new rule: brake pedal strength test?

[Yunlong Xu]

Hi guys,

I've got some questions about the new rules regarding brake pedal strength for 2011.

As stated in rule B7.1.8:

The brake pedal shall be designed to withstand a force of 2000 N without any failure of the brake system or pedal box . This may be tested by pressing the pedal with the maximum force that can be
exerted by any official when seated normally.

I don't know what other team's pedal ratio is, but 2000N sounds a lot to me considering typical strength of a driver! I think we have been using something like 550N in our calculation and doubt if our driver (or any design judge)will ever be able to kickout a 2000N force on the brake pedal.

So the problem is: apprently we won't design a brake system to work with near 2000N brake pedal force. Even though we can design the pedal to be so strong, the brake calipers won't take that! (say we design for 550N, then it will generate about 4 times the line pressure when a 2000N force is applied!)

Does that mean we have to run our brake system at a pressure much less than the rated pressure of the caliper in order to get a reasonable pedal force?

Of course I am aware that the judges shouldn't be able to put 2000N on the brake pedal(hopefully), but just a bit worried about the chance of meeting a 'monster' judge who's capable of doing that... Also that's in the rule.

[Brian McGreevy]

2000 N is high but most definitely not out of the question in a panic stop situation. Relatively in-shape men can easily leg-press 400-500 lbs, and the impact force you'd see from a panic stop would surely be higher than 250 lbs (one leg).

[NickFavazzo]

~200kg, considering there are some people who weigh 100kg and go to the gym regularly 2000N doesn't seem too bad.

What I'm more worried about is that the pedal box needs to withstand that so does that mean teams need to document tests proving carbon boxes can withstand this?

change the brake master dimensions to match your caliper, thing is though 2kN is no were near what a driver would be regularly putting in. the rule is there so the brake system wont fail, doesn't mean you have to use the full force, just cater for it.

[KyleD]

If I may quote Steve Fox, "never underestimate the strength of a scared driver".

[brettd]

550N is too low for a design pedal force anyway.

Assuming your brake system is set up decently you'd expect to see about 1.5-1.6 G's of braking for at least some amount of time, take your average 70kg driver and ask him if he or she has ever just had their left foot on the brakes and no force on the accelerators heel stop.

If it has happened (or if you think it could), then your design load becomes 70*1.6*9.81 =~ 1100 N.

I realise I'm ignoring friction between the drivers suit and the chassis and more importantly the harness holding the driver in plus whatever else you have holding them in but a slightly loose harness would probably let you get away with all of that weight (if you had a stiff brake pedal) and I wouldn't put the coefficient of friction between the driver and the tub at any more than 0.2-0.3 (still around 900N).

[GianVioli]

Let's do some math about this:

We calculated the maximun input for a regular driver and it should be about 200kgf (otherwise a 150kg guy couldnt jump in one leg), so 2000N is fairly easy to achieve. I totally agree with KyleD quote, btw. I would be really carefull to make an alluminum brake pedal, since the Yield Strength and the Ultimate Stregnth in AL is closer than with Steel, AL is not so good as Steel when it comes to fatigue (specially under "impact" conditions).

About the calipers, let's say that this is another reason to build your own as we do http://fsae.com/groupee_common/emoticons/icon_smile.gif

[Yunlong Xu]

But I think 550N is something reasonable to put down as the maximum that a driver needs to do during normal laps

Assume a safety factor of 2, so not breaking at 2000N means the design failure load is 4000N on the pedal

so the normal operating pressure(force needed to lock up) in the caliper can only be 1/7 of the rated pressure.

Take Brembo P32G as an example. It's rated at 70bar, which means we can only run the brake at 10bar as a maximum for normal operation at 550N pedal force for lock up? that's way too low given the 32mm piston size of the caliper

And we aren't that rich to afford making our own calipers I'm afraid...

[Adambomb]

550N sounds like a reasonable "regular" maximum braking force (lockup), although there are other times (brake test at competition, for example) where actual brake pedal force was probably close to 1500N or more(I believe the surface at brake test was a lot better than the lot we tested ours at). We've also had very large men panic and stomp the brakes like you wouldn't believe (to further validate the Steve Fox quote). And the last thing you want to happen then is to have something in the pedal system fail and lose all brakes.

As for pressure loads, as our advisor would say, manufacturers already have a bit of a factor of safety in there as well to cover their butts. We haven't had any problems with lockup being something like 2/3 rated pressure. Destructive testing would give you a better answer on that though.

