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Kestrel Wiring

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Kestrel Wiring

Post  Bill on Tue May 01, 2012 3:08 am

Hi I have 2 wiring diagrams for my Kestrel but neither says what amp rating wire I should use. Question
1) from the Magneto to the switch and 2) for the lighting system.
I was going with 10amp as thats what I have but thought I should check.
Can anyone help.
Regards Bill
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Re: Kestrel Wiring

Post  Rifleman on Sun Jun 10, 2012 1:50 am

Bill - hope this info is not too late, but I've only just seen your post.

There's three main factors to take into account with wiring a bike with a 6 volt system. Current carrying capacity, mechanical strength, and (the real bugbear) voltage drop.

Current carrying capacity is quite straightforward; at maximum current available from the battery and / or generator, will the wire overheat?

You don't say whether your bike is direct or battery lighting, so I'll start with direct. On the generator coils used on the E and A type engines, each lighting coil is rated at a nominal 18 watts, which equates to 3 amps at 6 volts. Even where the headlamp bulb is being fed from two coils in parallel, a 30 watt filament is only drawing 5 amps, so your 10 amp wire is well within its limits as far as current goes. All other bulbs draw far less current.

If your bike has a battery, it's a bit more complex, as even a small battery can shove out a heck of a lot of current into a very low resistance, such as a short circuit, if only for a few seconds - whereas a lighting coil can't. Even if a lighting coil is feeding a short circuit, the coil winding has its own resistance (pretty small, I grant you - but it's still there) which puts an upper limit on how much current it can supply, no matter how small the resistance of the load.

You can also get the odd hiccup, if, for example, the dipswitch bridges the contacts for main and dip as it switches across. If the headlamp is switched on, and such a dipswitch has the lever sitting right in the middle, the feed wire to the dipswitch may be feeding two 30 watt filaments at the same time - about 10 amps by itself. Add in the current drawn by the tail lamp, speedo lamp - and stoplight, too, if you're holding the bike on a hill with the footbrake - and the total current draw may add up to about 14 amps! If the engine, meantime, is only doing a sluggish tickover, virtually all of that will be coming through the main feed wire from the battery to the switch via the ammeter.

Another problem is that, sometimes, when a bulb blows, the broken filament can fall across the metal posts inside the glass, and give you a partial short circuit - i.e., very low resistance - and draw a lot more current than you would normally expect that bulb to do.

For both the above reasons, I always err a bit on the cautious side, and make sure that the wire I use is rated well above the worst case scenario, as described above.

The second point I mentioned was mechanical strength. For example, a typical speedo light bulb would be around 1.5 to 2 watts (nominally a quarter to one third of an amp).

One wire which could easily handle the load of a speedo bulb would be RS Components Part No. 177-0700. Seven strands, each one only 4 thou thick, overall diameter 32 thou. How long would that last being shaken around by a single cylinder engine, and being hit by a 40 mph slipstream? Sad So I always make sure the cable is able to cope with a bit of knocking about, too.

But the one which tends to catch people out is volts drop. Even the finest copper cable, in perfect uncorroded condition, is not a perfect conductor - it still has measurable resistance. To give you some idea of the losses this can entail, I did a rough calculation for a modern type of automotive wire, rated at 11 amps. If that wire was carrying 6 amps, the volts drop would amount to about a quarter of a volt per metre - in other words, if the supply voltage at one end of the cable was 6 volts, and the current was 6 amps, the voltage available at the other end would be down to 5.75 volts. That doesn't sound too bad . . . except that current doesn't flow from point A to point B and then stop; and it doesn't just flow through a perfect copper cable!

(if you have a 12 volt system - or even a 1200 volt system - if the current is 6 amps, the voltage drop is still a quarter of a volt per meter. That's why big trucks hauling 40 foot trailers use 24 volt systems - it keeps their tail lights nice and bright!)

What I mean is that current flowing from a battery to a headlamp bulb also has to flow back, via the earth connection, to the battery - and you're getting a volts drop every inch of the way. So, if the distance from the battery to the headlamp is 1 metre, the total current path is 2 metres - and, in the example cable above, you've lost half a volt! And it gets worse if you use the frame as an earth. The current has to get through steel, rather than copper, connectors with a tinge of corrosion, slightly rusty bulb holders - in some cases, the return for the headlamp has to get through the grease-coated headstock bearings, and each of those obstacles is robbing your bulb of another fraction of a volt.

