Bigger tires, heavy duty bumpers, winches, and armor. These are all common modifications that make your Jeep more capable and fun. But they all share a common often-overlooked negative side-effect: they reduce the effectiveness of your stock brakes.
It’s more than just the inconvenience of pressing the brake pedal harder to come to a stop. When the brakes have to work harder to stop, there’s less stopping power on reserve for an emergency. It also heats the brakes up quicker and can lead to brake fading, especially when driving in hilly/mountainous areas where prolonged downhill braking is common. It’s not just annoying. It’s a safety concern.
How your Jeep mods reduce braking performance
So how much harder are you working your brakes with your modded Jeep? First you need to understand the main contributors to what it takes to slow your Jeep:
- Total weight (mass) of the Jeep.
- Diameter of the tires.
- Weight (moment of inertia) of the tires and wheels.
The effect of the weight of the Jeep is pretty straightforward. It’s basic high-school physics: F = ma
. The force required to decelerate the Jeep (negative acceleration) is directly proportional to the mass of the Jeep. For our purposes here, we can interchange mass and weight (as long as you promise that you’ll always be driving our Jeep on Earth’s surface). If you add 10% more weight to the Jeep, your brakes have to work 10% harder to slow you down the same.
The diameter of the tire affects the amount of mechanical advantage, or leverage, the brakes have for slowing down your Jeep. Imagine a straight line drawn from the outside edge of the brake rotor, through the center of the wheel hub, to the opposite outside edge of the tire. This is an imaginary lever, with the center of the wheel as the fulcrum, that the brake caliper works with to slow your Jeep. The larger the tire, the less leverage the brakes have. This is also directly proportional relationship. A 10% increase in tire size requires 10% more braking power to slow the same as stock.
The weight of the tires and wheels is a bit more complicated. They contribute to the total weight of the vehicle, but they are also rotating. Similar to the F = ma
equation for linear acceleration, there’s a formula that describes rotational (angular) acceleration: = I
, which reads, “torque equals moment of inertia times angular acceleration”. I’ll leave it as an exercise to the reader to research “moment of inertia
” more. Long story short: a lot of math and some approximations about the shapes of wheels and tires
will lead you to discover that the impact of the tire and wheel’s moment of inertia on braking and acceleration is equivalent to adding about an e
xtra 80% of the weight of the tire and wheel. A 100 pound tire and wheel combo “feels” like a total of about 180 pounds to the Jeep when accelerating or braking.
Bringing it all together for a practical example
So how does this all work out when you add more weight and bigger/heavier tires to your Jeep. Let’s start with a stock JKU weight about 4200 lbs, with 32” tires. The stock wheel/tire combo weighs about 60 lbs. Let’s add 400 lbs of bumpers, armor, lights, and a winch, and upgrade to 37” tires weighing about 110 lbs each. Skip ahead to the bold text if you j
ust want to see the final result.
- Increase in tire/wheel weight: 50 lbs.
- Stock Weight = 4200 lbs
- Modified Weight = 4200 lbs + 400 lbs + (5 x 50 lbs) = 4850lbs
But we also need to add in the effect of rotational inertia of the 4 spinning tires/wheel to simulate how much weight it “feels” like the brakes are trying to slow down. Add 80% of the weight of the 4 tires and wheels:
- Stock Effective Weight = 4200 lbs + (4 x 0.80 x 60 lbs) = 4392 lbs
- Modified Effective Weight = 4850 lbs + (4 x 0.80 x 110 lbs) = 5202 lbs
Divide Modified Effective Weight by Stock Effective Weight to find out how much extra power you are demanding from your brakes based on vehicle weight and rotating mass of the tires/wheels (#1 and #3 from the last section):
5202 lbs / 4392 lbs = 1.18 = 18%
Divide new tire size by the original tire size to find out how much the tire diameter itself impacts braking (#2 from the last section):
37” / 32” = 1.16 = 16%
Multiply both results together to get the total impact on braking:
1.18 * 1.16 = 1.37 = 37%
That’s a total of 37% increase in braking demand
! You’ll have to press the brake pedal 37% harder. The brakes will produce 37% more heat, greatly increasing the chance of overheating the brakes. And when you really need to stop quickly, you’ll have 27% less maximum capability to slow down. Why not 37% less? That’s just how it works out when calculating the inverse of a percent change. For example, if you doubled the braking demand (100 % increase), you would have half the slowing capability (50% less).
How brake upgrades improve braking capability
So how can brakes be upgraded? There’s several aspects of the brakes that can be changed to improve braking capability.
- Larger brake rotors increase the mechanical advantage, or “leverage”, that the brake calipers have to slow your Jeep. They also have more surface area for dispersing heat.
- Drilled/slotted rotors disperse heat better, reducing the chance of experiencing brake fade from overheating.
- Larger brake pads have more surface area to grab the rotor.
- Fixed calipers (as opposed to stock floating calipers) have caliper pistons on both sides of the rotor, squeezing evenly from both sides for more consistent and even pressure.
- More caliper pistons spread the force more evenly across the brake pad for more effective use of the entire surface of the brake pad.
- Brake calipers can be designed to disperse heat better.
- Braided steel brake lines give better braking feel because they don’t expand as muchas stock rubber lines. Braided lines are also more durable, which can be a plus for off-road driving where brake lines could get caught on trail obstacles/debris.
(Be sure to look for minimum wheel size specifications for brake kits that you are considering. Larger rotors and calipers often require larger wheels for clearance.)
This all can add up to substantial improvements to braking performance. One manufacturer of Wrangler brake upgrades, for example, has demonstrated a 23% decrease in stopping distance, which suggests approximately a 30% increase in braking performance.
A brake upgrade may not be as exciting or visually appealing as many other Jeep modifications, but don’t underestimate the importance of reliably slowing your beast with ease. Check out the “Brakes” section of the Prodigy Performance online shop
for a variety of upgrade options.