+Tire Diameter 4 Better Look/Lower RPM
 

Just bought a set of used style 87s. Had them evaluated and "spun" at an independant wheel shop - OK. Now looking at my tire options.

I'd like to address a few issues with my new tires . . . for looks, I'd like to fill in a some of the large clearance between our 29.1" tires and the fender . . . also want to reduce the high rpms of my 5speed.

Check out the pic of one member's use of 285/50 R20 tires on page 138 and 139 of "Show Us Your X" thread. His tire diameter is 2.03" more than standard issue on our E53s, 31.13"!! And he claims to have no issues, don't know how long he's been using this setup on his 200k+ miles X.

So, a few questions for the incredible knowledge base here:

1. Is the large clearance between tire and fender necessary? For seriously stressful off-roading, I imagine so. But some here have lowered their E53s and maintained 29.1" tires in widths of 275+, thereby reducing this space and clearance. The gentleman above filled the gap with larger tires. At what point would one expect to run into inner fender tire rubbing in tight turns with normal street driving?

2. Larger tires = Lower RPM. 31.13" tires have a 6.9% larger circumference than our stock. At 80 mph, my rpm could be reduced from 3800 to 3535 with this larger diameter. I know this would affect the OBC estimates for mpg, range, etc. . . . but are there any damaging repercussions if the OBC is NOT reprogrammed for the new diameter?

3. Transfer case considerations - With staggered 87s and directional tires, the tires will stay in place for the life of their service. I'm towing a 5987# load and anticipate more wear on the fronts than the rears. Would it be prudent to find a tire combination for the set that starts with a SLIGHTLY larger diameter in the front? Thus the fronts could wear down to equal diameter . . . and then only approach a diameter disparity limit when the set of four is due for replacement . . . ideas?

2. the MPH, the MPG are calculated from the "clicks" coming from the speed sensors. The quantity of the "clicks" per mile is programmed into the FRW file - if you can calculate the new number (you need to pull your file with NCS Expert) based on the origianl number, and program it into the car, you should be ok.

I have not done it, and have not heard of anyone on the BMW forums do it, but I have heard someone on VW forum may have reprogrammed their speed sensor "clicks"...

1. Is the large clearance between tire and fender necessary?

Only fitting by trial and error will determine if you will have sufficient clearance.

2. Larger tires = Lower RPM. . . . but are there any damaging repercussions if the OBC is NOT reprogrammed for the new diameter?

Not to the OBC. Your odometer will read under the actual. The speedometer won't be out as far, as it is optimistic to start, by design.

Apart from the OBC, you will have some negative effect of such a heavy tire. Consider the increased wear on suspension components, and inertial effects of the increased tire at that position on the rim, which will mean slower acceleration and braking.

3. Transfer case considerations - would it be prudent to find a tire combination for the set that starts with a SLIGHTLY larger diameter in the front? Thus the fronts could wear down to equal diameter . . . and then only approach a diameter disparity limit when the set of four is due for replacement . . .

No. Always try for matched. Don't expect that they are going to wear down evenly in any case.

I always appreciate your feedeback JCL.

My experience has been that the front tires wear more than the rear. I know this is subject to driving habits . . . but the fronts are subjected to the demands of turning friction and the lions share of the bracking friction.

If I always start with equal diameter tires in a staggered set, I'll expect the fronts to be done when the rears still have ~ 50% tread left. Has anyone here experienced otherwise?

Haven't used a staggered set up, but both my X5 and X3 wore four tires evenly over 75,000 km (original tires in both cases). The X5 tires were still in good shape at that point (sold the vehicle). The X3 tires were replaced at 4/32", not worn out, but I wanted to replace them for another reason. I would have gotten 90,000 - 100,000 km on them, by my calculations. I never rotated tires on either vehicle.

Wearing fronts out first isn't from braking, it is from aggressive cornering with a heavy vehicle, so YMMV.

Based on all research and feedback, now looking at the following tires for my offset 87s:

General Tire Grabber UHP -

Front 275/45 R20, 29.7" diameter
Rear 295/45 R20, 30.5" diameter

Will these .8" larger diameter rear tires place any inappropriate stress on my Mechanical transfer case (NV 125)?

It seems to me the transfer case must be designed to accommodate slightly different rolling diameters front to rear. Is this not true?

685 revs per mile vs 699. I would try and get it closer than that.

Depending on what engine you have you may lose or gain some economy with the lower engine RPM at 80, especially when towing. While slight you will be in a different place in the engine 'power band.'

bcredliner - I'm not too concerned about the engine power band. The 3.0 has plenty of power for me and I'm not trying to win any races. I rarely tow my load over 65 mph.

At this point, I'm more concerned with differential revs per mile front drive to rear drive and any affect on my Mechanical (Non-Xdrive) transfer case.

Searching this site, online, youtube videos, my owner's and Bentley manuals, . . . I can't find a detailed explanation of the transfer case's ability (or inability) to mechanically adapt to differential front and rear axle rpms. I figure that it must be able to . . . otherwise anything that throws front to rear tire diameters out of sink (tire pressures, hvy rear load, hvy front load, . . . perhaps even as experienced during heavy braking) would cause a binding stress that would greatly reduce mpgs, or worse, strip TC gears. If this is this the case, tire pressures and absolutely even tire wear are critical.

So, what am I missing? What is the component in the drive train that allows for different front and rear drive axle rpms? If this component only allows for a "slight" differential in the rpms, what is the tolerance, and how would it most likely fail if the tolerances were exceeded?

I have read some threads that state differences in diameter can cause traction control and braking errors. Tread from new to bald would be about .68" so the X5 can handle near that for some period of time. I think you should be safe at about half that but I have not seen BMW transfer case tolerances. I wrestled with this and decided to go with a lowering kit on my 4.6. Not as easy to get there but does not mess with the computer, speedometer, mileage calc, or put greater load on the suspension. Plus is in spec for the transfer case. If you are only interested in going the tire route, I suggest you keep searching until you find a match. I don't know what component would be the problem but anything involved with the transfer case is going to be costly. If you are doing a lot of towing that is going to magnify the risk. If it helps my clearance between tire and fender lip is about 2 3/4' front and rear. Rims are 87s and tires are 275/40 on the front 315/35 on the rear, same brand, same diameter. I also have 7mm spacers front and rear that I added after I had the lowering kit in place for some time. Never any problems with tire rub. As you mentioned I replace front tires twice as much as rears so there is a period time where front to rear diameter is larger by about half the tread. Never had a problem with transfer case and have clocked a little over 100,000 mi.

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.