Dec 11, 2007

Motorcycle Helmets: The Indian Standard: IS4151

I just went out and purchased the Indian Helmet Standard (IS 4151) for Rs 260 from the sales counter at the BIS office in Andheri (East). And I read it twice and here is a summay of what it says.

So, what does the Indian helmet standard say, then?
It begins with a definition of what a helmet is. And it is more or less in agreement with this post. It also defines some inherent performance standards for these components. To give a quick, more or less complete list. The shell may not be metallic. The protective padding is defined as being of expanded polystyrene or any material of similar properties. I read one study that used PU foam for some helmet crash sims and then found that EPS did a far superior job. The retention system definition, surprisingly, has no mention of there being an adjustability requirement. All metal parts are required to be inherently non-corrosive. The chin bar is always referred to (and not just in IS 4151) as the lower face cover and at no point, will it be tested for protective ability or strength.

Measurement references
Now to some detail. But before that you need to grasp these um, planes which are used for measurement references. First, the basic plane. This is a horizontal surface that will pass through your ears and the lower edge of the eye sockets. The reference plane is another imaginary horizontal surface that runs parallel to, and above, the basic one at a given distance. This distance (always in mm) is a function of the size of your head. The bigger your head is, the farther the reference plane is from the basic plane. And finally, there is the vertical axis, a line perpendicular to the basic and ref planes that is exactly in the center of the head, that means equally distant from front, rear, left and right of the head.

Shell design/construction
The shell shape is predefined. To roughly put it, the shell must cover your head from just above the eyebrows. The horizontal lower edge will move downwards at an angle to cover your ears. After the ears, the shell can, in theory, curve back up and needs to cover the back of your head only up to the reference plane and not any further below. The shape is defined as convex and smooth, without any proturbances more than 5mm above the surface. Any rivets and stuff have to have rounded heads and stick out 2mm or less.

There is a further requirement, which I do not fully comprehend, but assume is about the ability to turn your head without restriction that says a 100 mm diameter cylinder placed perpendicular to your neck (think can of coke held to the nape of your neck so that the aluminium coloured top (or bottom) touches your neck) should not touch any part of the shell. There are some minimal requirements given for ventilation and detailed regs for visibility. The upward visibility is supposed to be uninterrupted at an upward angle of 7 degrees from the ref plane while the downward visibility is 45 degrees downward from the basic plane. The side to side visibility has two requirements. If a plane ran through the vertical axis so that it touched the front and rear of the head and divided the head symmetrically in half, the angle of visibility is 105 degrees on each side from both the ref and basic planes (which is where the angle measurement is taken).

The protective padding is again supposed to cover the same minimum area as the shell. There is only one proviso, the padding should not impair the hearing beyond a defined standard. I will get the visor standards soon (IS 9974) so hold on for that.

Finally, the chin strap is required to be a 2 cm wide, skin-friendly, pretty strong piece of work. The fastening mechanism also has corrosion and load bearing requirements. Further, there should be no way the mechanism can open unless a deliberate action is made.

Pre-test helmet conditioning
Before the actual crash performance testing happens, the helmets are exposed four situations. The first is called solvent conditioning. Where strips of cloth soaked in 70 per cent octane and 20 percent toluene are applied to the helmet. Regions 50 mm from the chin strap mountings are wet for 7.5 seconds (±2.5 seconds), while the rest of the shell gets the same treatment for 12.5 (±2.5 seconds). The helmet is then sat for half hour. When that time is up, the tests must be conducted within five minutes.

A second helmet receives what is called ambient conditioning. That's 25 degrees C (±5) and 65 per cent relative humidity (±5 percent) for four hours minimum.

