Let's Nerd Out On Some Calculations

At the end of the day science and theory doesn't mean much if you don't do something with it. That's what engineers are all about. Getting the science and theory out the door

 

’lectro magnetics nearly ruined me. What will i ever need this electron at time t in space calc for…

and here i sit, thumbing a touch screen.

and dang it, my lawn is …. Argh, get offff…..

 

and some days, it all clicks,


Others, well, lets just say, i didnt sleep at the Holiday Inn Express…

And no, i don’t engineer much these days (only held software engineer as a title) other than expensive home cooked dinners…

i dabble in bubbafied jeep fixins, and i gotta get back at that sometime…

 

My setup using Fireballs calculator comes out to a CR of 65.9:1 and a ground speed of .89 mph at 600 rpm idle.
Dauntless V6, 6.32 T18, D20 with 3.15 low, 33" tires and 3.31 gears.

 

Piling on with numbers. These are my current Jeeps and the one I sold to Duffer:

1968 CJ-5 (sold to Duffer): 225 V6, T-90, D18, 4.88 axles, OD, 32" tires, 600 rpm idle

Crawl ratio: 33.59
Crawl speed at idle: 1.7 mph
RPM at 60 mph: 2307​

1969 CJ-5: 231 V6, T-14, D18, 3.73 axles, 31" tires, 600 rpm idle

Crawl ratio: 28.44
Crawl speed at idle: 1.95 mph
RPM at 60 mph: 2427​

1971 CJ-5: Buick 350 V8, T18, D18, OD, 3.73 axles, 32" tires, 400 rpm idle

Crawl ratio: 57.99
Crawl speed at idle: 0.66 mph
RPM at 60 mph: 1820

That 400 rpm idle makes a big difference!​

2006 Rubicon: 4.0L I6, NSG370, NP241OR, 4.10 axles, 33" tires, 750 rpm idle

Crawl ratio: 72.97
Crawl speed at idle: 1.01 mph
RPM at 60 mph: 2100​

Now on to hypothetical stuff.

First the '71. Even though the Buick 350 in it can pull the highway with 3.73 gears and an overdrive (effective 2.8 gearing!), the intention is to eventually re-gear it to 4.27s or 4.56s and 33" tires:

1971 CJ-5: T18, D18, OD, 4.27 axles, 33" tires, 400 rpm idle

Crawl ratio: 66.39
Crawl speed at idle: 0.57 mph
RPM at 60 mph: 2018​

1971 CJ-5: T18, D18, OD, 4.56 axles, 33" tires, 400 rpm idle

Crawl ratio: 70.90
Crawl speed at idle: 0.55 mph
RPM at 60 mph: 2090​

I lean toward the 4.56s.


Finally, the flatfender project. I've already got a T98 with adapter, large hole D18, 5.38 gears for the D60/D44 axles. Still need to get an overdrive. Planning on 35" tires.

1946 CJ-2: T98, D18, OD, 5.38 axles, 35" tires, 600 rpm idle

Crawl ratio: 83.64
Crawl speed at idle: 0.75
RPM at 60 mph: 2325​

If you add Tera-Lo gears to the D18...

1946 CJ-2: T98, Tera-Lo D18, OD, 5.38 axles, 35" tires, 600 rpm idle

Crawl ratio: 107.11
Crawl speed at idle: 0.58
RPM at 60 mph: 2325​

It doesn't really seem like I'll need the Tera-Lo gears, so I can save a grand there.

 

Ready to see some serious...? Cam is for 2500rpm to 5500rpm

 

FYI, Dana20 hi ratio has a non-number character causing problem with calc.

 

Thanks. I'll fix it.

 

It's fixed. I fat-fingered the entry when I was making all the significant digits match throughout the form.

You may need to refresh the browser page to get the fixed version.

 

My Willys has a ATV 30% OD stock T-90 D-18 28.5" tires and my crawl gear ratio is 37-1 in 1st direct low range My engine will run well at 550 rpm though I normally set the idle at 600 rpm for a blazing speed of 1.37mph

Of course on the other end of things at 2800 rpm I am flying along at 63+ MPH my 134L powered CJ-2A on a CJ5 chassis has done 72mph comfortably on the freeway. Well as long as you are razor focused on what your doing. LOL

And at 60mph with the hard top on it I get 17.25mpg. Done this the other evening when I drove 189 miles to meet up with some guys in Longview WA. ALMOST made it on a tank of fuel but I was down to a wet tip on my stick 11 miles from home. It took 10.4 gallons.

 

Here's some more nerding out over gearing. (James, I must confess I used a spreadsheet to calculate the data).

Three other things I find interesting when you have high/low range, granny gears, and maybe an overdrive:

1. How many gears overlap between high and low range?
2. How many "useless" gears are due to overlapping gears being very close to each other?
3. How fast can you go in low range before you need to come to a complete stop and shift to high?

I don't have good answers to the questions, but it's the sort of stuff I like to think about. Almost all setups with a granny gear transmission will have overlapping gears in high/low range. The 6:1+ first gear is more than even and Atlas 5:1 transfercase can cover. Lots of overlap and otherwise redundant gears sounds bad, but can allow you to stay in the same range over a wider range of conditions and can be beneficial.

Below are some calculated examples. The first two are hypothetical gears for an early CJ5 with a T18, an overdrive, and a Dana 18 with either the standard 2.46 or Tera-Low gears. For contrast, the last is a 2006 TJ Rubicon with a 6 speed transmission and a 4:1 transfercase.

