Test driver Dave Royce answers questions about driving the IRS.
First race win for the newly-IRS-equipped MM American Iron racecar came with the first race entered, and in the process a new AI track record was set at Buttonwillow Raceway. After the race, MM driver Dave Royce answered some questions about the experience: He compared driving the MM racecar with the IRS to driving the same car when it had a Solid Axle, Torque-Arm, and Panhard Bar (SATAPB).
Q: Were there any differences during corner entry between the SATAPB and the IRS?
A: Corner entry is changed in a way I didn't expect. Like with the SATAPB, the car has great initial turn-in. However, once the car is set, it seems more stable from turn-in to mid-corner, or the apex. This allowed for faster corner entry and earlier throttle application.
Q: How does the IRS compare to the SATAPB during mid-turn?
A: Long, high-speed turns are always a challenge in a Mustang, and the entry and exit play a huge role in how well the middle of the turn works. However, sticking to the question; Mid-turn is quite good. With the SATAPB, I would have to wait just a little, through the middle, and then use the incredible launching capability of the Torque Arm to get great straightaway speed. With the IRS, I was able to apply the throttle just after turn-in, and maintain some throttle application throughout the middle of the corner.
Q: How well did the IRS cope with transitions, such as the quick right-left-right changes found in S-curves or slaloms.
A: The key to a great handling chassis is how it transitions, or handles the weight transfer. The IRS maintained composure throughout the esses. I was able to run full throttle through the esses, and would often touch the curbs without having to apply corrective inputs.
Q: Was there any wheel hop during the standing start?
A: The launch was probably the most surprising to me. I had done zero launches with the IRS, so I was ready for one of two things; either a mild to severe wheel hop, or a hazing of the tires. I was sure it wouldn't just free-spin, as it had decent grip out of the low speed corners, so I put that possibility out of my mind. Much to my surprise, the car hooked up with absolutely no wheel hop. I believe the tires were hazing, or slightly spinning, but I was able to keep rolling on the throttle until it was time to pull second gear, and so on.
Q: Compare the SATAPB and the IRS in how the car reacts to hitting berms, and other variations in the track surface, such as large bumps or dips.
A: When hitting "berms" the SATAPB, or more specifically straight axle, as I don't think the Torque-arm and Panhard Bar is a major contributor to this change, would usually set the back of the car over, or upset the balance to the point where I would have to induce correction. The IRS does what you would expect. It leaves the outside tire with full contact, while the inside tire rides over the berm. This was very noticeable in high-speed corners where there is a small steep berm that usually must be avoided. I was able to allow the inside rear tire to be on the berms, and not have to lift off the throttle.
Q: How was the braking with the IRS, compared to the SATAPB? Was there any rear brake hop?
A: Braking was a little better. The biggest advantage was the ability to do more trail-braking, deeper into the corner. No detectable wheel hop under braking was experienced.
Q: What were the best features of the car with the IRS?
A: Terrific transition and mid corner grip. I am able to roll onto the throttle earlier, and have the ability to negotiate berms and curbing without upsetting the car.
Q: People often complain about wheel hop during acceleration with IRS cars. Was this a problem you saw with the MM race car?
A: With the setup from MM I completely forget that IRS has a wheel-hop issue. In fact I can say there is no longer a need to associate wheel-hop with IRS, at least with the set-up I drove from MM.
Q: How did the MM car, in its current IRS form, compare to other road race cars you have driven?
A: The MM Mustang was comparable to the GT3 Porsche in a lot of ways.
Q: Did the MM products on our IRS cure any issues or complaints you observed when testing any other IRS-equipped cars?
A: Yes. Some IRS cars have undesirable bump-steer. That was not a problem with the MM IRS.
Q: What difference did you notice, between a car with MM IRS products and an unmodified, stock Cobra IRS?
A: As compared to the stock Cobra, with a street set up, it's night and day. The OEM car seemed to float around in the middle of the corner, and exhibited unpredictable traction or wheel spin. The MM suspension, felt positive. Mid-corner predictability seemed vastly improved.