Wheel Fitment Tips - Staying Fit - MMF Tech | Wheel & Tire Upgrades
Muscling Up Your Rolling Stock? Measure Twice; Spend Once
From the April, 2010 issue of Modified Mustangs & Fords
By Dale Amy
Photography by Dale Amy
The right rolling stock can...
The right rolling stock can make or break a project, but if going large, be prepared to check and re-check tire/wheel clearance measurements before ordering up those expensive alloys and rubber molecules. The fully packed wheelwells on Classic Design Concepts' Bullitt-themed, Dynacorn-based Flashback beautifully demonstrate the results of doing it right.
Few modifications can have as profound an impact on a classic Mustang or Ford as the right set of rims and tires-a declaration that holds true for both looks and performance. In fact, we can think of no other single investment so effective at personalizing a ride. Back in the 1960s, names like Cragar, Ansen, E-T, and American Racing were common buzzwords when talking about "mags" for one's muscular Ford. Like most other aspects of life, things were simpler then-rim diameters beyond 15 inches were practically unheard of, hardly anybody upsized their brakes, and wheel manufacturers could be counted without even taking your socks off.
Today, the wheel market abounds with brand names, and the range of available diameters, styles, offsets, prices, and even materials and construction is positively staggering. For the truly custom-minded, some companies will now build rims to customer specs. But while it's easy to get all wrapped up in the appearance equation, your first and foremost task in all this is to be sure that whatever hoops and rubber you decide upon will actually fit. Few things are more disheartening than to hear or feel that pricey new rolling stock rubbing or scraping on something even before their large Visa bill arrives. This fit thing may be relatively straightforward if you haven't, or don't plan to, modify your ride's brakes or other major suspension components, and are sticking with near-standard-size wheels that are specifically engineered for whatever classic Ford you're working on (like maybe going from 14x6s to 15x7s). Naturally, such off-the-shelf simplicity goes out the window when shopping for a truly custom look, when sizing after track width-altering suspension mods, or when trying to work around manhole-sized brake rotors or giant mono-block six-piston calipers.
An example of Plus-3 sizing-going...
An example of Plus-3 sizing-going from this Mach 1's stock-size 14-inch stuff to 17x8 American Racing Torq-Thrust IIs in a custom offset. Showing how important it is to get overall height right, P225/55R17 rubber was first tried on this project, but proved too tall (by about 0.9 inches) and rubbed. Going to lower profile P225/50R17 skins solved the problem by being almost exactly the same height as the Kelly 14s. Notice, though, that any excess ride height becomes more obvious with the short sidewalls of the 17-inch rolling stock. (Time to drop that Mach into the weeds!-Ed.)
So, where to begin? As with any project, the best place to start is with an overall plan, especially in relation to chassis and braking upgrades. For example, if your ride is to remain otherwise nearly stock, odds are you may not want to go too radical with its new footwear, especially if you're not thinking of lowering ride height. The tall tire sidewall that comes with a stock 14-inch rim is visually forgiving of any gap between it and a wheel lip, whereas the ultra-low profile rubber necessary for something like 18-inch rims can make stock ride height look both geekish and agricultural. On the other hand, a slammed and radical ride will in all likelihood demand more than 15-inch footwear.
Practically speaking, we can't foresee a lot of application for rim diameter greater than 18 inches on a classic Mustang. Beyond that, the required tire sidewall height becomes so short as to start looking out of place on something that was designed more than four decades ago. Remember also that such short sidewalls exact a stiff (pun intended) ride penalty. But, hey, if you're bound and bent on a set of 20-inch blings, maybe the following info will help you determine if it's even possible.
We can't talk about rims without some mention of tires, and that brings us to a quick discussion of "plus sizing"-the concept of going to a larger wheel diameter while staying as close as possible to original overall tire diameter and circumference. Proper plus sizing retains both speedometer accuracy and good visual proportions, while also avoiding any change in effective final drive ratio, and hopefully taking height-related clearance issues out of the equation.
Classic Design Concepts' (CDC)...
Classic Design Concepts' (CDC) modern Bullitt project, as shown in our lead photo, wears P245/40R18 rubber on custom-spec'd 18x8 Torq-Thrust IIs up front. The rear Torq-Thrusts measure 18x9.5 and mount P275/35R18 skins. Measuring for this project also had to take into consideration its huge Baer brakes and a complete front coilover/rear three-link RRS suspension.
CDC's modular GT500-powered...
CDC's modular GT500-powered Flashback also uses 18x8 rims up front and 18x9.5 out back, this time from Forgeline, with the same tire sizes as on the Bullitt. This car also wears RRS suspension hardware and brakes.
Though it may appear much...
Though it may appear much like an original '67 GT, this is CDC's first Dynacorn-bodied project, wearing 15x7 Styled-Steel wheels from Scott Drake with P215/65R15 rubber. Such a Plus-1-sized upgrade should work with most Mustangs without much need for clearance measurement. But if it was our money, we'd still measure.
