>Dean maybe we are not communicating. I have talked to other owners of low
>mileage coaches and those that have restored there entire suspension to
>original condition and they are not experiencing these rut running problems.
I restored my 76 PB suspension to like new condition over two years ago. I
still got launched on the Texas panhandle and a few other locations. I
suspect that you may not be experiencing the same "quality" of ruts that I
run into going cross country. I can't think of any major East coast
highways the I drove on in the last two years with Texas sized ruts. I
definitely haven't driven all or even a significant portion of the East
coast roads. Amarillo to Albuquerque has some of the damnedest ruts you
want to see. The roads are being resurfaced but there's still plenty to
experiment with.
Part of the issue is travel speed vs rut characteristics. I tend to drive
the limit or slightly above when going form Ca to Pa. The rut problem
mostly went away for me if I dropped speed down to 50-55 MPH on all but the
most seriously rutted roads (we're talking an inch or more of rut with
definitive ridges). I had problems with an International Traveler
(stretched Scout) on this same section of road and a Range Rover. The
problems were not as severe but were still dangerous.
>I can tell you that I am not experienceing them with mine. So what is the
>difference if some are?? I dont know.
You mentioned that when you run off the edge of the road you do have the
expected handling issues. Some of the Southwestern ruts are very much like
running off the road surface and onto the shoulder in parts of NY.
I think that there are potentially several differences between rebuilt
suspensions:
1. the ruts may be different
2. difference in torsional stiffness of the GMCs.
3. differences in weight distribution
4. different suspension components (shocks for example)
5. different tire construction and therefore different tire spring rates
6. different loads presented by the torsion bar due to age
Some time ago I developed a back-of-the-envelop suspension model with 2
degrees of freedom. I did so because the differences in how our coaches
responded to changes in the suspension made no sense to me.
My 76 Palm Beach can be jacked on either front side and not have the
opposite side front wheel move off the pavement - even if I jack it pretty
high. A number of guys have reported that their chassis is stiff enough to
start lifting the opposite front wheel off the ground. I don't know what
the lateral weight transfer is but I know that in any case it's significant.
The importance of this factor is simple: each coach has a (very) different
torsional stiffness. That in turn means that the transfer function for
dynamic weight redistribution can be significantly different between two
otherwise identical coaches.
Bottom line is that what works on one coach may not work on another to
correct handling problems.
Other factors that we haven't discussed: front spring stiffness vs worn out
springs, bushings, and rear airbag spring rates. All will have an impact.
When GM made the switch to steel belted radials from polyester, they
increased the tire spring rate (a lot). This in turn means that the ride
will be harsher, assuming that all other factors remain constant. The only
solution to this factor is to lower tire pressure which will move another
problem to the forefront (maybe all of a sudden when we least expect it).
And, as the rotational speed increases, the stiffening of the sidewall
begins to overcome any advantage that is gained by lowering pressure.
Bilstein and other high performance shocks coupled with Alcoa wheels worsen
ride stiffness. This is experienced as vibration between 4 and 8 Hertz,
where human senses are most sensitive. The problem is that the lower
unsprung weight needs less damping, but we have increased the damping by
using Bilsteins etc. So, Alcoas+Bilsteins= more ride stiffness which in
turn transmits more road irregularities to the coach.
Wheel hop (10-20 Hz) is strongly influenced by wheel eccentricities, out of
balance tires, out of round tires, etc. The older heavier wheels served to
reduce the wheel hop at numerous frequencies as did the polyester plys.
Alcoas help out of round wheels and out of balance wheels but nothing else
related to tires. So, Jim Bounds contention that tire trueing is a good
thing is probably right - and more right than it was when the coaches were new.
RIDE HARSHNESS ISOLATION - For frequencies above 20 Hertz the vibrations
can be both felt and heard. The frequency range between 20 and 25 Hertz
corresponds to the minimum threshold for human hearing. These vibrations
are absorbed by the tires, suspension springs, suspension bushings, and
friction damping in the suspension systems. Conventional hydraulic dampers
(shocks) have small effect on the ride harshness isolation. Ride harshness
isolation depends greatly on the stiffness of the tires, the sprung mass,
loose or damaged components and friction in the suspension system, and
depends moderately on the unsprung mass and damping characteristics of the
tires. The ride harshness isolation balance from side-to-side depends on
deviations in tire stiffness, tire damping, friction in the suspension
system, and the sprung and unsprung masses.
Ride harshness isolation does not account for the dynamic effects of the
rotation of the tires such as the dynamic tire spring rate, the dynamic
tire damping rate, roundness, wheel imbalance, or aerodynamic effects. The
sprung mass consists of the vehicle frame, body, and load. The resonant
frequency of the sprung mass is usually between 1 and 3 Hertz - and there's
not a whole lot we can easily do about this. The ride will seem a lot
"nicer" if the resonant frequency of the air bad in the rear suspension and
the sprung mass resonant frequency do not coincide - otherwise oscillations
can happen.