The third lid is heated to 50 degrees C (±2) for 4-6 hours

The fourth lid is sat in -20 degrees C (±2) for 4-6 hours

And a fifth is first exposed to UV rays from a xenon-filled quartz lamp,125w for 48 hours at a range of 25mm followed by 4-6 hours being sprayed with water at 1 litre per minute

Crash performance instrumentation
Now, the all important crash requirement. The measurement of acceleration is made by tri-directional accelerometer mounted near the center of gravity of the headform (that's the dummy head that's put inside the helmet (yes, there is an IS document for that too). During the crash tests, the recorded acceleration may not exceed 300g at any time. 150g levels are allowed for a maximum sustained interval of 5 milliseconds. The curve, usually a bell-curve that slopes steeply up and then comes down a little more gently with a few small spikes, the entire action sequence taking about 20 milli seconds.

The tests are done on this schedule. All four lids are subjected to the impact tests (that's four impacts on four lids). The worst performance lid is then used for the penetration resistance test. So, if the highest peak gs were recorded in the cold lid, say, a new lid is frozen over and then tested for penetration resistance. Two ambient lids are also tested for rigidity, and finally one ambient lid is used to check the strength and efficacy of the retention system.

The sound tests require that no more than 10dB is lost over 250 to 2000 Hz.

The mounted visor's fully open angle is also required to meet a standard. At the fully open position, a line passing through the top and bottom edges of the visor (through the center of the visor) should make a 5 degree included angle to a horizontal plane that sits tangential to the top of the lid.

If the lid has peak, there's a test for that too

The final requirement is a printed document that comes with a lid that informs the junta that the helmet must fit closely and that the chin strap should be fastened snug against the jaw. And that the helmet is designed to absorb one impact, and one impact only. After said impact, you need to get a new one. And finally, for greatest crash performance, you should not alter the lid. While no details are given, Shoei et al, say no re-painting, no holes drilled in the shell.

Finally, there is no weight requirement in the Indian standard and the manufacturer only needs to display the shell's weight to the nearest 20 grams.

So, to the tests, then

Impact absorption testing
The format is pretty simple. The helmet is mounted on a headform and dropped on to a immovable surface (called anvil, officially). The fall is solely under gravitational force, but is guided so that a pre-selected impact point makes first contact. The headform, as you know, has the accelerometer inside that does the measuring.

The impact points are called B, X and P. Each is actually a set of two points. B is on the crown on the helmet, usually where the logo of the manufacturer is. There will be two impacts here, the points separated by 15mm (±5). P is as the rear of the helmet where the basic plane would meet the rear of the shell. Same 15 mm (±5) req exists. X is on the side of the helmet. Enigmatically described as 45 degrees rearwards and upwards. On the diagram, it appears to be about 12-13 mm below the reference plane, on the plane that would bisect the helmet front to rear into half exactly.

Two kinds of anvils are employed. The flat anvil offer the helmet the opportunity to spread the impact better and is usually the anvil that is used for the first impact. In real life though, the thing is a cylinder with the impact face defined as having a 130 mm (±3) diameter. The other is the hemispherical or rounded anvil, which concentrates the impact and makes the lids job tougher. This thing supposed to have a rounded face of 50 mm (±2) diameter.

The height through which the helmet drops is defined by the impact velocity. The flat anvil speed is 7 m/s, roughly 25 kph. The rounded one goes even slower, 18 kph. Remember this the speed of your head (inside the helmet) hitting the ground, not the motorcycle's crash speed.

The sequence of tests of also defined.

First the ambient lid is tested at points B (crown set). The helmet is first smashed up on the flat anvil and then on the rounded one at the second B point. Repeat for points X. Then any one of the conditioned lids is tested (flat followed by anvil) for performance at P, more or less the only time the rear of the lid is tested under the Indian standard. The other lids, then, are subjected to the same impact test at B and X in sequence (hot, cold and then UV/water).

Penetration resistance test
From the data recorded in the previous test, the worst performance is noted and a lid is conditioned to that situation and used for penetration resistance. Okay, the standard is a bit vague on how the selection of the impact point is to be made, but one has to assume that the impact takes place at B, X or P wherever the highest gs were recorded, but that's just my guess. The helmeted headform is mounted in the test rig so that the test point is horizontal. A metal punch (300 gm, 0.5 mm radius at point, 60 degree cone, 45-50 hardness on Rockwell scale) is placed 45 mm above, vertically, the impact point. A drop hammer (think heavy cylinder, 3 kg) is dropped from 1 metre on the cone. Two tests are made, 75 mm from each other and from any previous impact test location. The cone should not come more than 5mm closer to the headform at any time, measured vertically.

