- Sheathing is a material we use to cover the bare wires of our harness.
It provides another layer of protection from abrasion while keeping
all the wires together and bundled tidily.
Aesthetically it's important too as a tidily sheathed wiring harness with
logical branching transition points is a great thing to see in an engine bay.
The two most common sheathing types I use are expandable braid and heat shrink.
We'll discuss the pros and cons of each in detail to allow you to make a selection
for your build.
Expandable braid is a plastic woven sheath constructed in such a way that as you
compress it along its length, its cross section expands.
To install this sheathing we expand it from its relaxed size,
allowing us to slide it over our harness where it then relaxes back
to its smaller size.
This means we can also re expand the braid to remove it from our harness
to correct an error or make a modification.
The plastic strands expandable braid is woven from,
are quite tough, providing good abrasion resistance.
However when selecting an expandable braid to use,
attention must be paid to its maximum temperature rating.
This needs to be at least 125 degrees celsius to ensure reliability
in the engine compartment.
Working with expandable braiding can initially be slightly challenging,
as when its cut with a knife or scissors the ends fray apart quite quickly.
The correct way to cut expandable braiding is using a hot knife that melts
the plastic fibres as it cuts through, stopping them from fraying apart.
A hot knife is not a piece of equipment commonly available.
So in the practical demonstration section of the course,
I will run through some tips and tricks I've picked up to make working
with the material using common hand tools a little bit easier.
You need to be conscious that the braided construction means this sheathing
cannot be used on a wiring harness that is required to be completely sealed.
Completely sealed harness are typically only required at the highest levels
of motorsport.
Where the sheathing of choice is the other option we've touched upon,
heat shrink.
The industry standard heat shrink used as a wire harness sheathing
is Raychem DR25.
It's different to commonly available heat shrink used in electronics
for insulation purposes in that its wall thickness once it has been shrunk
or recovered is much thicker.
This allows the heat shrink to retain its flexibility and provide the required
abrasion resistance.
DR25 is most commonly available in a shrink ratio of 2:1
This means that a half inch diameter DR25 will shrink to a diameter
of a quarter of an inch when it is fully recovered.
When using DR25 to sheath a wiring harness, extreme care must be taken
that every required wire has been laid and is in the right place,
as once the loom is sheathed and the DR25 recovered,
the wires can no longer be accessed to make repairs or modifications.
Recovering the DR25 onto the harness is a one chance procedure.
And if changes need to be made afterwards it will need to be carefully
cut off and then replaced.
Whether you're using expandable braid or DR25 heat shrink,
the rule for choosing the correct size to fit a particular harness section
is the same.
First you measure the diameter of the harness section you are sheathing,
and then select the largest size of expandable braid or DR25
that will relax or recover to tightly fit this diameter.
This will ensure the braid or DR25 you use is as easy as possible to install.
And in the case of the DR25 it will recover to its full wall thickness
to provide the required abrasion resistance and flexibility.
While we always strive to rout our wiring harness as far away from
engine bay heat sources as possible, sometimes it is unavoidable.
In these situations you will need to provide some heat protection
to these areas.
It can be accomplished by adding additional high temp silicone sheathing
over the existing sheathing at these points.
Then securing at either end to make sure it stays in place.
This sheathing is not shrinkable so it's typically a fairly loose fit
on the harness.
It's also not expandable so it needs to be installed before the harness
is pinned and terminated as it will not fit over connectors.
That was just one of the 65 modules from our EFI wiring fundamentals course,
which is the perfect place to start if you're interested in learning how to design,
and build EFI wiring harnesses.
This course will teach you the fundamental electrical principles that you'll need
to know, how to select the correct materials to ensure they can withstand
the automotive environment, the wiring requirements of different
sensors and actuators, and how to design a power supply and grounding scheme
that will protect the harness in the case of a component failure.
You'll learn how to correctly splice and insulate connections, how to bundle
a harness and then sheath it to keep it tidy and protected.
For more information and to purchase the course, click the link now.