Safety Factors Used in Formula One

[Callum]

I'm a fourth year student mechanical engineer and I am currently doing a six month engineering placement. Here I have been properly introduced to safety factors of particular equipment used for lifting things. During the conversation the safety factor used for air planes was mentioned (around 1.1 I believe) and it got me thinking...Does anyone know what safety factors are commonly used for parts of F1 cars?

[Greg Locock]

No I don't.

FoS are better described as factors of ignorance. If you know exactly what the loading regime is, and you are 100% confident of your material, and manufacturing process (etc etc) then you can use a an FoS of one, that is you can design to the exact stresses, which may even involve some plastic behaviour.

Obviously in the real world there are many sources of uncertainity so FoS>1. Also different parts of the car will be deliberately or accidentally designed to different FoS.

Finally not all parts are stress limited, for example the floor/firewall/roof/A/B/D pillar/rear bulkead system in a passenger car is largely stiffness constrained, not strength. The spring towers on the other hand tend to be strength limited.

Incidentally that 1.1 figure is way off for commercial or military a/c.

[Callum]

Interesting, Greg. What do you mean by stiffness constrained?

[Greg Locock]

It means the structure's primary target is stfiffness, once that is achieved there may be a few local hot spots to beef up for strength, but the design process is stiffness based.

[Greg Locock]

FAA have a blanket FoS of 1.5 (or greater) for civilian a/c. Military a/c used to use 1.5, but can choose what they like, and in the case of fly by wire they typically drop to 1.38, as they are less likely to see flight overload conditions. Bear in mind that to some extent, except when landing, an aircraft is rather more the master of its own destiny than a car, if the pilot can't physically move the joystick to generate a 5 g turn, and stays out of thunderstorms, then it isn't going to do 5g, whereas a car designed for 4g sideswiping a kerb will easily see instantaneous loads ~20g or so.


There's at least 2 other factors involved - what material property are you working from, and what manouevres are you applying. These can both be conservative, giving an extra margin of error. Or not, and not.

[Richard]

I think Greg means that if you have deflection as your limiting criteria (ie serviceability state) then the FoS for failure (ultimate limit state) is often not critical.

Also the FoS is usually compounded from many factors to take into account variability of input forces, material properties and consequence of failure. So life critical systems with a long operating life have a much higher safety threshold than temporary systems with no life safety consequences. Hence Colin Chapman's adage that the perfect car falls apart on the line would not work for mass production. It would also be somewhat unacceptable in aviation or structural engineering.

By the way, talking of lifting things, have a look at the rating plate on a lift/elevator. It's impossible to get humans packed densely enough to generate that load. Then the lift engineer and structural engineer whack on 1.6 FoS.

[kilcoo316]

Interesting, Greg. What do you mean by stiffness constrained?

Greg (obviously) has it spot on - but this reword might be a bit clearer for someone not versed in the finer details:

Its designed to deflect by so many mm under a load of so many Newton (or inches under lbf if you use a zimmer frame or are a yank/canadian).

Since this will be within the elastic zone of the material, it means the structure is easily capable of withstanding the maximum static load (which is where the factor of safety usually comes in).


[We'll leave aside fatigue for the moment!]

[kilcoo316]

It would also be somewhat unacceptable in aviation or structural engineering.

Ach, sure just kick the tyres a few times and declare "its a 100 per cent there, tear away, she'll be grand".


Never fails.

[Jersey Tom]

It's up to the designer I suppose. I've heard of low FOS's for both military and commercial aircraft, don't know if it's a legit number or not though. Good point on stiffness-limited design, though.

In any event, depending on what you're doing... 1.5 can be a big FOS. 50% over your worst case load scenario is a lot!

[Greg Locock]

To come from the other extreme, man rated gear such as handrails and harnesses will be 5, minimum, and 10 wouldn't be out of order.

There are other criteria used for structural targets, for instance dynamic stiffness is often important, and there may even be specific modal requirements, eg first torsional to be more than 20 and less than 24 Hz, or no modes below 300 Hz. I doubt that f1 is much concerned with that sort of target, although a modal target for engine parts would make sense.

[Just_a_fan]

By the way, talking of lifting things, have a look at the rating plate on a lift/elevator. It's impossible to get humans packed densely enough to generate that load. Then the lift engineer and structural engineer whack on 1.6 FoS.

Really? I was in one recently that said "10 persons or 900kg". Well, I'm 105kg so 9 of me takes us straight past the stated maximum. I reckon a group of "enthusiatic" lads could have filled it with a tonne of bodies. That's where the FoS comes in of course...

But you are, in normal circumstances, correct about loading densities though

[Jersey Tom]

Oh absolutely. Lot of code stipulates very high FOS. Or if I were designing some consumer product knowing it's probably going to be abused, not knowing what your worst case scenario is - go big.

If you don't have code to worry about though, and if you have pretty good idea of your materials and max load cases, 1.5 is a big number IMO.

Suffice to say I'd guess F1 designers push it pretty hard. The case with Red Bull (??) a couple years back ripping out suspensions over kurbs illustrates that well.

[Greg Locock]

Which comes back to Factor of ignorance. If they'd measured the loads from kerbing a car they could design to them and then use an FoS of 1. You could argue that incrementally increasing the strength of parts after each failure is a form of measurement, trouble is it is costing you races.

