Front parallel leaf spring design questions

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PackardV8
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Front parallel leaf spring design questions

Post by PackardV8 » Sun Sep 02, 2018 6:06 pm

On the older solid front axle pickups with parallel leaf springs, some have the fixed eye at the front of the frame horns and the shackle at the rear. My Studebaker has the shackle at the front and the fixed eye at the rear. Can anyone detail the reasons for choosing one or the other and the tradeoffs?
Jack Vines
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Re: Front parallel leaf spring design questions

Post by peejay » Sun Sep 02, 2018 7:40 pm

My understanding is that shackle in the rear is better for handling reasons as well as ride quality reasons, but most vehicles had shackles in the front for simple space reasons - the frame generally needs to be higher at the shackle end and it is hard to do that when it is not at the extreme end of the vehicle.

Modern Ford trucks with leaf spring front ends have the shackle at the rear, incidentally.

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Re: Front parallel leaf spring design questions

Post by enigma57 » Thu Sep 20, 2018 6:08 am

Jack, peejay's comments reflect my recollections on this as well. Placing shackle at rear spring eye will generally handle better, be less prone to bump steer and be more stable at speed, whether straight line or cornering.

Not sure what you are working on. Could be anything from a boat trailer to a vintage track car, but here goes......

If you are fabbing a new solid axle front suspension setup for a street rod or track car, you might consider welding a piece of pipe or tubing through 2" wide X 4" tall boxed frame rails (near top of frame rail) and have shackle halves drop on each side of frame rail so as to position rear spring eye within a couple inches of bottom of frame rail for ground clearance (as close to bottom of frame rail as is practicable, allowing for shackle to move through its arc of motion). You can use wider 2-1/2" spring leaves as well and this will make the type installation I am describing easier to fab. Wider spring leaves requiring fewer leaves than narrower, taller spring packs also result in lighter overall weight of spring packs.

If frame rails kick up at front, just position perch for fixed front spring eye directly beneath frame rail. Doesn't hurt a thing for front spring eye to be higher than rear spring eye so long as you position spring perches on front axle to provide correct positive caster (king pin inclination of 5 degrees or so).

And don't make springs overly short nor highly arched. Something similar to 1960s MOPAR rear spring design with shorter run from axle to front spring eye and longer distance from axle to rear spring eye with stiffer spring stack up in shorter front portion of spring would probably help ride, handling and stability noticeably at higher speeds as well. Keep springs close to flat (maybe 2" of arch) when loaded. Good shocks and anti-sway bar fitted, as well. Might even consider a panhard rod setup to better keep front axle centered in high speed turns.

In other words, avoid the typical 'gasser' style solid front axle / parallel leaf spring setup with short, highly arched leaf springs like the plague.

That's pretty much how Uncle Frank explained it to me before he passed (Dad and my Uncle Frank built and raced a car in the Memphis area in the late 1920s). My mention of 1960's MOPAR style leaf springs, anti-sway bar and panhard rod at front are my own ideas.

FWIW...... In those days, Dad and Uncle Frank's race car didn't run shock absorbers (neither friction nor hydraulic) as we know them today. They ran 'snubbers'. Uncle Frank described these as wide leather straps which were adjustable. They would get several guys to sit on race car and depress springs whilst Uncle Frank cinched up the snubbers from under each axle to over frame rail. This is how they pre-loaded suspension to suit track conditions in 1928. :shock:

Hope this gives you some ideas,

Harry

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Re: Front parallel leaf spring design questions

Post by pdq67 » Fri Sep 21, 2018 4:13 am

Leather, "snubbers"...

I think stage-coaches were sprung and shocked this way too if you look closely at them..

pdq67

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Re: Front parallel leaf spring design questions

Post by enigma57 » Sat Sep 22, 2018 5:05 am

Image

:D Could be...... Wouldn't be surprised, pdq!

Best regards,

Harry

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Re: Front parallel leaf spring design questions

Post by af2 » Sat Oct 06, 2018 4:48 pm

PackardV8 wrote:
Sun Sep 02, 2018 6:06 pm
On the older solid front axle pickups with parallel leaf springs, some have the fixed eye at the front of the frame horns and the shackle at the rear. My Studebaker has the shackle at the front and the fixed eye at the rear. Can anyone detail the reasons for choosing one or the other and the tradeoffs?
Must be rear steer.
GURU is only a name.
Adam

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Post by dwilliams » Sun Oct 07, 2018 10:51 am

The main difference is whether you get roll oversteer or roll understeer. That is, the axle cocks slightly during roll.

In practice, the effects are probably down in the "barely notice" category.

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