IRS vs. Live Axle

[USAF-Sergeant]

It has been talked about quite a bit in many of the threads... but from what I read on Car&Driver's web site... The way the rear has been designed, there may not big a big difference between the two now. I could be wrong and I guess we will have to wait until the release to test drive and find out for sure. I'm am curious though if I am the only one that feels this way. Like alot have said... save the IRS for the Boss (crossing fingers).

I also wanted to ask... if one hand the green to shell out.. what kind of cost and how much work would be involved in changing a Live Axle into IRS? Anyone ever tried this and to what success? Just wanted to put that out there.

 

[rhumbline]

I voted for option 3, though I think ideally Ford simply should have focused its design, development and production resources into just a very good IRS rather than spreading it out over two systems. My apologies to the hard core drag racers, but I think a good IRS offers the best ride AND handling solution for the vast majority of drivers, even if most of them don't know why their car handles so well and rides nicely.

In lieu of that, Ford definitely should offer the IRS they will have to develop for the Cobra to make it a credible world-class performance car in a much wider range of models, especially if the Cobra is going up to the $40-50K price range.

As for retro fitting or designing a custom IRS, that would be rather involved and expensive I imagine.
]
There is an interesting site somewhere where someone dug up the blueprints and specs for what was supposed to be an IRS on the original Stang, which in its original conception was probably more akin to a low-cost, V8 analog of today's BMW 3-Series quite in contrast to a simple big cube straight line muscle car that many conceive the Mustang to be today. But the Mustang really has become an automotive Rorschack test over the years -- is it a pony car, muscle car, sports car, GT cooupe, stylish coupe/convertible, all of the above, some of the above, none of the above????

In any case, the design was clever in using the stock leaf spring mounting points to anchor a leading and trailing longitudinal links per side with a lower A-arm and the up lateral/parallelogram link being the axle itself ala Jag E-Type. The latter and the diff were all mounted on a subframe.

It was a clever design, used a lot of off the shelf pieces and existing chassis elements and would have really cemented the Mustang's intended identity as a sort of low-cost V8 2+2 sport coupe. But the explosive sales and whatnot caused Ford to forego that extra step and it was never put into production.

It was only years later that the Mustang started to evolve a more "muscle car" aspect personna with the advent of the big block 390s in '67 in addition to the very small for its day big-bore, short-stroke, high-winding 289.

 

[63_Fairlane]

BMW's 3-Series is really the best modern equivalent of the 1960s Falcon/Fairlane/Mustang platform and driveline sharing ideal. Using one platform, BMW builds everything from family sedans to high reving road burners. The only real difference in the BMW formula is the use of the I6 throughout the lineup instead of upgrading to a V8 for the more sporting versions. OF course, BMW did build some special racing M3s with the 5-Series V8 for for GT racing when it became apparent that the I6 would not be competitive.

It is a bit unsettling that the same basic formula that served Ford and other US car makers so well during the 1960s is what puts BMW on top today. I know that there is a cost difference, but that is a consequense of lower volume and marketing hype.

I voted for optional IRS.

 

[Mach460]

I've made my feelings known more than often. Even though I feel the IRS should be standard on the GT.....I voted for IRS as option.

It makes everybody happy....the drag racers keep their old tech rear end, and those who want it...can pony up the extra couple of bucks for the IRS.

 

[shatner saves]

BMW's 3-Series is really the best modern equivalent of the 1960s Falcon/Fairlane/Mustang platform and driveline sharing ideal. Using one platform, BMW builds everything from family sedans to high reving road burners. The only real difference in the BMW formula is the use of the I6 throughout the lineup instead of upgrading to a V8 for the more sporting versions. OF course, BMW did build some special racing M3s with the 5-Series V8 for for GT racing when it became apparent that the I6 would not be competitive.

It is a bit unsettling that the same basic formula that served Ford and other US car makers so well during the 1960s is what puts BMW on top today. I know that there is a cost difference, but that is a consequense of lower volume and marketing hype.

I voted for optional IRS.

I believe the 3 series platform is also what they use for the new X3 SUV.

What's interesting about the V8 powered M3 was how they did it. They managed to convince the ALMS people that they were going to produce a road going version the following year. The sanctioning body, wanting to see someone give porsche a run for it's money, ignored their own homologation rules and let them run it anyway. They won the championship, pulled out the following year and never built a road going V8 powered M3. Nice.

I wouldn't say BMW is on top but they are certainly taking bigger bites out of the market. They do this because they build a really well engineered car and the dealers, generally, know how to treat their customers.

