Master Cylinder Position

[Du Machado]

Hi guys.. This is my first topic and post here. I´m desining a new pedal box to our car, and I have found some problems.
I´m trying to position my master cylinder as a triangle, but when I calculated the brake pedal ratio, it decreases as the pedal course increases. Is this true or am I miscalculating? For exemple, when the course is 1mm the ratio is 1/4,5, when 20mm the ratio is 1,85mm. As you can see the difference is high.
We´re using a Tilton 77 and my pedal has 170mm height. The distance between the pedal and cylinder´s pivot can be 40 to 60mm

to exemplificate -> http://formulasae.stanford.edu/images/pedal_box/Pedal_Box-162.jpg

Thanks all

[Du Machado]

Hi guys.. This is my first topic and post here. I´m desining a new pedal box to our car, and I have found some problems.
I´m trying to position my master cylinder as a triangle, but when I calculated the brake pedal ratio, it decreases as the pedal course increases. Is this true or am I miscalculating? For exemple, when the course is 1mm the ratio is 1/4,5, when 20mm the ratio is 1,85mm. As you can see the difference is high.
We´re using a Tilton 77 and my pedal has 170mm height. The distance between the pedal and cylinder´s pivot can be 40 to 60mm

to exemplificate -> http://formulasae.stanford.edu/images/pedal_box/Pedal_Box-162.jpg

Thanks all

[DART-CG]

Hi,

it is in the nature of a floating actuation, that the brake pedal ratio decreases but not that massive! We also use such a design with AP CP5558 MC's but try to gain an advantage from that degression as the driver is able to control the car on the brakes better on the limit while reaching tire locking as fast as possible. Our pedal rations differ from 5,1 in the beginning to 4,8 in the end at 10mm displacement of the pedal.

[Du Machado]

Dart, could you give some information about the angle between your CM and floor and pedal height?

If you can´t that´s no problem http://fsae.com/groupee_common/emoticons/icon_wink.gif

Thanks

[DART-CG]

It looks very similar to your design http://fsae.com/groupee_common/emoticons/icon_smile.gif

[Brian Evans]

That installation is just so totally wrong in so many ways...

The angle between the MC pushrod and the pedal should be approaching 90 degrees as the pedal is moved, ideally at exactly 90 degrees with the pedal fully applied at max brake pressure. Yours is about 25 degrees and approaching zero as the pedal is moved. Just totally wrong.

Brian

[DART-CG]

@Brian: Uhm... than nearly every Formula racing series uses a totally wrong designed pedal box, including F1 http://fsae.com/groupee_common/emoticons/icon_smile.gif
I think your thought is that the sine fraction of the pedal force is lost. This would be the case if you use a static mounting of the MC's and than this would be a real design flaw. But with a floating mounting the force has nowhere to go as into the MC's.
You can calculate the dynamic pedal ratio with a simple excel-sheet. We use such a system in our monocoque for two years now and it works perfect. Look at teams like Munich or Tu Graz, they use it, too.

[DART-CG]

Here is a picture from David Coulthards RB3 I took a few days ago. Although there are many different connectors etc you can basically see the floating MC's from Alcon which are actuated by a perpendicular brake pedal.

http://farm3.static.flickr.com/2341/2700712847_63c7facc8b_o.jpg

[Brian Evans]

Ok, I take my comment back. Very interesting. Does the pedal ratio increase if you move the pivot point of the MC closer to the pivot point of the pedal?

Brian

[Du Machado]

Yes... Closer the pivot points, higher the pedal ratio.. Dart thanks for the answer.. I did a model on Adams and found a ratio around 4:1. The Coulthard´s picture is interesting... Do you know how it works?? Is the pedal on the left?

Thanks

[DART-CG]

Yup, pedal on the left. F1 use quite massive bearings as drivers can apply up to 250kg (!!!) of pedal force during an emergency braking. For which pedal force for a full lockup did you design your brake system?

[terra_dactile]

Hey Du machado,
I believe you have a bigger problem with the pedal assembly you posted a picture of, the bias bar mechanism is set to pivot perpindicular to the pedal and not in the plane that the masters are attatched meaning that the setup will not function. The trunion style bias bar in your picture from tilton is supposed to allow the movement in one plane of the brake system to ensure that their is not a crazy amount of bias variation as the brake force is applied, a sperical bearing type system allows movement in all axis and when placed with pivoting master pushrods or pivoting masters lots of bias variation can been noticed when graphed against front or rear pressure. You can imagine that if your are setting your bias bar to hit the optimal pressure curve of the system for a certain maximum G value attained by your vehicle the actual dynamics of the system will be very uncontrollable if the bias is varying more that a few %, in 2006 with a spherical setup and pivoting masters we had a variation of +/- 6-8% with a slope to the bias variation graph when compared to the front pressure on th x axis. Our new setup with tilton trunion setup has +/- 2% bias variation.

