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millsd60
29-05-01, 10:29 AM
I am very close to buying a Cobra kit to build. The short list is now down to 3, AK,DAX & GD. Having obtained a 1986 XJS 3.6 manual gear box donor car and completely stripped it down, and inwardly digested the Haynes manual.

My focus has turned to the rear suspension tie bars. Two of the above manufactures fit tie bars to the Jaguar rear suspension on their kit, however one doesn't.

Two trains of thought here: -
One: Why did Jaguar decide there needed to be a tie bar on the rear suspension. Presumably to feed longitudinal loads experienced by the lower wishbone through the chassis, therefore reducing the stress on the wishbone and reducing the risk of if failing.

Two: Does there need to be a tie bar at all.
Talking to a lot of people and asking questions of one of the above kit manufactures like 'why did you put a tie bar on the rear suspension', the answers I receive are 'well Jaguar didn't feel they really needed it either, but we thought we'd put one on anyway'.

So the question is this. To buy a kit with no tie bars on the rear suspension, or buy on with tie bars?

Having said all this I have had the privilege of driving one of the cars from the manufacture who have no tie bars, and it handled excellently and drove better than any other car I have been in.

But. What I don't want to happen is to build one and find after 30,000 miles, whilst cornering at 80 miles an hour, one of the rear wishbones fails due to the lack of a tie bar, with potentially disasterous results. After all we are feeding 300 plus BHP through a component that Jaguar only feed 200 plus BHP through, and they feel it necessary to fit a tie bar!

Anyone got any thoughts / technical knowledge about this subject?

If so any advice would be appreciated.

cobraman
29-05-01, 07:48 PM
Hi there...I dont know of ANYONE that has had a rear wishbone fail...EVER!...UJ's..yes...But not the arm itself!...If you consider the geometry of the link,Its rubber bushed and works in a single arc..it provides limited assistance,which is why people just leave them off.
The one area when they would come into play,is when you put the setup in a 2 tonne car and brake heavily(Jag XJ6 etc)...the link WOULD then assist to spread the longitudinal load that the axle assembly experiences....As you will find out when you build your Cobra...the problem isn't the excess weight of the back end...Its how to get MORE weight in the back end.The rear of Cobra's are SO light,that the stresses experienced by the Jag axle assembly are a tiny fraction of those that they were designed for.......DONT WORRY!!.....If you wanted to be REALLY professional,you should build a 4bar link into YOUR kit!!(POP BROWNS will supply one for you!)....All the best...DJ

callum
29-05-01, 09:07 PM
This is quite simple! (Would you believe I am building A GD?!) The tie bars locate the subframe on a Jaguar to which the diff is mounted. They effectively locate the entire subframe assembly and have 'not a lot' to do with the suspension itself - which is why their role in a Dax etc., is pretty much cosmetic and GD have dispensed with them altogether. The wishbone is a torque tube and its roller bearings at the diff end need no help from a tie rod mounted to a squishy bit of rubber (rose jointing both ends would triangulate and 'lock' the arc of suspension travel which surely tells you something.) In the Jag they spread the loadings from the subframe across the width of the monocoque. A tubular chassis needs no such help (unless it's a ladder frame which has inherent torsional rigidity problems of its own whenever you load up the suspension). Look at the original Lotus Elan back bone design if you doubt that these tie bars owe their role to monocoque/subframe designs.

Either way this should not be a factor in your final selection. I eventually decided on the basis of engineering, handling and a slightly more elegant, less muscle bound body. If cost is a factor then Dax & GD are much of a muchness but look and see which ones are always up for resale! AK also have their devotees - you pays your money..............

So my single piece of advice is to visit all the makers on your shortlist. This clinched it for me.

ROBD
30-05-01, 12:03 PM
Hi

I am of the thought that alot of what happens to our cars comes from the Track and testing and as I can see there are only really 2 UK companies that have thier cars in regular compitition and that is RAM & GD of which neither have tie bars. And one of the companies you mentioned told me when I went to see them that they only build fast road cars not race cars that sold it for me so I went with a RAM.

Go Steady

Rob

Antony
07-03-06, 04:46 PM
I had a setup out of an E type Jaguar in my first Cobra, and with reasonable tyres, I went through the process of making the radius arms (wishbone locating/support arms) softer so I went for the Jaguar arms with the big complient rubber bush. Problem was with the diff solid mounted or firmly mounted which I think it should be, the factory Jaguar arm was letting the wishbone move and flex considerably under hard take off, and became wishy washy handling at over 80mph, that could also be due to other contributing factors in the car suspension design.

