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Steering
Ratios
What's hot now and why
It is worth mentioning that current North American
short track cars tend to be equipped with considerably quicker steering than was
the case twenty years ago. At that time, the conventional wisdom had it that
anything faster than a 16:1 Corvette worm gear was "too quick" for the
driver to keep up with. In the case of the steel-bodied behemoths of the 1970s,
this was at least partly true, since those cars (in addition to their high
unsprung weight) had cumbersome steering linkages. These were prone to
deflection because of all the rubber bushings that were used throughout front
ends in those days, and were pretty sloppy by present standards. Lighter cars,
coilovers, and spherical rod ends made rack and pinion steering practical for
stock cars, and the more positive linkage provided by rack and pinion
steering made quicker ratios practical. These days, it is not uncommon to find a
2.62 rack used with 5-1/2" steering arms at a one-mile, 175-mph paved track
like Phoenix, while on dirt, saying the steering is too quick is about like
saying the tires are too wide - in other words, not likely.
The case for quick steering can be summed up with the
common sense observation that it multiplies your reaction speed. The limiting
factors are equally obvious, if you think about them: the degree of precision
and smoothness with which you can apply your reactions to a steering
wheel, and the relative consequences of overcorrection and/or disturbing the
cornering attitude of the race car (a few degrees’ error committed on a
superspeedway can be irrecoverable, whereas far greater errors committed while
backing it in on some potholed bull ring, on the other hand, may be easily
recoverable).
Figure 7 illustrates the case for quick steering on
dirt:
Here the car with the push has steering that is just
too slow for the driver to either break the push or effectively change corner
entry to compensate for it. The other car has steering quick enough to enable it
to be thrown in under poor handling conditions but still caught by the driver
quickly and easily enough to gain position. Note the position of the driver’s
left hand (at ten o’clock on the steering wheel) versus the steering angle of
the front end. This is a 3.92 ratio rack and pinion, which in 1984 was highly
experimental. Today it’s a production item available from the catalog.
Bear in mind that adding power assist does not
quicken the steering; it only decreases the input effort. Remember, also,
that the steering ratio required for your car is a function of the radius of the
turns of the race track, and on dirt, the slide angle necessary to steer into.
In general, converting from manual steering to power assisted steering will
permit quickening the steering ratio by at least one step, and usually two.
Typical applications of the various rack ratios appear in the chart (Figure 8).
The "ratios" (1.57, 2.09, etc.) in the chart refer to rack and pinion
gearing and are given as linear inches of rack shaft travel per turn of
the pinion (or steering wheel). Since the rack steers the front wheels by means
of levers (the steering arms out on the spindles), the actual overall
steering ratio of the car depends as much on their length as on the diameter of
the pinion gear driving the rack.
How to measure the RACK ratio
If there are no numbers stamped on the caps, or if
you have reason to believe a different pinion has been installed, just measure
the distance from one tie rod hole (or from the end of the rack shaft) out to
some stationary object (a piece of flat stock clamped to the frame rail). Turn
the steering wheel (or the pinion) one full revolution and remeasure; the
difference is the linear travel of the rack, which is the "gear ratio"
of the rack and pinion. For reference, each additional tooth on the pinion
increases the linear travel in one turn by slightly more than 1/4 inch.
How to calculate the OVERALL steering ratio
The overall steering ratio (12:1, 14:1, etc.) is
measurable using turntables under the front wheels. Beginning with the front
wheels pointed straight ahead, rotate the steering wheel one turn (360 degrees)
one way, and read the turning angle of the front wheels from the turntable
scales. You will have to resolve the difference between the right and left due
to the Ackerman or steering toe; the usual method is to read the angle of the
inside wheel, which is the maximum value. In road racing some prefer to average
the two.