Hope this helps you

Jude Berthault
ETS FSAE 2003-Current
Team Leader

[oz_olly]

After a little searching I found a picture of Jude's set up. Here is the link:

http://www.auto123.com/en/multimedia/photos/gallery/for...?bid=96005&binding=3 (http://www.auto123.com/en/multimedia/photos/gallery/formula-sae-the-school-side-of-racing?bid=96005&binding=3)

In the same gallery is a picture of the front upright. To roughly quote Claude 'This is the best upright in FSAE'

Cheers

Olly

UNSW@ADFA 2004-07

[pucksaver]

Does anyone have any information on that front upright? It doesn't seem custom but as if they took it from another vehicle and modified it.

[C.Zinke]

OT: Regarding the upright:
I doubt this is the best design you can make in FSAE untill somebody can explain why. I think that the design is not very efficent regarding lateral contact patch forces. In my opinion casted uprights are the best alternative. If you have no possibilities to cast, look at the upright of TU Munich. This upright should be the best design for a milled upright. BTW You should be critical with Claude's opinion.
/OT

Here is a picture of our setup:
http://img244.imageshack.us/img244/2166/20070406erstebildervomlce3.jpg

Christopher

[oz_olly]

Christopher,

I know this is getting off the topic of master cylinder arrangements. I agree with you that being critical of opinions is an essential traite in a good engineer (I was more commenting on what Claude thinks is awesome, and what's wrong with a complement like that http://fsae.com/groupee_common/emoticons/icon_smile.gif). I like the TU Braunschweig upright design as it gives a very efficient second moment of area for mass of the material. As far as making a stiff upright goes it is dependent on the young's modulus (E) and Ixx. Which you obviously know. I am in the process of thinking of a more efficient design at the moment. Have you posted your casting method before? If not I am very interested, I especially like your dry sump plate I spent a couple of hours looking at your stuff the other day (that crash must have broke a few hearts).

As far as master cylinder arrangements go and the balance bar design. I have noticed doing some work with a Formula Ford team that the bias tends to vary quite a bit. If someone has a way for me to post a picture I will send you screen shot of a brake application at 220km/h and you can see how much the bias varies. I am very interested in methods of taking this variability out of the system. I haven't thought into it too much yet but I know it's a problem.

Cheers

Olly

[terra_dactile]

Hello gentlemen,
I am in no way an expert on the topics mentioned above but over the past 5 years of Formula SAE I have seen quite a bit and could try and assist in some way.

Uprights ETS 2005-2008
The upright design was originally made from an upright we saw while visiting Bar Honda in 2004 for the 1st of 3 most desirable engineering product award we won in formula student. The design has been physically validated to ensure that the camber variation does not exceed a certain preset value, whole wheel assembly included ( what the tire contact patch would see as camber loss). As far as lateral contact patch movement i dont think we have a problem when we compare our variation with tire data, however the design is not perfect for steering forces as the round outer rib shape from top ball joint to steering arm pick up is not otimized with in the wheel, it could be a longer arm and it should only go straight from this point to the top as any other material is wasted. The upright are machined from 6061 t6 aluminum, sand blasted and anodized clear in the picture. They weigh about 1.2 lbs and the whole assembly has a camber variation of only 0.25 degrees for 1.5 g,s of lateral acceleration.

Brake system variation:
Design system to work with a bias bar setting of 50:50 using different master cylinder and caliper bores to get the desired theoretical bias setting.This allow bias bar to be more efficient with less angle in it.
Use trunion style bias bar which incorperates needle bearings in all pivots.AP and Tilton make nice units.
Get rid of any friction or loose in brake system, incorperate needle thrust and radial bearings into the pivot of the pedal.

Put a pressure sensor on front and rear brake lines to see the effect of th changes mentioned above, graph the bias at every point of the data obtained against the front or rear pressure, Claude Rouelle has a good article explaining this in his tech tips.

As for Claude Rouelle, he is a motivator, he is not here to make himself feel smarter, he makes comment to challenge us to do better, in 2004 he told us our car was fat and to big, now in 2008 FSAE East we had the 2nd lightest car in the world behind TU delft our car now weighs 335 lbs compared with 510lbs(2004).
In 2004 we won the design event at formula student, but he still managed to criticise or car which helpd us evolve,I would much rather a critical judge then someone who like every thing you do and has nothing to say, in 2007 UK and 2008 detroit we finsihed 2nd behind TUG in design but have no idea what so ever what the judges did not like about our car!

Sorry for the essay

Jude Berthault
ETS FSAE 2003-Current
Team Leader

[Kirby]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by terra_dactile:

"2008 detroit we finsihed 2nd behind TUG in design but have no idea what so ever what the judges did not like about our car!"

</div></BLOCKQUOTE>

Jude,

It's very true that there was not much feedback during the initial design judging.

However, on Sunday (I recall) there was a call back to the design building for any team that wanted detailed feedback to take their car over.

We had a great chat with some of the judges about the pros and cons of our design.