[Yunlong Xu]

Originally posted by Adambomb:
550N sounds like a reasonable "regular" maximum braking force (lockup), although there are other times (brake test at competition, for example) where actual brake pedal force was probably close to 1500N or more(I believe the surface at brake test was a lot better than the lot we tested ours at). We've also had very large men panic and stomp the brakes like you wouldn't believe (to further validate the Steve Fox quote). And the last thing you want to happen then is to have something in the pedal system fail and lose all brakes.

As for pressure loads, as our advisor would say, manufacturers already have a bit of a factor of safety in there as well to cover their butts. We haven't had any problems with lockup being something like 2/3 rated pressure. Destructive testing would give you a better answer on that though.

Yes, I believe that manufacturers have put in safety factors in their design already. But assuming a 550N lock up force giving 2/3 rated pressure. Then for a 2000N force would give about over 2.5 the rated pressure. Would the calipers be designed with over 2.5 safe factors?

[MalcolmG]

Originally posted by Yunlong Xu:
But I think 550N is something reasonable to put down as the maximum that a driver needs to do during normal laps

Assume a safety factor of 2, so not breaking at 2000N means the design failure load is 4000N on the pedal

why do you feel the need to apply a safety factor of 2? Your target strength requirement is 2000N, this in itself has a reasonable safety factor, why go adding more? Do you do the same with your impact attenuator? I.e. where the required energy absorbtion is around 7500J, do you design it to be 15,000J?

[Demon Of Speed]

Originally posted by Brian McGreevy:
2000 N is high but most definitely not out of the question in a panic stop situation. Relatively in-shape men can easily leg-press 400-500 lbs, and the impact force you'd see from a panic stop would surely be higher than 250 lbs (one leg).

I know college football players and wrestlers that can't dead-lift the ~340lbs (1500N). And you gain an advantage with using two legs, plus the left leg is usually not as strong as the right. With a 170lb driver, (s)he will have to be able to push about twice (2) his/her weight with one leg to achieve this, which is out of the realm for most people.

[mp24]

Woah, people are designing thier stuff with 550N? Damn!! We are working with 3000N as our "Crap your pants" force.

Also, if your calipers cant take the force generated by 2000N, wouldn't you just decrease your pedal ratio?

[nowhere fast]

Originally posted by mp24:
Also, if your calipers cant take the force generated by 2000N, wouldn't you just decrease your pedal ratio?

Then normal brake operation would require more pedal force than may be desirable.

[Adambomb]

Originally posted by Yunlong Xu:
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Adambomb:
550N sounds like a reasonable "regular" maximum braking force (lockup), although there are other times (brake test at competition, for example) where actual brake pedal force was probably close to 1500N or more(I believe the surface at brake test was a lot better than the lot we tested ours at). We've also had very large men panic and stomp the brakes like you wouldn't believe (to further validate the Steve Fox quote). And the last thing you want to happen then is to have something in the pedal system fail and lose all brakes.

As for pressure loads, as our advisor would say, manufacturers already have a bit of a factor of safety in there as well to cover their butts. We haven't had any problems with lockup being something like 2/3 rated pressure. Destructive testing would give you a better answer on that though.

Yes, I believe that manufacturers have put in safety factors in their design already. But assuming a 550N lock up force giving 2/3 rated pressure. Then for a 2000N force would give about over 2.5 the rated pressure. Would the calipers be designed with over 2.5 safe factors? </div></BLOCKQUOTE>

Hard to say really, that's why I recommended destructive testing.

[brettd]

Originally posted by Demon Of Speed:
I know college football players and wrestlers that can't dead-lift the ~340lbs (1500N). And you gain an advantage with using two legs, plus the left leg is usually not as strong as the right. With a 170lb driver, (s)he will have to be able to push about twice (2) his/her weight with one leg to achieve this, which is out of the realm for most people.

Can you jump (or hop) off one foot? Or take the subsequent landing on one foot without collapsing?

[Demon Of Speed]

Originally posted by brettd:
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Demon Of Speed:
I know college football players and wrestlers that can't dead-lift the ~340lbs (1500N). And you gain an advantage with using two legs, plus the left leg is usually not as strong as the right. With a 170lb driver, (s)he will have to be able to push about twice (2) his/her weight with one leg to achieve this, which is out of the realm for most people.


