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Science, Maths & Technology

People, automobiles and statistics

Updated Friday 9th October 2015

Understanding probability and statistics is vital in many branches of engineering, particularly in production and design. We look to automobile design and component production, but the underlying principles could apply to any activity where large numbers of measurements are involved.

Automobile manufacturers typically want to sell as many vehicles as they can make. This means that they need to appeal to the widest possible market. Let's examine the way manufacturers might use statistics and probabilities to influence some aspects of vehicle design and to ensure they meet market requirements.

There are many considerations which feed into automotive design but one of the key drivers (sic) has to be the customer’s physique.

Two images of a tall man and a mini that is too small for him and an image of a large monster truck, too big for an average person. Creative commons image Icon Chris under CC-BY-SA licence under Creative Commons BY-NC-SA 4.0 license

Figure 1: Both a, b) small and c) large vehicles present access and drivability problems.

Obviously, the driver has to be able to access and fit into the vehicle Figure 1. This can be a problem for very large or very small people. In fact, even people who are only a little taller than average may struggle in the case of some sports cars (see Figure 2) where vehicle height might be restricted by styling or performance requirements.

Two images of a tall man trying to squeeze into a small sports car doorway and seat well.

At the other end of the scale, access is a major consideration in the larger 4x4 vehicles where stepping up into the cab can be an issue for some people due to height, disability or infirmity. In fact many 4x4 vehicles are designed to lower themselves to facilitate access. Once inside, the driver must be able to reach and operate the controls: steering wheel, pedals and switches. Also, the vehicle safety systems must be appropriate to the occupants, this influences things like seatbelt locations and airbag deployment which could themselves cause injury if there is a poor match between design and occupants’ build.

It’s clear that people’s physique will be a key influence in selecting a car. So manufacturers need to understand how factors such as: height, leg length, arm length, weight and even physical strength influence sales and this requires understanding how such metrics are distributed. If I’m designing a vehicle’s interior I should like to know how much of the market I’m likely to exclude when I take decisions which might exclude large or small drivers. I could choose to build the interior around some sort of mean value but how much adjustment do I need to incorporate to accommodate, say, 95% of the market? To understand this I need to engage with the branches of applied science called anthropometrics (or anthropometry), which is the study of the measurement of size and proportions of the human body. Measuring people is far from simple.

Measure the length of your forearm. This is the distance between your elbow and your wrist.

This is far from simple. You’d maybe like to measure the length between the axis around which your elbow hinges then a similar axis for your wrist, or simply from the back of your elbow with a bent arm to a bone in your wrist. But, either way, these are not well defined and the proportion of error is quite significant. All we are doing here is identifying the need for consistency in what we do.

Here is a link to Adultdata, the handbook of adult anthropometric and strength measurement.

On pages 21 to 28 you will see the many different dimensions which are used to characterise human beings. If you look at page 30 titled ‘Stature’, you will see a table identifying:

  • the country from which the data was obtained
  • the sex of the participants from whom the data were obtained
  • four columns of figures describing the mean, standard deviation and the 5th and 95th percentiles
  • the source of the survey – this turns out to be vital since some of the data is taken from quite narrow sections of the population.

The main part of the handbook, which is section 7, uses this format to list data for all the dimensions shown in section 6.

If you look at section 5 you will see the data do not necessarily describe the same populations. For example, the age ranges vary and in some cases were never recorded. Clearly a person’s racial origin is an issue when evaluating anthropometric data; you can see this by comparing say Dutch statures to Sri Lankan. Gender is another obvious factor, but there are many other less apparent factors. For example, in many countries the population is getting taller over time. This means that much of the data in this handbook is likely to be out of date. Also this only covers adults, things get a lot more complicated when dealing with children where the rates of growth and growth proportions vary widely.

Use the Adultdata handbook to find a definition of how to carry out the forearm measurement you attempted in earlier.

This is given on page 123 as ‘back of elbow to wrist crease’. Although it’s not perfect it does provide a means to be consistent.

Now think of yourself sitting in the driver’s seat of a car and identify three factors which might influence your ability to drive the vehicle.

The three I got were:

  • distance to pedals
  • weight of steering (force required to turn the wheel) at low speed
  • mechanical handbrake lever location.

Your three will probably be different because there are so many possibilities but this exercise is purely to help you think about the possibilities. 1. relates to height 2. to strength and 3. possibly to a combination of the two.

