DIY Si
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| posted on 28/3/06 at 07:21 PM |
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Quote
"Might have just confused myself..."
What i was trying to say was the way it has now been put. Syd says the bushes etc limit things. Others say the system itself is the limiting factor.
Seems a bit strange, as any and all suspension systems have their limiting factors. This just happens to be the limit of this type. If the rod ends
have a big enough rotational movement the limit is the shockers topping/bottoming out surely? Personnaly I agree with Syd, but I think this'll
be a let's agree to disagree jobbie.
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Syd Bridge
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| posted on 28/3/06 at 07:42 PM |
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quote:
Originally posted by britishtrident
In theory both rose joints and Polly bushes are wrong but in the real world you can get away with either.
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Mr. Trident,
Well then, come on. Tell us what the 'ideal' joint is for this application.
A Universal Joint?Or maybe one of those flash jobbies made from unobtainium!
Cheers,
Syd 
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JoelP
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| posted on 28/3/06 at 07:44 PM |
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i think that with equal length parallel trailing arms, and rose joints, you could twist it til the roses themselves run out of play. It would be
similar to a wire cube being collapsed, as the 4 verticals lean over, the top will remain perfectly flat. Unequal or unparallel arms will resist in
most cases.
To explain my poor example, take two pieces of card each 20cms square, tie a piece of string to each corner to turn it into an odd cube. Pull the
pieces of card apart til the string is taut, then as you twist them relative to each other they will be pulled together, but both remain flat. The
fact that they stay flat shows that no forces are acting on the axle or chassis.
So im with syd on this one 
However, add a panhard rod an my example falls flat on its face
[Edited on 28/3/06 by JoelP]
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Marcus
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| posted on 28/3/06 at 07:50 PM |
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I used metalastic bushes on mine and it works
As for the above argument, I'm firmly on the fence!
I'm going to model this up tomorrow and see!
Marcus
Marcus
Because kits are for girls!!
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Peteff
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| posted on 28/3/06 at 08:04 PM |
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I have poly bushes at the chassis end of my trailing arms panhard rod and joints at the axle end and it bounces up and down quite nicely (live axle
though but de dion will work same won't it?) I think as Jon says the shock absorbers will stop the sphericals getting to their limits. I have
been told that my panhard joint is in the wrong orientation as well but as long as it works I don't care. In theory bumble bees can't fly
as they are aerodynamically wrong, but it never seems to stop them as nobody has told them yet.
yours, Pete
I went into the RSPCA office the other day. It was so small you could hardly swing a cat in there.
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britishtrident
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| posted on 28/3/06 at 08:25 PM |
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quote:
Originally posted by Syd Bridge
quote:
Originally posted by britishtrident
In theory both rose joints and Polly bushes are wrong but in the real world you can get away with either.
.
Mr. Trident,
Well then, come on. Tell us what the 'ideal' joint is for this application.
A Universal Joint?Or maybe one of those flash jobbies made from unobtainium!
Cheers,
Syd
All to do with location geometry -- Nothing what so ever to do with the type of joint -- in theory no joint actually will work in a 4 link plus
panhard setup without some compliance some where. But in the real world something will give or bend just a enough to let it work.
Only 4 trailing link system that would locate and move perefectly is is 4 links plus watts linkage with the pivot point of the linkage bell crank
slap bang in the centre of the rear cover of the diff.
Me being human I just stick to two of those miracles of engineering called silentbloc bushes.
[Edited on 28/3/06 by britishtrident]
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DIY Si
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| posted on 28/3/06 at 08:33 PM |
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Since you say the watt's linked 4 link system is the only perfect one, and I'm trying to build one in the near future, what would be best
for the joints? Iknew something I'm planning would be good in the end! Huurah
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britishtrident
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| posted on 28/3/06 at 08:41 PM |
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quote:
Originally posted by DIY Si
Since you say the watt's linked 4 link system is the only perfect one, and I'm trying to build one in the near future, what would be best
for the joints? Iknew something I'm planning would be good in the end! Huurah
Not just a transverse Watts linkage but one with the centre point in the centre of the diif backplate --- not ideal as it gives a higher roll
centre than desireable.
The Locost book set up actually works well enough in practice.
Before we get another sermon from on high I will point out we all already know a roll centre is just an imaginary construct to aid the designer.
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Mix
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| posted on 28/3/06 at 08:47 PM |
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Firstly a big thank-you to Cheffy for sparking this one off.........I'm finding it particularly thought provoking.
The more I think about this the more I think I may be missing something 
I just can't imagine how a fully rose jointed system will 'lock up', (assuming the joints don't run out of radial
movement).
I'm off to model this in CAD
Mick
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DIY Si
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| posted on 28/3/06 at 08:59 PM |
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So, the centre point would be inline with the diff centre? Or does it have to be speciafically the back plate for some reason? Just so I get the right
pic in my head, the ideal system would have trailing links an equal distance above and below the diff with the shafts horizontal? or does the
horizontal bit not matter?
