[Technical question] Why would damage occur when being towed?

[autoque]

On the manual's towing section it says

"Do not exceed a towing speed of 45 mph/70 km/h and a towing distance of 90 miles/150 km, otherwise the automatic transmission may be damaged"

Obviously this wouldn't apply when using a conventional flatbed tow truck, but it got me asking why the automatic transmission may be damaged when pulled while in Neutral.
My guess was the transmission cannot be cooled down properly when driven with the ignition off and in neutral for too long.

Anyone know the answer?

[Thunder22]

The driveline isn't lubricated when towed with the wheels on the ground, hence the need for a flatbed.

[JCL]

Fully agree, but to add some detail....very early designs of automatic transmissions had an oil pump driven off the output shaft of the transmission. These days, automatics only have an oil pump driven off the input shaft (engine side). Thus, with the engine not driving the transmission there is no pressurized oil lubricating the transmission, but only splash lubrication.

[Weasel]

I still like when I saw a tow truck *try* to hook a 2000-2003 X5 by the front wheels only and drive off, he didn't make it far and the truck hit the ground! Thanks to his lack ow knowledge and my being too far away to stop him in time I got to change a transfer case and front driveshaft due to stripped splines. (and on the tow truck companies dollar)

[JCL]

Originally Posted by Weasel

I still like when I saw a tow truck *try* to hook a 2000-2003 X5 by the front wheels only and drive off, he didn't make it far and the truck hit the ground! Thanks to his lack of knowledge and my being too far away to stop him in time I got to change a transfer case and front driveshaft due to stripped splines. (and on the tow truck companies dollar)

Being a former tow truck driver, I always check every new vehicle I own as to towing restrictions so that I know in advance if I ever break down. My 2003 was rated for 30 mph/50 kph maximum speed, 95 miles/150 km maximum distance, with one axle raised. No slings permitted (related only to body damage, and they aren't common anymore anyways), so you needed to use a wheel lift, but BMW did expect owners to be towed with only one axle in the air, up until the X-drive models came out in 2004. Even with the 2004 models, using dollies under the non-lifted axle allows safe towing without a flat deck.

Your customer's vehicle may have been in park, or the transfer case or splines may have been already damaged, not sure, but he should have been able to tow it according to the manual. As a tow truck operator, I used to check every owner's manual if the vehicle wasn't in my AAA towing guide book, as it transferred liability to the vehicle owner from me. Saved my employer from writing cheques.

[Weasel]

Originally Posted by JCL

Being a former tow truck driver, I always check every new vehicle I own as to towing restrictions so that I know in advance if I ever break down. My 2003 was rated for 30 mph/50 kph maximum speed, 95 miles/150 km maximum distance, with one axle raised. No slings permitted (related only to body damage, and they aren't common anymore anyways), so you needed to use a wheel lift, but BMW did expect owners to be towed with only one axle in the air, up until the X-drive models came out in 2004. Even with the 2004 models, using dollies under the non-lifted axle allows safe towing without a flat deck.

Your customer's vehicle may have been in park, or the transfer case or splines may have been already damaged, not sure, but he should have been able to tow it according to the manual. As a tow truck operator, I used to check every owner's manual if the vehicle wasn't in my AAA towing guide book, as it transferred liability to the vehicle owner from me. Saved my employer from writing cheques.

Note that I am talking about a non x-drive X5, no clutches in the transfer case to allow give, just planetary gears. With the X5 in neutral there is still a physical connection between the front wheels and the rear wheels.

[Mujahid]

Originally Posted by Weasel

Note that I am talking about a non x-drive X5, no clutches in the transfer case to allow give, just planetary gears. With the X5 in neutral there is still a physical connection between the front wheels and the rear wheels.

Hi guys, this reply is a bit confusing - if the 2003 model can be towed with one axle lifted,(as per owners manual) and x-drive was released after 2004. It means that a 2003 X5 with planetary gears (pre x-drive) cannot be towed with an axle lifted or it will be damaged, as related in this post.
Does that mean that the owners manual is indeed incorrect.

Can some one please maybe answer these questions:
1) How does one tow a non x-drive X5
2) How does one tow and x-drive X5.

thanks, and great forum.

