Can anybody explain to me the science behind those little tr6 hub bolts pls?

[jogger321]

Ok i'm not an engineer firstly, understand how to use a torque wrench and kind of competant car mechanic/DIY'er but no expert on laws of forces etc

 

Secondly I dont see the Internet littered with stories of TR6's losing their back wheels & hubs

 

I am mystified though

 

We all know that people put wheels on and many tighten these bolts onto the hub as tight as they can because they are worried about the wheel falling off..so much so that if they break down and try to change the wheel its sometimes impossible to loosen the bolt without jumping on the lever socket.

 

.. What i'm really mystified is that I find on the TR6 is the reality is that the whole wheel/hub seems to be retained by the four small small nyloc bolts done up to a tiny torque.. As a non engineer I'm amazed that these appear unless i'm wrong to hold the whole wheel/hub/slider joint is this correct?

 

When people are whizzing around a race track with all the side forces being transferred to the rear how on earth do the wheels not fall off?? :-)

Edited March 9, 2016 by jogger321

[RobH]

A simplistic back-of-the-envelope calculation:

 

Bolts are surprisingly strong. A 3/8 bolt in grade 5 steel has a tensile strength of around 4 tons, and you say there are four of them (?). If they are grade 8 it is even higher so lets say total tensile strength is 16-20tons.

The cornering force cannot much exceed the weight of the car on the wheel even given wide tyres with 100%grip. Lets say a loaded '6 weighs 1.5 ton so under very extreme cornering with only the outside tyres loaded that is 0.75ton per wheel. Road tyres can give a grip factor up to about 1 so that is the absolute max sideways cornering force per wheel. Leverage on the bolts will be multiplied by the relative diameters of the tyre and bolt PCD - say x 3. so the load on the bolted joint could be 2.25 tons. That gives a safety factor of around 8 times or maybe more given that we have taken worst case. Probably adequate.

Edited March 9, 2016 by RobH

[peejay4A]

There are 6 of them so the problem is shared a little. The tightening torque is low because the studs are threaded into the aluminium trailing arm but you probably knew that.

Edited March 9, 2016 by peejay4A

[Phil Pope]

Aren't there 6 of them? Over-engineered :-)

[RogerH]

Hi Jogger,

there are 6 x 5/16 UNF nyloc nuts. I think the answer lies in the nyloc. The 4 x wheel nuts have no locking.

 

Although the studs go into fairly soft ali and have fine(ish) threads the majority of the forces are at 90' to the stud. So the stud is basically in shear.

Obviously when cornering there are some tensile forces. These are seriously smoothed out by the action of the tyre. There are no abrupt/sharp forces.

 

You will notice that the only thing holding the four wheel nuts in place is the highish torque loading and the coned surface.

The coned surface is there for centering but also provides a greater contact area.

As there is no additional locking for these nuts they need to be tight - but NOT everso.

 

Roger

[AlanT]

Few steel parts fail because of tension or shear. This is because it's really tough stuff.

But the enemy is fatigue which leads to cracks.

 

Fatigue comes when cyclic loading, like cornering, is able to bend an item, first in one direction and then in the reverse direction.

You will probably have broken a bit of body panel off by wiggling it back and forth.

 

Fatigue is a cumulative thing that builds up over the years. It's the reason stuff like reactor vessels and aircraft wings have a "service-life".

 

There are several well known "fatigue" points on TR's. More may come to light.

[John Bracher]

Adrian, just to clarify,

are you referring to the studs and nuts that secure the drive shaft bearing hub (6 x 5/16 UNF into the trailing arm) or the 4 x 3/8 UNF HT bolts and nylok nuts that hold the Drive shaft Inner (Universal) joint flange to the differential Output shaft flange?

 

I suspect possibly the latter??

 

Oh, and my general comment on wheel nuts, is that all too often, they are waayyy tighter than is necessary!!

[jogger321]

Thanks for the replies everyone and that has been really enlightening!

 

John I mean the 6 unf nylocs that bolt into the trailing arm... Sorry I said 4! It was so blooming miserable weather wise I couldn't face going into the garage to check the numbers

 

Mine will be coming off again shortly when I do the trailing arm bushes & I am am aware of the low torque settings they have through reading this forum

 

Thanks guys