Given the three points I have suggested, how might a designer alter the vehicle in order to accommodate a wider range of physiques?

The solutions might be:

  • Distance to pedals. This one seems quite straightforward. I simply have to ensure the driver’s seat will adjust over an appropriate range, but what does that do to the driver’s position in relation to the steering wheel?
  • Weight of steering at low speed. Again seat position can be a factor but this is usually overcome by either altering the gearing or adding some form of power assistance.
  • Handbrake lever location. This one might not have been a problem, except that I solved the first problem by making the driver’s seat adjustable. There are all sorts of solutions possible here. Fix the seat so that it doesn’t move, put a long lever on the handbrake. Some compromise, or maybe rethink the handbrake design and location entirely.

Here is a recording of an interview with Dr Paul Herriotts. Dr Herriotts is an ergonomist who designs vehicle interiors for Jaguar Land Rover. His job is to ensure that the vehicle is designed around the user. He is being interviewed by me, Dr Tony Nixon, senior lecturer in information systems and author of this article. I hope this will give you one perspective on how statistics play a role in industry.

Transcript 

Tony Nixon: So it’s a damp wet morning here at the Land Rover Driving Experience at Gaydon. I’m looking at a Land Rover Discovery Four. We are gonna get this white vehicle that’s looking absolutely spotless pretty filthy I guess. The track’s some extreme slopes. I must admit the place looks quite intimidating. What I’m here really to find out about is the human element and exactly how these vehicles are designed to accommodate a wide range of heights and builds of different people. We know that the car can cope with this environment – but how do human beings cope inside the vehicle, and how is the vehicle engineered in order to enable people to cope?

We are going to meet Paul Herriotts, who deals with vehicle ergonomics and essentially makes sure the passengers are safe and comfortable in the vehicle whilst it is doing all sorts of ridiculous things.

So Paul, we’re interested in statistics and people and how one might apply statistics in the context of vehicle design or something like that or ergonomics where you’re struggling to try to ensure that you’re accommodating as many people as possible – ouch – we are certainly going down a hill!

Paul Herriotts: As we’re driving I’ll try and relay what we use our statistics for.

So – I’m in one of our Discovery cars and I just wanted to remind you a little bit about how we designed this car from the point of view of designing it from the inside out. We are very keen on user-centred design and designing around the human being and we really are trying to cater for that wide range of customers that we have. A very small Japanese or Chinese or an Asian lady may be only forty/forty-five kilos, perhaps approaching one metre fifty. And at the other end of the scale we may have for example in, um, the Netherlands we’ll have the tallest people. From a width perspective we’d be looking at some really heavy guys and we’re really trying to design the car around those people. And by laying out the vehicle interior around those drivers – excuse me while I’m braking – so we may have a vehicle like this with seven people in and I have to be aware that when we are off-roading we’re doing it now and you can hear me moving around. Our heads are moving around in the car and our bodies are swaying and I’m kind of cognisant of that movement that we have, and also of the anthropometry of the populations that we’ve talked about. So if I put those two together we should be able to design an upper canopy that has enough space so that no one hits their head when we are off-roading.

We’re a very data-driven specialism in ergonomics and we’re really looking for data from populations around the world about physical sizes. Now we’re not so interested only in stature. We really are thinking about people’s back length to stature or their leg length. We’re thinking about people’s proportions and we have significant data on that from people all around the world. And we can then begin to get a data set that really describes the population so we can start looking at a measure of central tendency. So then of course we talk about means or medians and we can also look at how the population is dispersed and there we might be looking at the standard deviation of the population. Sorry – need to concentrate on my driving for a moment!

Tony Nixon: We’re going down a steep slope here and we’re about to tip up on our sides so –

Paul Herriotts: We’re not about to tip up on our sides. We’re gonna take it steady.

Tony Nixon: This is getting quite hairy! So how do you want people to feel when they’re driving a course like this?

