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Discussion Starter #1
There is one upgrade I would like to make that I have not seen mentioned.


I would like to swap out the antiquated live axle rear suspension for the more capable Independent Suspension out of the Cobra. Based on prior experience, limited slip differentials are almost unnecessary with IRS. And certainly the Bullitt's jiggly handling over bumps would be eliminated.


From what I have read the Cobra IRS utilizes a subframe that bolts in to normal suspension pivot mounts. It also utitilizes the same shock/spring mount locations. I would guess that with the stiffer suspension under the Bullitt that Cobra spring/shock/anri-roll bar settings should be pretty close to perfect.


Any Cobra owners want to trade??? (-8
 

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Man don't do that. You will have endless wheel hop,plus add about 80 lbs. of extra weight. I find the Bullitt suspension to be just fine!
 

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You must not be much for the drag strip because wheelhop is terrible on IRS suspensions. Personally, I prefer a live axle because of the performance advantage. Granted, the Bullitt is a bit rough on bad roads (& NY is FULL of em), but if I wanted a cushy ride, I wouldn't drive a Mustang. That's the trade off with sports cars. A guy I work with drives an M3 & his is rough too & that's a $50,000 car!
 

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Vroom,

I am a bit curious as to what you wish to gain by going to the Cobra IRS.
Do you drive the car only on the street? do you auto-x, or open track? Do you ever see the drag strip?

Also, what sort of experience did you have that told you limited slip was not necessary on an independant suspension? (I for one am frustrated every day that my Contour SVT did not come with a limited slip differential, and they are very expensive to add on a fwd car)
 

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Discussion Starter #5
Ok... clarifications.


My driving will be entirely on the street. I expect to enjoy a good twisty mountain road occaisionally and might try an autocross sometime. But I have no interest in drag racing.


I am not looking for a softer ride out of the IRS. That is a function of springs, shocks, and anti-roll bars not the suspension type. What I am looking for is better composure on rough surfaces. I want to feel that everything is still "hooked up" when the road is less than racetrack smooth.


I am curious about the proverbial "wheel hop". Can someone explain why this would be any different with IRS?


I do know from experience that one of the limitations of a live axle is that engine torque directly reduces RR traction and increases LR traction due to the action/reaction windup. This is IMHO the major reason to have limited slip in a street car. Of course since the differential is bolted to the frame in IRS, this windup is contained within the frame and never gets to the wheels.


Once upon a time I owned an old Triumph TR4A sports car. This was an early model with the IRS frame and a live axle (it seems that the Brits did not think we Yanks would pay the extra $$$ for IRS). After driving it awhile I put a set of IRS out of a TR6 under it. It makes a big difference when both tires get the same power/traction. I did not have limited slip and frankly never missed it. Nor did I ever notice the lack of limited slip in my 280Z.


Of course I have to acknowledge that there is a little different power curve in the Bullitt. (-8 And this could impact the need for limited slip. The other major factor is how the suspension is trimmed for weight transfer. Often live axles suffer from a raised roll center, which leads to more weight transfer. This is further suggested by the relative difference in roll stiffness between front and rear: the front likely has a lower roll center and has much more roll stiffness than the rear.


So limited slip may or may not be required depending upon a number of factors, one of which is rear suspension design and setup.
 

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you might as well have bought a Cobra. the cost and effort to switch out will make your Bullitt just as expensive (if not more so) as a Cobra.

_________________
mystang99
(Marty)
2001 Bullitt DHG #648

<font size=-1>[ This Message was edited by: mystang99 on 2001-08-10 14:41 ]</font>
 

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Kenny brown has a all new design IRS for the Mustang. Not sure it is available yet. He also has modified parts for the Ford IRS. You must send the rear end to him for the mods.

The IRS will certainly work better over bumps..especially in the corners. That is the ONLY advantage the stock Cobra IRS will give. It has some bad toe change problems and on smooth surface, the std live axle actually handles better than the stock Ford IRS according to Kenny Brown.
Call them at 317-247-5320

Brian
 

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Hey Vrooom, Kenny Brown e-mailed and said that the IRS adaption kit for the Standard GT and Bullitt would not be ready until the end of the year. It will be about 60 lbs. lighter than the stock set up on the Cobra and have better ride and handling qualities.
 

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The only major improvement when using the Cobra IRS is road feel over bumps. If that is what your after, go for it.
The Cobra IRS has really no improvement in performance as far as cornering or acceleration. Though I cannot comment on Kenny Brown's upcoming setup, I do hope it delivers as promised, but I'm not holding my breath.

As to the swap, it is not really that difficult, it is basically a bolt-in affair, you need the IRS module, and a cobra-style cat-back, and maybe a new fuel tank, and a few other odds and ends. There are a lot of Cobra owners that swap over to solid-axle for drag racing, If you could find someone who would like to swap, it makes things much easier. Though I personally would not consider the swap.

The hop thing with the IRS is more related to driveline windup, and is fairly typical of independantly suspended drive wheels, and in fact it is a bit more like a shudder than strictly a hop.

As to limited slip, a high torque/hp car will always benefit from a limited slip differential.
 
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