What's bad with a solid rear axle?

[g8gxp]

Sorry if this is a dumb question, but what's so bad about a solid rear axle? I keep seeing these threads about how the new 'stang is awesome if only they would change the solid rear to an IRS. Is it a noticeable difference in daily driving? or is it just a racing thing? cause from what i understand a solid rear is more reliable for putting down power in a straight line - most of the kill stories i see here are dig or roll races, and anybody who is tracking these cars gets suspension mods anyways.

Is a solid rear really that bad, or is it just a gm vs. ford thing?

[SRG963]

Here ya go


IRS is much better at keeping the car tracking in the same line over bumps, because when one tire moves over a bump, the camber on the other wheel isn't affected, so grip is retained. In a live axle system, both wheels lose a little grip over bumps. Modern live axle systems are getting close to IRS in performance, but will never meet it (its inherent in the design). For drag racing though...live axle all the way!

There are many reasons that IRS is better. The main thing is that the Camber Gain curves can be controlled. When you are in a corner the wheel will tuck up and through the camber change you will be able to maintain an optimal tire contact patch on the inside and outside wheels. You also have the advantage of being able to build in bump steer should you want to. So for example you can get the outside wheel when it compresses to toe out and help the car rotate, it will also make the slip ratio slightly higher on that tire allowing you to get some more power down off the corner. The advantage that Solid rear axles have is that they can build their anti-roll into the rear axle; but any IRS with any form of anti-roll will beat a Solid rear axle on a track... Plus, the calculations you have to do on a solid rear axle to figure out wheel movement are retarded and require iteration.

[Eidolon]

Jason Kavanaugh at Edmunds InsideLine also weighs in on this with his blog post A Live One (http://blogs.insideline.com/roadtests/2011/02/2011-ford-mustang-gt-50-a-live-one.html).

[todds87ss]

Rent "My Cousin Vinny". Best discussion ever regarding the IRS! :)

Seriously, IRS has it's good and bad points. Solid rear axle is cheaper to fix, is somewhat less prone to wheelhop, and easier to modify to support real HP.
IRS handles better in the twisties when setup correctly.

[Rayvan]

Another reason is ride quality. Lots' more un-sprung weight with a live axle, so when you hit bumps at high speeds, you'll notice that the front-end takes larger bumps with ease, then when the rear wheels hit it, the whole car goes "boom" as the shocks send all that energy they're trying to control (from the motion of the axle) into the chassis...

[Virus]

Solid is better on the drag strip :)

[GeorgeInNePa]

Considering that the new GT 5.0 is .02 slower than an M3 around a track, not much is bad with a solid axle. (<According to MotorTrend)

;)

I went for the Mustang specifically because of the engine and the fact it had a solid axle.

IRS= Do Not Want Anymore.

The people that say an IRS car handles better than a swing axle, most of the time they are parroting what others say. Many couldn't drive either hard enough to be able to demonstrate the difference in either design.

I wanted SRA because of the strength and the fact that it will be cheaper to fix when I break something, or cheaper to beef up beforehand.

[locrzn#92]

Only way to drag race and not break shit is to have a solid axle. You can buy a complete solid rear with the best stuff from strange, currie, etc. for about 2500.00 for an fbody. Just a bad ass set of axles from gforce are 1800.00. I can tell you from my vet I just sold last fall, irs is not for high hp cars that drag race. You will break shit and its expensive to fix. I will eventually set up a solid axle for the g8 if I decide to make more hp. I'll take it to a chassis shop and have it done and then won't have to worry about it anymore. You would probably spend about 5-6k or so having it converted to a solid axle. Somebody will probably do it this year I'd bet on it.

[00 Trans Ram]

I road raced a solid rear in my 2000 Trans Am. And, I competed against mostly IRS cars. I also researched putting IRS in the TA (it's possible - anything is, if you cut/weld enough).

To say that "an IRS is better than a solid axle" is like saying that beef is better than fish. Certainly I'd argue that a Kobe porterhouse is better than a Filet-O-Fish. However, I don't think that a skirt steak is better than blackened redfish.

Some IRS designs are good, and some suck. C4 Corvettes have terrible IRS designs. They have very little camber gain, so that the inside tire in a turn can actually have only the edge of the tire making contact. And, they have significant bump steer issues that they have to make up for with heavy spring rates, which limit body roll, which limits ultimate cornering G's (body roll is bad for transitions, but good for G's). The 2nd and 3rd gen RX-7s have great IRS, as does the C5. Nissan 240s have OK ones, but not great. BMWs are pretty good, but most FWD cars suck (notice how they carry a wheel around corners occasionally).

A solid axle has some major advantages on a smooth racetrack. If it's smooth, then there are no bumps to make one tire affect the other. In a solid axle, both tires are ALWAYS perpendicular to the ground. The outside tire will have more weight on it, but the full tread of the tire is on the ground. In an IRS car, the inside tire has very little rubber on the ground.

I saw this at my local track (Circuit at Grand Bayou, formerly No Problem Raceway). We had a few turns that had major bumps (exit of turn 5, if you want to look at a map). My car was horribly slow there, because the backend always wanted to step out. I'd lose ground on IRS cars. However, when we got to the smooth turns (Turns 8-9), I could carry 10-15mph more than an IRS car. I'd fly by them.

As mentioned, a live axle is also better at putting power to the ground. When IRS accellerates, weight is shifted to the rear. As the rear squats, the tires gain camber. Camber gain means that the outside of the tire lifts, and only the inside is on the ground. This means less tire on the ground. With a live axle, the entire tire has traction.

So, which is better for turns? If you have the right car and lots of money (suspension parts and diffs are more expensive), an IRS is better. If you have only a little money, then solid axle is better.