BMW has a really great way of keeping its dealers in line with respect to customer service. Any time they come out with a new model, M car, or special edition, the first dealerships to get them are the ones that score highest in customer satisfaction survays, not the ones that sell the most cars or are in the biggest markets. The big 3 could learn a lot from this.

As for the IRS debate, I voted to have it as an option. IRS as standard equipment would have been better but this is a good compromise for the drag racing folks. I also don't see the cost hit being that high for IRS.

 

[SVTdriver]

I probably am over thinking the issue. But I don't think that I could vote for either. The problem being we have no idea how the new suspension will handle. And how much if any the IRS would improve upon that. I can't really vote for IRS as the only choice. Because that to me that says I don't trust Ford to build a good car. So why should I even be looking at it? The other thing I have issue with is tires are nearly as much responsible for handling as the rearend design. And at least the stock tires on my year cobra STUNK. As soon as I changed them things got much better. I appreciate that people think the IRS will handle better. But those people that talk about the car going out of control when they hit a bump. I tend do there after discount what they are saying. The other part of it as an option. Is how would you know if you wanted the option until you test drove them both. Assuming the dealer stocked an IRS model.

 

[Sean`03///Zed4]

I probably am over thinking the issue. But I don't think that I could vote for either. The problem being we have no idea how the new suspension will handle. And how much if any the IRS would improve upon that. I can't really vote for IRS as the only choice. Because that to me that says I don't trust Ford to build a good car. So why should I even be looking at it? The other thing I have issue with is tires are nearly as much responsible for handling as the rearend design. And at least the stock tires on my year cobra STUNK. As soon as I changed them things got much better. I appreciate that people think the IRS will handle better. But those people that talk about the car going out of control when they hit a bump. I tend do there after discount what they are saying. The other part of it as an option. Is how would you know if you wanted the option until you test drove them both. Assuming the dealer stocked an IRS model.

I agree...my current Zed4 and the E46/E36 have/had IRS and I would say while it gives great precision for the rear, not to break loose while accelerating round the apex, great ride comfort as well as handling but there are some core needs for IRS to work very well.

IRS requires a very near 50-50 weight distribution because true benefits are reaped when the chassis is tuned to neutral.

It's sensitive to -ve camber adjustments, ride height and it's an "art" to lower the rear or adjust the camber just right to get great straight line acceleration and have it handle well in the corners.

Because the IRS is independent of each arm movement, the lateral forces cause greater stress on the rear shock mounts and rear sub-frame mounting points.

In fact early edition E36 92-95, rear shock mounts are known to tear through; the M-coupes, 3'er coupes had classic tearing up of subframe mount points right throught the chassis. This problem has plagued the E46 models upto model year 2k as coupes absence of the b-pillar contributes to less torsional rigidity.

Metal fatigue is greater in IRS. The chassis has to be really tight and bound together by upper strut/shock bars.

IRS is dead without necessary sway bar linkage and chassis torsional rigidity, it requires advanced lower and upper control arm geometry as well as location of the rear shocks w.r.t to the springs. The two must work in tandem, i.e one can get away by lowering springs without changing shocks in solid axle rear but in IRS such a decision may become a disadvantage.

If limited slip or ABS /electronic traction control is taken out of IRS, it's pretty much game over (another reason why BMW has always offered either LSD or the latter two as standard equipment)

Solid axle:
Because the axle is solid, the point of power delivery (from the drive shaft) is at the centre means that this is the axle's natural axis of lateral rotation.

So if the outer wheel moves up, the axle naturally plants the inner wheel down. It is helped more so by the inner wheel spring which open up. Placing the rear sway bar links to pass below the axle but linking it directly to the chassis say near the lower traverse arms means during cornering the axle will pull down the inner wheel, while the sway bar will pull down the chassis to reduce body roll.

On the other hand an IRS needs advanced chassis dynamics. aggressive spring rates/shocks to help push the opposing end (the inner end) down in a corner. It gets no help from the axle itself. An IRS must be equipped with a anti-roll bar in the rear with simliar stiffness and thickness as the fronts to dial in neutrality. Well because of this it's more comfortable and literally eliminates wheel-hop.

A solid axle does not have to impart a large upforce on the chassis points above the outer wheel because it naturally pulls the opposing end down.

I will put my money on the chassis stiffness, weight distribution and the front l-arm setup. It should reduce nose dive especially during late braking.

