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Discussion Starter #1
I've recently re-wired a 72 T100R, using a new 'Lucas' loom and old but serviceable wires from the original loom All Ok, works just fine.

I'm now embarking on a complete restoration of my 73 TR7. I had to do some re-wiring when I bought the bike as a non-runner, and that was enough to show me that the loom is a mess, so it's re-wire time. I have Boyer and a reg/rectifier. There seem to be redundant wires that I think shouldn't be redundant, so I'm going to start from scratch with cables of the correct color and the correct wiring diagram. Should keep me out of the pub for a week or 2. Having said that, the bike runs just fine.

I have crimp tools, strippers, cutters, soldering kit etc, so I thought I'd make my own and fabric wrap it.

Rusty1 has shown how he did his T100r and that's very helpful.

I seem to be able to get the necessary components from Autosparks and Vehicle Wiring Products here in the UK. Whilst I can carry out a forensic investigation of the original wiring specs on the bike now, may I assume that the cables are generally 28/0.30? If there are others, does anyone have a simple list of what's what? For example, the black/white and black/yellow to the points chest look lighter - 14/0.30 maybe?

Had a look for a thread that explains all, but couldn't find one - apologies if I've missed it.

Many thx.
 

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Hi Andy, this is what I used for my Boyer and Tympanium equipped Daytona, all from Autosparks. Hefty wire only required for main feeds and returns, skinny wire for all other stuff and on my bike (at least) super skinny for threading returns along the indicator stems. Nothing smoked when I ran the bike for the first time.

25 Amp Cable - Thinwall 2.00mm². Equivalent to 28/0.30 Brown/Blue (feed from battery negative to ignition switch)
25 Amp Cable - Thinwall 2.00mm². Equivalent to 28/0.30 White (feed from ignition switch into headlight shell, 14/0.30 thereafter)
25 Amp Cable - Thinwall 2.00mm². Equivalent to 28/0.30 Green/Yellow (from alternator to Tympanium)
25 Amp Cable - Thinwall 2.00mm². Equivalent to 28/0.30 White/Green (from alternator to Tympanium)
25 Amp Cable - Thinwall 2.00mm². Equivalent to 28/0.30 Red (main return)
5.75 Amp Cable - PVC 9/0.30. CSA - 0.65mm2 Red (indicator stems return)
8.75 Amp Cable - PVC 14/0.30. CSA - 1.00mm2 White
8.75 Amp Cable - PVC 14/0.30. CSA - 1.00mm2 Brown/Green
8.75 Amp Cable - PVC 14/0.30. CSA - 1.00mm2 Blue/White
8.75 Amp Cable - PVC 14/0.30. CSA - 1.00mm2 Brown
8.75 Amp Cable - PVC 14/0.30. CSA - 1.00mm2 Green/Red
8.75 Amp Cable - PVC 14/0.30. CSA - 1.00mm2 Green/White
8.75 Amp Cable - PVC 14/0.30. CSA - 1.00mm2 Black/Yellow
8.75 Amp Cable - PVC 14/0.30. CSA - 1.00mm2 Black/White
8.75 Amp Cable - PVC 14/0.30. CSA - 1.00mm2 White/Yellow
8.75 Amp Cable - PVC 14/0.30. CSA - 1.00mm2 White/Brown
8.75 Amp Cable - PVC 14/0.30. CSA - 1.00mm2 Lightgreen/Brown
8.75 Amp Cable - PVC 14/0.30. CSA - 1.00mm2 Purple/Black
8.75 Amp Cable - PVC 14/0.30. CSA - 1.00mm2 Red/Black
8.75 Amp Cable - PVC 14/0.30. CSA - 1.00mm2 Brown/White
8.75 Amp Cable - PVC 14/0.30. CSA - 1.00mm2 Blue
8.75 Amp Cable - PVC 14/0.30. CSA - 1.00mm2 Blue/Red
 

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Hi Andy, On my original from new '73 Tiger the harness is not fabric covered, but taped with common black plastic electrician's tape.

