Wheels turning when off the ground (FALC)

gorrie · Post by **gorrie** » Thu Oct 04, 2007 4:28 pm

Was gonna pop this in technical, but as it's about the Fiesta, I thought General Chat was more suitable.

A couple of months ago, I had the Fiesta down to the garage to put a bigger bore flexi section on the exhaust. They had one of those lift things, not a ramp (the ones where the wheels droop). The guys did the job and put the car down... started her up and it was blowing.

He then hoisted her back up again with the engine on to see if he could see which joint it was blowing from. While up, the front wheels started turning... getting quicker, the front left a bit quicker than the right. Now, I could understand the difference in speed being down to the diff.... but I can't work out why they would have been turning at all because the car was out of gear ??!?

I said to the mechanic about it, and he just replied to say "aye, some do that". I'm still confused... no gearbox expert, but surely if a gear wasn't engaged then there should be no drive to the wheels??

I had shrugged it off a month or so ago... but last night, just as I was about to fall to sleep... I got a fecking picture of a drooped spinning wheel pop in my head and that was me awake again. Can someone explain this so I can get to sleep tonight?

Andy

caleebra · Post by **caleebra** » Thu Oct 04, 2007 5:07 pm

I'm not a mechanical genius and dont know much about gearboxes, but I'd imagine it's because with the clutch out the engine is rotating the input shaft on the gearbox, and somehow that rotation is transferred to the ouput shaft, but I'd imagine the wheels are rotating with a force low enough to stop with your boot (obviously it's not enough to overcome the interia of the vehicle at standstill).

Have I just stated the blindingly obvious? I'll get me coat...

[sparky faction]

thinfourth · Post by **thinfourth** » Thu Oct 04, 2007 5:09 pm

Its oil drag in the gearbox which is spinning the output shaft around

Rich H · Post by **Rich H** » Thu Oct 04, 2007 5:13 pm

The gearbox syncro's mean the gears are always engaged. There is enough drag in the bearings on the synchros to make it all spin.

caleebra · Post by **caleebra** » Thu Oct 04, 2007 5:26 pm

thinfourth wrote:Its oil drag in the gearbox which is spinning the output shaft around

I was gonna take a stab at something like that, so is the input shaft always spinning thus spinning the oil round thus spinning the ouput shaft?

Rich H · Post by **Rich H** » Thu Oct 04, 2007 5:53 pm

No not the oil as the shafts are spinning in opposite directions, the gears are constantly meshed and it is the synchros that select the gears. The meshed gears run on bearings around the input (Or output I can't remember) shaft. Any drag in these bearings with cause the shaft to rotate.

Any force applied to the wheels will stop them.

gorrie · Post by **gorrie** » Thu Oct 04, 2007 6:27 pm

aaahh .... this place is great ....

Matelotman · Post by **Matelotman** » Thu Oct 04, 2007 7:24 pm

All my fiestas have done it as Richard said, if you just touch the wheel it will stop (then slowly start again)

Mr Momo · Post by **Mr Momo** » Thu Oct 04, 2007 8:26 pm

As all of the above - remember the gearbox oil is more viscous (about 80 SAE probably) and will do this, but will stop as soon as you put your hand on the tyres