya72]

Hey guys, I was wondering if anyone else has had a similar problem. I recently rebuilt the suspension on my '72 Road Runner front and rear. On the back I reused the stock sway bar and replaced the leaf springs with MP HD big block springs from Mancini. I also replaced the shocks with some KYB's. Ever since then the car rides really rough and bouncy in the back. I can hardly bounce the back of the car at all by pushing down on it. It's especially annoying because I live in an area with a lot of cement streets and every time I go over a joint in the road the whole back end of the car bounces and I get a big clunk. I read somewhere else online that the MP springs were junk and that I should just get another set from ESPO. Any comments or similar experiences?

 

[tallhair]

I'm looking forward to feedback on this post also... and hearing how your suspension mods plays out.

P.S. post some pics of your 72 already newb!

 

[RustyRatRod]

What did you torque the u bolt nuts to? They only torque to about 45 LB FT. If you over tighten them, the leafs cannot slip through the spring perch and shock plate and that will give you exactly what you describe. Most people make the mistake of running the nuts tight with an impact. Not the right way.

 

[jeremya72]

I will check that. I didn't use an impact, but I bet I tightened it too much. I'll get some photos soon. The steering column is still out so I cant pull it out of the garage until I put it back in.

 

[72RoadrunnerGTX]

Those KYB’s have a reputation for a harsh ride. Run that combination myself, handles pretty well and is a bit harsh.

 

[ater]

Hey guys, I was wondering if anyone else has had a similar problem. I recently rebuilt the suspension on my '72 Road Runner front and rear. On the back I reused the stock sway bar and replaced the leaf springs with MP HD big block springs from Mancini. I also replaced the shocks with some KYB's. Ever since then the car rides really rough and bouncy in the back. I can hardly bounce the back of the car at all by pushing down on it. It's especially annoying because I live in an area with a lot of cement streets and every time I go over a joint in the road the whole back end of the car bounces and I get a big clunk. I read somewhere else online that the MP springs were junk and that I should just get another set from ESPO. Any comments or similar experiences?

I went with the super stock springs and KYB all round on my 69 Roadrunner, tight and right, works well at track and really good on street.

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I went with Mopar super stock springs and KYBs on all four corners, good combo at the track and great on the street, 496ci 4-speed Roadrunner.

 

[Pistol Pete]

Those KYB’s have a reputation for a harsh ride. Run that combination myself, handles pretty well and is a bit harsh.

Last set of KYB's I bought was the last set I will ever buy. Made the car ride like a dumptruck.

 

[RustyRatRod]

Somehow this doesn't sound like shocks. He says he can hardly push down on the rear of the car. Sounds like something is in a bind.

I've even gone so far as to completely disassemble leaf springs, coat the contacting sides, all over the plastic spacers and where they contact the spring perches and shock plates with permatex anti seize and reassembled everything. This really makes the suspension work and the anti seize lasts a LONG time.

 

[Pistol Pete]

Somehow this doesn't sound like shocks. He says he can hardly push down on the rear of the car. Sounds like something is in a bind.

I only commented on the shocks because of my experience with them. That said, I can't say as I disagreee with you there Rusty. He certainly should be able to jounce the car from the backend.

Maybe this will help

 

[Roadbird]

last set of kyb's i bought was the last set i will ever buy. Made the car ride like a dumptruck.

lol.

 

[jeremya72]

You sure were right about this. I really overtightened all those nuts. I'll re-torque everything and hopefully be able to test drive it in a couple days when I get a day off. I'll let you know

What did you torque the u bolt nuts to? They only torque to about 45 LB FT. If you over tighten them, the leafs cannot slip through the spring perch and shock plate and that will give you exactly what you describe. Most people make the mistake of running the nuts tight with an impact. Not the right way.

 

[RustyRatRod]

Good. I hope that straightens it out. Caint get any simpler fix than that.

 

[68 Sport Satellite]

What did you torque the u bolt nuts to? They only torque to about 45 LB FT. If you over tighten them, the leafs cannot slip through the spring perch and shock plate and that will give you exactly what you describe. Most people make the mistake of running the nuts tight with an impact. Not the right way.