Yet another CDC-built project-the...
Yet another CDC-built project-the Equus Tom 1-uses the same 18x8, 18x9.5 combo as the company's other radical Flashbacks. These rims are form Electri-Cal Machining and again were custom spec'd for offset based on the car's suspension and brake hardware. Do you detect a pattern here? Suspension design and track-width, along with braking hardware and sheetmetal location/modifications, are major determining factors for successful wheel sizing.
Don Gasiunas' tricked out...
Don Gasiunas' tricked out '70 Mach 1 takes a different approach with 17x8 Raceline GTs up front and 18x10 versions out back. Front rubber measures P255/45R17, while the rears are generous P295/35R18s, likely made possible by the SportsRoof's Total Control Products coilover suspension system.
Another Total Control-suspended...
Another Total Control-suspended Don Gasiunas project was this '67 ragtop. PS Engineering made these rims in 17x8 for the front and 17x9.5 in the rear, wearing P245/45R17 and P255/45R17 rubber, respectively. You can see that the chosen offset left a little extra real estate between tire and rear fender lip.
This illustration should help...
This illustration should help clarify some of the wheel terminology we've been talking about, like that pesky offset stuff. (Courtesy Vintage Wheel Works)
Going up from, say, stock 14-inch rims to 16s would be an example of Plus-2 sizing, and would require a tire having a significantly shorter sidewall (lower aspect ratio) to maintain a similar overall diameter. So, if your Mustang now has F70x14 rubber (roughly P215/70R14 in modern-speak), you'd have to go with something like a P225/55R16 or P245/50R16 tire to get close to the same overall diameter and circumference when using a 16-inch rim. A tire's "aspect ratio" (the second number in its size designation) describes its sidewall height as a percentage of tread width and, for any given overall diameter, a tire with a lower aspect ratio will also be wider, which in turn may require a wider rim width for proper bead mounting. In our modified Mustang world, wider is often better-but only if everything fits.
The key to a successful marriage of custom wheel and Mustang is pre-purchase measurement, measurement, measurement. Measure with the suspension at normal ride height (not fully drooped), and, up front, it's important to measure with the wheels pointing straight ahead, as well as with them turned to full lock in both directions. Many wheel manufacturers and vendors now want you to supply specific measurements of your car's brake hardware as well as clearances to chassis/suspension hardware, wheelhousings, and so on, front and rear, when ordering their rims. Such pre-order scrutiny is a very good thing for all parties involved.
The amount of backspacing...
The amount of backspacing is easily measured by placing a straight-edge across the inside edge of a rim, and then with a tape measure or ruler measuring to the wheel mounting surface. The greater the backspacing, the more inboard the wheel and tire are positioned. Offset is not directly measured, but can be calculated as backspacing minus 1/2 of overall rim width. A negative result means negative offset.
Enthusiast discussions of wheel fitment most always go straight to the question of what overall size will fit their ride-16x8? 17x8.5? 18x9? The inevitable answers are: maybe, maybe, and maybe. Take two different 16x8 wheels out of the box and one may fit your car perfectly while the other won't even come close. The difference often comes down to two related terms: backspacing and offset. Backspacing is the position of the wheel mounting surface in relation to the back face of a rim, usually expressed in inches. Offset is the position of that mounting surface in relation to the centerline of rim width, and is confusingly enough normally stated in millimeters, positive or negative. A rim that has its mounting surface positioned outboard of its width centerline is said to have positive offset, so that an offset spec of +32, for instance, means the mounting surface is 32 millimeters outboard of the center of rim width. A mounting surface right on the rim centerline has zero offset, and one that is inboard has negative offset.
Backspacing/offset, then, determines how far inboard or outboard a rim of any given width is positioned, and this can make all the difference when trying to fit a wheel without contacting chassis bits or wheelwell lips. Wisely, many manufacturers now offer the same rim in a variety of offsets. In addition to simply avoiding clearance issues, choosing within an appropriate backspacing/offset range can also help position the rims so they properly fill the wheelwells for an aggressive stance. Remember that when measuring for clearance, that the overall width of any wheel is about an inch greater than its stated (nominal) width, because nominal width is measured across the inside (bead) surfaces of the inner and outer flanges. So a 16x7 wheel is actually closer to 8 inches in overall width.
You're Braking Up
As brake packages for classic Fords have grown in variety and size, so has the need for rims of commensurately large diameter, simply to clear the circumference of rotors and calipers. It should be fairly obvious that if 13-inch rotors and multi-piston calipers are in your plans, a 15-inch rim just isn't gonna cut it. But caliper overhang-the distance the face of the caliper extends laterally beyond the wheel mounting surface-is another critical factor of brake bulk that must be measured. You might find a wheel that would otherwise fit just fine, except for the fact that the back of its spokes won't clear your huge calipers.
This tight spoke/caliper clearance...
