Installing a fuse

Introduction to motorcycle fuses

Installing a blade fuse on my Matchless

I have suggested fitting a fuse to your classic motorbike in a number of articles on this site (such as Replacing the regulator and Converting to 12 volts), so I thought it would be worth taking a little time to go into this in a bit more detail.

Most classic motorcycles built up until th 1950’s and 60’s were never fitted with a fuse when they were new. Well there are lots of things on these bikes that wouldn’t be designed the same these days, that’s part of their charm after all. Whether the original omission of a fuse was intentional to save costs or whether it was just not thought of at the time, I’m not sure. But as every single modern vehicle on the road now has tens of fuses protecting every conceivable circuit, it seems sensible to retro-fit at least one to your old bike.

This guide to installing a fuse (or fuses) will cover the following topics:

• Introduction to motorcycle fuses
• Why a fuse is so important
• Where to fit the main fuse
• What fuse rating to install
• When and where to fit additional fuses
• The types of fuse available
• Conclusions and your comments

Why a fuse is so important

So if the original manufacturer didn’t think it necessary to fit a fuse within the bike’s electrical system, then why should you want to add one now? Two words – ‘safety’ and ‘protection’. I was going to say ‘just in case’, but then I remembered that in the couple of years since having fitted a fuse to my old Matchless motorbike I have accidentally blown this countless times whilst tinkering. It’s very easy to accidentally short one of the terminals to the frame whilst working on the bike, especially when fiddling inside the cramped headlamp cowl. So a fuse is as much about offering protection for when you do something silly as it as about being there for when something goes wrong.

I also remember working on a friends car with him some years back, a Peugeot diesel with a huge battery. Somehow he managed to touch the wing with the end of the ratchet as he was tightening the clamp on the positive battery terminal which blew a small hole right through it in a big shower of sparks! Now a fuse wouldn’t have helped at all in this situation, but it does show the potentially destructive force that a short-circuit could have on your vehicle.

What a fuse does is take over the role of the weakest link in your motorcycles electrical system from whichever length of wire, terminal, switch or connector previously held this rather dubious honour. Then when something goes wrong and an excessive current flows from the battery, it is this cheap, easy to locate and easy to replace component that takes the hit. That way the rest of the wiring and components in the fragile electrical system survives to last another day, rather than leaving you stranded at the roadside with an acrid cloud smoke from fried insulation wafting from under the saddle.

The maximum current that a normal 12v motorcycle battery could potentially deliver through a low resistance such as a short-circuit is quite surprising.  Take a fairly standard modern lead-acid pack that is designed to fir within one of the original-looking black rubber battery cases. It is rated for a maximum of about 10 to 15 Amps of output current in normal use, but the maximum 5 second discharge current is a whopping 200 Amps! That would easily fry a typical motorcycle wiring loom that might only be rated for somewhere around 20 to 30 Amps.

There is really no good reason for not installing a fuse if your bike doesn’t already have one.  The only possible argument I can imagine is that someone wants their bike to be ‘original’, but unless it is still on the same tyres, bulbs, battery, brake-shoes, etc, etc that it had when it first left the factory, then the addition of a well-placed hidden fuse is going to make it any less so in my opinion.

Where to fit the main fuse

So that brings me neatly to the question of where you should fit the fuse on your classic motorcycle. Ideally it should be hidden from site so as not to look odd, but be somewhere easily accessible for when you do need to check or replace the fuse. The main fuse should also be as close to the battery as possible so as to minimise the amount of unprotected wiring (i.e. that before the fuse) that remains vulnerable in the event of a short circuit.

Normally you would install the main fuse on the live side of the battery in-line with the wire which goes from the battery and up to the ammeter connections for distribution to the lights and other accessories. However the wiring in the fuse holder I bought was red (to suit modern applications I guess where positive is live) and my Matchless is wired positive earth. So I had the choice of either fitted a red fuse assembly in place of the normal black negative terminal, or else fitting it to the earth side of the battery instead. For me, a red negative battery connection was just asking for trouble and confusion further down the line, so I opted for fitting the fuse to the battery’s earth terminal instead. Realistically it doesn’t make too much difference as the same current will be flowing into and out of the battery in the event of a short-circuit. But stick with convention and fit the fuse to the battery’s live side unless, like me, you have a good reason to do otherwise.

There are a few options for positioning this main fuse depending upon the exact setup of your battery and wiring loom. If you have one of the black rubber battery cases fitted which lets you install a modern battery inside which maintaining the look of the original unit, then inside this case is one possible choice. The fuse will then be hidden from view and protected from the elements and dirt, plus it would be convenient to keep a supply of spares alongside. The downside is that you would of course have to open up the battery clamp each time you needed to check or replace the fuse.

