Differential Crack & Oil Spill / Tyre Size Issues? BMW E70 X5

[atomicking]

Hi,

I had changed my tyres to the slightly higher profile, as I am in the Caribbean where the roads are very bad, not long after (a month or so) the rear diff blew up. Before this about two weeks ago the main belt pulley came off as the inside rubber was rotten, could this be connected with the differential as just after replacing it I then had this differential issue the same day or a day after installing a new pulley?

Someone said it could be the circumference on the tyres but I made sure to check the tyres before purchasing and putting them on the vehicle to make sure that they are compatible (new ones are sized 275/45 ZR20 & 305/40 ZR20). I would hate to go back to the smaller sidewall tyres I had ie. 275/40 R20 (front) and 315/35 R20 (rear) as the ones I have now are brand new.

You can see the other thread and images on: https://www.bimmerforums.co.uk/threads/ ... st-3436305

Thanks for your help, AK

[X5Sport]

One issue with all but the very latest xDrive cars is they are very intolerant of the ‘wrong’ rubber. In BMWs car that means anything other than tyres with a ‘’ on the sidewall that indicates they are BMW approved. If the overall diameter is 2mm different between any one tyre or axle set and the other then there is an increasing risk of either terminal damage to a diff or to the transfer box.

The simplest way to check if your sizes are close is to mark each sidewall with a line pointing straight down to the road surface and then move the car forward 10-20 feet (on a level road with the steering straight ahead) to get one axle set of lines pointing at the ground again and then see whether the lines on the other axle line up. If they don’t then you will be getting transmission wind-up and that breaks things.

Alternatively wrap a piece of string around each tyre and note the circumference. Some ‘correct’ sized brands can actually be as much as 32mm different in their circumference - hence not being BMW Approved - and that difference causes damage.

An absolute giveaway is if you move the car forwards under power, at walking pace, and in full lock and the car has any sign of jerkiness then that’s a good indicator of tyres being the issue.

Your tyres are not one of the approved sizes and that may well be the cause, or a main contributor to what happened. There are ‘taller’ side wall options shown in the tyre pressure plate, but these are for 19” wheels. You may find it’s better to either return to the correct size, or source a set of 19” wheels and fit the same size all round.

BMW are not the only brand with this problem.

Your crank pulley failure is not connected with this. That was just bad luck to happen coincidentally.

Separately, have the transmission fluids from front to rear (diffs x2, transfer box and automatic gearbox) been replaced? BMW say they are Sealed for Life. Sadly that ‘life’ as far as they are concerned is just three years. The transmission manufacturer states all fluid should be replaced every 50,000 miles or eight years, whichever comes up first.

[atomicking]

One issue with all but the very latest xDrive cars is they are very intolerant of the ‘wrong’ rubber. In BMWs car that means anything other than tyres with a ‘’ on the sidewall that indicates they are BMW approved. If the overall diameter is 2mm different between any one tyre or axle set and the other then there is an increasing risk of either terminal damage to a diff or to the transfer box.

The simplest way to check if your sizes are close is to mark each sidewall with a line pointing straight down to the road surface and then move the car forward 10-20 feet (on a level road with the steering straight ahead) to get one axle set of lines pointing at the ground again and then see whether the lines on the other axle line up. If they don’t then you will be getting transmission wind-up and that breaks things.

Alternatively wrap a piece of string around each tyre and note the circumference. Some ‘correct’ sized brands can actually be as much as 32mm different in their circumference - hence not being BMW Approved - and that difference causes damage.

An absolute giveaway is if you move the car forwards under power, at walking pace, and in full lock and the car has any sign of jerkiness then that’s a good indicator of tyres being the issue.

Your tyres are not one of the approved sizes and that may well be the cause, or a main contributor to what happened. There are ‘taller’ side wall options shown in the tyre pressure plate, but these are for 19” wheels. You may find it’s better to either return to the correct size, or source a set of 19” wheels and fit the same size all round.

BMW are not the only brand with this problem.

