Live axels vs. IRS

[daepp]

I know this isn't 911, but it is technical. Too OT?

As far as reasonable priced cars go, the new Mustang has been getting great reviews for some time. It is fast, cheap and handles well. I understand a few models prior to 2007 had IRS. But all new models, including the Shelby's, have a live axle. Do you think this is due to cost only?

It's hard for me to believe that Ford couldn't just order an IRS axle assembly from a supplier (Dana?) or make it themselves for a reasonable cost. Make it an option? Add it to the Shelby's?

Or is it just not necessary. As a kid in the 70's we always sought out the IRS cars...

[Bill Verburg]

Originally Posted by DAEpperson

I know this isn't 911, but it is technical. Too OT?

As far as reasonable priced cars go, the new Mustang has been getting great reviews for some time. It is fast, cheap and handles well. I understand a few models prior to 2007 had IRS. But all new models, including the Shelby's, have a live axle. Do you think this is due to cost only?

It's hard for me to believe that Ford couldn't just order an IRS axle assembly from a supplier (Dana?) or make it themselves for a reasonable cost. Make it an option? Add it to the Shelby's?

Or is it just not necessary. As a kid in the 70's we always sought out the IRS cars...

It is but lets bend the rules a little,

IRS vs live axle debate has been going on forever, and there is no doubt that a well developed IRS has some great attributes, but a well controlled live axle can work well, particularly on smooth roads, is cheaper and in certain circles is a known therefore desirable attribute

This is just me, a burned out piece of toast from the '60s speaking, but I really like the new 5L mustang a lot, just the way it is.

[daepp]

I have to agree. I saw a flat black 5.0 the other day - I think if was a 2010 - it was stunning.

But having driven an 07 GT500, that one didn't handle well at all. I understand the new ones are lighter in the front, but I still can't understand why Ford wouldn't at least option a IRS set up. Or is it just not necessary?

[1QuickS]

As I recall Ferrari used a DeDion axle for smooth tracks on the 1973 312 F1 cars.

[RWebb]

live axles are fine if you never drive on a road with bumps

I prefer IRS, having had an experience with some bumps in a GTO in the '60s, but I am just a washed up lump of bread from back then. My family couldn't afford a toaster...

[Matt Monson]

Having watch the spec series Mustangs run at Miller last weekend I can tell you that I wouldn't want a live axle in one of those cars. The cars that would hit the curbs on the tighter corners were just all over the place afterwards. By comparison, any number of the Continental Tire cars (like Golfs, STis, Mazdaspeed 6s, etc. etc.) could hit those same curbs and not be an out of control freight train.

[Bill Verburg]

Originally Posted by Matt Monson

Having watch the spec series Mustangs run at Miller last weekend I can tell you that I wouldn't want a live axle in one of those cars. The cars that would hit the curbs on the tighter corners were just all over the place afterwards. By comparison, any number of the Continental Tire cars (like Golfs, STis, Mazdaspeed 6s, etc. etc.) could hit those same curbs and not be an out of control freight train.

I think we're mostly talking street use here

[Wil Ferch]

Each has its advantages and disadvantages......

The live "axle" .... has the advantage of being cheap, rugged and has no weird toe, camber or caster changes as it goes up/down ( well...to some degree this is not true, but compared to *some* IRS systems that change toe/camber a great deal with suspension movement, one can say live axle changes very little in this respect in comparison, and the wheels stay fairly perpendicular to ground). However, it is heavy and the differential "pumpkin" has to have allowable movement up and down too...so the masses involved to be controlled are very big. This makes it a bear to control properly on a ripply surface. Lots of unsprung weight to control.

The IRS claim-to-fame was that the up/down movement of one wheel would *not* affect the opposite wheel, as live axle does. But the basic car ads of the time never talked about the camber, toe and maybe even caster changes as each wheel goes up and down, which in itself can lead to goofy handling, even though it moves "independently" . The differential "pumpkin" can also be solidly fixed to the car and becomes "sprung" weight instead of undesirable "unsprung" weight whose movements have to be controlled. This allows a more supple approach to springing/tuning that can be used and still give good ride and handling.

My guess would be that lack of IRS availability for Mustang is based on multi-factors
1.) less cost
2.) roads for the most part in developed countries are good and not ox-cart rough as in the model T days.
3) the advantages of IRS would have you drive the car perhaps near its limits to fully notice its dynamic superiority....and the manufacturers figure most of us wouldn't drive like that on the street, or wouldn't notice.
4.) With proper 4 or 5 bar linkages attached to a live axle ( instead of it being located simply by leaf springs as in the old days where the axle bounced around alot in an unconrollable fashion)......you can get reasonable response from a live axle.

