The Geo Storm was designed to use a 14x5.5 inch steel wheel, with a +40mm offset, 4x100 mm bolt pattern, 56.6mm
hubcentric bore, and a 185-60R14 tire. This is the standard wheel and tire combination used on the Base model
Storm. Most of the GSi models were equipped with 15x6 inch aluminum wheel, with +40mm offset, 4x100mm bolt
pattern, 56.6mm hubcentric bore, and a 205-50R15 tire.
Aftermarket wheel selection is reasonably plentiful, because these sizes are shared with early 90’s Mazda Miata
and Honda Civic, although the hubcentric bore size is unique to the Geo Storm and Isuzu cars. Thankfully, most
aftermarket wheels are made with a much larger bore, and use hubcentric rings to match the many, specific sizes
required by various vehicles. In past years, 56.6mm hubcentric rings were not overly common, shared only with
some Deawoo vehicles. The only alternative was to buy the more common 56.1mm ring (Honda size) and carefully
enlarge it to the required size with a sanding drum on an electric drill. However, It appears that the Chevy
Cruze and Chevy Sonic, as well as several Opel and Vauxhall small cars, use a 56.6 mm hubcentric ring. These
GM small cars use a 5 bolt lug pattern, but they may be a good reference to find the 56.6 mm hubcentric ring,
which should be interchangeable between all the wheels of a given aftermarket wheel manufacturer.
The typical North American autocross and road course wheel and tire size combination is a 15x7 wheel with a 35-38
mm offset, and a 205-50R15 tire. These sizes are most commonly available, but, many have found that these vehicles
are not heavy enough and not powerful enough to get a 205 width R compound tire up to its optimum operating
temperature. A smaller size would be better, but a smaller size is not available.
The typical Japanese gymkhana and circuit wheel and tire size combination is a 15x6 or 15x6.5 wheel with 35-38
mm offset, and a 195-50R15 or even a 185-55R15 tire. The Japanese market has a better selection in smaller,
racing compound tires.
Light weight racing wheels that are recommended include: Volk CE28, Volk TE37, Enkie RPF1, Kosei K1 TS, OZ
Ultralaggera, Kosei K1.
R compound tires that are recommended include: Hoosier A6, Hoosier R6, Hankook Ventus V214, Kumho V710, BFGoodrich
R1.
Street tire class (extreme performance summer) tires that are recommended include: Bridgestone Potenza RE-11A,
Toyo Proxes, Direzza Sport Z1 Star Spec.
Plus Sizing For Appearance
Both 205-45R16 tires on 16x7 wheels, and 205-40R17 tires on 17x7 wheels, with 37-38 mm offset, will fit the Geo
Storm. These sizes tend to yield less performance, because of the added weight of the wheel and tire, and narrower
availability of true performance tires in these sizes. 16 inch offers a few choices that may be tempting, but
when compared to a 50 series tire on a 15 inch wheel, the 45 series tire on a 16 inch wheel will be less gradual
and less predictable when they reach the limit of their grip (Break loose with less warning, causing a more violent
spin, and are generally less forgiving to the driver).
Tire Pressure
Street
For street use, best tire wear, fuel economy, and mildly spirited driving, set the hot tire pressure
to match the maximum tire pressure rating printed on the sidewall of the tire. For a 185-60R14 or 205-50R15
tire, this rating is probably going to be 31-32 PSI. Hot pressure, means the pressure at the maximum tire temperature that the vehicle will experience. This
does not mean driving off of the street, into the gas station, and airing the tires. This means running the car
on the interstate, on a hot day, at the maximum speed that it is to be driven at, getting the tires up to their
maximum temperature, then pulling off at an exit or rest area and bleeding the tire pressure down to the maximum
pressure. When the tires cool after the car has been parked, that pressure will drop several degrees. But they
will come back up to that pressure the next time that the car is drivin on the highway again. Set the tire
pressure for street driving based on what the pressure will be when the tires are at their maximum temperature
for the most stressful use they will encounter. When airing up cold tires, compensate for the difference in
temperature by airing the tires 2-3 PSI lower than the pressure expected when the tires are hot.
If softer ride quality is desired, reducing the tire pressure py a pound or two will usually reduce the
accomplish a less bumpy ride.