There are some smooth wire sleeves also. The points wires, oil pressure switch wire, & some heading over rear fender are sleeved. There is a fat smooth sleeve that passes through left frame grommet & enters head lamp grommet. Some others also.

Looks like my harness is tapped together inside the "head tube" sleeve. I personally have found over the last 40 years a bundle of wires don't fracture the conductors as easily if they are loose in the sleeve. Allows each wire to find its own place & are not nearly as stressed. Ok(good) to tape harness together before & after the sleeve. But inside the sleeve I don't. I like to zip tie sleeve to harness at lower end. This doesn't make a perfect seal, so any water that might find it's way into headlight shell riding in rain, if it gets into sleeve, it can drain out. I find riding in rain, my shell takes in water.
Don
 

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Discussion Starter #4
Thank you Gentlemen. Brilliant. My current loom works, but it's stiff, grubby and has a number of dead ends. It's an unscheduled stop just waiting to happen, somewhere out of the way just when you need to be somewhere else by a certain time. It's got to go.........

Rusty1: many thx - that gives me a good place to start. I'll have to go and have a count - I thought most of the cables were 28 conductor - clearly not. My new indicators will be here in a day or so so I can see what size wire will go along the shaft to wire in a dedicated earth.

Don: interesting that yours is taped. Looking at pattern looms they are all fabric covered. Not sure what mine should have been, but it's at least partly taped now. I used sleeving quite a bit on my T100R and it looks really neat - so I'll be getting more, particularly for the connections into the headlamp shell and the runs down to the rear light and the points chest.
 

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Hi Andy,

cables are generally 28/0.30?
If what's on the bike now is original, all wires in all original '71-on looms were/are 14-strand, including all the Red wires and the Brown/Blue. :( Moreover, original strands are slightly smaller (32SWG) than modern metric (0.3 mm.), so Lucas rated original wire for only 7.5A ... exceeded by the standard alternator above about 2,000 rpm. :Not again It's one of the "cheap shortcuts" I've posted about before, even modern 14/0.30 is only rated for 8.75A, all modern off-the-shelf looms copy original Lucas in this respect so it's one reason I don't use modern off-the-shelf looms.

re-wire time.
Suggestions:-

. The standard Lucas (original Lucas) implementation of Red return wires is over-complicated and fails if more than one wire breaks ... :Not again

. I use two of these, one in the headlamp shell and the other under the seat. Each individual component Red wire is connected to the nearest, they're connected together with two lengths of 28/0.30 'thinwall' (higher-rated but thinner overall than standard PVC-insulated) that join attached to an engine head-steady bolt/stud. On a pre-'79 kickstart-only bike, this simplified network is joined to battery +ve by a 28/0.30 Red wire from the underseat "Sleeve Connector", through the main fuse/holder.

. In addition to the aforementioned Red wires, I also use 28/0.30 thinwall for the Brown/Blue wire and the White wire between the ignition switch and Sleeve Connector in the headlamp shell that's the junction of individual component White wires.

. I fit relays in the headlamp supply as a matter of course, even if the existing headlamp is the crap standard BPF; if you don't fit relays, I advise 28/0.30 thinwall also for the Brown/White between Ignition Switch and Lighting Switch (on a '73).

. Apart from those wires, all other wires are to/from individual components could be 9/0.30 as its 5.75A rating isn't exceeded by the consumption of any component, the only thing that precludes this is not all colour combos. are available in 9/0.30. :(

. I use the above wire sizes plus 14/0.30 because crimp-on bullet terminals that work are available for all three from both Autosparks and VWP. The "1 sq.mm." (aka 14/0.30) ones don't work reliably on original Lucas wire (e.g. switch cluster wires, because of the aforementioned smaller strand diameter); for those, I have/keep some http://www.vehicle-wiring-products.eu/product.php/453/w-crimp-type-4-7mm-bullet.