Paul Herriotts: Really the – the crux of designing vehicles like this is to let people feel in control and one of the things we’re trying to do is to really afford people with a great sense of all round vision. We talk about Land Rover Command driving position and what we’re talking about there is giving people an understanding of the size of the vehicle and the vehicle extremities so they have great confidence in the car but they also have great vision of the outside environment. So we’re driving now – I know exactly where my bonnet edges are. I can see the edges of the bonnet very well. I know where the edge of the car is. You can hear me tapping it here. But I’ve also got great forwards and side vision so I can see all these features that we’re driving through at the moment. And I know exactly where to place the car and exactly where to place the front wheels and that’s really because we try to design that in during the design process. So we’ve been able to put a range of people in the car so they have an understanding of where does the bonnet end. Actually we’re looking now at a metal bonnet but when that existed only as a clay in the design studio we could put small driver surrogates into the driving position and I could ask them as I’m looking across now ‘can you see the edges of the bonnet?’ And I would nip out and I’d put my little five pence piece on or a ten pence piece and I’d slide it along and I’d say ‘how much of the bonnet can you see?’ And – and that really has dictated the form of this car that we’re in. And so we have a very low, flat bonnet that’s very easy to read and we’ve got two great feature lines that run along it so that also gives you a great visual cue and so helps you understand where the front of the car is. And of course in normal driving it’s important to understand extremities of the car whether ones parking or manoeuvring. In off-roading it’s essential to have that understanding of where my car is and where the environment is around it so that I can drive in a safe manner. And the wonderful thing is we get towards the end of that design process and we start driving prototypes and then we have production cars, we really can see a tangible output for all that hard work. So all those years of discussion and poring over data and looking at statistics and at the end we have a physical vehicle, as we are driving now. And there’s a tremendous sense of pride that I can point at a car as it goes down the road and say ‘I did that bit and I did that bit.’ And it really is a satisfying thing to have done.

Vehicle design requires an understanding, not only of how to measure physique, but also how to handle data which is distributed – one size will not fit all. The content of the Adultdata handbook provides information which is useful but needs some interpretation. In particular, most of the measurements are expressed in terms of things called ‘percentiles’ and their use is based on some knowledge of statistics.

The sort of question we would like to be able to address is:

‘What range of physiques must we accommodate in order to be sure that 95% of the population can drive the vehicle comfortably?’

In doing this you will see that we can also address other questions which are of interest to engineers, such as:

‘Given a batch of components, some of which are faulty, how many do I need to sample to ensure the number of faulty components is below a given threshold?’

For example, if I am manufacturing rivets and I can tolerate 0.3% being over size, how many from a batch of 1000 do I need to measure to be 95% sure that I know when I fail to meet this condition? This sort of question arises frequently in manufacturing and the mathematics required to answer it is the same as that required by the car designer.

 Finally, an interview with Dr Herriotts, and his colleague Louise Malcolm, who is an ergonomist. This will hopefully give you a little insight into how anthropometrics are interpreted in automotive design.

Transcript

Tony Nixon: Paul, just tell us what we’re walking through here.

Paul Herriotts: We’re in our ergonomics lab and in this facility we helped develop our early cars and prototypes of cars or models of cars to ensure that they are optimised from an ergonomics perspective.

Tony Nixon: So before we get stuck into the detail what is ergonomics?

Paul Herriotts Ergonomics is a user centred discipline design products and workplaces to fit the needs and abilities of people. We can call it the science of designing for people. So we’re doing a lot of testing in here with a range of people of all sizes. We get them to come up to the car, to open the doors, to get in and out, to get a comfortable driving position, to get a view of the road.

And as you are aware we’re in an enclosed laboratory but up against the wall we’ve got our driving simulator and when people are sat in a car or a model of a car and they look forward in the windscreen and they do see something that feels very realistic and it really is to simulate what driving a car would be like in the real world.

So we’re looking fundamentally at a tarpaulin covering a big block so I can’t really tell you what’s exactly underneath there but you can see by the volumes that it’s a large Land Rover car that we’re working on. We take a model like that and just as we’re modelling the car we’re also going to model our customers. So we would bring in thirty, or forty or fifty people who are a representation in size of our customers from around the world. And we’re looking at ease of entry and exit. We’re looking at driving position. We’re looking at being able to see out of the vehicle. We’re looking at being able to have enough space and room and we take the feedback from those people in a very structured way. We look at what percentage of people of which size have said this and on the basis of that we make design recommendations and design changes.

The reason we’re doing this early on in the design process is it is at a stage when we can change the design. Things like what we call the belt line of the vehicles so where the windows start, crudely put, and the proportion of glass to metal. We can change things like door opening angles. We can make changes to apertures. We can look at form of the bonnet so I could try three or four different bonnets on there and we can tune the vehicle to be more ergonomically optimised. And as we’ve done that early on that can then feed into tomorrow’s cars.

