Cheffy
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| posted on 28/3/06 at 07:54 AM |
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Poly Bushes vs Rose Joints on trailing arms
I am in the process of fitting a De Dion axle to my chassis. I was planning on making up simple trailing arms with bush tubes welded on at either end
and poly bushes and crush tubes the same as on the front wishbones.
Now the stupid question.......
When cornering, the chassis will be trying to roll around a longtidudinal centreline while the axle is staying relatively parallel with the road
surface - i.e. - the axle will not be simply travelling directly up and down in relation to the chassis.Won't this be exerting twisting forces
on the trailing arm mounting brackets, bushes, bush tubes etc. Surely in time this twisting is going to weaken the brackets/welds, and therefore
isn't it better to use rose joints?
On the other hand I'm just a simple chef - am I missing something?
Thanks in advance,
Martin.
Farts are like Rock'n'Roll. You love your own but you hate everybody else's. Lemmy, Motorhead.
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Peteff
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| posted on 28/3/06 at 08:25 AM |
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The poly bushes should be compliant enough to allow this much twisting movement of the arm.
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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procomp
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| posted on 28/3/06 at 08:47 AM |
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Hi you can't rosejoint that setup as it will lock up solid you have to use a compliant bush set
cheers matt
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Kissy
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| posted on 28/3/06 at 09:15 AM |
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Sorry - rosejointed trailing arms and panhard rod are pefectly acceptable - if not a little pricey. The benefit is sharpness/precision of axle
positioning, downside is a harsher ride.
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Syd Bridge
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| posted on 28/3/06 at 09:37 AM |
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quote:
Originally posted by procomp
Hi you can't rosejoint that setup as it will lock up solid you have to use a compliant bush set
cheers matt
Please explain?
Martin,
If anything, rose(or ball) joints are perfectly acceptable.
If anything, they are preferable for the reasons you give. The ball joint does not transmit torsion, so the arms are left to transmit pure tension or
compression.
Although the twisting with the poly or other joints is small, it is there nonetheless, and has been known to lead to the failure of the arms or
brackets.
Cheers,
Syd. 
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procomp
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| posted on 28/3/06 at 09:48 AM |
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Hisorry kissey but if you think that a book 4 link setup will work with rose joints you should do some more research as it allows so little roll of
the axle and any movement in bump or rebound with a slight amount of roll is just trying to rip the brackets of either the chassis first and then the
axle. search on the westfield forum wscc where this has been seen hapening for the last 20 years. all of the fast 7 type cars with tis setup use
compliant bushing of some sort.
you may be getting confused with 4 link systems such as on mallock's ans so on where it may look simalar but it's totaly different
geometry setup all together.
cheers matt 20 years of building live axle cars for competition.
sorry if that sounds harsh but in this area i do no what im'e talking about
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procomp
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| posted on 28/3/06 at 09:53 AM |
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hi syd ime confussed are you for or a gainst rose joints in this aplication.
sorry but ime speed reading and typing as my day job seems to be getting in the way
cheers matt
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Syd Bridge
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| posted on 28/3/06 at 10:03 AM |
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Matt,
I'd like you to explain how rose(or ball) joints would 'lock up'.
Read what I put, carefully. If you undestand tension, compression, and torsion; then you'll understand how wide of the mark your reply is.
As the major amount of movement is vertical, with only a minor( though not insignificant) amount of twisting, when a rear trailing arm is looked at.
A spherical type joint would have no problems, and is preferable as it would have no compliance, giving pinsharp handling. Downside is harshness, so a
poly bush is softer and less noisy. But, the polybush imparts extra loads which are unnecessary(and potentially dangerous), into the arms.
Cheers,
Syd.
If Westfields with Rose joints are experiencing failure with brackets, then it is a problem with the bracket or its attachment, and definitely
NOT with the joint setup. As example, if the axle is in single wheel bump, and moves say 4", across an Escort axle this is less than 10
degrees, which is what the joint sees as torsion. Most, if not all, joints would have this in their movement. The only exceptions are some of the
very cheap joints.
[Edited on 28/3/06 by Syd Bridge]
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zilspeed
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| posted on 28/3/06 at 10:15 AM |
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I don't get that either.
I've seen rod end bearing used in both axes (axes - not axles) on front setups (although I know which I prefer) and they work fine.
Why would these joints not be more suited to twisting than any form of plain / rubber / metalstic / poly / nylon bush ?
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Syd Bridge
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| posted on 28/3/06 at 11:05 AM |
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The answer to the problem Mr.Procomp posts, is in his own explanation.
The arms are 'ripping the brackets off', either the chassis or axle. If it was a joint problem, then the joints would be failing, even if
oversized as most are. Se7en style cars with rose joints and solid axles have been around since the very first, and an overall problem is not
known.
