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We could all use a little more grip. The more we enhance suspension performance and driving technique – the easier it is to point at a smaller contact patch for holding us back. We’ve been planning this upgrade – to a 275/40/17 wheel and tire combination for quite a while. The Fox Mustang platform doesn’t allow for this upgrade in stock form, so we haven’t been in a hurry to make the necessary modifications, but we stumbled across a used combo that was too good to pass up. Luckily, we had our camera handy when the sparks began to fly.

Editor’s Note: Since this post - we’ve opted for even larger fenders. We’ve compared the Maier Racing Fender Flares, and the Tiger Racing Fender Flares, and provided a write-up of the installation.

There are several factors that can complicate upgrading the wheel size on a Fox Mustang:

Fender Opening – All Fox Mustangs are too tight in front of the wheel to allow for clearance of 275 tires. Pre-1991 models have slightly smaller fender openings in the front. Especially around the fender extender ( the part below the molding, in front of the wheel well, and behind the bumper.)

Wheel Offset – Aftermarket suspension components can alter the location of the wheel in relationship to the fender. Many manufacturers relocate the wheel outward, and forward to increase track, wheelbase, Ackerman, caster and camber.

Backspacing – There are so many aftermarket rims available, but not all of them were designed to work on the Mustang. You can’t have the wheel sit too far inward – or you’ll get interference with the suspension components (especially if you’ve already installed coilovers in the front.) If the wheel is sticking too far outward, you’ll really have a tough time getting clearance around the fender.

Tire Size – Higher profile tires will require more room for clearance. Often tires from different manufacturers are different circumference and width even at the same tire size.

 

We opted for an OEM wheel because of its combination of size, weight, construction and backspacing (6.12”). These wheels were available on the 2003 Cobra.

Test fitting the wheel on the car presented no surprises. There was ample clearance for the Cobra Brakes and 96+ spindles up front. The rear wheels fit into the fender relatively well on stock length Fox axles. There is a minor amount of interference with the way our exhaust is currently hung. The cat-back exhaust provides a lot of room for adjustment, so we’ll position the pipe inward a bit more.

The real problem is evident up front. The fender tops and fronts were never intended to accommodate any tire beyond 245 width. We’re definitely not the first to try this modification - but we had a hard time finding an example of our specific application. There are plenty of great online sources that we reviewed before our modification. CornerCarvers.com have a several great posts. We reviewed:

General Overview: http://corner-carvers.com/forums/showthread.php?t=162

Specific Modification: http://www.corner-carvers.com/forums/showthread.php?t=27194)

And one of the pioneers of online sharing, DariusRudis.com: http://www.dariusrudis.com/mustang/2007_Splitter/)

All of the previous write-ups we found feature three common elements: Cutting the fender under the trim to allow flexibility to the fender (indicated in red,) Pushing the fender out with a main stretcher bar (this is an “after” photo showing the aluminum stretcher bar,) and pushing the lower fender extender out with an additional bar (indicated in yellow.) While our alteration is similar to these samples, we made a couple of variations to suit our needs.

Fit

First things first. We placed the 2003 10^th Anniversary wheel on the 1990 Mustang. This coupe has the front 5 lug conversion via 96+ Mustang spindles. We loosened up the fender extender from the fender to measure for interference in front of the wheel. (Our fender extender had previously been trimmed slightly when we installed the Griggs K member and Maximum Motorsport Control arms. The car was wearing 245 tires at the time, and was contacting the inner fender extender on turn-in) We’re also noting how far the tire sticks out from under the fender.

NOTE: During the modification, it was necessary to lower the car back to ride height several times. We found it easier to work with wheel ramps instead of jack stands.

Disassembly

The trim panel is held in place by 3-4 (depending on year/body style) panel nuts on the inside of the fender. On one side of our car, they came off with very little persuasion. On the other side, two of the fasteners broke with the slightest touch. It is an 18 year-old car after all, so be prepared to improvise come reassembly time if your car is similar.

