Dangerous to life split pins

[Bainford]

I appreciate this discussion, and thanks Gavin for the explanation. The subject of axel loads in tension (and the fact that in normal driving, this is never the case) has long puzzled me. My understanding has been the same as Mr Sparkrite's, though I have always known this thinking is flawed as a 5 mm (or whatever size) steel pin could never reliably retain those loads. I am somewhat embarrassed to admit that I am still having trouble getting my head wrapped around it, because this isn't rocket science. In fact, it should be very simple to grasp.

This weekend I shall do just as Gavin suggested; have a sit behind the car, with his explanation in hand, and stay there until I can 'see it'.

[EuropaTC]

Rather than sitting in a cold garage floor, how about this....

I think of it as in the image below.  This is the S2 layout whereas the TC has the damper/lower link on the same bolt, but it's close enough.

The weight of the car is the blue line through the spring/damper.  The reaction from the ground through the tyre is yellow arrow A, which is outboard from the lower mount of the damper which forms a pivot point. If there were no top link/driveshaft then the wheel would pivot and the top go inwards as shown on yellow arrow B. This means the driveshaft is always going to be in compression as long as there's a ground reaction at A. The more the force on the wheel, the more compression on the driveshaft and in theory the lower link should go in tension.

Even when you're cornering fast and the tyres sliding, unless you lift the opposite wheel clear of the ground, there's always going to be some compression in the driveshaft. If you do manage to corner fast enough to lift the opposite wheel then yes, the driveshaft could go into tension but I think the load would only be at most the weight of the wheel trying to go into positive camber

[GavinT]

Yep.
The fundamental forces in play in my super simple pic are gravity (red arrow) and the reaction from the spring (green arrow) – Isaac Newton's third law.
The core principal of the upright's function is that the spring acts on a lower pivot point and, thus, aways maintains a leverage advantage.

My slide-rule is in for service but someone smarter than me could calculate the leverage effect.
As a side note, the spring is laid over somewhat, so the actual spring force at the lower pivot will be something less.

So, if Colin walked into his engineers office with this upright, how would they stop it from rotating clockwise?
It's getting on for summer here, but to avoid a cold backside in the Northern hemisphere, I'd sacrifice a cornflakes packet to some cardboard aided design.

[Sparkrite]

I have now seen for myself. With the car jacked up and the suspension at full extension and the slot pins removed there is in fact tension on the axle as it separates from the gearbox by a quarter inch or so. But the minute you raise the wheel about half inch, the axle butts up nicely against the gearbox.
So unless you go over a hump at speed and your car gets sufficient "air" then the axle will be in compression.
I do still wonder though if any of the driving forces might be sufficient to overcome the set up and momentarily apply tension force to the axle and thus stress the pins.

[EuropaTC]

I do still wonder though if any of the driving forces might be sufficient to overcome the set up and momentarily apply tension force to the axle and thus stress the pins.

Unless you lift the wheel I don't think you will move the driveshaft into tension and even then, the load won't be a great deal.  But you've had an obvious failure on a pin and apart from not being fully shimmed in place I am struggling to work out a mechanism.

I'm not convinced it's a poor quality part, these things are in shear and I would guess that plain mild steel bar would do the job because the drive is supposed to be transmitted by splines, not the pin.

So I go back to shimming. Mine is so tight that I have to really drive the pins in and getting them out is equally tough. If yours drove out relatively easily then I'd add a bit more shimming.

The other, very blue sky thinking idea, was if it was possible to go from a very high compressive load to much lower stress and the cycling effect was enough to initiate a fatigue failure. That's very close to your proposal but I'm struggling with that one because you've always got some compressive stress. 

But if you are driving fast enough to lift wheels on our potholed roads, you deserve a medal alongside a free set of super high tensile steel pins !

[GavinT]

But the minute you raise the wheel about half inch, the axle butts up nicely against the gearbox.
So unless you go over a hump at speed and your car gets sufficient "air" then the axle will be in compression.

Or, to look at it another way, if the dampers were ~ half an inch shorter, there would be no situation where the yokes would escape the gearbox output shafts.
Dunno the answer, but did the stock dampers permit this at full droop?

I do still wonder though if any of the driving forces might be sufficient to overcome the set up and momentarily apply tension force to the axle and thus stress the pins.

These are 50 year old cars.
I wouldn't be surprised to find a few thou of rotational play in the yokes - that'd do it, methinks.

[Bainford]

Thanks much Brian and Gavin. Things are very much clearer. I see that my earlier thinking was not so much flawed, as it was the imagining of incorrect values to some of the force loads, and not giving enough credit to the lever action of the shape & orientation of the upright. This is an issue that has been gnawing at me for several years now. I'm going to sleep much better tonight. Cheers.