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March 2003
FREE SEMINAR MARCH 17
For readers in the Charlotte area, I will be presenting a one-hour
talk at UNC Charlotte, in Fretwell 126, Monday, March 17, at
12:00 noon. The title is "Minding Your Anti: Understanding Roll
Centers, Jacking Forces, and Other Factors in Weight Transfer".
This will not be a purely standard treatment of the subject.
I will include discussion of "lateral anti" and shed important
light on the much-discussed topic of lateral roll center location
and migration. This is a free presentation of the UNCC student
SAE chapter.
TIRE WARMERS LESS EXPENSIVE THAN I THOUGHT
In the January issue, I mentioned I'd found tire warmers, supplied
by Chicken Hawk Racing , 866-HOT-TIRE or www.chickenhawkracing.com.
I said they had a standard model for around $1500 and an adjustable
one with temperature readout for around $2000. I was under the
impression that those prices were for a single warmer, but actually
they're for a set of four.
INDEPENDENT REAR SUSPENSION FOR DIRT?
QUESTION:
Would there be any advantage to running an independent suspension
on the rear of a dirt car? This refers primarily to a modified,
but would it help on a dirt Late Model, also? We were wondering
if a design similar to a Corvette would work.
ANSWER:
There is no doubt that independent rear suspension can work
very well on dirt. This is provable not only by theory, but
by example. Independent rear suspension is used with great success
in off-road buggies, rally cars, and Unlimited hill climb cars
at Pikes Peak. The only place IRS isn't used on dirt is in oval
track racing.
The biggest single reason for this is that in most classes,
and in most sanctioning bodies, independent rears are illegal,
presumably for cost containment. I'm not sure about the high-dollar
mods that D.I.R.T. runs in the northeastern US, but in IMCA,
UMP, NASCAR, and WISSOTA, there are specific rules against independent
or "sports car" rear ends. They don't even allow quick-changes.
Down here where I live, we have the Carolina Modified Tour,
which runs similar cars, but with quick-changes permitted. For
Late Models, the rules vary. I haven't checked all the sanctioning
bodies that run these cars, but WISSOTA abolished all suspension
rules in the Late Model class a few years back, after previously
prohibiting independent rears. To my knowledge, everybody still
runs live axles, partly so they can go on the road and race
in other series, and partly because they are mostly car buyers,
not builders, and no Late Model builders offer independent rears.
Far as I know, all sprint car and midget sanctioning bodies,
including World of Outlaws, now require beam axle suspension
front and rear.
So the first obstacle to overcome is to find a sanctioning
body that will let you run IRS. You have to think about not
only what the current rules are, but also how the organization
is likely to react if you are successful with an independent
rear, and everybody else faces the prospect of having their
cars obsoleted. You will have to invest a lot of time and money
in building your own car and developing it. If it's outlawed
as soon as it starts winning, you take a big loss.
You will face another problem that besets all innovative owner-builders:
when you tear up equipment, you can't just order replacement
cars or parts; you have to make them. If you have a heavy schedule
and are running for points, or you're on tour, this is a major
concern.
Twenty years ago, there were some attempts to build independently
suspended sprint cars. These efforts were mainly the work of
backyard builders, who had little formal training. They attempted
to build systems that looked like what they'd seen on road racing
cars of the era, with little real understanding of what they
were doing. I recall one case where the builders didn't realize
they'd still need tire stagger, and blamed the suspension when
the car went straight into the wall the first time they ran
it.
The lesson here is that a mediocre concept, executed and set
up well, will beat a superior concept, executed or set up poorly.
Independent suspension has the potential to win races on dirt,
but only if it's done right. Since you'll be pioneering a new
idea, you won't be able to rely on conventional wisdom; you'll
have to study sufficiently to understand the principles of the
system. I will be happy to help you as a consultant, but those
actually doing the project will need considerable knowledge
as well.
Okay, assuming you aren't daunted by the practical aspects
of trying something radical, and assuming you've found a class
where IRS is legal, what are the pros and cons of IRS, and what
sort of design would be best?
Independent rear suspension is good, but it is a mixed blessing
in some respects. In general, overall weight is greater for
independent suspensions than for beam axles. However, unsprung
weight is much less for an independent suspension, especially
if the brakes are inboard, and most Late Models run to a minimum
weight rule that requires them to add ballast. So in terms of
weight, the only drawback to IRS is that you have somewhat less
ballast to move as desired. There is a big benefit in roadholding,
meaning ability to keep the tires in contact with the track,
and minimize tire load variation, on bumpy surfaces - and dirt
tracks are often bumpy, though not always.
