...To have a truly capable rig, you need far more than just a super-low low.  Especially if you want it to be capable, drivable, and fun over a variety of terrain. 

Strangely, what, in my opinion, is one of the most important aspects of a 4x4s performance is never talked about.  In fact - I've never seen anyone discuss it - the concept only occurred to me because of experience in another sport.

Many, many moons ago I used to do a bit of bicycle racing.  As you can imagine - gearing and gear choices are extremely important to the cyclist - especially since it's your legs that are providing the power.  Just as in building 4x4s, serious cyclists build their own gearing by building custom combinations of sprockets on the front and rear cogs of the bike.  Too low or too high a gearing for the combination of bike, rider, and course - and it's almost impossible to win.  But cyclist know there's much more to it than that.

So here's the secret to not only winning bike racing - but to great 4x4 performance as well - the concept is called "gear stepping".  If you've ever ridden a bicycle you are already familiar with the concept.  Take, for example, your average 18 speed bicycle - it will have 3 sprockets on the front (crank sprockets) and 6 sprockets on the back (freewheel sprocket) for a total of 3 x 6 = 18 gears. HOWEVER - not all 18 of these gears are useable - the progression from one to the other is not linear.  Say you start peddling on the smallest front sprocket and the largest rear sprocket - this is the lowest and easiest gear the bike has.  As you pick up speed you shift the rear derailleur through the 6 gears on the rear sprocket.  If you were to then shift the rear derailleur back to the largest while simultaneously shifting the front derailleur to the next larger sprocket - you would find that the gear this put you in was not a step higher (harder to peddle) than the last gear you were in - in fact, it would most likely be a step backwards and be a lower gear.  This is because of the fixed number of teeth on the sprockets which determines the actual gear ratios available.

So this gear ends up being unusable, and the experienced cyclist skips it and goes on to another combination he knows to be a higher gear.  In fact, the serious cyclist will calculate out the ratios achieved by each possible combination, including the percentage jump from one to the next.  The resulting table looks something like this:

By considering this table, the cyclist can determine which gears are "useable" and which are not, and furthermore, for a given course, will plan which gears to use in which order - a plan in which the percentage jump becomes critical - you don't want to suddenly shift to a gear that is 50% harder as you would lose your momentum and stall.

Take particular note of the ratios between 6th gear and 11th gear.  Notice that if you were pedalling along in 6th gear (chain on the small front sprocket and smallest freewheel sprocket) and then just shift into 7th gear (chain on large front sprocket and largest freewheel sprocket) it actually gets easier to pedal (gear ratio goes down instead of up) - we've probably all experienced this, and it would spell disaster in a race (in a car race - like downshifting along the straightaway instead of upshifting).  Also note, that in order to "upshift" this bike from 6th gear we have to go all the way to 11th gear - making gears 7-10 essentially useless, and leaving our fancy 12 speed bike really an 8 speed.  If I was planning to race this bike, having charted the ratios, I would know my planned shift pattern would have to be 1-2-3-4-5-6-11-12.

GUESS WHAT?! ALL of these concepts apply equally well to the 4x4.  With multi-speed transmission and one or more 2 speed transfer-cases we have a wide array of gear choices, often spread all over the place.  The thing is, we often talk only of our lowest gear and seldom, if ever, does anyone chart their whole range of possible gear ratios and which are useable (and in what order).

Well, before I decided to upgrade my T-cases gear set, I did just that, and then re-did it with the numbers from the Tera Low kit.  The results were dramatic, to say the least - and convinced me that this was the right route to go  - even over changing to an aftermarket transfer case.

Here are my available gears before the Tera Low upgrade:

(The Wolf's axle ratio of 4.1:1 is not shown because it is consistent for all cases, though it is included in the calculation of the final ratio; in addition, both the NP 203 and D20 high (H) and low (L) ratios are 1:1 and 2:1 respectively, and the SM 465 gears are: 1 - 6.55, 2 - 3.58, 3 - 1.57 4 - 1.00)

Note that 4 of the gears are completely wasted since the low ratios of the 2 transfer cases in the Wolf are the same.  Note also that 26:1 and 27:1 are so close as to be indistinguishable, as are 15:1, and 16:1, so if we delete those, we are left with only the following final ratios:

Looking at this table, a couple of things should become immediately apparent:

1) There are still "unusable", or at least "indistinguishable" gear ratios - experience tells me anything less than about a
10-15% change is pretty hard to detect and therefore pretty meaningless - so the actual number of useable gears I have at my disposable, even with dual transfer cases, is actually pathetically low.  Out of a possible number of combinations totalling 16, I really only have 7 gears (just like my 12 speed bicycle is really only an 8 speed)

2) On the other side of the coin, there are some HUGELY wide ratio jumps - from 59:1 to 107:1 for example, (an 81% jump) - leaving a big gap in drivability - which can not only kill performance over a variety of terrain (especially as it changes from gnarly obstacle to mixed trail to easy spots - as most trails do), but can also make the rig a pain in the a$$ to drive.