A live axle, with good suspension that controls the "location" of the live axle reasonably well, actually works fairly well on smooth surfaces, where the IRS advantages are lost.

So...that begs the question...what sort of suspension linkages are used in today's Mustang?

[daepp]

Will - great analysis. Unfortunately I don;t know what the current setup is.

Nonetheless, it make me wonder why IRS is in every Ford SUV, the Vette, Bimmers, MBZ's etc if a live axle is sufficient for road use.

[Bill Verburg]

Originally Posted by Wil Ferch

...
So...that begs the question...what sort of suspension linkages are used in today's Mustang?

the '11 Mustang has a 3 link rear axle, coil springs, trailing arms and panhard rod(just like the NASCAR stockers)

the BOSS is back in'12

[Wil Ferch]

Some cars HAVE to have IRS because a large piece....like an ENGINE....is in the way being in the middle where a live axle may want to be !!!..

So...mid-engine cars need IRS, in general.

As to the others...there is no doubt that IRS is "generally" superior since it allows independent action altogether. It's just that early IRS versions were poor, like the early Beetle swing axles that had no inboard CV joints. Camber change was horrible with up/down movement. However, when done *right*... and when applying a lot of thought/engineering....you get the most advantages and least disadvantages of IRS. Some sophisticated cars like Benzes and Bimmers wouldn't think of ever going "backwards" toward a live-axle design once they had IRS before, even if the earlier IRS versions were "poorly done". Their DNA says...."OK, we had IRS to show the advantage of "independence" of wheel movement....now we work on the next problem of limiting toe, camber and caster changes WHILE keping the "independent" advantages of IRS...". That's why you see fairly complicated 5 bar links on EACH side for some IRS cars....to retain independent springing yet chasing down the remaining camber/toe problems, etc. Remember the "Weissach-axle" for the 928? It also was complicated and even used a double-articulated front hinge pivot that would minimize toe change under braking . The "next gen" IRS's like this began looking at the remaining , second-order problems while retaining the earlier advantages of IRS independent springing.

[gtc]

If I recall correctly, the Mustang went back to live axles after 2004 becase people were snapping axles on the IRS equipped supercharged Cobras.

[wdfifteen]

Live axles are simple until you try to make them behave. The pinion tries to climb the ring gear when you accelerate, the brakes try to wrap the axle around the springs when you try to stop - all of this can be controlled, but by the time you add the assorted links and bars to get it under control it's as complicated as IRS.
OTOH I had a TR4A IRS - let's just say IRS alone does not equal adequate handling.

[RWebb]

Wil - re: heavier -- I often people talking about the lower wt. of a live axle. I assume this relates to front engine, rear drive cars.

[racer]

Two words - Drag racing! The solid rear axle is fantastic for this application and well, lets sterotype here, most Mustang drivers are more concerned about the 1/4 mile than driving at SPA-Francorchamps.

It is also arguable that on a SMOOTH surface, such as a racetrack should be, a solid axle shouldn't give up much since there isn't a natural disturbance other than a driver hitting something (like a curb).

I would also imagine that there could be a packaging benefit to the solid axle vs a multi-link independent set up.

Cost, i have to believe considering the production qty, wouldn't make too much difference overall.

[Dantilla]

Originally Posted by racer

Two words - Drag racing!

You beat me to it. A live axle keeps both tires firmly perpendicular to the road under hard acceleration.

[Matt Monson]

Originally Posted by Bill Verburg

I think we're mostly talking street use here

Sure, but the cars they are using in this series are not terribly far off a gutted and caged street car in the sort of trim one of us hot rodders might put one into if we bought our own car for play:
Mustang Challenge // About the FR500S

The point of the comment was that if a track curb upsets them in that way, some of the undulations of a backcountry road or canyon run that one might take in the their equivalent street Mustang, like a GT500 or Super Snake, would introduce the same unsettling I witnessed seeing these cars run in anger.

It's just my bias. I'm all about the curves, and feel that an IRS design is superior for that.

[AlfonsoR]

Does anyone know if the Mustangs that have recently competed on road courses wear IRS or live axle? I thought I heard that Ford was campaigning a few mustangs in ALMS or something similar, but that was 1 or 2 years ago, iirc.

[DanielDudley]

Originally Posted by Bill Verburg

the '11 Mustang has a 3 link rear axle, coil springs, trailing arms and panhard rod(just like the NASCAR stockers)

the BOSS is back in'12

This is a good setup, and a lot of the racers were modifying the IRS Mustangs to this spec. I am not sure what was lacking with the IRS they were using, but this is a popular setup.

[GrantG]

The worst part about live axles is that it is impossible to add negative camber to the alignment. My friend used to track a Mustang with Hoosiers and even with very stiff springs he would wear the outside shoulders of the tires to the cords before lunch.