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January
2003
SHOCKS WANTED FOR RESEARCH
In previous newsletters, notably August 2000 and
December 2001, I have discussed acceleration sensitivity of
shock absorbers. (Acceleration is the rate and direction of
change in velocity.) I noted that just looking at the difference
between the extended end of the stroke and the compressed end
of the stroke in a standard sinusoidal shock dyno test will
give you a crude indication of a damper’s acceleration sensitivity.
If the two ends of the stroke look substantially different,
that suggests a high degree of acceleration sensitivity.
It is reasonable to suppose that differences
in acceleration sensitivity are a big part of the reason why
shocks that generate similar traces in the most common dyno
test (sinusoidal motion produced by a crank, 2” stroke, 100
rpm) can act so different on a car.
I have for some time been interested in investigating
the matter more systematically. I would like to come up with
test procedures that will give us a way to measure and quantify
sensitivity to acceleration, and also investigate the importance
of jerk sensitivity. (Jerk is the rate and direction of change
in acceleration.)
It now looks like I’ll get the chance. At least
one, and quite possibly two, shock dyno manufacturers are interested
in working with me on this. At least one up and coming damper
manufacturer has expressed interest in building shocks for test.
At least one racing team has expressed interest in working with
us. That’s enough to get started.
What I’m looking for now is additional teams actually
running cars, who are interested in collaborating on this. As
things stand, teams won’t pay us, and we won’t pay them. Teams
will furnish shocks they already run, and/or experimental ones,
for dyno testing, and provide us with feedback regarding how
the various shocks affect their car. We are particularly interested
in obtaining shocks that are reported to dyno similarly but
act different. The team gets free shock dyno testing and a better
understanding of how shocks work. We get free shocks to test,
and a better understanding of how shocks work. At some point,
perhaps I will write a feature article in Racecar Engineering,
and get some publicity for the consulting business. Teams interested
in being on the inside of this cutting-edge effort are invited
to contact me at the address, e-mail, or phone at the top of
this page.
TIRE WARMERS FOUND
Back when the forum on www.RacecarTech.com
was running, somebody asked me where to get tire warmers for
their dirt Late Model. At that time, I pursued several leads,
but they all ultimately came up dry.
Finally, at the PRI show last month in Indianapolis,
I found a US source. They are Chicken Hawk Racing, at 249 Hapeman
Hill Rd., Red Hook, NY 12571. Their phone is 866-HOT-TIRE (468-8473).
They have a website at www.chickenhawkracing.com.
For those unfamiliar with tire warmers, they are
basically high-temperature electric blankets that wrap around
a tire’s tread and heat it up. Apart from the obvious advantage
of giving you sticky rubber right from the green, tire warmers
also allow you to heat and cool the tires gently, and keep them
hot between runs. This reduces the effects of heat cycling,
keeping the rubber soft longer. Additionally, they allow you
to set your “cold” pressure at a controlled temperature, rather
than ambient. This temperature can be high enough to assure
that the tire won’t have any significant liquid water in it.
Regular readers may recall that water in a tire does not cause
any unusual pressure rise if it’s in the vapor state when you
set the pressure.
Why wouldn’t you use them? First of all, many
sanctioning bodies and tracks have outlawed them as a cost-containment
measure. And they aren’t cheap. Chicken Hawk sells two models,
one for around $1500 and one around $2000. That’s each, and
you need at least four for a car (they make them for motorcycles
too). The less expensive model has a pre-set thermostat, ordinarily
175deg F (80deg C). The more expensive model has an adjustable
thermostat, and a digital thermometer so you can see if the
tire’s up to temperature yet.
Whether the performance gain is worth the money
depends on your personal situation, but the performance gain
is real.
SPRING PLACEMENT ON TRIANGULATED
4-LINK
Question:
I have a question on rear spring placement
on a stock metric 4-link suspension. I have built several chassis
and have been mounting the rear spring centerline forward 2½
inches of the centerline of the axle. I’ve started on a new
chassis and thought I would go back to mounting the spring directly
on the axle centerline. Since the housing does not rotate under
power I don’t feel I’m gaining anything. Does mounting the spring
forward of centerline affect the static rear percentage or in
any way change the motion ratio of the spring?
Answer:
US oval track racers will need no introduction
to this type of suspension. For readers unfamiliar with it,
this is what is sometimes called a triangulated 4-link, or Chevelle-style
4-link. It has been used on various GM cars, including the “metric”
series referred to here, and also recent Mustangs. It is illustrated
on p.648 of Milliken and on p.260 of Gillespie. It uses four
angled trailing links to locate a beam axle both longitudinally
and laterally, with no Panhard bar, Watt linkage, or other purely
lateral locating device.
