Brake System - Caliper stiffness/failure pressure

[markocosic]

Hose burst pressure: in the region of 900 bar for the Goodridge stuff – rated to 300bar operating pressure.
Line burst pressure: in the region of 1250 bar for Cupro-Nickel ˜kunifer' line – rated to 250bar operating pressure.
Caliper: rated to 100bar (70bar) operating pressure for the P32F (P32G) – but no absolute maximum ratings given.
Master-cylinder: no ratings gien.


As per the other post, my crude spreadsheet reckons on 220mm front discs and a 205mm rear (effective diameters 201.5 and 186.5mm respectively), assuming those 32mm piston diameter P32 Brembo calipers. 4.5:1 or 5:1 pedal ratio and 15.875mm (5/8"�) bore master cylinders for front and rear give "balance"� adjustable from 0.7g to 1.9g deceleration assuming 63-37 to 37-63 adjustment available on the balance bar.

(50-50 is set for 1.3g – a little lower than the maximum decal predicted for predicted klutzy drivers, some initial stability/front lockup first and to allow some braking into the corner – though not 100% sold on this yet)


Here comes the question though:

My line pressures run around 40 bar on the 50-50 balance bar setting and up to 70 bar at the extremes of the balance bar adjustment. This is with ~400N pedal force. Under ˜blind panic' (an assumed 3500N pedal force) conditions this will shoot up to 600 bar. (!)

The pipes and hoses can take this, and fagpacket pressure-vessel calcs (~160MPa hoop stress) say that the master-cylinder body should be able to, but I'm not sold on how well these cheap cast alloy callipers will survive 45kN (10,000Lbf) trying to prise them apart.


More a curiosity than something that'll change the design (˜course you can have nicer callipers sir – that'll be the phase one upgrade at $$$ http://fsae.com/groupee_common/emoticons/icon_wink.gif ) but does anybody have thoughts on this? Anybody done stiffness measurements on these calipers? Anybody broken broken one? Do seals fail/weep at these pressures?

[markocosic]

Hose burst pressure: in the region of 900 bar for the Goodridge stuff – rated to 300bar operating pressure.
Line burst pressure: in the region of 1250 bar for Cupro-Nickel ˜kunifer' line – rated to 250bar operating pressure.
Caliper: rated to 100bar (70bar) operating pressure for the P32F (P32G) – but no absolute maximum ratings given.
Master-cylinder: no ratings gien.


As per the other post, my crude spreadsheet reckons on 220mm front discs and a 205mm rear (effective diameters 201.5 and 186.5mm respectively), assuming those 32mm piston diameter P32 Brembo calipers. 4.5:1 or 5:1 pedal ratio and 15.875mm (5/8"�) bore master cylinders for front and rear give "balance"� adjustable from 0.7g to 1.9g deceleration assuming 63-37 to 37-63 adjustment available on the balance bar.

(50-50 is set for 1.3g – a little lower than the maximum decal predicted for predicted klutzy drivers, some initial stability/front lockup first and to allow some braking into the corner – though not 100% sold on this yet)


Here comes the question though:

My line pressures run around 40 bar on the 50-50 balance bar setting and up to 70 bar at the extremes of the balance bar adjustment. This is with ~400N pedal force. Under ˜blind panic' (an assumed 3500N pedal force) conditions this will shoot up to 600 bar. (!)

The pipes and hoses can take this, and fagpacket pressure-vessel calcs (~160MPa hoop stress) say that the master-cylinder body should be able to, but I'm not sold on how well these cheap cast alloy callipers will survive 45kN (10,000Lbf) trying to prise them apart.


More a curiosity than something that'll change the design (˜course you can have nicer callipers sir – that'll be the phase one upgrade at $$$ http://fsae.com/groupee_common/emoticons/icon_wink.gif ) but does anybody have thoughts on this? Anybody done stiffness measurements on these calipers? Anybody broken broken one? Do seals fail/weep at these pressures?

[chris_uwa]

3500N panic load? That would mean a driver standing on one foot carrying about 280kg on his back..
I doubt you'll find any driver capable of exerting that much force, I'd also question the ability of most pedals to withstand that sort of loading.

