Math guys: Question on seat belt strength

[RickM]

As I research the replacement of my 911 seatbelts I have a question regarding breaking force for the belt webbing material.
According to the DOT specs (FMVSS-209) the breaking point must be at least 26,689 N (Newtons). Does this translate to 5,999 pounds of force?

Is this the same a tensile strength? This is the terminology that the webbing material manufacturers use.

Thoughts?

[Bill Verburg]

5,999.925903 # is a little closer

[RickM]

LOL...good one.

Same as Tensile strength?

[Wayne 962]

One thing to remember - never buy used belts, unless you know specifically the history of the car they came out of. Belts are designed to stretch on impact, and if they were taken out of a wrecked car, then they are no longer useful (one-time use, like airbags).

Buy new ones for safety.

-Wayne

[savagekabbage]

My car has bucket seats and racing harnesses. I assume these are better for zipping around the track, instead of seatbelts, but how am i going to fare if I get in a head-on collision?

[RickM]

Originally posted by Wayne at Pelican Parts
One thing to remember - never buy used belts, unless you know specifically the history of the car they came out of. Belts are designed to stretch on impact, and if they were taken out of a wrecked car, then they are no longer useful (one-time use, like airbags).

Buy new ones for safety.

-Wayne

Wayne, Point taken. In fact in some states it's illegal to sell used seat belt equipment. I just purchased webbing material that appears to be superior to the original product. I've sourced kevlar thread to replicate the stitch pattern on the ends. I'll be using a computerized Bernina machine to program the pattern.

The DOT is very specific with stretch characteristics as well. These specs are given by the webbing material MFR in addition to recovery specs.

My Strength question is to insure my info is 100% accurate.

BTW, If I purchase new belts must I buy the entire mechanism as well? A mouse chewed into my existing belts.

[RickM]

Originally posted by savagekabbage
My car has bucket seats and racing harnesses. I assume these are better for zipping around the track, instead of seatbelts, but how am i going to fare if I get in a head-on collision?

If anchored properly and DOT approved Belt/mechanisms are used then it's probably better than standard equipment. Is it 4 or 5 point?

[JRISER]

Good point Wayne! That's kinda what I was thinking.

[Wil Ferch]

Tensile stress...in Mechanical engineering terms ( careful...don't know how the belt manufacturers quote or use tensile strength)...is that load that begins to tear the fibers of the host material apart. I suspect that is what they mean. It is likely the point where the belt has taken on a permanent ( and non-recoverable) stretch...or has gone beyond its elastic limit. Contrast this with ultimate stress where you go even further and the belt arctually breaks apart in two or more pieces.

Unknown use of words..or assumed semantics..is dangerous in cases like this. I'd contact a manufacturer and ask if they mean its the point where elastic deformation has been exceeded, or when it "breaks". There is a difference !

---Wil

[RickM]

Wil, Thanks for the info. Feds make it clear that breaking point is at 5,999 lbs. I'll let the manufacturer translate for me.

BTW, if I decide to use my "homemade" belts I probably won't share the info as it could introduce unwanted liabilty issues.

[KobaltBlau]

On a similar note, It seems advisable to replace belts on an older car. My '81 has never been crashed, but the belts are looking a little fuzzy and tired, probably like most of yours. Anyone done this as a preventative measure, or, alternatively, has anyone seen how older belts hold up in crashes?

TIA

[RickM]

Andy,

One of the required tests (micro organism attack test) for belt manufacturers is to bury the web material in soil for two weeks and to retest for breakage. Another is to cut 10% into the belt and test.
No mention of effects of aging. A very good point.

[KobaltBlau]

That's interesting info, Rick, thanks. Might not be a problem, but although my belts appear to be made in the same way as the belts on a new car, the definitely don't look as fresh (and they aren't).

[mikes72t]

Yes seatbelt webbing degrades over time. The stitching on seatbelts is very tightly controlled and it is not adviseable to sew your own. I would be very cautious of buying webbing from anyone. Personally this is one of those things that I think is worth spending the money for at the dealer. They are just not worth messing around with.

Racing belts hold you much tighter and actually put additional forces on the body because they slow you down faster as there is less stretch, but they are stronger and will hold you tighter so you are less likely to hit the steering wheel or windows or whatever.

Thanks

Mike