Upright pedal box forces

[boxin]

Currently designing an upright master cylinder pedal box. We are trying to distribute the forces in the brake pedal. We are encountering difficulties in the locations of the forces. We know the human input force such as the foot force, and the force upon the master cylinder. We are trying to work out the forces upon the pivot and the brake pedal bracket. Any help would be appreciated. Thanks.

[boxin]

Currently designing an upright master cylinder pedal box. We are trying to distribute the forces in the brake pedal. We are encountering difficulties in the locations of the forces. We know the human input force such as the foot force, and the force upon the master cylinder. We are trying to work out the forces upon the pivot and the brake pedal bracket. Any help would be appreciated. Thanks.

[Nick Renold]

free body diagram that sh!t

[Adambomb]

Just make it all out of 3/4" rebar, and it will be fine.

[RBS]

Even i wanted to know.. We're wanting to change our assembly to a more vertical one but i'm unsure about the movement of the whole mechanism.. It's slightly difficult to visualize..

[Simon Dingle]

+1 to Nick. Everything is clearer with a freebody diagram.

@RBS, how much movement are you expecting? With most (all?) fs pedal box setups, the only movement in the brake pedal is due to compliance in the system.

While I'm guessing that Adambomb is joking, the best brake pedal I've ever seen was a length of 1" aluminium box section with two holes in it; one for the master cylinders and one for attaching it to the chassis. Must have weighed all of 100g, cost 50p and taken a total of an hour to design and make.

[cvargas]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Adambomb:
Just make it all out of 3/4" rebar, and it will be fine. </div></BLOCKQUOTE>

http://www.myfacewhen.net/uploads/954-not-sure-if-serious.jpg

[Chapo]

If you treat the whole thing as a pin jointed frame the calculations become easy with a simple FBD. Just remember that the angle its self acts as a lever (as obvious as it sounds, it took me a little while to get this and now im kicking my self)...

[RBS]

@Simon, we don't want too much travel, a couple of cm at most just so we have some feel.. Last time round our pedal travel was virtually non existent.. Hopefully it'll be better this time..
And as for the FBD, I've been giving it some thought and it makes sense..

[kcapitano]

Someone please correct me if I'm wrong, but my understanding was that your brake pedal shouldn't move at all. This is because your brain is pressure sensitive, ie. it responds to force, not displacement. So if it's "feel" you're looking for, you want as rigid a system as possible.

[Adambomb]

Of course I was joking about just using 3/4" rebar. Everyone knows rebar works better when reinforced with concrete http://fsae.com/groupee_common/emoticons/icon_wink.gif

As for travel, there are a couple things to consider: Yes, you don't necessarily want to have travel, but there is both a fixed distance the pedal will move before the brakes begin to work, and after that a system compliance that, in the end, will result in the pedal moving by a certain amount.

As for the best design, rectangular tubing works pretty awesome, the brake pedal on our 2007 car broke and in a span of two hours we went from saying "hey, let's build a new brake pedal for this" to looking for some paint for the finished product. And with almost no design work, it weighed something like 0.5 lb, and will easily withstand 450 lb of foot force.

[Simon Dingle]

@RBS, have a think about where that "couple of cm" is coming from. As Adambomb said, there a distance that the brake pedal needs to travel before the brakes will start to work. Off the top of my head this is the distance required for the piston in the master cylinder to cover the holes that feed the system from the reservoirs and the distance for the brake pads to bind on the disc. During this initial travel there is no work being done on the brakes and so you want to make sure that this isn't too long.

Once the brakes start binding, any travel in the pedal is normally down to bending in the pedal and it's mounting, but to a lesser extent flex in the brake lines.

As an aside, we had a very rigid pedal in 2010 and all our design judges with motorsport backgrounds loved it, all the judges with automotive backgrounds hated it. As always in design judging, it's not about doing the right thing, it's about knowing why it's the right thing to do.