. When ordering from VWP, they also sell a variety of button contact that can, if necessary, be used in the insulated part of bulb holders (couldn't find the link :().

have Boyer
. Red wire from the B-B "Transistor Box" connected directly to battery +ve terminal.

. Red wire from coils' series connected either directly to battery +ve terminal or underseat Red wires' Sleeve Connector.

. Fuse in either the Transistor Box White wire (preferably) or Red wire.

and a reg/rectifier.
. Connect its DC wires (Red and Black?) directly to the corresponding battery terminals, one or other through a fuse (not the main fuse protecting the loom); don't replicate the standard Lucas rectifier connection in the Brown/Blue wire to connect the reg./rec.

fabric wrap it.
. Bear in mind wrapping was/is only to keep a manufactured loom together during transport (and original fitting on the bike-building production line). Once a loom's on the bike, any wrapping's just a pita.

. I've never wrapped any loom I've built. On a bike, I start by securing wires to top frame tube and drive-side seat tube (on a 'dry-frame' keeps the wires away from possible oil contamination) with releasable reusable cable ties. My first loom, having built it, I just couldn't be arsed to take it apart to wrap it. Turned out to be an inspired choice because I still own the bike, it's had some upgrades over the years, :whistle which were wa-aa-ay easier than if the loom had been wrapped.

. Any loom I build, once it's complete, tested and mostly covered by tank, seat and sidepanels, any wires still exposed I cover with lengths of what Autosparks calls Split Convoluted Conduit (scroll down the page to see the different sizes) and VWP calls Slit Convoluted Tubing ("Slit" is in the "Options:" drop-down); the Split/Slit is lengthways so the Conduit/Tubing can be fitted or removed from wires without needing to disconnect and thread them. :thumb

. If the headlamp shell has three wiring holes, I always split the wires to/from the frame and thread them through the left and right shell holes (and OIF frame gusset holes?). To allow Conduit/Tubing beside other wires or cables, Autosparks sell a grommet with a larger centre hole, or you can simply cut out standard grommet centres?

There seem to be redundant wires that I think shouldn't be redundant,
Off the top of my head:-

. Twins with points originally had a supply wire (White/Yellow on your bike) to each coil "-" terminal; only one's required for the B-B Transistor Box White wire so the other White/Yellow is redundant.

. Points require condensers, their connection was (on your bike) one of the two Black/White and Black/Yellow wires originally attached to each coil "+" terminal. The other Black/White and Black/Yellow wires originally to the points are now being used between the Transistor Box and the (B-B) "Stator"? So the condensers and wires to them are redundant.

. If the reg/rec. DC- (Black?) wire was connected to the original Brown/Blue wire rectifier connection, as I say, imho both reg./rec. DC wires should be connected directly to the corresponding battery terminals, so any break in the new Brown/Blue wire isn't required. :thumb

. Similarly, with any reg./rec., the original Brown/Blue connection to the Zener is redundant, as is the Red wire (usually) attached to your bike's airbox between the original rectifier and Zener mountings.

recently re-wired a 72 T100R, using a new 'Lucas' loom
interesting that yours is taped. Looking at pattern looms they are all fabric covered.
It's one of the changes Wassell ("Genuine :rofl Lucas") make to suit themselves; ime, Autosparks and British Wiring looms are wrapped correctly for their application; afaict, all 'original Lucas' looms were taped '71-on; however, it's becoming harder to be sure because even replacement looms can be old and tatty.

As Don posted, taping the wires between frame and headlamp shell is a crap idea; on my first T160, the White/Yellow wire broke there within a couple of years, because the wires had been taped too tightly by whoever built it at Lucas. :Darn Simply covering them with tubing, wires can move and flex individually as required when the forks are turned.

sleeving
to
points chest.
If your bike still has the original points cable, with the Black/White and Black/Yellow wires encased in black insulation, bear in mind the rubber seals in crankcase and timing cover were intended to work with that. Otoh, if you fit the seals over sleeving, it folds between the seal ID and over the wires inside, allowing a space for water to enter.

