Only thing I saw was, you'll get the same amount of elongation... Come to think of it, that's definitely true. Now time for beer, bowling and pizza.

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Grant Hughes

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starion887
For belts, this is not really the case. Presumably, they return to their original shape regardless, unless they break. But the idea of the rate of enlongation does change very markedly with load changes; that is the non-linearity.

Not according to what I've read. Good belts have an elastic zone (phase?) and then a non-elastic zone and then they break. In fact, you are supposed to discard any belt that has been subjected to enough force to do some permanent stretching, since that second, steeper phase is important to the overall goal of spreading the force (on the person) out over time.

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Iowa999
If we're going to get serious, then I suppose we ought to be clear about the three phases or zones (or something-or-other - I don't know the technical terms) for applying tension to a semi-elastic thing like a seat-belt. The first phase is when the tension is below the yield point. No stretching occurs. The second phase is when you do succeed in stretching the material, but you're still within the elastic zone. The stretching that occurs in this phase is pretty much like stretching any other elastic material, such as a rubber band or car's coil spring. And it's linear, so you can describe the stretch as a percent of original length. Note, however, that the elastic zone for a racing seat-belt is rather thin unless it has one of those gizmos like a Schroth Rallye belt has. Finally, you've exceeded the ability of the material to stretch elastically and you start stretching it permanently. This goes on for a little while (with a much shallower slope than the elastic phase - as in: more tension is needed for a given amount of stretch) and it's often non-linear for reasons I don't understand at all. Then, finally, the belt snaps as you surpass the breaking tension.

This is what I mean by it being non-linear. The function relating tension to stretch is, at first, flat, then linear, then shallower and non-linear, and finally infinite (when the belt breaks).

At least with metals, I don't think there is "No stretch zone". With metals you get 3 zones:
1) Within the elastic limit. This is where the elongation is linear with the load. Starts from 0 pounds of load to some number depending on the material. If you release the load, the metal will spring back to the original state.
2) Past the elastic limit but before the ultimate breaking strength. This is where the curve is not linear. For metals in particular you get great increase in elongation with very little increase in load. From what I understand, all materials do this, but to a lesser extent. Again, this is non linear part, so, you get progressively more elongation with increasing load. From what I understand, if you release the load here, you are left with permanently deformed material, it won't return back to the original shape.
3) You get to the ultimate breaking strength. Shit breaks. You are not getting the original part back.

Klim

Agreed as to ductile metals. There's no flat zone before the elastic phase. In fact, now that you say that, what I said before was technically wrong as there is no truly flat phase for a belt, either. It's just that there are two very different elastic zones and the first is so close to flat (like a metal) that I said it was flat, but it isn't. The second part, which is what I called elastic before, has a slope that is probably 100 times as steep and is also elastic. And, then, once you're exceeded the elastic limit, you start doing permanent damage, just like when you enter the ductile phase for a metal.

I have some plots for belt, but they're stress-by-strain, not length-by-force. You can clearly see the transition from the first elastic zone to the second. Both are linear with 100:1 slopes. Then you get the non-linear zone where the belt is being permanently stretched. This is sub-linear and is just about flat at the moment the belt breaks. It's actually quite impressive, when you think about it. The first, very steep zone is so you can strap yourself in without the belts stretching much if at all. The second absorbs small hits without trashing the belt. The third is when you take a really hard hit and the slight extra permanent stretch saves your life.

That's cool, I would be interested in seeing some strain/stress plots for belts. I have seen a bunch for metals, but the only thing I could find on the net close to belts was nylon. Couldn't find any belt stress/strain plots.

Klim

Well, I told you I would think about this all night. And something finally did hit me; I checked the clock by the bed and it was 3:27 AM!!

This is a dynamic problem, not a simple static one like the 100 lbs hanging from belts of 10' or 100'. The belt is absorbing the kenetic energy from the body it is restraining, and ultimately it all has to go into the belts (unless the belt mounting moves). The energy absorption is presumably spread evenly through the length of the belt. The short 2' belt has to absorb a 7 times more energy per inch than the 14' foot belt.

