A Lesson On
(Not meant to be a end all explanation, just enough to get you over the rough spots…)
By Doug Chartier
**This is a
companion article to one printed previously in 4x4 Low Down
and available in the SHR website Tech Section - Understanding
Gear Ratio’s vs. RPM http://www.southernhighrollers.com/tech/articles/gearratios.html
All the gears,
including 1st, 2nd, 3rd, 4th, and 5th (or even D, R, etc on an Automatic)
will be lower when using the 4:1 kit and lowering your low side in the
transfer case. The transfer case and the transmission are two
different pieces of machinery. The transmission takes rotation of the
engine crankshaft as input. You change that rotational speed with your
throttle. The rotational speed comes in, and the transmission alters
it based on the gearing. Suppose 1000 rpms come into the transmission.
If your gear was a 2:1 ratio, that means that it reduces the input by that
ratio - for each 2 revolutions that come in, one revolution is passed to the
output shaft, 1000 rpms out of the engine and into the transmission, 500
rpms out of the transmission. A gear of 4:1 would take the same
1000 rpms out of the engine and output 250 rpms from the transmission.
Over the years
different gears have been given specific names. "Direct" or
"high" gear is normally associated with a 1:1 ratio (1000 rpms in
- 1000 rpms out), "overdrive" with any gear less than 1:1. A
.79:1 (1000 rpms in - 1255.82 rpms out) would be an overdrive gear.
On the low side any gear that is lower than one normally used for
starting from a dead stop is called a "granny" gear, "low
low", "compound low or just compound"; although
“compound” is often associated with another low gear lower than the
"granny". Heavy equipment, trucks etc., may have several
gears that are lower than the "low" gear or "first"
Ok, now we have the
operation of the transmission. On a normal 2-wheel drive vehicle the
output from the transmission goes directly to the axle where the rotational
speed is changed again. Suppose the transmission is exporting 1000
rpms. If the gear in the axle is 2:1, we have a further reduction of
rotational speed - 1000 rpms into the differential, 500 rpms out to the
Go back to the
engine. 1000 rpms out of the engine into a 2:1 transmission
produces 500 rpms out to the axle. Using the 2:1 in the axle again
causes the 500 input to be reduced by half to 250 rpms from 1000
down to 250 is a 4:1 ratio. If you multiply the axle ratio by the
transmission ratio, you get 4:1.
we introduce the transfer case. The transfer case is a 2-speed
transmission. It's just another transmission between the main box and
the axle. In addition it outputs from the front and the rear so that
you can operate 2 axles from it. It takes input from the main
transmission, alters it, and transfers it to the axle or axles. The
transfer case has 2 gears, low and high, and again the "high"
conforms to the above mentioned specific gear name. High gear is 1:1
or direct. It does not alter the input, so it is as if the
transmission was connected "directly" (direct) to the axles.
If you are in high gear in the main transmission, it is as if the engine is
connected "directly" to the axle because you could accomplish
the same thing with no transmission at all - a direct connection.
The final bit of
math. We have 1000 rpms coming out of the engine. The
transmission is in a 2:1 gear. 500 rpms out of the transmission and
into the transfer case. The transfer case is in low gear, and for this
explanation, it will be another 2:1 ratio. So, 500 in, 250 out.
The axle is also 2:1. 250 in 125 out. 1000 rpms out of the
engine and 125 rpms to the wheels. That is an 8:1 ratio. If you
multiply all the gears - 2 X 2 X 2 you get 8. If you reduce the
1000:125 ratio to it's lowest form, you get 8:1.
Back to the big trucks for a moment. Where the Jeep has a 2 speed transfer case or second transmission, big trucks have a 5 or 6-speed second transmission, and sometimes even 3 transmissions as well as selectable gears in the axles. (Note: Sometimes on off-road vehicles especially it seems on Toyotas you will see a setup with multiple transfer cases sometimes referred to as “Crawl Boxes”. These adapters and cases come from many sources but Marlin and Klune are two of the better known ones.)
I might add that
the term "crawl ratio" is one invented by off roaders or 4
wheelers. It has no official or semi official meaning that I know of.
More correctly the term should be "final ratio" or "overall
ratio" because it refers to the cumulative ratio of the various gears
from the power source to the point where the work occurs - engine to
wheels - regardless of which gear is used.
This ratio is
useful to 4 wheelers to determine the lowest overall gearing of the vehicle,
but it is just as useful to answer your original question. You
questioned the drivability of your vehicle in the higher gears on a
relatively smooth trail where low speed was not a factor.
Let's do a bit more
math and combine the results with real world data. The following are
not real numbers, but they will make the point.
