pwned in design

[Ecks]

All right vets, lets hear it. What kind of crazy design questions have you gotten owned by in the past?

[Ecks]

All right vets, lets hear it. What kind of crazy design questions have you gotten owned by in the past?

[conekilr]

Last year our suspension guy got pwned by Claude asking something simple like distribution of roll stiffness front to rear and also which tires are more heavily loaded when steering with caster......ya, I don't think he even got to any crazy questions. That's what happens when someone practically rebuilds a previous design and doesn't attempt to understand the theory.

[Kyle Walther]

i too have been pwned by claude in design. He asked me what the roll stiffness distribution was, and i grabed my comp. to see what the exact number was, and he got offended and said "if you can't remember these things about your car, you must not love it. What is your girl friends phone number." and i had to reached for my phone. http://fsae.com/groupee_common/emoticons/icon_confused.gif

[Steve Yao]

The one i was not able to answer at the time was:
"What is the elastic modulus of your brake pads?"

[pablo180]

We didnt get owned but one question that we got last year at West was how would you adjust the brakes if you were running in the rain.

[Wesley]

With the brake pedal? http://fsae.com/groupee_common/emoticons/icon_biggrin.gif

[absolutepressure]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Kyle Walther:
i too have been pwned by claude in design. He asked me what the roll stiffness distribution was, and i grabed my comp. to see what the exact number was, and he got offended and said "if you can't remember these things about your car, you must not love it. What is your girl friends phone number." and i had to reached for my phone. http://fsae.com/groupee_common/emoticons/icon_confused.gif </div></BLOCKQUOTE>

Wow! He straight up dissed you and proceeded to tear you a new one! I think he was a little out of line in saying that, though.

[Steve Yao]

Judges at FSAE West did not do much 'owning'. At any rate, our judges pretty much waited for us to talk to them rather than ask us detailed questions as they do at Detroit.

The pad modulus question is one I got in Michigan 2004.

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by pablo180:
We didnt get owned but one question that we got last year at West was how would you adjust the brakes if you were running in the rain. </div></BLOCKQUOTE>

[Christopher Catto]

yeah, yeah, i know. replying to old posts. how sad.

but....

by chance i found the brake bias one in Olley Chassis Design because I am finishing to read one chapter i long left due to falling asleep in the middle of it. yeah. how sad too that I dont read Playboy un sundays.

so, Olley says you can use for example 60:40 in the dry and 55/45 in the rain. his exact words are:
----------------------------------------------
On a dry road, with 60/40 brake distribution the rear brake force meets the brake capacity line at point A (this is just relative to the car params of some kind of 50s saloon car. Chris). From then on the rear wheels are locked, but extra deceleration can be obtained from the front brakes.

With 55/45 brake distributionthe rear wheels lock sooner. Extra deceleration is obtained from the front brakes.

On the wet road with 60/40 distribution the front wheels will lock first.
With 55/45 distribution the rear wheels will still lock first (but with a higher value of deceleration. Chris)
Probably there are some reasons for keeping the 55/45 distribution even though it sacrifices some stopping power on dry roads.
-------------------------------------------

I am somewhat confused as to why the good ol god of vehicle dynamics accepted the fact that the rear tyres will lock first. possibly at low speed this is ok but from my experience at high speed this may cause fishtailing and general mad twirling of the steering wheel while pumping the pedal to avoid skidding!

oh well. is that what Claude would like to hear?

LOL for the phone no. of the gf tho! To be honest who needs to know the number by heart unless they carry a US army Vietnam wireless phone in their backpack??

I wouldnt get too upset about this stuff, even if it ruins ones chances in the final. fact is, sometimes shit happens.

[Bill_Murray]

I was just curious if anyone could explain why it is important to know the modulus of your brake pads.

[DJHache]

I think before you start measuring the elastic modulus of your brake pads you should try measuring the distortion in your brake calipers when you mash the pedal.

At any rate, as long as the pedal is stiff who cares if the thing distorts 1 or 2 thou?

By the by Chris, you are still one crazy Catto

[Storbeck]

Chris, I would guess that the judge was looking for a response along the lines that in the rain you use less front bias because you have less weight transfer due to lower overall grip.

About the brake pad modulus question. That is, in my opinion, a silly question.

