Differential Gear Ratios for 4WD and AWD Vehicles

For example?

A 1% disparity to overcome issues when turning and prevent one axles overun from interfering with the other. In this model the front axle would turn slightly faster than the rear; better to pull the rear than to push the front; when in the 4 wheel low gear selection. Unlike a torque distribution control system.

I suppose that vehicles with a viscous coupling could tolerate a mismatch in ratios, why would you want to?

In my earlier Jeeps the front axle turned slightly faster than the rear, remember these were all manual locking hubs etc..disparity of ratios between front and rear axles

in earlier 4 wd you were not supposed to drive on road with four wheel drive as it caused damage to the drive

the use of a diff on the gear box means you can now have diffs of varying ratios.

i suspect the difference is down to the amount of power that is transfered to front and back.

have you tried googling the question ?

for instance

http://www.google.co.uk/search?q=differential+gear+ratios+fo+4+wheel+drives&sourceid=ie7&rls=com.microsoft:en-US&ie=utf8&oe=utf8&rlz=1I7GPCK_enGB411&redir_esc=&ei=yUkoTa-ZC4yIhQfZj53SAg

this ones good

http://www.ajeepthing.com/how-4wd-works.html

This is a statement commonly made having to do with part time 4WD drive trains. Why would the wheels need the ability to slip as provided in loose off-road conditions? Because it lacks a center differential, a part-time 4WD system can only be used in low traction situations where the wheels have the ability to slip as needed.

Sorry, but I think you are confused. On the 4 WD vehicles that I have owned (GM) the front and rear axle ratios were/are identical. They were part time and I agree that driving them on pavement is not a good idea. However I drove many mile down dirt roads "locked in" without ever a problem.

I'm not aware of any vehicles, or reason for vehicles to have, different gear ratios front and rear. That doesn't mean they don't exist, I admit.

I think Bwire is correct, although I have not worked on or owned systems in which the ratios are different. My 1990 F150 had identical ratios and part time 4wd with 2 speed transfer box.

I theorize that the difference he mentions may be mainly to take up slop in the transfer case dogs. In a note to me, he mentioned 4:10 in the front 4:11 in the rear. Although even on a dry road, this small a difference would be overruled by the large differences in turning (from the front axle being outside the rear), on average this would keep the system under consistent load... maybe.

Another theory is that it would enable the freewheel hubs to work more consistently, perhaps making them easier to disengage, by ensuring that they are always (more often, really) loaded in the driving direction.

NVH issues may come into play (and the backlash issue would fall into this -- in fact I think I have been in 4wd trucks where I have heard slight gear load changes -- more or less whine, etc.).

Resonances? Beat frequencies?

I'd be interested in hearing from someone who designed these systems.

In AWD (with center diff or fluid coupling) I can't imagine that the slight difference in ratio could make any measurable difference at all. A large difference could determine front/rear torque split, but that split is handled by other means on most systems. Having the differential always spinning differentially would be an efficiency issue.

Nope. I just doesn't make sense. It would be much easier to adjust the ratio of output shaft speed, front to back, in the transfer case, if there was any logical reason to vary the speed of front wheels vs rear wheels. Why on earth would anyone produce two different differentials with virtually no difference in ratios. Think of the logistical nightmare involved.

Now, consider the fact that I used to run 40/50 mph down dirt roads in 2 WD with the front hubs locked in and then shift "on the fly" into 4 WD with no gear grind. Even 1% difference would not allow this.

I'm skeptical, but as I said earlier, I could be wrong.

Even 1% difference would not allow this.

Actually 1% would allow engagement without grind. In fact it would speed engagement much of the time because the transfer case has dogs instead of shynchros, and dogs will only engage when they have a slot to drop into, which is not all the time -- it's quicker if there is some relative motion. (this is why, incidentally, on the few systems that used a dashboard button to engage on-the-fly-4wd there is a variable lag in the actual engagement)

There would be no detectable difference in function between a 4:11 and 4:10. The f/r differences from going around corners are, in a many turns, more than an order of magnitude greater (than that 1/4% difference), and tire sizes, inflation, load all differ by more than that. So I agree with those who say that it just comes down to what's available: the front and rear axle assemblies are not the same, and so can come from different vendors; the front could use a smaller ring in the interests of compactness, etc.