The human body is the most sensitive to tactile vibrations in the frequency
range from 4 to
8 Hertz and drops rapidly for frequencies above 8 Hertz for the same
accelerations. The ride stiffness is highly affected by the damper (shock)
characteristics, the equivalent spring rate, and the sprung mass. Our
sprung mass is a relatively variable quantity (although reducing mass can
worsen the 4-8 Hz vibration). The two variables left to us are the shocks
and springs. I suspect that we would find that a good number of the front
end springs (torsion bars) are not providing the spring rate that was
designed - on some of the coaches. This has been my experience with older
passenger cars employing coil and leaf springs. So, the springs will tend
towards a softer ride (and more understeer) while the shocks are matched
for a stiffer ride. The result is an over-damped system that works the
shocks harder. For the rear bogies, we only have weight and shocks as the
controllable factors unless we replace the airbags with another system of
springs (4-bagger etc).
In my simple 2 degrees of freedom 1/4 model (one wheel) of the GMC, it's
pretty clear that the system as a whole is technically marginally stable
(but is more stable in the OEM configuration) from a systems design
standpoint. This is not unusual for automotive suspensions since the
engineering is always a compromise of handing qualities. Keep in mind that
I'm not talking about a complete model of the suspension, so some of my
analysis could be wrong.
The "launching" phenomenon of the GMC when encountering a rut is partly
explained by the relative sensitivity of the system. (technically, there
are poles and zeros pretty close to the imaginary axis, which means that
there *could* be instability). My guestimate on this "problem" is that if
the GMC is put back to the original configuration - meaning bias ply tires
etc - it will handle well. GM did a pretty good job of engineering in
general! Steel sidewalls and stiffer shocks (I think) move the system to a
point of instability under some transient loads (hitting a rut, potholes,
etc). When combined with the widely varying torsional stiffness exhibited
by the 25 years old GMCs it's no wonder that we see different handling
results. The front and rear tracking width difference DOES NOT have a
direct impact on rut handling as far as launching is concerned, from a
design point of view. Increasing the front track MAY move the front end to
a more stable point as regards bump steer - which means load transfer when
hitting a rut becomes a lesser concern. I haven't done that analysis yet.
Summary
1. Difference in torsional stiffness between coaches means that there is no
cure-all.
2. Steel sidewalls are required per GM, but add handling problems for some
coaches, and adversely affect ride quality relative to bias ply poly tires.
3. Stiffer shocks mean a harsher ride
4. Alcoa wheels increase the magnitude of some transient responses.
5. Some of the handling problems may be caused by the bump steer problems
common to mid 70s front wheel drive cars.
Henry
>mileage coaches and those that have restored there entire suspension to
>original condition and they are not experiencing these rut running problems.
I restored my 76 PB suspension to like new condition over two years ago. I
still got launched on the Texas panhandle and a few other locations. I
suspect that you may not be experiencing the same "quality" of ruts that I
run into going cross country. I can't think of any major East coast
highways the I drove on in the last two years with Texas sized ruts. I
definitely haven't driven all or even a significant portion of the East
coast roads. Amarillo to Albuquerque has some of the damnedest ruts you
want to see. The roads are being resurfaced but there's still plenty to
experiment with.
Part of the issue is travel speed vs rut characteristics. I tend to drive
the limit or slightly above when going form Ca to Pa. The rut problem
mostly went away for me if I dropped speed down to 50-55 MPH on all but the
most seriously rutted roads (we're talking an inch or more of rut with
definitive ridges). I had problems with an International Traveler
(stretched Scout) on this same section of road and a Range Rover. The
problems were not as severe but were still dangerous.
>I can tell you that I am not experienceing them with mine. So what is the
>difference if some are?? I dont know.
You mentioned that when you run off the edge of the road you do have the
expected handling issues. Some of the Southwestern ruts are very much like
running off the road surface and onto the shoulder in parts of NY.
I think that there are potentially several differences between rebuilt
suspensions:
1. the ruts may be different
2. difference in torsional stiffness of the GMCs.
3. differences in weight distribution
4. different suspension components (shocks for example)
5. different tire construction and therefore different tire spring rates
6. different loads presented by the torsion bar due to age
Some time ago I developed a back-of-the-envelop suspension model with 2
degrees of freedom. I did so because the differences in how our coaches
responded to changes in the suspension made no sense to me.
My 76 Palm Beach can be jacked on either front side and not have the
opposite side front wheel move off the pavement - even if I jack it pretty
high. A number of guys have reported that their chassis is stiff enough to
start lifting the opposite front wheel off the ground. I don't know what
the lateral weight transfer is but I know that in any case it's significant.
The importance of this factor is simple: each coach has a (very) different
torsional stiffness. That in turn means that the transfer function for
dynamic weight redistribution can be significantly different between two
otherwise identical coaches.
Bottom line is that what works on one coach may not work on another to
correct handling problems.
Other factors that we haven't discussed: front spring stiffness vs worn out
springs, bushings, and rear airbag spring rates. All will have an impact.
