why must you disconnect the prop shaft before towing?
Think it’s something to do with when the engine is off, there’s no oil being pumped round the gearbox. Leaving the prop connected would probably damage the gearbox in a similar fashion to running your engine with no oil in it. Think that’s the right answer but I could be wrong.[emoji4]
i asked the same question a few years ago and this was the answer i recieved, they disconnect the propshaft (or half shaft from diff) because if they didnt it makes the engine run backwards and also ■■■■■ up the gearbox. how true it is i dont know but it was a mechanic who told me
damoq was given the same answer as me when I was given a suspended tow off the m6 . however as the problem was a failed front wheel bearing and not the motor , we simply left the motor running while I was towed off the road works to Gallagher park 
When mine broke down in Kingston, the recovery guy told me that it would turn into a large box of scrap after about 30 miles if you didn’t disconnect it.
If its a manual then you dont have to bother,if you did imagine the cry of E.E. ,Stobart,flip flop and agency.
My understanding is if the gearbox is in neutral, which is the case if it is being towed, nothing is splashing around inside the transmission to lubricate the bushes and bearings as only the mainshaft is turning. Result is after a few miles the other items in there start to suffer significant permanent damage, from lack of effective lubrication.
alamcculloch:
If its a manual then you dont have to bother,
Why if it’s a manual?
alamcculloch:
if you did imagine the cry of E.E. ,Stobart,flip flop and agency.
What are you talking about? 
alamcculloch:
If its a manual then you dont have to bother
Actually you do if you still want a gearbox by the other end of the tow unless the engine still runs and you’re happy having it sat there ticking over whilst you’re being towed down the road.
Euro:
why must you disconnect the prop shaft before towing?
The Oilpump of gearbox is powered from the Engine. If Gear is in neutral than is no oiling and it runs Hot.
My understanding (which may well be flawed) is that gearboxes are designed to provide drive TO the wheels, if you tow a vehicle without the prop (or halfshaft) disconnected then the wheels are providing drive to the gearbox. This causes torque damage to the box.
the maoster:
My understanding (which may well be flawed) is that gearboxes are designed to provide drive TO the wheels, if you tow a vehicle without the prop (or halfshaft) disconnected then the wheels are providing drive to the gearbox. This causes torque damage to the box.
This ^^^ Maoster is the winner ,the half shaft is the preferred option .
Half shafts everytime
alamcculloch:
If its a manual then you dont have to bother,if you did imagine the cry of E.E. ,Stobart,flip flop and agency.
What was that random bs for.
Colin_scottish:
alamcculloch:
If its a manual then you dont have to bother,if you did imagine the cry of E.E. ,Stobart,flip flop and agency.What was that random bs for.
the first Gearboxes were different designed and it didnt matter. If here is a 60 Year old who started driving with 20 he may remember Lorries towed without attention towards the Gearbox other then shifting it into Neutral.
If the engine would run then it wasn’t neccessary to drop the prop as the layshaft would still be splashing oil around inside the gearbox, with Fodens we just had to ensure that the splitter was in mid or high range though else the gearbox overheated when towing at motorway speeds! The only times that we took the prop off was if the engine was dead, then of course all the brakes had to be wound off as well, but if the engine was only knocking then we usually risked it! 
I was fortunate one day, I dragged one in with a bad knock and left it ticking over while towing, while reversing it into the workshop it decided to chuck a conrod through the block when the TM was standing next to it!! 
Pete.
the maoster:
My understanding (which may well be flawed) is that gearboxes are designed to provide drive TO the wheels, if you tow a vehicle without the prop (or halfshaft) disconnected then the wheels are providing drive to the gearbox. This causes torque damage to the box.
so how do you explain the overrun and using the gears to slow down or indeed the use of the retarder ? all these provide reverse torque on the box
in traditional gearboxes the cogs are spinning in a bath of oil, this splashes oil all over everything inside the box keeping it lubed and cool. The default design is for the gearbox to be properly lubed when the engine is running. But with the engine off and the box being been spun via the prop, then in some/many/all boxes the danger exists that parts of the gearbox will be spinning and may not be getting enough oil to keep them lubed, hence damage. So since it may cost you many thousands for a new box, then if you are towing it for more than a few miles then disconnecting the prop is the wise thing to do.
I guess (but wouldn’t try it on my own truck) selecting a gear suitable for say 35mph, jamming the clutch down and then towing it at 35mph would probably work (could well bugger up the thrust bearing but not sure) … but do take professional advice first boys and girls 
far easier to just disconnect the prop then you know the box will be safe.
Bluey Circles:
the maoster:
My understanding (which may well be flawed) is that gearboxes are designed to provide drive TO the wheels, if you tow a vehicle without the prop (or halfshaft) disconnected then the wheels are providing drive to the gearbox. This causes torque damage to the box.so how do you explain the overrun and using the gears to slow down or indeed the use of the retarder ? all these provide reverse torque on the box
Well I did say that my understanding may be flawed. You can rest assured however that the next time I am on the overrun for 30+ miles at a time I shall report back my findings. 
It’s usually take out in mauala in case the box slips into gear whilst being towed autos it must be removed
alix776:
It’s usually take out in mauala in case the box slips into gear whilst being towed autos it must be removed
Yet it is never done when towing manual transmission cars - which would surely be just as (un)likely to “slip into gear” as a lorry?
The reason is down to lubrication. Manual car gearboxes are lubricated by simply running them in an oil bath, so they won’t come to any harm by towing them. Modern lorry gearboxes (and traditional automatic car transmissions) have pumped lubrication systems, driven off the input shaft. So if the engine’s not running (and/or the clutch is disengaged) the 'box is not being lubricated. Not a problem over short distances at low speeds, but likely to cause damage if done over a longer distance at normal driving speeds.