force on pedals

[solex]

hi........m handling the design of pedlas of my team........
pls tell me for wat maximum force the accl and clutch pedals shud be designed?????????????
(we are having inexperienced drivers so adrenaline inputs form them wud be higher)
am planning to use 300 lbs of force for the brake pedal.

[Dash]

How much do you think you ( or any of your team members ) could squat?

[tougeferio]

you should probably read this article...
w w w.sae.org/servlets/pressRoom?OBJECT_TYPE=PressReleases&PAGE=showCDSNews&EVENT=FORMULA&RELEASE_ID=1311

[mp24]

300lbs is like 136kg. This is fair enough, and with your inexperienced drivers, add a fair factor of safety, say like x3.

BTW tougeferio, cool article.

[tougeferio]

Thanks but i cant take any credit for it... I'd been looking around for ergo papers two weeks ago and a team member sent me that link, it is a pretty good paper, very well written and perfect for someone just starting up on a team and wants to learn more about ergonomics of our cars.

[solex]

guys for wat maximum force(a nnumber) shud a acclerarter and clutch pedal be designed for......
m assuming 40lbs

[billywight]

guys for wat maximum force(a nnumber) shud a acclerarter and clutch pedal be designed for......
m assuming 40lbs

6

[Old Greg]

That's probably a fair assumption, but be sure take your pedal stop into account. Depending on where it's located you may end up with significantly more than 40lbs of force acting on your pedal/pivot.

[mmcdermott]

On a leg press machine that puts me in a position almost identical to a FSAE/FH car driver I can exert 340# with one leg slightly bent at the knee. I'm a 70 year old 155# male, so a large 21 year old should be able to exert more.

[Old Greg]

340 lbs on the gas pedal? That's one heck of a lead foot.

P.S.
One thing to keep in mind about leg press #'s is that the weight is slid up on a ~45 degree incline, so you really ought to be multiplying those numbers by 0.7

[oz_olly]

Not all leg press machines are 45 degree incline, many are horizontal that lift a weight stack vertically through a cable and pully arrangement.

I do agree that all pedals should be designed to about 150kgf (~1500N), we design the brake pedal with FoS of 2 as it is safety critical and others (clutch and throttle) with FoS 1.5. These are "No permanent deformation" load cases. The rationale we use is that in an emergency situation (spin and heading for barrier, btw I have seen exactly this situation happen to Curtin at FSAE-A) any one of the pedals could be subject to that load if the driver accidentally hits the wrong pedal. You have to ask yourself: Are you willing to have a broken clutch or throttle pedal in this situation? I have also bent a throttle pedal by getting a little over excited exiting a steep down hill corner (high speed, plenty of grip) and mashing the throttle.

It is important to note that on the clutch and throttle pedal the high design load needs to be reacted by the travel stop and not the cable actuation point. A simple triangluar pedal is a very easy way to achieve this.

Cheers

[mp24]

oz_olly, u use a FoS of 2? i used like a little more, cause when discussing with a senior member, we came up with a number we called, "Crap your pants" force, basically to stop the "over excitement."

[murpia]

Originally posted by oz_olly:
I have also bent a throttle pedal by getting a little over excited exiting a steep down hill corner (high speed, plenty of grip) and mashing the throttle.

Off topic a bit but I have a couple of anecdotes:

One driver I heard about bent all the spare throttle pedals in the truck at one race weekend. Maybe he thought the car would go faster if he pressed it harder!

Another driver needed to be continually reminded to push the throttle pedal harder. He used so little pressure that bumps on the straights would bounce his foot off the pedal so he lost a little straightline speed.

People come in all shapes, sizes and behaviours...

Regards, Ian

[oz_olly]

I don't have a reference handy but I am reliably informed that aircraft control systems are designed with FoS=2. For example the rudder control is designed from both ends: identify the maximum aerodynamic load expected on the rudder and follow the load path back to the pedal and then compare to the maximum pilot load on the pedal and follow the load path back to the rudder (assuming a jammed rudder). The load case that puts the most force on the components in the system should be used as the design load.

