braking developments
Testing the Effect Installing Bigger Wheels and Tires has on Stopping AbilityWhile the domino theory typically refers to a foreign policy, the concept also has an application that’s closer to home. It goes something like this: You put a lift kit on your truck or SUV so it climbs over tall boulders in a single bound. This super-sized suspension makes room for larger wheels and tires, which also help in tackling tough terrain with improved traction.
But now it’s time to head down the mountain, and the domino effect brought about by all the increased rotational mass of those big rims and knobbies suddenly becomes obvious in your braking ability — or rather, the lack of it. Another example is if you’re towing a toy hauler chock full of ATVs and dirt bikes with this same rig. This added trailer load is more evident in the loss of stopping power. Now’s not the time to consider whether you should have upgraded your brakes as well when you swapped to larger rolling stock.
Since being forewarned is forearmed (yet another foreign policy approach), we decided to follow along with Power Performance Group during some recent track testing to see just how much impact non-stock wheels and tires have on braking performance both in stopping distances and higher rotor temperatures. The latter is important, since brakes work by converting wheel rotation to heat, and high heat degrades stopping ability, otherwise known as “brake fade.”
During track testing, the factory system was overwhelmed by the larger wheels and tires. Brake fluid began to boil in the caliper and the ABS system could no longer modulate wheel lock-up. There was visible brake pad smoke and a strong burning smell as the vehicle came to a stop.
Power Performance Group conducted a series of braking tests from 60-0 mph with a 2004 Ford F250 Superduty 4x4 crew cab fitted with a 10-inch lift. (While not your typical trailride rig, it does serve well as a tow vehicle and could conceivably see some moderate off-road use.) These tests measured stopping distances, deceleration forces, rotor temperatures, and the Delta T (the change in the front brake rotor temperature before and after a run). In addition, with each stopping test, the driver noted subjective observations such
as vehicle control and pedal feel.The overall goal of these tests was to reveal the true effect adding larger/heavier wheels and tires had on the Ford and to determine if any improvements could be realized by upgrading to performance brake pads and rotors. The hard numbers gathered from the tests tell the whole story. (see accompanying charts and explanations) But lets get right to the bottom line and answer some basic questions about the affect of adding larger wheels and tires to a vehicle.
The difference in size between the stock tire and the off-road rubber is dramatic, so we’d expect to see a big difference in stopping performance as well, when using the stock factory brakes.
Do larger wheels and tires put an additional load on the OEM brake system?
No surprises here. They cause a noticeable increase in the operating temperature of the brake rotor. They also contribute to longer stopping distances and brake fade as temperatures climb into the 700 to 800° range.
Do larger wheels and tires hurt a vehicle’s stopping performance?
Well, yes and no. Big wheels and tires can cause a greater amount of variation in stopping distances from one stop to the next. On the other hand, larger tires also put a larger contact patch on the ground and improve traction. At lower brake temperatures, the added traction from 15.5-inch wide off-road tires actually reduced the stopping distances. But at higher temperatures the stopping distances were worse than stock.
Can Hawk Performance brake pads and Power Slot brake rotors help improve stopping performance?
Definitely. Look at the differences between the best stops from test number two and three. When the stock pads and rotors were replaced with Hawk LTS pads and Power Slot rotors, the stopping distances were between 3 to 12 feet shorter.
The Power Slot rotors were also slightly better at dissipating heat, recording an average Delta T (change in temperature) that was 8.7 degrees lower than the factory rotors. Finally, the test driver felt that directional stability was best with the Hawk Performance and Power Slot combination. Only minor steering corrections were necessary to keep the vehicle traveling in a straight line.
So what’s it all mean? Whether you’re on a trail or towing, having bigger rims and off-road rubber can have some performance advantages, but you should upgrade your brakes as well.
COMMENTS:
Ideally this test should have
been conducted with the vehicle
at its stock ride height. However,
the 10-inch lift kit was already
installed on the vehicle in preparation
for the switch later to larger
wheels and tires.
The OEM Ford brake pads and
rotors perform consistently when
used with the stock wheels and
tires. At lower brake temperatures,
the stopping distances
range from 225 to 237 feet. The
vehicle was hard to control
under braking and fishtailed on
every single run. Steering corrections
were constantly
required to keep the vehicle in a
straight line.
At higher brake temperatures,
the stopping distances actually
improved to a range of 210 to 220
feet. By runs number six and
seven, the brake rotors have
reached their maximum operating
temperature and start to lose the
ability to absorb additional heat.
As a result, more heat is transferred
into the brake pad and
fluid. At this point there is visible
smoke and a burning smell coming
from the brake pads as the
vehicle reaches a stop.