Can you jump (or hop) off one foot? Or take the subsequent landing on one foot without collapsing? </div></BLOCKQUOTE>

Obviously I can. Best case scenario, when I was in the best shape was when I was wrestling at 149lb in college, which meant I really weighted around 165lb. At this time I could dead-lift (which is high than squat) around 385lb which is around 233% of my 'real' body weight. As I said getting close to or above lifting twice your body weight in dead-lift/squat means that you are quite in shape (no one on our FSAE team is in that kind of shape, and I know the judges are not going to be). Because we are looking at best case scenario we will assume that the person can lift half of this weight with one leg (I am left leg dominant {which is unusual}) which would come out to 192.5 lbs (we designed our pedals for 200lbs). That is around 43% of the 450lb (2000N). Additionally, how most drivers sit in the car (lied back and knees bent) reduces the force that a person can push with there legs (at legs bent around 90deg you have a little less than half the strength in your legs). This is why people use chains sometimes when lifting, so it adds weight as your knees (or elbows for bench) become more strait.

For your theory to work the guy is going to half to weigh around 400lb. I have never seen any FSAE drive with 100lb of that weight drive a FSAE car. I am pretty sure a ~400lb driver couldn't fit in almost all cars without modifications. We have problems getting a 210 lb guy at 6' tall into the car to drive. Additionally, try your jumping on one leg idea with a couple of sand bag (to get your weight to around 400lb) and doing it with bent knees around what is seen well driving, cause I know I can't do it.

I completely understand the point of the rule, in that the brakes should never fail. However, a good driver knows that the vehicle will stop faster when the tires are not locked (which means a somewhat low, compared to the ~450lbs, force on the pedal). Additionally, turning the car out of the way of whatever is going to be hit is a much better idea then locking the tires. If it is a situation where there isn't time to turn out of the way then there probably is not time to get on the brakes for long enough to break them. Also, wouldn't you want the pedals to fail in a crash as to not crush your legs (if the chassis is being crushed from the force)?

I would also like to point out that almost everyone is designing with yield or fatigue strength. The ~450lbs should be a ultimate strength (IMO) thing, which means that in shouldn't ever have a test force of the full ~450lb on it. We designed our brake pedal for a fatigue strength of 100lb applied and a max ultimate strength applied of 200lb. We have never had a problem with our brake pedal, and it is quite rigid.

[brettd]

Two things: shock loads and how much have you seen people hack squat?

[BeunMan]

I'll take a look at the straingauge data from the DUT10 this week (placed on the rod's connecting to the brake cylinders) to see the force during normal driving.

This rule is in place to prevent carbon or badly designed aluminium brakepadels to bend and break under stress. There is a reason we don't use carbon brake pedals http://fsae.com/groupee_common/emoticons/icon_wink.gif

[Lorenzo Pessa]

but just a bit worried about the chance of meeting a 'monster' judge who's capable of doing that...

Be worried. The "monster" judges exist!
One of them entered in our car at design event in FSG and try our brake pedal.

[Zac]

Originally posted by brettd:
Two things: shock loads and how much have you seen people hack squat?

Seriously. Comparing an olympic lift like a deadlift or squat to the maximum force someone can apply to a pedal seems like a mistake. The limiting factors to what a lot of people can squat or deadlift are bad form and other muscle groups that are underdeveloped. An isolated exercise like a leg press would be a better indicator I think.

I'd be surprised if any driver in FSAE could squat 600 pounds, but a 600 pound leg press is likely for most, particularly if the guy is scared out of his mind.

[Crispy]

At OSU we went through this discussion several years ago and came to the same result as the new rule. I (average build 5'9" 160 lb) sat between two walls and pushed on a bathroom scale (back on one wall, scale on the other, with a nearly straight leg). 300 lb with one leg is a lot of force, but it was near the upper end of what I could do. We multiplied that by 1.5 (scared/larger driver factor) to set our design point at 450 lb.

[Dave Evans]

I'm glad this rule is coming in to force, I've seen far too many teams with flimsy pedals that are just dangerous. When judging at UK, we always made sure we had a judge sit in the car and push the pedal hard and watched for flex in the pedal and mounting area.

Sit with your back against a wall, some bathroom scales placed vertically against something sold to mimic pedals and find out how hard your drivers can push, you'll be surprised! 2,000 N is perfectly reasonable, I'm sure brake calipers will be able to take the extra pressure, it's the student built bits I'd be worrying about!

[brettd]

Originally posted by Zac:
Seriously. Comparing an olympic lift like a deadlift or squat to the maximum force someone can apply to a pedal seems like a mistake. The limiting factors to what a lot of people can squat or deadlift are bad form and other muscle groups that are underdeveloped. An isolated exercise like a leg press would be a better indicator I think.

I'd be surprised if any driver in FSAE could squat 600 pounds, but a 600 pound leg press is likely for most, particularly if the guy is scared out of his mind.

Sorry, should've said hack squat machine but yeah, 1500N easy with one leg on that.

[Drew Price]

Originally posted by Zac:

I'd be surprised if any driver in FSAE could squat 600 pounds.....