FoS is used to add some margin of safety to allow for all the unknowns and the fact our analysis is an abstraction of reality. The higher your confidence in the magnitude of the max load, manufacturing methods, maintenance, service environment and the lower the consequence of failure the lower your FoS can be. FoS is not really to cover the designer stuffing up the analysis, that is why designs are checked.

Using the definition in FAR25.301 I would call the brake pedal load a limit load "the maximum load expected in service". Under these circumstance the structure must be capable of withstanding the load with no permanent deformation i.e. design to yield. I would call this 150kg, then considering the definition of an ultimate load: "limit load times FoS". The structure must be able to withstand the ultimate load for 3 seconds with no catastrophic failure. The base FoS for FAR25 is 1.5 unless otherwise specified.

Compared to the above we are still being conservative as we are ensuring limit load X FoS is below yield. So long as we don't make the pedals out of a very brittle material this adds additional conservatism.

We're designing race cars not buildings.

[mp24]

agree with you olly, race cars, BUT, brakes is not really something you want to not work or break. :P

[oz_olly]

At the end of the day if you are designing the components you have to be satisfied that you have designed them to be safe given all the information you had at the time.

Neil Roberts gives a great line in Think Fast: If you want the power to kill a lot of people, be an engineer.

At the end of the day if an engineer wants to be a little more conservative then sure go ahead you need to do what ever is required to be comfortable about your work. It's unlikely an engineer could be found negligent for being over conservative (except where that may bring a problem of it's own). Interestingly enough it is very easy to be put in gaol for being negligent but very difficult to be put in gaol for being incompetent.

Anyway I am derailing this thread a bit. I just offered the way we do it, I'm sure (as the many existing threads on this topic suggest)there are many opinions on this topic. I think the good thing is we are discussing it and we all seem to agree that you should choose a fairly stout load and be conservative. The controls are not the place where skimping on margin of safety is going to give you 0.01 sec quicker acceleration time.

[solex]

thanks u so much oz olly.............u r rite.....
i need to be confident about my design.........very true....
i have another question...........
we were planning to buy a pedal force tranceducer for getting these values.......but we cant afford it rigth now and probably wont buy that coz of monitary constraints...........is der any method by which i can get the readings of the amnt of force the driver puts on the accl and clutch pedal????????????/

[solex]

[QUOTE]Originally posted by Old Greg:
That's probably a fair assumption, but be sure take your pedal stop into account. Depending on where it's located you may end up with significantly more than 40lbs of force acting on your pedal/pivot.[/QUOTE
on cad m being shown an FOS of 4.31 in the entire design with the pedal stop having the least fos of 4............ is this FOS safe enuf or shud i probably increase the load bearing capacity of the pedal

[Rex]

Originally posted by solex:
we were planning to buy a pedal force tranceducer for getting these values.......but we cant afford it rigth now and probably wont buy that coz of monitary constraints...........is der any method by which i can get the readings of the amnt of force the driver puts on the accl and clutch pedal????????????/

Wow, gives some insight into how thought processes differ between teams/countries/whatever. Maybe my old teams were just unsophisticated, but we took a bathroom scale and put it between driver's foot and pedals to roughly approximate the force exerted. Obviously not measured during driving conditions, but we tried to be realistic with our static test and it seemed reasonable enough at the time. The idea of buying an expensive transducer for this never purpose ever crossed our minds...lots of other things on the car that would've been more interesting to measure.

[Old Greg]

Originally posted by solex:
...is this FOS safe enuf...?

Like I said, it depends on your setup. Your pedal will see different different amounts of force based on how much leverage the throttle stop has (think statics and 3 point bending). If you are already taking this into account in your FEA, then feel free to ignore me.

If you are designing specifically for 40 lbs, a FOS of 4+ is overkill, but since this is apparently a 3-pedal car you might want to listen to oz_olly about the forces. I've gotten so used to our 2-pedal cars that a panic stop with the wrong pedal hadn't even occurred to me.

That said, I'm not sure I'd be comfortable with the throttle stop being the weakest point in the system.