Rear-Pivot master cylinders

[FraserK]

Can anyone give me some tips on using rear pivoting master cylinders as part of the pedal assembly. Last year we used fixed mounted Tilton 75-series masters. I'm looking to angle the masters upwards by using the rear pivoting style to cut down on the assembly length to meet with the new template rule for 2013.

This is sort of what im going for:
http://seniordesign.engr.uidah...ip_image002_0002.jpg (http://seniordesign.engr.uidaho.edu/fsae/car08_clip_image002_0002.jpg)

My biggest question for people who have used this design is what kind of components are you using to compensate for loss of mechanical advantage (pedal ratio) when using a brake pedal design like this? (i.e. 4 piston calipers?, smaller master bores? bigger rotors?)

Basically I'm just wondering what kind of success people have had using this design and whether it could reliably lock up all 4 tires. Right now im looking at Tilton 77-series master's with the matching balance bar, but if anyone could reccommend some alternatives that would be great too. thanks

Kirk

[Dash]

I've never used one of these systems myself, but I have seen a fair number of good teams running a similar system.

EDIT: I stand corrected.

[acedeuce802]

What makes you think you can't design an upright brake system with a pedal ratio similar to one of a typical design?

[Owen Thomas]

There isn't as much of a loss in pedal ratio as you may think. Get some ballpark values (foot length, uncompressed MC length) and hash out the math and you'll see that it's fairly easy to get a PR over 4. It can be easily changed by altering the distance between the pedal pivot and MC pivot (effectively the angle of the cylinders).

I can also tell you that the Tilton 77-series cylinders are VERY nice (and light!), but with a price to match. The balance bar will run you around $600 alone, and the cylinders are $200 each after discount. They also only go down to a 5/8" bore, so keep that in mind.

[MCoach]

FraserK,

Let's work through this together.
I see that you want to advance your team a bit, but I'd like to not just give away the answer.

So, step one:

Identify the components of the system we are working with and their constraints. We have a lever that is rotational constrained to move only in one axis.
We have a link (the MC) constrained to increase and decrease it's length AND can rotate around two pivot points.
The confusing part comes in here, the imaginary link between the MC rear pivot and the pedal pivot. This essentially creates a triangle that supports a force at the tip of the lever (about perpendicularly) and reacts it axially through the MCs.

Step two:

FBD that shit.


Let us know what your approach is to the FBD. It took me a long time to figure it out correctly just because it isn't exactly obvious the first time around.
It may look like a normal pedal ratio isn't possible, but after working through it a few times and understanding the affects of changes to a few components, it becomes pretty clear how sensitive the system is to certain changes. After your reply I'd be more willing to talk a bit more freely about it.

[onemaniac]

My biggest question for people who have used this design is what kind of components are you using to compensate for loss of mechanical advantage (pedal ratio) when using a brake pedal design like this?

Kirk

Kirk, this is exactly why FSAE exists:
The problem you're facing is a first-year mechanics level problem. I bet you already took a course like that in your school and probably passed it with success. However, a lot of people just forget about what they learned in school and try 'screwing around' too much and end up wasting all the precious time trying to find the answer on Google or various forums. This is why FSAE is a good learning environment because more often than you think you can apply alot of school material to it.

I'll give you a hint. Draw a right angled triangle the smallest angle pointing upwards (as if you're looking at the brake pedal setup from the side). Then, assuming the bottom two corners are pin-jointed, you apply a horizontal load at the top corner from the adjacent side (as if you're pressing down on the pedal). Now, using whatever method you know (probably the joint/ node analysis method is the quickest for this case) and figure out how much load the hypotenuse member will see.(because that is your master cylinder!)
It literally takes less time than logging-in and posting your question on this forum.

Plus, think of other areas where you can gain extra mechanical advantage other than just the 'triangle'. It's pretty simple. try drawing more detailed diagram of your final setup, then you'll see it in a sec!

[onemaniac]

Originally posted by MCoach:

Step two:

FBD that shit.



Ah, MCoach beat me to it.

Anyway Kirk, give it a try. What I wrote above is basically just asking you to do up a FBD of the loading point of the linkage to the MC.