If you don't have/use an original points cable, Autosparks has the larger selection of sleeving OD; use the one that completely fills the passage through the crankcase and timing cover and leave out rubber seals.

Hth.

Regards,
 

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Discussion Starter #6
Thanks Stuart - very helpful. I stripped out the looms today and they are sitting in a crate, as an oily, disheveled, knotty mess. I can do better!

I bought the non-runner, scruffy TR7RV as a project to have something do, being retired. I got it running and on the road, now it's time for this winter's project. I suspect the loom will be a significant part of that. Looking forward to making the loom to be as neat and tidy as I can make it.

Your advice, together with that from Don and Rusty1 gives me lots to chew on.

It's a non matching-numbers bike, so its value is limited, but this summer showed that it's loads of fun to ride, relaxed, torquey, an easy ride, and a complete contrast to my T100R (which is loads of fun in a different way) - so it's worth the effort to get it as smart and reliable as I can. Even if it'll never be worth what I'll put into it.

You can't take it with you so you might as well enjoy it while you are here.
 

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Discussion Starter #8
Hi Andy, On my original from new '73 Tiger the harness is not fabric covered, but taped with common black plastic electrician's tape.
Don
Don, I've now removed all of the wrapping from my 73 TR7RV harness. Whilst most of it was sticky, PVC tape, some of it (the first layer) was non-adhesive, shiny, black pvc tape - is this what you mean by original? I quite like it - it was still shiny, and it unwrapped really easily to leave clean wires beneath.

If so, I can get that over here from Autosparks.

I'm about to catalogue each and every cable to identify its length, where to goes and where from, and the terminations. I can then check my stocks and buy whatever else I need to reproduce the loom.

It would stand a bit of rationalisation too. For example, there are 4 earths from the frame earthing point just below the tank which run into the headlight - one to each of the instrument lights and two more to the headlamp. It seems to me that they can be rationalised down to 2 and that would help with the rat's nest in the headlamp shell. Similarly, there are redundant terminals in the coils area that can be rationalised. A good project on a cold and wet winter's evening with a coffee in hand!
 

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Hi Andy,

rationalisation
4 earths from the frame earthing point just below the tank which run into the headlight - one to each of the instrument lights and two more to the headlamp. It seems to me that they can be rationalised down to 2
:Huh Why 2?

As I posted for you above:-

. The standard Lucas (original Lucas) implementation of Red return wires is over-complicated
If you're sensible, you'll add a return (earth) wire to each indicator, so that's two more at the front. No idea why there are "two more to the headlamp" on your bike specifically but, as standard, the main beam idiot lamp return/earth is also in there.

Given these are all on the forks (if not in the headlamp shell already), why would you want two return/earth wires across the gap between headlamp shell and frame?

Then what's the point of connecting anything electrical to the frame, which doesn't have anything electrical on it that earths through its mounting? :confused: Otoh, spark plugs and oil pressure switch on the engine do require a return/earth path from engine to coils and battery +ve respectively.

help with the rat's nest in the headlamp shell.
As I posted in your restoration thread, my T100 has the same disc-brake front end as your bike, with a similar-size headlamp shell. However, as it's pre-'71 (basically :whistle), the headlamp shell also mounts an Ammeter and I've added indicators. Using the "Four Way Sleeve Connector" I linked for you in my previous post and some thought, my T100 also has a headlamp with an H4 plug (much bigger than the BPF) and two standard automotive 'cube' relays and wiring plugs. But it isn't difficult to fit the headlamp into the shell.

catalogue each and every cable to identify its length,
Mmmm ...

. Both Autosparks and VWP only sell cable/wire in multiples of one metre; three metres is ample from headlamp to tail-lamp (e.g. Brown/Green from pilot to tail or Brown wire from front brake switch to lamp), two metres is ample from either headlamp or tail-lamp to under the seat, one metre usually neither use nor ornament ...