I would be much more concerned with the stiffness of the caliper itself (from both its design and the elastic modulus of the material it's made from), the stiffness of the lines leading to the calipers (diameter, wall thickness, material, and length all matter) the bulk modulus of the brake fluid (and if it is well bled) the stiffness of the master cylinder mounting, pedal assembly and it's mounting, master cylinder quality, and correct sizing of master cylinders and brake calipers.

Also the friction properties of the pad and rotor material, and how they are affected by temperature.

In fact the elastic modulus of the caliper pistons and the sole of the driver's shoe probably matter more than the pad material.

Perhaps the question is fishing for something like the answer above, otherwise I would have gotten it wrong. Even if I know what the modulus of my pad material was.

Luckily for me I was never asked that question.

[DJHache]

I don't understand the logic behind letting the rear wheels lock up more quickly in the wet. Once the rears are locked the car spins. Have you ever seen a single-seater lock a rear wheel? It doesn't happen because that would cause the car to spin if it has even the tiniest amount of lateral force. If the fronts lock than the car goes straight and at worst you miss the apex by a few meters, but at least the car is still facing approximately the right direction. I've only ever seen drivers add brake bias to the front in the rain.

[Kyle Walther]

DJ:

by adjusting the brake bias rearward your not attempting to make the rears lock up first your just trying to increase the brake efficiency of the system. since in the rain the surface CoF goes down the potential weight transfer goes down. therefore require less bias towards the front. the best way to visualize this is to plot a performance diagram. Gillespe(sp)talks about this is one of his online lectures. all this is is a plot of the weight transfer and how close your brake bias is to the actual weight transfer of the vehicle. what you'll see is that if a system is tuned to brake at 1.6 to 1.8 g's the efficiency of the system is very low in the region of a wet surface or a low road CoF. when adjusted this higher efficiency will then allow you to squeeze out just alittle more accel from your brakes. But i could be wrong, and knowing this forum someone will let me know. http://fsae.com/groupee_common/emoticons/icon_smile.gif

[DJHache]

Hey Kyle, do you mind pointing me in the direction of Gillespie's online lectures?

[RiNaZ]

Kyle, this Gillespe(sp) talks, is it free and is it still online?

[Kyle Walther]

i wish i could remember what the website was called. a member on these forums called "fade" told me about it sometime ago. you have to login and then put in some special password. i don't even know if it is still there.

there is a section in brake design and safety by Rudolph Limpert called Comparison of Optimum and Actual braking forces (chapter 7.5) that goes over the same thing he just calls it something else.

[Bill_Murray]

That is what I thought just wanted to see if i missed something. Stoptech has some pretty good free papers on brakes basics:

http://www.stoptech.com/tech_info/tech_white_papers.shtml

"Brake Bias and Performance: Why Brake Balance Matters" has some diagrams covering what Kyle was saying (I think).

The only other thought I had on the modulus was if brake it is somehow generally related to COF, but that would be a backwards way to ask pad COF in design.

[Alan]

I went to the FSAE Claude seminar in '02 or '03. If I remember correctly the brake pad modulus thing had something to do with a particular driver he worked with not liking the feel of the brakes and it ended up being a change in brake pad that had a different modulus. For some reason this particular incident must have stuck in his head so he must feel its important. I feel if this is the limiting factor in your FSAE brake system, good on you.

I remember getting "owned" for not knowing the MOI of the car and the castor thing (which I feel really stupid about now).

[Christopher Catto]

LOL DJ Hache in the mix. Yeah I am still one crazy cat.

Just got back from Brno after free practice day one. Some nice DTMs out there for those who are into superlow front V8 engines and interiors that look out of a spaceship

Personal stuff aside, what Kyle is saying is perfectly correct (?!) since the driver just shifts the balance a bit to account for a lower min deceleration so less wt transfer. I would guess this is also linked to the springs you are running and the ride heights. Oh dear, I ve already found more ways to make the problem complex.

As for the pad modulus, I would say in something like sportscars, F1, Indy etc it would be quite noticeable. I would say even changing to a harder compound on your day car might have some effect (provided there are no air leaks in the system making your pedal already very soft.)