A couple of things happen:
- The short belt enters the 'less strechty' zone of the stretch curve a lot sooner. The long belt may not ever enter that zone. (That's all dependent on the speed, load, deceleration distance of the car, etc.) A(And yes, I am sure there are better words than 'less stretchy zone' and 'more stretchy zone'.)
- The deceleration rate of the body is less for the long belt, which ties in directly with the longer absolute stretching (and our intuition). Hence, the total force on the long belt has to be lower; otherwise it would violate the law of physics F=mA. So, indeed, the total force, and force per unit length, is lower for the long belt, as we have to look at deceleration force, not static load force. The key is to see it as a dynamic problem.

I still am back to being puzzled as to why the % longer stretch for the longer belts. Re-reading my orrginal thinking, it looks like it would be the opposite. Morrison, did your number results get inverted? I have not looked at the actual data myself.

Mark B.



Edited 1 time(s). Last edit at 09/28/2013 09:17AM by starion887.

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Iowa999
I have some plots for belt, but they're stress-by-strain, not length-by-force. You can clearly see the transition from the first elastic zone to the second. Both are linear with 100:1 slopes. Then you get the non-linear zone where the belt is being permanently stretched. This is sub-linear and is just about flat at the moment the belt breaks. It's actually quite impressive, when you think about it. The first, very steep zone is so you can strap yourself in without the belts stretching much if at all. The second absorbs small hits without trashing the belt. The third is when you take a really hard hit and the slight extra permanent stretch saves your life.

Are you saying that the 2nd zone has a slope that is 100 times steeper than the 1st zone? These curves would be very instructive if you could scan and share. And I see your point about the permanent belt stretch, I had totally forgotten about that.

BTW, your stess-strain curve gives you essentially the same info as force vs length in this case of a single axis of force colinear with the axis of the material, and a uniform cross-section material. Stress is just the internal force between particles (which is presumably the force uniformly spread across the material area in this case), and strain is the % elongation.

Anf I have to wonder about belt stretch after an accident. Does anyone actually mark their belts and make a 'pre-wrecked' reference measurement for later comparison?
Thanks!

I'm going in to work tomorrow and will post the plots when I get there. I know that they provide the same information as what we want (which is why I've been using them for this conversation). However, for all I know, I am translating stress/strain to length/tension incorrectly because, like most humans, I'm lousy at non-linears and/or derivatives in my head.

But if I'm reading them correctly, you can put the belt under a measurable amount of tension with little or no elongation. Then you enter a steeper, linear zone that is still elastic. Then it starts stretching permanently.

And, yes, some people are very careful to mark or tear off the date-tag when a belt is involved in a serious accident, just as you should peel the sticker off any helmet that you drop on tarmac, etc. A belt that has "used up" its permanent stretch is nowhere near as safe. If you're parsimonious, that's when you sell it to an autocrosser, but you really should warn them about it, so they don't use it at speeds above 50.

Finally, it's definitely dynamic, which is the whole point of stretchy belts. The goal is to spread the force out over time, since it is peak g that causes internal injury. This raises the question of whether the standard tests that involve stretching the belt by .001% at each time-step and measuring transmitted force is really appropriate, but that's most of what we have to work with, it seems.

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starion887
I still am back to being puzzled as to why the % longer stretch for the longer belts. Re-reading my orrginal thinking, it looks like it would be the opposite. Morrison, did your number results get inverted?

I don't have any results, the only thing I did here was to ask John for a clarification of his assertion that belt stretch should be expressed in "% per length." The reason I asked is because it seemed fairly counter intuitive.

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starion887
I have not looked at the actual data myself.

Which is really John's point - that the installation propaganda from Schroth was lacking in information about what the test sled photos showed. (even though the dummy weight and crash angle/speed/distance/duration were in the document two pages earlier. Yah, ok, the data isn't explicitly tied to the pictures but for the purposes of what they are there for it's reasonably safe to assume the data is for the pix.)

But as John would say, this is all a bunch of mental masturbation. Wanking around data just for wanking's sake. At the end of the day we have to use belts that conform to one standard or the other, including being valid by age. Installation needs to follow the belt manufacturers instructions, at least according to most rules I've read, so little room for interpretation there.