I posted about this same symptom with my 68 satellite a few months ago. I am running Hotchkis rear leafs. This is not something I considered, so thanks for the tip!

 

[RustyRatRod]

You bet.

I posted about this same symptom with my 68 satellite a few months ago. I am running Hotchkis rear leafs. This is not something I considered, so thanks for the tip!

 

[72RoadrunnerGTX]

I’m having trouble following along with this, “If you over tighten them, the leafs cannot slip through the spring perch and shock plate”. What movement are you referring to at the spring perch? The leafs are bolted together with the centering bolt at the point. Shouldn’t be any movement of the leafs in relation to each other at the spring perch/shock plate. Movement is taking place the ends of each leaf.

 

[RustyRatRod]

Yup, they sure do move some. That's why the torque spec is so low. The springs are in an arc. when they compress they have to be able to "slip" a tiny amount on each side of the pin in order to do their job. If the nuts are overtightened, they are clamped too tightly and cannot do this. You can prove it by taking two yardsticks and holding them together and bending them. The one on the outside of the bend will be longer in the bend. Even if they are pinned together, you can bend them on either side of the pin and the result is the same. The individual leaves must be allowed that slip or the suspension will not work.

I’m having trouble following along with this, “If you over tighten them, the leafs cannot slip through the spring perch and shock plate”. What movement are you referring to at the spring perch? The leafs are bolted together with the centering bolt at the point. Shouldn’t be any movement of the leafs in relation to each other at the spring perch/shock plate. Movement is taking place the ends of each leaf.

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Movement is taking place the ends of each leaf.

The leaves are all one piece. For the ends to move, the whole leaf has to move. If not, they would be stretching everytime the suspension flexed, and that sure doesn't happen.

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Tell you what you do. If you don't believe it, go jack your car up, stick jack stands under the rear frame, let the rear axle drop as far down as it will. Then loosen the u bolt nuts, then tighten up your u bolt nuts to 125 LB FT. Let the car back down and enjoy the "ride".

 

[72RoadrunnerGTX]

Movement of the leafs at the u-bolts?

"Correctly torqued U-bolts make the center section of the spring immobile"

 

[RustyRatRod]

Movement of the leafs at the u-bolts?

"Correctly torqued U-bolts make the center section of the spring immobile"

That's right AT THE PIN. But you're talking what? a 5-6" span where the springs are clamped. I'm tellin you this is why the u bolt nuts only torque to 45 LB FT. If you wanna believe everything you find on the internet, great. The more area the springs are clamped together, the more they tend to bind. This is something I've done and experienced. Not something I've dug up on the internet. I'm no engineer so my terms are limited. I cannot explain it any better than that. Do like I said. If you don't buy it, try it yourself. Then you'll understand.

 

[72RoadrunnerGTX]

45 foot pounds is the designed application limit for a grade-5 U-bolt with 1/2"x 20 rolled threads. In this Mopar application, 45lbs provides the full clamping force necessary to prevent movement of the leafs in the roughly 12.5” square area centered by the center bolt without exceeding the thread limits. Any torque above that threatens to stretch or fatigue the threads leading to clamping force failure. Multi-leaf spring technology of this type, in any application is based around this required clamping force at the center of the spring to prevent movement of leafs at the U-bolts, bigger the spring, spring load, number of leafs the bigger U-bolt requirements are to insure there is no movement. Movement at the U-bolts can and will lead to fractures of the leafs at the center bolt, the weakest point. As with any other suspension fastener, setting torque specs should be done at normal ride height only and should never be done with suspension hanging. Doing so will lead to binding of bushings and such will lead to undesirable ride characteristics of all types.
Dug up on the internet? That’s just one page of one site of countless sites all stressing the importance of achieving/maintaining clamping force at the center of leaf springs. Don’t have to be a mechanical engineer to see the endless information available on this subject.

 

[RustyRatRod]

Ok.