The extent of our caliper stiffness measurements to date has been to hold ~500N on the pedal and measure the spread of the caliper using good ol verniers.

[James Waltman]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by chris_uwa:
I doubt you'll find any driver capable of exerting that much force... </div></BLOCKQUOTE>

There was similar skepticism when this topic came up once before.
http://fsae.com/eve/forums/a/tpc/f/125607348/m/61510812821/p/1

I don't think that Marko is too far off.

[Parker]

Assuming the driver could apply that much pressure, would the piston seals even remain sealed?

[markocosic]

Percy is a 95th percentile male - and a North American one at that...


Drefuss ('The Measure of Man and Woman' - ISBN 0-471-09955-4) reckons on:

99th percentile male as 112kg and 1920mm tall
95th percentile male as 102kg and 1870mm tall

1.92m/100kg (6'2@ and 220lb or 15st10) blokes who aren't just raw fat can leg-press 2500N without giving themselves hernias. (100kg man with 150kg on his back - piece of cake) The 1000N margin allows for some factor of safety, but nothing particularly huge. Easiest thing is probably to find a big guy/leg press and measure the numbers rather than debating them? Watch they don't stomp on the kit and smash the weights though - not popular with the gyms.

Granted it's a far cry from the 'FSG standard' 68kg driver (approx 1725mm tall?) but should probably be considered as worst-case.


What kind of caliper spread are you seeing if you don't mind me asking? I'm beginning to think that the rated pressure on the P32F caliper vs the P32G caliper is down to the extra material taken out to fit the larger pad and decreased pad/claiper stiffness as a result.

[KU_Racing]

I think you are forgeting the one-leg position of the brake pedal. 3500 newtons is approximately 800 pounds. I can almost guarantee that no one on your team can leg press 800 pounds; I doubt that more than a few can do even half that. Since the pedal is operated with one leg, this equates to a team member capable of squatting almost 1600 pounds- pretty much impossible for a professional strongman, so I dont think your team drivers will be able to do it. I think this is just a case of gross over-estimation on marco's part. Unless you can find me a someone who can fit in an SAE car and maintain 800 pounds of force on the brake pedal with one leg, I think you should your maximum panic pedal force to maybe 800 N, which I think is a little more accurate.

800 N, which is about 180 lbs, is still probably more than most of your team members can manage for any length of time with one leg applying the force. Also dont forget that the position that a driver is in is not optimized for applying force with the legs- just because a driver can squat 400 pounds does not mean that he could apply 400 pounds of force onto the pedals from the driver's seat.

[Ehsan]

As an extension of driver position, can your seat-back even take 400 lbs? Cause if you're putting 400 lbs on the brake pedal, thats where the force is propogating from.

I've certainly seen my share of paper thin seats that would flex just under the weight of the driver. another couple hundred pounds would wreak havoc.

[Dr Claw]

dont forget about loosened safty belts, and the "stomp on the pedal and pull up with the steering wheel" effect. A friend of mine, who's working for Continental's brakes division, told me of some surprising measurments that they took (though i've forgotten what the exact numbers are) but the underlying story is dont be surprised when doing brake calcs - it ends up bad when you are..

[Rex]

Yep, I agree with KU. Unless you know someone who can stand on one leg and support 800lbs you probably don't need to worry about 3500N forces at the brake pedal. Half that number is probably a reasonable estimate unless you have some exceptionally strong drivers. But measuring it is a great idea.

[Marshall Grice]

we've put a scale in the pedal box area before and measured the force. I was able to generate 200lbs of force pretty easily with out any external acceleration or adrenalin effects at work. For the record i can barely squat 250lbs with both legs. So i would be careful trying to relate what someone can squat to what someone can do to a brake pedal.

[Rex]

Is your 250lb squat including your weight? I would assume not. I weigh 150lbs and can barely do 250lbs (or at least I could several years ago, maybe not now - ha!), so that's a total of 400lbs being lifted including my weight, or a per-leg force of 200lbs. Sounds just about right relative to your scale test.