Cobra cars using the Jaguar IRS should either keep it with the Jaguar subframe rubber mounted everything, including locating arms, or go solid mounted diff unit, and fit a nice locating arm on the same plane as the inner suspension arm, this will need at least a firm urethane bush for a little complience. Another mistake in my opinion is people take out the little needle roller bearings on the inner end of the suspension and replace them with urethane, urethane is going to allow for to much movement. Other ways if you have room is to make a suspension locating system called a "Jacobs Ladder" the principle of this is in some of the handling books etc.

dingocooke
07-03-06, 04:57 PM
Hi

I am of the thought that alot of what happens to our cars comes from the Track and testing and as I can see there are only really 2 UK companies that have thier cars in regular compitition and that is RAM & GD of which neither have tie bars. And one of the companies you mentioned told me when I went to see them that they only build fast road cars not race cars that sold it for me so I went with a RAM.

Go Steady

Rob

My Ram has tie bars, not sure when this changed though, my car was finished in 1994, chassis built around 91-92 without looking through the receipts. Or have I misunderstood?

dave
07-03-06, 05:35 PM
The jaguar wishbone or Torque tube to give it's correct name and description is not designed to carry loads from the sides of the diff. If it was don't you think they would have triangulated it.

the Jaguar is propelled forward by the radius rods/ trailing arms because the rear axle is held in place by a rubber mounted cage.

These cars dispense with the rubber mounted cage and mount the whole rear axle assembly solidly to the chassis. This looks like there is then no need for the radius rods as the drive from the hub is passed along the lower arm to the sides of the diff and then into the chassis.
The lower arms are very strong but are still only designed to pass the power through the trailing arms. The large diamiter tube is there to prevent the arm from twisting under high torque loads (Hence the name "torque tube") due to there being no upper triangulated upper wishbone.

As far as I know the Dax is the only UK kit to correctly mount the radius rods. The real reason many manufacturers do not fit them is because they have not had any problems and also it make the chassis design quite a bit more complicated.

Notice that the people who say they are not needed are the people with kits that don't use them!
Everyone here knows just how much Dax like their profits, so why do you think they make life difficult for themselves by fitting radius rods and carrying out all the relavent chassis work if it has not benefit.


You only have to think about it really.


I have known of people who have broken rear arms. The longer the arm the greater the stress on the inner weld....Not really rocket science.

mylesdw
07-03-06, 05:59 PM
Rubber bushed radius arms on a solidly mounted Jag IRS really do nothing to unload stress from the lower wishbones and do cause the suspension to bind unless they are hinged in the right plane (as Dave is alluding too above).

A few guys in the states have managed to break the lower wishbones through mega motors, sticky tyres and drag racing. Attached is a nice piccy of a mod to the wishbones that should give you peace of mind (and a very strong IRS)

TonyD
07-03-06, 06:13 PM
Rubber bushed radius arms on a solidly mounted Jag IRS really do nothing to unload stress from the lower wishbones and do cause the suspension to bind unless they are hinged in the right plane (as Dave is alluding too above).

A few guys in the states have managed to break the lower wishbones through mega motors, sticky tyres and drag racing. Attached is a nice piccy of a mod to the wishbones that should give you peace of mind (and a very strong IRS)

Myles what make is chassis in the picture, is it yours?

Cheers,

Tony

dave
07-03-06, 06:16 PM
Thanks Myles.
i was thinking the world was all insane and i wasn't.................The first sign of madness.
The chasis ibn the picture is a RAM..And nicely prepared it is too.

And if you are not running a radius rod then that (In the picture) is how it should be done.

mylesdw
07-03-06, 06:17 PM
No, it's not mine, it is the 'new' Ram chassis from Realm, sort of Mk2 Ram. Anthony on this forum has one. Here's the front view:

Purple AK
07-03-06, 06:19 PM
As far as I know the Dax is the only UK kit to correctly mount the radius rods. The real reason many manufacturers do not fit them is because they have not had any problems and also it make the chassis design quite a bit more complicated.