3:50 vs 3:55 on the other hand, is enough of a spread to seem intentional. Then the issues mentioned by me and others could make sense: fronts pulling, engagement/disengagement timing and load issues, etc.

That there have been f/r ratio differences in production vehicles is fact. As far as the why?, I am just guessing.

(It is actually relatively hard to adjust the f/r spit in a conventional transfer box, in which one speed is straight through, with no reduction at all. It is comparatively easy in the variable torque split systems -- but they are entirely different things.)

OK, I concede that 1% may be beneficial in non-synchro gear transfer cases.

Once again, I just don't know. But, I also concede that you seem to know a lot more about the subject than I.

So, I'm not going to lose any sleep over it, since I've started my 4WD vehicle just once in the last 5 years, to move it out of the garage.

Cheers.

Thx it does make sense

If there is a center differential (or fluid coupling), the F/R ratios do not need to be the same, but in ordinary four wheel drive systems the ratios and tire rolling sizes must be the same. I've never worked an any four wheel drive systems (full time, part time, AWD. etc ) in which the ratios have been different.

Here's a link, to a discussion of such stuff.

From this and another couple sites, I'd agree with the people who say is is just a tooth count issue (from different manufacturers, different pinion sizes. etc.). The front pulling vs rear pushing for off road use makes sense, but the difference in factory ratios are too small to have an effect. Given the loose surfaces, you'd want at least a 10% difference to be able to detect anything off road.

Years ago when the Novi was running in the Indy 500 in 4 wheel drive configuration the front wheels were 2% ahead of the rear for stability. My 4x4 trucks all had identical ratios front and rear. It may depend on the system of driving the axles never knew there was a difference on modern road cars. Cheers

I remember in the 50's a 4WD Land Rover being driven on Tarmac for 250 miles scrubbing a set of tyres. 4WD was meant for off road use only. Tractors in 4WD mode had the front axle going 5% (approx.) faster than the rear. You have to pull pushing makes the unit unsteerable. The typical Japanese 4 x 4 such as the Honda CRV etc has 4WD drive automatically cutting in based on wheel speed. i.e. wheel(s) accelerates the drive goes to the slower wheels. This has been a common method on turfcare machines in the UK. 4WD drive only when necessary.

After all you only need 4WD/AWD to provide better traction (A farm Tractor)and /or apply excessive power(rally car).

I do not see the difference between AWD and 4WD unles you have six wheels!

I do not see the difference between AWD and 4WD unles you have six wheels!

It's semantics and convention: 4wd is part time, AWD is full time. AWD must have a center differential or fluid coupling (etc) to allow for f/r differntial when cornering.

i agree with 15. i have had 3- 4X4 , a 85 Subaru for the first and two Trucks and all had a fasters front diff. everyone lurched when i made a right or left turn over a 45% degree turn. like on #15 above, Subaru has the central differential for it's AWD cars. all the older 4X4's had different ratios for slip. slip is what they anticipated only in 4WD. this is old Enginerring. the new we have central differencials.

There is no reason to deliberately mismatch the drive ratios. All it will do is scrub tires, power, and mileage. On an open differential power is going to be sent to the wheel traveling the least distance. Go around a turnon a RWD vehicle, and power will drive the inside wheel because it covers less ground than the outside one. Add an unlocked all wheel drive vehicle to the same situation, and the power will still be sent to the inside rear whell. It is still the slowest turning wheel.

The differentials should only have to adjust the differences in tire size, and turning distances.

As for the Novis, the split ratios might have been to compensate for different tire sizes.

I have never seen non matching ratios going back as far as the 60s in the 1 ton and under classes.

On an open differential power is going to be sent to the wheel traveling the least distance. Go around a turnon a RWD vehicle, and power will drive the inside wheel because it covers less ground than the outside one.