How we get data from people is quite critical and one of the things we have in the lab is our measuring equipment. We’re interested in the discipline of anthropometry, the measurement of human beings. We can measure ourselves as adults. But we also have data on the small children to understand the space requirements and how we have to design our vehicles to accommodate this range of people.

Tony Nixon: Okay well let’s have a go at measuring my sitting height.

Paul Herriotts: Yeah you can Tony. If we can go over to our stadiometer and we also have our – our complex anthropometer for measuring such measurements as sitting heights. So if you want to take a seat we’ll take a measurement of you now.

Tony Nixon: Excellent.

So I’m with Louise Malcolm who is an ergonomist and she’s now going to measure my sitting height.

Louise Malcolm: What I’ll do is I’ll just get you to sit here on this platform with you facing this way. So if you keep your knees up there so we’ll try and get a ninety-degree angle between your upper and lower legs. What I’ve got here is a calibrated measurement that I can just bring down - are you sitting up nice and straight there because it makes a difference whether you’re slumped or upright - bring it down to the top of your head – hold it there and your sitting height is nine-seven-seven millimetres.

Paul Herriotts: Very nearly a metre!

Tony Nixon: Wow that’s me. One metre of …

Paul Herriotts: I can tell you what we can now do in our book of anthropometry is to look you up and see what percentile sitting height you are for a UK male population or within a UK male population.

Tony Nixon: It seems like people are getting bigger I mean if you look at sort of medieval doorways and things like that they appear to be very small, they’re either badly made or people are changing shape. I mean is there something in that?

Paul Herriotts: You’re right to identify that. Crudely speaking over time people are getting taller and this has some consequence for us in product design and in automotive design so we have some human modelling software which predicts the size of people in the future. And we can select populations from around the world. We can look at the sizes of those populations five, ten or fifteen years in the future. So let’s go and have a look at that now.

Louise Malcolm: So I’m sitting at a computer terminal here and what I’ve got up and running is our RAMSIS digital human modelling software. We’ve developed a JLR mannequin. It ranges from our shortest female up to our tallest male. And then in between in the image that we’ve got showing here, we’ve got some larger mannequins to represent people who are bigger in the population. And we’ve also got some Chinese mannequins because they’ve got different body proportions to our UK and US mannequins. So this next image that I’m showing is a picture of our Range Rover Evoke driving position.

So what I’ve done here is loaded in very basic CAD data so we’ve got a seat in there, the floor, the accelerator brake pedal and brake pedals and also the foot rest, a knee bolster so to look at knee clearance going forwards and then the steering wheel. And what this software does is it will position within the seat movement envelope a person of a particular stature and body dimensions into their appropriate driving position. When we’re looking at creating new mannequins we spoke earlier about having to project future sizes and so within a mannequin creation option we can choose what nation they come from. So for us China is a really important market so we might want to look at a Chinese female of medium height and then we can choose a reference here. So its SIT 2014 but we’ve got the option to project all the way up to 2030.

We aim to fit as many of our customers as possible so the seats are designed based on fitting a range of people and so by choosing the largest and the smallest we’re making sure that we include all of the extremes of the population. We generally design from a fifth percentile female to ninety-fifth percentile male.

Tony Nixon: So is there such a thing as an average person? You know when you’ve got all these different dimensions combined?

Louise Malcolm: There’s not really such thing as an average person and we would never design for an average person because you’re automatically excluding everyone who’s taller or smaller than that which is why we design for extremes. But if we were to take someone for example who was of fifty percentile stature it’s very, very unlikely that their sitting height, their leg length, their arm length would also fall in that fiftieth percentile range. So generally we’ve got different proportions.

Paul Herriotts: So Tony, when discussing percentiles as you heard Louise describing just now, if we talk about ninety percentile stature we always have that word associated with the percentile. So we’re looking at specific measurements and percentiles of those measurements. We don’t talk about ninety-fifth percentile men or fifth percentile females because that’s quite inexact and really doesn’t describe things in an appropriate way. So percentiles relate to specific measurements. So generally when we say ninety-fifth percentiles if that’s used in the motor industry people are referring to a ninety-fifth percentile stature.

This article is an extract from T276- Engineering: Professions, Practice and Skills. 

 

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