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Author Topic: Rear suspension: why? how?  (Read 12352 times)
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DavidLaver
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« on: 02 February, 2008, 11:38:31 AM »

Folowing on from a few bits on the timing chain thread:

Rear suspension - who's got a good scan of the parts book? 

Then we can try and work out if it was a "errrrmmm, we got no space for a dynamo boss, what djarspect meado?" or if it was "the very best way to suspend a car is..."

David
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ColinMarr
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« Reply #1 on: 02 February, 2008, 07:07:01 PM »

Sorry - no scans, no parts books, only the images in my brain.

What a pity we don’t have access to an on-line archive (any committee members bothering to read this stuff?) – if we did I could guide you to an article by Paul Frere, Belgian Grand Prix driver of note from the late 1950s and highly revered motoring writer. Frere’s article, which was published in an LMC Journal many years ago, was a technical appraisal of the Aprilia rear end that explained just how brilliant and advanced it was. I don’t think he used these words, but I think he made the point about how the ‘soft and slow’ transverse spring worked to allow the ‘hard and quick’ torsion bars do their stuff – and with in-board rear brakes integral with the diff, the un-sprung weight was kept to the minimum. Maybe someone can find the article?

Of course, one of the design objectives here, as with sliding pillar front suspension, was to ensure that the plane of the wheels stayed at 90 degrees to the road at all times. A fine idea, but drivers being drivers these things get tested to the limit – judge for yourselves in this photograph from 1978. It’s Dave Scheldt driving Barry Waterhouse’s just acquired and un-modified car.

Colin


* aprilia(small).jpg (64.64 KB, 567x471 - viewed 185 times.)
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Scarpia
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« Reply #2 on: 02 February, 2008, 07:50:17 PM »

this is what is so good about the forum. I've seen more interesting photos about Aprilias in the last 10 days on here than I will se in VL in the next 10 years. (I don't mean to criticize VL, its just the reality of timing and the internet of course)

Colin, your collection of heart warming and thought provoking/memory envoking photo's seems inexhaustable...If everybody was to contributed their similar items we would have a book well worth publishing and great fun to browse (TB legacy....?)
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Sliding Pillar
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« Reply #3 on: 03 February, 2008, 03:21:28 PM »

David, I will email you a scan from the parts book of the rear suspension, just remember NEVER EVER try to remove the rear spring without the proper tool as you are very likely to do yourself a serious injury......if not worse!!!
I look after the Club rear suspension tool, which you can borrow, subject to collecting & returning etc.
Ade
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« Reply #4 on: 03 February, 2008, 04:29:42 PM »

Ade's right about the almost legendary challenge of releasing the spring and the associated danger if done incorrectly.Having benefited from the kind loan of this tool I would simply add the note of caution that you may also do yourself a serious injury lifting the thing..... Wink

Only kidding, it works well and the robust  design is comforting considering the forces at play.If you don't have a bridge to raise the car on its quite exciting as the tool is about 1/2 a yard long in the vertical sense which meant for me elevating the back end of the car on a wobbling collection of wooden blocks and jacks to achieve the height necessary to release the spring.  Working under the car was a little concerning with ever creak leading to a rapid exit...
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ncundy
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« Reply #5 on: 03 February, 2008, 05:26:07 PM »

One of the side effects of using torsion bars on IRS is that the roll siffness is very high. Usually you would counter this by adding a front anti-roll bar, but you can't do this on sliding pillar suspension. Therefore the only option is to reduce the rear anti-roll stiffness, but doing this by reducing the stiffness of the torsion bars would reduce the bounce stiffness. One effect of the traverse spring is to conteract the natural stiffness of the torsion bar in roll and lower the roll stiffness, as it actually pushes the unloaded wheel down onto the road against the torsion bar. A sort of anti - "anti-roll bar" if you will !
« Last Edit: 03 February, 2008, 06:46:06 PM by ncundy » Logged

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DavidLaver
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« Reply #6 on: 03 February, 2008, 06:49:31 PM »

I had to wrap a cold towel round my head to contemplate anti-anti-roll bars   Undecided   That is starting to sound like a "three spring" setup like some single seaters have - the contemplation of which is a sure substitute for counting sheep. 

I've attached a favorite Aprilia picture - and some "internet size" versions of the rear suspension scans from Mr Rudler mentioned above.   These are a good start but anyone got a good picture of the entire rear end?   

By the look of TAV29a the leaf is free to pivot in the middle which makes it the "two wheel bounce" spring (eg over a hump back bridge) and also an interconnector such that a single wheel bump (hit a brick) will result in the other side of the car being lifted - much like a 2CV or Morris 1300 interconnection front and rear to kill pitch this will kill roll when one wheel hits a bump - the back will bounce up and down more (pitch more) but roll about less over a difficult surface - as such it will disturb the front wheels less which is great for control.

Cornering:  as the car rolls onto the outside wheel the pivot in the middle means some of the load is going to push the inside wheel down - which is good for traction.  A modern car might have an antiroll bar which is going to lift the inside rear which then has even less traction which is ok if the axle has a limited slip diff - and ok if the outside tyre is fine on its own - and ok if you don't worry about having "all your eggs in one basket" with regards the grip from the road being there.   On prewar tyres on prewar roads without an LSD I expect you'd rather the two tyres shared the load as evenly as possible...