I'm pretty sure Adam from Iowa State could do it....


http://fsae.com/groupee_common/emoticons/icon_biggrin.gif

[t21jj]

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

I'd be surprised if any driver in FSAE could squat 600 pounds.....



I'm pretty sure Adam from Iowa State could do it....


http://fsae.com/groupee_common/emoticons/icon_biggrin.gif </div></BLOCKQUOTE>

? While I suppose that's possible Adam is not all that big compared to some of our other team members.

As for the rule, it is what it is. We've had some pretty big guys drive our cars including my self. I think it's pretty reasonable.

[Kirby]

We discussed this at length this year.

I believe we ended up similarly with a 2kN load simulation at the tip of the pedal.

[Adambomb]

Originally posted by t21jj:
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Drew Price:
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Zac:

I'd be surprised if any driver in FSAE could squat 600 pounds.....



I'm pretty sure Adam from Iowa State could do it....


http://fsae.com/groupee_common/emoticons/icon_biggrin.gif </div></BLOCKQUOTE>

? While I suppose that's possible Adam is not all that big compared to some of our other team members.

As for the rule, it is what it is. We've had some pretty big guys drive our cars including my self. I think it's pretty reasonable. </div></BLOCKQUOTE>

LOL, yeah, although I used to push my Jeep around the parking lot every other day when I was active duty Army, otherwise they would put one of those annoying "abandoned vehicle" stickers on it and tow it. Great lower body workout!

Although it's true, I'm pretty much average build for an ISU driver at 6'0" 200 lbs. We've had a couple people at or pushing 300 lbs drive regularly, even on our pre-template cars, and one of our competition drivers this year was 6'5" with size 17 shoes. From what I hear those are hard to find with the proper spec.

But our design load, which is pretty close to the new rule as I recall, was more based on a certain very large man who DID panic and stomp the brakes. Actually ended up wrecking the car, sadly. Unfortunately you can't always absolutely guarantee that every person that ever drives the car has good "panic braking" reactions.

As for the yield vs. ultimate strength discussion, don't forget to take into account torsional loading and twist-buckling on the pedal...

[Yunlong Xu]

So, if your brakes are designed for 2000N ultimate load (including calipers and pedal box)

what is your normal design load for lock up?

[2BWise]

Originally posted by Demon Of Speed:

I completely understand the point of the rule, in that the brakes should never fail. However, a good driver knows that the vehicle will stop faster when the tires are not locked (which means a somewhat low, compared to the ~450lbs, force on the pedal). Additionally, turning the car out of the way of whatever is going to be hit is a much better idea then locking the tires. If it is a situation where there isn't time to turn out of the way then there probably is not time to get on the brakes for long enough to break them. Also, wouldn't you want the pedals to fail in a crash as to not crush your legs (if the chassis is being crushed from the force)?


What does the driver do in a spin? If the car isn't pointed forward he can't steer away from impending doom.

[Dettle]

When comparing the maximum brake effort with some workout exercises you should think about the angle of the drivers leg when he sits in your car... e.g. when you're doing leg press you'll start pushing (lets say 600lbs) from an angle of min. 90 degrees between the lower and upper leg (at least you should do http://fsae.com/groupee_common/emoticons/icon_wink.gif ). And you will stop the motion at nearly stretched position.

So I guess most of you guys know by experience which phase of the leg press motion requires the most concentration and tension in your muscles!?
Obviously the more stretched you're legs are the easier the motion gets...so if you would start at an angle of under 45 degrees you could press close to 1000lbs instead of 600lbs.. or at least lift it a bit!

Actually you can see a lot of guys in the gym cheating this way! http://fsae.com/groupee_common/emoticons/icon_wink.gif

I would claim that for a sporty person it is possible to generate a Force of about 5000N (with both legs) if the angle in the legs is lower than 45 degrees.. and of course if he is able to somehow react this force with the rest of his body! So with one leg this will be eventually a little bit less than the half of 5000N. So 2000N sounds pretty realistic for me if someone really slams on the brakes in a panic situation!

[Brian McGreevy]

Originally posted by Demon Of Speed:
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Brian McGreevy:
2000 N is high but most definitely not out of the question in a panic stop situation. Relatively in-shape men can easily leg-press 400-500 lbs, and the impact force you'd see from a panic stop would surely be higher than 250 lbs (one leg).

I know college football players and wrestlers that can't dead-lift the ~340lbs (1500N). And you gain an advantage with using two legs, plus the left leg is usually not as strong as the right. With a 170lb driver, (s)he will have to be able to push about twice (2) his/her weight with one leg to achieve this, which is out of the realm for most people. </div></BLOCKQUOTE>

Again "Never underestimate the strength of a scared driver".