The fender extender is held in place by two fasteners at the fender, and two at the front bumper cover. We loosened/removed them to allow the fender extender to move around during the stretching. Once again, we had two of our fasteners snap during removal. One at the bumper cover, and one at the fender (on different sides of the car.) The stud that broke off the fender extender is more critical so we were quick to grab some gorilla glue, and put the stud back on. The fastener at the bumper cover will be replaced with a new nut/bolt/washer combo.

Stretch

A wooden dowel (size is trial and error depending on surface texture that you can use to wedge the dowel in temporarily) is used to force the fender outward. Our goal is to place an aluminum brace behind the fender approximately in the middle of the area covered by the trim piece. The wooden dowel is wedged in to push the fender outward while you are measuring/cutting/fitting the stretcher bar.

We always try to save weight – so we’ve elected to use aluminum instead of steel for the stretcher bar. A quick trip to the hardware section of our local home improvement store yielded a wide selection to choose from. None of the flat bar material was strong enough in the length we expected to need – so we opted for the L shaped stock in the longest length they sold.

After locating the positions on the fender and chassis that would provide the mounting location – we measured our required span of 15.5” and went to work cutting, notching, and bending the ends of our stretcher bar. We’re going to attach it to the fender first, and then to the chassis. We’ll keep the wooden dowel in place until after the both ends of the bar have been fastened.

We’re drilling a hole in the recessed area of the fender behind the trim. This will allow plenty of room for clearance for the head of the small machine screw. The order of materials was: Machine screw, washer, fender, washer, nut, stretcher bar, washer, second nut.

Now we’ve pulled the chassis end of the bar as tight as we dare without taking the pressure off the wooden dowel and predrilled a pilot hole for the self-tapping sheet metal screw that will attach to the chassis. The angle is awkward, and the drill bit is small, so take your time and let the drill do the work. In no time you’ll have the pilot hole drilled and the drill bit intact. Once you’ve screwed the bar into the chassis – remove the wooden dowel (as pictured above) and put the wheel back on the hub, lower the car to ride height, and test for fit above the tire. Naturally you allowed for a large enough stretch, the fit is perfect, and it’s time to move on to the next side, right? Yeah, well, it took us two tries on the first side also.

Cut & Grind or Push & Brace?

Depending on your application, you’re now left staring at minimal or severe interference with the lower fender and fender extender. If it’s minimal – the technique of adding a second bar to push the bottom of the fender out works for many – and alleviates the need for cutting/grinding. Because our wheel is offset so dramatically forward with the aftermarket suspension components, we’re forced to break out the Dremel and grinder.

Here you can see the amount of material that needs to be removed for this application. We’re fortunate enough to have a white car. The material for the extender is white, and we won’t need to run to the body shop right away for a touchup after the grinding/reshaping. We’re fortunate enough to still have one mounting location from the extender to the fender that won’t need to be removed.

We start with the fender first. During our test fitting, we marked up the interfering areas with a thick black sharpie. Using a Dremel and cut off wheel we cut the bolt off the lower mounting stud to get access to the fender – then the fender to get access to the fender extender…

and then finally cut off the bottom stud of the fender extender. (You’ll notice how much of the fender extender we’ve ground off already in this photo. We’ve tapered the grind from top to bottom. Near the top - less material was removed, toward the bottom we ground more off.)

Next, we grab the grinder and finalize the shape of the front fender opening. We worked slowly, and smoothly concentrating on the shape and consistency of the new surface. We kept the grinder perpendicular to the cut to keep the surface smooth, and retain as much of a factory look as possible. We focused our grinding below the area of the trim molding. We stopped to test fit several times – attaching the wheel, lowering to ride height, turning the wheel in and out.

Results

The final fit and finish has benefits and challenges. First off, we got the size tire to fit that we’ve been looking for. There’s ample room up top, and in front of the wheel now. Unfortunately – even the side that didn’t break a fastener – the fender extender has a visual gap next to the bumper cover. If we were going to leave the front of the car alone – that would probably be a problem. But – an aftermarket bumper cover is in our future to allow for better brake duct functionality – and since many of these pieces replace the fender extender, cosmetically we should be ok. As for the touchups to the paint? That’s a pain in the ass that we can put up with for now.