Why is a ground strap a strap ?

[michel richard]

The question is not quite as deep as wondering about the meaning of life, but I am still wondering.

I figure it's not for mechanical reasons (more flexibility or such) becasue there are cases where the part to be grounded does'nt move relative to the grounding point.
But then there are a lot of grounds done through wires in our cars.

So . . . why is the ground strap between the trany and the trunk floor a strap ? and why is the equivalent a strap in most cars I've seen ?

Thanks

[DDS]

Probably cost as compared to insulated cable assembly of equivalent ampacity and flexibility.
Braid is very flexible so it will have a long service life, since it is braided rather that just laid up like a rope, (uninsulated fine-strand wire would unlay in production or use) you needn't pay to insulate it, you dont need failure prone terminals in many cases, and it is compact.
Just some guesses based on my past working life in the wire and cable industry.



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-- Dave
Pics of my '73 project here: http://members.tripod.com/mike4g/dds73_b4pics.htm

[DonNewton]

Because "Ground Beam" was taken?

[michel richard]

I hear you guys, but I somehow believe that there is an actual electrical, or current, reason. Straps are just better at conducting current in certain cases, and I'm just trying to know what the details are.

Again, no big deal, but if anyone knows . . .

Thanks

[John Rogers]

If wires are braided in a flat assembly they are call a strap, but if they are round then it is a wire. Braided flat straps will carry more current at less voltage so they are used for grounding. Try a Yahoo search on "ground straps" and you'll be amazed at what you'll find.

[DDS]

Straps are better at conducting current in this application because they last longer than the equivalently priced cable assembly that would otherwise be used.

The only significant impediment to current in that ground strap or wire would be resistance at the connection points, or resistance as a result of insufficient conductor size. Remember, the oxidized magnesium tranny housing, and the rusting steel screw on the chassis are part of the circuit too. There's no leakage, loops, shielding, HF, RFI or other issues here, and older cars probably arent all that sensitive to higher impedence paths to ground anyway. The 914 isn't exactly high tech - an appropriately sized wire in good repair would work just fine.

Most constructions of copper braid are extremely flexible when compared to stranded wire of a given 'circular mil area' (conductor size) and braid is generally tin plated so it won't readily corrode. (corrosion equals resistance)
For reliability in high flex applications, fine conductor stranding and insulations that not only resist the stuff found in automotive applications but also survive at high and remain flexible at low ambient temps are essential -Insulated wire that would perform as well as braid in this application would be expensive. You'd be paying for fancy insulation just to keep the strands bunched.
You will notice that another major grounding path - the ground wire from most battery posts to the chassis - is just that, yer basic dirt cheap automotive wire. No real vibration there and little heat.
IIRC, the 914 ground strap doesn't even have terminals - it is just tin dipped and punched. Cheap, simple, and reliable.

Jeez, did I just write all that about wire...
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-- Dave
Pics of my '73 project here: [URL=http://members.tripod.com/mike4g/dds73_b4pics.htm]

[This message has been edited by DDS (edited 07-21-2001).]

[Roland Kunz]

Hello

The straps are just a other design. They have a much lower resistance then normal connector ended units. The used material will be to "soft" and brake if you make it to a massive "rope". And the best from all is that every hobby mechanic can make them with just a bowl to heat up the Sn and then dive the ends in. The crimp tool for the connectors is not everywhere to find.

However on the battery strap is a other material used becouse the battery acid will make soft copper more trouble then the steel rope, But old 911 have also a strap as ground cable on the batttery post. The benfit is that the wing nuts allowed to loosen the strap fast without removing the battery. On the soft strap a low clamp force on the nut will vibrate loose.

Grüsse

[michel richard]

Thanks, that's what I was looking for !

Michel

[AlexO]

So what guage wire should be used if the strap needs to be replaced?
--Alex

[John Rogers]

On both our cars I use straps between the tranny and body and I use 3GA Exide cables between the batt and the kill switch/starter and batt and it's ground. I se the ones with clear coating and when the green corrosion starts going up the wires I replace them, about every 18 months or so. Good luck.

[DDS]

John's got it right.
For the archives - Bearing in mind that the factory unit is metric I'll describe it in roughly equivalent NA units. It is made of tubular tin-plated copper braid rolled flat consisting of 36X15 strands each with an OD of around .009", or between 31 and 30 AWG. A 30 AWG conductor has a circular mil area of around 100, so multiplying 540 strands by 100 gives a total CMA of 54000. If the strands are 31AWG the CMA would be 43200. 4 AWG wire has a CMA of 41700, 3 AWG 52600, 2 awg 66400. Whatever you use, try to find tinned copper conductors and at least equivalent stranding. If you use something with fewer (in wire, 133 strand is more common) then put a loop in the wire to avoid stress concentrations and keep the bend radii as large as possible.

Dave



[This message has been edited by DDS (edited 07-22-2001).]