Still haven't quite sussed how the torsion bars interelate but I'm sold - its genious not "we need a bit more spring for when fat gran needs a lift".

David


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* Aprilia_rear_suspension_1_1_s.jpg (60.8 KB, 727x999 - viewed 197 times.)

* Aprilia_rear_suspension_2_1_s.jpg (58.06 KB, 729x980 - viewed 201 times.)
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David Laver, Lewisham.
fay66
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« Reply #7 on: 04 February, 2008, 12:37:09 AM »

Crikey! that looks a bit complicated, but what a wonderful example of Lancia over engineering.

Brian
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DavidLaver
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« Reply #8 on: 04 February, 2008, 09:18:31 AM »


To me a defining feature of all Lancias is that they are great in a straight line.  By that I mean that on a difficult surface you can go faster than lesser marques.  They also tend to have excellent traction, they get the power down well. 

It may require a bit more weight and so blunt acceleration.  It may require a compromise with outright steady state cornering power on a smooth surface.  However it will reward day in day out and show itself road racing or rallying.

David 
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David Laver, Lewisham.
ColinMarr
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« Reply #9 on: 04 February, 2008, 10:01:00 AM »


I think Neil’s explanation is correct. And it fits with the fact that an Aprilia can both lean out when cornering and yet go round as if it is on rails!

But I think David has missed something. The flexible cables that connect the spring ends to the trailing arms can only pull, and not push. So that one wheel hitting a bump will not be reacted to as a push up on the other side – it will be a pull down and hence add to the stability.

Colin
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DavidLaver
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« Reply #10 on: 04 February, 2008, 10:19:04 AM »


I didn't miss anything in my head for all the written words lack clarity. 

Think of a tug of war competition with two teams of big guys evenly matched.  They are heaving and sweating but nobody is going anywhere.   There's a rope in the middle with a pre-load.   Sneak round the back of one team and push the guy on the end.  You will be pushing both teams as effectively as if they were all holding onto a steel pole.  You will be able to move "the system" of people and rope by pushing one side or pulling from the other.

The pivoting leaf, with its preload, CAN transfer a load that is pushing the outside wheel upwards into a "push" on the inside wheel downwards.   It may not physically "push" but it will be pulling a whole lot less and that comes to the same thing.   

They'll be a limit defined by the preload and they'll be dynamic forces due to friction in the pivot and momentum of the spring will be a factor but using a cable in tension there's going to be no slop or wear or additional "vibration" as with rubber bushes.  A cable in tension is a very pure way, light and direct and self compensating for wear, to apply and vary a force.

David
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David Laver, Lewisham.
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« Reply #11 on: 04 February, 2008, 11:36:27 AM »

I suspect we are exchanging the mechanical equivelent of mixed metaphors with some of these discussions.
I have yet to digest the proposal that the rear torsion bars will push the inner wheel down due to the cornering forces on the outer wheel.I'm not convinced at the moment this is true but I need to think it through in practical terms.I believed the two shafts worked independently of one another when I dismantled them.As for the steel rope discussion...hmmm.
I shall retire to a darkened room to contemplate and emerge one this has all fallen into place.
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fay66
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« Reply #12 on: 04 February, 2008, 11:56:56 AM »

Grear Discussion, keep up the good work, I'm learning more with every posting. Grin

Brian
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DavidLaver
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« Reply #13 on: 04 February, 2008, 12:23:13 PM »


Imagine the cables are rigid link.  Imagine the leaf is a rigid "see saw" bar.  Just for now...

1 - Roll onto the outside wheel and it goes up inside the wheel arch. 

2 - The "cable link" (pretend it is rigid) gets pushed up.

3 - The "see-saw bar" goes up at one end and down at the other end.

4 - The oposing "cable link" goes down

5 - The inside wheel goes down.

Struggling with the cables being able to push?   Imagine a tower crane with a big load hanging on the end of the hook.   Push up on the counterweight end behind the driver and the top beam (the bit than the latest Bond has a punch up along) "see saws" and the load goes down.   The cable hasn't pushed the load down, but it is the same result as if the cable had been a rigid bar.

Think of old fashioned ballance pans - the sort you put the goods in one side and the weights on the other side.  It works the same with rigid links from beam to pans or with strings.  Scraps of gold in one pan, brass weights in the other pan, push up under the side with the weights and the pan with the gold goes down.  Those threads don't "push" the pan down, but they "hold it up less".

More pull, less pull is the same result as push a little, pull a little.  The cable is pulling that inside wheel up, as the outside wheel goes up into the arch and the leaf see-saws the inside cable is pulling the inside wheel up rather less.

For sure there is a load transfer to the outside wheel in cornering - but this system makes the transfer less, it keeps the inside wheel on the ground longer, it shares the cornering load between the rear tyres better. I can imagine situations where it will provide greater accelerative traction than with a limited slip diff.  I can imagine it providing better stability braking (or engine braking) in a corner than with an anti-roll system.

Its clever - just thinking about the leaf its clever and I'm sure the torsion bars and thinking about the range of loads (passengers and luggage) I'll come to think of it as even more clever (and worthwhile) again.

David
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David Laver, Lewisham.
DavidLaver
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« Reply #14 on: 04 February, 2008, 12:24:25 PM »


Of course if the leaf DOESN'T pivot I'm wrong - and I've yet to see a picture of the whole system - so maybe the whole of the above is invalid   Roll Eyes

David
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David Laver, Lewisham.
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