Secondly, having done both, many times, dead lift and leg press are two very different exertions. Dead lift requires substantial technique to lift any significant sum of weight. Any college football player or wrestler that can't leg press 500 lbs shouldn't be playing. Impact forces from one leg could easily reach those magnitudes when the adrenaline from a panic-stop situation comes into play.

Your argument about a good driver knowing that locked tires don't slow as well is rather moot. Someone who has time to think about pedal modulation while about to crash, has already reacted too slowly to avoid one.

[cjanota]

We were already pretty much designing to this. No yield at 400 lbs. We also set a max compliance at lockup.

If you are sitting with a fairly straight leg, I could see guys easily hitting 4-500 lbs.

[Andrew07]

For those worried about their calipers: why not just make sure something is done to prevent the hydraulics from reacting all 2000N? Something that wont come into play until after the kill switch of course.
Also, by using the idea above, someone could probably ease up the beefiness of the pedal if one knows that only some of that 2000N force will be reacted by the MC push or pull rod(s), which is obviously much closer to the pivot point of the pedal than the drivers foot. The rest of the force being reacted, perhaps, right on the other side of the pedal the foot is pressing down on (reducing the induced moment). Thoughts?

[Adambomb]

Originally posted by Andrew07:
For those worried about their calipers: why not just make sure something is done to prevent the hydraulics from reacting all 2000N? Something that wont come into play until after the kill switch of course.
Hmm, initial response is "ick, unnecessary complexity!" I can imagine some sort of pressure blow-off valve that routes fluid back to the reservoir, although in practice I can imagine Ol' Murphy taking effect and magically re-setting the blow-off pressure to 5 psi in the middle of endurance. Or just making it stick open all the time. Especially with the typical poor brake fluid hygiene a lot of college kids have. As for having something tied to the kill switch, double ick! Now you've got some sort of electro-hydraulic system, which sounds even more heavy and unreliable.

And when you say kill switch I'm assuming you mean the brake over-travel switch? If so, I wouldn't bet on that being activated just by stomping the pedal hard. If your brakes are well-designed there will be very little travel in the pedal from lockup to 2000 N. The reason the over-travel switch is there so in case you lose hydraulic pressure, resulting in the pedal going limp, it will shut off the engine.

The only other way I can think of right now to limit pressure mechanically would be some sort of stop for the piston in the master cylinder, but again with very little travel between lockup and 2000 N it sounds like it would be near impossible to get set so that it actually worked as intended without reducing braking capacity. Never mind the fact that if your brake fluid heats up you'll get a bit more travel.


Originally posted by Andrew07:
Also, by using the idea above, someone could probably ease up the beefiness of the pedal if one knows that only some of that 2000N force will be reacted by the MC push or pull rod(s), which is obviously much closer to the pivot point of the pedal than the drivers foot. The rest of the force being reacted, perhaps, right on the other side of the pedal the foot is pressing down on (reducing the induced moment). Thoughts?

I wouldn't count on any benefit from that. Also I don't like pedal designs that use the bottom of the pedal as a heel rest. Generally not so much of a noticeable problem with the brakes, but with the throttle the friction between your shoe, the bottom of the pedal, and the floor it tends to hold the throttle on after lifting your toe, at least for a second. Not cool. But you wouldn't gain much anyway, you'll have a ton more force on the lower pivot than the upper due to MC reaction forces being generally 3-5 times higher than foot force. To get a better idea of whether or not that's relevant, hop in your car, and without putting to much thought into it (to gauge your natural reaction) stomp on the brakes. Then notice what part of your foot is applying the force. I know for me, after I get through brake lockup test at competition I can certainly feel it in the ball of my foot; I don't even think my heel was touching.


But the more I think about it it seems kind of pointless. Last year our brake pedal weighed less than 0.5 lb, was designed for something in the ballpark of 2000 N with a factor of safety of like 1.5 (don't have the documentation handy), and was machined from 7075 Al on a 2 axis CNC. It's really not asking all that much; no need to make the problem more complex.

[Andrew07]

Originally posted by Adambomb:
<snip>

Hm, at the time what I posted made sense but I have not had experience in this area before. I assumed the compliance of all the braking components would be such that at 2000N the over travel switch would pretty close to be toggled over. That's way more force than what any of us use to size the MC.

Edit: Maybe this is implied but I figure I should reiterate. If the driver is applying 2000N to the pedal in an effort to stop the car maybe it is time for the engine to take a rest.