. The only two colours you'll need more than three metres are White and Red (between individual components and the respective bullet Sleeve Connectors) in your choice of either 9/0.30 or 14/0.30. Given having too much is going to cost you a few pennies but ordering too little is going to cost you wa-aa-ay more in both time and p&p ... :whistle

Of greater importance before finally cutting wires to length and terminating:-

. Don't forget to turn the forks to the opposite lock ... :bluduh

. Long wires (e.g. the aforementioned Brown and Brown/Green, wires from the headlamp shell to under the seat), make a small loop - e.g. around a finger, pencil/pen, screwdriver, etc. - at one end. After a few thousand miles, check the loops ... surprising how many won't be there ... Any still present, shorten and reterminate if desired - a lot cheaper and less time-consuming than replacing a long wire that turned out to be a centimetre/inch too short ... :whistle

Hth.

Regards,
 

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Hi Andy, I've never personally unwrapped a late tape covered wire harness, so I don't know. I don't know if Triumph did an under wrap of non adhesive tape or not.

In my life experience with old cars I've found old PVC type tape, the adhesive can "evaporate" & the tape basically falls off with glue behind. Sometimes it's a gooey mess of old adhesive. I don't know why. I know for certain on some than fell off, it was sticky tape when new.

Regarding wire lengths, on my bike the wires in the center of bike where harness passes down/by frame are just barely long enough. 3/4" more wire in this area would make routing smoother & easier to install in my mind, but I've never done it on late bike, so I really don't know until I get there, just keep it in mind.

Yes there is a lot going on inside headlight shell. As Stuart said there is room in shell without forcing anything. However I've heard of aftermarket warning lights & headlight switch that are larger than original. Maybe that was on earlier bikes though??

Here is my shell. All original from new. I rewired headlamp switch to prevent pilot lamp operation key off. I would forget to turn off switch & battery would go dead. We've had daytime headlamp law for years.

Don
 

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Discussion Starter #11
Hi Don,

That's neat! I've never seen inside a headlight shell as neat as that - so that gives me an idea of how to lay out the cabling. Many thx.

I don't think the non-adhesive wrap was an underwrap - I think that it was overwrapped later by a PO. A 20m roll of the stuff is only £3-4, so I'll give it a go and order some when I order the new cabling lengths. And you can be sure I'll not be making the runs too short. I threw away that T shirt years ago.....

I'll arrange all the necessary cables in that bundle that runs along the spine first, with over-long tails out to the front and back. I can then temporarily install the loom and cut the individual runs to length, then remove the whole lot and wrap as and when required, and add the sleeving down to the oil pressure switch and the points chest. I'll terminate the cables last.
 

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Hi Don,

aftermarket warning lights & headlight switch that are larger than original. Maybe that was on earlier bikes though?
:nah All original idiot lamps are as in your attached picture - one wire direct to the terminal that contacts the bottom of the bulb, other wire crimped and soldered to the 'lip' at the bottom of the metal part of the bulb holder. Otoh, modern 'replacement' bulb holders have two male spade terminals, females spade terminals 'straight' on the ends of wires prevent most reflectors fitting; even with 'flag' female terminals, many reflectors won't fit. :Not again When stripping old original Lucas harnesses, I make a point of saving usable bulb holders with as much wire attached as possible (on the basis that it's easier to shorten wire than extend it ...).

Ime, Lighting toggle switches larger than original are old pattern ones, even Wassell "Genuine :rofl Lucas" manage to get 'em the same size as 'original Lucas', albeit sometimes the lower male spade terminals are at the wrong angle and have to be bent to greater than 90 degrees as in your picture to clear some reflectors.

I rewired headlamp switch to prevent pilot lamp operation key off. I would forget to turn off switch & battery would go dead. We've had daytime headlamp law for years.
Brown/White wire on same Ignition Switch terminal as White wire, or Brown/White wire originally on Lighting Switch replaced by a White wire?

Hth.

