Off-roading Performance -- a comparative technique
Posted: Fri Oct 27, 2017 12:52 pm
As a warning, this is going to be long, possibly controversial, and geeky. Essentially I have come up with a way to graphically
compare off-road capability using key specification data. Scroll down to the chart and see if this looks interesting to you. I did
this research for myself, then thought some would find it useful. Just to be clear this is one way to look at capability and
certainly not the only important factor -- it did help me to put things in perspective. All this analysis is predominately 4x4,
Manual tranie's, torque converters are different. Typically rigs are 70's and 80's era. If you see something wrong with my
data provide me source data and I'll make corrections. Now the background ---
I bought my 710K with a TD Tranie + 2.7L/EFI, it's all I know. For me trails worth wheeling to are 200+ miles away, the
improved HWY performance is a plus. Although, after a very challenging run to Snake Lake in the Sierra's I decided to
revisit the Pinz drive train options. There are mixed reviews of the TD Tranie in the 1'st gen Pinz, some say it's a bad idea.
viewtopic.php?f=13&t=10003
First gear in the stock Tranie is 5.33:1, the TD is 4.25:1 that put's TD's 1'st gear between 1'st and 2'nd of the stock Tranie
and a 25% reduction in applied Torque -- as a spoiler an Engine upgrade has to be part of any move to taller gearing or you
will absolutely regret it. Even if the percentage of Torque gain from the 2.7L engine upgrade were to equal the gear ratio loss
it will NOT completely compensate for the difference, this is will become obvious.
I started thinking about a technical way to graphically compare the Pinz drive train options relative to each other and against
other rigs based on "wheeling" factors. I could not find anything published that represents this idea. What I came up with is based
on a fundamental principle -- moving a heavy mass with control over obstacles up steep grades. The first factor is, Force to
Weight Ratio (FWR), obviously important for any 4x4 facing steep grades. Calculated using total gear ratio, Tire size, Engine Torque,
and Curb weight, it's simple to calculate. With FWR=1.0, ground force equals vehicle weight. In theory, you could crank your truck
up a vertical wall. However, FWR=0.5 implies a grade limit of 30 degrees. FWR is the Vertical axis on my chart. The other factor,
Ground speed, measured in ft/sec. This is a measure of control. Fast presents less control negotiating obstacles, hitting an obstacle
too fast, or breaking traction on a steep grade. Slow is better, more latitude to reach peak torque from a dead stop. Calculated from
Total gear ratio, Tire size, and at a fixed RPM of 2,000. This becomes the horizontal axis. I picked 2,000 RPM as a baseline as that
is a fairly common RPM near peak torque, below 2,000 many engines have a sharp roll-off in Torque.
Trucks compared; 710 and various drive train options, 712 stock, Mog 404, C303, 716, CJ-7, Hafie, Series III, FJ Crusier, FJ40,
1979 G-461/280GE, VW Thing 181s with Portals, and for yuks a Mini-Cooper. The CJ-7 is factory with aggressive options, I also
show a lifted CJ-7 with 37" Tires and Dana 4:1 X-case. A Table with the data is included. In the table you can see why some look
for taller gears. A stock Pinz 710 is 3800 RPM @ 60 MPH, high rev's for the Mog and C303 as well. If not the extra work on the engine,
these Rev's are very noisy. This has lead to the TD Tranie swaps in the 710 where 5'th is OD now your down to about 2700 RPM at 60.
For tire diameters I assume nominal. For example a 285/75R16 = 32.8" but under load at 45PSI it's close to an effective 32", so I
assume 32" for 16" wheels, and 30" for 15" wheels etc.
Pinz options, TD Tranie swap, swap 712/710 X-cases, and Engine upgrade. I made up pseudo Model numbers:
710-TG-X10 = Stock 710, TG is stock Transmission Gas, and stock X-case
710-TG-X10-EFI = Same as above with 2.7L Engine conversion, a 20%+ Torque boost.
710-TD-X12 = 710 with TD Turbo Diesel Tranie, and 712 X-case
710-TD-X12-EFI = same as above with 2.7L Engine conversion
710-TD-X10 = 710 with TD Tranie, stock X-case
710-TD-X10-EFI = same as above with 2.7L Engine conversion
Before jumping to the results, there a few properties of this chart to understand. First as a broad observation, Rock Crawlers
are in the upper left quadrant of the chart, with high FWR and low ground speed. Passenger vehicles would be are in the
lower right, low FWR high ground speed. Military/all-terrain are in the middle, but with FWR>1 and ground speed typically < 6 ft/sec.