Notes:

• The 'Ratio' column is the overall gear ratio using the axle ratio noted in the header
• The Speed noted in the tables below is based on a comfortable 2500 cruising rpm and 33" tires
• The '% Change' change column is the percentage change from the previous gear when shifting up
• The bold entries are in low range to highlight overlaps with the non-bold high range entries

Observations:

• Crawl ratio is 70.9:1 with a highway gear of 3.42:1
• Four high range gears overlap with the low range gears
• 28.82:1 is the lowest high range ratio
• 8.41:1 is the highest overall low range ratio
• 2/hi is very close to 3/low/od and 2/hi/od is somewhat close to 4/low, otherwise the overall gear splits aren't too bad
• You can comfortably cruise at almost 30 mph in low range

Observations:

• Crawl ratio is 81.62:1 with a highway gear of 3.08:1 for a much wider overall range as expected
  
• Now only three high range gears overlap with the low range gears
• 25.91:1 is the lowest high range overall gear ratio
• 9.69:1 is the highest overall low range ratio
• 2/hi is very close to 4/low and 2/hi/od is very close to 4/low/od, otherwise the overall gear splits aren't too bad
• You can comfortably cruise at 25 mph in low range

Observations:

• Crawl ratio is 73.14:1 with a highway gear of 3.44:1 which is very similar to the T18/D18/OD setup
  
• Only a single high range gear overlaps with the low range gears
• 18.29:1 is the lowest high range overall gear ratio - meaning you can't do any serious offroading in high range
  
• 13.78:1 is the highest overall low range ratio
• The gear splits are pretty good across the range with no nearly duplicated ratios.
  
• You can comfortably cruise at 18 mph in low range - this is pretty slow meaning you'll need to stop and shift into high more often

Overall thoughts:

• In general super low range transfercase gears help get a good crawl ratio and also reduce high/low range gear overlap
• However, super low range transfercases come at the expense of the highest speed you can do in low range
  
• Granny gear transmissions are fantastic for widening the ratio spread in both high and low range.
• D18 overdrives help fill in the huge gap between 1st and 2nd on granny gear transmissions. I've found myself using the OD a lot offroad for this reason.
• The TJ Rubicon works very well, but when wheeling with Rich and James in Moab, I was often clear up in 5th or 6th gear while in low range just to keep up.

I'm curious what others think about this stuff.

 

THIS is the stuff that matters the most to me.
This is also the reason any rig I build in the future will have twin t-cases. A super low crawl ratio is great to have, and often mandatory if you’re getting into gnar, but more often than not, a mid-range case is not only all you need, but is beneficial; keeping you from constantly switching to low range at every obstacle, then back to high on your way to the next one.

 

Maybe it’s the fact that my teeth were cut in this sport(?) in the form of racing, where we didn’t have the luxury of switching from high to low to high. 99% of the guys ran in high range and fried their clutches. I took a different approach and built a Jeep that would do 55mph flat-out in low range, but still had 47:1 CR for the rock gardens.

Switching into low when you get to an obstacle isn’t bad; I’m almost at a stop anyway. But coming to a stop to switch into high range when there’s a mile of clear trail in front of you and you’re maxed out at 22mph in low… there’s almost nothing more annoying to me.

 

I thought I'd throw in Kyle's setup because it's also interesting.

My above combos are T18, Dana 18, overdrive combos with lowish axle ratios like a lot of us early CJ-5 folks run.

Kyle uses a T18, a Dana 20 with Tera-Lo gears, and a higher axle ratio in a setup like you would run in an intermediate.

Notes:

• Low range is green this time
• Speed is at 2500 rpm with 33" tires

Observations:

• Crawl ratio is 65:90:1 with a highway gear or 3.31 for a pretty good overall range of ratios
• One high range gear overlaps low range gears
• 20.92 is the lowest high range ratio
• 10.43 is the highest low range ratio
• 2/hi and 4/low are redundant for overall ratio, but that probably doesn't matter because they are in different ranges.
• You can comfortably cruise at 24 mph in low range
  
• Simple without the overdrive, less things to go wrong
  
• With the high final drive ratio, you have lower driveshaft speeds, but higher driveshaft loads
  
• There is a big jump between 1/low and 2/low, but 2/low at 32.22 is a pretty good non-obstacle off road ratio so it may not matter much

And here's the numbers if you take it the next step and switch to a LowMax equipped D300:


Observations:

• Crawl ratio is 83.68:1 with a highway gear or 3.31 for a great overall range of ratios
• Only one high range gear overlaps the low range gears
• 20.92 is the lowest high range ratio
• 13.24 is the highest low range ratio
• 1/hi and 3/low are almost redundant but do have some separation
• You can comfortably cruise at 18.5 mph in low range, similar to the TJ Rubicon
  
• There is a still a big jump between 1/low and 2/low, but 2/low at 40.91 it useful for mild obstacles

As I said, interesting stuff.

I personally lean towards the twin stick D18/OD setup for emotional reasons. It reminds me of being a kid in my Dad's Jeep and admiring all the sticks on the floor. It also works out well with the Buick 350 since it's happiest between 400 rpm and 1800 rpm. Having 16 speeds helps keep it in that range.

One downside to the D18 is that you can't do front digs while you can with a twin stick D20/D300. Not something you do all the time, but very handy when you need it.

Another thing to note is that most of this stuff is out the window with an automatic. The torque converter doubles your torque at stall and has an infinite range of speed/torque you can control using the brake and gas at the same time. A 40-50:1 crawl is probably fine for an automatic (pulling numbers out of my rear because I have no experience with this). Another advantage to an auto is that using the brakes to control speed, you don't have driveline lash issues when cresting things. The torque stays loaded up in the same direction.

And that may all be wrong. Comments from people with automatics welcome!