It would appear that Westfield have a car specific weakness, that a non compliant suspenson brings out by 'ripping off' the trailing link
brackets.
Syd.
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procomp
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| posted on 28/3/06 at 11:14 AM |
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Hi effectivly when the car is in roll and bump one side droop the other you are trying to tension one side and compress the other . at some point
this will restrict any further movment of the axle if however the axle trys to carry on moving further the loads are then placed in to the bracketry
system and the rest is obvious.This remember is talking about equal lenth trailing arms as opossed to long bottom and short top ones as in the case of
mallock's gp4 escorts and so on .set up with compliant bushing allows the axle to move further without resriction thus enabling the car to have
more available grip.
probably not verry well explaind and would need a working model to fully demonstrate .
cheers matt
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procomp
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| posted on 28/3/06 at 11:24 AM |
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hi syd i think you should build yourself a full size rig and play . if you streghen up the brackets so that there will be no failures all you are
going to end up with is an axle that has resricted movement and less grip available in cornering ussually lifting the inside wheel clear of the
ground.
i am afraid i think we will have to agree to disagree on this.
cheers matt
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britishtrident
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| posted on 28/3/06 at 11:30 AM |
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In theory both rose joints and Polly bushes are wrong but in the real world you can get away with either.
A locost book style a 5 link system has a redundant member (in theory you only need 3 links + a panhard rod) because of this there is a small conflict
of arc when the suspension is in bump and roll at the same time -- however as the chassis and axle isn't 100% ridgid it is compliant enough to
work.
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procomp
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| posted on 28/3/06 at 11:35 AM |
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Hi british trident is correct may be my first reply was not worded correctly . what i am trying to say is that there is more travel available whith
polly bushes than with rosejoints.
cheers matt
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NS Dev
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| posted on 28/3/06 at 11:42 AM |
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on the rally cars we used to run equal links (boreham group 4 style) but with bushes one end and rose joints the other, so you get the travel without
binding but keep the adjustability of the rose joints. it was just easy to do it that way, others put insitu adjusters in the links and ran bushes
both ends.
Retro RWD is the way forward...........automotive fabrication, car restoration, sheetmetal work, engine conversion
retro car restoration and tuning
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MikeRJ
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| posted on 28/3/06 at 11:51 AM |
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Procomp is perfectly correct, a system with 4 trailing arms does bind, it's simply a matter of geometry.
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Syd Bridge
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| posted on 28/3/06 at 02:22 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.
I'd like to hear what is correct, if ball joints and poly bushes are wrong.
Within the limits of their angular travel, rodends/spherical joints/balljoints WILL be the optimum end fixing.
I will concede that at and past their limits, rodends will lock up, and the end should fail, not the bracket, but that's bye the bye.
If a 7 type car is 4" up one side and 4" down on the other, (which is fairly drastic)the angle of the axle to the body is ~11 degrees.
This about the limit of most rodends. A racecar shouldn't be getting to these angles, nor a road car.
Not many 7's have 8" of travel in the rear shocks.
At any angle, poly bushes impart torsion into the trailing arms and thus the brackets, and this is undesirable. Up to their limit, ball ends impart no
torsion; then huge amounts past the limit, but no more than the poly's at the same point.
Within normal road driving, poly bushes are acceptable, for the mileages these cars would normally do, as are rodends.
Simply a matter of geometry. At the limit both poly and rodends will cause a failure of either the bracket, end, or arm.
To say that rodends are unsuitable because they 'lock up' is simply untrue. Check under a Nascar or Aussie V8 Supercar just to see how
useable rodends are in 4 and 5 link axle setups, with panhards or Watts links.
Cheers,
Syd.
Oh, and I have been working with 'full size test rigs' near every day for far too many of years. They're called
'racecars' and 'test mules'. 
[Edited on 28/3/06 by Syd Bridge]
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NS Dev
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| posted on 28/3/06 at 02:48 PM |
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I'm trying to get my head around this one Syd, and not being at home can't make a little model, but it's not simple!
At first I thought exactly the same as you are saying, but that's not actually quite right.
Matt wasn't talking about running out of roll articulation on the rose joints, he was talking about the fact that with 4 trailing links, all
parallel and the same length, in roll, the compression and tension in the links will actually cause the whole lot to bind up. This is due to the arcs
of movement of the 4 links, which will cause them to try and attain 4 different distances from the common mounting plane of the 4 links (if that makes
sense!) Because the axle is solid in torsion and the links are attached to it, this cannot happen. The bind will only be over a tiny distance, hence
bushes will enable it to work, as will bracket flex and chassis/axle twist, hence the system will work, but on a light car will cause fluctuation in
loads at the 4 wheels.