How noticeable is this gonna be? Ummm, I think the deformation in the hoses and the brake fluid will make up a large percentage of brake squidginess (new term for your Pi Toolbox channel). I know our FSAE car had a very stiff pedal but then we had a short travel AP master cyl and the leverage ratio was small so the pedal did not move much at all. Kind of weird. Dunno if good idea or not for the driver to brake relying on foot pressure and little/no foot travel.

Say hi to Jeff at Cranfield and enjoy your beer at the CSA.

[BryanH]

My .02 cents worth, have used many diff brands, compounds of brake pads. Modulus has a major effect on pedal travel and feel, BUT all modern race pads and more than a few road pads (eg TRW) are now carbon based and have the approx modulus of a brick - pedal moves until pads hit disk then clunks to a hard stop
His question is redundant

re Hashe"I don't understand the logic behind letting the rear wheels lock up more quickly in the wet".
In the wet if the fronts lock first you are looking at the scene of your accident.
If you can get the car to turn in, you can powerslide around the corner on the throttle but if it doesn't turn in.......
Cheers

[Christopher Catto]

so next time Claude will aks us how hard bricks are! http://fsae.com/groupee_common/emoticons/icon_smile.gif just taking the mick, no harm intended.

I was modifying some pads the other day. Our diamond grinder cut through them like cheese. they were Citroen C5 pads.

[JR]

It is impossible to know everything about your car... therefore you prioritize your time. Questions like "what is the modulous of your brake pad material" really show a lack of quality judging in my oppinion. I realize that essoteric questions can show just how well a team knows their car, but in the limited time that we have to be judged it is a ridiculous waste of time and a slap in the face for the many hours spent on the project. It borders on the illegal when a certain well known judge offers the answers to his essoteric questions if you will only pay him to come to his seminar.

Instead the judges should ask teams questions related to the engineering decisions they made. For instance... Why did you choose to make the part that way? What performance criteria did you use to choose that design? What was your load case? How was it tested? Under what conditions would you expect it to break?

Take even the best tested and understood part of your car and I can make up some dumb question you won't be able to answer. What is the relative magnetic permeability of your frame material????? What... you didn't induction harden the frame so you don't know the answer???? I'm shocked!

[exFSAE]

Given that some FSAE drivers can't feel when wheels are about to fly off etc I'd have to think the pad modulus is probably low on the priorities http://fsae.com/groupee_common/emoticons/icon_smile.gif

[Mike Cook]

JR, don't be too hard on the design judges...To be honest, I don't think I have ever got a bad question in 3 years of events at both Detroit and west. Sometimes I didn't quite agree with the judges and their POV but thats bound to happen..

[ben]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by JR:
It is impossible to know everything about your car... </div></BLOCKQUOTE>

The best teams do, end of.

Best answer I ever got to one of my questions was "It's on the spec sheet..." the response being "that's not the point, I need to see if you know it"

Claude asking the odd team what the modulus of their brake pads is isn't going to fundamentally change the order of things. I do agree though that the installed stiffness of the system as a whole is more important :-)

Having been a student and a judge I can safely say the every team that doesn't win thinks there's a reason other than another team or teams was better than them. The intensity of the situation does funny things to us all, but I firmly believe the system we have is the most workable solution within the time constraints.

Ben

[Superfast Matt McCoy]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by JR:
It is impossible to know everything about your car... therefore you prioritize your time. Questions like "what is the modulous of your brake pad material" really show a lack of quality judging in my oppinion. </div></BLOCKQUOTE>

It has been my experience that the judges ask questions that are raised by the answers to other questions. The questions get deeper and deeper until you don't know an answer and that's how they test your knowledge. Maybe SEY knew a lot about his brakes and so that was the next question. Maybe their team had found a solution to a problem that could have been more easily fixed with different pad modulus and the question was just a hintity hint hint.

Don't get me wrong, there is at least one judge that makes me want to head but the wall every time I think of his stupid-ass questions, but it certainly is not Claude and he is definitely an exception to the rule.

[VFR750R]

The brake pad elastic modulus question seems like a trick question which a few here have alluded to. He's not really asking what the modulus is, he wants you to explain that it is insignificant compared to the expansion of the calipers and lines ect.

I could imagine a judge asking an engine captain the thermal expansion of his stainless steel headers. And although its much higher then mild steel it won't change the engine tuning a significant amount.