Kwik-fit and a puncture.
in [Car Maintenance]
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From: Harry Bloomfield on 10 Nov 2009 15:45 Paul explained on 10/11/2009 : > Harry Bloomfield wrote: >> Dave Plowman (News) used his keyboard to write : >>> BTW, congrats to Audi for giving the wheel bolt torque in the driver's >>> handbook. 120 Nm If only they took their own advice - it was nowhere that >>> tight when I took it off. >> >> They can settle in a little, especially so on a new car. The advice is to >> recheck them after a period. >> > Am I being daft - or does it not take less force to UNDO a nut than to > tighten it? If it takes 120 Nm to get as tight as you like, does it take > 120Nm to get it undone again? Not something I've ever tried, but its never > occurred to me to use a torque wrench to undo a nut.... I would suggest that if undone straight away, that it would take a little less. However if left for some time and especially if there is corrosion, it can take more effort to undo them. -- Regards, Harry (M1BYT) (L) http://www.ukradioamateur.co.uk
From: Harry Bloomfield on 10 Nov 2009 15:48 Mrcheerful laid this down on his screen : > Paul wrote: >> Mrcheerful wrote: >>> Paul wrote: >>>> Harry Bloomfield wrote: >>>>> Dave Plowman (News) used his keyboard to write : >>>>>> BTW, congrats to Audi for giving the wheel bolt torque in the >>>>>> driver's handbook. 120 Nm If only they took their own advice - it >>>>>> was nowhere that tight when I took it off. >>>>> They can settle in a little, especially so on a new car. The advice >>>>> is to recheck them after a period. >>>>> >>>> Am I being daft - or does it not take less force to UNDO a nut than >>>> to tighten it? If it takes 120 Nm to get as tight as you like, does >>>> it take 120Nm to get it undone again? Not something I've ever >>>> tried, but its never occurred to me to use a torque wrench to undo >>>> a nut.... >>> >>> you need far more force to undo, mainly because of friction and >>> corrosion, using a torque wrench to undo would not give a meaningful >>> result and might overload and damage the accuracy of the torque >>> wrench. in addition most people do not understand how to use a >>> torque wrench to re-check the tightness of a bolt. >>> >>> >> Corrosion aside, that just seems to be 'anti' common sense.. >> >> Tightening, you are overcoming (increasing) friction and trying to >> make a piece of steel longer than it wants to be - so two components >> to the force in Nm required to get the nut to where you want it. >> >> Loosening, you need to overcome the same friction (decreasing) but do >> not have the stretching element of the material.. >> >> So in principle, if you tighten up new, greased nuts on a thread and >> then immediately take them off using a torque wrench - it should take >> less effort? >> This just seems so obvious, i can see I'm going to have to have a go >> to test the theory. ;-) > > the friction is the problem, when tightening you have the mating surfaces > already sliding and it easier to keep them sliding than it is to re-start > them sliding, especially in the opposite direction, the metal itself acts a > bit like a one-way clutch. > How about this for an analogy: Imagine two pieces of spiky carpet (giant size > version of the faces of the nut and wheel) if you were to rotate and > compress the carpet then the fibres would all get squashed and interlock, to > undo in the opposite direction you have to overcome the bent fibres and get > them going the other way. Or try interleaving the pages of two books. They push in fairly easy, but you try parting them again by pulling. -- Regards, Harry (M1BYT) (L) http://www.ukradioamateur.co.uk
From: Harry Bloomfield on 10 Nov 2009 15:50 After serious thinking Mrcheerful wrote : > the friction is the problem, when tightening you have the mating surfaces > already sliding and it easier to keep them sliding than it is to re-start > them sliding, especially in the opposite direction, the metal itself acts a > bit like a one-way clutch. Yes, but in the tightening process you are fighting against the two surfaces you are pulling together. In undoing the force (no jokes about the film please) is working with you. -- Regards, Harry (M1BYT) (L) http://www.ukradioamateur.co.uk
From: Chris Whelan on 10 Nov 2009 16:21 On Tue, 10 Nov 2009 20:41:33 +0000, Harry Bloomfield wrote: > Chris Whelan expressed precisely : >> On Tue, 10 Nov 2009 18:06:00 +0000, Harry Bloomfield wrote: >> >>> Dave Plowman (News) used his keyboard to write : >>>> BTW, congrats to Audi for giving the wheel bolt torque in the >>>> driver's handbook. 120 Nm If only they took their own advice - it was >>>> nowhere that tight when I took it off. >>> >>> They can settle in a little, especially so on a new car. The advice is >>> to recheck them after a period. >> >> By what process would a wheel bolt, tightened to the correct torque, >> become significantly less tight? > > Did I say significantly? No, *you* didn't, but the OP said "nowhere near that tight". > Check your cars handbook, most do recommend that the torque is rechecked > after a few miles. Not any I've seen in the last twenty years or so. > All/most car manuals also recommended that cylinder > head bolts are retorqued after 500/1000 miles it's all the same process > of settling in. The slightly high spots between the faces settle in and > the tightness declines. Hang on! I'm having a "Life on Mars" moment! Re-torquing head bolts went out with OHV engines, if not before. Chris -- Remove prejudice to reply.
From: Duncan Wood on 10 Nov 2009 16:36
On Tue, 10 Nov 2009 21:21:56 -0000, Chris Whelan <cawhelan(a)prejudicentlworld.com> wrote: > On Tue, 10 Nov 2009 20:41:33 +0000, Harry Bloomfield wrote: > >> Chris Whelan expressed precisely : >>> On Tue, 10 Nov 2009 18:06:00 +0000, Harry Bloomfield wrote: >>> >>>> Dave Plowman (News) used his keyboard to write : >>>>> BTW, congrats to Audi for giving the wheel bolt torque in the >>>>> driver's handbook. 120 Nm If only they took their own advice - it was >>>>> nowhere that tight when I took it off. >>>> >>>> They can settle in a little, especially so on a new car. The advice is >>>> to recheck them after a period. >>> >>> By what process would a wheel bolt, tightened to the correct torque, >>> become significantly less tight? >> >> Did I say significantly? > > No, *you* didn't, but the OP said "nowhere near that tight". > >> Check your cars handbook, most do recommend that the torque is rechecked >> after a few miles. > > Not any I've seen in the last twenty years or so. > >> All/most car manuals also recommended that cylinder >> head bolts are retorqued after 500/1000 miles it's all the same process >> of settling in. The slightly high spots between the faces settle in and >> the tightness declines. > > Hang on! I'm having a "Life on Mars" moment! > > Re-torquing head bolts went out with OHV engines, if not before. > > Chris > Most engines still have overhead valves:-) But not many need re-torquing nowadays. |