Assuming the alignment is straight, and nothing is broken, observe the tire wear over time. If the middle of
the tire wears more quickly it may be desirable to decrease tire pressure. If the shoulders wear more quickly,
it may be desirable to increase tire pressure.
Racing
Tire pressure is one of the only adjustments that can be done in the staging lanes to fine tune handling, and it
is often misunderstood.
There are three tools required for adjusting tire pressure: a bottle of white liquid shoe polish (liquid window
chalk), a 60 PSI dial face tire pressure gauge, and a pyrometer.
There are three primary concerns that affect the tire pressure adjustment. This will be written from the
perspective of autocross or gymkhana use, which is arguably the most stressful for tires, especially with regard
to cornering forces.
Preventing Sidewall Rollover
Tire pressures, especially front tire pressures, will need to be quite high to prevent sidewall rollover.
During cornering, the forces on the car are primarily to the side, was the vehicle changes direction. The tire
tread will (hopefully) grip the track surface, while the tire bead will (hopefully) remain seated to the wheel,
which is attached to the car. Between the two is the sidewall, flexing and struggling to hold the two
together. Increasing the tire pressure will reduce tire sidewall flex, and help prevent sidewall rollover.
Soft sidewall tires, such as all season touring tires, are designed for ride comfort, instead of high G cornering
and if used for racing, would require extremely high tire pressures. R Compound racing tires are designed to the
opposite extreme, with very stiff sidewalls, and require much less extreme tire pressures. Performance tires
tend to be between the two extremes.
On a basic level of safety, it is best to adjust the tire to prevent it from coming loose from the wheel, and
preventing a blowout, accident, and the ensuing damage involved. And from a more economical view, preventing
damage to the sidewall, and excessive wear to the outside shoulder of the tire, will save money that might be
spent replacing prematurely wore out tires.
At an autocross event, many drivers will mark the outside shoulder of their tires with white dots or lines
of liquid shoe polish or liquid window chalk. When they take the car out on the course, the white marks on the
tires will wear wherever the tire touches the pavement. Between runs, the driver will check the tire marks and
adjust the tire pressure up or down, in order to prevent the tire from rolling over onto the sidewall, while
making sure to make use of all of the treaded surface of the tire. The goal is to get the tire pressure
adjusted so that the tread all the way to the edge of the shoulder is touching the ground during cornering,
and not a bit past that point. This point is usually marked on performance tires with little triangle arrows
along the edge of the treaded surface.
The exact pressure will depend on the track surface, temperature, course design, and how the car is driven. So
the exact ideal pressure will be different for each event, different for morning and afternoon, and different for
each driver. In addition, if the course has more severe turns in one direction or the other (as course layout
usually dictates) then the ideal tire pressure will be different for the left and right sides of the car.
And to add another layer of complication, the ideal front and rear tire pressures will be different because
these tires experience different conditions and forces. The front tires do the additional work of turning left
and right with the steering wheel, to change the direction of travel. And with a front-engine, front-wheel-drive
car, such as a Geo Storm, most of the weight (two thirds of the weight) is over the front axle and rest upon the
front tires. Meanwhile, the rear tires are encountering much lower lateral forces, and supporting much less
weight. They are really only there to keep the rear bumper from dragging on the ground.
Setting tires pressures solely by this method will often result in rear tire pressures half, or less than half
that of the front tires. Race tire pressures might be in the range of 30-40 PSI front and 17-22 PSI rear.
While street tire pressures (
especially without the benefit of negative camber)might be in the range of 55-65+ PSI front and 20-27 PSI
rear. These are not suggested pressure settings, but it should not be surprising if the tire pressure settings
look something like these numbers.
Evening Out Tire Temperature
Once the tire pressures are roughed out to prevent sidewall rollover, a slightly more advanced measurement and
adjustment method involves looking at the friction forces on the face of the tire tread, by measuring the
temperatures across the face of the treaded surface. This step is usually accompanied by, or instigated by,
the addition of more adjustable suspension parts allowing for adjustment of
caster, camber, and toe.
This is done by measuring the temperature across the face of the tire: the inside tread face (toward the middle
of the car), the center tread face (middle of the tire), and the outside tread face (toward the outside of the
car). This can be seen at events when the driver jumps out of the car immediately after a run, grabs a pyrometer,
and starts shooting tire temperatures, or with a more organized method, a friend shoots the tire temperatures
before the driver can get out of the car.
