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Tire _ Wheel Plus Sizing

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Tire & Wheel
Plus Sizing
Upgrading wheels and tires
                                                                               By Kelvin Dodd
for fashion and performance
                                                                      Moss Motors offers Panasport wheels for classic style and
                                                                      performance. Plus-size fitments are available for many MG and
By Kelvin Dodd                                                        TR applications (MGB +2 shown).



I  f concours-quality restoration is
   your thing, or you believe that what
the factory offered is sacrosanct, then
                                            excitement at low speeds may not be
                                            everybody’s cup of tea, particularly
                                            during rush hour.
                                                                                          Some Popular First-Step
                                                                                           Performance Upgrades
this article is not for you. If, however,        Today, the many available wheel            CAR        ORIG.   ORIG. UPGRADE UPGRADE
you are open to change and are              and tire options offer increased                          WHEEL     TIRE WHEEL TIRE
                                                                                        Spitfire/Midget 4"      155-13 5" 175/70/13
interested in improving the looks and       stability and safety for performance-            MGB      4.5-5"   165-14 5.5" 185/70-14
handling of your car, then read on.         minded drivers, or the choices may                                               195/65-14
      There are two primary reasons         be merely adequate for those keeping        MGA, TR4, A-H   4"     165-15 5.5" 195/65-15
to upgrade wheels from their original       up with modern iron on American
sizes: increased performance and            freeways. Changing to a wider wheel              Even these minor upgrades can
visual impact. Obviously, the intent        allows the use of wider, lower-profi le      cause fit problems with the limited
of any change is to get the most out of     tires, which generally have far superior    clearance in narrow wheelwells.
your car without breaking your budget       handling characteristics compared           Correct-offset wheels must be used to
or creating more problems than you          to the original-sized tires. Typically,     ensure that the tire is centered in the
already have. As in any modification,        increasing the original rim width by        wheelwell. Generic wheels and older
a balance between cost, performance,        1” will allow the use of a slightly wider   American “Mag” wheels often have
and drivability must be determined,         70- or 65-series tire, the closest sizes    the incorrect offset and will cause
based on each individual’s needs. This      to the original 82-series (which were       fitment problems, even with stock
article will broadly cover some of the      the only factory-offered tires with any     tire sizes.
positive and negative aspects of wheel      kind of performance pretensions).
and tire changes.                           Note that the TR6 and MGB Limited                     How To Measure
                                            Edition were both equipped with                        Wheel Offset
Let’s Put Some Rubber                       more modern wider wheels from the            1. Find the overall width of the
On The Road!                                factory.                                        wheel. Divide this width in two to
When most of our classic cars were                                                          give the centerline depth.
new, the factory wheels and tires were      A Quick Note On                              2. Lay a straightedge across the
sufficient for the vehicles’ anticipated     Tire Sizing                                     inner rim of the wheel. Measure
performance and to define their              Modern tires are listed in Metric sizes.        from the straightedge to the
looks. Nowadays, that original look         The first group indicates the nominal            mounting rim of the wheel.
may be dated, and the performance           width of the tire, called the section        3. Subtract the centerline depth
expectations have increased to keep         width. The second group is the aspect           from the mounting rim depth.
up with current trends of high-speed        ratio, which gives the height of the            This gives the offset, which is now
stability and cornering. I’m a firm          sidewall as a percentage of the width;          usually designated in millimeters.
believer in skinny tires sliding around     this is also referred to as the profile of
mountain curves: For me, the “new”          the tire. So, 70-series tires have taller        Positive offset indicates that the
sport of sport-compact “drifting” was       sidewalls than 50-series sizes. Modern      hub center (mounting surface) is
pioneered by the MGs, Triumphs,             performance tires are generally only        toward the outside of the car; negative
and Healeys during the street races         available in low-profile 70-series and       offset indicates that the hub center is
and rallies of the ’50s. But this type of   lower aspect ratios.                        closer to the brake drum.

16 | Winter 2005 • British Motoring
                                                                                                       Tire & Wheel Plus Sizing

