Brake Master Cylinders and Pedal Travel

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Jon Updegrove
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Joined: Thu Feb 07, 2013 9:01 am
Location: United States, Pennsylvania, Allentown

Brake Master Cylinders and Pedal Travel

Postby Jon Updegrove » Thu Sep 19, 2013 5:08 pm

Ok, so I had the f440 out this past weekend and since the frame up rebuild, I needed to relocate and make new pedals to accommodate changes between a 6'3" driver and a 5'2" co-driver. My rear master cylinder has 1/2" of travel in the bore while the front has 1/8". They are 2 different Wilwood master cylinders but the same bore size (3/4"). How do I even out the adjuster bar with the brake pedal but not change the "effective" point. Do I need 2 exact (or very similar) master cylinders or am I just missing something? I want to add a bias adjuster but the threaded adjuster rod binds because its on such a hard angle. when I first got the car it was like this as well. I tried to upload a photo so you could see what I mean:
https://www.facebook.com/photo.php?fbid ... =960%2C540
https://www.facebook.com/photo.php?fbid ... =960%2C540
Willing to race anything with 4 wheels!

Scott Newton
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Postby Scott Newton » Thu Sep 19, 2013 5:27 pm

You can alter the relative pedal travel by altering the bores of the two master cylinders. If you increase the displaced area of the rear master cylinder, it will travel a shorter distance for the same fluid displacement. Another way of thinking of it is that a larger MC diameter makes a "firmer" master cylinder.

If you currently have 4x as much travel in the rear than in the front, then you need 4x the piston area in the rear than it currently has to make it "even". This can be done by using a sqrt(0.75^2 * pi * 4 / pi) = 1.75" bore. Hmm... ok, so something is wrong with my math here, as that can't be right....

It's the end of the day, so obviously I'm missing something... but that's the idea.

As for balancing out the two co-drivers - the solution I've come up with is to have replaceable pedal "pads" that are of various heights. The original intention was for me to support myself (5' 9") and a 6'2" co-driver. We found that using 1" and 3" offset pedal pads with a custom made quick-change design would allow both drivers to use the same setup with just a quick pin swap between drivers.

Jon Updegrove
Posts: 81
Joined: Thu Feb 07, 2013 9:01 am
Location: United States, Pennsylvania, Allentown

Postby Jon Updegrove » Thu Sep 19, 2013 6:19 pm

I made pedals extensions that release with a cotter pin already but I had to move the pedals to where I needed them so I only had to make 1 set of extensions. I'm wondering what are the factors for pedal travel? Size of the bore, braided or hardline, length of brake lines, pedal height, # of calibers, # of pistons, and Fluid? and does bigger bore mean less pressure more travel or the opposite?
Willing to race anything with 4 wheels!

Richard Schmidt
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Location: United States, Minnesota, Plymouth

Re: Brake Master Cylinders and Pedal Travel

Postby Richard Schmidt » Thu Sep 19, 2013 9:54 pm

Jon Updegrove wrote:My rear master cylinder has 1/2" of travel in the bore while the front has 1/8". They are 2 different Wilwood master cylinders but the same bore size (3/4"). How do I even out the adjuster bar with the brake pedal but not change the "effective" point. Do I need 2 exact (or very similar) master cylinders or am I just missing something? I want to add a bias adjuster but the threaded adjuster rod binds because its on such a hard angle. when I first got the car it was like this as well. I tried to upload a photo so you could see what I mean:


John,

If I understand it, you are auto crossing. I don't see a need for dash adjustment of the brake bias in your situation.

If the brakes are working as is, then I don't see any need to change. You don't show a photo of the brake pedal being depressed. Is the bias adjuster balanced at that time?

Richard

jtc161
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Postby jtc161 » Thu Sep 19, 2013 11:42 pm

The MC's should be .75" front and .875" rear, that's what the car headed to oregon with. That works well given the calipers on the car, unless those too have changed.

Next you need to even out the clevis' on each master cylinder rod. back em off and thread em back the same number of turns. That will straighten out the bias bar, which shouldn't be jacked sideways like that. From there, hook up your remote bias adjuster, which you should buy, and go figure out where to set stuff. You probably won't be adjusting bias on a run, but it's significantly better than ripping the nose off to fix stuff. Plus, you'll probably run a lot more rear brake than your co-driver.

As far as your different co-driver heights, good luck. Put the pedals on a sliding rack like FSAE teams, and make two seats to move her closer.
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Jon Updegrove
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Joined: Thu Feb 07, 2013 9:01 am
Location: United States, Pennsylvania, Allentown

Postby Jon Updegrove » Fri Sep 20, 2013 7:03 am

Thanks for the responses.
Jeff, I was wondering if they "should" be different. The previous owner screwed alot of things up from when you had the car, I have found out. The driver, co-driver change issue has already been figured out. I made pedal extensions and a custom seat insert. All in all takes about 60 seconds to make a driver change (which isnt too bad considering the difference).
Rich, there are two main reasons I would like to add one. First that, no two autox lots have the same grip level. On a smooth slick lot I tend to like a bit more front bias then usual. And in the same way, I like alot more rear bias than usual on a concrete lot. Also, I change the bias depending on the course layout. If there are alot of tight turns, I add a little more rear brake, if its a long wide open course, I run a bit more front bias. Second, like Jeff said, I'm a heavy left foot braker, always have been. I seem to like way more rear bias then most other people are comfortable with. I dont know why, it just feels and seems to be the fastest thing that works for me. I like my corner entry rotation.
Willing to race anything with 4 wheels!


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