Teraflex Manufacturing Dana 20 Low Range Kit

Part 2 of the Uber-20 project
(Building the ultimate low geared transfer case at home)
By BillaVista

The Heart of the Wolf Gets a Serious Gearing Boost!

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Page 1 - Introduction


Welcome to the second instalment of my ambitious project to build a strong, all-gear, compact, twin-stick, low-geared transfer case at home, bit by bit, as I can afford it - a project I call Uber-20; named for the core component - a '79 Jeep CJ Dana 20 transfer case that lies at the heart of my rock buggy.

In this part I upgrade the fairly tame 2.0:1 low range ratio of the Dana 20 to a far more respectable, and useful, 3.15:1 using Teraflex Manufacturing's "Low 20" replacement gear set.

Follow along as I detail the why, the how, and the results of my latest upgrade.

(it's not just what you'd think)

The Wolf takes on a waterfall...

...rocky, steep, and slippery - seriously demanding on rig and driver.

The need for super-low gearing in technical rock crawling and other hardcore wheeling is no secret and no surprise.  Long gone are the days when 100:1 overall gearing was considered as deep as anyone would ever need.  More technical courses and more extreme trails demand greater control and that means low gears - in my opinion there really is no "too low", BUT...


...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 4x4's 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 4x4's, 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:

Gear Selection Front chain wheel teeth (drive) Rear freewheel teeth (driven) Ratio (drive/driven) % jump from last
1 42 24 1.75 n/a
2 42 21 2.00 14%
3 42 19 2.21 11%
4 42 17 2.47 12%
5 42 15 2.80 13%
6 42 13 3.23 15%
7 52 24 2.17 -33%
8 52 21 2.48 14%
9 52 19 2.74 11%
10 52 17 3.06 12%
11 52 15 3.47 13%
12 52 13 4.00 15%

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:

NP 435 Gear NP203 D20 Final Ratio
1 H L 54
1 L H 54
1 H H 27
1 L L 107
2 H L 29
2 L H 29
2 H H 15
2 L L 59
3 H L 13
3 L H 13
3 H H 6
3 L L 26
4 H L 8
4 L H 8
4 H H 4
4 L L 16

(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:

Final ratio % Jump
6 50
8 33
13 63
15 15
26 73
29 12
54 86
59 9
107 81

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.

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Contact Info:

Tera Manufacturing, Inc.

5251 South Commerce Dr.
Murray, UT 84107-4711
phone/801.288.2585 -- fax/801.288.2571



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