Locking Diff

[camoyj7]

Is there any different numbers on axles that have a locking diff stock? On my M35A2c on loose gravel it will spin all eight rear tires if you hold the brake and let off the clutch.

 

[cattlerepairman]

I am not aware of stock locking diffs on the M35A2C. Some 5tons and wreckers had them, IIRC. What you see is likely the result of highway tire pressure and no weight in the bed. Not enough to prevent any of the tires from spinning.

 

[hdmax]

Holding the brakes or using the E-brake is a good way to help move a vehicle that has low traction, so what you are probably seeing is that. Using the brakes to help lock the tires with the least traction, forcing the others to gain some traction resulting in all tires spinning.

 

[RANDYDIRT]

Using the e-brake like that on a deuce will not add traction as it's located on the drive shaft.

Dirt

 

[mudguppy]

using brakes on open differentials will not add traction, period.

 

[spicergear]

Maybe he was "power-jacking" to see what it would do. The only M35 series trucks to have factory lockers were sent to Scandinavia on a limited amount of trucks. M35 Tom has a set in his truck. There are a few sets around but their pricey. You want a locking diff, you get a Detroit or a unit from Ouverson.

ALSO do a search and find lots on this topic.

 

[Mikey90744]

using the brakes will "add traction" only if you have a torsion diff, but like mudguppy said, wont do squat with an open diff exept wear yer brakes

 

[rickf]

using brakes on open differentials will not add traction, period.

This is a tried and proven method of gaining traction. Been done for more years than most of us have been alive. It forces the differential to transfer some of the power to the unloaded wheel. This is done by way of equalizing the force between the wheels by using the brakes.

Rick

 

[RANDYDIRT]

Yep, what rick said

Dirt

 

[nk14zp]

Yep, what rick said

Dirt

 

[mudguppy]

This is a tried and proven method of gaining traction. ...

yep, but not for open differentials.

Yep, what rick said

nope.

ditto to the 'nope.'



applying braking pressure (via hand/park/E- brakes or riding the whoa-pedal) only works for traction-aiding differentials. it does not help with open differentials.

here's why: limited slip differentials work off of a biasing principle. this means that the clutch packs (or gears if it's a Torsen or TruTrac) on the spider gears resist opposition by a certain factor, or bias. this bias is supposedly consistent for varying forces, or torque.

example: let's say we have vehicle that is driving off-road and one wheel drops into a slippery mud hole; now the vehicle is stuck. through engine power and transmission multiplication, let's say (for argument's sake) that 600ft-lbs of torque is coming into the rear axle and differential.

now, if the vehicle has an open differential, then the wheel on good ground may hardly get any of that input torque to propel the vehicle because the open diff is letting it all run out the other side. so let's say the wheel on good ground is only getting 100 ft-lbs of torque: not enough to move the truck thru the hole.

however, if this truck has a limited slip diff, then the clutch packs will resist opposition to one another. the amount of torque that is resisted (and therefore, seemingly 'transfered' to the other axle shaft) is dependant upon the torque bias. so if this is a 3:1 bias, then the wheel with good traction will see 300 of that incoming 600 ft-lbs of torque. this is much better than an open diff, but is it enough to move the truck? let's say no.

now apply braking force to the wheels. this increases the amount of total input torque that the differential is seeing. so instead of the diff seeing 600 ft-lbs, maybe now it's seeing 900 ft-lbs. so now this same diff is able to transmit 450 ft-lbs to the axleshaft who's tire is on good ground. this could help the truck get moving.

so what happens when you apply braking force to an open diff? absolutely nothing. unless you apply brakes to only one wheel, then you have changed nothing at all and will just bog your vehicle down while still spinning one wheel. now, this isn't magic and may not always work. the park/hand/E-brake is a better option as they are only applied to a single axle, whereas the wheel brakes will apply front brakes as well, which will greatly increase the amount of force needed to propel the vehicle.

also, higher quality limited slips usually have higher torque bias. an average limited slip may have a bias around 3:1, but an Auburn gear LS may have a 3.5:1 or better. a factory LS may be a measly 2-2.5:1. and the TruTrac or Torsen diff will have usually at least a 4 if not 5:1 bias.

and remember, LS's with clutch packs wear, obviously. gear oil additives help sustain the appropriate friction compound, but they still wear. that's why dudes that like to do lots of burn-outs with factory LS's soon have a car/truck that only seems to do the one-wheel peel; worn clutches lower the torque bias. i've seen guys take a new truck out into the mud, get it stuck, then lean on the throttle and smoke the LS. then they don't understand why the 'posi rear' doesn't grab anymore.

 

[rickf]

Works just fine with opens, done it many times. It is also well documented in many off road books dating back 40 plus years.

Rick

 

[Mikey90744]

so if I go power brake (hold the brake pedal and floor it) in my open diff ranger I will smoke both tires then right????


DOESNT WORK THAT WAY, sorry a lotta guys will tell you it does, but a standard open differential will not apply more power to the wheel with more traction due to the fact that the friction from applying the brakes is equal, and if its not you have an issue. meaning again the wheel with all the traction and braking pressure applied will still have more resistance then the wheel with no traction and just braking pressure


edit:

what guys do however in drag cars with open diffs is they preload the suspension more on the right rear, put the battery there and heres the kicker that gets people, apply the parking brake with the parking brake set right side bias to help apply power to both rear tires

 

[Recovry4x4]

If you want braking to work with an open diff you need independent brakes for each wheel. If the right side spins out start applying right side brakes until the left side starts driving. Travis is right on the money on this deal.