SRV8 and AK both use radius rods. I'm not saying that they are correctly mounted as in the Jag, But they are present and doing the job non the less ;)

dave
07-03-06, 06:27 PM
SRV8 and AK both use radius rods. I'm not saying that they are correctly mounted as in the Jag, But they are present and doing the job non the less ;)


Nah!. No rubber bushes allowed Chris. rubber bushing will only help on a rubber mounted axle, on a solid mounted one all they will do is induce stress.

mylesdw
07-03-06, 06:29 PM
SRV8 and AK both use radius rods. I'm not saying that they are correctly mounted as in the Jag, But they are present and doing the job non the less ;)

But the point is, if they are made of rubber they are NOT doing the job however they are mounted. If they have some sort of solid bushing or rose joint they MIGHT be doing the job if they are correctly mounted. The front hinge needs to be on a line projected forward from the lower wishbone inner hinge.

dave
07-03-06, 06:35 PM
Something like this:-

Purple AK
07-03-06, 06:54 PM
Nah!. No rubber bushes allowed Chris. rubber bushing will only help on a rubber mounted axle, on a solid mounted one all they will do is induce stress.
Better than Nowt ;)

russell_ram
07-03-06, 07:15 PM
'Better than Nowt '

No, actually much worse than 'nowt' because they stop the suspension working properly. I can't believe we go through this arguement about once every 18 months.

That RAM chassis is no different to my '97 vintage one by the way - so it may be new - but not new design.

And, late model XJS and DB7 Aston have a plate welded in (from the factory) to give additional f/aft stiffness to the lower arm like those bracing tubes do.

Russ

dingocooke
07-03-06, 07:16 PM
Heres the arm on my RAM, maybe the mkII doesnt have it, (the MKII also does not have the fabricated lower front wishbones, but has reverted to Jag items)

Dont know if Cheng has this control arm on his racer, but it was included on the original Reynard chassis as far as I know.

Steve

dingocooke
07-03-06, 07:19 PM
It helps if I upload the pic-doh!! Sorry..:D

dave
07-03-06, 07:24 PM
If nothing else russell, these sort of threads bring you out into the open.
Nice to see you are still with us.

mylesdw
07-03-06, 07:25 PM
Heres the arm on my RAM, maybe the mkII doesnt have it, (the MKII also does not have the fabricated lower front wishbones, but has reverted to Jag items)

Dont know if Cheng has this control arm on his racer, but it was included on the original Reynard chassis as far as I know.

Steve

That is like mine, what I would call a Mk1 chassis. It is a good example of exactly how not to fit a radius arm. I remember asking Adrian about it and he said it was there 'because people like to see it there' and agreed that is served no positive purpose.

dingocooke
07-03-06, 07:26 PM
I live and learn!!! Cheers Myles

Clarkson
07-03-06, 07:31 PM
Something like this:-

That's how SR do there rear arms. Must be better than nothing?? I have full movement, with no trouble at all.

RobinH
07-03-06, 07:38 PM
'Better than Nowt '

I can't believe we go through this arguement about once every 18 months.

Russ

Oh ****, I remember it too! Have I really been reading this forum for 18 months already? I really must get a life!

mylesdw
07-03-06, 07:39 PM
I can't believe we go through this arguement about once every 18 months.
Russ

Amen!


That RAM chassis is no different to my '97 vintage one by the way - so it may be new - but not new design.
Russ

I wasn't sure when the changes were made but there are certainly enough differences to call it a MK2

dave
07-03-06, 07:40 PM
That's how SR do there rear arms. Must be better than nothing?? I have full movement, with no trouble at all.


Sadly though, since the SR was taken over by Madgwick they have done away with the original radius rod set up (Or so I hear) and gone for one that doesn't work.....Your life in their hands etc!!!!!!

Clarkson
07-03-06, 07:43 PM
Sadly though, since the SR was taken over by Madgwick they have done away with the original radius rod set up (Or so I hear) and gone for one that doesn't work.....Your life in their hands etc!!!!!!

Oh I did not know that? I wounder why??

dave
07-03-06, 08:00 PM
...Easier to manufacture, less complicated floor pans. Less,cuts, less steel, less time, more profit.

gareth08
07-03-06, 08:04 PM
Ah,
So it would seem that all you Jag based boys without torque tubes or wrongly fitted ones and who keep raving on about it being so much better than a Ford based set up are actually driving around in something which is not a very good idea:rolleyes:

Good Luck:D :p

dave
07-03-06, 08:06 PM
But our top ball joints aren't over stressed!!!!!

Clarkson
07-03-06, 08:08 PM
Ah,
So it would seem that all you Jag based boys without torque tubes or wrongly fitted ones and who keep raving on about it being so much better than a Ford based set up are actually driving around in something which is not a very good idea:rolleyes:

Good Luck:D :p

Yeah that's why your running ford motor and we are chevy motors:D :confused: :p !