The location I chose in the end was a few inches along the live wire from the battery which meant that the fuse holder could easily be tucked up behind the battery clamp and under the seat, out of sight on my Matchless. By leaving an inch or two of slack in the cable it is easy enough to pull the fuse holder down for easy access when needed, then tuck it back up out of the way again afterwards. You could also maybe think about fitting it somewhere under the petrol tank if that is easier, but with the heat and grime from the engine, that’s probably not ideal.

Another location that I have heard suggested is within the headlamp assembly. This does mean that the fuse is both hidden and protected from the weather and dirt, but it also makes it harder to get to as you will always be needed to open up the headlight. Another disadvantage is that it is a long way from the battery and so won’t offer any protection if a fault or short-circuit occurs somewhere along the main battery connection, so I wouldn’t go down this route personally.

What fuse rating to install

In order to work out which rating of main protection fuse you should fit to you motorbike you first need to work out the maximum electrical load that it is expected to draw with all the lights etc on at once. Firstly draw up a list of the various electrical components fitted to your bike and write their rated powers (in Watts) at the side. The following list is for my 1951 Matchless G3LS so you can use this as a template and adapt as necessary for your own motorbike:

• Headlamp 35W
• Pilot light 1W
• Tail light 5W
• Brake light 21W
• Speedo light 1W
• TOTAL 63W

Next you need to convert the power (in Watts) to current (in Apms). To do this you divide by the voltage of your motorbikes electrics (the nominal battery voltage). For a 6 volt bike the maximum anticipated current draw would be 63 / 6 = 10.5 Amps and for a 12 volt bike it would be 5.3 Amps.  The current will be much higher (double) for 6v systems as for those running at 12v; this is the reason that 12v systems are more efficient. Also note that you are selecting a fuse based upon maximum load rather than the dynamo capacity as it is the battery that will be the primary source of high current in the event of a fault occurring.

Notice that I have not included the horn in the above calculations. The original Lucas Altette and Clearhooters type horns fitted from the factory can draw huge amounts of current. This is why they were usually wired separately, directly from the battery, so as not to overload the terminals at the ammeter where all other electrical loads are connected. To include the fuse in these calculations would result in a much higher fuse rating being selected which will then offer less protection to the rest of the bikes electrics. If you still have one of the original horn types fitted then it is worth giving it its own separate fuse, as discussed in the additional fuses section below.

Some electrical components, the headlight bulb in particular, will actually draw a much high current to begin with when they are first switched on, compared to that implied by the wattage stamped on the side. This is because the filament inside the bulb is metal and like all metals, its resistance increases as it gets hotter. So in order for a bulb to have the required power when the filament is glowing white hot, it must have a lower resistance (and thus higher initial power) when it is cold. This initial surge in the current drawn by conventional filament lamps can easily blow an otherwise correctly sized fuse, so we need to allow for it over-rating our selected main fuse size.

A good starting point is to over-rate the fuse by 50% above whatever maximum current you determined calculated from adding up all of the bulb wattages etc. So in the example I quoted above for my Matchless, 50% extra over-rating would mean that I needed a 1.5 x 10.5A = 15.8 Amp fuse if my bike was 6 volt and 1.5 x 5.3A = 7.9 Amp if my bike was 12 volt.

Clearly these ratings are rather odd numbers and so we would be hard pressed to find the exact fuse size we wanted. Fuses tend to come in standard ratings of 1, 2, 3, 4, 5, 7.5, 10, 15, 20, 25, 30 and 40 Amps although you may find more or less choice in a certain fuse style or from a given supplier. In order to select the most appropriate fuse for your bike you will need to round off the 50% over-rated value you calculated to the nearest available fuse size. It is always preferable to round down where possible as a smaller fuse will give better protection than a larger one, and the 50% factor we have already included is probably quite generous.

For my Matchless example I would go with a 15 Amp fuse if it was 6 volt (the calculated value was 15.8A) and a 7.5 Amp fuse if it was 12 volt (where the calculated value was 7.9A). These are fairly safe and standard values for most old bikes I think, but some trial and error needed. If the bike often blows a fuse in normal operation and you’re sure there’s not a fault, then try the next few size up instead. It is always better to start small and work up to make sure you have the best protection.

When and where to fit additional fuses

The main fuse fitted close to the battery is the most important one to get right, but there are some occasions when it might be worthwhile fitting one or more additional fuses to your motorbike. I mentioned earlier about the current drawn by the original Lucas Altette and Clearhooter type horns which are typically wired separately from the rest of the electrics with their own direct connection to the battery. Therefore adding a separate fuse to the wire going to the horn from the live terminal of the battery is also a very good idea. Sizing the fuse may take some trial and error to get right (I haven’t yet needed to do it myself as my original Clearhooter is still not working so I have substituted a modern low-current horn for the time being which is powered via the main fuse). The Lucas workshop manual for one of these horn types specifies a maximum current draw of 5 Amps for the purposes of testing, so I would be tempted to try a 5 Amp fuse to start with then increase to a 7.5 Amp (or maybe even a 10 Amp) fuse if needed.