Regards,
 

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Discussion Starter #14
Stuart, Don, Rusty1,

I've ordered (from Autosparks) all of the cables and terminals etc, plus the non-adhesive wrapping tape (I know, I know....), plus additional in-line fuses and fuse holders: 5 amp for the Boyer unit and 15A for the reg/rectifier (120W alternator), plus a 25A for the battery (which presumably should go in the -ve side).

I've used your advice for the cable specs. It's been a useful few hours as it's completely clear to me now how the electrics on this bike work - they are not exactly rocket science!

A question: the rh Lucas console switch (the big polished aluminium switch) as shown on the wiring diagram indicates a front brake light switch (Sw9). There's a white feed in (i.e switched 12V) and a brown out to the rear stoplight via the rear brake light switch.

There's no connection between the front brake lever, the master cylinder and the switch itself (other than that the master cylinder bolts to the back of the ali housing) so how does this switch work? What am I missing? I'm not that bothered about a front brake switch as when I did my RAC/ACU course many years ago, the local police instructors made us all use the back brake a moment before the front (and hence the stop light is operated by the rear brake switch). I've never got out of that habit so I don't need the front brake switch - but I'd like to know how it's supposed to work!
 

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Stuart, Don, Rusty1,

I've ordered (from Autosparks) all of the cables and terminals etc, plus the non-adhesive wrapping tape (I know, I know....), plus additional in-line fuses and fuse holders: 5 amp for the Boyer unit and 15A for the reg/rectifier (120W alternator), plus a 25A for the battery (which presumably should go in the -ve side).

I've used your advice for the cable specs. It's been a useful few hours as it's completely clear to me now how the electrics on this bike work - they are not exactly rocket science!

A question: the rh Lucas console switch (the big polished aluminium switch) as shown on the wiring diagram indicates a front brake light switch (Sw9). There's a white feed in (i.e switched 12V) and a brown out to the rear stoplight via the rear brake light switch.

There's no connection between the front brake lever, the master cylinder and the switch itself (other than that the master cylinder bolts to the back of the ali housing) so how does this switch work? What am I missing? I'm not that bothered about a front brake switch as when I did my RAC/ACU course many years ago, the local police instructors made us all use the back brake a moment before the front (and hence the stop light is operated by the rear brake switch). I've never got out of that habit so I don't need the front brake switch - but I'd like to know how it's supposed to work!


My 73 T140V has the front brake switch in the casing and is operated by the lever as you apply the brake. If you can’t see it, take the lever out and you should see a small plastic pin in the opening of the casting. Or open up the casting, the switch is a bit Mickey Mouse to be honest and mostly sticks in the normally open state.
 

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....plus a 25A for the battery (which presumably should go in the -ve side).
If your bike is positive earth then no, see Stuart's previous advice on my re-wire;

"Because the electrics are 'positive earth', the main fuse should be in the one-and-only Red wire attached to battery positive.

This protects against the not-unknown possibility of something metal accidentally touching the battery negative terminal itself and some other part of the bike. In this event, because the bike's electrics are 'positive earth', any metal parts of the bike can form a conductor to any wire attached to battery positive. If there isn't a fuse in the one-and-only Red wire attached to battery positive, this becomes an unfused short-circuit and damage will result very quickly. A fuse attached to battery negative cannot prevent this short-circuit because the short won't be through it."


I have already experienced the benefit of this set up. I was refitting the seat stay cable with the battery in place (stupid), the spanner connected the negative terminal and blew the fuse. Had the fuse been in the negative side the short would've been between my spanner and the battery with no protection.
 

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Discussion Starter #17
My 73 T140V has the front brake switch in the casing and is operated by the lever as you apply the brake. If you can’t see it, take the lever out and you should see a small plastic pin in the opening of the casting. Or open up the casting, the switch is a bit Mickey Mouse to be honest and mostly sticks in the normally open state.
Hi Ram,

I've bought a genuine Harris T140V master cylinder, c/w lever. The actuating rod that connects the lever to the piston passes straight through the casting - there's no pin, plastic or otherwise to operate any switch in the Lucas unit. Presumably the pin would press the contacts together in the console, but the Harris part won't as far as I can see.