Your crank pulley failure is not connected with this. That was just bad luck to happen coincidentally.

Separately, have the transmission fluids from front to rear (diffs x2, transfer box and automatic gearbox) been replaced? BMW say they are Sealed for Life. Sadly that ‘life’ as far as they are concerned is just three years. The transmission manufacturer states all fluid should be replaced every 50,000 miles or eight years, whichever comes up first.

Thanks very much for this info. I was thinking the same that probably the fluid was not replaced and my vehicle now has about 80,000 miles on it. I bought it with about 70,000.

I went online and used a tire compatibility calculator which measures the exact circumference of tyres. The circumference on the older original sizes I had on (the original tyres the vehicle came with) were only 0.07% difference whilst the new sizes I have on now with the higher sidewall are 0.46% difference in size.

According to other comments this could not have caused an issue, as this is a minimal difference?

[X5Sport]

It might have been triggered well before you bought the car. Many owners decide OEM tyres are too expensive or don’t follow the 2mm guidance. Sadly you seem to have got the bill though.

Use if non-OEM tyres aren’t always the cause, but they are frequently enough for warnings to be known in the motor trade and amongst owners. It’s a particular issue with staggered size setups as you (and I) have. We have had a couple of instances in the past where diffs have failed even with OEM rubber. It’s just one of those awful facts about anything mechanical. You can treat it right and the damned thing will still up and break in you.

Non-OEM tyres may change diameter too much due to centrifugal forces when the vehicle is on motion. They might look fine on paper, but the carcasses are built to a different spec which may be a trigger. Square setups are much more tolerant of rubber. With them the only concern is the 2mm tread depth difference,

[atomicking]

It might have been triggered well before you bought the car. Many owners decide OEM tyres are too expensive or don’t follow the 2mm guidance. Sadly you seem to have got the bill though.

Use if non-OEM tyres aren’t always the cause, but they are frequently enough for warnings to be known in the motor trade and amongst owners. It’s a particular issue with staggered size setups as you (and I) have. We have had a couple of instances in the past where diffs have failed even with OEM rubber. It’s just one of those awful facts about anything mechanical. You can treat it right and the damned thing will still up and break in you.

Non-OEM tyres may change diameter too much due to centrifugal forces when the vehicle is on motion. They might look fine on paper, but the carcasses are built to a different spec which may be a trigger. Square setups are much more tolerant of rubber. With them the only concern is the 2mm tread depth difference,

Thanks very much. Do you know what the 2mm difference relates to on a percentage level. The calculator gives percentages. Have attached result in screenshot. The guys in other forum don’t think this is enough to warrant the diff issues. Kind regards

[X5Sport]

The 2mm is from BMWs own manuals which states that there should never be a difference in tread depth between any tyres of greater than 2mm. It doesn’t matter whether that’s a new tyre, or a part worn. It’s what BMW themselves state in their Technical Instruction System (TIS) data sheets for ALL xDrive cars. Even my electric i4 M50 (it has dual motors but they aren’t linked), fitted as it is with a staggered wheel set, carries the same warning.

The very first measurement in your table shows a 4mm difference in diameter with an 11mm difference in circumference. It may not be a lot, but it might matter. Experience from owners and the number of broken diffs and damaged transfer boxes we’ve seen over the years says it matters. In almost every case, tyres may have been a contributory factor. OEM approved tyres have matched rolling radii. Tyre pressures and how a tyre behaves when rolling also have to be factored in. It’s not just about the figures you have, it must include dynamic effects upon those figures.

I understand that OEM rubber often has a specific carcass build in how the layers are assembled to control these dynamic effects. They are designed to keep all the fitted tyres within the OEM specification at all speeds.

It’s highly likely that the damage occurred sometime in the past and just really unlucky it failed in your care. It’s one of those things that builds up and then just fails. One of our members has just had a diff fail with no warning in an E53. Was it tyres? No one can say for sure. They haven’t had the car from new do can’t say what was fitted, by whom and when. They were the correct sizes. I had a warning ‘grumble’ with my X6 when I used too high a ‘full-load’ tyre pressure with winter tyres. Dropped the rear pressure by a couple of psi and the grumble vanished.