Purple AK
07-03-06, 08:09 PM
Sorry. I hadn't realised this discussion had been upgraded to argument status :rolleyes: :D Yes I can see that the Ram/Late XJS/DB9 bracing is the ulitmate answer. However any form of solid rosejointed trailing arm, be it angled in or straight ahead, will, whatever it's length, work on a swinging axis, and ultimately cause some binding or stress. On the other hand a rubber bushed arm will not give as much rigidity, But, will allow the nessessary compliance through the axial movement.
All that said! Unless we are pumping HUGE amounts of HP/Torque We have nothing really to worry about.

Clarkson
07-03-06, 08:11 PM
Sorry. I hadn't realised this discussion had been upgraded to argument status :rolleyes: :D Yes I can see that the Ram/Late XJS/DB9 bracing is the ulitmate answer. However any form of solid rosejointed trailing arm, be it angled in or straight ahead, will, whatever it's length, work on a swinging axis, and ultimately cause some binding or stress. On the other hand a rubber bushed arm will not give as much rigidity, But, will allow the nessessary compliance through the axial movement.
All that said! Unless we are pumping HUGE amounts of HP/Torque We have nothing really to worry about.

I think the same Chris.

gareth08
07-03-06, 08:12 PM
Yeah that's why your running ford motor and we are chevy motors:D :confused: :p !

Dave,
I think you'll find the Ford engine will be way to powerful for them dodgy Jag set ups:D :p

mylesdw
07-03-06, 08:27 PM
Sorry. I hadn't realised this discussion had been upgraded to argument status

Maybe 'argument' is putting it too strongly but it is a discussion that has been knocking about since the mid 70s at least, when hot-rodder Nick Butler claimed that the short radius rods were a bad idea for a solid mounted diff: and was immediately pilloried by the hot-rod community!

Purple AK
07-03-06, 08:38 PM
Maybe 'argument' is putting it too strongly but it is a discussion that has been knocking about since the mid 70s at least, when hot-rodder Nick Butler claimed that the short radius rods were a bad idea for a solid mounted diff: and was immediately pilloried by the hot-rod community!
I've changed my name since then ;)
Seriously, Unless you are Stripping it, Racing it, And putting serious Hp through it. Any of the "compromise" set ups ought to be fine.

Clarkson
07-03-06, 08:59 PM
Seriously, Unless you are Stripping it, Racing it, And putting serious Hp through it.

450HP be ok Chris??:D :p

Purple AK
07-03-06, 09:03 PM
Seriously, Unless you are Stripping it, Racing it, And putting serious Hp through it.

450HP be ok Chris??:D :p
Me to wonder :cool: You to find out :rolleyes: Never Gonna worry me with my little cooking motor ;)

dave
07-03-06, 09:08 PM
[.........However any form of solid rosejointed trailing arm, be it angled in or straight ahead, will, whatever it's length, work on a swinging axis, and ultimately cause some binding or stress.....


Sorry Chris but no. Not if the radius rod pivot point is on the same axis as the wishbone/lower arm..
The Dax radius rod is not just at an angle, the rose joint on the end is perfectly in line with the fulcrum pin through the inner wishbone. Even with no rubber bushing the suspension can work through its whole range without the slightest hint of binding.

On the picture i posted the masking tape runs along the hinge line, you can have as many hinges as you like and it will work perfectly so long as they are along this line. Anywhere else and it all goes pear shaped.

Purple AK
07-03-06, 09:23 PM
[.........However any form of solid rosejointed trailing arm, be it angled in or straight ahead, will, whatever it's length, work on a swinging axis, and ultimately cause some binding or stress.....


Sorry Chris but no. Not if the radius rod pivot point is on the same axis as the wishbone/lower arm..
The Dax radius rod is not just at an angle, the rose joint on the end is perfectly in line with the fulcrum pin through the inner wishbone. Even with no rubber bushing the suspension can work through its whole range without the slightest hint of binding.

On the picture i posted the masking tape runs along the hinge line, you can have as many hinges as you like and it will work perfectly so long as they are along this line. Anywhere else and it all goes pear shaped.
Dave.
I Bow my head so you can give it a good slap :( Stoneleigh it is I guess :D

Miket
07-03-06, 09:25 PM
[.........However any form of solid rosejointed trailing arm, be it angled in or straight ahead, will, whatever it's length, work on a swinging axis, and ultimately cause some binding or stress.....


Sorry Chris but no. Not if the radius rod pivot point is on the same axis as the wishbone/lower arm..
The Dax radius rod is not just at an angle, the rose joint on the end is perfectly in line with the fulcrum pin through the inner wishbone. Even with no rubber bushing the suspension can work through its whole range without the slightest hint of binding.