I personally don't necessarily agree with some of the things that were criticized. However, I know that I can be as opinionated as they are. http://fsae.com/groupee_common/emoticons/icon_biggrin.gif

[RBbugBITme]

Those uprights look really familiar! I didn't design them but they are beautiful.
http://img.photobucket.com/albums/v364/iwantawd/FSAE%20at%20VIR%2008/rearupright.jpg

[STRETCH]

Back on the topic of the thread, it seems that some feel this type of master cylinder packaging is ideal, others flawed. You should definitely avoid having such a large angle between the pedal force direction, and master cylinder, and that pic is on the limit of not functioning at all.
The immediate implication is a massive pedal ratio, which also varies with travel as you say. It does minimise the pedal effort (not sure if this is ur aim) but consequently demands minimal master cylinder travel for it to work. So assuming you have such a rigid brake system (and you will always have losses in hoses etc...) that you can go from zero to max brake on a few millimeters of pedal travel (even this will vary the ratio), you better hope you don't get any knockoff! Even with small bores 2pots, any knockback will cause an enormously long pedal, so getting on it will cause the pedal to swing over the max extension of the cylinder, and you can no longer refill the system - that means no brakes by the way! Of course you could place a physical stop to prevent this overtravel, but it still means hundreds of pedal pumps to pressurise the lines again, assuming you have the cylinder stroke to allow the reservoir to refill it! This effect is exagerated of course by larger caliper piston area.
All this is assuming you can bleed the system in the first place... you need to achieve full stroke of the master cylinder to bleed (also note the highest point of the cylinder is above the reservoir connection). Because this arrangement demands such a small stroke, you will struggle / be unable to bleed the system, unless you are sourcing some very small stroke/enormous bores cylinders. Either way the system is compromised one way or another, and I haven't even touched on getting the bias bar to work.
There may of course be an angle between horizontal and that image, which offers a better compromise between pedal effort and travel, whilst keeping everything working properly. But in any case, do make sure your choice of master cylinder is suitable for that arrangment.

[RBbugBITme]

Are you saying that the design DART-CG has described to have worked for 2 years on his car, won't work?

[Kirby]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by RBbugBITme:
Are you saying that the design DART-CG has described to have worked for 2 years on his car, won't work? </div></BLOCKQUOTE>

I don't think he is saying *that* at all.
It will work, I think everyone has seen a similar design for the sake of packaging around comp.

He's saying there are some (oft serious) issues in having such a massive rising rate setup for your brake pedal.

Its up to the individual designing it to decide if the trade-offs in packaging benefits outweigh the drive-ability and ideal function of the pedal box setup.

Personally, I think there are much better ways to package the cylinders such that you don't get such a massive rate increase over the travel of the pedal. And still have the desirable "shallow" pedal-box profile.

[STRETCH]

I never said it doesn't work, but its a lot harder to get everything to work properly. DART-CG, do you suffer any long pedal issues during driving - any pad knockoff would cause a major problem??

[Steve Yao]

I think a little too much fearmongering from Stretch here.

As people have pointed out it works. I built a vertically orient MC setup for U of Washington starting in 2006. The system has a falling rate pedal ratio of 4.33 to 3.92 at 20 degrees of pedal motion. So thats 10% increase in driver input required to maintain lock-up at 20deg of motion. We never see 20 deg of pedal motion and what force variation there is, is well within what a driver can deal with.

Honestly, our team did have some difficulty bleeding the system, but I've also talked to teams who have had no problems bleeding. Must be something we were doing wrong. You do not need to utilize the full stroke of the MC to bleed them...at least not with the Tilton 77.

It was mentioned that the varying in real-time bias can be pretty alarming. As i understand it, the rear-pivot MC and accompanying trunnion balance bar were created exactly to combat this by reducing friction/stiction(as Jude pointed out).

There were 2 reasons I implemented this:
1) It reduced pedal tray length by 4"
-4" less chassis tubing x 6
-4" less brake, brake bias, throttle, and electrical lines.
2) Brake pedal now in tension/compression instead of bending

Some cons:
-These MC's and the trunnion balance bar are expensive even with FSAE discounts.
-If for any reason you are not at 50:50 bias, the fixed pivot position of the trunnion bar mean that the MCs will put lateral forces into the brake pedal.

Re: Other short pedal box methods
Placing MCs above, below, or in between the driver's legs each presented issues with compliance or safety in my mind, but teams do them all.

[Michael Palaszynski]

3.2.5: In side view, no brake components on the sprung part of car can project below the bottom of the frame or tub.

from the 2009 rules change summary.

[duckei]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Michael Palaszynski:
3.2.5: In side view, no brake components on the sprung part of car can project below the bottom of the frame or tub.

from the 2009 rules change summary. </div></BLOCKQUOTE>

Yes, one of the reasons we made a switch from the 75 series to the 77 series was to package the pedal assembly above the lower rails. It also is a much simpler setup for us and we can now make use of a brake proportioning valve...