The question is, when the gearing is changed, what happens to a specific Vehicle. It's pretty obvious that increasing gear ratio
increases FWR and slows the rig down. Changing gears and/or tire size results in a given point on the chart moving in a straight
diagonal line. This is an important point as relative performance gains with gears or tires is basically a square law effect -- increased
gearing moves to the left and up. Changing Torque, or Curb weight moves up/down the vertically axis only. This is why when you
swap in a TD Tranie even with the Engine improvement you end with a compromise in off-road capability, the Torque gain didn't fully
compensate. And without the Torque boost, you are much worse off.
On the chart are all three variants of drive train options for the Pinz both with and without the Engine upgrade are shown. The
stock Pinz is FWR=1.35, at 4.8 ft/sec, compared with the TD with EFI the original equipment is simply better off-roading performance.
Although with the TD+EFI it's still better than a Series-III. Given a choice, the TD+712 X-case with EFI would be better balanced but
major surgery. The stock Pinz, C303, and G-461/280GE are very closely matched. The 716 is a major improvement over the original
Pinz. Then there's the little Hafie and shows it's quit capable. The FJ Cruiser is a significant improvement over the FJ40 which was
geared too tall, a rear diff change would make a significant improvement and some have gone this route. The Mog 404, with a FWR = 4.0
and speed of 1 ft/sec, you can see why they are great rock crawlers. From what I determined true crawlers have a FWR >3 and
move <3 ft/sec. A lifted CJ-7 with a 37" tires and a Dana 4:1 X-case has a FWR=5 and 3 ft/sec. A brief note on the 712 -- lower
gearing didn't totally make up for the additional 1,000 lbs of curb weight against the 710, but they are pretty equivalent only due to the
slower ground speed, the 712 with EFI would put the 712 closer to Pinz diagonal line. As far as drive train options for the 712, the
best trade-off would be a swap to the 710 X-case, the TD tranie in the 712 is approximately the same trade-off as the TD in the 710
even with the 712 X-case due to the higher curb weight, goes without saying EFI is a must.
My conclusion at this point is that a Pinz is not a Rock Crawler though they are very capable vehicles, the TD tranie even with a big
bump in Torque is a definite compromise but, from my experience still capable. I have no immediate plans to swap out the TD tranie,
again as I spend a lot of time on HWY. However, now with a method for comparison I plan on going down somewhat in tire size to add
an additional 5% in FWR and 5% reduced ground speed, airing down is also a big help reducing effective Tire diameter. I also now
know my Pinz's limitations better and when to turn around. If a 712 X-case shows up one day, I'll consider moving in that direction.
I enjoyed digging up all the data. I hope you find this interesting and helpful.
compare off-road capability using key specification data. Scroll down to the chart and see if this looks interesting to you. I did
this research for myself, then thought some would find it useful. Just to be clear this is one way to look at capability and
certainly not the only important factor -- it did help me to put things in perspective. All this analysis is predominately 4x4,
Manual tranie's, torque converters are different. Typically rigs are 70's and 80's era. If you see something wrong with my
data provide me source data and I'll make corrections. Now the background ---
I bought my 710K with a TD Tranie + 2.7L/EFI, it's all I know. For me trails worth wheeling to are 200+ miles away, the
improved HWY performance is a plus. Although, after a very challenging run to Snake Lake in the Sierra's I decided to
revisit the Pinz drive train options. There are mixed reviews of the TD Tranie in the 1'st gen Pinz, some say it's a bad idea.
viewtopic.php?f=13&t=10003
First gear in the stock Tranie is 5.33:1, the TD is 4.25:1 that put's TD's 1'st gear between 1'st and 2'nd of the stock Tranie
and a 25% reduction in applied Torque -- as a spoiler an Engine upgrade has to be part of any move to taller gearing or you
will absolutely regret it. Even if the percentage of Torque gain from the 2.7L engine upgrade were to equal the gear ratio loss
it will NOT completely compensate for the difference, this is will become obvious.
I started thinking about a technical way to graphically compare the Pinz drive train options relative to each other and against
other rigs based on "wheeling" factors. I could not find anything published that represents this idea. What I came up with is based
on a fundamental principle -- moving a heavy mass with control over obstacles up steep grades. The first factor is, Force to
Weight Ratio (FWR), obviously important for any 4x4 facing steep grades. Calculated using total gear ratio, Tire size, Engine Torque,
and Curb weight, it's simple to calculate. With FWR=1.0, ground force equals vehicle weight. In theory, you could crank your truck
up a vertical wall. However, FWR=0.5 implies a grade limit of 30 degrees. FWR is the Vertical axis on my chart. The other factor,
Ground speed, measured in ft/sec. This is a measure of control. Fast presents less control negotiating obstacles, hitting an obstacle
too fast, or breaking traction on a steep grade. Slow is better, more latitude to reach peak torque from a dead stop. Calculated from
Total gear ratio, Tire size, and at a fixed RPM of 2,000. This becomes the horizontal axis. I picked 2,000 RPM as a baseline as that
is a fairly common RPM near peak torque, below 2,000 many engines have a sharp roll-off in Torque.
