Mechanical Advantage on Floor Mount Brake Pedals

Distributor protectors seemed to bring about a slippery slope when it comes to modified derby cars. At first, the idea was that it acted as a safety device so that the literal FIRE WALL would not have to be cut out to prevent breaking a distributor cap. It didn’t take long to figure out that it also helped keep the car from bellying, and the snowball was rolling. A byproduct of this development was that the firewall was no longer available real estate to mount the brake master cylinder on. If the firewall is getting pressed and prestressed and stretched, it is no longer a semi stable place that you can count on to support the master cylinder and brake pedal.

Enter in dash bar and floor mounted brake pedals. I want to say that I used to see a lot more dash bar mounted setups 6-7 years ago, but now those setups seem to have gone the way of the dodo in favor of the floor mounted brake pedals. The benefits of the floor mount over the dash bar mount include easier install, additional stiffening of the floor/firewall, and in the catastrophic event of cage shifting, your brake pedal does not shift with it. The biggest con for me on the floor mount tends to be master cylinder access. It can be tough to refill while bleeding brakes, and it also makes access to the brake line going into the master cylinder very tight. Depending on how you route your brake lines, it can also make for some sharp turns to get from the master to where you need it to go.

The title of this post hints at the point I’d like to get into next. Some floor mount brake pedals don’t work very well. It doesn’t matter how much money you pay for your pedal set up if the geometry and design is not well thought through. A floor mount brake pedal is typically set up in the fashion of a second class lever, with the fulcrum at the bottom, the load in the middle, and the applied force (pedal) at the top. A standard firewall mount brake pedal is also a second class lever just with the pedal and the fulcrum switching sides. On a second class lever, the mechanical advantage is figured by dividing the distance from the fulcrum to the applied force by the distance from the fulcrum to the load. Mechanical advantage = (fulcrum to center of pedal distance)/(fulcrum to center of load distance). The google told me that a typical power brake setup advantage should be in the neighborhood of 5:1, and for a manual brake setup 6:1 or even 7:1 is ideal. What I’ve seen on some floor mount brake pedals is an advantage more on the order of 2:1. That makes for a very firm pedal and weak brakes. Not all pedals are created equal when it comes to this aspect, so I’m always judging them when I see new designs to see how the designer tried to improve it.

The second next important point to take into consideration is the arc of the pedal in relation to the master cylinder rod. If you pay attention to a stock brake pedal, the swing of the pedal does not start straight up and down. Instead, it is angled back slightly so that as the pedal travels through its arc it goes past straight up orientation. This allows the master cylinder rod angle to stay as close to straight as possible. If the pedal started with the rod straight, it could potentially bind up as the pedal is fully depressed. Look closely at where the master cylinder rod is attached, and the arm it is attached to should ideally angle back so that at full depression it creates an equal angle forward.

Other caveats to be aware of:

  • Some factory master cylinders may have residual pressure valves built in. If it was originally for a full drum brake car, it may not get along with disc brakes and hold too much pressure on them at rest.
  • The smaller the master cylinder bore, the less volume and more pressure the brakes can apply. 1″ or 1.125″ is a common factory master cylinder bore size, and 3/4″ seems to be a popular aftermarket master cylinder bore size. I like to split the difference at 7/8″
  • A floor mount brake design I like I purchased several years ago from a guy (not a big vendor) at Blizzard Bash. It moves the brake pedal rod arm to the bottom and reverses the master cylinder so it is mounted closer to your seat than the firewall. I like this design because it makes master cylinder easier to fill, work on, and run brake lines to. It also uses a short arm in relation to the pedal height for good mechanical advantage.

Thanks for reading and good luck guys!

Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s

%d bloggers like this: