What are the advantages of solid live axle suspension

[Grimace427]

Yes, no, and maybe.

IRS cars have a much easier to tune roll center height which can create more geometric anti-roll (for lack of a better phrase) whereas it is a little more difficult to tune that with a live axle. On the S197 the PHB's center point defined the majority of the rear roll center and it was reasonably high from the factory but it didn't leave much in the way of adjustment or tweaking. A factory IRS wont have much adjustment for roll center height either but the methods of changing it are much less complex and generally cheaper. The price of ball joints or offset bushings as compared to a watts link or custom PHB mounting configuration drives that point home quite well.

Live axles generally have much, much, much lower spring rates than IRS cars and this has to do with the additive nature of the spring rates in 2 wheel bump. It doesn't take much spring rate on a live axle car to drastically change how the car behaves and that goes back to the additive nature in 2 wheel bump and how that impacts body roll during cornering. Since an IRS is independent (mostly anyway) they will run more spring rate to compensate for the loss of roll resistance, or higher rear roll center or any combo of those. For instance, the Boss 302R or 302S race cars shipped out with 600lbs/in front spring rates and 300 lbs/in rear spring rates on coilovers front and rear. In contrast, my buddy's Evo IX SE's Ohlin Coilovers came with 500/400 spring rates. He runs 700/600 now so way more rear spring rate than any live axle car should have. Part of that is the motion ratio difference but his wheel rates were higher in the rear than the Boss 302R and 302S Race Cars in a factory street car.

The ride thing with an IRS comes from unsprung weight and it's effect on the chassis. In a live axle, all 250-300lbs moves upwards which requires a really well valved damper to reduce the upward jolts in the chassis. You'll notice in the 2013 GT500 reviews the reviewers complained about the vertical chassis movement over bumps... it's hard to defeat physics and momentum and the effects can only be mitigated, not removed. By contrast, the unsprung weight of an IRS unit is something like 10x less and thus there is less momentum carried through the springs to the chassis. That results in better ride quality. Damping wise, a live axle car tends to have more rear compression damping which is necessary to control the 250-300lbs of rear axle where an IRS will have far far less and need far far less.

Very cool info, thanks.

When we talk about lowering the S197 and the affects on geometry, we talk about other necessary mods to correct pinion angle, bump steer, LCA angles, so on. Now that an IRS is out back and double front lower control arms, what things will people need to consider when lowering or otherwise modifying the suspension? I expect similar changes in the front regarding bump steer, don't know how to change the ball joint angles compared to the single front LCA. Out back bump steer will need to be considered with the toe link, but simply getting extended ball joints won't work with the IRS rear lower control arm.

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[shelbystang]

So now that the 2015 Mustang will have IRS they still haven't listed the price on car. We can only expect it to cost alot more since it will cost more to produce?

 

[Grimace427]

Ford began the design process of the new S550 back in 2009, plenty of time to examine the production process to keep costs within reason. I expect base prices to go up some, how much would depend on how much standard equipment they include.

 

[Whiskey11]

Very cool info, thanks.

When we talk about lowering the S197 and the affects on geometry, we talk about other necessary mods to correct pinion angle, bump steer, LCA angles, so on. Now that an IRS is out back and double front lower control arms, what things will people need to consider when lowering or otherwise modifying the suspension? I expect similar changes in the front regarding bump steer, don't know how to change the ball joint angles compared to the single front LCA. Out back bump steer will need to be considered with the toe link, but simply getting extended ball joints won't work with the IRS rear lower control arm.

Basically the front is the same. The S197 never suffered from much bumpsteer issues when lowered, only when using a taller ball joint but like all strut based suspensions, lowering the car is going to change where in the camber curve the front suspension is. The fix is going to be ball joints, maybe two... I don't know, the dual ball joint setup is going to change some because the tension link has a different ball joint configuration (it's on top of the spindle instead of the bottom of it).

The rear should be pretty much the same as the front only we have upper and lower control arms we can play with lengths and angles of the control arms. The camber curve, toe curve, etc are going to be changeable, maybe not from the factory but certainly with the aftermarket. The rear alignment should be changeable. The front is probably changeable too.

We will know more when the aftermarket starts releasing parts and we can actually analyze what the suspension is doing first hand. This is all guestimation at this point.

 

[shelbystang]

Yes to both.

Cheaper to fix because it's a simpler design, which also means it should have less problems on the whole. Also much more durable than IRS. It can take a beating.

As for the lighter part, live axle can actually be 85-150 pounds lighter than IRS.

Perhaps this may the reason my 1988 Mustang GT in factory condition has lasted so long and running good.:clap2:

 

[Stuntman]

The price of ball joints or offset bushings as compared to a watts link or custom PHB mounting configuration drives that point home quite well.

WL are expensive but aftermarket brackets to relocate PHB are not and its only 1 bolt to change the RC. Its pretty easy to change the RC in a PHB layout and difficult in a WL.

 

[Whiskey11]

WL are expensive but aftermarket brackets to relocate PHB are not and its only 1 bolt to change the RC. Its pretty easy to change the RC in a PHB layout and difficult in a WL.

It's still only one bolt on a Watts Link setup:

At most it is two maybe three:

The rear RC on a watts link is essentially the center pivot bolt of the watts link. The Fays2 Watts link is $650. Not horrible considering the changes it makes to the car in comparison to a PHB configuration. Once the Watts link is setup, you don't have to change anything except the center pivot point.

Also, I have yet to see a PHB relocation brackets that allow adjustment in the manner you describe for the S197 chassis... I've seen ones that move the PHB away from the axle but not up or down. The older chassis' benefited from the aftermarket where the PHB was superior to the quadrabind mess.

 

[Stuntman]

You can actually drill a hole ~1" lower than stock on the factory axle mount to lower the RC a little for free. A quick search came up with this kit, however you can mix and match the R&L brackets for a lot of adjustment of the RC. I'm sure you can find just the brackets somewhere:

http://store.kennybrown.com/product...-kit-heavy-duty-double-adjustable-panhard-bar

 

[shelbystang]

Maybe the 2014 Mustang GT is the one and the last one to get. I prefer the styling so far on that car vs. the new 2015 but I have only seen the pics and haven't seen the 2015 in person. When do the 2015 models come out?

 

[Grimace427]

Maybe the 2014 Mustang GT is the one and the last one to get. I prefer the styling so far on that car vs. the new 2015 but I have only seen the pics and haven't seen the 2015 in person. When do the 2015 models come out?

http://www.mustang6g.com/forums/showthread.php?t=1104

 

[UOP Shadow]

Perhaps this may the reason my 1988 Mustang GT in factory condition has lasted so long and running good.:clap2:

Mine too .

 

[shelbystang]

Guess the advantages of LSA are that it keeps costs of repair and initial price of car down. Suppose to be better for drag racing???:shrug:Still waiting to see what the 2015 Mustang GT will cost? But I like the look of the 2014 much better especially those tail lights.

 

[Norm Peterson]

Wheel hop is curable, I'm not sure what you mean by powering the wrong wheel in a corner though

An IRS doesn't suffer from the effects of driveshaft torque, where the RR unloads and the LR gains load. This behavior is also somewhat asymmetric, with inside rear traction on a stick axle car suffering more in right turns where the roll of the sprung mass also unloads the RR and plants the LR. I guess if anything, an IRS car doesn't need as much bias ratio in its limited slip mechanism, which at least sounds like a good thing.

Seriously after all these years of complaining that the Mustang needs IRS it finally gets it and people want to know the advantages of a solid axle?

The devil was in the details, and a well done stick axle has been at least the equal of and was frequently superior to an indifferently or poorly done IRS. At least on smooth pavement.

When we talk about lowering the S197 and the affects on geometry, we talk about other necessary mods to correct pinion angle, bump steer, LCA angles, so on. Now that an IRS is out back and double front lower control arms, what things will people need to consider when lowering or otherwise modifying the suspension?

Rear camber and camber gain
Toe steer (which is roughly comparable to stick axle roll steer except that the two IRS wheels don't have to have identical toe changes and usually don't).
There may or may not be any adjustment reasonably possible for anti-squat (roughly analogous to LCA and UCA inclinations in a stick axle suspension) or the rate at which it varies with suspension bump/rebound travel.
Pinion angle becomes a non-issue since both the transmission and the differential housing are chassis mounted (i.e. no suspension motion interfering with the angular relations here). But there will be changes in half shaft inclination and half shaft U-joint angularity to consider, and those U-joints will be carrying torques roughly equal to [final drive ratio] x [driveshaft torque] divided by two. A bit more than that if the LSD is doing its job.

WL are expensive but aftermarket brackets to relocate PHB are not and its only 1 bolt to change the RC. Its pretty easy to change the RC in a PHB layout and difficult in a WL.

Varying the RC height with a PHB means you need to relocate both PHB fasteners, else the PHB will pick up some inclination and the car will become slightly (slightly more) asymmetric in its handling under some conditions.

This wouldn't matter in street driving, but if that's all the car would be used for you wouldn't be bothering to play with roll center heights (and axle steer) anyway.


Norm

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