Jag rear suspension tie bars?

[millsd60]

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]

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]

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]

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]

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]

Originally Posted by ROBD

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]

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]

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]

Originally Posted by mylesdw

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]

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.