IRS or Solid Axle for roadrace/autocross?

[Sharad]

Let's just pretend that I actually have the ability to keep my '99 Cobra for more than about three days before I decide I want something else. Let's assume I want my '86 to be the drag car and the '99 to be my handling car...

For occasional autocrossing and open track school events, is the IRS more trouble than it's worth? Seem like a procharger would put this car around 500rwhp. That makes me lean toward a solid axle, but this car will never sit on slicks. IRS seems like a no brainer, but I thought the AI and AIX guys usually use a solid axle?

Thoughts?

[Black Hole]

If you build it correctly, either setup can "handle"....although I think the IRS is the better for a more comfortable "street suspension".

My 03 is a street car.....always will be. And as long as I don't change my mind about that, it will keep the IRS.

HOWEVER, if I was building it to race....straightline or opentrack, I'd probably still go with a solid rear. JMO.

Dave should be able to give some good feedback...he has both. ;)

[Sharad]

This car will probably NEVER see slicks. It will NOT be a race car. It would be cool if it was an all around car though...

I like that it doesn't have a "rear end snap" coming out of corners. (there is a "clunk" though.. I'm hoping the spare tire is loose)

If Md says a 31 spline diff and half shafts will make this car handle 500rwhp, that would definitely be the route to go... unless Dave says IRS sucks on the open track. See my dilemma? :lol:

[Gene]

I have done both as well...in the same car. ;)

The stick axle is still more developed than the IRS, so if you're looking at building an all-out race car, then I'd do a live axle/torque arm combo.

But if you're going to be just using it as a fun AX/OT car with street use, just keep the IRS and throw a bunch of MM parts at it. They've done quite a bit when it comes to making the IRS work.

FWIW, if you're not going to be drag racing it, you don't really even need to upgrade the diff and half-shafts. Drag launches are what cause them to break, open track is very easy on those parts.

[Black Hole]

The 99's did have the weakest of the IRS's....28 spline shafts I believe.

99-01-03....each year got better and stronger, but the overall design is the same.

With my 01, I had FRPP 4.30's in the rear, stock 31 spline half-shafts, and a stock suspension aside from some cut Eibach Pro-kit springs and some MM poly bushings where the IRS subframe connected to the frame. Launching at 6k on 275-40-17 BFG DR's I was netting 1.70 sixties with just popping the clutch......had a good 25+ launches without a problem. Brandon broke an axle on one of his first passes in his 03 (wheelhop after launching at 6k on MT DR's).

My 03, with MM subframes, a Billetflow IRS brace, and Eibach springs has been launched as high as 6k on 17" MT DR's....no problems other than another clutch going out. :rolleyes:

Wheel hop is the killer, not so much axle twisting torque although I have seen pics of twisted halfshafts. Good tires are key, especially in cold weather.

I had a "clunk" in the driveline after Chad and I did my gear in the 01.....I also had a slight vibe between 80-100 mph. I also used to experience a "suspension twisting" effect when at high RPM's at the shift.....like there was some energy that was released on the shift, but one characteristic of the IRS is that it usually stays "home" even if you get a lil loose.....

With my 03, I do have a much smaller vibration over 70 mph, but it's just enough to vibrate the rear view mirror.....not car shaking or anything. I have MM subframes, a Billetflow IRS brace, and Eibach springs as well, so that helps with keeping everything tight. My biggest problems are wheelspin, bad gas mileage, and a non-shiftin fool behind the wheel. :D