[MCoach]

My biggest question for people who have used this design is what kind of components are you using to compensate for loss of mechanical advantage (pedal ratio) when using a brake pedal design like this? (i.e. 4 piston calipers?, smaller master bores? bigger rotors?)

ACtually, just to assure you that it's possible to achieve the same pedal ratio that you are currently running -- when we switched to this style set up, only the MCs changed.

[Dewi Griffiths]

Our team had a similar brake pedal arrangement for 2012 competition. It's a nice design. Make sure you consider the pedal ratio and pressures produced though. We had a couple of joints in the system leak since pressures went too high. Maybe a lesson to never use carryover parts!

[Decker]

I have easily achieved a 4.66 to 1 pedal ratio using an upright setup.

It is all about the FBD, just spend a day making a correct diagram and make a spreadsheet from that.

Oh, one tip that will really save you time - in brake calcs you use the surface area of the pistons on ONE side of the calipers, mult by mu, then mult THAT by two to get total friction force =)

[MCoach]

4.66 seems a little low, even for me.

What input force are you designing to?
I suspect that your MCs are quite smaller than ours.

[Decker]

125 lb driver input force. It is going to vary a lot depending on what our braking setups and car weights are.

[FraserK]

Thanks for all the responses guys. I definitely made it more complicated then it had to be and after a little more playing around with some FBD's got really close to the Pedal Ratio I was looking for.

Unfortunately the price is pretty high for the Tilton components ($300/master and $450 balance bar), so just looking for any suggestions for alternative master's or balance bars that you've used and had success with that are a little less expensive. I've looked at the AP Racing CP7855 and a couple others but still prefer Tilton. Again, any suggestions are appreciated

[Dash]

Why not make some kind of bracket to adapt wilwoods to be rear pivot?

[Kevin Hayward]

Fraser,

The masters are very high quality from either AP or Tilton, I would just go with the one you can get the best price on. AP does some fantastic deals for FSAE teams.

It is pretty easy to make your own balance bar. They are not very complicated, and you just need to make sure you limit the degrees of freedom where necessary. Making your own allows you to make it either part of the pedal or part of the mount, which can improve packaging and reduce the number of fasteners.

Kev

[Kirk Feldkamp]

The Brembo masters are quite nice as well. They pop up on eBay all the time, apparently because NASCAR teams buy spares and liquidate parts frequently. I've gotten brand new Brembo masters for $65 before... retail is close to $600. There are screamin' deals out there, you just have to be on top of things to snag 'em.

-Kirk

[Cardriverx]

Both the AP racing trunnion and Tilton trunnion setup will cost the same. It is worth it in my mind tho, my balance deviation over a run is maybe 2% with those setups.

One thing to keep in mind is that AP components take a long time to get to you because they are in the UK. Last year it took over a month.


Originally posted by FraserK:
Thanks for all the responses guys. I definitely made it more complicated then it had to be and after a little more playing around with some FBD's got really close to the Pedal Ratio I was looking for.

Unfortunately the price is pretty high for the Tilton components ($300/master and $450 balance bar), so just looking for any suggestions for alternative master's or balance bars that you've used and had success with that are a little less expensive. I've looked at the AP Racing CP7855 and a couple others but still prefer Tilton. Again, any suggestions are appreciated

[Cardriverx]

Originally posted by Cardriverx:
Both the AP racing trunnion and Tilton trunnion setup will cost the same. It is worth it in my mind tho, my balance deviation over a run is maybe 2% with those setups.

Either way an upright MC setup is the way to go, it saves so much room.

One thing to keep in mind is that AP components take a long time to get to you because they are in the UK. Last year it took over a month.

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by FraserK:
Thanks for all the responses guys. I definitely made it more complicated then it had to be and after a little more playing around with some FBD's got really close to the Pedal Ratio I was looking for.

Unfortunately the price is pretty high for the Tilton components ($300/master and $450 balance bar), so just looking for any suggestions for alternative master's or balance bars that you've used and had success with that are a little less expensive. I've looked at the AP Racing CP7855 and a couple others but still prefer Tilton. Again, any suggestions are appreciated </div></BLOCKQUOTE>