BrakeFilter

March 20, 2005 6:14 PM

BrakeFilter: My dad and I need to settle a bet about driving habits affecting brake pad life.

He says that sudden stops from high speed wear out brake pads faster than gradual, slow stops, because F = ma means sudden decelleration equals high force on the brake pad.

I say that gradual braking wears out the brakes faster, since the brake pads are in contact with the disc for a longer period of time.

Whose explanation is correct? (Neither?)

posted by AlexReynolds to travel & transportation (13 comments total)

this article shoss that there's no simple general answer, but also seems to imply that, since brake pads are designed for particular scenarios, you should use them in the manner for which they were designed. so if you are using standard equipment pads you should probably brake reasonably progressively; if you are using "high performance" pads you should brake agressively.
posted by andrew cooke at 6:23 PM on March 20, 2005

My personal experience, when compared with that of a friend who treats his brakes exactly the opposite, shows that gradual stopping results in longer-lasting brakes.

Obviously, this isn't a scientific experiment, since I have a different type of car with different brakes, but I've replaced my pads half as often as my friend who brakes aggressively at nearly the last moment possible. In fact, I've even gotten comments from the mechanics who work on my car that they're amazed at how little my brakes are worn.

I'm very conscious about how I stop my car. I usually don't even apply the brakes until I've been slowing down naturally. When I do apply the brakes, I do so in a way that results in constant deceleration, no jitters, and I end the stop with very little force on the brake.
posted by odinsdream at 7:18 PM on March 20, 2005

Heat kills brake pads. Hard stops generate more heat. Hard stops will kill your brake pads faster than gradual stops.

However, brake pads are cheap in the scheme of things so I really wouldn't worry about it. If you brake too hard, too often though you may start to heat up and warp the rotors unless you have high performance brakes (which carry their own limitations) and that is $. This takes pretty aggressive driving.
posted by caddis at 7:49 PM on March 20, 2005

Friction is the tool and the primary enemy of your brake pads. Aggressive or fast braking will create higher disc / drum temperatures, and even possibly warp the shape of your brake pads/linings and the rotor/drum. The entire braking system has to be designed for the type of braking it's used in, so it can compensate for those extreme temperatures.

So, driving a Porsche? I'm sure you can get away with quick hard taps before the curves, and 50 foot dead-stops at the red light, without worrying about the pads so much.

Ford Taurus? Take it nice and easy.
posted by id at 7:52 PM on March 20, 2005

This is a tough argument. On the one hand, I'm tempted to say that the time the pads spend in contact with the disc means little, and that your dad is right. The only possible thing I can think of in your favor is that the pads probably wear more at higher temperatures, so perhaps quick in-and-out braking is better, since a larger proportion of the time they spend in contact is cooler. I don't know.

But I'm reminded of another argument I got into (with my high school French teacher, of all peope). I was arguing that 55 is the most fuel efficient speed to drive at, because that's the lowest speed at which you can drive in your highest gear. He said nope. It takes more energy to get somewhere faster, so it's always more efficient to drive slower. I'm not sure if he was right, in practice, but he definitely had a point in principle. As answers here have pointed out, frequently pencil-and-paper physics misses some real-world factors. I highly doubt that driving 1,000 miles at 15mph is very fuel-efficient.

But one real problem I see here is that you haven't defined "brake pad life" thoroughly. Quality of the pad plus type of system / electronic braking assistance can all affect the true answer - which may, in the end, be: no measurable difference at all.
posted by scarabic at 8:57 PM on March 20, 2005

If you combine F=ma with at=v-u and rearrange, you get Ft=m(v-u). Since v, u and m are the same in both scenarios, we can say Ft=k, or that halving doubling F halves t. In order for your dad to be right, doubling F would have to more than double the rate of wear, which seems reasonable. For you to be right, halving F would have to reduce the rate of wear by less than one half. I know nothing about the mechanics of friction-wear, but intuitively that seems wrong.

(Of course this model is far too simple to discern anything useful)

scarabic: Your professor is right that moving an object a given distance requires less kinetic energy the lower the speed, but the efficiency of producing kinetic energy from fuel also decreases, so there's a sweet spot where the engine is running reasonably efficiently and the energy expenditure is not too great.
posted by cillit bang at 9:36 PM on March 20, 2005

For you to be right, halving F would have to reduce the rate of wear by less than one half. I know nothing about the mechanics of friction-wear, but intuitively that seems wrong.