Measuring that sort of load is always a fun exercise, as for some reason the guys don't like us using million dollar sets of wheel force transducers in a test like that. But, we aren't really interested in contact patch forces, we are interested in the load in each arm, so we strain gauge them, and off we go. My job is then to work back to an input force so that on a similar size car but different suspension we can come up with likely loads before we can measure them.

[Ciro Pabón]

Well, you can call it factor of ignorance, but perhaps that's a pretty "antique" concept.

I do not design mechanical pieces, but I do design once in a while metallic pieces for buildings, bridges and some structures.

I don't know if mechanical engineers use the Von Mises stress concept, but I would say yes. The text explains the (kind of) new approach to analysis: "All other items of interest will mainly depend on the gradient of the displacements and therefore will be less accurate than the displacements".

In that case, you do NOT increase forces. You use an increase in permissible strain. This means you do your analysis based on the context of finite elements. This is very important if you're designing complex frames and (over all) shells, as I imagine is the case for many elements in Formula One. After all, beams are "passé".

The concepts for finite analysis, afaik, were started by civil engineers, so allow me to give a picture of a master here...

Pier Luigi Nervi under the Stadio Flaminio, 1960, way before monocoques... and using concrete, a more difficult material


The general idea is that you use safety factors as strains allowed because materials accommodate. Even when you go over the elastic limit, there is a (not residual, but effective) plastic resistance. This is called plastic design.

(I apologize if some people here, people like Pup, Greg or Callum roll their collective eyes, but perhaps not everybody is familiar with this).

This means that the building bends a little and it will be bent for life, actually. However, who cares? It can take it.

You actually "dare" to extract the last drops of resistance. Hence you use a safety factor measured in strains: you go into the plastic zone and you assume the material is going to be permanently "damaged", that is, it'll go over the maximum deformation that allow total recuperation of the form of the material. How much? Well, I've seen people going into half an inch. It depends on the material and the part.

After some decades of study of materials structural engineers have realized that there is not such thing as a mathematically exact strain-stress relationship when you go into the atomic realm. All curves are approximations to the real behavior of materials.

So, I would not call it factor of ignorance, but factor of intelligence. Brembo uses around 1.1 but in plastic design, afaik.

Allow me to digress a little from the thread question, please, if you're so kind.

Long digression you do not have to read (actually, believe it or not, you do not have to read anything in this post!)

I will never forget when our teacher of soil mechanics explained to us the concepts of elasticity, plasticity and thixotropy. I may have mentioned here before (sorry, guys).

While he carry on with the lesson on stress-strain curves, he was playing all the time with a ball made of Silly Putty.

Field trip of awesomeness!


He explained elasticity and plasticity (elastic means it will recover from stress, while plastic will have a permanent deformation).

Then he bounced the Silly Putty ball against the floor. Let me explain that Silly Putty bounces a lot. He asked us: ¿what's this material? We answered in one voice: "elastic!".

Then he stretched the ball. It behaved like chewing gum, of course. He asked again: "¿what do you think now? As you can see, this material exhibits permanent deformation...". Some said: "It's plastic!". Other said (smartypants): "It's elasto-plastic!".

Then he explained thixotropy. He told us that thixotropy means (I'm trying to abbreviate here) that a thixotropic material had a resistance that depended on the speed of the load.

For example, ketchup: when you apply a fast load (that is, you hit the bottom of the bottle) ketchup flows, but if you apply a constant load (that is, you turn the bottle upside down) it won't flow.

Thixotropic material (bentonite) showing "roll waves" depending on gellification under shear


For example, when you apply a slow load to asphalt, it will flow, but under a fast load it won't. That's the reason why there are so many potholes in toll plazas or parking lots (or the reason why you make them of concrete, which has negligible sensitivity to speed of load): asphalt will flow when trucks go slowly.

Golden Gate Bridge Toll Plaza: notice the concrete surface in the stopping area


Anyway, when he finished he called our attention to the Silly Putty ball. He had left it on top of a table.

Under constant forces (that is, gravity), the silly putty flows (the exact opposite of ketchup behaviour), so there was a puddle of Silly Putty on top of the table. It had "melted" under its own weight.

The lesson? Materials are not that simple. They are not simply elastic, plastic or thixotropic... and soil less than anything. Soil is made of dirt, for the love of Pete. Who knows what it contains?

So, I think we are barely "scratching the surface" of material science these days. Hence, safety factors are not used (at least not in my profession) in more modern designs as simple factors that you apply to loads, but as permissible strains that actually take you under the "old" safety factor of 1, perhaps into designs that would be justifiable if you used (in the "old methods") safety factors of 0.9 or 0.8.

Anyway, as you have no idea what will happen in real life, you use them ("old" safety factors) to build structures that will be able to withstand forces you did not imagine it will take (like a hurricane in New York and a huge drop in temperature á lá "The Day After", or, as Greg clearly explains, planes that have to dodge a missile when a wind shear comes into play, or "standard" cars that have to take a kerb)

However, that's a different approximation: the one of durability and safety. In racing, you don't use that point of view that much (that is, you are not worried about building for the centuries: the car will barely last for one season, and that's if you're lucky and FIA do not decide to change everything).

Duration is a virtue: Puente de Alcántara, a structure that has taken many things in the last 20 centuries... including relatively modern trucks. It's not safety factor, it's "durability factor" what plays here


I love materials.

[bhall]

This is only to see how many times you edit.
It's never, ever just right.