Hard to imagine someone of average (a.k.a race-car-driver) size doubling that, let alone quadrupling it to get to 800lbs. Use a panic factor of maybe 150% on your 200lbs test number and you get to a reasonable result that I would feel comfortable using in max force/pressure computations.

[markocosic]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">an extension of driver position, can your seat-back even take &lt;X&gt; </div></BLOCKQUOTE>

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">the "stomp on the pedal and pull up with the steering wheel" effect </div></BLOCKQUOTE>

Both valid calls http://fsae.com/groupee_common/emoticons/icon_smile.gif


<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Hard to imagine someone of average (a.k.a race-car-driver) size doubling that, let alone quadrupling it to get to 800lbs. </div></BLOCKQUOTE>

http://fsae.com/groupee_common/emoticons/icon_confused.gif You're designing for anything up to and including Percy.

Strength vs the mass of the person is decidedly non-linear in my experience too - adding 70lb of muscle to a 150lb body doesn't increase strength by just 50%... At the end of the day you can debate as you please to be honest - it'll be the real world and measurement that counts though.

For a given STIFFNESS requirement at the design load, how much extra material are you adding to meet the STRENGTH requirements? (as far as I can see with designing for 3500N - none...)


...back to calipers? http://fsae.com/groupee_common/emoticons/icon_wink.gif

[KU_Racing]

I thought we were talking about calipers.... if you dont see 3500 N of force at the pedal, than you dont need to worry about 45,000 N spreading the calipers do you?

Get the pedal calculations right, and everything down the line will follow. If you design for 45,000 N loading spreading the calipers, you might want to call freightliner for some of your racecar parts.

[terra_dactile]

Hi all,
A very easy way to end the contraversy of what amount of force is used during braking is to just by some pressure sensors and lsap them on your fron and rear brake line of one of your old cars, then you will fget from the system hydraulic measurement what kind of forces are being applied,

we did so and did not get very much higher then what we designed for using the available grip from the tires,

what kind of maniac would lose grip while braking and continue to push 8- 10 times harder?

Dont believe me thought, go test it for your self!

Jude Berthault
ETS FSAE 2003-Current
Vehcile Dynamics Leader

I was able to dicuss quite a bit with brake experts at FSAE EAST 2006 and FSAE West during the continental teves brake awards and managed to do fairly well so i am pretty confident saying that 3500 N is exagerating things by a long shot,

Hope some of my phsycho babble has helped this discussion!

[markocosic]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">I thought we were talking about calipers.... if you dont see 3500 N of force at the pedal, than you dont need to worry about 45,000 N spreading the calipers do you? </div></BLOCKQUOTE>

True http://fsae.com/groupee_common/emoticons/icon_redface.gif

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">we did so and did not get very much higher then what we designed for using the available grip from the tires, </div></BLOCKQUOTE>

Drivers that are cool, calm and collected even when it has gone wrong - sorted. http://fsae.com/groupee_common/emoticons/icon_smile.gif http://fsae.com/groupee_common/emoticons/icon_wink.gif

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">what kind of maniac would lose grip while braking and continue to push 8- 10 times harder? </div></BLOCKQUOTE>

Agreed, I find myself asking the same question too - along with 'what 95th percentile North American male will AutoX a Formula SAE car that weighs not much more than him', or 'what driver would drive into a wall with front wheels locked and the engine flat-out if the throttle sticks'. We're told that's the case by the rules and by accident reports on here.

Then again, as my housemate and I are trying to weld up tears in the floorpan and straighten a 10 degree kink in the towbar I had to wonder what the folks who designed these (http://upload.wikimedia.org/wikipedia/commons/d/df/Rover_800_02.jpg) would make of customers complaining that they don't make good rallycars. Or I look at the number of bent rear axles I picked up for my Land-Rover before finding a good one and you have to wonder what the bl**dy hell the previous owners had done to them. Or I ask a friend why they ran off the road only to hear 'the brake pedal made a horrible rattling sound so I let go.' Then you figure if it's physically possible, it'll probably happen in the hands of the public... LOL! http://fsae.com/groupee_common/emoticons/icon_smile.gif http://fsae.com/groupee_common/emoticons/icon_frown.gif