Gyroscopic coupling 1

[SMOKEY44211]

My shop services a large # of medium duty cab over comercial trucks. They all have Ibeam straight axels with leaf springs. Perplexing problem occurs on some of them (about 30%) at some point in thier service life if you hit a bump between 55-70 mph. the steering wheel goes into an uncontrolable shake until the vehicle slows down. I've explored the obvious possible worn king pins, tie rod ends, shocks ect. but find nothing out of spec. Tire balance, air pressure, and alignment have little if any affect. The only way I have been able to cure the problem is by installing a steering damper by custom fabricating the needed brackets. This has worked out well for those that see only service on pavement. Some of my customers supply building materials for new home construction and therefore travel over some irregular terrain. I've had a few come back damaged (dampeners)as a result of these off road excursions. My concern is possible liabilities I may experience if one of these damaged units should cause a loss of steering conrol. The question is there any other method of eliminating the problem without installing a damper and why is it that some of them don't ever have the problem? -------Phil

 

[GregLocock]

I'd have a look at the leaf springs themselves, particularly the eyes and the brackets.

That problem is supposedly caused by excessive camber gain on an IFS, I doubt that it applies here.



Cheers

Greg Locock

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

 

[patprimmer]

I would be looking at shackle bushes, and also be checking castor and camber. overloading or high speeds over bumps when loaded might cause the axle to bend a little giving excess negative camber.

Compliance due to wear in the shackle bushes could give all sorts of dynamic changes to steering, toe and castor.

Regards
pat pprimmer@acay.com.au
eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[SMOKEY44211]

I took your advice and double checked the most recent vehicle with this problem taking particular notice of springs, spring bushings, and shackles. Everything appears in good shape. On one of the trucks I sent over to an alignment shop making them aware that I was trying correct this problem. They had installed a couple of wedges to cause more negative castor. Doing this seemed to aggravate the problem. I tried switching the wedges to provide positive castor. This seemed to help but the problem persisted. The front rims are offset in such a way that it appears the design goal is to provide zero scrub. I should also mention that the problem occurs when the trucks are empty. Is there a toe in/out setting and or positive/negative castor trend that would make this less likely to happen?-------Phil

 

[JakubMech]

Ive experienced this with older four wheel drives using a solid front axle with two longatudinal links going back behind the axle for the longatudinal restraint and a panhard rod for lateral restraint. As in your case if the vehicle hit a bump or a series of undulations at a certain speed, the steering wheel would start to oscilate and shake the entire car at increasing severity until the car is slowed down.

I put it down to the stiffness of the bushes in the longatudinal links, seting up an axle type tramp resonance which shakes the entire car and feeds its tramping displacements back through the steering wheel.

What do you guys think?

JMech

 

[PTwizz]

Smokey,

I assume from your original query that this problem is not observed in new vehicles. Could it be that the geometry and stiffness issues which cause the problem are always present, but are masked by friction (in the steering linkages) on newer vehicles? Older motorcycles often used simple friction steering dampers to prevent 'wobbles'.

Pete.

 

[SMOKEY44211]

PTwizz There may be some merit to what you said. I remember reading somewhere that passenger cars of the late 20's and early 30's commonly experienced this problem as paved roads became more commonplace and vehicle speeds rose. I think they reffered to it as axel tramp. If I recall correctly this was the motivating factor for implementing independant front suspension. The trucks in question that I've been dealing with seem to manifest this problem around 45,000 miles. At first I reasond that tire wear was the culprit as they were nearing or at the end of thier usefull service life. Replacing them with new ones didn't seem to help much. The thing that is most curious to me is that some of them have very high milage, slightly bent rims, uneven tread wear, and a number of other defects, go down the highway without a hint of this phenomenon occuring.--------Phil

 

[PTwizz]

Phil,

I would guess that steering linkage wear might not be related to mileage or other abuse. Driving my (non PAS) Landrover tells me that high steering loads occur at low speeds, i.e. when parking or tuning in a yard etc. Some trucks may do many highway miles with few low speed manouvers, causing little steering link wear. Is it practical to lift the front axle and measure, or just 'feel' steering friction?

Pete.

 

[Tmoose]

Den Hartog "Mechanical Vibrations". p 329 - 33?

He says for cars of pre-independent era there often was a vertical component and at least 3 degrees of freedom involved.

Were any of the problem vehicles given fresh shocks (providing some added vertical damping), even though the originals seemed fine?

 

[PJGD]

I have had this experience too, some years ago in my 1953 Jowett Bradford van. I would be driving down the road at perhaps 30 MPH; one wheel would hit a pothole, and then the steering wheel would be wrenched from my hands as the front axle went into this lock-to-lock oscillation at perhaps 3 Hz with no hope of stopping it through grabbing the wheel, unless you wanted broken wrists!

The first time it happened, I was petrified since I was totally out of control and the vehicle was shaking violently. The brakes in the Bradford were mechanical (rod linkage), and due to geometric shortcomings in the linkage, the steering oscillation caused brake pedal oscillations too, so that it was initially impossible to slow down by braking either. You could only let the vehicle speed and the wheel oscillations die down naturally. Later, after I became used to the effect, I took some pleasure in watching the faces of motorist coming towards me who couldn't quite believe what they were seeing.

