IRS Bumpsteer Kit, 1999-04 Cobra

In stock
Regular price $177.97

Adjustable outer tie-rod ends for correcting rear bumpsteer to improve handling of IRS-equipped Mustangs.


  • Allows adjusting bumpsteer to fine-tune the IRS geometry for improved handling.
  • Allows adjusting the bumpsteer curve to tune the IRS' oversteer characteristics.
  • Exclusive MM spacers are stronger than others, will not buckle when the nut is tightened.
  • Assortment of spacers allows precise fine-tuning of bumpsteer.
  • Measurement and adjustment of bumpsteer is required during installation. MM's Bumpsteer Gauge is the right tool for that.

Product Features

  • Direct bolt-on installation, no drilling of the IRS spindle.
  • MM-designed tapered stud with rolled threads (not the weaker machined threads found on other tapered studs).
  • Studs are heat treated for maximum strength.
  • Spacer bushings were designed specifically for this kit.
  • Longer spacer bushings are made of thick-wall Chro-Moly steel to ensure they will not buckle when the 5/8" nut is properly tightened.
  • A variety of spacer thicknesses ensure the ability to properly adjust bumpsteer.
  • Black anodized aluminum adapter sleeve is threaded to match the stock IRS inner tie-rod.
  • Hex on the aluminum adapter sleeve is easily accessible because it is at the inner end of the aluminum sleeve, clear of the tire.


  • Your Mustang will need to have the rear toe setting properly adjusted after installing this part.

IRS Bumpsteer Tech

  • Besides the commonly required adjustments to camber and toe, an IRS can greatly benefit from fine-tuning its bumpsteer characteristics. Just as with the Mustang front suspension, the IRS can exhibit undesirable handling behavior due to bumpsteer characteristics.
  • Ford designed the rear suspension to function as a complete system. The static bumpsteer curve was designed to work in conjunction with the compliance of all the rubber bushings in the suspension. When those bushings are changed to a less compliant material, the dynamic bumpsteer curve will change and no longer be what Ford had intended.
  • With the installation of urethane or Delrin suspension bushings there will be far less compliance than there was in the stock rubber bushings, and therefore the alignment changes when the car is driven will be smaller in magnitude. The original stock static bumpsteer curve will no longer allow the same dynamic curve that the car had with the original rubber bushings.
  • While other companies advocate the simple fix of duplicating the 2003 Cobra bumpsteer curve for earlier IRS Cobras, that becomes less and less desirable as the compliance of the OEM suspension is reduced. MM recommends matching the 2003 Cobra static bumpsteer curve only for cars that retain the original factory rubber bushings. And as a starting baseline prior to measuring and adjusting bumpsteer. The MM kit includes instructions to adjust the bumpsteer to match that of the 2003 Cobra.
  • Rear bumpsteer has a much larger effect on the car's handling balance than front bumpsteer has, which is why MM recommends measuring and adjusting the rear bumpsteer on your Mustang as an integral part of your performance modifications.
  • For cars with any performance upgrades, such as springs, IRS subframe bushings, or rear control arm bushings, MM recommends that the rear bumpsteer curve be adjusted to minimize any toe changes with suspension movement. After you test your car with this initial set-up, you may wish fine-tune the handling balance of the car by adjusting the rear bumpsteer curve. Only your continued testing will confirm what works best for your car and your driving style.

What is bumpsteer?

  • Bumpsteer is the term for the situation when the toe angle of a wheel changes as the suspension moves up and down, such as when driving over bumps, or from body roll during cornering. This happens when the arc that the spindle travels during bump and droop is not the same as the arc of the outer tie-rod end.
  • If the toe changes more than a very small amount, the rear wheels begin steering the car. Bumpsteer is commonly measured and then plotted on an X-Y coordinate graph as a curve showing suspension travel vs. toe change. The resulting curve is an illustration of the bumpsteer characteristics.
  • To measure bumpsteer you will need a bumpsteer gauge. MM's Bumpsteer Gauge is accurate and quite affordable.


                    Quickly find what you need by searching with the SKU. Our streamlined search makes it faster and easier to locate your desired part!

                    Click To Search

                    Can't find the part you need? Simply text us at 843-484-3617 or a swift response and assistance with product lookups. This applies to inventory checks as well!