Question - Why not?

"HammerheadFistpunch" (hammerheadfistpunch)
06/09/2016 at 18:10 • Filed to: Subaru

Okay, so more specifically - Why not offer a “low range” on a few Subaru models?

Photo credit: !!!error: Indecipherable SUB-paragraph formatting!!!

“Duh, cause they’re cars!”

Well yeah, but it terms of their target audience especially XV, Forester and (to a lesser degree) outback they are seen as inexpensive adventure mobiles. No you wont be doing crawling in those anytime soon but many people take these and really use them, including climbing steep hills, going through deep sand and over undulations that are going to be beyond the ability of the engine to power over. Wouldn’t low range be sweet?!

Typically engine power goes:

Engine -> Transmission -> Final Drive -> Wheel.

Where the lowest possible ratio of these combined elements is know in off road circles as the “crawl ratio”. I.e. a Jeep Renegade TrailHawk has a crawl ratio of ~20:1 because first gear is 4.7:1 and the final is 4.334:1 (4.7*4.334 = 20.37:1). Therefore engine torque is is multiplied 20X in first gear across the rev range. The Subaru Forester, for example, is ~18:1 in “X-Mode”. While that sounds like a lot...it isn’t. The reason it isn’t is because of three factors:

1. “Small” cars are heavy. The Renegade weighs 3500 lbs

2. “Small” cars have small engines with relatively little power, especially low down.

3. Power + traction control / 4 wheels = minimal available power.

Traction control is awesome, X-Mode included and it reduces wheelspin and allows for good ol cheap open diffs to perform much better than would otherwise but they all have the same problem - they use the brakes. What this means is that you aren’t actually moving engine power around (excepting that power doesn’t exists at all on a stopped wheel) what you are doing is absorbing engine torque into the brake system turning what little power there isn’t and throwing it away. This combined with relatively low engine speeds means that, when the going gets tough, !!!error: Indecipherable SUB-paragraph formatting!!! .

If you !!!error: Indecipherable SUB-paragraph formatting!!! you find out that even as fast as 5 mph, way above crawling speed, the engine just isn’t spinning near fast enough to make enough power to move the vehicle forward, especially if one or two wheels are being braked to improve traction. We’re talking AT BEST 300 lbs-ft per wheel(before diving by the tire radius). !!!error: Indecipherable SUB-paragraph formatting!!!

So, low range. Low range is another gearset in the equation so that now it looks like this:

Engine -> Transmission -> Low range gears -> Final Drive -> Wheel.

So, taking the Jeep as an example, if you added a modest low range 2:1 you get (4.7*2*4.334 = 40.7:1). Now instead of 300 lbs-ft per wheel you are talking 600 lbs-ft. Big difference...now factor in that your engine is spinning 2x as fast for the same speed and you change the math even more so instead of 60 lbs-ft at 2000 rpm you are looking at 120 lbs-ft OR MORE at 4000 and now that 600 lbs-ft becomes 1221 lbs-ft per wheel.

So obviously you want low range, even if all you are going to be doing is climbing an lumpy hill to an overlook so why doesn’t everyone do this? Simple: Its hard to add a second set of gears after the transmission in a transaxle...there are serious packaging issue before you add in the complications of packaging AWD anyway. Traditional 4x4's solve this by using a transfer case behind the transmission that houses the gears and splits power for and aft, and this requires a longitudinal layout.

Now is the part where I remind you that Subaru’s are longitudinal AND use a transfer case already. Symmetrical AWD yo.

This idea isn’t foreign to Subaru either as they’ve been doing low range for a long time, but that was back in the tiny power days of 60 hp or so, even still it made a difference. Actually, they’ve had low range up until very recently even in their smaller engines or in the forester which is the companies “tow vehicle” outside of North America.

The SH forester had this, as seen by the 2 gears with the shift fork above the center differential. In this case, this is a legit 4X4 low range setup but a little backwards since it comes before the transmission. This example is a 1.2:1 multiplier, but they also made a 1.5:1 system.