The desire is that the three temperature readings be close, which will indicate that the entire treaded
surface of the tire is working evenly. In practical application, if the difference is within 5 degrees from
one point to the next, everything is adjusted pretty well. But if the differences in temperature are more
than five degrees from one point to its neighbor, or ten degrees across the face, then things need there are
adjustments to be made.
Hot on the outside shoulder would indicate a need for more negative camber or toe out. Hot on the inside
shoulder would indicate a need for less negative camber or less toe out. Both of these adjustments are
concerned with alignment.
Once the alignment adjustments are made, the tire pressure adjustment comes in to get the temperature
measurements even between the center and the outside edges of the tire. Hot in the center would indicate
too much tire pressure, while cold in the center would indicate not enough tire pressure.
Oversteer and Understeer, Adjusting For Grip
After preventing sidewall rollover, and evening the tire temperature, the final measurement becomes the
subjective opinion of the driver regarding the balance of the handling characteristics, and the need to
increase or decrease grip on one end of the car, or the other, to adjust oversteer and understeer.
The tire will have a pressure point where it will have its optimal lateral grip and offer the maximum
cornering ability. Grip will decrease above and below that point. The result is a bell curve. The peak
will be somewhere in he middle of the pressure range, usually somewhere around 30-35 PSI (or we will
use that point for this example). Meanwhile, the previously discussed concerns of preventing sidewall
rollover, and evening out tire temperature, have already forced the tire pressure settings outside of
that optimal range, in order to keep the tire mounted to the wheel, keep the wear even across the
treaded surface, and deal with the realities of a front-heavy vehicle. Most likely the front tire
pressures are above that optimal lateral grip setting, and the rear tire pressures are below that optimal
lateral grip setting. Meanwhile, spring rates, damper rates, and sway bar rates, are all combining
and likely requiring just a little help to get everything just perfect. This is where adjusting tire
pressure for grip comes in.
In the event that the vehicle is experiencing understeer, reducing the front tire pressure can be reduced
by a pound or two to increase front grip.
If the rear of the vehicle is coming loose, increasing the tire pressure a pound or two will increase rear
grip.
In this way, the handling can be adjusted and balanced by moving the tire pressures slightly closer to, or
farther away from, the pressure setting for optimal cornering grip.
Be aware that R compound tires have a minimum tire pressure around 20 PSI. Below that pressure, the tire
will have no grip whatsoever. Looking back at the sidewall rollover method for setting tire pressure, the
rear tires of a front wheel drive car support relatively little weight and encounter much less lateral force.
Setting the tire pressure by the sidewall mark alone, will likely lead to reducing the tire pressure so much
that the rear looses grip. As discussed directly above, this is when to ignore the marks on the sidewall,
and increase the rear tire pressure to improve rear grip. Tire pressure for race tires might end up right
around 38 PSI front, and 28 PSI rear.
Space Saver Spare Tire; Upgrade Tread Diameter and Fitting Over Big Brake Kits
The spare tire is generally ignored. A 25 year old tire, even if never used, will probably be dry rotted
in the trunk of the car. The spare wheel is wider than the tire, and will likely have sawed completely through
the trunk bottom insulation and halfway through the sheet metal floor of the trunk. Rust may have eaten back
the rim to the point it is no longer serviceable for replacing the tire. And used spares from the junk yard are
usually in worse condition.
Unfortunately, the tire well space provided in the car will not allow for the width of a full sized spare without
the top of the tire sticking up past the flat surface of the trunk floor and creating a high spot, reducing
trunk capacity, making storage a mess, and eventually destroying the trunk liner and carpet.