[Stocky Fast 1]

Easy way to solve the argument. If you have been running your car with pressure transducers back calculate. You will be surprised what your driver input will be. Depending on your system it could be 50-75 lbs, or it could be 100-250 lbs. Add a small factor of safety and the oh crap factor for a stuck throttle 400-500lbs is not crazy.

[Spetsnazos]

Originally posted by Zac:

I'd be surprised if any driver in FSAE could squat 600 pounds, but a 600 pound leg press is likely for most, particularly if the guy is scared out of his mind.

LOL! no...

No one in FSAE can do a 1 leg 600lb leg press....

I'll put money on this.

[Spetsnazos]

Do teams have to show test results and report for this new rule or are they going to be destroying brake pedals at the competition as part of testing?

EDIT: Also doesnt this new rule create unnecessary redundancy where there is already a requirement for a brake over-travel switch?

Seems highly unnecessary...

[Dash]

What i'm pretty sure this is trying to say is don't make your pedal out of cardboard. Just make it beefy enough to be able to take a 2kN load. It might just be me being unexperienced with the braking system, but I don't see how this will make any difference to the system as a whole, except making the pedal itself stronger. ( talking to you people asking about changing caliper forces, etc )
@Spetsnazos This is not redundant. The pedal breaking, and your brakes loosing pressure are two totally different things. If the top of the pedal snaps off, you more than likely won't be able to push on the nub hard enough to stop.

[Dsenechal]

Originally posted by Spetsnazos:
Do teams have to show test results and report for this new rule or are they going to be destroying brake pedals at the competition as part of testing?

EDIT: Also doesnt this new rule create unnecessary redundancy where there is already a requirement for a brake over-travel switch?

Seems highly unnecessary...

Your master cylinders can malfunction/lose fluid, allowing your pedal to over travel. Doesnt have to break in half...

[Zac]

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

I'd be surprised if any driver in FSAE could squat 600 pounds, but a 600 pound leg press is likely for most, particularly if the guy is scared out of his mind.

LOL! no...

No one in FSAE can do a 1 leg 600lb leg press....

I'll put money on this. </div></BLOCKQUOTE>

Did I mention anything about a 600lb single leg press?

and Drew, I did my free body diagram with a 150 pound driver. Depending on how large the ISU guys are they'd have to do somewhere between a 600 and 0 pound squat to reach the critical load.

[Spetsnazos]

Originally posted by Zac:

Did I mention anything about a 600lb single leg press?

I figured you tried to compare a gym leg press to pressing on the FSAE car, my mistake.

[Mike Cook]

I'm pretty sure that most of the calipers used on these cars will not hold up and/or rated for the pressures seen during a 300lb brake stop. At about 100lb, we are very close to the limit.

[cjanota]

Posted September 22, 2010 04:35 PM Hide Post


EDIT: Also doesnt this new rule create unnecessary redundancy where there is already a requirement for a brake over-travel switch?

Seems highly unnecessary...

If the pedal breaks off, it likely wont hit the switch in most designs that I have seen.

Also, you don't have to be able to do 600Lbs for a full leg press. A lot of the drivers sit with their legs fairly straight in the cars. The straighter your leg is the more you can push. Just standing on one leg and using my calves to lift my heel off the ground (a very similar motion to pushing a brake pedal) I have no problem lifting 500Lbs (body weight included.) That is more than this rule calls for.

[BFaz]

In my opinion, you can not use the dead lift analogy to judge the maximum amount of force that a scared driver can exert.

Last year, a student was conducting her fourth year thesis (on I think the strength of clothes) which required a test of just this. She organised members from a local karate group and who ever else was around on the weekend (so the SAE team...) to perform a front kick as hard as possible on a flat slightly padded surface. We each did 5 or 6 kicks and the maximum force was calculated.

I can't remember her total findings, but I kicked about 2kN (being of average height, weight and strength) and I saw numbers of up to 3.5kN.

Admittedly this was from a standing position where you can utilise other muscles, however I daresay that it is an indication that 550N is a bit low for a safety critical control interface design load.

[Hossam]

no one can press the pedal with 2000 N. and i don't know how can they test it for that .
it is not important because it is hard for the pedal to be broken . but the problem is that the other components will not withstand this high pressure generated from this force.
this components including calipers , hoses.
it is too hard for these hoses to withstand this high pressure.
to avoid this, we can reduce pedal ratio and increase master cyl. diameter .
but for normal braking (550N) with this new adjustment , the system will not be able to lock the wheels
please suggest what can we do

[RollingCamel]

Rules committee should clarify this. If not, what about pressure relief valve? At your selected pressure return the fluid to the tank.