Applied to the Forster with “x-mode” we are talking about going from an ~18:1 crawl ratio to a more useful 23-27:1. All the power needed for getting up and over that sketchy loose hill and at lower, safer speeds. Granted, these systems were only available on manual transmission models that had “crawl ratios” much lower down in the 15:1 range so its not entirely applicable, but even still, with the 1.5 reducer its still a better crawl ratio (22.5:1) than either the “x-mode” forester or the Jeep Renegade trailhawk. A system that would be compatible with the CVT is forthcoming as a small planetary gearset in a similar spot, or before the center differential in the transfer case. Planetary gears are tiny, they can hold up to a lot of torque and they allow for torque multiplications without having to use chains, or shift forks. Basically you just pull up on the lever (or with an actuator), braking a part of the gear and changing its ratio same as an automatic gearbox. Highly efficient, smartly packaged and rugged. Basically its an old fashioned “overdrive” but its an underdrive. These types of gears are being used in torque vectoring differentials like SH-AWD and The Focus RS to great effect.

Now obviously the answer to the question above is that the market isn’t demanding it, but...what a thing to offer to customers! After all the #1 complaint of the popular XV is lack of power and while this wouldn’t really affect engine performance at highway speeds it would sure be a major selling point against competitors like the Renegade if you could address the power issue and push the product even further into the adventure market.

Great crap on a stick this was longer and more drawn out than I thought...enjoy some 80's Subaru off road

DISCUSSION (9)

extraspecialbitter > HammerheadFistpunch
06/09/2016 at 18:27

I’m still kinda annoyed my 2011 imPretzel didn’t come with it. But I guess I should just be glad it came with a stick.

RallyWrench > HammerheadFistpunch
06/09/2016 at 19:07

This would be really helpful in my Outback, or even just a crawler 1st, because it’s hard to get that thing moving uphill when it’s loaded off road without torturing the clutch.

DynamicWeight > HammerheadFistpunch
06/09/2016 at 19:11

I would like to say, that as an owner of a manual Crosstrek who has taken it on steep dirt roads, this is sorely needed. Trying to get that thing started on any sort of semi serious grade is an exercise in abusing the shit out of the clutch. And going up rutted dirt grades means the car goes slower.... and slower.... and slower.... until even the passengers start freaking out. The only option is to either rev the shit out of it or keep up momentum, which is tough on really rutted roads. Not to mention it beats the shit out of the chassis and suspension.

I am going to trade it in. Probably for a Tacoma. I cant stand how weak it is or the seating position required to reach the clutch.

If only EssExTee could be so grossly incandescent > HammerheadFistpunch
06/09/2016 at 19:19

I’ve done the math, in 2wd our 1990 Wrangler has crappy a crawl ratio of 13:1, but throw it in the 2.72 low range and you’re suddenly dealing with a 35:1 ratio, or ~1800 torques/wheel.

HammerheadFistpunch > If only EssExTee could be so grossly incandescent
06/09/2016 at 19:32

At what rpm? My crawl ratio is 31 but the engine has lots of low torque so at 1500 rpm it’s like 1850 lbsft/wheel.

Sam > HammerheadFistpunch
06/09/2016 at 19:44

Counterpoint: 99.99999999999% of Subarus will only ever see dirt in the form of, at most, an unpaved back road.

If only EssExTee could be so grossly incandescent > HammerheadFistpunch
06/09/2016 at 19:49

Peak torque- 210lbs/ft @ 2000rpm

FTTOHG Has Moved to https://opposite-lock.com > HammerheadFistpunch
06/09/2016 at 19:53

I'd be interested if it was part of a package that had more underbody protection. Otherwise it would probably just tempt me to take my Forester places where I would just destroy it.

NipperDawg > Sam
06/09/2016 at 20:20

Mt 1989 GL had a 4wd low. I used it all the time on straight roads. But it was in bumper to bumper rush hour traffic in NYC at 2mph. it mad commuting a dream.

But seriously, how many people even know what 4wd low is? 20, maybe 30?