Using only three links will remove the problem (but not really possible on a 7), and, if my confused brain is working right, using divergent links
will alleviate the effects over the normal working range, but not remove them altogether.
I think though that with parallel links attached to a solid axle which is stiff in torsion, bind up in roll using totally non-tension-compression
compliant links will happen, which is what Matt was explaining.
Retro RWD is the way forward...........automotive fabrication, car restoration, sheetmetal work, engine conversion
retro car restoration and tuning
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DIY Si
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| posted on 28/3/06 at 03:42 PM |
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JUst being nosey, but some of you are talking about a 3 link type thing, as opposed to 4 trailing links. Where would these go? Would they be 3
seperate trailing arms?
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procomp
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| posted on 28/3/06 at 03:48 PM |
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Thanks nat for explaining what i was trying to say i cant stop work to respond at the moment . clearly syd dose not understand the geometry of the 4
link setup whitch is why he cant understand the point im'e trying to make .
cheers matt
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Jon Ison
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| posted on 28/3/06 at 03:50 PM |
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After much head scratching I think I understand, but, for it too bind up its got too travel prob further than the shocker etc would allow ?
or don't I understand........?
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DIY Si
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| posted on 28/3/06 at 04:27 PM |
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The way I see it, and I do understand what both sides are saying, is that the rose joints are better to a point, ie their rotation limit, at which
point somthing breaks. However, poly bushes will always transmit some twist to the chassis, but have a generally higher rotational limit, and as such
may be better at higher levels of bump/droop. BUT, how much bump/droop do any of you have at the limit? To get the +4"/-4" situation
stated above you'll need 8" of travel at each side. And I seriously doubt anyone has near that! Neither system is ideal. So, basically
what I'm saying is for most it makes bugger all difference.
At this point he grabs a tin hat and dives for cover......
Forgot to say, I think rose jointing both ends will limit things more. Best bet seems to be rose joints at one end and poly bushes at the other. This
should overcome some of the 'vagueness' inherent with poly bushes, but still allow the ease of movement.
Might just have confused myself reading that back.
[Edited on 28/3/06 by DIY Si]
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britishtrident
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| posted on 28/3/06 at 04:42 PM |
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quote:
Originally posted by DIY Si
JUst being nosey, but some of you are talking about a 3 link type thing, as opposed to 4 trailing links. Where would these go? Would they be 3
seperate trailing arms?
Chrysler Europe used 3 trailing links + panhard on some models but I can't for the life of me remember which exact models it was either the
Simca 1301/1501 and 180 or the estate version of the Avenger (estate used different suspension from the saloon.
The lower links and panhard rod were as per book locost the upperlink was as mounted to a similar position to the anti tramp rods fitted to the RS
escorts and Capris and MK2 Cortina Lotus.
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MikeRJ
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| posted on 28/3/06 at 05:52 PM |
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quote:
Originally posted by DIY Si
The way I see it, and I do understand what both sides are saying, is that the rose joints are better to a point, ie their rotation limit, at which
point somthing breaks. However, poly bushes will always transmit some twist to the chassis, but have a generally higher rotational limit, and as such
may be better at higher levels of bump/droop.
No, that's is what Syd is saying.
What everyone else is saying is that the 4 link rear axle location (as used in the locost) has an inherent design problem that requires the use of
compliant bushes to make it work correctly.
The problem is nothing to do with the limited articulation of rose joints, it's a simple geometrical issue that puts a torque loading on the
axle when the car is in roll. Without some kind of compliant member in the suspension (i.e. a rubber/poly bush) things will be put under a large
amount of force which will at best compromise the suspensions performance, or worse still break something.
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Syd Bridge
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| posted on 28/3/06 at 06:26 PM |
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quote:
Originally posted by procomp
. clearly syd dose not understand the geometry of the 4 link setup whitch is why he cant understand the point im'e trying to make .
cheers matt
Syd does clearly understand what is being said! Equal length parallel links, think about it!
Of all of you lot, Nat should know that I would be the person who may know just a little about these things.
Just jack up a car with four link back end using rodends, then waggle the axle sideways, up and down , any which way. Make sure the lateral support is
disconnected first.
I'll leave this to all of you 'experts'.
But first off, you better go tell all the Nascar and V8 supercar teams that their 4 trailing link rear suspensions aren't working...and must not
have done for quite a number of years!!! Not to mention the rally teams, the sprint 7's and similar, the sprint cars, the hotrods, ........and
on and on.
Cheers,
Syd.
Now, Procomp are supposed to have a good reputation....but after hearing this load of bovine exhaust, a modestly intelligent person cannot help but
ask' How did this good reputation come about???'
[Edited on 28/3/06 by Syd Bridge]
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