Rear suspension: why? how?

[ColinMarr]

I think we are in danger of arguing that what we have here is an ant-anti-anti-rollbar effect!

Some points: the two torsion bars are independent of each other – they are each clamped separately near the centre line of the car and they don’t interact other than through the rolling action of the car. The transverse leaf spring pivots through silent—bloc bushes at the centre line below the diff.

I fully understand that the cables don’t go slack and the loading on them can be reduced – that is not in dispute.

Forget tower cranes and tugs of war, just go back to the simple example David first quoted “a single wheel bump (hit a brick) will result in the other side of the car being lifted”. I am sure this is wrong! Think about it – wheel A hits a brick, it is deflected upwards, the trailing arm that side pulls upwards on the cable-end, which causes an upward force and deflection of the transverse spring at that end. Because of the pivot, this upward force causes an almost equal downward force on the other end (see-saw), hence the cable tension increases with an increase in the downward force on wheel B, not a lifting one!

Must get back to work……

Colin

[DavidLaver]

Drivers side wheel hits a brick - it goes up into the arch. 

The leaf goes up on the drivers side, down on the passenger side.

As the leaf goes down on the passenger side it changes the load on the wheel such as to lift the passenger side of the car.

The brick sends the drivers side up.  The see-saw action of the leaf sends the passenger side up also.  The result is "pitch" fore-aft instead of "roll".  Pitch will upset the front tyres less than roll.  It might upset passengers but it will please the driver. 

In the case of a bump its an anti-roll device.

In the case of cornering load it works in the oposite way to a conventional anti-roll bar.

It's just occured to me the Dedra used to bounce about at the back more than seemed sensible, but it did track and ride very well on a B-road...


David

[ColinMarr]

David,

I am with you part of the way – but we are not all the way there yet!

Following your example: You say: “As the leaf goes down on the passenger side it changes the load on the wheel such as to LIFT the passenger side of the car.” Sorry, but I don’t think it does.

As the transverse leaf-spring is constrained to go down on the passener’s side, there will be two consequences – there will be additional downward tension on the cables fitted to the trailing arm near the hub and there will be an upward force on the centre of the spring acting on the diff trying to raise the whole car. The result will surely be an increase in downforce on the passenger side wheel.

The attached photo shows the cables as they fit up to the trailing arm. At the lower end the cables go into threaded ends that screw into the bush-end of the spring, unfortunately not shown in this photo. The car is a delectable Aprilia that came from Italy to the Sliding Pillar Rally in 2006. Note those beautiful Italian drive shafts! Sadly most UK Aprilias had Hardy Spicer shafts that weren’t nearly as nice.

Colin

[DavidLaver]

Re-read what I said and stand by it - the leaf goes down, the contact force between that tyre and the road will be higher, and they'll be a reaction to that which lifts that side of the car.

Photo from Ade attached.  I found another in LaLancia but no time to scan it right now.

David

[ColinMarr]

No, don’t think we yet have a meeting of minds. For sure the rear of the car will rise a bit, as I have said, this is because of the upward force transmitted by the spring centre – but that’s not because of “a reaction to that which lifts that side of the car”.

I am going to be off-line for a couple of days. I have to cycle down to the Isle of Wight to sort something out – well actually most of the miles will be by train, but it will feel like 200 miles and will be good for me. Just in case this exchange rumbles on in my absence, let me suggest a way to resolve it.

David, if you are going to venture north of the river to the woodwork show at Alexandra Palace this weekend, please call by and we can sort it out. All we need is a few levers, fulcrums, diagrams and some wine and/or beer (all of which is here) and I am sure all will be harmony. And you might end up wanting to put an Aprilia rear suspension on your Augusta Special!

Colin

[DavidLaver]

No Aprilia rear suspension....this time... 

An Augusta holds the road very well, and is of the era where it is almost as "at home" off road as on and part of the plan is to try some VSCC trials in it.   
 
David

[DavidLaver]

If there was a conventional anti-roll bar pushing up on one end (driving over a brick) would result in the other side being pulled up into the wheelarch.  Lift one wheel you lift the other.   With an Aprilia it is the opposite, because of the pivot you lift one you push the other down.

Another way to think about the load transfer: because the leaf pivots in the middle the load on each end has to be the same, if it isn't the same it moves until it is.   

The drivers side wheel hits a brick.  There is an extra load (thrust) on the drivers side that results in the drivers side of the car being pushed upwards.   That side of the car is "thrown upwards" by the brick.

The leaf will pivot until the load on each end is the same. 

At that point there will be a thrust upwards on both sides of the car.   The passenger side of the car is also thrown upwards.

The car is pushing the drivers side wheel back down on to the road behind the brick. 

The car is pushing the passenger side wheel down on to the road to counter the rolling motion from the single wheel bump.

David

[Scarpia]

I'd stop the car and move the brick if I were you....its buggering up the suspension.

[DavidLaver]

Far from it...

Then again - I've never had to unwind one of those springs or sort out stripped or siezed splines on the torsion bars so maybe I'd best defer.

David

[inthedark]

Listen to Scarpia he knows these things.....................

[DavidLaver]

Anyone got pictures of the adjustable rear dampers?  Were they on all Aprilias?   LaLancia has a picture with the control (or at least the indicator) just under the dash with "Town and Touring" or something written on it.

I've heard that competiton Aurelias had secondary adjustable rear dampers - were these Aprilia units?

David

[Scarpia]

The dash controlled adjustables were on first series only I thought but my second series handbook describes their adjustment so perhaps there was some overlap?

In any case my 2nd series has Houdaille dampers which are actually very good.I had them bench tested off the car a couple of years ago and they still perform well within the MOT requirements here.In fact they don't really wear out as far as I can tell .Unlike a sealed unit these have a filler cap for the oil and work by forcing oil to pass through varying sized holes in the same principle as other types of unit.There is also an adjustment on them to vary the action.Very few places will service them though if the seals fail.(one in Paris I believe and one in the US.)

[DavidLaver]

In small letters at 2-o-clock it says BREVETE which a quick google tells me is a type of French Patent. 

I've just come in from painting Bibendum wheels which also have BREVETE stamped in and up to now I'd assumed to be a place name.  As an aside the rims are stamped MADE IN ITALY and one of the centres has a tiny LANCIA in an oval.  If asked to guess I'd have said they'd bought English centres and French rims but I'd have been wrong...

Anyone know if those dampers are Lancia made under licence or a bought in component?

Anyone got some pictures of the adjustables?

David

[Sliding Pillar]

David, You really need to get yourself an Aprilia parts book!!!!  I will email you the exploded diagram of the rear dampers (won't be until tomorrow) all the parts are available from Vintage suppliers, I have seen similar units on British cars like Lagondas.

[Scarpia]

Houdaille is/was a french company that existed from the beginning of the century (last one ) and only made this sort of damper with a circular drum and lever arm.They still exist under the name of Spiral. You see them on many other vintage cars , even american cars.I have seen them on early fiats and they were fitted to certain ferraris also.No specific lancia connection that I am aware of.