Question about Jag IRS

[cpd427]

I'm attaching a quote that I pulled from a thread over at ClubCobra that caught my eye. The author was responding to prior comments about the need for an IRS in a Cobra set-up. The quote is:

I do have to say from an engineering point of view, the Jag IRS design, where the axel serves as the upper control arm is as unsafe a design as anything I ever saw on a car. I understand failure is rare, but I don't trust them at all.

I know that several manufacturers (Unique and ERA) promote the use of the JAG IRS on their vehicles and this system has been used for many years on many cars. Not knowing the inner workings or "from an engineering point of view" as the above author states, I'd like to hear comments from the forum on this statement of unsafe design and mis-trust of the JAG IRS, a system used on all Unique cobras, I believe. I know I have taken this single comment from an entire thread, so for those interested, the thread at ClubCobra is called "Who makes the better mouse trap/car?"

Thanks,

Chris

[eliminator]

I don't know who the author of that thread is but I think he has no clue of what he is talking about.

"I do have to say from an engineering point of view, the Jag IRS design, where the axel serves as the upper control arm is as unsafe a design as anything I ever saw on a car. I understand failure is rare, but I don't trust them at all."


One would have to question Ford, with the Thunderbird IRS, or GM with the Corvette IRS. What rare failure is he talking about, Engineering point of view??? I would question if or what kind of an engineer he claims to be. I am not going to wring my hands and worry about my JAG IRS, thats a bunch of Pelosi!!!!

[TurnpikeBoy]

Failure is always a possibility, in any thing we make. The difference is probability - and that's where that poster's perspectives are just too blind.

If he thinks the Jag IRS is unsafe, then why is he in a Cobra at all? Short of a motorcycle, there isn't another vehicle out there that's likely more dangerous. The IRS portion is a non-starter, as it would require mechanical failure to induce trouble, whereas operator failure is almost always a given.

The Corvette and Ford IRS units anchor the top of the outside hub with either spring (Corvette) or a control arm (Ford). Theoretically, if the half-shaft breaks then these elements retain the hub, keeping the rear wheel vertical and control intact. That posters' issue is when the Jag universal joint breaks, the hub then pivots on the lower control arm and the wheel is free to flop from vertical. Yes, it could happen - but the probability is so awful low that it borders on the bizarre. One look at the size of the u-joints used here will quickly put that fear to rest, especially when compared to the Corvette and Ford joints.

The engineering of this unit is actually pretty elegant, and efficient. That it is strong enough for our uses is a foregone conclusion - there are a helluva lot of these units out there, used in Cobras and hot rods and even Jaguar sedans and roadsters - and failures of the type this guy dreads just don't happen as casually as he seems to envision. So - of all things, you do not need to concern yourself with the Jag rear - as regards catastrophic failure, anyway. If you were to ignore it for, oh, 30K miles or so, it would probably remind you that service is a good thing by leaking or whining or some such event.....but breaking the u-joint and losing the wheel just ain't gonna happen.



-Roger

[cpd427]

Turnpike Boy,

Thank you for that response. You certainly have a lot of information that speaks to the design. It's not that I questioned the use of a JAG IRS in the first place, as this has been successfully used for many years, but the posters remarks really made me want to request feedback from a forum that would have the proper insight. By the way, the poster stated that he had never personally driven a cobra with a JAG IRS, so I guess his remarks are based on something other than a true evaluation.

Thanks for your insight.

Chris

[TurnpikeBoy]

I think this is why forums like this are great - the opportunity to think about alternative viewpoints is huge. The guy who posted the original misgivings about Jag rears was thinking out loud and saw a downside he just wasn't comfortable with. I kinda think he hadn't been up close and personal with one and had the opportunity to see for himself that these things aren't made for bicycles. And he surely didn't understand the designed capacities of the parts involved.

All this said, the Jag is hardly the pinnicle of IRS design; after 40++ years that's to be expected. It is efficient, using the least number of parts to accomplish the task. It is strong, and will easily withstand 450 hp on a daily basis. It can be made to fail - just put good slicks on the car and do 5000 rpm holeshots. It can be improved, as CWI and AMP and others have done, largely to remove the thrust loads imparted on the center section by normal vertical movement of the half shafts. And it is, comparatively, a b*tch to work on.

But all said and done, it is car-and-period-correct, it is strong as hell, it works well in the application, and parts are plentiful and (mostly) reasonable. There will always be better alternatives - a new ZO6 will stomp about any Cobra's tush - but when measured against the desire and use, they're way more than adequate.



-Roger