 

[rhumbline]

"I agree...my current Zed4 and the E46/E36 have/had IRS and I would say while it gives great precision for the rear, not to break loose while accelerating round the apex, great ride comfort as well as handling but there are some core needs for IRS to work very well.

IRS requires a very near 50-50 weight distribution because true benefits are reaped when the chassis is tuned to neutral......."

I think most of the points you present regarding IRS are, in fact, pretty much just as valid for a good live axle design too. Much, of course, depends greatly on the overall quality of engineering of either configuration, both an IRS or live axle can be done very well of very poorly, and that needs to be considered in any discussion ("my grand daddy had a '63 Corvair with IRS and its handling was evil, so I don't want any IRS in my Mustang...").

In terms of loads and chassis loading, that really is more a matter of simple proper engineering of suspension pickup points, whether those loads are fed by the links of an IRS or live axle. Indeed, because of the need to use significantly higher spring and shock rates to control the high inertial loads, a live axle design may well have higher stresses than an IRS.

An IRS is more complex certainly, and this can be either a blessing or a curse depending on ones needs.

For those seeking greater overall handling and ride capability/quality, an IRS generally offers much greater flexibility to optimally tune a suspension for a wide range of situations and conditions.

On the other hand, for other folks, drag racers especially, who real don't need much from a rear suspension other than to hold still, straight and together, an IRS's greater complexity will offer no real benefit in that particular narrow realm and in essence is money wasted and just a few more things to break and worry about.

Aside from car enthusiasts, be they corner carvers, strip burners or something else, what is the best setup for the average Joe who neither knows nor cares about what's holding up the arse of his ride? I would submit that a good IRS (see aforementioned caveat about design quality) will offer a better overall ride/handling envelope/compromise. That is, for a given ride quality, an IRS will give better handling over a wider range of conditions than a comparable live axle design. Or looking at it from the other side, for a car with a given level of handling capability, an IRS will offer you a better, more compliant ride over a wider range of road conditions than a live axle.

That all said, I am sure that the live axle in the '05 will be a fine suspension, likely a marked improvement over the current Stang and probably the most optimal for some particular segments of the Mustang market (drag racers).

But is it the best suspension that might be hoped for, expected or wanted for the broader customer base?

That's a question with a lot of variables (cost, use, expectations, competition, etc), but I still think that overall, Ford should have consolidated its design, development, engineering and production and monetary resources into a single superior IRS design that would have best met the needs of the broadest market.

For whatever reasons, and it will be interesting to really find out the details of the '05s development, Ford has chosen to develop two rear suspension designs. Whether this, in the long term, will be a good thing overall (greater choice for all the Mustangs customer bases) or a bad thing (dissipitated scarce resources over two designs rather than a single optimal one) will be something only time will tell.

 

[TrueBlueCajun]

I say put the IRS in as an option. Best of both worlds. I wish the GT would come in a 6-speed or have the option of one.

 

[mp67]

It has been talked about quite a bit in many of the threads... but from what I read on Car&Driver's web site... The way the rear has been designed, there may not big a big difference between the two now. I could be wrong and I guess we will have to wait until the release to test drive and find out for sure. I'm am curious though if I am the only one that feels this way. Like alot have said... save the IRS for the Boss (crossing fingers).

I also wanted to ask... if one hand the green to shell out.. what kind of cost and how much work would be involved in changing a Live Axle into IRS? Anyone ever tried this and to what success? Just wanted to put that out there.

Ford is about making the masses happy with the Mustang and thats why its still here and the camaro/firechicken is gone. From Muscle Mustangs-When it came time to decide on the rear axle, the team spent an inordinate amount of time at media events, Fun Ford Weekends, and talking to its customers. As you might imagine, a very vocal majority told them they needed to keep a solid rear axle in the car. Otherss did not know what kind of axle their car had, nor did they care to as long as it worked.

Remember, it is all of those six bangers that account for a large portion of sales. Hopefully they will offer it as an option on the GT soon, but remember that will cost money. Besides, if they offer it on the GT, next we will want a six speed, and we always want more power so where would that leave the future cobra and hopefully shelby edition?

 

[Sean`03///Zed4]

"..whether those loads are fed by the links of an IRS or live axle. Indeed, because of the need to use significantly higher spring and shock rates to control the high inertial loads, a live axle design may well have higher stresses than an IRS.."

