Axle Wrap point of rotation

[heavytrucker]

When a vehicle with a conventional hotchkiss suspensionsexperiences axle wrap and resulting "S" shaping of the leaf springs, does this roation happen about the axis of the axle or the point where the leaf springs mount to the axle?

Any help would be appreciated.

 

[evelrod]

Reaching back to the late 50's when I did a bit of drag racing in my much modded '49 Mercury...
That car tended to rotate about the leaf spring mount below the 2 1/2" lowering blocks I used. For that car, and considering at the time I was only a first year engineering student, the cure seemed to be to bolt/clamp a length of 'T' bar to the front portion of the leaf spring. Seemed to cure the axle wrap up and the attendant wheel hop.
I don't think that I would do that today. Of course I would not use lowering block and I have nothing worthy of drag racing should insanity come visiting. Now a days a good set of leading/trailing rods with urethane or rod ends would do the job without the attendant 'rock crusher' ride.

Rod

 

[FeX32]

When I picture the torque exerted from the axle held vertically by the leaf-springs I would expect the reaction torque to be transferred through the springs to the chassis in turn creating a moment about the leaf spring mounting points.
But naturally, the point of wrap (maximum rotation of the leafs) should be the axle. (hance, the "S")
I would expect the axle to rotate about the mounting point only if the leaf spring had one mount.


Fe

 

[GregLocock]

Is this caused by braking or traction?

This is a non linear FEA of a leafspring showing it at the design load on the left, and then with a torque on the right.







Cheers

Greg Locock

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

 

[FeX32]

Nice! That FEA adds dimension to the topic.
I have never done any analysis on leafsprings. By non-linear, are you referring to the analysis in general or to the fact that the leafsprings may exhibit a non-linear stiffness?

Fe

 

[GregLocock]

It is a non linear analysis in that the material properties could be non linear (they aren't in this case) and that it accounts for contact between the leaves and also the secondary, and that it accounts for the changes in geometry, which a linear FEA doesn't.

Cheers

Greg Locock

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

 

[FeX32]

Oh I see. Makes sense.
Thanks,

Fe

 

[evelrod]

If the center of rotation were the axle housing I would not expect to see lateral movement or change in the wheelbase. In actual practice, that was not the case---accounting for memory loss over the last 60 years. Be that as it may, the tires in that very much lowered Merc did move laterally. I expect that throwing in the lowering blocks may have altered the suspension and thus the spring wrap up in a negative way, thus the reaction an wheel hop...both under acceleration and, to a lesser degree, braking.

Rod

 

[Bribyk]

It doesn't really make sense to define a "point of rotation". What you have is a beam with a vertical (normal), horizontal force applied (axial) at some distance and a moment. The axial force at a defined distance and the torque reaction (wrapping) of the axle contribute moments to the beam. If we remember our statics, it doesn't matter where you apply a pure moment to a beam, the result is the same. The resulting deflections and slopes are dependent on the force/moment magnitudes and the geometry of the beam.

 

[heavytrucker]

"Is this caused by braking or traction?"

It is caused mainly by braking. And braking on a 60 percent slope at that.

 

[GregLocock]

OK, in that case what I posted is a reasonably valid model of the excitation.

As you can see there isn't really an axis of rotation in any helpful sense.

Incidentally the angle of the shackle (not shown in my picture, it is present in the model) has an enormous influence on this sort of problem.




Cheers

Greg Locock

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

 

[NormPeterson]

Would it be accurate to consider this situation as being an instantaneous center of rotation? I'm thinking that such an axis would start out close to the centerline of the leaf, but migrate as the spring develops its "S" shape and moves the axle longitudinally.

The axle center can't be the answer except perhaps by coincidence, as it is at the end of a cantilever and cannot offer geometric constraint (other than mostly ensuring that the two springs wrap/unwrap in unison).


Norm

 

[JohnLear]

I was thinking the same thing as Norm. I can't see the axle shaft axis as being definitive, surely the centre of rotation must be a product of the spring geometry and the distribution of stiffness within the spring?

 

[FeX32]

I think Bribyk summed it up in my view. If it were purely a moment then possibly the center of rotation could be the same as the applied moment. However, there are other forces that shift this.

Fe

 

[GregLocock]

an applied moment doesn't have a point of application. Statics book chapter 1.

Cheers

Greg Locock

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

 

[FeX32]

surely when constructing a simple moment diagram we must take into account the point of application.

Fe

 

[FeX32]

By the way, in statics book chapter 1 applied moments do have a point of application. ONLY in the case where the moment is a COUPLING moment you can say that the moment is the same everywhere. (Here you would have to prove definitely that the moment created in fact is created by 2 equal forces in a couple which don't move)



Fe

 

[FeX32]

I would like to add that the lack of correct definitions by some posters can cause this confusion. (this is just to clarify)
Furthermore, in this particular problem one can easily prove that there will exist a COUPLING moment in the leafsprings. This justifies my agreement with Bribyk as before...


Fe

 

[GregLocock]

If you apply a torque to a rigid body the spatial coordinates at which you apply it are irrelevant.

Whatever other things you are talking about cannot confuse this fundamental point.






Cheers

Greg Locock

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

 

[FeX32]

There is no confusion on my side. By definition it is a coupling moment. If you do not imply a coupling moment then forces are associated with it. (you did not define it correctly)
Statics book chapter 1.

Fe