Anti-squat in independent systems is different than in live
axles. In a live axle system, we can separate rear jacking forces
under power into thrust anti-squat and torque anti-squat. In
a typical Late Model, torque anti-squat is the lift we get from
the torque arm, and thrust anti-squat is the lift we get from
the geometry of the linkages at the ends of the axle, which
most commonly attach to birdcages (brackets that can rotate
on the axle). With independent suspension, we only have thrust
anti-squat to work with, because axle torque reacts through
the differential mounts and does not act through the suspension.
This leads some people to suppose that overall anti-squat is
necessarily less with independent suspension, and that therefore
independent suspension would be at an inherent disadvantage
compared to current state-of-the-art dirt Late Model live axles.
I question this myself, although I do agree that in theory at
least, a live axle can probably be made to lift more under power
than an independent system. As I have mentioned at various times
in the past, the advantages of anti-squat are often over-estimated,
and it is possible to get ample lift from an independent system.
It is safe to say that the live axle has some edge in terms
of anti-squat properties, particularly as regards the potential
to manage variation in anti-squat properties as grip varies.
However, current systems do not exploit the possibilities in
this area as fully as they could, so this potential advantage
of the live axle is hypothetical until somebody decides to exploit
it. These possibilities might be a future newsletter topic.
Compared to current live axles, an independent system could
have similar, or at least adequate, anti-squat, and much better
adhesion over bumps. The independent system might reasonably
be expected to compare most favorably on a bumpy track, and
least favorably on a smooth and slippery one.
For Late Models, there are rules about transmissions, at least
in WISSOTA. They have to be mounted to the engine, so transaxles
are out. That means the diff would be an IRS quick-change, with
either a spool or a Gleason.
You mention Corvette rear suspensions. There are two basic
styles of independent suspension used on Corvettes. The C2 and
C3 used the halfshafts as upper lateral or camber-control links,
a lateral link sometimes called a strut rod below the halfshaft
to complete the camber-control linkage, and a trailing arm for
toe location, longitudinal location, and brake torque reaction.
A variation of this system, with a third lateral link for improved
toe control near the front of the trailing arm, was used in
second-generation Corvairs.
The C4 and C5 Corvettes have a 5-link system. There are three
transverse links to control camber and toe, and two longitudinal
links to provide longitudinal location and react brake torque.
On the C4, the halfshaft is still used as the upper camber control
link. On the C5, the model currently in production, the halfshaft
is only used to transmit power, and the five links are all purely
suspension parts. Similar 5-link systems are used on the Viper
and most purpose-built race cars. On some current race cars,
two pairs of links are combined into upper and lower a-arms,
with a toe-control link. The system then visually resembles
a front suspension.
If I were designing an independent rear for any form of racing,
including dirt oval-track, I would use a five-link system, or
the a-arm and toe-link variation of the 5-link. Using the halfshafts
as camber control links saves a little weight and cost, but
it compromises geometry. Specifically, it forces you to choose
between a high roll center or meager camber recovery in roll.
Also, the consequences if you break a shaft or U-joint are particularly
nasty, though of course they aren't pleasant regardless.
One key decision is whether to use inboard or outboard rear
brakes. The advantage of inboard brakes is that you reduce unsprung
mass, and thereby maximize the system's roadholding advantage
on bumps. The advantage of outboard brakes is that you can have
ample anti-squat under power, without having excessive anti-lift
under braking. A lot of anti-lift in braking tends to cause
wheel hop when used with generous rear brake bias, and many
dirt drivers like to use a lot of rear brake to get the car
to turn in. Typical 4-bar Late Model rears have more than 100%
anti-squat and zero or negative anti-lift.
To get such properties with an independent system, you need
geometry that makes the hub travel rearward approximately .15"
to .20" per inch of suspension compression, and makes the upright
rotate rearward approximately 0.6 to 1.0 deg per inch of suspension
compression. In terms of side view geometry, this means a side
view instant center something like 80" behind the rear wheel,
and at or slightly above ground level.
That's with outboard brakes. With inboard brakes, you'd want
the hub to move rearward no more than .10" per inch of suspension
compression, unless the driver never uses a lot of rear brake.
Upright rotation doesn't matter with inboard brakes. Probably
the simplest approach would be to make the whole upright move
along a line inclined about 5 deg rearward, and not rotate at
all.
In either case, I'd consider having a bit more anti-squat
on the left than on the right, to make the car gain wedge under
power.
For lateral location, I'd try instant centers between 70 and
100 inches from the wheel and try to keep the force line slopes
between zero and 10 degrees, upward toward the center of the
car, in all combinations of ride and roll. This would correspond
to a static roll center height of 3 inches, give or take an
inch. I would try to make the lower control arms as long as
possible - all the way in to the center of the car if possible
- and have the upper arms shorter than the lowers by as much
as needed to achieve least possible force line slope changes
in both ride and roll, with perhaps a bit more emphasis on roll
than ride.
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