The 4th gear of a
standard shift 5 speed Jeep transmission is 1:1 or "direct" or
"high gear". That is a real number, BTW. Another real
number is the 2.63:1 gear ratio in the low side of your transfer case.
A Jeep TJ Transfer Case has a ratio of 2.73:1. Suppose you are able to
drive comfortably at 30 mph in 4th gear/low side transfer case. Your
concern was whether you could continue to drive on the smooth trail in
low range, or whether you would have to change into high range if you
changed to a 4:1 gear in the transfer case.
Here's the math.
The 30 mph is a number from thin air, but will make the point.
The overall or final gear ratio in your current vehicle using 4th gear, the
low side of the transfer case and we will assume a 3.73:1 axle ratio (that's
a common ratio. I don't know what yours is.) is 1 (transmission gear)
X 2.63 (transfer case) X 3.73 (rear axle) or 9.8099:1. So, at a
particular rpm you can drive 30 mph on the trail. The question is - at
the same rpm how fast would you be going with a 4:1 low in the transfer case
instead of a 2.63:1.
The easy way
to figure that is - a ratio of ratios, or 4:1 is to 2.63:1 as "X"
is to 30, or 2.63 is to 30 as 4 is to "X". The math:
2.63 divided by 4 is .6575 multiplied by 30 is 19.725. If you changed
from 2.63:1 to 4:1 in the transfer case, and you formally went 30 mph in 4th
gear, you can now go 19.725 mph in 4th gear at the same rpms. If you
changed to a 4.3:1 ratio, your speed at the same rpms would drop to
18.35 mph. OR you could do it using the "overall ratio".
The final or overall ratio of a 4:1 under the same circumstances would be 4
(transfer case) X 1 (transmission) X 3.73 (axles) or 14.92:1. Again we
look at the relationship of the ratios. 9.8099:1 is to 30 as 14.92:1
is to "X". The math: 9.8099 divided by 14.92 times 30 equals
19.725 - same result.
Having the real
world data of your own Jeep - all the transmission ratios, the axle ratios,
the transfer case ratios and the speeds at various rpms in various gears
will allow you to mathematically determine the speeds under identical
conditions if you only change one factor - a single gear ratio - the
transfer case gear ratio, for example. You can do it very quickly
as you drive if you use a calculator. Predetermine the relationship of
the two transfer case ratios. If it is 2.63:1 vs. 4:1 the number you
will use is .6575 - 2.63 divided by 4. If the ratio is 2.73:1, the
number would be .6825. As you drive attain a comfortable speed in each
gear. That would be a speed that does not over-rev the engine.
If it is comfortable to drive 27 mph in 4th, simply multiply 27 by .6575 or
whatever the correct number is. The answer will be the speed at which
you will be comfortable in the same transmission gear with a lower gear in
the transfer case.
How to figure your vehicle crawl ratio:
__________ x __________ x
__________ = __________
Of course the one thing overlooked on this overall ratio is tire size. Tire size plays a lot into the off-road performance of a particular crawl ratio.
For example if I had a Jeep TJ with a stock 5-Speed, a stock t-case. 4.10 axle gears and 33” tires. I would have a crawl ratio of 43.98849.
In order to
“upgrade” to 35” tires and keep the same performance I would need a
“crawl ratio” closer to 46.6545.
I can determine this by a the following
(__________ x __________ )
/ __________ = New
(43.98849x35)/ 33 = ~46.6545
In order to achieve
that crawl ratio one could either modify the transfer case to a numerically
lower gear or the axles.
One more bit of info. The lower gears multiply the power delivered to the wheels. In the 4 wheeling community the goal is to drive VERY slowly. High gears will only allow a vehicle to drive at a certain low speed before the engine stalls. Lower gears allow the vehicle to drive at a lower speed before stalling the engine. Our goal is to drive SLOW. Trucks do the same thing. They gear LOW, but their goal is not slow speed. Their goal is to increase the power to the wheels. Different goals, but the results of each satisfy the goal of the other. In some respects they are counter productive though. The 4 wheeler starts breaking things because of all the extra power that results from his "go slow" low gears, and the trucker has to shift a whole bunch of gears to get up to speed because his quest for power has resulted in a very low maximum speed in the low gears. Some of the very low gears in a big trucks only have a speed range of a very few miles per hour - the lowest, from 0 to about 2 or 3 mph, the next from 0 to about 5 mph etc. When you get into the higher gears they are only good for ranges of speed like from 34 mph to 45 mph, but that's a factor of the engine power band. The engines, despite their size and power, cannot "pull the load" if the engine revolutions are allowed to go too low. Same is true of your Jeep. Overdrive will not "pull the load" below a certain speed. The engine is not strong enough.