This is great reasoning, thanks. I'll bet that a more nuanced equation takes into account the friction effect determined by the brake pad material, which would explain why some pads work better for different braking applications.
posted by AlexReynolds at 9:51 PM on March 20, 2005

slighly OT, but you're less likely to get rear-ended if you stop gradually than if you stop suddenly. And since body work is always going to be more expensive than replacing brake pads...
posted by pmbuko at 11:57 PM on March 20, 2005

Given the very low cost of brake pads and the fact that an ordinary person can change them - I agree with pmbuko that the most important rule in this situation is to drive safely. I was taught that one important aspect of safety is to drive in the most predictable manner possible so that other people can accurately anticipate what you're doing and thus give you the proper room, etc.
posted by Slothrop at 4:58 AM on March 21, 2005

if you want to do the hand waving physics, then you can look at momentum, as cillit bang did, and force x time = change in momentum, or you can look at energy, with force x distance = change in energy. my guess is that the best simple description of wear is to consider it proportional to force and distance - imagine the contact area between pad and disc and it seems reasonable that the amount of material "dragged off the surface" of the pad is proportional to the force applied and the amount of dragging. since the energy to be lost remains constant, force x distance is constant (double the force requires half the distance), but the simple model suggests wear = force x distance, so wear is constant too.

however, we've ignored both variations in force and temperature. if you're breaking smoothly, you're not limited by anything in particular but presumably you want the deceleration to be pretty much constant (except for a taper at start and end of braking). so force would be constant over time (force = mass x deceleration). if you're breaking hard, you're limited by traction (too hard and you skid), so there's a hard upper limit and again force is pretty much constant (just below that upper limit, if you judge it right). so let's keep assuming force is constant.

temperature depends on two things. the thermal mass of the system, which slows the rise in temperature, and dissipative cooling, which is proportional to temperature (something hot cools faster than something warm - there's no paradox because as its temperature lowers to "warm" its rate of cooling drops as expected). the easiest thing to assume here is that the thermal mass is pretty small, so that things are always in equilibrium between heating and cooling. cooling, as i've said, is proportional to temperature. heating is proportional to energy input, which is force x speed (from force x distance per unit time). so temperature at any moment is proportional to both speed and force.

to recap, i started by arguing that wear is proportional to force x distance which is constant (higher force meaning less distance). however, that was ignoring temperature. i've heard that increased temperature is bad for brakes (fading), so all other thing being equal (as our earlier argument suggested), it seems that whatever gives lower temperatures should be better. and since temperature is proportional to force, that suggests that lighter braking, with lower temperatures, is the best solution.
posted by andrew cooke at 5:33 AM on March 21, 2005

(i've assumed, but never stated or proved, that the distance travelled by the brake pad "along the surface of the disc" is proportional to the distance travelled by the car. this is true - if the wheels, discs etc all stay the same size, which is a pretty reasonable assumption)
posted by andrew cooke at 5:35 AM on March 21, 2005

I would submit that the best method of stopping a vehicle is to coast as long as possible before applying the brakes (in a slow, gentle manner). As you coast, your speed tends to drop. If you brake from 20 mph instead of 35 mph, the wear on your brakes will be much less.

I cannot count the number of times that I am passed by cars as I am approaching a red light who then have to brake sharply. I know from talking to friends who drive in a similar way that they have to replace their brake pads more often than I do, as well.
posted by achmorrison at 9:39 AM on March 21, 2005

Gradual braking implies more fuel efficiency and less brake wear precisely because it must be done over a longer period of time. With sudden braking, you are spending that extra time putting fuel and energy into forward motion that must ultimately be undone. You are using gas to fight friction that would otherwise help slow the car and save brake pads. With problems like this, I find it helpful to imagine the extreme case. If you look at the traffic lights well ahead, and the idiots don't pass you only to stop in front of you at the lights, you can back off the gas occasionally and drive through town without using the brakes at all.
posted by weapons-grade pandemonium at 11:09 AM on March 21, 2005