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Mix
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| posted on 28/3/06 at 10:18 PM |
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Got it...........I think ........apart from why the Watts linkage is different
When the axle moves in bump and droop at the same time the trailing links impart a torsional load in the rear axle and corresponding loads in the
chassis. My CAD doodlings suggest that the twist with 1.5" of roll and droop would be in the order of 0.2 degrees or 0.2mm.
Or have I missed the point.........again
Mick
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MikeRJ
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| posted on 28/3/06 at 11:02 PM |
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quote:
Originally posted by Mix
Got it...........I think........apart from why the Watts linkage is different
When the axle moves in bump and droop at the same time the trailing links impart a torsional load in the rear axle and corresponding loads in the
chassis. My CAD doodlings suggest that the twist with 1.5" of roll and droop would be in the order of 0.2 degrees or 0.2mm.
Or have I missed the point.........again
Mick
Your description is spot on!
I'm not at all convinced that a Watts linkage eliminates the bind though? Have to think about that one. Have you tried to model it?
[Edited on 28/3/06 by MikeRJ]
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NS Dev
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| posted on 28/3/06 at 11:03 PM |
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quote:
Originally posted by Mix
Got it...........I think........apart from why the Watts linkage is different
When the axle moves in bump and droop at the same time the trailing links impart a torsional load in the rear axle and corresponding loads in the
chassis. My CAD doodlings suggest that the twist with 1.5" of roll and droop would be in the order of 0.2 degrees or 0.2mm.
Or have I missed the point.........again
Mick
PERFECT!!!!!!!!
YAY!
This is exactly what Matt (Procomp) is saying, and I was agreeing with after a head scratch. I'm just a bit agricultural about it and less
precise!
Likewise I am not sure on the watts linkage bit, as far as my brain-crometer can tell without making a model or drawing it up, it will bind up either
way, due to the torsion set up in the axle as you say.
If you've drawn it Mix, can you have a fiddle with making the links divergent towards the axle end, out of interest?
PS the torsion does not sound much, but with rose joints you need bugger-all to make them bind up, 0.2mm would be plenty enough.
Retro RWD is the way forward...........automotive fabrication, car restoration, sheetmetal work, engine conversion
retro car restoration and tuning
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NS Dev
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| posted on 28/3/06 at 11:11 PM |
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quote:
Originally posted by JoelP
i think that with equal length parallel trailing arms, and rose joints, you could twist it til the roses themselves run out of play. It would be
similar to a wire cube being collapsed, as the 4 verticals lean over, the top will remain perfectly flat. Unequal or unparallel arms will resist in
most cases.
To explain my poor example, take two pieces of card each 20cms square, tie a piece of string to each corner to turn it into an odd cube. Pull the
pieces of card apart til the string is taut, then as you twist them relative to each other they will be pulled together, but both remain flat. The
fact that they stay flat shows that no forces are acting on the axle or chassis.
So im with syd on this one
However, add a panhard rod an my example falls flat on its face
[Edited on 28/3/06 by JoelP]
wrong Joel I am afraid. Your string is too compliant and your card not stiff enough in torsion! If it were on both counts then one of the strings will
go slack, and not because of the panhard rod.
Retro RWD is the way forward...........automotive fabrication, car restoration, sheetmetal work, engine conversion
retro car restoration and tuning
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Cheffy
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| posted on 29/3/06 at 12:46 AM |
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Being following this all day. Just checked it having got back from the pub! Sorry for opening up a whole can of worms, but glad it wasn't the
daft question I thought it was going to be! Thanks for all the input guys!
I think from all this I've deduced the following - if I'm going to use it on the track go for rose joints, for road use go with the poly
bushes. Would that be a fair assessment?
PS - Thanks Mix - I aim to please!
[Edited on 29/3/06 by Cheffy]
Farts are like Rock'n'Roll. You love your own but you hate everybody else's. Lemmy, Motorhead.
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JoelP
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| posted on 29/3/06 at 07:40 AM |
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nat, im just pondering one thing now! If the system i described is symetrical in all dimensions, and the twisting is round the axis through the centre
of the cards, how could the strings decide which would go slack, seeing as they are all in an identical situation?
possibly making it square has taken it away from the car simulation, as the cars 'cards' would be rectangular.
[Edited on 29/3/06 by JoelP]
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NS Dev
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| posted on 29/3/06 at 07:46 AM |
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ahh, now if it were symmetrical in all directions then I think you are ok, should move fine, but as you say, on the car its far from that layout!
Retro RWD is the way forward...........automotive fabrication, car restoration, sheetmetal work, engine conversion
retro car restoration and tuning
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Syd Bridge
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| posted on 29/3/06 at 08:17 AM |
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This really is unf**kingbelievable!!
Grade 8 geometry and trig, and all of you professors seem incapable of simple reckoning!
All of what has been said on this previously put aside.....Someone answer me this...First from you Matt, as you want to be seen as the all knowing
expert on this subject..