Not a problem for me anyway, and since the bike is MOT exempt, I'll not be having some mechanic failing it because it doesn't have a front brake switch.
 

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Discussion Starter #18
If your bike is positive earth then no, see Stuart's previous advice on my re-wire;

"Because the electrics are 'positive earth', the main fuse should be in the one-and-only Red wire attached to battery positive.

This protects against the not-unknown possibility of something metal accidentally touching the battery negative terminal itself and some other part of the bike. In this event, because the bike's electrics are 'positive earth', any metal parts of the bike can form a conductor to any wire attached to battery positive. If there isn't a fuse in the one-and-only Red wire attached to battery positive, this becomes an unfused short-circuit and damage will result very quickly. A fuse attached to battery negative cannot prevent this short-circuit because the short won't be through it."


I have already experienced the benefit of this set up. I was refitting the seat stay cable with the battery in place (stupid), the spanner connected the negative terminal and blew the fuse. Had the fuse been in the negative side the short would've been between my spanner and the battery with no protection.
I must be being particularly thick here. The wiring diagram, which in all other respects looks correct, shows +ve earth and the fuse in the -ve.

I need to understand why it should be in the +ve on a +ve earth bike. Say you have 12V at some point on the bike and this is then shorted to the frame (or an earthed cable) without passing through a component that would reduce the current by virtue of its resistance - so a dead short has ~0 ohms resistance and hence the battery will supply whatever current it can, at 12V.

If the frame is connected to the +ve side of the battery (being +ve earth) the unlimited current will pass wherever it likes on its way back to the battery and maybe damage something else. However, if the fuse is in the -ve, then the current will be cut off as the fuse blows before it gets anywhere else.

Or is this a matter of electrons passing current the opposite way to what you'd expect - I seem to remember something from my A level physics, some 50 years ago!

I'm quite happy to do as recommended and put the fuse in the +ve but me being me likes to understand why!
 

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I must be being particularly thick here. The wiring diagram, which in all other respects looks correct, shows +ve earth and the fuse in the -ve.

I need to understand why it should be in the +ve on a +ve earth bike. Say you have 12V at some point on the bike and this is then shorted to the frame (or an earthed cable) without passing through a component that would reduce the current by virtue of its resistance - so a dead short has ~0 ohms resistance and hence the battery will supply whatever current it can, at 12V.

If the frame is connected to the +ve side of the battery (being +ve earth) the unlimited current will pass wherever it likes on its way back to the battery and maybe damage something else. However, if the fuse is in the -ve, then the current will be cut off as the fuse blows before it gets anywhere else.

Or is this a matter of electrons passing current the opposite way to what you'd expect - I seem to remember something from my A level physics, some 50 years ago!

I'm quite happy to do as recommended and put the fuse in the +ve but me being me likes to understand why!
Not as thick as me Andy! I didn't think it made much difference and I certainly can't comment on how electrons behave but there is one scenario which I understand. With the fuse in the -ve side on a +ve earth bike if the seat (for example) closes onto the -ve battery terminal there's nothing protecting the dead short, the contact is between the earthed seat (effectively the battery +ve terminal) and the battery -ve terminal. If you put the fuse in the +ve side, and it must be in the only wire going to the battery +ve terminal, the fuse will blow. I think that's it but Stuart will be along soon.
 

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Discussion Starter #20
I get that scenario, the seat contacting the -ve terminal on the battery would effectively short both terminals of the battery - if there were a fuse in the +ve connection that would blow and that would be the end of it. It would be the same as dropping a spanner onto the -ve and it contacting the frame - that would be bad news if there was no fuse in the +ve.

My thinking was if a 12V supply in the loom earthed to the frame (say through chafing) - I'd have thought that a fuse in the -ve feed near the battery would be best. But there's no harm in fusing both the +ve and the -ve at the battery. Other than added complexity?
 
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