Your thoughts on IRS

[bluebeastsrt]

I remember all the S197 guys proclaiming the IRS is garbage for drag racing. Now those IRS cars are running 7's on a stock super 8.8, whereas the s197 guys need serious upgrades to the diff to run the same.

If you’re building a purpose build drag car. You can’t beat a good old Ford 9 inch for drag racing. But the IRS cars are doing pretty good.

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[Matthewneedforgreen]

I have had other cars with the solid rear, and I liked how easy it was to break it lose in a corner, however the grip that I have with my 2019 PP1 in the turn is out of this world and I love curves.

 

[RazzaRossa]

Just slap this baby in there, good for 10K HP...

 

[Doc2020GT]

Soon we’ll be racing for pinks.

 

[bluebeastsrt]

My 95 had a 9 inch.

 

[RazzaRossa]

Heck, even Chevy guys new to run the 9 inch....

 

[bluebeastsrt]

Heck, even Chevy guys new to run the 9 inch....

Haha quoted for truth.. But we ran their transmissions so everything evened out in the drag universe.

 

[Hack]

For any given amount of wheel rate (springs) and sta-bar stiffnesses, IRS cars generally do roll a bit further at any given cornering g level. Has to, since the IRS's geometric roll center is located several inches below production stick-axle geo roll centers. This also helps make the IRS car "feel" softer.

There's somewhat more going on as well. Like how much the suspension stiffnesses affect the progression of understeer while roll is in the process of developing. Cars with higher rear geo-RC heights initially start out being slightly "looser", with the forward bias of roll stiffness dragging that in the understeerish direction as roll develops. More than you'd set up an IRS car to do. What happens between the initial steering and the final attitude of the car in roll matters - transient handling is by nature dynamic and (hopefully) brief, but it's something that you absolutely should be feeling/sensing from the driver's seat.


Norm

The newer IRS Mustangs function better controlling roll in a corner than the older SRA cars do. All the way from the base to the top handling models. All of them work better in roll now with IRS. That's why a 2015 PP1 GT that rides like a Cadillac is faster on a road course than the higher hp, stiffer suspension Boss 302 was.

Basically with a SRA the car can't handle irregularities in the road very well, or it can't control body motion well. IRS can do both quite a bit better.

I think the angry guy I replied to was talking about a drag car with the sway bar disconnected, but I'm not sure about that. That's why I asked the question. I think for drag racing the SRA might be the superior option.

 

[Norm Peterson]

The newer IRS Mustangs function better controlling roll in a corner than the older SRA cars do. All the way from the base to the top handling models. All of them work better in roll now with IRS. That's why a 2015 PP1 GT that rides like a Cadillac is faster on a road course than the higher hp, stiffer suspension Boss 302 was.

That's at least partly because the geometric roll center heights are more similar in the IRS. Meaning you don't need the front to rear balance of springs/bars/dampers to be quite as understeerish in order to end up at the same place TLLTD-wise at steady-state.

Plus, you can now dial in rear negative camber. Some of this is to oppose the effects of the sprung mass rolling, but some of it is also opposing camber change that effectively happens as load transfer compresses the outside rear tire and allows the inside rear tire to relax back toward its unloaded shape (this effect is typically worth something like half a degree per lateral g, maybe a bit more).

Basically with a SRA the car can't handle irregularities in the road very well, or it can't control body motion well. IRS can do both quite a bit better.

IRS can do both better, but simply "having IRS" does not guarantee it. The details and the geometry still have to be done up right, and I can tell you that that has definitely not always been the case.

I think the angry guy I replied to was talking about a drag car with the sway bar disconnected, but I'm not sure about that. That's why I asked the question.

What he said was irrelevant as far as I was concerned - I was just going for a straight tech explanation of the differences that should be expected.


Norm

 

[Hack]

What he said was irrelevant as far as I was concerned - I was just going for a straight tech explanation of the differences that should be expected.

Norm

Got it. I wasn't sure what you were saying about equal springs/bars and wheel rates WRT theory. That's why I was trying to distill down to functionality in the SRA Mustang vs. 2015+ Mustang. I think the specifics for the Mustang platform address the OP's question.

 

[526 HRSE]

VW Bugs went to IRS in 1969. Now you know.

 

[Norm Peterson]

VW Bugs went to IRS in 1969. Now you know.

They always had IRS.

Swing axles are, in fact, a type of IRS. Just not a very good type, with serious limitations.


Norm

 

[Norm Peterson]

Got it. I wasn't sure what you were saying about equal springs/bars and wheel rates WRT theory. That's why I was trying to distill down to functionality in the SRA Mustang vs. 2015+ Mustang. I think the specifics for the Mustang platform address the OP's question.

I went back to post #1, and without intentionally trying to be harsh . . . OP should be able to feel significant differences while cornering (if he's driving with enough "enthusiasm" to start building much in the way of slip angles, anyway), and that's before even thinking about stick-axle behavior over bumps and other road roughness.

I'm not at all sure how to describe the differences, other than IRS having a slightly softer 'feel' in roll.

My WRX is sprung at least 25% firmer than my S197, which now has wheel rates comparable to a GT350R, and the WRX still has a softer, maybe smoother, cornering feel about it.


Norm

 

[Hack]

I went back to post #1, and without intentionally trying to be harsh . . . OP should be able to feel significant differences while cornering (if he's driving with enough "enthusiasm" to start building much in the way of slip angles, anyway), and that's before even thinking about stick-axle behavior over bumps and other road roughness.

I'm not at all sure how to describe the differences, other than IRS having a slightly softer 'feel' in roll.

My WRX is sprung at least 25% firmer than my S197, which now has wheel rates comparable to a GT350R, and the WRX still has a softer, maybe smoother, cornering feel about it.


Norm

I tend to agree that if you drive with enough enthusiasm you will notice the differences. And I don't think you even necessarily even need to drive with enthusiasm to notice. But it's common for the roads here to have a lot of imperfections.

I think some people just like to ask questions and have a conversation. I've seen enough posts about what color car someone should buy, etc. to not be surprised by it.

 

[Sivi70980]

I had a 2011 GT500 so not exactly the same as my 2019 GT but the GT is faster out of the box soooo. Anyway, the GT500 was great going straight but zero confidence around a corner. Also beat me up as a daily driver. The much newer GT with IRS just feels like a class above. Doesn’t have the punch of the supercharged GT500 but it feels much more refined and light years more confidant in the corners.

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