[Whis]

So I messed with our brake spread sheet, and upped the force to the panic stop of 2000N. (~480lbf). I saw about 2700 psi in part of the brake system.

As far as I know, that is below what most calipers and brake lines are capable of holding. I'd love to hear different though.

Also, the rules say just the brake pedal must take the force. Make sure your pedal won't break with 2000N applied to it with a normal brake sensitivity (approx 75 lbf/g) and call it a day. Why are you worrying so much about the rest of the system?

[Mikey Antonakakis]

Yeah, after following this discussion it seems that the thing they are going for is to not have brake pedals fail. So design your mounting for the indicated load. Perfectly fair rule. Now, it would be nice to know how this rule is going to be enforced...

[PatClarke]

I cannot believe some of the nonsense I am reading in this thread!

We have had several recent brake pedal failures, some resulting in crashes (Wollongong a couple of years ago and TU Graz massive crash in Silverstone this year). We have had brake pedal failures when Design Judges were inspecting for pedal box compliance. Obviously, some teams were under designing a critical component.
Some action had to be taken before someone got hurt.
The judges were particularly worried about composite pedals (Both UoW and Graz).

After quite some deliberation (I was in the discussion) a figure of 2,000N was agreed on, for all the reasons outlined in this discussion, and composite pedals were banned That was when Steve Fox made his oft repeated statement 'Never underestimate the strength of a scared driver'.

No one is ever going to test for 2,000 in actuality! However, when a scrutineer or judge pushes hard on the pedal, it better not fail!

The stuff about potential caliper failure, pressure relief valves etc is simply nonsense. Any brake component, designed as such, will withstand whatever pressure Andrei the Giant could apply.

Now that you will have nice strong and stiff brake pedals, I guess the judges will be looking closer at pedal box compliance! (Hint hint)

The FSAE rules are written as they are. That means there are no hidden agendas and no attempt by the Rules Committee to steer design in some special direction. Stop looking for something hidden between the lines. This is not Formula One where you can interpret the rules any way you wish as long as you have a bank of very expensive lawyers to back you up.

By definition, the FSAE Designers are amateurs and a group with credentials from the University of Hard Knocks is trying to prevent you making expensive and painful mistakes.

Rant over.

Pat

[Yunlong Xu]

Thanks Pat for your input. I understand your concerns about safety issues and you are definately right! I was honeslty quite shocked when I heard the Graz incident in Silverstone.



Originally posted by PatClarke:

The stuff about potential caliper failure, pressure relief valves etc is simply nonsense. Any brake component, designed as such, will withstand whatever pressure Andrei the Giant could apply.

Pat

But I'm still a bit concerned with the caliper problem. For a caliper that is rated at 70bar, I'm not sure how much trust I should give for it to survive a test at over 200bar...

[Demon Of Speed]

Originally posted by PatClarke:
The judges were particularly worried about composite pedals (Both UoW and Graz).

After quite some deliberation (I was in the discussion) a figure of 2,000N was agreed on, for all the reasons outlined in this discussion, and composite pedals were banned

How does it make any sense to ban composites from brake pedals when they are allowed to be used as side impact structures and now for roll-over protection? If we trust college students to make a survivable structure in the event of a crash then we should allow the use of it for a brake pedal.

As I said before, if someone is 'freaking out' where they are pressing the brake pedal with 450lbf they are probably where they are going to hit something anyways. I would be worrying about the composite side impact and roll-over structure before worrying about the composite brake pedals.

Additionally, aluminium (and titanium, depending on which metallurgist you are talking to) have a fatigue life. It is possible that the metal brake pedal designed to withstand a maximum force of 450lbf will brake suddenly due to fatigue at a lower force.

I would like some clarification on the "The brake pedal shall be designed to withstand a force of 2000 N without any failure" does this mean design to the ultimate strength (as airplanes are done)? An applied strain rate clarification would be nice.

[PatClarke]

Sadly for you DoS, the Rules Committee don't have to justify their decisions.
However, teams DO have to abide by them. It is the same for everyone and if you don't agree, there is another option....Well, two options if you consider opting out one of them.

Simply write to the Rules Committee c/o Kathleen MacDonald as you would with any rules query. They will debate/discuss your request and give you an answer.

Certainly trying to debate the issue on this forum will have little impact.

Cheers

Pat

[RollingCamel]

Then judges should start hitting the gym for the 2KN press next year. (j/k)

http://axon.blogg.se/images/2009/muscles_29811953.jpg

[Mike Cook]

Pat,

As I mentioned before, I'm not sure that most systems will handle the pressure seen at 300lb of pedal force. You might think that any system "designed as such, will withstand whatever pressure Andrei the Giant could apply" but this may be like 3 times the rated pressure for some of the components. I'm not disagreeing with the 300lb number, seems reasonable to me, just advising teams that you may want to check your line pressures at some point.