How do the many hundreds and probably thousands of solid axled racecars that race every weekend around the world, using just the very setup that you
say doesn't work, manage to get themselves around a circuit for many laps, without the major failures you say must happen??
And how have those same racecars done so for many many years???
Four link with panhard or Watts is the norm in Nascar and Aussie Supercars, AND THEY ALL USE RODENDS!!!!!
If this is the mentality at Procomp, then my high opinion of the company has just dropped to unknown lows.
Cheers,
Syd.
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NS Dev
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| posted on 29/3/06 at 08:26 AM |
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quote:
Originally posted by Mix
Got it...........I think........apart from why the Watts linkage is different
When the axle moves in bump and droop at the same time the trailing links impart a torsional load in the rear axle and corresponding loads in the
chassis. My CAD doodlings suggest that the twist with 1.5" of roll and droop would be in the order of 0.2 degrees or 0.2mm.
Or have I missed the point.........again
Mick
has Mix drawn it wrong then Syd?
If that's what you are saying then I am going to have to resort to autocad tonight!
Retro RWD is the way forward...........automotive fabrication, car restoration, sheetmetal work, engine conversion
retro car restoration and tuning
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Syd Bridge
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| posted on 29/3/06 at 08:45 AM |
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[quote
has Mix drawn it wrong then Syd?
In a word, yes(well, maybe). The drawing is maybe correct, the interpretation is a little out.
You'd have to draw it in 3d, very carefully. Then animate it properly.
The bit that everyone is missing, is that the separation at each end of the arms does not change, nor want to. That is, the axle end sees the same
changes(or not) as the chassis end.
Look at what's happening as though the axle stays still, and the car is moving. Then reverse it. Look at the changes as though the car is fixed,
and the axle moves.
Each end sees the same effect, hence no torsion into the axle casing.
As I keep saying, thousands of these setups race every weekend, with rodends and have done so for far too many years, and if there were problems they
would certainly be well publicised by now!!!
All of a sudden, one 'expert' says it doesn't work, and you lot all think he's reinvented the wheel!
The Watts link argument is unfounded. In single wheel bump, or bump/droop couple, you still get the axle twisting about an axis somewhere, relative to
the chassis, or vice versa.
ALL COMMON SENSE WOULD TELL YOU THAT MATT'S STATEMENT AND REASONS ARE GROSSLY FLAWED, JUST WORK OUT WHY.
Cheers,
Syd.
[Edited on 29/3/06 by Syd Bridge]
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JonBowden
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| posted on 29/3/06 at 10:05 AM |
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Syd,
I have just discarded a long post explaing that four trailing links will bind very slightly. I was wrong. Four equal length trailing arms will not
bind!
Jon
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Syd Bridge
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| posted on 29/3/06 at 11:24 AM |
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Thank you for posting that, Jon. I'm humbled that you took the time. It's just a shame that the rest have neither the ability to reason
this out, nor the strength of character to write as you have.
Thanks Again,
Cheers,
Syd.
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NS Dev
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| posted on 29/3/06 at 11:26 AM |
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God I am so geometrically confused now!!!
Still running this one through my head and STILL can't suss it!
I never was very good at maths!
Retro RWD is the way forward...........automotive fabrication, car restoration, sheetmetal work, engine conversion
retro car restoration and tuning
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NS Dev
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| posted on 29/3/06 at 12:24 PM |
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aaarrrrrrrrggggggghhhhhhhh!!!
can't stop myself getting more and more muddled now!
Now convinced myself that it won't bind up, but I'm still totally confused!!!
If the axle mounts move in a common circle in roll then everything stays right, but they won't surely, as (and this is where I am struggling
again!) surely the roll centre with panhard rod migrates in roll????
but then the roll centre is not real and the links are...?????
aaarrgghhh
I give up, need to make a model now just to satisfy my own curiosity/confusion!!
Retro RWD is the way forward...........automotive fabrication, car restoration, sheetmetal work, engine conversion
retro car restoration and tuning
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NS Dev
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| posted on 29/3/06 at 12:28 PM |
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also:
"Roll-steer
When a car rolls into a corner, it is very important that no steering component is added to the suspension geometry. Short trailing arms used to
locate live axles are the worst offenders (300 mm trailing arms used on the Locost will induce 5 mm of roll steer in 50 mm travel). A car will never
feel 'in-the-groove' or 'take-set' in a corner if the suspension is deficient in this area. The Striker and Phoenix use
longitudinal Watts-Linkages to locate the rear axle. This arrangement ensures that the rear wheels never steer. There is a downside with this
arrangement. When the car rolls, the 2 watts linkages try to twist the axle in opposite directions. This means that this arrangement can never be
completely rose jointed, there has to be at least one compliant joint in the system. "
from Sylva autokits website....................what are the differences between the longitudinal watts linkage that they use and the 4 parallel link
that most locosts use in terms of binding in roll????? I can't see any difference in terms of binding but they say theirs does??
anybody please???
Retro RWD is the way forward...........automotive fabrication, car restoration, sheetmetal work, engine conversion
retro car restoration and tuning
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