     Here are some typical offsets        oval tires to their cars.                       the rear axle will move sideways,
necessary to keep the wheel centered      Austin-Healey: Big Healeys’                     aggravating clearance problems. On
for popular British sportscars:           wheelwells on the BN2 onward are                this car it is very important to have
                                          wide open. Their rear suspension                the wheel centered in the well, as
    Typical Wheel Offsets                 has limited movement, which allows              suspension travel and axle movement
       Sprite/Midget: +20mm               fitting wider wheels and tires without           can be major problems. Minor fender
        MGA/MGB: +22mm                    major rubbing problems. However,                clearance issues can be addressed by
          TR2-TR6: +6mm                   overly wide tires can spoil the clean           rolling or grinding the rear fender lip.
          TR7-TR8: +15mm                  lines of the car when negative-offset           TR2-TR4A: Fender clearance is an
          Spitfire: +20mm                  wheels are installed, which push the            issue, and a wheel with less positive
                                          tire edge past the fender lip. BN1              offset is required to clear the front
      By keeping the wheel centered       fenders are very tight, so wider tires          brake calipers. This restricts the
in the wheelwell, there is less chance    are not advised. On all models, check           maximum wheel width.
of the wider tires fouling the inner      fender clearance.                               TR250-TR6: Again, a less positive-
fender, suspension components, or         MGA: Like the Healey, the rear wheel            offset wheel is required to clear the
outer fender—this is most important       arches are open and can accommodate             front brake caliper; fender clearance
with the narrow rear fenderwells of       tires up to 195 cross-section without           is improved and tires up to 205 cross-
most British cars. For street driving     problems.                                       section can be installed as long as the
where there is likely to be a lot of      MGB: The rear fender lip on early               car is not significantly lowered. Rear-
suspension and body movement, it          cars and lowered late cars is at the            fender clearance can be a problem on
is a good idea to stay conservative       widest point of the tire, so tire bulge         lowered cars.
on width. Otherwise, the smell of         can be an issue. Raising or lowering            TR7/TR8: A positive-offset wheel is
burning rubber and the sounds of          the suspension may allow extra                  required, but the wheelwells are quite
tire-rub are going to accompany any       clearance, but suspension travel can            spacious. 205/60-13 tires will fit on the
spirited driving. Make it a point to do   cause clearance issues, particularly            original factory alloy wheels without
your test-driving with your spouse or     on the left-hand side of the car.               any clearance problems.
significant other in the car. This way,    The inner bumpstop structure and                Spitfire: Open wheelwells allow
you won’t experience the unpleasant       front swaybar can cause clearance               the use of 5.5” wheels. These can be
surprise of their added weight            issues for wheels that have too much            mounted with 185/70 series tires, but
causing the tires to rub.                 positive offset. Under hard cornering,          they may stick out beyond the fenders.
      If you want that maximum-
rubber, road-racer look, be aware                                           STRAIGHT EDGE
that most racecars have had some
radical surgery and are often fitted
with stiffer springs and panhard rods                                   BACK SPACING
or other axle locators to ensure that
suspension movement is limited.                                                                                           WHEEL
You can’t have the same look and                                                                                          WIDTH
performance without making the
same sacrifices.

What Are Some Of
The Pitfalls?                                       CENTER LINE                 CENTER LINE                 CENTER LINE
Sprite/Midget: The square-arch
rear-fender cars have very restricted
wheelwells. Even a 165-section tire
is probably going to rub on the back
under hard cornering. The round
wheel-arch cars have much less of a       BRAKE                                                                               WHEEL
problem, and 5.5” wheels can be fitted      SIDE                                                                                SIDE
with wide tires as long as the springs
do not allow the tire to contact the
fender lip. This led to many a Bugeye
that looked like it got mated to a
steamroller. Perhaps Austin moved to
the square wheel arches specifically              POSITIVE OFFSET               ZERO OFFSET               NEGATIVE OFFSET
to prevent Americans from fitting          Backspacing and offset refer to the position of a wheel’s mounting surface in relation to its
Chevy Vega wheels with fat, wide-         centerline. (Courtesy Yokohama)