As an example, if your reading is 36 degrees, dividing
this into 360 gives you a quotient of 10, and thus a 10:1 overall steering ratio
(if it is not possible to get a full turn out of the steering wheel, use three
quarters of a turn and divide into 270). With a six-inch steering arm, a result
of 10:1 is roughly what you could expect with a 3.14 rack. This measurement
becomes more approximate with quicker racks and shorter steering arms, and
because of the prevalence today of rack and pinion steering in short track stock
cars it is common now to refer simply to rack travel numbers.
Rack and Pinion Ratio Chart
1.57 inches per turn:
Very slow steering, mainly for superspeedways (Daytona Dash cars) or road
courses (GT classes) where top speeds exceed 160 mph. Ordinarily applied as
manual steering. Also used to compensate for the short steering arms on small
cars. With long steering arms, the overall ratio can range down to 24:1. For
power steering with this rack on short tracks, use a #850 or #855 servo valve
with a KRC standard 7.2cc pump and #8 and smaller output fitting; The same servo
valve with a KRC 5.9cc pump and a #4 or #5 fitting will handle a ratio like this
on superspeedways.
1.83 inches per turn:
Slow steering for paved tracks 5/8 mile and over; usually run as manual
steering. Formerly popular in GT classes, as this ratio closely approximates
that of OEM sports car steering when used in conjunction with short steering
arms. Use #850 or #855 servo valve with KRC 7.2cc or 5.9cc pump and #8 and
smaller output fitting.
2.09 inches per turn:
Formerly the most popular ratio for manual steering pavement applications in
both stock car and road racing. Known as the 16:1 rack, it feels about like a
Corvette. Used with power assist about half the time. Use #855 servo valve, KRC
7.2cc pump, #6 and larger output fitting
2.36 inches per turn:
Quicker steering for paved tracks, especially those 1/2 mile and under. Useable
as either manual or power steering, although run as power steering in 80% of
cases. Its overall ratio usually works out to around 14:1. For power steering,
use a #855 servo valve with a KRC 7.2cc pump #7 and larger output fitting
2.62 inches per turn:
Ten years ago this ratio was commonly used as manual steering on dirt late
models. At 12:1, it still gives relatively easy steering on dirt, and is the
best entry-level choice for manual steering limited late models. It's most
common application today is on pavement with power assist. Use a #855 servo
valve with KRC 7.2cc pump and #8 and larger output fitting.
2.88 inches per turn:
Quicker than average manual steering for dirt tracks. At about 10:1, this will
require setting the car’s caster and camber for minimum effort, unless power
assist is used. Much faster steering than any road vehicle. For power assist on
pavement, use a #855 servo valve, on dirt use a #850, with a KRC 7.2 pump and #8
and larger output fitting.
3.14 inches per turn:
For powersliding around bull rings. This is about the fastest ratio usable as
manual steering (and was considered radical in the early 1980’s). If used
without power assist, most front ends will need the positive caster backed way
off to street-vehicle levels (under two degrees), and the caster split reduced.
With power assist, use a #855 servo valve on pavement, #850 on dirt, with a KRC
7.2cc or 8.5cc pump and #8 and larger output fitting. If a spline drive pump
setup with a slow drive ratio is used (as is now common on dirt cars) a 9.6cc
pump is required for this and quicker ratios.
3.40 inches per turn:
Very quick response; generally applicable only to dirt and only with power
assist. Cuts your reaction time in half if you’re ready for it. Use a #850 or
#855 with a KRC 9.6cc pump and #8 and larger output fitting. If used on pavement
a this would require a #860 servo
3.66 inches per turn:
Instant response with even less wheel movement, standard equipment on many new
late models. Best with large amounts of power assist, such as 100-125 PSI. Use a
#850 or #855 servo valve (depending on the make of chassis and steering arm
length) with a KRC 9.6cc pump and #9 and larger output fitting. With short
steering arms, this will virtually turn a late model into a sprint car. Most
comfortable with servo torsion bar sizes 235-225.
3.92 inches per turn:
See Figure 7. Needs a soft torsion bar (220 and smaller) and a KRC 9.6cc pump
with #10 and larger output fitting.
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