8 replies to this topic

[MrBounce]

I'm coming to the stage where I will be starting to put the Midas back together, and one thing that's concerning me is earth points. When I got the car it was a rolling shell and the only earth point I could find which had been used at some point was a few ring terminals on one one the shock mount bolts, which I thought probably wasn't ideal.

 

The monocoque is entirely fibreglass - there is no metal in it other than the stuff attached to it. It uses a single bolt front subframe and a bespoke beam axle. My thoughts (and please excuse my poor terminology as I don't understand electrics much) would be to have a specific single earthing point (potentially somewhere on the subframe) for the battery. I would then connect a block terminal to this point, and could run all other earths to this block.

 

Happy to accept all advice on this and any alternative suggestions.

[nicklouse]

I would have something like this in the cabin so you can run your dash earths to it. Then continue the big earth wire to the subframe. Then earth the engine to the subframe. Then all front of car earths can attach to the subframe where ever needed.

Ps the picture is just a fancy one from a boat. You can just drill some holes in a bit of steel.

[Dusky]

You ll need a big wire between the front and rear subframe, the rest of our 'idea' is good imho, corvette solves it in a similar fashion.

[MrBounce]

You ll need a big wire between the front and rear subframe, the rest of our 'idea' is good imho, corvette solves it in a similar fashion.

Do you mean using something similar to a battery cable?

[nicklouse]

No you don't.

Just battery to front subframe with an earthing point near the dash like above. So you don't need to run earths through the bulkhead or back to the battery.

[Dusky]

If you feel better running a lot of wires from te tank, brake Light,.. Then fine for you nick. 1 cable is a lot less messy.

[tiger99]

Absolutely not! If you have anything electronic on the car, especially stuff like an ECU for fuel injection or mapped ignition, severe damage can result due to earth voltage drops in the wrong places. I currently work for a manufacturer of such things. There is also the need to consider any unsafe failure modes that will happen if one of the wires breaks.

The only satisfactory way is to create a "star point" which will be a nice meaty bolt somewhere suitable. The driver's side bulkhead or inner wing area may be suitable. The battery earth and engine earth strap, both as thick as the main battery cable, must have heavy crimp lugs fitted and be the first on the bolt. I would put them together directly, lock washer and nut on top. Then on top of that, stack the various earth returns that I shall list shortly, each with its own properly crimped tag, followed by a lock washer and nut. A 5/16" or 8mm zinc plated bolt is adequate, a brass or copper bolt even more so, if it will take enough torque to stay tight for ever. Give it a good coat of vase line or Waxoyl when finished.

Now to the individual earths. One from each front light cluster, same thickness as the blue/white headlight feeds. Go up a size if using spots. Left side, indicator, head, spot on one wire. Right on another. Dashboard, depending on type, may be more than one black wire. Extend them individually to the star point. Give the radio a nice meaty ground of its own.Each tail light cluster an individual wire. One of these will carry number plate lamp too, if there are seperate left and right fuses that will determine which. Do NOT share the fuel tank earth with anything else. Add extra wires for odd things like interior lamp as needed. Individual wires from any ECUS that are fitted.

That way there should be no spurious interactions due to earth voltage drop, nothing damaged, and both headlights not failing simultaneously.

Avoid the temptation to fit the star point bolt to the front subframe. It will cause some starter and alternator current to take the alternate path through the diff, driveshafts, hub bearings and top arms, and that will very seriously shorten the life of at least the hub bearings. The subframe should be electrically isolated.

[nicklouse]

If you feel better running a lot of wires from te tank, brake Light,.. Then fine for you nick. 1 cable is a lot less messy.

 all you are doing is using the cable as a replacement for the car body.

 

 

Tiger seems to have missed the whole point of the body being fiberglass.

[tiger99]

I have not missed the point at all and I know the body is fibreglass. You are NOT simply replacing the body with copper. With practical sizes of cable there is far less conductivity, more voltage drop, and no redundancy in the event of a breakage. If the earth potentials of certain expensive items like ECUs differ from other connected items by too much in the wrong direction, they are destroyed. I know this to be absolute fact, as I design engine management ECUs for a living at the moment. I can not disclose proprietary information about the innards, but suffice it to say that MANY electronic items are similarly vulnerable.

 

It is absolutely imperative that a star point earth is used if any modern electronic systems are used, or may be in the future. It is even done on large trucks, with a massive chassis, for very good reasons.