                                                                                        Winter 2005 • British Motoring | 17
Tire & Wheel Plus Sizing

Plus-Size Wheel Options                           TR6 Plus Sizing                   mean you won’t need a body hammer
One way to increase the availability          STOCK WHEEL DIAMETER: 15”             and cutting torch to remove the wheel.
of high-performance tires is to                  Typical Tire: 185/80-15            A very tasty combination for the
increase the diameter of the wheel,                (diameter 677 mm)                MGB is a +1 package of a 15” spline-
which allows the use of lower-             Performance Alternative: 195/65-15       drive wheel and a 185/65-15 tire. This
profi le tires while still maintaining             (diameter 634.5 mm)               will clear on most cars; the larger
the correct overall tire diameter.              +1 WHEEL DIAMETER: 16”              195/60-15 will likely rub under hard
Only baseline modern vehicles are                Tire Choice: 205/60-16             cornering.
equipped with 13" and 14" wheels,                  (diameter 652 mm)                Wire Wheel Options: The first option
so high-performance tire availability                                               is to look at the factory racing history
in these sizes is very limited. By        TR7/TR8: The look of the “wedge” is       and see if a wider wheel was available
increasing rim diameter one inch          really improved with more modern          as an option. In the case of the MGB,
(+1) or even two inches (+2), the         tire and wheel combinations. This         the factory offered a 5.5” 72-spoke
availability of lower-profile high-        “shape of things to come” really was      wire wheel, which has the correct
performance tires becomes much            ahead of its time and looks dated         offset to handle larger rubber without
greater. In some cases, larger-           mainly due to the small 13”-diameter      clearance problems.
diameter wheels allow the use             wheels. A number of sticky 205/60-13           Most early cars originally
of wider tires because the tires’         tires are on the market, but increasing   equipped with 48-spoke 15x4 wheels
sidewalls don’t flex as much, so           wheel diameter gives the car a whole      can be easily upgraded to 60-spoke
clearance under load may be greater.      new image.                                15x4.5 wheels or even the 15x5 72-
MGB: An inch-larger wheel allows                                                    spoke wheel originally specified for
the use of high-performance low-              TR7/TR8 Plus Sizing                   the MGC. The exception is the Austin-
profi le tires without compromising            STOCK WHEEL DIAMETER: 13”             Healey, with front 2” drum brakes:
looks and performance. A popular                 Original Tire: 185/70-13           Where 60-spoke wheels foul the brake
performance option for the 14” wheel              (diameter 589.2 mm)               drum, the wider MGC wheels will fit
is the 195/60-14 tire, which has a         Performance Alternative: 205/60-13       without a problem.
much smaller diameter than stock.                 (diameter 576.2 mm)                    The 72-spoke 15x5.5 wire wheel
This reduces the overall gear ratio             +1 WHEEL DIAMETER: 14”              was originally fitted to the TR6 and
and does not fi ll the wheelwell from             Tire Choices: 185/60-14            the necessary reduced offset may
front to rear.                                    (diameter 577.6 mm)               cause tire-rubbing on the outer fender
                                             195/60-14 (diameter 589.6 mm)          of cars originally fitted with 48- and
       MGB Plus Sizing                             RECOMMENDED +1                   60-spoke wheels.
    STOCK WHEEL DIAMETER: 14”                 205/55-14 (diameter 586 mm)                Center-lace wheels are available in
        Typical Tires: 165/80-14              Not many tire options available       15x5.5 for the small sportscar hub and
         (diameter 619.6 mm)                    +2 WHEEL DIAMETER: 15”              also for the larger Jaguar hub. These
   185/70-14 (diameter 614.6 mm)                 Tire Choices: 185/55-15            wheels look stunning, but also have a
      +1 WHEEL DIAMETER: 15”                      (diameter 584.5 mm)               much-reduced offset. So, outer fender
        Tire Choices: 185/65-15              195/50-15 (diameter 576.0 mm)          clearance must be carefully checked.
         (diameter 621.5 mm)                  205/50-15 (diameter 586 mm)           The unobstructed rim was a highlight
   195/60-15 (diameter 615.0 mm)                  Recommended +2 size               of the AC Cobra, so these wheels are
   There may be clearance problems                                                  often called “Cobra” wheels and look
                   with                   What If I Have Wire Wheels?               stunning on a TR250 or TR6.
   this width of tire on some wheels.     Although the classic wire-wheel crowd          Tires and wheels are one of the
      +2 WHEEL DIAMETER: 16”              doesn’t have as many performance          most popular topics for discussion,
        Tire Choices: 195/55-16           options, they shouldn’t feel left out.    and I hope that this article may
         (diameter 620.9 mm)              Here are some suggestions that will       clear up some of the questions about
   205/50-16 (diameter 611.4 mm)          get you going with even more style.       possible fitments. Each car is different,
 This width of tire could only be fitted   Spline-Drive Alloy Wheels: An             so always check clearance before
  to a car with modified rear fenders      expensive option, but the best alter-     allowing a tire to become damaged
 and some type of axle location device    native to converting to bolt-on wheels    and unsafe. If you are looking for the
         such as a panhard rod.           for maximum strength and a look that      ultimate in wide rubber, be ready to
                                          shouts CLASSIC PERFORMANCE!               do some body modifications; if in
TR6: Moving up an inch in diameter             Clearance is a major issue as        doubt, stay conservative and minimize
allows the use of a wider wheel due       the 13”, 14”, and 15” spline-drive        headaches. The difference in handling
to increased suspension clearance.        wheels are 5.5” wide and there must       between one tire size to the next may
Note that the only available 15”          be enough clearance to the outer          be insignificant compared to the
performance tire has a much smaller       fender for the wheel to be removed.       problems caused by tire-rub when
diameter than stock.                      Just because a tire fits the rim doesn’t   cornering.

18 | Winter 2005 • British Motoring

				
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