Leaf spring rear suspensions

[GregLocock]

If I were to redesign a light truck's rear suspension I would like to reduce the compliance oversteer.

It seems to me that one thing that would help would be to put the hanger at the front of the leaf spring.

Has anyone tried that? What problems did it cause?

I realise it would tend to cause more driveline plunge, and ground clearance might be an issue, but other than that I can't see a downside.

Alternatively, other than fitting a Panhard rod, has anybody got any neat suggestions to get the same effect?

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[Fabrico]

A similar reduction in compliance oversteer, while maintaining ground clearance, may be available by using a substantially flat spring mounted on top of the rear axle.

Jeeps have used front mounted shackles on the front springs for many years, so it can work. However, it is fairly popular among off-roaders to change the shackle to the rear in order to improve handling. How this would relate to the rear springs is uncertain.

Of course a 3 or 4 link with coil or quarter-elliptic springs is also an alternative.

 

[BillyShope]

How about quarter elliptics, fixed at BOTH ends, with a triangulated upper link? This forces the spring into an "S" shape, thereby doubling the stress, but there might prove to be some other advantages for your application.

 

[GregLocock]

Ah, perhaps I ought to explain I am talking about a production vehicle, so the budget is 20 bucks.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[patprimmer]

I presume you have seen some trailer springs where the front eye is located by a bolt and the rear edge floats back and forward in a boxed section rather than using a shackle. Could you let both ends float and use a radius rod on each side. The radius rod could be located so as to create the toe change you require.

Radius rods will be somewhat more expensive than the shackles and bolts they replace, but the springs should also be a little cheaper if no eyes need to be rolled into the top leaf. I do not know what a simple radius rod costs at OEM level, but it might be doable.

Also as the springs are now not used to the same extent to locate the axle, you might be able to put more emphasis on load carrying and less on location characteristics when deciding their dimensions.

Regards

eng-tips, by professional engineers for professional engineers
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[GregLocock]

A simple radius rod is about 10 bucks. But it doesn't really solve my problem, I need to stop the rear of the axle from moving, and let the front steer away from the lateral force at the CP, which is what a Panhard rod does for me.





Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[BillyShope]

Perhaps I didn't adequately describe my quarter elliptic design. First, forget the "triangulated" business. The upper link would be solidly mounted to the axle housing with a pivot at the front. This would be the ONLY bushing as, again, the single leaf springs would be solidly mounted at both ends. The springs would, of necessity, have an unconventional aspect ratio in cross section, which, with the solid mounting at both ends, would make them more than adequate to control deflections in "Y."

Don't see how you could achieve any further cost reduction and maintain pinion angle over the life of the springs.

 

[Fabrico]

Transverse leaf spring with desmodronic shackles?

 

[Warpspeed]

My understanding is that roll understeer is usually built into the rear by having the rear spring mounting points located higher than the front spring mounting points. Each wheel moves forward in bump and rearward in rebound.

But moving the spring hangers to the front will certainly increase the built in rear roll understeer, it may just not be the ideal method to reduce a compliance oversteering problem. Much better would be to try to unload the springs from lateral forces altogether somehow, rather than compensate for any compliance steer.

Perhaps more body roll may allow the built in roll understeer to work as intended ?

 

[GregLocock]

Well, you've rather got things backwards. The roll understeer is put in, primarily, in order to compensate for the compliance oversteer. Because these two effects have different time constants this causes a two stage feeling in the cornering.

Typically what would happen would be the driver would enter the corner with a certain amount of steering wheel angle, then when the vehicle rolls over he'll have to add some more.

This is rather unsatisfactory.

Now there are other ways round this, but it would be nice to get rid of the root cause.



Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[BossBill]

Greg,

Have you considered removing the shackles altogether and using a slider?
Here is a hard slider assy:

http://www.secureperformanceorder.c...D=14&ClassID=180&SubclassID=851&ProductID=985

and there are also poly sliders available.
Use a very flat spring and a slider and you may not require a panhard.
It could be mounted on the front or rear.
Hmmm...use radius rods and do both?
It does somewhat blow your $20 budget.

 

[GregLocock]

Not durable enough for me, but that is a great idea, thank you.


Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[carnage1]

wider springs with a stiffer narrower bushing inside to reduce side flex? have the upper poriton of the shackles rotate on non compliant bushings or bearings? mount the shackles higher on the frame and use the frame to keep the shacles form deflecting? you have three joints could you make two of them non compliant and leave the third one compliant to prevent overconstaining?

 

[Triaged]

How about giving the leaf springs some toe-out. Obviously the bushings won't like you much.

 

[GregLocock]

Sure, we can muck about with the spring geometry, that is all old-hat. Typically you toe them in, for a rear shackle spring, and tilt them down towards the front.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[ghamish]

The slider idea made me think maybe you could get rid of the shackle, & replace it with a large rubber bush that's compliant in the directions you want, & stiff in other directions?

 

[Warpspeed]

Greg, I think you have answered your own question.

If the problem is rear compliance steering, and compensating for this by introducing some deliberate roll steer is not acceptable, then a panhard bar, or other positive axle location linkage would be about the only alternative.

 

[GregLocock]

ghamish, yes Olley suggested much the same thing in the thirties. It turns out there are quite few solutions, of various degrees of expense and difficulty.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[hjkort]

A very simple question:

I can't find a formula to determine the springrate of a parabilic leafspring...

Anyone?

Cheers,

HJKort

 

[BillyShope]

Pages 301 through 306 of "Design of Machine Members," by Vallance and Doughtie cover it pretty thoroughly. You'd better hope it's in your company library, though, as it's what I used as a student in the early fifties.