Rear suspension / drive shaft design

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Rather than go totally OT on Ron's thread, I thought I would start a new discussion....

So I asked about the angle of the driveshafts from the diff to the hubs, my suggestion was that ideally they should be straight and level when the car was in a normal operating condition, and then deflect during bump / droop.

Chris gave this answer

Hi Rick

No, the driveshafts lie at a considerable downwards angle from the diff to the hubs - ie the diff is much higher than the hubs. This is clearly to allow clearance for the discs - you may remember that the Irish rallying P6 actually bottomed out onto the discs recently and destroyed a disc.

This has one major drawback in that the hook joints (UJ's in common English) are running at an angle and the rotational velocity therfore varies cyclicly. This did cause problems during the car's development with vibration. There were a number of changes to compensate for this, one being the substitution of hollow driveshafts for the original solid ones. I've wondered whether it is now possible to substitute constant velocity joints for the hook joints, but I can't think of any other vehicle that transmits lateral loads through cv joints, as would be necessary in the P6 - remember that the cornering loads pass up the driveshafts into the diff. The 2200 had a new design of diff rear mounting using a voided bush, which I suspect was also designed to tackle this issue. This bush didn't spread to the V8 though, so I suspect they are not as tough as the normal solid rubber type.

Chris
Click to expand...
 
Soooo, my next question is, with the drive shafts travelling down from the diff, and of course transmitting loads to the diff as they do in a p6, doesn't that cause a potential problem in cornering, a bit like the old triumph set (but obviously nothing like that, if you get what I mean)

I.e. the shafts already point down, so as you apply a load towards the diff, wouldn't they be inclined to tuck under rather than go over-centre ?

I don't know why I'm discussing this, probably just a bit bored, but anyway..... :LOL:
 
I wonder if it's just down to the roll, i.e. by the time you're loading up the outside wheel, the car has rolled to a point where the shaft is much more in line.
 
Hi, although as you say the driveshafts point downhill the wheel is kept upright
by the elbows and de dion tube at all times. by comparison the Triumph's method
of control is the transverse leaf spring which when it gets to a certain point just
bends all out of shape. Plus I don't think the tall skinny tyres on the Triumph help.

Colin
 
Don't worry I wasn't in any way suggesting that the P6 setup is anything like as bad as the triumph. Just that once you apply a load to shaft like that below the centre line, it will inherently want to go futher under, rather than back up past the centre.
 
The Triumph system was vastly improved when they added a second joint (the Rotoflex coupling) to the driveshafts in the Mk2 Vitesse and GT6. It was only the earlier Vitesse, Herald, etc., that were so bad. On those, there was a UJ up against the diff, then a long driveshaft which caused the wheel to prescribe an arc, ending up at extreme angles to the road. Once the driveshaft was split into three, like the P6,the wheels could stay much more vertical and the handling became safe and predictable.
As well as the narrow tyres as mentioned by Colin, I think the short wheelbase was a big factor.
But even then Heralds were never powerful enough for the infamous tuck-under to be a danger for the average driver. It was only the Mk1 Vitesse that was a genuine liability.
 
Another really bizarre phenomena concerning universal joints is that they rotate with non uniform velocity. Now that seems totally counter intuitive but it has been mathematically proven to be the case.

http://en.wikipedia.org/wiki/Universal_joint

Quoting from the following piece,.."Robert Hooke is commonly thought of as the inventor of ‘Hooke's joint’ or the ‘universal joint’. However, it is shown that this flexible coupling (based on a four-armed cross pivoted between semicircular yokes attached to two shafts) was in fact known long before Hooke's time but was always assumed to give an output exactly matching that of the input shaft. Hooke carefully measured the relative displacements of the two axes, and found that if one were inclined to the other, uniform rotation of the input produced a varying rate of rotation of the output."

http://rsnr.royalsocietypublishing.org/ ... 2/219.full

http://kmoddl.org/machinesandmechanisms ... rsal_Joint

Ron.
 
webmaster said:
It was my understanding that that was one of the reasons why they developed the CV joint.
Click to expand...

And it's also why they always need to be correctly aligned with one another.
 
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