The manufacturer of the DVR2 electronic voltage regulator I installed on my Matchless also suggests fitting a second fuse in-line with the output (ammeter) wire from the regulator, presumably to further protect the charging system and dynamo. They recommend a 13 Amp fuse for 6v and a 10 Amp fuse for 12v operation which would seem a little high based upon the calculations we have made above. If you do decide to install the extra fuse then I would go with the same rating as the main fuse next to the battery’s live terminal as a starting point. I have bothered with this additional fuse as yet, but will revisit the logic behind it again when I eventually get around to rewiring my bike.

The only other situation I can think of when you might need to install extra fuses other than those mentioned about is when you are fitting extra non-standard electrical items to the bike. Anything that is wired directly to the battery,by passing the main fuse, should have it’s own specific fuse rated accordingly to the maximum power draw required. One example would be if you fitted an anti-theft alarm or other such device, or if you were to wire in a connection for a sat-nav, intercom or other such modern accessory. These devices are much more sensitive and require significantly less current than the standard electrical items on the bike, so fitting a small (say 1 or 2 Amps maximum) fuse will ensure they are well protected. This fuse could be parallel to the main one if you take a separate supply from the battery, or in series if you take the supply from somewhere within the headlamp that is already protected by the higher-rated main fuse.

If you have a sidecar fitted to your motorbike, then it would also make sense to fit a fuse in the live supply cable between it and the motorbike. This will ensure that the motorbike will be unaffected in the event of a fault with the electrics in the sidecar, rather than losing all of your lighting at once.

The types of fuse available

There are a number of different types of fuse available from different manufacturers in different eras around the world. The two most common ones which I will consider here though are blade fuses and glass fuses.

Glass fuses are small cylinders with a metal contact each end and a thin thread of fuse wire running through the centre. They look much like the fuses found in UK-type domestic mains plugs, only they are usually transparent so that you can see the fuse wire inside in order to determine whether or not it has blown. They are available in several different lengths of which 25mm (1″) and 30mm seem to be about the most popular. The length is not particularly important and does not affect the operation of the fuse, but will usually be determined by the style of fuse holder you select.

There are also various different ‘speeds’ of glass fuse available to suit different applications. The speed refers to how quickly the fuse will blow once its rated current has been exceeded. Fast-blow fuses are used with more modern electronics where even the shortest excess current pulse can cause damage. Standard or slow -blow fuses are more suited to the noisy general electrics of classic bikes where spikes and ripples in the current drawn (for example when turning on headlights) are common. A fast-blow fuse will theoretically offer better protection, but in practical terms may blow unnecessarily with a small voltage spike that would cause no damage to the motorbike. Standard glass fuses designed for normal automotive applications should be more than fine. Glass fuses were fitted to older British vehicles so may be a more authentic choice for installing on a classic motorcycle.

Blade fuses are a modern design made of plastic which makes them much more robust than the older glass fuses. They can also be fitted more densely in a small fuse box which is why they are more used in modern vehicles, but that won’t really make much difference when you’re only fitting one or two. Another advantage of blade fuses is that they are colour coded so that each Amp-rating has its own unique colour. This makes it much easier to make sure you install the correct fuse, rather than squinting trying to read the often feint text stamped around the end of a glass fuse. That might be an important consideration if you’re or a similar vintage to your bike and your eyesight isn’t quite what it used to be.

Blade fuses are readily available and come in a wide range of fuse ratings. There are also several different variants available including ‘mini’ (smaller), standard and ‘maxi’ (larger) sizes. Again you can be more guided by what fuse holders you can find, but I’d stick to the standard size if possible.

The choice of which type of fuse to install on your bike is a personal one. Glass fuses are a little more authentic as they were the choice for many older British vehicles, but then true authenticity is to not have a fuse at all! Blade fuses are more robust and possibly a little easier to get hold of being fitted to so many modern vehicles. The main determining factor will probably be the fuse holder you chose based upon what is available locally, where you intend to install it and how you would like to wire it in. In terms of offering protection to the electrics, either type of fuse will do exactly the same job. I used a blade fuse on my Matchless when I first installed it back in India as the blade fuse holder was the only one I managed to pick up from a petrol station shop whilst on holiday in Australia. However I have since bought some glass fuse holders which I think are more in-keeping with the style of the bike and I shall be fitting these instead when I eventually get around to rewiring. The choice is yours!

Conclusions

So there we have it; hopefully everything you need to know in order to install an appropriately sized protection fuse (or fuses) in the electrics of your classic motorcycle. Even if your bike already has a fuse fitted it would be worth using the calculations above to ensure that it is correctly rated to offer full protection, or maybe it would be best served with the addition of a couple of extra fuses in specific circuits.

If you have any thoughts, suggestions or feedback on this article then I would love to here from you. There is a comment form below so please get in touch!
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