I have to agree with you on this. It missed me perhaps because i have been pampered by the IRS

A solid axle does have a disadvantage in that the front being IFS (A-arm or l-arm Mcpherson setup)and the rear being solid, the nature of chassis compression differs twixt front and rear; thus best benefits from a solid axle cannot be reaped without SFC's..

on second thoughts dang it all...Ford should just go ahead and slap on IRS(because the 05 has a good chassis and neutral weight distribution anyway), but unlike the GTO, the front and rear should have forged aluminum lower and upper control arms, with lightweight springs.

I also hope Ford publishes some torsional rigidity figures for the coupe and the vert.

 

[shatner saves]

I found a website (I am trying to find the link) that quoted a torsional stiffness of 15,500 Newton-meters/degree. I have also found numbers for the E46 3-series cars of anywhere from 13,500 to 16,000 Nm/degree, for the coupe. The BMW Z4 is 14,500 Nm/degree, an amazing number for a convertable (3X as stiff as the Z3).

Hot rod magazine quotes a "global stiffness" of 7000 lbs/degree. I know what torsional stiffness is and how they measure it but I am a bit fuzzy on what they mean by "global stiffeness". The units are different as well. The torsional stiffness is expressed as torque (Nm), the global stiffness is is experessed as a load (lbs). It's likely that it's just a different way of expressing the same thing. Can anyone shed some light on this?

 

[351CJ]

I have not seen and hard figures wuch as Nm/deg, lb-ft/deg or resonant frequency.

But C&D says that the 2005 convertible will have a 100% increase in torsional rigidity.

So this is another path to follow, are the #'s that are being thrown around for the coupe or vert?

 

[shatner saves]

I have not seen and hard figures wuch as Nm/deg, lb-ft/deg or resonant frequency.

But C&D says that the 2005 convertible will have a 100% increase in torsional rigidity.

So this is another path to follow, are the #'s that are being thrown around for the coupe or vert?

Ok,
From: http://www.hotrod.com/featuredvehicles/113_0402_must/index3.html

"Big chassis improvements have also been made to the current Mustang's notoriously flimsy unit-body structure. According to Thai-Tang, the torsional rigidity of the body structure in the convertible model has been improved by 100 percent over the existing model; the global stiffness of the new platform is now about 7,000 pounds per degree of deflection as measured at the wheel as opposed to about 3,500 pounds per degree for the current chassis."

Take 7,000 lbs/deg. (load), multiply it by some distance to get a torque (lbs-ft)/degree and you get some value. How that distance is determined, I don't know. I'm working on it.


I couldn't find the site where I got the 15,500 nm/degree number but I stumbled on this:

http://translate.google.com/transla...torsional+nm&hl=en&lr=&ie=UTF-8&oe=utf-8&sa=G

It sites a torsional stiffness of 21,000 nm/degree (!) for the coupe. That converts to approx. 15,500 lb-ft/degree, so, either I messed up on the units or the site I quoted it from did. That number seems a bit optimistic, considering Ford's claims for the Ford GT:

"The aluminum space-frame chassis is similar to Ferrari 360 in technology but only stiffer. Torsional rigidity is 29,000Nm per degree, surpassing the Ferrari’s 23,000Nm/degree."

...or maybe not. That number isn't so out of line for a lot of new chassis coming out these days, especially considering the cars weight. I don't know.
That comes from: http://autozine.kyul.net/html/Ford_US3.htm

If the new coupe does actually have a torsional stiffness of 21000 nm/deg.

The numbers for the e46 3 series come from a BMW thread:
http://www.bmw-m.net/Digest/Archive_2001_03_digest_1012-1112/digest1102.htm

"static torsional rigidity:
sedan 18000Nm per degree
sedan w/fold downs 13,000Nm
sport wagon w/fold downs 14,000Nm
coupe w/fold downs 12,500
convertible 10,500Nm
An interesting note here is that folding rear seats always entail some loss
of rigidity)."

I can't vouch for the accuracy of these numbers. These could be E36 #'s or the guy who posted them might be a bit unstable.

 

[65conv50]

It absolutely should have IRS to bring it into the 21st century. The ONLY (repeat only) time live axle is better is in competition drag racing, and it is only marginally better then! The V-6 absolutely should have IRS - nobody is going to drag it, but buyers WILL know the better ride and handling of the competitors who do have IRS. Ford can do it, for the same price range: they have done it before. To say it should only be in the GT (as in the survey) is not correct. It is needed in the V-6, the V-8, and the GT. They could have a live-axle option for racers (maybe market an "R" version, like Shelby did). IRS rides better, handles better, absorbs bumps better, and is substantially safer, giving you a better ride in a quieter car, all other things being equal.

