Them's the Brakes · 9 January 06

Every car uses mechanical brakes. Even an electric car with regenerative braking (using a generator to turn momentum into electricity) has mechanical brakes as a backup. You also need some sort of brake for parking, since an electric motor doesn’t have the back-pressure of its gas counterpart.

Now that Eve’s dash is all pulled apart it’s a great time to get in and check out the brake system, rebuilding or augmenting whatever is needed along the way.

That and I was a little curious what the inside of the brake system looked like…

The photo above shows what you might think of as the brake system in a car. A big ol’ black cylinder with a bit of metal and piping sitting under a reservoir of brake fluid. Thin metal pipes snake along the firewall and out to each wheel. (btw, click the little images in the article for a zoomed view…I forgot to add the magnifying glass)

Somewhere behind each wheel you’ll find either a drum or disc brake assembly. Here’s how Eve’s front disc assembly (minus the disc) looks.

In the front left is one of the brake pads. The assembly in the back holds the two pads on each side of the metal disc attached to the wheel. The gadget on the right is the workhorse of the bunch, it’s the hydraulic cylinder which squeezes the pads together whenever someone steps on the brakes.

These cylinders pretty much follow the main cylinder.

What happens when you step on the brake pedal is that you are pushing a plunger into this hydraulic cylinder. I’ve removed the “C” ring and pulled the plunger part of the way out so you can see it better.

Inside the cylinder is hydraulic fluid, basically it’s stuff that holds up well under lots of temperature fluctuations and doesn’t really compress. The plunger is a lot like a water pump shaft, with a gasket that seals tight along the inner metal walls as it pushes and pressurizes the fluid.

The brake fluid transfers pressure wherever it can, most notably down the skinny pipes and into the disc or drum brake cylinders. The brake cylinder starts to expand, in turn squeezing the brake pads against the spinning wheel disc/drum and slows things down.

Here’s a closer look at the setup. This is typically referred to as a Master Cylinder. If you were leaking brake fluid or not getting good brake action you might buy a rebuild kit or replace it altogether. Over the weekend I checked both; one place quoted $39 for a replacement, another said $89 for a rebuild kit. Makes no sense to me either.

The wire coming off the side is part of the brake fluid sensor. Inside the plastic chamber is a little float valve which turns on or off depending on how the brake fluid level. Too low and it triggers the dummy light on the dash.

This is the power assist or brake booster. It’s sole job is to make your life easier.

This view of the booster shows the rod that extends into the passenger compartment and actually hooks to the brake pedal. When you step on the brakes you are PUSHING this shaft in.

On older cars and trucks you might not see one of these, just a master cylinder bolted to the firewall. What the brake booster does is take vacuum from the motor and use it to make your foot heavier. Well, not really heavier, but stronger. The chamber builds up a reservoir of vacuum and when you press the brake pedal the vacuum is triggered to assist in pushing the rod into the master cylinder

This is the side that pushes into the master cylinder. You can see that there’s been some brake fluid leaking and eating the paint. That’s something to remember whenever you top off the brakes yourself…brake fluid loves to eat paint!

The booster is upside down in the picture.

You can see a metal pipe coming out of the booster. This is the vacuum connection. You’ll find a thick hose going from here to the engine and somewhere along the way should be a check valve. The check valve let’s the engine pull a vacuum and keeps the vacuum from draining out of the booster when the engine is off (or at a reduced vacuum level).

We have an article elsewhere on the site covering Vacuum Assist for electric cars. Since the electric motor doesn’t generate vacuum like its gas counterpart we have to come up with another means of creating the vacuum, or start working out at the gym to toughen up our brake foot and leg.

I was wondering how much force it took to engage the brakes with and without the vacuum assist and decided to do a little test.

Actually, I’ve been watching Mythbusters lately and decided to try doing a little more investigative research. It’s all for the advancement of science, or at least our understanding of it. Rather than just regurgitate things I’ve read I’ll actually try a few of them! ”:^)

This is the setup. I used the DC-DC converter for a power source. This is from the first EV and was used to turn the 144vdc pack voltage into 12vdc to charge the system battery and help run lights and signals. It’s a heavy duty 30amp Battery Charger/Supply that can run from either AC or DC voltage since it is a switching power supply.

Paul Compton recently posted on the EVList that switching power supplies (usually what your laptop uses) have the following stages:

• input noise filter
• input rectifier (turns any AC into DC)
• input capacitor (smooths DC)
• switching element (turns DC into AC, go figure)
• high frequency transfomer (20Khz up to 2Mhz in some cases) provides isolation (important to keep high voltage system separate from low voltage)
• output rectifier (back to DC again, whee!)
• output inductor (smooth)
• output capacitor (and smoother)

Sorry about the tangent. Anyways, I wired the input up to a wall plug (110vac) and the DC output to the vacuum pump. The vacuum hose is hooked to the booster.

Before starting up the vacuum I set the rod on a bathroom scale and pushed down on the booster: 50-60lbs of force needed to get the rod moving. Started up the vacuum, let it go for a while and measured again: only 10-15lbs of force needed. Cool!

Obviously this doesn’t measure the force needed to push the real master cylinder while hooked to all four brakes, but it’s a good indicator of how much the booster can help out.

Since it’s a small pump and we might be going down a long hill (I know, regen would be nice) we’ll want to have even more vacuum buffering/assistance. Most EV’ers typically add another vacuum tank to the system. I have a chunk of sealed PVC pipe from the first EV.

Oh, the other reason I conducted this test was to see if there were any leaks in the booster. The first EV developed a nasty habit of running the vacuum pump for a long time and I was never quite sure why. Turns out from tonight’s testing that the vacuum shut off switch is a little flaky. It’s an Air Logic Vacuum Switch and I’ll be checking around for a replacement.