When GM made the switch to steel belted radials from polyester, they
increased the tire spring rate (a lot). This in turn means that the ride
will be harsher, assuming that all other factors remain constant. The only
solution to this factor is to lower tire pressure which will move another
problem to the forefront (maybe all of a sudden when we least expect it).
And, as the rotational speed increases, the stiffening of the sidewall
begins to overcome any advantage that is gained by lowering pressure.
Bilstein and other high performance shocks coupled with Alcoa wheels worsen
ride stiffness. This is experienced as vibration between 4 and 8 Hertz,
where human senses are most sensitive. The problem is that the lower
unsprung weight needs less damping, but we have increased the damping by
using Bilsteins etc. So, Alcoas+Bilsteins= more ride stiffness which in
turn transmits more road irregularities to the coach.
Wheel hop (10-20 Hz) is strongly influenced by wheel eccentricities, out of
balance tires, out of round tires, etc. The older heavier wheels served to
reduce the wheel hop at numerous frequencies as did the polyester plys.
Alcoas help out of round wheels and out of balance wheels but nothing else
related to tires. So, Jim Bounds contention that tire trueing is a good
thing is probably right - and more right than it was when the coaches were new.
RIDE HARSHNESS ISOLATION - For frequencies above 20 Hertz the vibrations
can be both felt and heard. The frequency range between 20 and 25 Hertz
corresponds to the minimum threshold for human hearing. These vibrations
are absorbed by the tires, suspension springs, suspension bushings, and
friction damping in the suspension systems. Conventional hydraulic dampers
(shocks) have small effect on the ride harshness isolation. Ride harshness
isolation depends greatly on the stiffness of the tires, the sprung mass,
loose or damaged components and friction in the suspension system, and
depends moderately on the unsprung mass and damping characteristics of the
tires. The ride harshness isolation balance from side-to-side depends on
deviations in tire stiffness, tire damping, friction in the suspension
system, and the sprung and unsprung masses.
Ride harshness isolation does not account for the dynamic effects of the
rotation of the tires such as the dynamic tire spring rate, the dynamic
tire damping rate, roundness, wheel imbalance, or aerodynamic effects. The
sprung mass consists of the vehicle frame, body, and load. The resonant
frequency of the sprung mass is usually between 1 and 3 Hertz - and there's
not a whole lot we can easily do about this. The ride will seem a lot
"nicer" if the resonant frequency of the air bad in the rear suspension and
the sprung mass resonant frequency do not coincide - otherwise oscillations
can happen.
The human body is the most sensitive to tactile vibrations in the frequency
range from 4 to
8 Hertz and drops rapidly for frequencies above 8 Hertz for the same
accelerations. The ride stiffness is highly affected by the damper (shock)
characteristics, the equivalent spring rate, and the sprung mass. Our
sprung mass is a relatively variable quantity (although reducing mass can
worsen the 4-8 Hz vibration). The two variables left to us are the shocks
and springs. I suspect that we would find that a good number of the front
end springs (torsion bars) are not providing the spring rate that was
designed - on some of the coaches. This has been my experience with older
passenger cars employing coil and leaf springs. So, the springs will tend
towards a softer ride (and more understeer) while the shocks are matched
for a stiffer ride. The result is an over-damped system that works the
shocks harder. For the rear bogies, we only have weight and shocks as the
controllable factors unless we replace the airbags with another system of
springs (4-bagger etc).
In my simple 2 degrees of freedom 1/4 model (one wheel) of the GMC, it's
pretty clear that the system as a whole is technically marginally stable
(but is more stable in the OEM configuration) from a systems design
standpoint. This is not unusual for automotive suspensions since the
engineering is always a compromise of handing qualities. Keep in mind that
I'm not talking about a complete model of the suspension, so some of my
analysis could be wrong.
The "launching" phenomenon of the GMC when encountering a rut is partly
explained by the relative sensitivity of the system. (technically, there
are poles and zeros pretty close to the imaginary axis, which means that
there *could* be instability). My guestimate on this "problem" is that if
the GMC is put back to the original configuration - meaning bias ply tires
etc - it will handle well. GM did a pretty good job of engineering in
general! Steel sidewalls and stiffer shocks (I think) move the system to a
point of instability under some transient loads (hitting a rut, potholes,
etc). When combined with the widely varying torsional stiffness exhibited
by the 25 years old GMCs it's no wonder that we see different handling
results. The front and rear tracking width difference DOES NOT have a
direct impact on rut handling as far as launching is concerned, from a
design point of view. Increasing the front track MAY move the front end to
a more stable point as regards bump steer - which means load transfer when
hitting a rut becomes a lesser concern. I haven't done that analysis yet.
Summary
1. Difference in torsional stiffness between coaches means that there is no
cure-all.
2. Steel sidewalls are required per GM, but add handling problems for some
coaches, and adversely affect ride quality relative to bias ply poly tires.
3. Stiffer shocks mean a harsher ride
4. Alcoa wheels increase the magnitude of some transient responses.
5. Some of the handling problems may be caused by the bump steer problems
common to mid 70s front wheel drive cars.
Henry
