Let's Nerd Out On Some Calculations

Another round after talking to ITLKEZ. He likes the idea of doublers to have a decent speed overall range in low, but a really low crawl for obstacles.

This is something like he's thinking about:

• NP435 transmission
• NV205 1.96:1 transfercase
• NV203 1.96:1 doubler
• 3.73 axles
• 37" tires

Notes:

• The 'Ratio' column is the overall gear ratio using the axle ratio noted above
• The Speed noted in the tables below is based on a comfortable 2500 cruising rpm and 37" tires
• The '% Change' change column is the percentage change from the previous gear when shifting up
• The green entries are with both the 205 and 203 in low
• The yellow entries are with only the 205 in low
• The red entries are with only the 203 in low
• The white entries are with both the 205 and 203 in high

Observations:

• Crawl ratio is 95.72:1 with a highway gear of 3.73:1
• The crawl ratio with one of the transfercases in low is 49.83 which is pretty decent and you can comfortably cruise at 37 mph in that range!
  
• 24.92:1 is the lowest high range ratio
• 7.31:1 is the highest overall low range ratio in a single low range
  
• Due to the nature of both transfercases being about 2:1 and the jump from 1st to second on the NP435 also being 2:1, there are no less than 6 duplicate gears
• The non-duplicate gears have a decent spread
• You would typically pick one of the low ranges to use on the trail, and only kick in the second one at a tough obstacle
  
• All in all, this seems like a nice setup but you are carrying extra weight duplicating a bunch of gears to have that 95:1 crawl when you need it

Thinking about the above observations and doublers, it seems like having one low range being around half the other would be the way to go. So if you want to throw $4100 at the problem, a 4 speed Atlas with a 1.5:1 low range and a 2.72:1 doubler is a nice setup.

Notes:

• The 'Ratio' column is the overall gear ratio using the axle ratio noted above
  
• The Speed noted in the tables below is based on a comfortable 2500 cruising rpm and 37" tires
• The '% Change' change column is the percentage change from the previous gear when shifting up
• The green entries are with both the Atlas and doubler in low
• The yellow entries are with only the Atlas in low
• The red entries are with only the doubler in low
• The white entries are with both the Atlas and doubler in high

Observations:

• This is expensive
• The 18" length is a problem for SWB Jeeps
  
• Crawl ratio is 101.66:1 with a highway gear of 3.73:1
• The crawl ratio with the Atlas in low and the doubler in high is 37.37 and you can comfortably cruise at 49 mph in that range! That's great for medium difficulty trails.
  
• The crawl ratio with the Atlas in high and the doubler in low is 67.77 and you can comfortably cruise at 27 mph in that range. That's great for tougher trails.
  
• You have the two great trail setups mentioned above, but you can still pull a 101.66 crawl ration out of your hat for really tough obstacles
  
• 24.92:1 is the lowest high range ratio
• There are no duplicated gears, but a couple are close-ish together
  
• You would typically pick one of the two low ranges depending on the difficulty of the trail, and only kick in the second one at a tough obstacle
  
• Really nice if you can afford/package it

 

Thanks for doing the math for me!

Not to be the turd in the punch bowl, but 205s and 203s have 1.96:1 ratio, not a true 2:1. It’s not a lot, but it does compound into a few digits when you start doubling.

This talk really puts the fire under me to start on this project.

 

You're welcome and thanks for the 1.96 reminder. I went and fixed the original post with the right numbers. It doesn't change the observations at all but changes the numbers a little.

 

I really think this is the key.

I can play with some other visualizations. I started in this manor because I was curious about all the ratios from top to bottom and overlaps/duplicated gears. In practice, you're not shifting in/out of ranges to get to the next numerical gear, so as you said a more range specific visualization is more real-life useful. I added the bold/colors so you can kind-of see what you get in each range.

 

Another interesting thought is the Warn overdrive basically acts as a doubler, but with less ratio change and a synchro so you can split gears everyewhere which you can't do with an actual doubler. It trades overall ratio for not having to stop to shift.