For each 60-0 mph stopping
event, the rotor temperature
increased by an average of 61.1°
F. The best stop in this sequence
was 210.2 feet recorded on run
number seven. The worst stop in
this sequence was 237.9 feet
recorded on run number one.
COMMENTS:
At brake temperatures below 774
degrees, the stopping distances are
better than the factory wheels and
tires. How is this possible? The
wider and taller tires have a larger
contact patch, increasing available
traction under braking. Also, at this
point, the OEM brake system can
still take advantage of the improved
traction and transfer the extra kinetic
energy into heat. Look at the Delta
T (change in temperature) of the
brake rotor for runs number one,
two, and three. The brake rotor is
absorbing 25 to 30 degrees more
heat per stop compared to the earlier
test number one.
However, when rotor temperatures
remain in the 700- to low 800-degree
range, the factory system is overwhelmed
by the larger wheels and
tires. Brake fluid begins to boil in the
caliper and the ABS system can no
longer modulate wheel lock-up. There
is visible brake pad smoke and a
strong pad burning smell as the vehicle
comes to a stop.
Flash temperatures (the boundary
layer of heat just above the surface of
the rotor) soar to nearly 1100 degrees
in the first few seconds after the vehicle
is stopped. Directional stability
becomes a problem as the truck darts
left and right under heavy braking.
Stopping distances increase dramatically
with the worst run (number five)
approaching 259 feet. On our final run
(number six) the brake pedal sinks to
the floor and the truck needs 243 feet
to come to a stop.
For each 60-0 mph stopping event,
the rotor temperature increased by an
average of 104.6° F. The best stop in
this sequence was 206.8 feet recorded
on run number 1. The worst stop in this
sequence was 258.8 feet recorded on
run number 5.
COMMENTS:
The Hawk Performance LTS pads
and Power Slot slotted brake rotors
clearly have a “sweet spot” in the 400
to 700°F temperature range where
they consistently outperform the stock
rotor and pad. Runs number one
through four show a very tight pattern
of stopping distances that increase
gradually on each successive stop as
heat builds up. The best stop in this
beginning group was 203.4 feet,
nearly 3.5 feet shorter than the best
stop in the entire Test number two.
Cutting the 60-0mph stopping distance
down by over three feet just by
changing brake pads and rotors is a
dramatic improvement.
On runs number six through eight,
we experienced brake fade. The brake
pad reached its maximum operating
temperature and stopping distances
increased. The worst run was number
six where the stopping distance
stretched to 265.2 feet. However,
directional stability was only a mild
issue, unlike tests number one and
two where the steering darted left and
right. There was never a complete
loss of ABS function and the brake
pedal feel remained firm throughout
this test sequence.
After run number eight was completed,
we allowed the brakes to cool
for a few minutes while driving. Our
intent was to determine how well the
Hawk LTS pads and Power Slot rotors
recover after being subjected to
extreme temperatures. The answer is
delivered to us on run number nine
when we achieve the best stop of the
entire day: 194.7 feet!
We also observed a deceleration
rate of 0.609G, and 156 degrees of
heat absorption by the Power Slot
rotor. Perhaps this was the equivalent
of golf’s hole-in-one shot, but this was
12.1 feet shorter than our best stop on
stock pads and rotors (see the first run
on Test number two.)
For run numbers one through eight
(the toughest portion of the test), the
rotor temperature increased by an
average of 95.86° F during each 60-0
mph stopping event. The best stop in
the entire sequence was run number
nine. The second best stop was run
number two.
left: At the conclusion of test number two, new pads
and rotors were installed on the vehicle (as
shown here, stock rotor is removed, and the
new Power Slot rotor is already installed). The
vehicle was driven for a few days to bed-in the
new pads and rotors. Test number three was
then conducted one week later.
right: Real-world measurements
on the track are valuable,
because lab tests of brake
pads and rotors are unable
to account for variations in
wheel/tire sizes and
weights, vehicle traction,
and ABS activation.
Distances and deceleration forces were measured using a Vericom VC3000 decelerometer, a device that’s approved by the FAA.
left:Starting and ending brake rotor temperatures were obtained from the
passenger-side front rotor on each test, using a Rebco Accutech 0-
1500° F digital pyrometer and an Omega 88010K temperature probe.
right:Just by exchanging the factory pad (left) for Hawk Performance LTS
pads (right) and installing Power Slot rotors cut the 60-0mph stopping
distance down by over three feet.
SOURCE
Power Performance Group
20645 Bahama Street
Chatsworth, CA 91311
818-709-4800
www.powerslot.com
Irwindale Speedway
500 Speedway Drive
Irwindale, CA 91706
626-358-1100
www.irwindalespeedway.com