Rigidity testing
For the rigidity test, the helmet is mounted between two vertical plates and basically squeezed. The initial load is 30 N. Then, it is jacked up by 100 N every two minutes until you reach 630 N. The 630N is held for 2 min, and then the distance between the two plates is measured and compared to the initial distance. Then the load is backed down to 30N again for five minutes and the distance is measured again. This is done on the sides of the helmet as well as front-rear. The diff in measurements should be less than 40 mm at full force (630 N) and 15 mm after the force is backed down to 30N again.

Retention system strength check
The retention system is checked by mounting the helmeted headform (chinstrap fastened and tightened; though there is no requirement that the chin strap actually be adjustable) vertically, suspended from the shell. The headform is preloaded to weigh 15 (±0.5) kg. A 10 kg mass is attached to headform and allowed to fall 750 mm. The point at which is the force is applied should not move more than 35 mm. After two minutes, the preloaded point of force application (yeah, I don't know what they mean either) should not have moved more than 25 mm. Actual damage to the retention system is considered acceptable if the helmet can be taken off easily (foreign standards elaborate on this saying that a paramedical staffer with basic tools should be able to get the lid off - cut the strap is the meaning).

Sound attenuation test
The sound test is done with a meter mounted inside a helmeted headform at the left or the right ear location. The location should be uninterrupted for 12 m radius without any acoustic materials, as in open ground. A horn is sounded (90-115 dB, ambient sound should be 10 dB or less) 2 metres from the helmet, mounted rigidly 1.2 m off the ground on a stand. The sound level is measured at the sound meter location with and without the helmet covering it. 10 dB loss or less is acceptable.

Helmet peak test
The peak of the helmet is tested by putting it in the rigidity test setup with a load of 120 N to hold in place. By means of a small hook and light strong thread, 115 kg is suspended from the peak. But not directly. The thread runs perpendicular to the attachment peak (horizontally) over the pulley and then comes the suspended weight. A 1 kg mass is added gently and left for 2 min. Then the vertical displacement of the 115 kg mass is measured. The peak should not break or detach. And the mass movement should be between 6 and 32 mm.

Is it a good standard then?
Actually, yes. Adopted in 1993, the IS 4151 was based closely on the ECE 22.03 (sorry unable to find a link to the document), which is the 2003 European standard. Since then, the ECE 22.05 came out in 2005, which is probably the toughest helmet standard in the world today. And which I will detail in another, equally tedious post. The 22.05 regulation has been adopted across Europe and from whatever I can tell in places as far and wide as Japan and New Zealand (correct me if I am wrong, please). As you can guess, the ECE 22.05 is pretty comprehensive, and leaves the Indian standard far behind. But I must say that I was more than impressed by the Indian standard and was not actually expecting it to be this, um, current.

On the other hand, enforcement and compliance are another issue altogether. Let me work on that and I am working towards returning with a post on how easy/difficult it is to get approval and details of the process. So, if you're still awake after this massive post (2374 words at this point), watch out for that.

Related links in this series:

Other links on helmets

4 comments:

Prasanna said...

quite singularly, the most complex post I have read here.

This one's really really tough to understand, most of the time. Quite unlike your other posts. Would have worked miles better if you had broken it up, I reckon.

rearset said...

Hey Prasanna, thanks for the comment. Am working on it. Will split it up and repost it in series, just like the helmet series. Watch out for it, okay?

Prasanna said...

Sure thing. Although it might just scare the shit outta me if I find my helmet's tested at less than 50kmph.

Btw I remember watching a looong time ago about bike being subjected to some X kilo of weight fallin from a height of 12 metres or so - isnt that on anymore?

Thibaud said...

Very interesting review, thanks for the details!
Would you have a soft copy of the standard?
What about the standard for visors?

Have a nice week end.

Tibo