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Kirkham Motorsports

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  • 1 Post By LoBelly
  • 1 Post By stephen low
  • 2 Post By Treeve
  • 2 Post By Treeve

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  #1 (permalink)  
Old 04-09-2018, 03:59 AM
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Default Jag front & rear.... good, bad or indifferent?

Hi guys and Gals,
Just in the early stages of looking into purchasing either a Kit or Pre Built Cobra and have seen a Cobra that looks nice in photo's on carsales and has jag front and rear. I know a lot of the newer Kit's are using their own designs but I was wondering how do the older Kits that have used the jag front and rear stack up. Any things to watch out for or issues? How do they handle and stack up to the newer kits?
I guess I ask this as I am not 100% confident in my abilities to build a kit and cant afford to have one of the companies build it for me and this car looks to have a lot going for it for the money.
Any input greatly appreciated.
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Old 04-10-2018, 08:38 PM
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the Jag running gear is fine

you have to look at the use of this gear in context, a lot of the earlier kits would have been conceived/designed in the late 80's when that stuff was plentiful/cheap and it is a vastly superior set up to trying to build in McPherson struts and solid rears that would have otherwise been available from local models.

the jags we're way heavier than the cobras the components were going into so the builder can have confidence in the relative robustness of the items.

scroll forward to now and the supply is not so plentiful and is now much older - although I would not classify it as a significant problem yet.

I have only driven my own cobra but the components used in construction typically have less influence on the overall performance than the care/precision/ fetling to get the suspension dialled in.

If I were in the market for a cobra the jag running gear would not be an issue for me.

good luck with your search..

LoBelly
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Old 04-10-2018, 09:53 PM
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LoBelly thank you for your reply. Great information, I had not looked at it from that perspective. Certainly gives me far more confidence.
So are supplies likely to become an issue in your opinion?
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Old 04-10-2018, 11:08 PM
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Plenty of parts/suppliers around for new and used also plenty of options to modify and upgrade depending on your requirements.
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Old 04-11-2018, 01:16 AM
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G'day Brewy

My G-Force has S2 front and XJ6 rear.

To-date I've swapped out the front top arms for fully adjustable jobbies for greater and easier adjustability.

The std xj6 rear is solid skinny inboard discs and twin shocker set up with a lousy handbrake system.

The common swap and which I am doing is use of XJ8 - X308 rear, same hubs and mounting but with outboard big vented discs, larger calipers and an effective handbrake.

I've also gone from very basic std gas monroe shockers and fixed coilover to double adjustable Konis with adjustable coilover, with single shocks to the back.

Like LoBelly said the original stuff worked in my car, well maybe not the handbrake, but to vastly lift performance what I have and am doing is typical performance enhancement better dialling in performance.

Not cheap though the Koni-Eibach springs have cost me nearly $5K but were set up and installed by a suspension expert.

So the standard stuff will get you going without concern.

Steve
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Old 04-11-2018, 04:54 AM
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My chassis is an RMC I have a Watts link on either side of my XJS rear end to do away with trailing arms. The front upper wishbones are built wider to let the shocks stand upright through them. Not a major modification. Uses all the jag suspension bearings and really improves the ride / handling. The hand brake I modified with some linkages that made it too light for RTA specs. ( a bigger spring on the handle fixed that. )
The handbrake pads are about as big as a 50c coin so you can't do any "tricks" with them. Nothing wrong with a jag suspension other than weight.
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Old 04-11-2018, 05:05 AM
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I’d be concerned about running a jag rear with only a single spring. Failures from this setup change are not unheard of.
Treeve
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Old 04-12-2018, 06:25 AM
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Hey Treeve Can you explain further on this please ?
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Old 04-12-2018, 08:04 AM
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The ultimate set up for a Jag with a solid mount differential is radius rods with a watts link. The watts link is mounted high on the hub and minimzes rotational loads from the half shafts on the lower control arm bearings. The radius rods minimize tension and compression loads on the lower control arm. Some Cobra designs make facilitating the installation of radius rods a challenge.
Curt Hamilton is rebuilding my diff.while I'm traveling on business. On my return I will begin the assembly process and modifications of the rear end in my Contemporary. It will include radius rods a watts link, replacement of bearings with bushings,lca gussets,dual shocks,sway bar and a structure to support the rear fulcrum bars that will tie into the side brackets. I think this will stabilize the rear end in the chassis.
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Old 04-12-2018, 02:40 PM
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The Jag rear end is different to most double A-arm setups as the driveshaft is used to carry loads and support the upright. The lower arm plays an equally critical role in supporting the upright. The inboard brakes are where they are to limit torsional loads at the upright, and move the force paths back to the inside of the diff carrier. The trailing arms are free moving so do nothing to prevent torsion of the lower arm. Finally, you have symmetrical springs in front and behind the lower arm to evenly distribute any loads and prevent torsional forces from wheel bump on the lower arm.
By replacing this symmetrical layout with a single spring, every single bump / drool action puts a torsional load through the lower arm and bushes at each end. There is minimal resistance to this as the entire design philosophy of this rear end was that with a dual spring setup and internal brakes there wouldn’t be any... and if there were, the symmetrical springs would act to resist them. By having a heavier single spring you remove the torsional resistance of the lower arm and in fact cause torsion over every bump.
Edit to add: the later single spring jag rears with outboard brakes have a different designed lower arm and different spring geometry to change the force paths again to minimise this.
Treeve
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Last edited by Treeve; 04-12-2018 at 02:44 PM..
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Old 04-14-2018, 09:07 PM
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I agree with Treeve on the load and torsional twisting of the lower arm with one shock only per side. It will load the small bearings at the end of the arms more. The only upside I can see is that the cobra is a hell of a lot lighter than the jag. I will also add that the lower arms in my G Force cobra were zinc plated when I bought the car. I had them replated again as corrosion had formed on the top of the arms due to the car sitting for a long period of time. When I picked them up the platter said he had to plate them a number of times due to blistering. Plating can cause hydrogen embrittlement. One sign of that is blistering of the surface. AAARRRRGGGG, I'm not going to take a risk. I know of one chromed arm failing in a Hot rod in WA. new arms being built as we speak.
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