Independent rear suspension system converstion ? - Page 2 - Vintage Mustang Forums

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post #16 of 52 (permalink) Old 04-13-2017, 01:47 PM
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We seem to have this debate every year, and that track comparison comes out. No modern Road Race car would choose a solid axle, unless mandated by the rules. But IMO that's not where IRS really shines, it's on bumpy back roads that it sees the biggest benefit. A stick axle can get very lively negotiating a bumpy turn, and not in a good way.
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post #17 of 52 (permalink) Old 04-14-2017, 12:41 AM
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I like the idea of an IRS for overall handling and ride quality. Possibly a well engineered modern irs in a modern car is a no brainer.

This article seems to indicate that the live axle is faster; Independent Rear Suspension for Early Mustangs


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post #18 of 52 (permalink) Old 04-14-2017, 06:54 AM
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I don't know why people keep making this "definitive" conclusion based on a system designed in the 60's, which they admit needed more development. If you read "unfair advantage" by Mark Donohue you will realize just how little was understood at that time. For starters: using the axle as a suspension link, while convenient and cheaper, is a flawed design right from the start. In addition, it was designed around the leaf spring pickup points, hardly ideal. Do that test with a modern clean-sheet design and I'm confident the results will be different. The Roadster Shop IRS looks such a system, but appearances doesn't necessarily mean anything
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post #19 of 52 (permalink) Old 04-14-2017, 07:57 AM
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My stepson and I have gone to VIR several times with his 2014 GT trac pack and then new a 2015 GT trac pack after selling the 14'. He runs in HPDE-2 class. He was running lap times in the 14' avg 1:24 and in the the 15' is 1:16. His first experience with the new setup IRS was that going thru the S's and rumble strips the car stayed planted much better than the 14' and pushed his speed up to 120 mph thru that area feeling confident. He's not a pro by any means and the 15' is an overall better handing car than the 14' but going thru the "S's" proved the IRS was a much better setup to him.

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post #20 of 52 (permalink) Old 04-14-2017, 08:43 AM
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Gordonr, Was he on the exact same tires?
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post #21 of 52 (permalink) Old 04-14-2017, 09:34 AM
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Gordonr, Was he on the exact same tires?

I dont know. Both were fitted with the factory wheels and tires. He later up graded the 15's wheels and tires, suspension and to a full exhaust then dropped his times a couple more of seconds.

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post #22 of 52 (permalink) Old 04-14-2017, 11:52 AM
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Originally Posted by stephen_wilson View Post
We seem to have this debate every year, and that track comparison comes out. No modern Road Race car would choose a solid axle, unless mandated by the rules. But IMO that's not where IRS really shines, it's on bumpy back roads that it sees the biggest benefit. A stick axle can get very lively negotiating a bumpy turn, and not in a good way.
Perhaps a modern car beckons?

It'd be interesting if instead of trying to justify it- since literally less than five guys on the entire forum have even come close to needing IRS- people just said, "I did it because I wanted to." Which is the reason why we've all made the awful, terrible choice of having 50-year-old cars to begin with. We polish turds. It's fun.
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post #23 of 52 (permalink) Old 04-14-2017, 12:22 PM
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Perhaps a modern car beckons?
What are you talking about ? This is the Mod & Custom forum isn't it ? None of us NEED IRS, or a 3-link, or 400+ H.P. What's your point ? Are you the judge of whether I deserve Vintage Mustang ownership ?

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post #24 of 52 (permalink) Old 04-14-2017, 12:36 PM
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I'll just throw this out there from griggs:

http://www.griggsracing.com/pdfFiles...e%20Debate.pdf

I agree with Bruce and also feel like the cost/benefit isn't there to convert a car to IRS. I also agree with the above comment about we do this ish because we want to, not because it makes sense lol.

Do what you want but when guys like Bruce and Mike Maier and such who can run literally build and run anything they could imagine still choose a stick axle... well....
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post #25 of 52 (permalink) Old 04-14-2017, 12:47 PM
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I used the NM12 TBird IRS under my FFR Cobra, basically because it has much less unsprung weight vs the 8.8 live axle and will give a better ride and is more compliant over irregular surfaces. However, the design of the later 2015 Mustang IRS is a much better design and can show handling improvements over the NM12 design.

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post #26 of 52 (permalink) Old 04-14-2017, 01:20 PM
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This is the Mod & Custom forum isn't it ? None of us NEED IRS, or a 3-link, or 400+ H.P. What's your point ?
You realize this is exactly my point and you said the same thing I did, right? Nobody is putting IRS on their Mustang because it's a reasonable thing to do.

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Are you the judge of whether I deserve Vintage Mustang ownership ?
I'd have to see a picture of your Mustang to make that decision.
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post #27 of 52 (permalink) Old 04-14-2017, 01:34 PM
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I'll just throw this out there from griggs:

http://www.griggsracing.com/pdfFiles...e%20Debate.pdf

I agree with Bruce and also feel like the cost/benefit isn't there to convert a car to IRS. I also agree with the above comment about we do this ish because we want to, not because it makes sense lol.

Do what you want but when guys like Bruce and Mike Maier and such who can run literally build and run anything they could imagine still choose a stick axle... well....
Despite Grigg's reputation, that evaluation is a bit flawed, and oriented towards smooth race tracks. I will agree that a stick axle puts power down better, but the Roll Center argument makes little sense. A high RCH reduces compliance on a solid or independent suspension, and last I checked, modern Trans-Am cars have low RCH's ( non-adjustable Watt's links ).

Also, it can't be considered an unbiased review because, while he and Maier could build anything they want, their Products are designed around the more affordable solid axle platform.

We could debate until the cows come home, but my earlier statement stands as proof enough to me: No purpose-built road race car uses a solid axle unless required by the rules.
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post #28 of 52 (permalink) Old 04-14-2017, 01:35 PM
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You realize this is exactly my point and you said the same thing I did, right? Nobody is putting IRS on their Mustang because it's a reasonable thing to do.



I'd have to see a picture of your Mustang to make that decision.
Now I'm even more confused .............

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post #29 of 52 (permalink) Old 04-14-2017, 02:31 PM
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I'm just lol'ing over here about Griggs and affordable being in the same sentence...

I'm unsure on the roll center issue, so take this with a grain of salt, but my understanding is that the RC of an IRS basically functions well in a range that is all lower than the range that a stick axle functions in.... going higher than a certain point causes jacking in the irs, thus you need to be stiffer sprung when using an irs since you can't raise it to the levels that would be considered "low" for a stick...

does that make sense?
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post #30 of 52 (permalink) Old 04-14-2017, 02:57 PM
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You may have something there. Admittedly my Jag E types were using 50+ year old technology, but the rear ends on those cars used a 4 shock and 4 coil spring set up on a very light car, and those springs weren't weak either.

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