Thanks bcredliner -

I appreciate your real world experiences with these size tires on your 87s.

I understand your conclusions regarding TC tolerances relative to your experiences. But what I do NOT understand is this:

How is it that ANY difference in front to rear wheel diameter doesn't rip the TC (or another part of the drive train) apart? I mean, either it can deliver differential drive shaft rpm or it can not. And if it can NOT, then the moment the average wheel diameters of the fronts differ from that of the rears, by more than a gnat's ass, something is going to break.

Ok, here is how your transfer case works. I don't have a drawing so will try with words. Then, maybe the parts drawings will make sense. Edit: found a schematic, posted below

You don't have a fixed connection between the front and rear output shafts. It is not like a manual 4wd transfer case in a pickup truck that is locked, and where you can get axle windup. It is an open differential that allows relative rotation between the two output shafts, front and rear. The two outputs are connected through a single planetary cluster, and it is that cluster that provides the fixed front/rear torque split. That isn't a fixed rotation, just a fixed torque ratio.

What happens is that the power flows to where it divides, and then it goes 38% to the front and 62% to the rear. But for a moment, pretend it is equal, 50 : 50, and we'll take that variable out of the discussion. If the input from the transmission is spinning at 100 rpm, usually each output shaft spins at 100 rpm. But if one spins at 99 rpm, and the input is still at 100 rpm, then the other has to spin at 101 rpm.

When there is a relative difference in rotational speed between the two output shafts, then the planetary has to spin. Continuously. And it isn't designed to. It wears. If it wears enough, it fails. It doesn't have lubrication designed for continuous spinning.

Think of a rear diff on a RWD car. One wheel is on ice. You gun it. The wheel with traction stays still. The wheel on the ice spins at twice the speed. Until the diff carrier fails. Because it is only designed to allow small relative rotational differences when cornering. Same principle.

Now look at the parts book or service manual. Follow the power flow in a schematic.

The tolerance varies by manufacturer. Some state 1%. Some say 2%. Some say maximum 2/32" tire tread difference.

The BMW spec equates to about half tread depth, from memory. I would use 1% as a safe figure. I like to see 4 revs per mile delta, personally. That is within all the above guidelines, except the 2/32" one. And it isn't from BMW.

Hope that helps.

Jeff

OK, let's see if a schematic helps. This is not a BMW, it is a Ferguson. Same principle. It had a 37:63 torque split. That is due to the geometry of the planetary cluster used.

Ignore the limited slip, you don't have that feature.

See the planetary cluster on the right. The input shaft is connected to the carrier. The rear driveshaft is connected to the ring gear. The front driveshaft is connected through a chain to the sun gear.

The planet gears don't rotate on their shafts unless there is relative rotation front/rear, although they always spin with the carrier.

Jeff

Nice explanation Jeff.

I've edited your schematic and attached 3 screen shots of Cobra Transmissions youtube on the TC breakdown, one of which I've edited to match the schematic.

I want to understand this better, so let's talk about the planetary gear bearings (A), the moment arm (B) from the input shaft, through the planetary gear to the outer ring gear (rear) drive, and the moment arm (C) from the input shaft, through the planetary gear to the inner gear (front) drive.

As I understand it, the input shaft turns the planetary gear. And for further discussion, let's say at a hypothetical rpm of 100 as you did. The three small outer gears/bearings (A) of the planetary gear are each splined in two places . . . to the large outer "ring" gear driving the rear, and the smaller inner shaft driving the front through the chain.

Now, . . . we agree that as the entire planetary gear rotates, if these 3 small gears do NOT rotate, then the rpm of all three shafts will be identical . . . input 100, fwd 100, rear 100. The differential torque is achieved via the difference in the moment arms B & C driving the front and rear shafts . . . T = Force X Moment . . . longer moment arm (B) - more torque . . . shorter moment arm (C) - less torque.