More importantly, the T115/70D14 size space saver spare tire is about 20.3 inches in diameter, compared to the
22.6-23.1 inch diameter road tires the car came with on all four corners. The undersized spare should never be
used on the front if the vehicle. The difference in size will cause one tire to turn more revolutions than the
other when the car is driven in a straight line. The mildly annoying part is that the car will pull quite hard
to the side, and the steering wheel will have to be held firmly to the side to keep the car going straight. The
more important mechanical problem is that one axle will be traveling at a different speed than the other, and
this will cause the differential to allow one axle and wheel to turn more revolutions than the other. The
differential is designed to do this, for short durations of time, while the car is turning. But if it is forced
to do this for extended periods, it will get hot and this excessive heat can cause failure of the differential
(especially if a viscous coupling type, clutch type, or gear type differential is installed), and eventually
failure of the transmission gear clusters. For this reason, the OEM undersized spare should only be used on
the back of the vehicle, and dealing with a punctured front tire means switching out a good road wheel from
the rear axle to the front, and then installing the undersized spare wheel and tire on the back.
For cars with AWD, like the Impulse RS AWD, or Storms that have been converted over to AWD using the Impulse
RS AWD drivetrain, even using the spare on the rear is a problem, because the rear and center differentials are
viscous coupling type, and the car must have the same diameter tire or the excessive heat of binding the
differentials for extended periods of time will cause damage, and those parts are no longer available.
Unfortunately, buying space saver spare wheels is not easy. They are not commonly offered new from
aftermarket companies or by wheel and tire vendors. It seems that the sole source for space saver spare wheels
is the auto makers and at dealership parts departments.
There are several good donor candidates (other vehicles which use 4x100 bolt pattern wheels). And as wheels and
tire sizes have grown on newer vehicles, so have the space saver spare wheels and tires. Post-2000 models of
Mini, Honda, Toyota, and Nissan are all good candidates, with similar offsets, and most were available with 15
inch spare wheels. The first down side is that many are 56.1 mm center bore, and will have to be carefully ground
with a large diameter grinding bit to increase the bore size to 56.6 mm for use on a Storm (or Isuzu Impulse or
Stylus sister car). The other hurdle seems to be that the OEM tires on these 15 inch space saver spares is most
often a T115/70D15 size, which is 21.3 inches in diameter. This is a little undersized.
A T125/80D15 size tire has a diameter of 22.9 inches in diameter, which is about as close to the 195-50-15 and
205-50-15 tires that are most commonly used on the Geo Storm and Isuzu sister cars. The only cars found to be
using a 15 inch wheel spare, 4x100 bolt pattern, and a 125-80-15 tire, seem to be the 1993-1998 Volkswagen
Golf and Jetta. An added bonus is that these appear to have a 57.1 mm center bore, which is a hair larger than
the 56.6 mm of the Geo Storm and Isuzu sister cars, but should not be an issue for temporary use, which is
what the spare tire is intended for.
The spare tire can become a serious issue if larger brakes are installed. The OEM spare wheel is a 14 inch wheel.
It fits very nicely over the stock rotors and calipers even of the Impulse and Stylus sister cars which have
OEM four wheel disc brakes. But install larger rotors and/or larger calipers, and the spare sill no longer
fit.
A big brake kit designed for actual racing use, and meant to fit under a 15 inch aluminum racing wheel (11 inch
rotor), will not fit behind a 15 inch steel wheel. This is because cast aluminum structure is more compact than
stamped and welded steel. A 16 inch steel space saver spare wheel is required.
The best donor for this situation seems to be the a group of Toyota vehicles which all share the same 16x4
compact spare with a 4x100 bolt pattern: 2000-2002 Toyota MR2, 2003-2005 Toyota Echo, 2004-2006 Scion XA,
2004-2006 Scion XB, and 2012 Toyota Prius Hatchback. These also use a T125/70D16 size tire, which happens
to be 22.9 inches in diameter, and is the best match to the 195-50-15 and 205-50-15 tires that are most commonly
used on the Geo Storm and Isuzu sister cars. The draw back again is that these will have a 56.1mm center bore,
which will have to be carefully ground with a large diameter grinding bit to increase the bore size to 56.6 mm
for use on a Storm (or Isuzu Impulse or Stylus sister car).
For those installing 12+ inch diameter rotors on their Geo Storm (which is primarily for appearance, because a
properly built brake setup with 11 inch rotors will nearly yank the fillings out of the teeth of the driver
and passenger under hard braking), there basically is no combination of steel wheel and compact spare tire that
will fit over a brake rotor and caliper that is 12 or more inches in diameter, and carrying a full sized 16+
inch aluminum wheel and road tire is the only option.
