[Vwdiesel] What exactly holds the A2 front wheel hub in place? Just the b...

Wed Sep 2 15:24:22 PDT 2009

Loaded question, but let me try to answer:

First of all, the ideal way to load a fastener is in "double shear".  That means the holes are set up as "guillotines" with one such interface on each side of the loaded member trying to chop the fastener in SHEAR.  Using one such on either side means that there is no bending load on the joint (only a small bending load on the fastener itself) and 2x the actual shear strength of the fastener available to the joint (because there are two such situations).  That is exactly what the lower control arm to balljoint has, x 3.  So, the actual shear strength of the joint is 6x the shear strength of the bolt.  BIG numbers.

However, as you note, the bolts are tightened and squeeze the two pieces of sheet metal in the control arm against the cast flange of the ball joint.  What we REALLY want is for the friction in these areas underneath the head of the bolt/nut and washers to be sufficient to hold the joint through its normal range of loading - so the reciprocating loads on the joint dont pound out the bolt hoses.  

Now, every bolt has a design torque.  To get best friction in this case, we use something with a fairly fine thread.  To get better shear strength, we also use a finer thread (more bolt cross section when shear load is being passed through a threaded area).  To get the best clamping force, we use a bolt of a fairly fine thread (lower pitch - greater clamping force for given torque).   The specs given for shear strength assume that the bolt is already at the correct torque (which actually reduces all strengths of the bolt - tensile and shear - thus why overtorquing is a really bad idea).  

Believe me, VW understands this kind of design detail very well, and the A2 balljoint mounting is an example of really good design.

Pat

----- Original Message -----
From: mark shepherd <mark at shepher.fsnet.co.uk>
Date: Friday, October 2, 2009 4:33 pm
Subject: Re: [Vwdiesel] What exactly holds the A2 front wheel hub in place? Just the b...

> Does this take into account the oodles of lb force applied to the 
> bolt when 
> clamping?
> 
> Mark
> ----- Original Message ----- 
> From: "Patrick Dolan" <pmdolan at sasktel.net>
> To: "Bryan Belman" <dieselwesty at yahoo.com>
> Cc: <vwdiesel at vwfans.com>
> Sent: Tuesday, September 01, 2009 11:14 PM
> Subject: Re: [Vwdiesel] What exactly holds the A2 front wheel hub 
> in place? 
> Just the b...
> 
> 
> 
> Bryan:
> 
> First of all, you DO have the snap rings on either side of the 
> wheel 
> bearing, right?
> 
> The three little bolts on the bottom control arm that hold the 
> ball joint 
> need to handle about 1.25 time the cornering and braking loads fed 
> into that 
> wheel.  If you consider the extreme, you could have 100% of the 
> front end 
> load on the outside wheel at a cornering acceleration of maybe 
> 0.8g, and a 
> loaded car weight of maybe 1500 lbs. on that end, or 1,200 lbs.  
> The little 
> 7mm bolts EACH have more shear strength than that, and I am too 
> lazy to 
> calculate the clamping force to show that by friction alone, they 
> will do 
> the job.  You are now fully appreciating what light weight 
> vehicles are all 
> about:  if the car isn't some kind of bloat pig (i.e., like an 
> A4), you 
> don't need spare locomotive bits to hold the suspension together.
> 
> ----- Original Message -----
> From: Bryan Belman <dieselwesty at yahoo.com>
> Date: Tuesday, September 1, 2009 10:27 am
> Subject: Re: [Vwdiesel] What exactly holds the A2 front wheel hub 
> in place? 
> Just the b...
> 
> > I guess that is enough, I am not knocking the design as it does
> > work. Maybe I am not German enough to see how it all works
> > together or my Irish side is getting in the way here.
> >
> > I am looking forward to my new driving experiance once I put my
> > new/slightly used brake pads in and crank that axle nut on to 190
> > ft. lbs. tomorrow night.
> > thanks all.
> >
> > Bryan Belman, Pt. Pleasant, NJ
> > 04 Jetta Wagon TDI PD, 100hp, 5sp -- running :)
> > 92 Jetta 1.6 Eco-Turbo Diesel -- running :)
> > 82 Diesel Westy 1.9NA -- Not running :(
> > 70 Type 1 stock Beetle -- Not running :(
> >
> >
> >
> >
> > ________________________________
> > From: "LBaird119 at aol.com" <LBaird119 at aol.com>
> > To: vwdiesel at vwfans.com
> > Sent: Tuesday, September 1, 2009 11:39:29 AM
> > Subject: Re: [Vwdiesel] What exactly holds the A2 front wheel hub
> > in place? Just the b...
> >
> > In a message dated 9/1/2009 7:19:01 AM Pacific Daylight Time,
> > dieselwesty at yahoo.com writes:
> >
> > >
> > > So, the only thing I see holding the front wheel to the car is
> > these 3
> > > small bolts that hold the lower ball joint in place. They
> > torque to only 18
> > > ft-lbs to the triangular plate that sits on top of the control 
> arm.> >
> > > That just does not seem like enough for the forces that are put
> > on the
> > > front wheels?????????????
> > >
> >
> > Well, that IS a clamp of sorts. The ball joint piece is
> > sandwiched
> > between the two stamped halves of the lower control arm so
> > friction
> > gets kinda "multiplied" there.
> > If you don't like 3 bolts holding stuff, there's only three that
> > hold
> > the control arm on, there's only ONE that holds the steering
> > knuckle to the ball joint. :-)
> > The strut's fastened pretty firmly to the steering knuckle as I
> > recall.Your pivots are the inner control arm bushings, ball joint
> > and the
> > top of the strut.
> > Don't forget the tie rod end is there too! :-D
> > Loren
> > _______________________________________________
> > Vwdiesel mailing list
> > Vwdiesel at vwfans.com
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> 
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