As you said, when one of the 2 drive shafts wants to turn at a different rpm than the other, the gears/bearings (A) roll to make up this difference. Imagine the stress on these 3 (A) with driving loads applied. With NO DIFFERENTIAL rpm, all 3 (A) would never turn. The force it applied to the outer ring gear and inner shaft gear would always be from/to the same teeth. But this is hypothetical because there are constantly minute differences in rpm front to rear for many reasons. Thus turning at (A) occurs constantly.

So, we know the bearings/gears (A) are continuously transmitting torque to the two shafts, and that they are designed to roll while doing so to accommodate differential rpm between these two shafts.

I've replaced the TC fluid once to date. I notice the fluid level is well below all the TC gearing accept where the chain connects to the front drive shaft gear. Thus, I assume, the chain carries the oil up to the higher components of the TC including the planetary gear and the large outer ring gear it drives.

Here's what I DON'T understand. Since the 3 planetary bearings/gears (A) are continuously rolling a little, why can't they (A) continuously roll a little more? They have bearings . . . they are lubricated with oil . . . they're enduring the same amount of force to their bearings whether turning or not. It seems like the bearings and all the teeth involved would benefit from continuous movement thus dispersing the heat of friction more evenly on each bearing and gear, as opposed to very little movement where the heat of friction might concentrate into one section of teeth on the gears.

Also, the service manual highlights a paragraph saying it is critical to use the same make and tread pattern front to rear for proper operation of ABS/DSC. That doesn't mean you can't use larger tires just that they should be the same diameter front to rear. JCL has addressed transfer case and suspension load. If I wanted larger tires I would only go that route with tires of the same diameter. There will be enough difference over life of front to rear wear and enough potential greater issues if I ignore the other caution flags.

The common "tolerence" figure is approx 3% (+/-) overall diameter/circumference for side-to-side or front-to-back.

One should try to stay within this 3% (+/-) difference when tire upsizing or going with a staggered wheel/tire set-up. This should satisfy any calculations etc and stay within specs of ABS/DCS/speedometer/odometer/transfer case issues.

Difference greater than 3% could cause issues like warning lights or malfunctioning components.

Qsilver7 - I'm still looking for an understanding as to why these bearings/gears (A) can't handle continuous rotation under load.

But, to address your 3% tolerance guidance, I'm looking at a set of 275/45 R20 (diameter 29.7") front and 295/45 R20 (diameter 30.5") rear.

30.5 - 29.7 = .8". I could divide the fronts or the rears by this difference. But let's go with the fronts for the highest percentage. .8/29.7 = .026936 or 2.7%. Would you anticipate troubles with this setup?

There are at least two limiting factors in the ability of the system (vehicle) to tolerate different rolling diameters front/rear.

The first is the transfer case. You are correct that the planet gears rotate a small amount, ie every time the vehicle turns a corner and the front wheels and rear wheels follow a different track, the average rotation of the fronts and the average rotation of the rears is different. The thing is, the planet gears are not designed to rotate constantly. As you note, it is a splash lubrication system. So, I would look to lubrication and heat as likely failure modes. I haven't looked at any failure analysis of these transfer cases to know what the sequence was, whether it was the axles or the gear teeth, for example.

The second limiting factor is the computer system. The ABS sensors monitor wheel speed, if they sense a big difference, there is intervention. There will be a setpoint (delta) at which it intervenes. Say it is x%. If there is a 1% difference to start with due to different tire sizes, you will just get to x sooner, ie turning a corner, or a wheel slipping. Then, depending on the delta, you will get brakes applied and warning lights. This can make the vehicle undrivable if it is constantly intervening, even if the transfer case can withstand the relative rotation difference.

What is interesting is there are reports of transfer case failures on X3 models that didn't have the lights on, ie it just went long enough with different sized tires to fail the (admittedly smaller) transfer case. So, my theory is that if the lights are coming on you have a problem, but even if they aren't, you may have a problem over the longer term.