[JCL]

There is little discussion of towing with one axle lifted, as X5 owners tend to prefer flatbeds, because then there is no question of potential damage. You don't actually need a flatbed, however, and flatbeds can cause other damage if not deployed carefully.

These are in the reverse order to your questions, for reasons of clarity.

1) For an x-drive, from 2004 onwards, you need to ensure than axles are turning at the same speed (or both stopped) when the X5 is being towed. One way is to tow flat on the ground, with all wheels turning. Alternatively, if you don't have a flatdeck available, then you need to use wheel lift equipment on one axle, and put a dolly under the other axles (so that all axles are stopped).

2) For pre x-drive, up through 2003, the manual states that you can tow with one axle raised, for a limited distance and speed. Weasel's point is that the two axles are joined by gears. It isn't a fixed connection, but rather a planetary diff. As long as the transmission is in neutral, I can understand that it would work (but it would spin the input shaft). As an example, your left and right axles are joined by gears as well, and they can turn relative to each other. It isn't all locked together like a 4wd truck, with the two driveshafts geared together 1:1, as the front and rear axles need to be able to turn relative to each other to prevent binding in corners. The centre diff is used to create a torque split (32/68 on early X5s, IIRC). Again, when towing with one driveshaft fixed and one turning there is a distance and speed limitation, as you aren't lubricating the transmission. To be completely safe, treat it as an x-drive, above.

Weasel may be able to chime in and clarify the comment, as to the accuracy of the manual and my interpretation.

Edit: The following refers to the New Venture 125 transfer case that my 2003 X5 had. This was a Chrysler/GM joint venture company, now owned by Magna. The operating description is lifted from another board.

The NV125 is a full-time, permanent All Wheel Drive (AWD) transfer case with a fixed 32:68, Front : Rear torque split. TheNV125 transfer case has the following features-:
Permanent AWD.
Single Speed (i.e. No low range ratio).
Constant mesh centre differential with 38:62
torque split.
3/8" Pitch chain with 35 Tooth sprockets.
Splash lubrication via chain rotation.
Weight 23 kg.
The transfer case has no external controls and requires no input from the driver to operate. The unit contains a centre differential that allows permanent connection of the front and rear wheels to the driveline to achieve constant all wheel drive. The major components of the transfer case are-: the front case half; the rear case half; the planetary gearset; the main shaft; the input and output shafts;and the chain.

The front and rear cases are manufactured from cast aluminium. Both cases are bolted together and provide the bearing locations for the main shaft, drive sprocket, planetary gearset and front and rear output shafts. The front case assembly also houses the mounting bush, which is connected to the transmission cross-member when installed in the vehicle.

The transfer case receives torque from the transmission output shaft. The transmission output shaft is connected to the transfer case input shaft via a 38-tooth spline. Torque (62%) to the rear output is transmitted via the input shaft to the carrier (which carries the planet gears) then to the annulus (or ring gear), which is connected to the rear output shaft. Torque (38%) to the front output is transmitted via the input shaft to the carrier, then to the sun gear. The sun gear carries a 35-tooth drive sprocket, which drives the chain. The chain then drives another 35-tooth sprocket integral with the front output shaft. Because both sprockets have the same number of teeth, the rotational speed of the input shaft and front output shaft are the same.
The fixed torque ratio is arrived at by the leverage ratio of the annulus (ring) gear of the differential over the sun gear.



Item Description
1 Rear Output - 62% Torque
2 Sun Gear
3 Chain Drive to Front Output
4 Front Output - 38% Torque
5 Input - 100% Torque
6 Annulus or Ring Gear

[autoque]

Ah~, so it wouldn't be advisable to go down hill in neutral even with the ignition on. I liked doing that 'cause I thought it saves some fuel while doing so. Never did I think it could damage the transmission and transfer case.
I'll use neutral only when going through the car wash tunnel from now on.

[Thunder22]

Originally Posted by autoque

Ah~, so it wouldn't be advisable to go down hill in neutral even with the ignition on. I liked doing that 'cause I thought it saves some fuel while doing so. Never did I think it could damage the transmission and transfer case.
I'll use neutral only when going through the car wash tunnel from now on.

*waits patiently for criticism about using car wash to begin