[PatClarke]

Mike,
Perhaps you didn't understand my point.
The new rule changes nothing relating to the pressure in the brake system. A scared driver will push like hell regardless of how the system is specified.
Any thought that the brake pedal should be the 'fuse' in the system to prevent over pressurisation of the brake system is crazy.

Simply, the Rules Committee don't want to see any more crashes as a result of brake pedal failure and have constructed a worst case scenario test.

The TU Graz crash in the UK in July scared the crap out of a lot of people! That the car left the confines of the course and hit a passenger car on an adjoining road was an event that could seriously threaten the viability of the event!

The Rules Committee have no idea what pedal leverage or master/slave cylinder diamaters are and so cannot factor that into the rule.

Nothing has changed except the pedal must now be stronger! Noone has ruptured a brake system in the past and I don't see it happening in the future unless the system design is inferior.
In that case, the hefty judge push on the pedal that causes the rupture might prevent such a happening on track.

I really don't want to debate this any more.

Pat

[Mike Cook]

Pat, I understand you and I'm not trying to debate anything. I was simply suggesting to others that if you are going to design one component in your system for a certain strength, all the components downstream of it should be able to handle the stresses as well.

[Whis]

MC,

I'm not so sure that it does. A) no one is saying that the actual force at the pedal to lock up the tires has to be 2000N~480 lbf. So, in a normal situation, no driver should ever use 480lbf. The only time a driver would ever use that much force is if he/she is panicking. Which leads me to B) if all of the brake components in your system can't take 3000psi just once or twice, then I'm not so sure you want to run them.

All the rules committee is trying to insure is that, in case your brake system takes a huge load from a really scared driver, the first part to fail will not be the brake pedal.

[Stocky Fast 1]

Pat you are correct when you state the worries about calipers are nonsense. No one seems to be taking into consideration that everything in a brake system design is adjustable. If you need 450 lbs of driver input on the pedal, so what. Your line pressure is not dependent solely on that figure, you have to consider pedal motion ratio and master cylinder sizing to determine line pressure. You also very likely will start at the tire, rotor, and caliper and work backwards to get a line pressure to design a pedal assembly too.

It is a shame about no compositess but even so I know for a fact that a complete braking system to stop a 500lb car can be made to weigh around 13 lbs with an aluminum pedal. I've done it, i've seen lighter.

[Rex]

I think what's giving people concern is that the rules imply they might actually put 2,000N on the pedal (the aforementioned monster judge scenario). If common FSAE caliper or brakeline rated pressures would be exceeded by this force in a system designed around a normal/reasonable pedal effort to lock the tires, then the concern about how the test would be administered is valid. Surely any concern based upon a chance of exceeding rated pressure in a hydraulic system is a valid concern.

If the goal is to prevent the brake pedal from being the system "fuse," perhaps teams could disconnect the hydraulics before the 2000N is applied. Or some other method to isolate the pedal. Along the lines of what a couple of folks have said/implied earlier.

Alternatively, if the goal is to ensure the braking system as a whole can withstand the worst case panic stop scenario (not an unreasonable requirement), then perhaps teams should investigate other line/caliper designs going forward. Clearly this is the right answer anyways if you believe a human is capable of exerting anything near 2000N with one foot (p.s. sit in a narrow hallway, back against one wall, and press with one foot on a bathroom scale against other wall to draw your own conclusions on this issue - takes just 5 minutes and prevents embarassing yourself by spouting incorrect handwavy BS on the internet).

If you're worried about it, just write the rules committee regarding how this test would be (or could be) administered, and implement your braking system based on their response. Maybe next year they'll provide additional clarity in the rules on this point. But don't believe any handwavy internet forum BS about whether they will/won't/can/can't put 2000N on your brake pedal - ask the rules committee.

[DOA]

I have to say that I was quite happy when I saw this rule in there following the failure of our brake pedal at last years event during the brake test. This was due to a complete lack of understanding by last years team of the level of side force a driver can exert on a wide pedal when giving the pedal a good poke in order to lock all four wheels. It only cost us 100g to fix it with a bit of bar rammed inside the pedal and a bit of welding but its rather embarrasing, not to mention scary in the light of Graz's of track excursions.
All of those moaning that other components may fail before the pedal may consider the off axis forces a bit more carefully now as our team should have done last year http://fsae.com/groupee_common/emoticons/icon_smile.gif.