Oh. One other point. Cable resistance is usually quoted at an ambient temperature of 15 degees C. As the temperature rises, so does the resistance - and wiring looms run right above the cylinder heads of most bikes . . . Sad

So, first thing I do is to use a really hefty cable, especially on 6 volt systems. Roughly, if you double the cross sectional area of the wire, you halve its resistance (there are other factors which make it not quite half, but let's not get too technical!) This is especially important for wires carrying the most current; headlamps, battery connections, and connections via the ammeter to the light switch, and to and from the dipswitch.

Second thing is, never ever ever rely on the metalwork of the bike as an earth return! That is a typical cheap and nasty motor trade bodge, even on new vehicles. On 50 year old bikes - forget it, and fit a hefty wire for all earth connections. I know, "it's not original". But it works. Reliably. For a long time.

In any case, how 'original' is your bike going to look if some Erbert shunts you off the road because he didn't see your 'jamjar full of glow-worms', masquerading as a tail light?

Hope this is not too late to be of help, and best regards,

Jack
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Re: Kestrel Wiring

Post  Bill on Sun Jun 10, 2012 5:03 am

Many thanks Jack for your complete answer I have gone with the 10amp wiring almost finished the wiring. I had already decided on running earth wires as didn't want to remove new paint just to get an earth. Yes it has a direct lighting system although I dont plan on being out after dark. I am however a fan of running lights in daytime for saftey. I will be fitting stop light although don't think they originally had one.
Regards Bill
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6v wiring

Post  myquest on Sun Jun 10, 2012 10:50 am

Rifleman Jack.
That was a MOST enlightening dissertation - thank you.
Note to self: Check Falcon's earth wiring - now!
MQ
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Re: Kestrel Wiring

Post  Rifleman on Sun Jun 10, 2012 9:44 pm

Bill and MQ - you're welcome!

Bill, ref. fitting a stoplight. I don't know about your particular model, but many bikes of that era didn't have a stoplight fitted, especially the low-cost lightweights. On some bikes, it was an optional extra. I know some people feel it very important to put their bikes back to absolutely original condition, but the reality is that many bikes in those days barely made it out of the dealer's door before the new owner was modifying it - and, of course, adding a stoplight was a pretty popular mod, especially for those having to cope with big city traffic!

If you look at adverts in motorcycle papers and magazines from the 1960s right back to the 20s, there were stacks of companies offering all sorts of extras and mods - and people then were just as keen as they are these days on making their bike just that little bit different from everyone else's. Different handlebars, handlebar mirrors, quick action twistgrips, carb bellmouths, in-line bowden cable oilers, chain oilers, special waterproof covers for plugs and carbs, special seats, nailguards for tyres, special exhaust pipes, extra loud 'silencers', handlebar fairings, leg-shields, crash bars, spotlights and foglights, panniers - the list is endless. So you could say, in truth, that fitting one or more of those 'non-original' bits and bobs is just as historically accurate as having the bike in catalogue condition!

Oh, and living in the Peak District, where 1 in 4 gradients are no big deal, it seems only common sense to do what I did with my old C.15 when cling rubber tyres came out - especially as I had to ride to work over cobbled streets, even on rainy days. The original ribbed front tyre was junked in favour of an Avon cling just as soon as I got paid - and what a revelation that was! If a bike is permanently on display in a museum, then sure, fit original type tyres - that they don't grip as well as modern tyres doesn't matter, if the bike never goes on the road. But if you use your bike in everyday modern traffic, especially in built up areas, old-fashioned tyres may be pushing your luck. After all, modern car drivers have servo brakes with ABS, disc brakes all round, and radial tyres - not cable operated drum brakes and crossply tyres - and they use them to full advantage, on the assumption that you can pull up just as sharp as they can.

Which reminds me of a hooligan I used to know in London, who had an Ariel Arrow - noted for their feeble front brakes. Del had the front hub from a Honda 125cc sports laced into his front wheel rim - which gave him a 7 inch twin leading shoe brake! Cool
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Stop lamp

Post  kerabo on Mon Jun 11, 2012 6:44 pm

Just so you done have to repeat your work

If the vehicle was made after 1st jan 1936 and can exceed 25mph and has other lights then it must have a stop lamp that neets to requirments in the MOT manual.

Regards Ken
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Re: Kestrel Wiring

Post  Rifleman on Mon Jun 11, 2012 9:00 pm

Thanks, Ken - I didn't know that.
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Re: Kestrel Wiring

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