Crawl Ratio
The
crawl ratio is a measure of the overall gearing, or final
drive ratio, of a vehicle in first gear, low range. The term
is usually applied to four-wheel drive vehicles, such as SUVs
and jeeps or other off-road vehicles, or trucks. Crawl ratio
is calculated by multiplying together the first gear ratio,
the transfer case (in low range), and the axle differential.
The lower the crawl ratio, the slower a vehicle can 'crawl'
in first gear and low range. Low crawl ratios mean that higher
torque is available
at the wheels, and is especially advantageous if there is
a need to crawl very slowly (less one mile an hour) over rough
terrain, climb steep gradients, or descend steep gradients
so that engine braking can be used instead foot brakes. The
ideal crawl ratio depends much on the conditions a vehicle
is likely to encounter and how much low-end torque the engine
develops. In most off-road situations, crawl ratios in the
range 40:1 to 60:1 are adequate for manual transmissions (slightly
less for automatics). Some large rigs have a ratio as great
as 350:1 and are capable of crawling at a barely perceptible
rate.
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