IRS: A Wheel Hoppin’ Good Time

In 1999, Special Vehicle Team (SVT) at Ford put in an independent rear suspension (IRS) in that year’s Mustang Cobra model, and this suspension setup remained in the Cobra through 2004. This setup is used in many other high performance vehicles, such as the Corvette. Mercedes and BMW also use it in many of their vehicles. The IRS is known for its fantastic ability in cornering around the race track. However, it is also known for wheel hop, also called tramp, which is most undesirable, unsafe, and bad for anyone’s investment in a car. Is there a solution for wheel hop in IRS equipped Cobras? We will examine costs, benefits, thoughts, and opinions about the IRS from around the web, so that Mustang enthusiasts will be able to determine if the IRS is for them. We will consider things like the involved factors leading to wheel hop and proposed solutions. We will also consider financial costs involved given a few scenarios. In the end, we will see that there are many opinions as to the costs and benefits of the IRS, and it really only comes down to a matter of personal preference and driver style. Before we examine the costs and benefits of having IRS, let’s take a look at wheel hop in action.


Costs

Broken Half Shaft, Result of Wheel Hop

Wheel hop not only makes driving dangerous but it is very hazardous to the drivetrain. As we will see later, one proposed solution is to get tires with better traction, because if the wheels don’t spin, there is no wheel hop. However, this solution means a sacrifice in burnouts or tossing the car sideways. Thusly, wheel hop comes with a cost of more conservative driving apart from aggressive cornering to prevent damage to the drivetrain. Read more at Modular Fords, which is where the picture on the right was found.

Benefits

The Auto Channel highlights the benefits of IRS in the Cobra:

The ride and handling characteristics of the Cobra were pretty great before–and now are world-class in every respect. Important ride and handling benefits are derived from IRS and the modified MacPherson strut front suspension. Steering response and on-center feel are improved–including a reduction of the turning circle to 38 feet [previously 40.8 feet (Edmunds)]–critical to the enthusiast driver.

Along the way, Team Mustang made the rear track wider by 1.2 inches, uninterrupted suspension travel was increased, and weight distribution was improved. The Cobra’s handling is especially refined on uneven or bumpy road surfaces and rear-end lift under hard braking is reduced.

In sum, the IRS provides better handling, steering, and overall control. One problem with live axle setups is that a bump in the road can throw the car in the direction of the bump. The IRS is not as nearly susceptible to such road conditions, making for a much improved driving experience.

Thoughts

One website claims to have engineering solutions for wheel hop in IRS setups and not just “band-aids” or mechanical masks. It notes how Mercedes, BMW, and others use traction control to mask wheel hop, but it really isn’t a true solution. It notes how GM has apparently solved wheel hop in the Camaro and CTS-V by using different sized axles from left to right, preventing frequency resonance that seemingly causes energy to disburse through wheel hop. One engineer suggested stiffening engine mounts even for an IRS setup. Others have suggested, as many have before, it is due to bushings. What causes this wheel hop in terms of real world road conditions? What kind of masks are there or actual fixes for Cobras with IRS? Is swapping to a live axle a viable solution? Let’s first examine the cause in the real world.

Cause and Effect

According to the same aforementioned website, cars that have a sizable amount of rear wheel horsepower will commonly experience wheel hop “when accelerating out of corners in damp conditions.” On a different website, one person suggested wheel hop is the result of cold or poor tires with too much throttle. At Corral, another individual thinks it is related to an unbalanced wheel. It was reported at StangNet that an ’03 Cobra experienced severe wheel hop when launching at 3,000 RPMs. More causes of wheel hop and be explored here.

No matter what the condition, wheel hop is something that is destructive and dangerous. Given that it is a common issue for all IRS equipped vehicles, Cobra or not, some solutions must be utilized. For Cobra owners, refer to the Cobra FAQ, which provides links to the following two solutions: swap to a live axle setup or properly build up the IRS. Let’s look first at building up the IRS.

IRS Masks

One person at StangNet reports there is no solution for wheel hop, only masks or band-aids. However, according to SVT Performance, there are are up to 11 steps that can be taken to “dramatically reduce wheel hop on all surfaces.” According to Full Tilt Boogie Racing (FTBR), a combination of tire selection and IRS components cause wheel hop, so there are only two main steps needed to stop it. First, strengthen the IRS and make it rigid by removing polyurethane bushings with stiffer, non-rubber material. Second, choose a tire that does not have stiff sidewalls. Strengthening the IRS involves more than just the removal of rubber, as FTBR later discusses in its FAQ, so we will return to the 11 step path proposed by SVT Performance. The table below shows the steps and costs:

Step Modification Cost
1 Billetflow differential cover brace $160
2 Maximum Motorsports differential bushings $50 or $250
3 Maximum Motorsports full length subframe connectors $250-300
4 Maximum Motorsports IRS bushings with 14mm bolts/nuts $65
5 Billetflow IRS mount brackets $50
6 Used coupe springs (vert only) $50
7 Nitto drag radials $375-425
8 Paul’s High Performance toe link bars $425
9 Maximum Motorsports upper and lower control arm bushings $550
10 Level 5 shafts $1500
11 Strange ADJ Shocks from HBH Billetflow mount bushings $168
Total $3928

It seems that the 11th step is ill-advised as it can break some of the components in the IRS and actually ruins the ride quality. One individual at StangNet suggested not bothering with Level 5 shafts due to the cost and the possibility of them still breaking.

