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-   -   Using 4L for Engine Braking/Torque with FWH Disengaged (https://www.team-bhp.com/forum/4x4-technical/130183-using-4l-engine-braking-torque-fwh-disengaged.html)

Hi All,

The Thar DI with 3.73 ratio offers absolutely no engine braking as the gearing is to tall. When coming down a steep slope in 1st Gear, the vehicle almost touches 20 kmph without any throttle input. After I changed gearing to 4.27 it is much much better, but in certain tricky off-road situations I found the crawl speed too high for comfort and end up using a brakes.

In my land in Ooty, there is a extremely steep road with lot of hair pin bends. When going down and up, the speed in 1H is too much and I used to put the vehicle in 4L just to slow the vehicle down and not for traction. However the surface is hardened mud and 4WD is not required for traction especially since I have the LSD in the rear. Another problem in using 4WD is that in the hairpin bends, it causes the front axle to bind as the surface is not slippery in the summer.

This time I decided to engage 4L without locking the Free Wheeling Hubs, effectively creating a 2WD Low range gear and it worked just great. I could crawl down an up without affecting steering or having to pump brakes or accelerator.

This is actually a bonus of Free -Wheeling Hubs that I never really thought of before.

The question I have is, will it cause any damage to the drive train to engage 3WD with FWH unlocked? Other than the extra stress on the rear axles?

I guess it shouldnt be a problem as long as you use it only for coasting downhill (hence less stress on axles). This will definitely overload/stress the rear axle if you try climbing up using the same setup.

Lets wait for inputs from users with experience in similar situations.

Dear Tini - please refer the original Jeep manual. 2WD low is not to be engaged. Axle shaft will break. I have broken it, just to see, it snapped like a matchstick! In your specific case it worked because you were going downhill so you were not torquing the axle shafts. That's why T18 transfer case has an interlock. As long as 4WD is not engaged, the smallest lever will not go towards the front, means low will not be engaged.

Best regards,

Behram Dhabhar

Great Question Tini! Infact, I didn't know this would be an issue until you brought it up.

There is also a similar place in my hometown, where the path is made by concrete, hence the traction is really good. However, the incline is very steep, that, if you have some load in your Jeep, it will climb only in 1st LOW. The incline is about 200 meters with sharp turns.

So.. What happens is that, if my FWH is in lock position, while turning, my steering is pushed straight violently. So, what I do is to put it in the free mode, slot gear in 1st Low and drive up. There used to be no problems. However, I've noticed that even with this much traction, in this much steep inclines, if I drive with power in only two wheels, it slips and burns tyre.. But, I was of the impression that my tyre is less precious than my original front axles/gears etc..

Another place where I use this technique is when coming up from my farm. On top of an incline, there is this gate with no gate keeper. So, on approaching the gate, if I'm alone what I do is: Put the Jeep in 1st low, let her craw up, in the meanwhile, I walk to the gate open it, and then get back into the Jeep. (And, yes same thing while closing). stupid:

Sir, my axle is not broken yet. However, even with T-18 TC, I guess I should stop this practice?? Please suggest a simple YES or NO. :D

I actually climbed up in 2Low as well, the climb is extremely steep with dips, ruts and rocks along the way. However, while going down and up, I went in 2nd Gear with Low range.

The gear ratios of the NGT520 is given below:
1=> 3.787
2=> 2.09
3=> 1.38
4=> 1
5=> 0.79
R=> 3.52

The T18 Transfer Case has a low range gear ratio of 2.46 to 1. So in 2 Low your gearing is 5.14 to 1. I don't think that this amount of Torque will break the rear axle in a jeep without loads.

The first gear of the KMT90 gearbox supplied to the Army 550 is something like 4 to 1. So will a 25% lower gearing break the rear axles?

Dear Dhanush - simple answer in the format as you have requested is: YES. PLEASE DO NOT USE 2WD LOW RANGE.

Best regards,

Behram Dhabhar

This is where center differential comes very useful. But most full-time 4WD (like GV or Fortuner) lock the center diff in low range. Why not allow center diff to be open or in limited slip in low range?

Dear Sharath - this is not done due to huge cost involved and availability of aggregates only in left drop configuration, not in right drop as required for the "MM family" platform, including Thar. Worldwide trend is left drop.

See the new Safari Storme 4WD. Also study the torsen differential of the Aria Pride 4WD. Now decide who has the platform! :D.

Best regards,

Behram Dhabhar

Dear DB Sir,
If i am not wrong whether its T18 or KMT18, you have to be in 4WD to engage it to low gear ratio ??

Shubhendra

Sharath was asking about Grand Vitara and Fortuner which have center diff.

In normal mode its 4WD with center diff not locked, called full time 4WD.

You can go to 4 high locked, or 4L locked

So your modes are
4 high unlocked center diff, 4 high locked center diff, 4 low locked center diff

Why can't there be a 4th mode, 4 low unlocked center diff?

Second query have is that

Whetner front hubs are locked or not locked, 4L torque will remain same. So why will it damage real axle
Is it because now the entire torque will be on rear since front hubs are not locked, and front driveshaft is free to rotote without resistance.

You can refer the below post for understanding how the interlock works.

http://www.team-bhp.com/forum/4x4-ve...ml#post1141430

In 2WD, torque is transmitted only via rear propshaft to rear axle. In 2WD High-range, transmitted torque is below critical limits for which axle & propshaft is designed, hence stress is below limit.

In 4WD, having the FWH open is equivalent to 2WD as the front wheels are not powered. In 4L, torque which is now higher than in High-range, is transmitted only via the rear (FWH open, remember?). Higher torque = higher stress = axle failure. Front axle remains unmolested since it is disconnected from the wheels at the FWH, and hence not torqued.

Mr. Dhabhar, I hope this is a satisfactory answer? :D

As an aside, even European 4WD vehicles with fully electronic transfercases do not enable 2L (atleast in stock form).

Cheers,
Rahul

Going by this logic, even if FWH is locked, if, any one of the front wheels are spinning freely.. Meaning, when front diff is open, the rear axle can break if load is high and both rear wheels have traction?

Well, things get even messier if you got a rear locker and one front wheel is spinning and one rear wheel has no traction. So, all the effective torque has to be borne by one rear axle, and it should break like a matchstick. Right?

The locker splits the power to both wheels. That's why it's called a locker (locks the open differential to evenly distribute power to both wheels instead of all of it going to the spinning wheel).

Tejas, I know that.. That is why, I mentioned 'effective torque' or torque which is needed to move the vehicle.

Read the first part of my post. You will get an idea about what I'm trying to ask!