The beauty of upgrading the guts of my T-case with the Low 20 kit is that it not only significantly improves my overall lowest low, but also has a huge effect on these two problem areas. 

Results coming at the end of the article.

Dana 20 Teardown

Once everything is torn down, carefully clean and inspect all parts, ready for installation of the Tera Low kit.  You want to thoroughly degrease the case so that any grinding dust created when you clearance the case for the new rear sliding gear does not get trapped and form a grinding paste.

Tera Low Low-20 Kit

Clearance checking and case grinding

Installing the kit

Once you have the old case torn down, clearanced, cleaned and inspected, it's time to install the new gear set.

Installing Main Drive Gear

Reassembling the Case

Checking Case Operation

To check the case operation and ensure that you have all the parts in the right place, proceed as follows:

1. Assemble the case completely, except for the bottom inspection cover.
2. Place the case upside down on the bench with the shift rods towards you.
3. Place some sort of support (a chunk of 2x4 worked for me) under the rear output housing so that the rear output flange can turn freely.
4. Arrange the case so that you can turn the main drive gear (input gear).  I accomplished this by bolting my doubler adapter to the case and turning the NP 203 output gears by hand - thus driving the input gear of the transfer case.
5. Dana 20 shift positions are controlled, via the shift rods and shift forks, by the positions of the front and rear sliding gears. For both sliding gears, towards the front of the case is high gear, in the middle is Neutral, and to the rear of the case is Low gear.  I deleted the interlock pill on reassembly, enabling "twin stick" operation - i.e. the ability to shift the front and rear outputs independently.
6. With the transfer case installed the right way up in the vehicle, the following are the shift lever, shift rod, and sliding gear positions for each gear possibility (interlock pill removed).  Note that these listings are for the case "as installed" in the vehicle, with the CAN/US drivers position the left of the vehicle. Note that because the fulcrum of the shift levers is above the shift rods when the transfer case is installed in the vehicle (i.e. the shift levers pivot about a point that is above the shift rods), the shift lever positions are the OPPOSITE of the shift rod positions for high and low gears.  With the transfer case on the bench, make sure you can smoothly shift between all of the following positions, with no binding, ratcheting, grinding, etc.  It shouldn't be hard to shift - you should be able to do it with strong hand pressure on a punch or lever inserted into the front shift linkage hole of each shift rod.  You may have to rotate the gears slightly to get the teeth to line up as you shift.

Gear	Rear (left) shift rod & sliding gear position	Rear (left) shift lever position	Front (right) shift rod & sliding gear position	Front (right) shift lever position	Picture
Rear Low	rear	forward	middle	middle
Rear High	forward	rear	middle	middle
Front Low	middle	middle	rear	forward
Front High	middle	middle	forward	rear
4 Low	rear	forward	rear	forward
4 High	forward	rear	forward	rear
Neutral	middle	middle	middle	middle

Installing the case in the vehicle

Results

The most important part - what are the results of this upgrade?

What you gain is:

• A 58% reduction in lowest achievable gear ratio – in my case from a low/low of 107.4 to 169.2 !!
• Even in single-case configuration, a low range lower than any factory/stock ratio.
• If you have a similar doubler setup - four extra gears (one in each transmission gear), now that the 2 cases low ranges are no longer identical. i.e. now L/H and H/L are no longer identical.

But the greatest improvement relates back to what I was saying at the beginning about "gear stepping" or "ratio spanning".

Recall my original gear table:

What I was really missing was a gear between my low / low and the next lowest gear. That is, something between 59 and 107, as well as something between the next 2 gears i.e. (29 and 54). I knew this just from driving the rig - even before charting the math involved.  In other words, my lowest gears (deeper than about 25:1 which is what I use most on the trail) used to be

27, 29, 54, 59, 107.

Now I have

27, 29, 41, 46, 54, 85, 92, 169

You just don’t get choices like that with a fancy aftermarket transfer case. If I had, for example a 4.3 low ratio Atlas II, I would have:

27, 28, 63, 115, or in effect, only 3 really usable gear ratios, for twice the price. To me, that is far from the ideal setup.

In my opinion, it’s all about 3 things:

- Options for different terrains
- Ratio spanning or % jump between ratios; and
- The lowest low you can get

Judged by that criteria, and by real world budget constraints, with the option of building over time, the low buck doubler with aftermarket gears in one transfer case comes out as an awesome option - and one I am extremely pleased with and don't regret one bit.

Testing

References:

• TeraFlex Installation Instructions for the Tera low range Dana 20 (LOW20) (.pdf)