It seems that $3500 to continue to use the IRS is steep and silly. Even if steps 1-8 are followed, it will still cost $1725. These steps, this cost, is not even a guarantee to resolve wheel hop. It may improve the effect, but it may not completely get rid of it. How does this option compare to a live axle swap?

Live Axle Swap

Wheel hop is not exclusive to the IRS. Even in live axle S197 Mustangs, wheel hop has been reported, such as at All Ford Mustangs. Quad shocks have been used in the SN95 Mustangs to prevent wheel hop; for the most part, especially as it pertains to New Edge SN95s, wheel hop is not an issue for these live axle Mustangs. As such, many people have suggested swapping the Cobra’s IRS for a live axle, such as one person at StangNet.

To have a Mustang shop swap the IRS for a live axle could cost around $5000, but if you do the work yourself the parts would likely cost around $3000 according to SVT Performance. The swap is ill-advised, not only because of resale value but also because the IRS is a better daily driver setup. Here’s the list of parts and cost in table form:

Number Part Cost
1 8.8 Ford Rear Axle housing, stripped $400
2 8.8 T/A reinforced cover with bolts, washers, and gasket $170
3 Moser 31 spline axles with ABS adapters $285
4 31 spline Eaton Posi unit with metal clutch plates $380
5 FRPP 3.73 ring and pinion $195
6 Timken wheel bearing $20
7 CK wheel seal $12
8 Timken bearing kit $100
9 Welding tubes, additional weld on brackets and assembly of rear unit $150
10 4oz Mopar friction modifier and 3qts 75/140 synthetic gear oil $25
11 ABS tone rings $28
12 Cobra pinion flange $24
13 RH brake bracket $64
14 LH brake bracket $63
15 RH ABS sensor $36
16 LH ABS sensor $29
17 GT ABS sensor bolts $4
18 LH exhaust hanger $20
19 RH exhaust hanger $20
20 Exhaust hanger clips $10
21 Exhaust hanger bolts $10
22 Shock mount brackets $20
23 Shock mount bolts and nuts $10
24 Strange Eng. 10-way adjustable shocks $130
25 Fox body Mustang springs $82
26 Urethane rear spring isolators $18
27 X2C upper and lower control arms $163
28 Differntial-to-upper control arm bushings $18
29 Rear upper housing bushing set $22
30 Upper control arm bolts $20
31 Upper control arm nuts $16
32 Lower control arm bolts $24
33 Lower control arm nuts $20
34 Rear damner kit $23
35 Rear sway bar 23mm $88
36 Sway bar bolts $16
37 Sway bar nuts and retainers $12
38 Dust shield $8
39 Dust shield screws and washers $32
40 Fuel filter $12
41 7mm nylon push pins unknown
42 Borla catback exhaust (over-axle) $449
Total $3228

Since the total to swap out the IRS for a live axle is $3000, it seems that it is not worth it. The resale value will drop, the overall drive quality will decrease, and all we will have done is remove the wheel hop. If a Mach 1 buddy wants to trade, that could be great, but resale value and ride quality still end up suffering even if the wallet does not.

Opinions

Burnouts. Sideways. Donuts. Exhibition usage of the Mustang may be fun, but it is certainly illegal on the streets. The Cobra is not a toy, it is a daily driver street car with a nod to the racing enthusiast. It is my own opinion that the Cobra should not be driven with a view towards burnouts, getting sideways, or doing donuts. If the driver is somewhat conservative on launch, wheel hop would not even be an issue. But who am I kidding? What Cobra owners in their right minds would drive conservatively? Cobras have power, and their drivers intend to put it to good and fun use. In my view, swapping for live axle setups really is not an option. Instead, masking the wheel hop appears to be the way to go. It takes equal or less money to strengthen the IRS and rear end setup, and the car would retain a better resale value and ride quality. If wheel hop is really so undesirable, the Mustang enthusiast would be better off getting a Mach 1 and putting a supercharger on it. There are other drawbacks to that, however, such as transmission or even appearance differences. It could be thought to just get a Shelby GT500, but with a few reports of wheel hop on the S197 platform, this undesired effect might still occur. I, for one, am rather drawn towards the ’03-’04 Cobras with IRS and large power potentials. I would opt to keep the IRS and strengthen it. What is the best solution for wheel hop? Driver style and intention is the key factor for deciding on the best solution. Yet, it simply seems more feasible to keep the IRS.

How about you? Have you swapped to live axle? Do you regret doing so? Have you strengthened your IRS? What worked and what didn’t? Have you never experienced wheel hop or is it a constant factor in your driving? When do you notice that it occurs the most? Chime in and let your experience be known in the comments below.

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