This tight spoke/caliper clearance highlights the importance of careful measurement of "caliper overhang"-the distance the caliper face protrudes outboard of the hub mounting surface. If your project will have serious calipers, disregarding this one measurement can ruin your day, even if you get all your other clearance measurements right, especially up front where the brakes are at their beefiest.
A potential time-saver is...
A potential time-saver is this multi-adjustable Wheelrite tire and wheel simulator from Percy's High Performance (www.percyshp.com). You mount it on your hub and use its adjustability to experiment with various rim diameters, widths, and backspacings. The wire can be set to approximate tire profile. Then (at normal ride height) you rotate the hub and see if there are any clearance issues-pretty handy for around $100.
Here we have some rims off...
Here we have some rims off of two of CDC's Dynacorn-based '67s. One is 15x7 with a P215/65R15 tire; the other has P245/40R18 rubber on an 18x8 wheel. Both are of roughly the same overall height and demonstrate the concept of plus sizing, but notice also that the wheel mounting pad is notably more outboard on the 18-inch combo, meaning its offset is more positive than that of the 15-incher. In this case, the offset difference required had to do with differing track widths and brake packages between the two vehicles.
Up front, with a stock-style...
Up front, with a stock-style suspension (as on CDC's '67 GT clone here), the upper ball joint and the tie-rod end are two areas of critical clearance measurement. Though the suspension is in full droop here on the hoist, it's important to measure for wheelhousing/body clearance with the suspension at ride height. Oh, and remember that the wheels don't always point straight ahead, so check clearances at full steering lock too.
Which sort of takes us back to the beginning: Have an overall plan and consider implementing any desired chassis or brake mods before you invest in those perfect-looking new alloys. Thoroughly measuring your Mustang or Ford for a new set of shoes can be a tedious process, but so can trying to sell or send back a new set of wheels and tires that don't fit.
How Much Is Too Much?
The earliest Mustangs wore some pretty scrawny factory rubber-heck, that's all that was out there at the time-on rims no taller than 14 inches (at least until the Shelbys came along.) And their wheelhousing areas were engineered and sized accordingly. These early cars are the most dimensionally challenged when it comes to accepting larger rolling stock, but can still be substantially upgraded. By 1969 and the advent of factory F60x15 rubber, a bit more space was provided. Though individual measurement is still the best way to proceed with your own rolling stock selection, we've stumbled across a website that has catalogued wheel and tire fitment on various years of early Mustangs with various brake and suspension combos. Have a look-but don't throw out that reliable old tape measure . . .http://dodgestang.com/mustang.htm
Also, for a well illustrated look at wheel fitment tips and terminology, you may want to check out: www.rsracing.com/tech-wheel.html
Calibrating For Calipers
Clearly, the decision to upgrade/upsize brake rotors will have implications on minimum diameter when it comes to choosing rims. But caliper design will also play a major role in determining which rims will fit. The following illustrations detail the specs of two different Stainless Steel Brake Corporation (SSBC) kit offerings for early Mustangs. Both have 13-inch rotor diameter and require wheels of at least 17-inches, but one (the three-piston Tri-Power) is of floating-caliper design while the other (the four-piston Extreme) uses fixed calipers. The fixed-piston Extreme caliper (on the left) protrudes 0.92 inches outboard of the hub's wheel mounting surface (this is called caliper overhang), while the floating Tri-Power caliper (above) actually tucks 0.12 inches behind the hub face, therefore giving a lot more leeway in spoke-to-caliper clearance. It is absolutely essential to consider-and specify-caliper overhang when ordering rims. By the way, SSBC also has a 12.5-inch rotor braking system that will work with a variety of 16-inch rims.
Something to bear in mind:...
Something to bear in mind: Aftermarket suspension designs may alter vehicle track-width for handling benefits, and as such may require notably different wheel offsets than a stock setup. Such is the case with CDC's GT500-powered Flashback, which uses an RRS coilover front setup. Notice too that with this car's RRS 14-inch, six-piston Phase 4 brakes, nothing smaller than 18 inchers would have cleared the brake hardware diameter.
To demonstrate the whole offset...
To demonstrate the whole offset thing to ourselves, we took one of the Flashback's 18x8 front rims/tires and tried to mount it on another CDC '67 having stock-style suspension. The high degree of positive offset on this rim caused two major issues: first, the inside edge of the wheel and tire was so far inboard that it couldn't begin to clear the upper ball joint area. Second, said offset would have left an enormous outboard gap between tire and wheelwell lip.
Out back, wheelhousing dimensions...
Out back, wheelhousing dimensions and the proximity of leaf spring hardware have a great bearing on required backspacing/offset dimensions as well as maximum wheel/tire width. This is CDC's stock-suspension '67 with 15x7 rims and P215/65R15 tires.
CDC's blue Flashback has an...
CDC's blue Flashback has an RRS three-link suspension that eliminates the leaf springs, but the wheelhousing limitations are still there. The wheel is 18x9.5 mounting P275/35R18 rubber.