I never did discover what feature of the pretty basic steering on the Bradford caused this effect. Certainly the vehicle was old, well travelled, and worn. Nevertheless, it was always maintained well enough to pass the UK MOT test. As I recall, it had a retrofit friction damper on the track rod, and eventually I tightened that up which helped the situation. The Bradford was essentially a pre-war chassis design with rolled chassis rails, half eliptic leaf springs and forged I-beam front axle. In retrospect, not very different in fundamentals from the modern trucks that you are referring to; my word, we have come a long way in 50 years!

PJGD

 

[c2s]

The problem is that the stiffness of the steering system has changed. This could be from steering gear wear (even adjusting preload may not fix this) or worn rag joints (if so equiped), etc. The root of the problem lies in the natural frequency of the system as designed. Since freq = sqrt(k/m) (simplified) one solution is to change m (mass). Try adding a pound or two to the steering wheel rim (for example, tape wheel weights to it).
This is actually the old Omni/Horizon CU Reports problem. The solution was to increase the weight of the steering wheel.
Kevin

 

[alwieser]

The couple involved is between the natural frequency of the axle/wheel assembly in tramp and the wheel/steering linkage in shimmy. When the design of the system results in bringing the natural frequency of these two motions into proximity, damping is critical in controlling the potential unstable oscillations. Wear in the suspension and steering components lessen the available damping.

It would seem to me that adding weight at the steering wheel would be the least effective place to alter the frequency of the system as it is furthest away from problem. I know from first hand experience, that using my hands, arms and shoulder muscles to increase damping at the steering wheel does very little to control this situation once it developes.

After checking for wear in steering and suspension bushings and joints, changing the mass of the tire and wheel is probably the most effective way to solve the problem. A larger tire or change in the wheel offset can help reduce the shimmy frequency. Although these changes also affect tramp frequency, the ratio of change is different. Thus, the natural frequency of the two oscillating systems move further apart.

 

[NormPeterson]

It would seem to me that adding weight at the steering wheel would be the least effective place to alter the frequency of the system as it is furthest away from problem.

Mass placed there is made more effective by virtue of the ratio in the steering box or rack. Since that increases the rotational acceleration of the steering wheel relative to what the road wheels are experiencing, a little steering wheel mass goes a much longer way than would half that mass added to each front wheel.


I'll speculate that the Omni/Horizon solution was influenced by the need to maintain the original EPA inertia weight class, to minimize unsprung mass in that relatively lightweight automobile, and possibly to improve other steering column vibration modes (all of which are likely to be less critical considerations for medium-duty commercial trucks).


Norm

 

[damienmavis]

I had this same problem on an offroad race type truck I built. I solved it! but I did two things at the same time so I'm not sure which one fixed it. 1 I installed a hydraulic ram assist in the powersteering system, I assumed this acted like a steering damper. 2 this is what I think solved the actual problem. All the steering was on rodends (brand new, insignificant amount of play) The pitman arm was connected, through a short shaft with rodends on both ends, to an intermediate link which through a bellcrank was connected to the upright. The problem was, the connection to this intermediate link did not occur on the tubes centerline. It was conected about an inch off the centerline, this meant that there was a little bit of slop as the pitman arm moved. I cured it by eliminating the rotation of the shaft and hence the slop by useing a bronze bushing instead of a rod end. I think at a certain speed and road input the wheels and hence steering oscilated through this slop, undamped. The tires probably acted as the springs in the system, twisting in thier contact patch and loading up. Interestingly enough there was no shake when in the dirt. This seems to make sense because the lower traction would prevent the tires (springs) from preloading then dumping the energy into the system and causing the shake. With the front wheels jacked off the ground and free there would be some play in the steering gear downstream of the box. I bet there is a corralation between the amount of this play and the shakeing experienced. My suggestion is minimize play in the steering system and as we both found out a damped system vibrates less that an undamped system. So even though the damper fixes the symtom, not the cause maybe thats ok.

As for gyroscopic progression, I was hungover during that lecture in dynamics class.

Damien

 

[notnats]

With a solid beam axle, I don't see how you can prevent this without heavily damping the steering.

As I remember my theory of machines classes, if the right wheel (for example) rebounds from a bump, the gyroscopic precession will force the steering to the left. Forcing the steering back to the right will try to force the right wheel back down, but the down movement will then turn the steering to the right which will then continue the process.

I am happy to be corrected, but it seems that worn joints may only play a secondary role in the process.

I remember in the 60's seeing a pre war Austin 7 special open wheeler suffering amazing front axle tramp under braking, lap after lap. I don't know how he could stand it

Jeff

 

[Triaged]

In off road circles this is often referred to as "Death Wobble" and I think for good reason. I just about soiled myself when it first happened to me. Searching google will come up with many interesting "solutions" to the problem.

This is one of my favorite threads on the topic.

http://coloradok5.com/forums/showthread.php?t=34596

Most interesting is the statement that the power steering system might be one of the contributing factors to the problem.

Has anyone looked into the power steering system for a solution?