Trucks compared; 710 and various drive train options, 712 stock, Mog 404, C303, 716, CJ-7, Hafie, Series III, FJ Crusier, FJ40,
1979 G-461/280GE, VW Thing 181s with Portals, and for yuks a Mini-Cooper. The CJ-7 is factory with aggressive options, I also
show a lifted CJ-7 with 37" Tires and Dana 4:1 X-case. A Table with the data is included. In the table you can see why some look
for taller gears. A stock Pinz 710 is 3800 RPM @ 60 MPH, high rev's for the Mog and C303 as well. If not the extra work on the engine,
these Rev's are very noisy. This has lead to the TD Tranie swaps in the 710 where 5'th is OD now your down to about 2700 RPM at 60.
For tire diameters I assume nominal. For example a 285/75R16 = 32.8" but under load at 45PSI it's close to an effective 32", so I
assume 32" for 16" wheels, and 30" for 15" wheels etc.
Pinz options, TD Tranie swap, swap 712/710 X-cases, and Engine upgrade. I made up pseudo Model numbers:
710-TG-X10 = Stock 710, TG is stock Transmission Gas, and stock X-case
710-TG-X10-EFI = Same as above with 2.7L Engine conversion, a 20%+ Torque boost.
710-TD-X12 = 710 with TD Turbo Diesel Tranie, and 712 X-case
710-TD-X12-EFI = same as above with 2.7L Engine conversion
710-TD-X10 = 710 with TD Tranie, stock X-case
710-TD-X10-EFI = same as above with 2.7L Engine conversion
Before jumping to the results, there a few properties of this chart to understand. First as a broad observation, Rock Crawlers
are in the upper left quadrant of the chart, with high FWR and low ground speed. Passenger vehicles would be are in the
lower right, low FWR high ground speed. Military/all-terrain are in the middle, but with FWR>1 and ground speed typically < 6 ft/sec.
The question is, when the gearing is changed, what happens to a specific Vehicle. It's pretty obvious that increasing gear ratio
increases FWR and slows the rig down. Changing gears and/or tire size results in a given point on the chart moving in a straight
diagonal line. This is an important point as relative performance gains with gears or tires is basically a square law effect -- increased
gearing moves to the left and up. Changing Torque, or Curb weight moves up/down the vertically axis only. This is why when you
swap in a TD Tranie even with the Engine improvement you end with a compromise in off-road capability, the Torque gain didn't fully
compensate. And without the Torque boost, you are much worse off.
On the chart are all three variants of drive train options for the Pinz both with and without the Engine upgrade are shown. The
stock Pinz is FWR=1.35, at 4.8 ft/sec, compared with the TD with EFI the original equipment is simply better off-roading performance.
Although with the TD+EFI it's still better than a Series-III. Given a choice, the TD+712 X-case with EFI would be better balanced but
major surgery. The stock Pinz, C303, and G-461/280GE are very closely matched. The 716 is a major improvement over the original
Pinz. Then there's the little Hafie and shows it's quit capable. The FJ Cruiser is a significant improvement over the FJ40 which was
geared too tall, a rear diff change would make a significant improvement and some have gone this route. The Mog 404, with a FWR = 4.0
and speed of 1 ft/sec, you can see why they are great rock crawlers. From what I determined true crawlers have a FWR >3 and
move <3 ft/sec. A lifted CJ-7 with a 37" tires and a Dana 4:1 X-case has a FWR=5 and 3 ft/sec. A brief note on the 712 -- lower
gearing didn't totally make up for the additional 1,000 lbs of curb weight against the 710, but they are pretty equivalent only due to the
slower ground speed, the 712 with EFI would put the 712 closer to Pinz diagonal line. As far as drive train options for the 712, the
best trade-off would be a swap to the 710 X-case, the TD tranie in the 712 is approximately the same trade-off as the TD in the 710
even with the 712 X-case due to the higher curb weight, goes without saying EFI is a must.
My conclusion at this point is that a Pinz is not a Rock Crawler though they are very capable vehicles, the TD tranie even with a big
bump in Torque is a definite compromise but, from my experience still capable. I have no immediate plans to swap out the TD tranie,
again as I spend a lot of time on HWY. However, now with a method for comparison I plan on going down somewhat in tire size to add
an additional 5% in FWR and 5% reduced ground speed, airing down is also a big help reducing effective Tire diameter. I also now
know my Pinz's limitations better and when to turn around. If a 712 X-case shows up one day, I'll consider moving in that direction.
I enjoyed digging up all the data. I hope you find this interesting and helpful.