<VV> Fuel Leak - Now Lug Nuts, etc.

[Hugo Miller]

Ok, I’m going to take an aspirin and have a lie down now. But I just remembered an old car I bought on ebay for a friend. It had been diagnosed with a collapsed wheel bearing by the AA (=AAA). I decided it would last the 20 miles home so drove it for her. I thought it sounded and felt like the lug nuts were loose, but I couldn’t see them as it had a wheel trim covering the nuts. I stopped and tried to rock the wheel but couldn’t feel any movement with the weight of the car on it, and there wasn’t a jack. We got home ok, & I prised the wheel trim off to find that I was right – the ‘collapsed wheel bearing’ was nothing more than loose lug nuts. The reason I am relating this story now is that I was simply amazed that the car had driven twenty, maybe twenty five, miles with loose wheel nuts, without losing the wheel altogether. They were no worse when I got home than when I started (and it had presumably been driven a bit like that before being diagnosed in the first place). The wheel holes were worn, but the studs only sustained slight damage and were fine. I just fitted the spare wheel & that was it – job done.
These were right-hand thread lug nuts on the left front of the car (front wheel drive at that). Why didn’t the wheel fall off?

From: roboman91324 at aol.com 
Sent: Saturday, September 29, 2018 10:38 AM
To: Hugo at aruncoaches.co.uk ; virtualvairs at corvair.org 
Subject: Re: <VV> Fuel Leak - Now Lug Nuts, etc.

Hugo,


When I was imaging this in my mind, what you are thinking puzzled me as well.  I thought more about the pressure distribution and concluded otherwise. If the force wave moves 360 degrees around the lug-wheel interface, your statement would hold.  However, the wave doesn't move in a circular fashion as much as it moves from the rear of the car forward.  Think of it as two 180 degree semi-circles.  The semi-circle furthest from the center works as you describe but the one closest to the center works in the opposite direction.  These tend to cancel out but the slight difference favors your contention.

However, there is another, more significant effect. This has to do with the fretting/mechanical precession we discussed.  The primary load bearing surface of the lug-wheel interface is between the contacts furthest from the center of the wheel.  The lug oriented at 6 o'clock is taking more of the load but all lugs are effected to varying degrees at any point in time.  This tends to "flatten out" the hole in the wheel where the lug nut rests.  This effectively increases its diameter ever so slightly which contributes to mechanical precession.  While the measurable diameters of the mated surfaces remain the same until the nut loosens, the compressive forces within materials mimic precession.  These are the force waves I mentioned but was less than clear.  This is why I stated in a previous post that this isn't true mechanical precession.  Think both of the wheel and each lug-wheel interface as constantly changing ovals.  In a 5 lug system there are 6 imperfect constantly rotating ovals while the vehicle is moving.  A way to visualize this is to look at the rubber tire where it meets the road.  It flattens out where the forces are greatest.  It is an imperfect oval.  Because a steel rim is much stiffer than the rubber tire you can't detect the deformation with the naked eye but it is there in both the overall rim and the lug holes.


Of course, once the lug nut loosens, true precession takes over.


The dynamics of this change with a spigot arrangement.

Doc


In a message dated 9/29/2018 12:46:21 AM Pacific Standard Time, Hugo at aruncoaches.co.uk writes: 

  I knew I was too quick to agree with you! I said this made sense, and it does, but your ‘pressure wave’ that is going round the wheel is going the wrong way! If the wheel is turning anti-clockwise, the ‘pressure wave’ is going clockwise, and this will tend to undo left-hand nuts.
  For some reason this occurred to me while I was lying in bed last night!



  From: roboman91324 at aol.com
  Sent: Friday, September 28, 2018 12:41 PM
  To: Hugo at aruncoaches.co.uk ; virtualvairs at corvair.org
  Subject: Re: <VV> Fuel Leak - Now Lug Nuts, etc.

  Hugo,


  Both the coriolis effect and gyroscopic precession are miniscule and probably irrelevant in the situation we are discussing.  The fretting effect (mechanical precession) is the major issue.

  What I believe is happening is there is cyclic compression and decompression of whatever material the wheel is made of as it turns.  The weight of the car is transferred to the wheel at the hub.  The weight is then transmitted across the radius of the wheel to the tire and eventually to the ground.  As the wheel rolls, the compression forces rotate as well.  On the left side wheel while rolling forward in a counterclockwise direction the force profile will compress material at the lug nut in a counterclockwise direction in waves repeatedly effecting each lug in turn.  This will tend to coax the nut in a counterclockwise direction if not held in place with friction.  Therefore, a left hand thread will resist this effect.  I would expect this issue to be more pronounced with a smaller lug bolt circle.  After working this out in my alleged mind, this may not be mechanical precession in the strictest sense.  Mechanical precession results from two circular surfaces of different diameters rotating together. 


  The introduction of left hand threads "back when" could have resulted from mere observation and experimentation.  Loose lug nuts almost always occurred on the left side but rarely on the right.  The difference was the direction of wheel rotation left side to right.  They experimented with left hand threads on the left wheels and the problem went away. 


  Your question regarding knock-offs with left hand threads on the right side wheels is a different issue.  This appears to be due to true mechanical precession.  If slightly loose, the hub's contact surface is smaller diameter than the knock-off nut/cap.  On the right wheel, this will tend to work the nut/cap in a counterclockwise direction.  in this case, you will want a left hand thread on the right wheels.  I imagine it might be a bad idea to tow a vehicle with knock-offs from its rear with the front wheels on the ground.

  Doc

  In a message dated 9/28/2018 2:04:03 AM Pacific Standard Time, Hugo at aruncoaches.co.uk writes:


    Funny you should say that, as I was about to agree with you that it might indeed be the Coriolis effect at work. In truth I’m not sure what forces are at work that make wheel nuts do what they do, or more specifically why you are better off with a left-hand thread on wheels that rotate anti-clockwise. I’m even more surprised that somebody figured this out so early on in the days of motoring, since the whole concept of lug nuts didn’t exist before the motor car.
    Now tell me why RIGHT-HAND knock-off wheels have a LEFT-HAND thread on the spinners 


    From: roboman91324 at aol.com
    Sent: Friday, September 28, 2018 4:20 AM
    To: Hugo at aruncoaches.co.uk ; virtualvairs at corvair.org
    Subject: Re: <VV> Fuel Leak

    Hugo,

    Please excuse my last post.  Mechanical precession is exactly what is going on with left-side lug nut loosening.  I assumed you were talking about precession as it pertains to the dynamics of rotating mass.  I. E. gyroscopic precession.  I have always known it as epicyclic fretting.

    By the way, mechanical precession is still a concern but the deeper tapered lug nuts added just enough friction to compensate.  In addition, the taper (acorn) itself, not just the added contact area, has been the solution to the issue in two ways.  First, the taper acts as a wedge which increases the applied force "N" in the frictional force equation.  Second, the taper allows for different diameters at different points of contact.  These interesting tricks of Physics were a game changer.

    Again, my apologies,

    Doc

    PS:  I believe the rest of my posts to be accurate but you never know.
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