That is a little misleading. The power is split evenly in a straight line and very nearly evenly when going around a turn. Thus in an open differential, when one wheel is on ice, the driving torque is limited to the torque that can be delivered by the wheel on ice. If no power is being delivered through one wheel, none can be delivered through the other. For this reason simple traction control applies the break on the wheel with less traction, increasing the torque required to turn it, which increases torque at the other wheel.

I agree with you on that. Perhaps I should use a different term. How about power will be sent to the wheel that causes the vehicle to travel the least distance.

According to the manual for my 1974 Wagoneer, there are 2 different ratios, the purpose of which is to prevent "bucking" between the two differentials due to the small amount of backlash required in each differential.This provides a slight tension between the 2 differentials,preventing the overlap of backlash.For this reason, it also recommends to limited driving on hard surface roads in 4 wheel drive. I am sure the new technology has made this method obsolete...or maybe not.

This was true gorilla engineering,but it was a very tough critter,with totally enclosed axles front and rear.It is still the chassis of choice for mud bogging.

HTRN

Well you have sparked my curiosity. Now I am going to have to do some digging in my older shop manuals. Using the 4.11 to 4.10 ratios, this would only be about 1 revolution per second difference at 60 MPH, but, still it does not seem to be necessary in other vehicles. But if the vehicle was going at 20 MPH, the teeth would be passing by each other slowly, and could probably help to allow them to align easier. My head hurts now.

It still doesn't make sense to have different ratios to me . If GM did it, they kept it a secret.

You have all hit it, but talked around it a little.

The front and rear axle drive ratios may differ from each other, but the final drive ratio is matched between the front and rear via the transfer case (or combined transmission/viscous coupler). In older vehicles, it was much simpler to use the same carrier and drive ratio in the front and rear to bring the cost of production down. As time went on, performance and efficiency took hold. In essence, front wheel drive provides a more stable platform for traction and directional control.

If the torque load to the front wheels is increased by providing a lower gear ratio to the front than the rear, forward traction is increased. the front wheels are not turning at a faster rate than the rear, but the torque distribution provided by the transfer case favors the front axle. This puts the power to the front wheels, which are providing directional motion.

If the front wheels were actually turning at a faster rate, you would be dragging your rear wheels down the road. That's not happening.

As time went on, performance and efficiency took hold. In essence, front wheel drive provides a more stable platform for traction and directional control.

I think that Bwire was writing mainly of older, truck-based 4wd drive vehicles which were normally rear wheel drive, and typically had manually selectable low and high range 4wd. (This system is still available.) In these systems, the 4wd f/r transfer box ratio in high was exactly 1:1, mechanically linked without any provision for slip. So in these vehicles a large difference in f/r axle ratios would be problematic in 4wd, when on pavement. However this (straight line) problem, with a slight ratio difference (4:10 vs 4:11 = .024%) would be minor compared to the larger (up to 3%) difference caused when the front and rear axles travel on different arcs, while turning. So the manuals for all such vehicles cautioned against driving on dry roads with 4wd engaged. This class of vehicles has not changed in this respect -- you can still buy a Ford F150 with part-time 4wd. There are no "real" trucks that are front wheel drive, with load carrying capacity being more important than efficiency.

With a center differential or slip coupling, none of this is an issue, and f/r ratios can vary, although I am not aware of any production cars in which this is the case (but haven't researched the Porsche and Lamborghini systems in which the front and rear tires sizes are different.) Some of the more modern transfer arrangements, like the Haldex, are not ideal for continuous f/r differences in drive shaft speeds.

I agree. The "original" Jeep Quadra Trac system (knob in the glovebox) locked the front and rear drivetrains together through the BW1339 transfer case. There was a very specific warning labeling it Emergency Drive. The minor differences in tire and gear train wear between front to rear (and the unforgiving short CJ wheelbase) would spell trouble and cause galling and eventually breakage in the weakest link, usually the chain.

In jeep Cherokees the mismatched front/back drive ratios were in existence to at least 1989. I had one. The in vehicle signage definitely said for off road use only. I do know the slightly faster turning front wheels made for great hill climbing ability. I'd still be driving it if it had not gotten in a losing argument with a semi truck, got me from behind and buckled the floor boards.