[Leslie]

The rear diffs are prone to failure at higher mileages and the earlier e70's had smaller diffs than the facelift cars from what I understand. You see a lot of e70's for sale now needing a rear diff in the uk. I looked up your sizes and while they are a little more of a difference than the standard tyre 20's sizes ( 0.46%), the difference is less than the 21's Bmw fitted as an option which was 0.61% so id say they are ok especially as they are new and all the same brand .
The crank pulley failing has nothing to do with the diff failing just unlucky to be co close together.
In an ideal world we would all fit star marked runflat tyres but these are way more expensive and the vast majority of old e70's now don't and are just fine but nothing lasts forever except maybe a really old merc, not the modern rusty ones after 2000
Check the end of the driveshaft too where the circlip goes as the groove can get snapped of when the diff fails and the shaft will pop out and lose drive or leak oil

[Greydog]

Just looking at the Tyre calculator chart posted the Rear tyres cover 3 complete revolutions more than the Fronts each mile covered that wouldn't help the diffs or Transfer box

[atomicking]

The 2mm is from BMWs own manuals which states that there should never be a difference in tread depth between any tyres of greater than 2mm. It doesn’t matter whether that’s a new tyre, or a part worn. It’s what BMW themselves state in their Technical Instruction System (TIS) data sheets for ALL xDrive cars. Even my electric i4 M50 (it has dual motors but they aren’t linked), fitted as it is with a staggered wheel set, carries the same warning.

The very first measurement in your table shows a 4mm difference in diameter with an 11mm difference in circumference. It may not be a lot, but it might matter. Experience from owners and the number of broken diffs and damaged transfer boxes we’ve seen over the years says it matters. In almost every case, tyres may have been a contributory factor. OEM approved tyres have matched rolling radii. Tyre pressures and how a tyre behaves when rolling also have to be factored in. It’s not just about the figures you have, it must include dynamic effects upon those figures.

I understand that OEM rubber often has a specific carcass build in how the layers are assembled to control these dynamic effects. They are designed to keep all the fitted tyres within the OEM specification at all speeds.

It’s highly likely that the damage occurred sometime in the past and just really unlucky it failed in your care. It’s one of those things that builds up and then just fails. One of our members has just had a diff fail with no warning in an E53. Was it tyres? No one can say for sure. They haven’t had the car from new do can’t say what was fitted, by whom and when. They were the correct sizes. I had a warning ‘grumble’ with my X6 when I used too high a ‘full-load’ tyre pressure with winter tyres. Dropped the rear pressure by a couple of psi and the grumble vanished.

Very useful info, thank you! Please see the last post, someone said "Just looking at the Tyre calculator chart posted the Rear tyres cover 3 complete revolutions more than the Fronts each mile covered that wouldn't help the diffs or Transfer box"

[atomicking]

The rear diffs are prone to failure at higher mileages and the earlier e70's had smaller diffs than the facelift cars from what I understand. You see a lot of e70's for sale now needing a rear diff in the uk. I looked up your sizes and while they are a little more of a difference than the standard tyre 20's sizes ( 0.46%), the difference is less than the 21's Bmw fitted as an option which was 0.61% so id say they are ok especially as they are new and all the same brand .
The crank pulley failing has nothing to do with the diff failing just unlucky to be co close together.
In an ideal world we would all fit star marked runflat tyres but these are way more expensive and the vast majority of old e70's now don't and are just fine but nothing lasts forever except maybe a really old merc, not the modern rusty ones after 2000
Check the end of the driveshaft too where the circlip goes as the groove can get snapped of when the diff fails and the shaft will pop out and lose drive or leak oil

Very useful info, the way you explained with the ratio makes sense ie. 0.061%. Please see the last post, someone said

"Just looking at the Tyre calculator chart posted the Rear tyres cover 3 complete revolutions more than the Fronts each mile covered that wouldn't help the diffs or Transfer box"