Front wheel turning while in neutral?

[Breezy_Pete]

Was checking the front brakes on a neighbour's 2006 Fiesta earlier, which were actually fine.  Check-sheet at last service/MOT visit in Feb had highlighted 'front discs below minimum thickness'. 20.6mm vs 21mm claimed minimum.

 

So I do a bit of research and find that fitting details (on ECP site) for every disc brand that fits the car says minimum is 20mm...

 

Sure enough both sides say 20.6 to the nearest tenth on my micrometer, nice and even, no massive lip, no bad scoring etc.    Pads are about 50% worn, all the same.

 

They offered to take £270 off him to fix this non-existent problem.  Glad he asked me about it.

 

Buuut, I had to shuffle the car about in the garage to get at the second side, and left it running after I moved it.  Was slightly surprised when the front wheel started turning all by itself with the car definitely in neutral, as the tyre left the ground.

it was only just going, and stopping and starting, but that shouldn't happen, right?!  I didn't mention it in case it's trivial and not worth worrying about, but what do the Brisky crew think?

I guess the other side probably would've done the same if I'd had the engine running then too, but I didn't, and didn't check.

[NJRJ]

4 hours ago, Wino said:

<SNIP brakes>

Buuut, I had to shuffle the car about in the garage to get at the second side, and left it running after I moved it.  Was slightly surprised when the front wheel started turning all by itself with the car definitely in neutral, as the tyre left the ground. it was only just going, and stopping and starting, but that shouldn't happen, right?!  I didn't mention it in case it's trivial and not worth worrying about, but what do the Brisky crew think? I guess the other side probably would've done the same if I'd had the engine running then too, but I didn't, and didn't check.

On older cars it was Just the clutch plate dragging a bit, as long as it stopped instantly when you touched the tyre it's nothing to worry about. If both wheels had been up in the the air you'd usually find neither would spin as two wheels have enough inertia to resist the tiny amount of drag, however if both wheels span and are a bit reluctant to stop easily then the clutch needs checking/adjusting.

[Breezy_Pete]

Much appreciated, thank you.

[J.R.]

Not clutch drag as the clutch would be completely engaged.

 

Simply viscous oil drag in the gearbox, all of the intermediate forward gears would have been free rotating on the mainshaft, every one of them imparting a little bit of drag, enough to turn the mainshaft, diff crown wheel and the planet wheels if only one wheel turned, its possible both of them would have turned with it up in the air but that depends on the drag of the pads on the discs.

 

Editted, at idle in neutral the first motion shaft turns at engine RPM, the layshaft is driven from it which is in constant mesh with the intermediate gears which free rotate on the mainshaft until one is engaged via the synchro hub locking the drive to the mainshaft at that given gear ratio.

 

Edited 19 June, 2020 by J.R.

[J.R.]

There is also the slight possibility that the gear linkage is not free moving and when in neutral is actually putting a small force on the selector fork & synchroniser hub of one of the gears, this would eventually cause wear of both items leading to synchro failure and/or gear selection problems.

 

Worth a check.

[NJRJ]

J.R. are you saying a car in neutral is really in gear?

[J.R.]

No.

 

I am saying that all the gears are in constant mesh, the solid laygear cluster rotates at crankshaft RPM, the intermediate gears are in constant mesh with their compatriots on the laygear so in neutral they are all rotating around the mainshaft albeit at different RPM, every one of them creating drag.

 

Dependant on the contact of the brake pads with the discs on the driven wheels said drag is enough to turn both wheels if jacked up off the ground, if one brake has more drag than the other then the differential planet gears may rotate and drive to the other wheel.

[Breezy_Pete]

Thanks guys for further info.

Relevant facts might be that the discs on this car have no retaining screws; and the hubs have studs/nuts to secure the wheels.

So, as soon as the wheel comes off, the discs can rattle around retained on the studs. As you take the wheel off you probably give the pads a nudge back via knockage of the free-to-rock disc?

I suppose that coupled with never usually having engine running in this situation caused it to play out the way it did.

Sounds like there's no need to alarm neighbour by mentioning the observation. 

[mrgf]

I think you found the answer, Wino but I was about to say that I have on occasions, had motorcycles running, whilst on the centre stand and the rear (Drive) wheel does have a tendency to rotate, albeit slowly and will stop at the slightest touch/resistance, then rotate again, if left to its own devises.

So I guess its just "Normal" in this situation.