On the picture i posted the masking tape runs along the hinge line, you can have as many hinges as you like and it will work perfectly so long as they are along this line. Anywhere else and it all goes pear shaped.

Why have Dax gone to so much much trouble (in your opinion) getting these things to work correctly, when by a majority opinion, they do f**k all anyway. :confused: :confused:

dave
07-03-06, 09:26 PM
Hi Chris.
If you are at a lose end at stoneleigh could you get me a burger?

Purple AK
07-03-06, 09:35 PM
Hi Chris.
If you are at a lose end at stoneleigh could you get me a burger?
Onions? And how many sugars in the coffee?? :D

dave
07-03-06, 09:44 PM
Chris.
Onions, ketchup, the works.


Mike.
There are none so blind as those that will not see.

You use the word"Opinion". Opinions are like a55 holes we all have them but backing up an argument/debate is another thing , just because it hasn't broke still doesn't mean it is right. There's plenty of design features on all the major kits that I don't like and could go into but not on an open forum.
I an not saying that I think that the Dax is without faults.
The WTC was fine until they discovered too late that there was not enough fire proofing on the girders.


Put enough power through them and you will see them work.

The fact that almost all Super 7 replicas have over stressed top ball joints and no one has been killed because of it still doesn't mean that it is right. As far as I'm aware Dax are the only manufacturer of these type of kits to address this potential problem.

"Oh, I'm sure it'll be alright" kind of arcs back to the Dutton days of kit car design, and there wwas me thinking things had moved on.

The example shown on the silver RAM chassis costs pence to do and could save a life/win a race some day.

Miket
07-03-06, 09:52 PM
Mike.
There are none so blind as those that will not see.
Christ came 2000 years ago to give "sight" to the blind, but still they refuse to "see". ;) ;)

dave
07-03-06, 09:56 PM
:D :D :D :D
Is that better?


This bl00dy discussion has cost me a cut off mitre saw at a bargain price I was trying to snipe on eBay.

B0ll0x!:mad: :mad: :mad:

Miket
07-03-06, 09:58 PM
:d :d :d :d :d :d :confused: :confused: :confused: :confused:

mylesdw
07-03-06, 10:03 PM
In order of merit, the solutions from best to worst are:

1. Dax rose-jointed and aligned radius rods.
2. Ram (mk2) braced wishbones, no radius rods.
3. No radius rods
4. Short rubber-bushed radius rods
5. Short solid bushed/ rose jointed radius rods.

Note that #4 and #5 are NOT compromises, they are worse than nothing (#3).

dave
07-03-06, 10:06 PM
Hasn't this filled an evening. (Or morning or whatever time it is in NZ)

mylesdw
07-03-06, 10:08 PM
Time for lunch. Sorry about the mitre saw!

Miket
07-03-06, 10:11 PM
In order of merit, the solutions from best to worst are:

1. Dax rose-jointed and aligned radius rods.
2. Ram (mk2) braced wishbones, no radius rods.
3. No radius rods
4. Short rubber-bushed radius rods
5. Short solid bushed/ rose jointed radius rods.

Note that #4 and #5 are NOT compromises, they are worse than nothing (#3).
All apart from 3 will stop your rear wheel travelling across the central reservation of a motorway at 100 mph :mad: :mad:

dave
08-03-06, 06:37 AM
All apart from 3 will stop your rear wheel travelling across the central reservation of a motorway at 100 mph :mad: :mad:


4+5 wouldn't stop the wishbone from breaking in the first place, which would then screw up the rear track drastically sending the whole car across the central reservation.:p :p :p :D About as much use as a broken leg when compared to an aputated one........Don't try running on it!!!!:rolleyes:

russell_ram
08-03-06, 07:43 AM
As a point of order, choice 1 actually relies on deflection of the lower arm fore/aft BEFORE the tie rod does anything.

If the lower arm doesn't deflect, the rod carries no load and is therefore redundant.

Russ

dave
08-03-06, 09:09 AM
But it does deflect in these applications...................Not a lot but it does deflect, and over time this could lead to fatigue and eventually failiure.

mylesdw
08-03-06, 05:45 PM
All apart from 3 will stop your rear wheel travelling across the central reservation of a motorway at 100 mph :mad: :mad:

If you're worried about whether the car crosses the central reservation in one or two pieces following the fracture of a lower wishbone, a short length of chain in place of the lower radius rod should do the trick (as long as it's not installed tight). Options #4 and #5 will increase your chances of such an excursion but you will at least end up with all the bits in one place.