Interestingly, I'm now able to visualize how - with improperly installed belts - lower back compression injuries could be exaggerated when using a HANS device. It is very conceivable that the position of the harness bar would need to be changed from a non-HANS configuration to a HANS configuration, particularly for taller occupants. Come to think of it, I don't think I've ever been asked to sit in the car and show belt fitment at tech. As a co-driver I usually skip on tech unless I expect rule based questions (usually at US events.)

---

First Rally: 2001
Driver (7), Co-Driver (44)
Drivers (16)
Clerk (10), Official (7), Volunteer (4)
Cars Built (1), Engines Built (0) Cages Built (0)
Last Updated, January 4, 2015

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john vanlandingham
Blame is for idiots. losers.

Ignore what JVL wrote about belts; that's my suggestion. Every version of it that he's come up with so far has been demonstrably wrong. Make fun of him for it, of course, but don't take it seriously. Think about the fawkin' children.

As to whether real data exist: they do. And the question of how long the belts should be is not "mental masturbation" since it can mean your life. Shorter belts will be more likely to keep you in your seat (due to less total stretch) but longer belts can reduce peak g by a larger amount (due to greater total stretch). An optimal length for a given belt does, therefore, exist and this isn't something you figure out by trial-and-error ... at least not with real people in the seat. If Schroth or SafeQuip says that there should be 8 to 16 inches (which is made up - I have no idea what they say) between the shoulder top and the anchor, then I'd listen, because they have the data.



Edited 1 time(s). Last edit at 09/28/2013 12:58PM by Iowa999.

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NoCoast
That reminds me of a band called Anal Cunt. As I recall, every song was like 5 seconds long and ended with the lyric "you're dead."

Not all of them.

"I Convinced You to Beat Your Wife on a Daily Basis" is a great song to listen to when you have to hammer a skidplate flat, incidentally.

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Pete Remner
Cleveland, Ohio

1984 RX-7 (rallycross thing)
1978
Silence is golden, but duct tape is silver.

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mulik52
That's cool, I would be interested in seeing some strain/stress plots for belts. I have seen a bunch for metals, but the only thing I could find on the net close to belts was nylon. Couldn't find any belt stress/strain plots.

Klim

Surely there are plots for climbing ropes and webbing if you can't find something specific to belts.

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____________________________________________________________-

One. Class -- 2WD

www.quantumrallysport.com

http://www.facebook.com/home.php?#/pages/Quantum-Rally-Sport/281129179600?ref=nf

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Morison
Interestingly, I'm now able to visualize how - with improperly installed belts - lower back compression injuries could be exaggerated when using a HANS device. It is very conceivable that the position of the harness bar would need to be changed from a non-HANS configuration to a HANS configuration, particularly for taller occupants. Come to think of it, I don't think I've ever been asked to sit in the car and show belt fitment at tech. As a co-driver I usually skip on tech unless I expect rule based questions (usually at US events.)

As a tall-guy, I have thought about this a lot and agree it is a huge deal, particularly if your shoulders are taller than the bottom of the harness slot

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____________________________________________________________-

One. Class -- 2WD

www.quantumrallysport.com

http://www.facebook.com/home.php?#/pages/Quantum-Rally-Sport/281129179600?ref=nf

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Josh Wimpey
Surely there are plots for climbing ropes and webbing if you can't find something specific to belts.

Even more, webbing is pretty much the same as seat-belts, but I still don't think his name is Shirley.

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Iowa999

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Josh Wimpey
Surely there are plots for climbing ropes and webbing if you can't find something specific to belts.

Even more, webbing is pretty much the same as seat-belts, but I still don't think his name is Shirley.

Well, to be really nit-picky most webbing used for climbing is tubular

rather than flat like a seatbelt.

Based on using it to rig slacklines, the tubular stuff can stretch somewhere in the 10% range and spring back... I don't think flat webbing (or seatbelts, or ratchet straps, or ...) will go quite that far.

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KF7RWG
http://www.utahrallygroup.com