Gyroscopic coupling
Gyroscopic coupling
(OP)
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





RE: Gyroscopic coupling
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.
RE: Gyroscopic coupling
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
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RE: Gyroscopic coupling
RE: Gyroscopic coupling
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
RE: Gyroscopic coupling
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.
RE: Gyroscopic coupling
RE: Gyroscopic coupling
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.
RE: Gyroscopic coupling
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?
RE: Gyroscopic coupling
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
RE: Gyroscopic coupling
This is actually the old Omni/Horizon CU Reports problem. The solution was to increase the weight of the steering wheel.
Kevin
RE: Gyroscopic coupling
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.
RE: Gyroscopic coupling
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
RE: Gyroscopic coupling
As for gyroscopic progression, I was hungover during that lecture in dynamics class.
Damien
RE: Gyroscopic coupling
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
RE: Gyroscopic coupling
This is one of my favorite threads on the topic.
h
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?