I suppose if you're really, really good at double clutching, you can shift ranges on the fly?

 

This.
I use the overdrive to split gears a lot, especially off road. With a low HP engine like the Dauntless it really needs more than 3 gears and AFAIK there's no 5 or 6 speed trans that'll work for a 5.

 

You're definitely not getting a TJ 6-speed Rubicon setup in a CJ-5:

 

I've done those calcs a lot because I like the appeal of a much smaller/lighter transmission, but even with 5.38s and Tera-Lo gears the crawl ratio is only 55:1 while high gear is a little high at 3.15:1 with a Warn overdrive. Really extreme 5.89 gears (if you run 44s front/rear) get somewhat reasonable but still not great numbers of 60:1 crawl and 3.45:1 high gear.

It sort of works if you switch to a LoMax D300. Then you get a crawl of 77:1 with 5.38s but your high gear drops to 4.2:1 since you lose the Warn overdrive.

Overall it really doesn't pencil out well. At 3.25/.78=4.1667, those overdrive 4 speeds have about the same overall ratio range as your 4:1 T18, but it's just shifted up too high. You need really extreme axle gears to make it work.

 

No different than getting down into 1st.

 

I have, sitting in my shed, a Ford RTS out of a Bronco. Considered going this route in my CJ, but as Fireball posted, the numbers don't work out as well and you still have 4 gears vs a theoretical 6 for a T14/OD combo or 8 for a T18/OD. That plus the fact that Herm is the only source for adapters, led me to do the T18 swap.
https://hermtheoverdriveguy.com/rebuilt-rts-4-speed-overdrive-31-1150

 

If only there was a www.Joetheadapterguy.com

 

I was scoping through this at one time. The 3speed od in the half tons/ vans is a Tremec T170. It shares a host of parts with the Jeep used T176 ( including the case) I was " reasonably" sure you could use the T176 mainshaft and factory D300 adapter. As Roy noted a 4:1 set would get you in respectable territory. Alas I could never source the factory D300 adapter at an affordable price point, gave up.

I almost never split. On the street I leave it in OD, offroad finds me in 2-3 direct low 90% of the time ( and in 2wd so much I forget the front is thrown out) I use first for the occasional technical obstacle and steep downhill to stay off the brakes. For getting up ledges and the like the short wheelbase just won't crawl the traction isn't there. Wheel speed is needed for the hump and goose tactic.

 

Get a loan, buy a CNC mill, buy a CNC lathe, start a new career.

It's the path to riches I'm sure. There are dozens of us who want these adapters. Dozens!

 

Following with interest. A few observations:
If we are talking about fitting any of this in a ECJ, there are some length and width issues with a lot of it. For example, isn't a 203/205 doubler a little long, no matter what transmission you have? I have only ever seen those in FSJ's and Blazers. And an Atlas does not fit between the rails of an ECJ. Frame notching required. My next thought is, (if we are only talking ECJ's), you can get all kinds of gearing with tried and true combinations of readily available 4 speeds, adapters, D18's, some with 3.15's, and overdrives. I think you will find that those combinations have almost too many options. As far as availability, T18's, SM420's, SM465's are relatively cheap and plentiful. As are D18's. Yes, you have to take a deep breath and buy Herm's gears, but the trick is to buy them through AA. Then you never talk to him. Overdrives are not rocket science, and AA has everything you need. And you can have: 1. A nice highway gear, you don't need to go over 55 in a SWB jeep. And splitting gears on a grade is a dream. 2. A great crawl ratio (119 in my case). 3. It fits with a not terrible DS angle without a CV. 4. Gear selection -16 forward gears. 5. Don't worry about the strength of lower diff gears, yes they have fewer teeth, but we're not tractor pulling here.
Anyway, love the math and the charts, thanks guys.

 

Stay tuned.

 

Oh ok. If I can dream, it's a 115" CJ8 with 5.13's, 6.0, 4L60E, Atlas.