In most such layouts, the side-view geometry gives
a substantial amount of anti-squat. The axle does rotate with
ride motion, nose-down in bump and nose-up in droop. However,
the only rotational compliance with drive torque comes
from flexure of the parts, mainly the bushings. When the questioner
here says the axle does not rotate, he means that there is no
highly compliant torque absorbing device such as a torque arm
or pull bar incorporating a spring.
In roll, there is little or no axle housing rotation.
The location of the springs has no effect on
static rear percentage, except that the mass of the springs
is positioned slightly further forward or back. Spring location
fore-and-aft does affect motion ratio a little bit in ride.
Moving the spring forward makes the spring-to-wheel motion ratio
slightly less than 1:1 in ride. In roll, the motion ratio is
the same as it would otherwise be, assuming the lateral spring
spacing is unchanged. Note that this motion ratio in roll is
always less than 1:1 for any beam axle, which means that any
beam axle without an anti-roll bar has a substantially softer
wheel rate in roll than in ride.
So on a stock metric suspension, moving the springs
forward softens the wheel rate in ride somewhat, without softening
it appreciably in roll. This makes the ride and roll wheel rates
less unequal. However, if the spring is moved forward only 2½
inches, that will have only a small effect.
Note that we are speaking here of springs (on
buckets, on coilovers, or on sliders) mounting directly to the
axle, not to a link or a birdcage.
Even in cars with compliant torque arms or pull
bars, mounting the springs forward of the axle does not add
a lot of rear jacking, and rear jacking only adds total rear
wheel loading due to the overall vehicle CG being slightly higher
when accelerating forward. Such effects tend to be small.
Remember that jacking up both rear corners does
not increase rear percentage, in and of itself. Remember also
that jacking one rear corner up more than the other also doesn’t
significantly change rear percentage, but it does change diagonal
percentage.
Correspondingly, fairly significant effects in
torque-compliant axles can result when the fore-and-aft spring
offset differs on the right and left, as when the left spring
is ahead and the right spring is behind. Then there can be a
meaningful change in instantaneous diagonal percentage as power
is applied. This in turn will affect the car’s cornering balance
under power.
ROLL CENTER WITH A J-BAR
Question:
Many books, forum posts, and websites go into
great detail on on the front roll center and only touch on the
rear. I run an IMCA modified with a j-bar [short, off-center
Panhard bar, bent into a J shape to clear the pinion snout –
usually mounted to the left side of the frame and the right
side of the pinion snout, with the left pivot somewhat higher
than the right]. I would like to determine where my rear roll
center is.
Answer:
This is actually a fairly complex question. First
let’s discuss what a roll center is, and isn’t.
A roll center isn’t a real thing. It’s a modeling
construct – an invented idea that helps us think and talk about
the suspension’s behavior. It’s a way of representing the geometric
roll resistance of a front or rear wheel-pair suspension system,
to simplify prediction of wheel loads when cornering. In the
simplest method of modeling wheel load changes due to lateral
acceleration, the suspension is imagined as a beam axle (which
yours actually is), and the roll center describes a height
at which lateral force is transmitted between the axle
and the sprung mass.
It is vital to recognize that we are not talking
about a point the car actually rotates around, or a point whose
lateral location determines how vertical forces react. The roll
center is best thought of as a point in a side view of the car,
that has no defined lateral location at all, or perhaps as a
point in the same longitudinal plane as the sprung mass CG.
In other words, we should imagine the roll center as the height
of a pin in a vertical slot, or the height of a horizontal Panhard
bar, not as a pin joint. It is a notional device that transmits
horizontal force only.
Okay, now with a Panhard bar that’s curved, offset,
and inclined, how do we assign that imaginary point to get the
best wheel load prediction? There are two answers to this, depending
on how much work we want to do, and how accurate we want our
model to be. In both methods, we disregard the bend in the bar,
and think of it as a straight link connecting its two pivot
points.
In the simpler method, we find the point where
the centerline of this imaginary straight bar intersects the
longitudinal CG plane, and take that point’s height as the roll
center height. With this method, we disregard effects due to
the off-center, inclined Panhard bar jacking the rear of the
car up or down.
In the more rigorous method, we take the midpoint
height of the imaginary straight bar as the roll center height.
We then must also take into account the vertical forces resulting
from bar inclination. We likewise consider these as acting at
the bar’s midpoint. When the left pivot is higher than the right
pivot, in a left turn the jacking force tries to raise the sprung
mass. When the bar centerpoint is left of the sprung mass CG,
this effect tries to roll the car rightward, reducing effective
roll resistance at the rear. So we have a higher roll center,
suggesting more roll resistance, but also a pro-roll moment
from the jacking. Net result will be similar to the load transfer
predicted from the lower roll center in the simpler method,
though not exactly the same. (As described here, both methods
have some inaccuracy due to the bar being forward of the axle.
Correction for this is possible, but beyond our scope here.)
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