 

[GaPonyFarm]

If Ford could have done it within the same price range, I'm sure it would have been offered. Don't give up on the solid axle of the '05... its not your daddy's solid axle setup. I think you might be pleasantly surprised...

 

[351CJ]

Thanks for the info, Shatner_saves. Especially the 3 Series comparison, if all the #'s are right.

I was trying to find the #'s for the Lincoln LS, as when it was introduced, it was the stiffest unibody that Ford had ever made. So far the only thing I have been able to find is the natural resonances 24 Hz bending and 29 Hz torsional. The higher the resonant freq. the stronger it is. IIRC I found #'s for a Crown Vic and they were something like 8 Hz & 12 Hz.

The only other thing I found was in C&D they said that the 05 Mustang vert was 15% stiffer than the 01 T-Bird, which isn't saying much since the T-Bird is suppose to be too flexy, so I wonder if that # is correct.

 

[shatner saves]

Thanks for the info, Shatner_saves. Especially the 3 Series comparison, if all the #'s are right.

I was trying to find the #'s for the Lincoln LS, as when it was introduced, it was the stiffest unibody that Ford had ever made. So far the only thing I have been able to find is the natural resonances 24 Hz bending and 29 Hz torsional. The higher the resonant freq. the stronger it is. IIRC I found #'s for a Crown Vic and they were something like 8 Hz & 12 Hz.

The only other thing I found was in C&D they said that the 05 Mustang vert was 15% stiffer than the 01 T-Bird, which isn't saying much since the T-Bird is suppose to be too flexy, so I wonder if that # is correct.

I'm mot sure if resonant freq. are a good indicator because it is also effected by mass, as well as stiffness. Definitely, the higher the better.

That doesn't surprise me about the crown vic; it is still a body-on-frame vehicle. Ladder style frames generally don't have good torsional rigidity (they are very two dimensional). I found an article on the Dodge Durango that talked about torsional stiffness and it said the body actually had 5X the torsional stiffness of the frame.

http://www.truckworld.com/Sport-Utility/98-Dodge-Durango/Durango.html

From a torsional stiffness standpoint, you could almost think of it as a unibody vehicle with a big subframe that carries the drivetrain as well as the suspension.

The new car is definitely stiffer than the T-bird. I can't remember if it was the CD or hotrod article, but they commented that doing a convertable, 2 door version of the DEW98 meant adding a lot of cross bracing behind the seats. As the mustang is a 4 seat car, this option was not open to them and they had to seriously rethink the chassis to achieve their torsional stiffness targets. The way ford tells it, the new chassis is more it's own platform as opposed to being a variant of the DEW98. It only shares 30% of it's content with other ford vehicles. Considering that number was about 0% for the sn95, that's a big improvement.

It's very easy to get hung up on torsional stiffeness. BMW claims that the new Rolls Royce has a torsional stiffness of 40,500 Nm/degree (Yikes). But, the car weighs 5000 LBS. The Lotus Elise has a torsional stiffness of 10,000 Nm/degree, but only weighs about 2000 LBS. With a car that light, I doubt you'd be able to load the chassis enough to flex. Too bad it'll sticker for $40k.

What's significant about the mustang, is that they made these gains and the car really hasn't gained any weight.

I don't know about the 3 series numbers. I suspect they might be of the e36. I can't seem to find anything on the e46 sedan or coupe.

 

[351CJ]

You are correct, the resonant frequency is only of use when comparing similarly sized vehicles. But the bending in Nm also depends upon size of the unibody, so the #'s only would make sense when comparing similarly sized vehicles.

The #'s for the Crown Vic are for the whole body frame assembly. The Crown Vic really does not have a ladder frame, like a F-150 has, it's a perimiter frame which is even more flexible. Also, IIRC the #'s I gave were for the last version, Crown Vic, I assume the updated one (2002?) is stiffer.

 

[Sean`03///Zed4]

yes the torisional rigidity figures of the E46 agrees to what i have:

sedan 18000Nm per degree of torsion

sedan w/fold downs 13,000Nm

sport wagon w/fold downs 14,000Nm

coupe w/fold downs 12,500

convertible 10,500Nm


and the Zed4 is indeed very rigid. They have added lots of cross member conectors underneath and above the front strut towers...rears are not necessary as the seats are not fold down...

so the new Mustang with 21k+ newton meters of force to twist the chassis through a degree is very very good...defnitely a vast improvement over the current chassis...