 

[spicergear]

Holy smokes...ever hear of traction control? Let's take for example a Lexus SUV with traction control. You would NOT EVER BELIEVE how well that could do in poor traction environments because of the traction control. There is NO limited or locking diff offered for one of the rears even though it's glorified Toyota. What it does is when one wheel loses traction and spins, the other OBVIOUSLY HAS traction. The computer will apply the brake to the spinning tire so that the tire WITH traction is forced to turn. The traction control keeps track of this spinning and will keep bumping the brakes as needed to stop the spinning wheel to aid in traction.

The same principal can be reversed if you have a peg leg (open diff) hot rod and one tire is spinning. You can stab the brakes quick and have it shock the rear into spinning both by forcing the tire that was planted to have to spin since the brake got applied short and hard. It's really hard on the rear especially smaller ones and has been known to break the retaining bolt of the cross pin in some GM diffs.

Sorry Mud Guppy as much reason and sense as you made, this brake trick works on open diffs now and then. But really, traction control on later model SUV's is very impressive even if they don't have crap for ground clearance.

Maybe do a google search for 'Trail Lex' which was (hope I got it right) a Lexus SUV that one of the 4x4 magazines built to shut people up about newer vehicles being all crap off road. The traction control worked so well the lack of posi or a locking diff really didn't bother them.

It IS traction control. My John Deere has split braking on it and if I'm grading something instead of stepping on the pedal to fully lock the differential I can bump the brake on the tire that spinning more so that the other tire has to start working again. It's as simple as an open diff explained: One wheels spins the other has traction. Stop that spinning wheel and what has to happen? The other wheel MUST spin or something must break or stall.

 

[rickf]

OK, This will be my last post on this subject because I can see the mechanical engineer in you is not looking at this with an open mind. I am not saying that holding the brake will give you an instant locker. What I am saying is this, IF your brakes are adjusted correctly and you apply some, not a lot, brake then you are equalizing the torque required to turn the rear wheels. Therefor with equal traction seen by the open differential it will send power to both wheels. If you are twisted up with the left wheel jammed in the fender and the right hanging in the air then this is not going to work. If you are sunk in the sand or mud it will work as long as both rear brakes are equal. I can point out a flaw in your L/S theory, you say when you apply the brake the torque jumps up to the rear, this is not true unless you also apply more throttle. I have been a master mechanic for 42 years and have raced all my life. I have traveled the deserts of the southwest and Mexico extensively many years ago when Limited slips and lockers were something the average person just could not afford. I could reference several books pertaining to this subject including the recovery manual from the military but it seems you are locked into this idea that it WILL NOT work. Trust me, it does work within reason and has done so for as long as open differentials have been around. I suggest you try it sometime, do not hammer it to prove I am wrong but approach it with an open mind. As to the person asking about powerbraking, try it, leave some brake applied and it will spin both tires until one tire gets stickier than the other and grabs tighter than the brake on the opposite side can hold. Better yet, straight line, dead stop, clean road. Assuming you have enough power just hammer it with no brakes. It will spin two for a short bit.

 

[Sevin7]

The traction control system selectively braking one wheel and the operator tapping on one brake pedal on a agricultural tractor is essentially the same thing. But punching the brakes (applying the brakes to both rear wheels equally and both front wheels equally) during a burn out on a non-traction controlled, open differential vehicle is something competely different. It's not an apples to apples comparison. Whether the latter increases traction seems pretty unlikely to me. You can't cheat physics.

 

[spicergear]

You can't cheat physics.

Yep, you can't cheat experience either. Think what you want and the rest of us that know it works will use it.

 

[mudguppy]

OK, This will be my last post on this subject because I can see the mechanical engineer in you is not looking at this with an open mind. ...

and you'd be wrong in your assumption. i did lots of reading of forums and other google search results looking for answers yesterday. there were many folks asking about whether this worked or not. a resounding majority of responses were [paraphrasing] "yes, mainly on limited slips."

i appreciate your experience. i have lots of open-diff offroading experience, too. every time i've been stuck i've been told to try this 'method' and it's gotten me nowhere time and time again. often, i've challenged these 'mentors' to show me how to do it more-better with the same results.

it's never worked for me, and i've literally given it the college try.

... I can point out a flaw in your L/S theory, you say when you apply the brake the torque jumps up to the rear, this is not true unless you also apply more throttle. ...

first, it isn't 'theory' - it's how LS's work.

second, that's not a flaw - you can't create torque w/o an opposing force or resistance. it's like letting the clutch out; your engine is not producing any torque at idle until a resisting force is applied.


the only non-measureable outlier is the comments and discussion around the theory of effect that the park-brake has on the spinning wheel. there exists the argument that, when the park brake is applied to both rear wheels, the energy from the slowing spinning wheel is transfered to the other wheel.


the only problem is that the static coefficient of friction is greater than the dynamic coefficient of friction. so even with truly equal braking pressure exerted on each wheel, the stationary wheel will have more braking force applied to it than the spinning wheel. so, somehow, the kinetic energy of the spinning wheel must be transfered in a way that it overcomes:

• the coefficient of static friction to get the opposing wheel to turn,
• the equalized braking forces (which nets in zero), and
• retain enough energy to transfer back into the vehicle to provide propulsion.

since this is a pretty difficult position for energy to be in, the variables are infinite (comparative ground pressure, surface conditions, wheel size/mass, wheel speed, braking force, vehicle mass, how deep the hole is, color of vehicle, amount of pocket lint, etc), and given the fact it has never worked in any situation that i really really wanted it to work in, i'm not aligned with this theory.

 

[rickf]

Travis, Please tell me you do not believe what you find on Google?! Ask most anyone over 55 if they have done this. I do not know how old you are but you really need to think outside of the engineering box. I am not going to continue to debate it. If I have time I will copy the section in the military manual.

Rick