Dynamo help please!

[Birdie]

You got there before I wrote my reply so well done but yes, the best solution is someone with soldering equipment.

Lovely, thank you. No one in my household though, other arf can't even change a plug! I'll look further afield!

 

[Birdie]

Back in Victorian times when I had a dynamo like that I was riding down a country lane that was pitch-black but quite adequately illuminated by said lamp. On a downhill stretch with my speed increasing I grabbed the lamp to raise the angle a bit and it came off the bracket. Not only did it plunge me into total darkness but I could distinctly feel the current flowing from the lamp body in one hand through to the bare metal of the handlebar in the other. Surprised me somewhat!

Oh my god! That would scare me to death! Note to self - don't touch anything whilst moving except handlebars.. If my hair starts standing on end, then I'll know I've done something wrong

 

[Birdie]

Designs vary but I'd take an educated guess that the wire was attached to the terminal where the tab on the base of the bulb its as marked by the arrow in my drawing. View attachment 478438
The red stuff insulates it from the rest of the bracket which would have been grounded using the body of the lamp and in connection with the frame of the bike. It should be soldered or possibly riveted into place. Someone handy with a soldering iron could easily fix this. Or if that rivet is hollow, you may be able to bodge it with a bare wire fed through it so it contacts the bulb but soldering would definitely be more reliable.

If the front lamp works, I can think of several different reasons why the tail lamp won't work.

1. Probably because of a bad connection on the bit of the wire and tab that have now broken off.
2. Blown bulb - as mentioned, swap with the front temporarily or test it on a battery. Don't ride with the rear bulb in the front as it will blow very quickly but a quick spin of the wheel to check operation won't do any harm.
3. The light most probably grounds through the frame anyway even if an additional wire is used (for reliability). In which case you have caused a short circuit.

Thank you for all the info. I pressed it on and kind of used my nails to get it slightly under the tiny rim and it stayed on long enough for me to test it. I have light, so it's not the bulb. Can't believe I've actually done something slightly technical! Might as well buy myself a soldering iron and try it myself! I recall using my dad's old one that you had to heat up on the gas cooker . BTW, what do you use to put an arrow etc on a photo?

 

[EltonFrog]

Thank you for all the info. I pressed it on and kind of used my nails to get it slightly under the tiny rim and it stayed on long enough for me to test it. I have light, so it's not the bulb. Can't believe I've actually done something slightly technical! Might as well buy myself a soldering iron and try it myself! I recall using my dad's old one that you had to heat up on the gas cooker . BTW, what do you use to put an arrow etc on a photo?

They’re not expensive and easy to use. Patience is the key.

 

[Birdie]

Sorry, just one more thing! The 'ground' wire is connected to the nut on the front lamp and wrapped round the screw in the back of the tail light. Is that right because from googling, it seems that everyone else says to run it from headlamp bracket to dynamo? but I don't see how.

 

[Chet Spiker]

Electrons can only flow in circles - they leave the dynamo via a couple of wires, pass through the front and rear bulbs then return via the metalwork of the frame. People with carbon frames can't have wiring like this - as much as they want to! Problems can arise when the frame forms a less than ideal conductor due to paint, corrosion, oil-films etc. The best way to wire a dynamo is with twin flex like doorbell cable (not too thin though). Find a way in each lamp to connect the twin flex across the bulb then bring both flexes back to the dynamo. One of the connectors in each lamp will be the "ground' or frame of the bike but that is now redundant as the wire will have less resistance than the frame. Identify which of the twin wires is the "ground" connector and fit this securely under a screw of the dynamo clamp. The other wires of the twin flex are connected together and then connected to the wire coming out of the dynamo body. Obviously, you need to run the flex safely and neatly - make sure it won't foul or snag on any moving part of the bike. This should be reliable.

 

[tyred]

Sorry, just one more thing! The 'ground' wire is connected to the nut on the front lamp and wrapped round the screw in the back of the tail light. Is that right because from googling, it seems that everyone else says to run it from headlamp bracket to dynamo? but I don't see how.

That is probably correct as you have metal lamps which are designed to earth through the bicycle frame back to the dynamo. Most bottle dynamos are made in this way with this assumption and have only the "positive" connector. Some (usually Axa) will have two contacts but most (including Millers from memory) will only have one connector and assume you are using the frame as the return path. Some dynamo and light mounting brackets have little pointy grubscrews to make better connection but I never use these as they damage the paint and invite corrosion.

What you need to achieve is to have a wire running from the connector of the dynamo to the connector on each lamp (which should be in contact with the small contact on the base of the bulb. The outside part of the bulb should connect (ground, or earth) through the lamp body. Run a separate wire from the lamp mounting bolt to the dynamo mounting bolt for a more reliable connection.

A lot of what you read online nowadays with regards to dynamo lighting is probably talking about hub dynamos where you wire the tail lamp from the headlamp and have a switch on the headlamp and in that way you can turn both head and tail lamps on/off as required using the switch. With a bottle dynamo, you have no need for a switch as the you pull the dynamo away from the tyre when you want to turn the lights off so it would be traditionally a separate wire from each lamp coming to the dynamo directly.

I normally use speaker wire when wiring dynamos. Easily and cheaply available from any car accessory or hi-fi shop. Just make sure it's reasonably thick.

Soldering is a skill well worth of learning. Get some old wire from somewhere (strip the flex from a broken electrical appliance or something) and practice "tinning" the strands of wire and soldering them together. Your lamp fixtures would probably benefit from being cleaned before soldering as it probably tarnished through age and corrosion makes soldering difficult to say the least. Use wire wool or similar.

 

[tyred]

Thank you for all the info. I pressed it on and kind of used my nails to get it slightly under the tiny rim and it stayed on long enough for me to test it. I have light, so it's not the bulb. Can't believe I've actually done something slightly technical! Might as well buy myself a soldering iron and try it myself! I recall using my dad's old one that you had to heat up on the gas cooker . BTW, what do you use to put an arrow etc on a photo?

The arrow is really quite simple using a piece of software which is a relic from the Windows 3.1 days - I copied and pasted your picture into Paint, added the arrow, saved the file temporarily and uploaded it to this site again.

 

[Birdie]

Oh paint!, yes of course. Lovely thank you.