front dynamo lamp breaks bulbs

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front dynamo lamp breaks bulbs

Postby avalanche » 15 Nov 2004, 7:58pm

Hi there. I have a problem with the front lamp on one of my bicycles. I have recently returned from a tour during which the bulb on my front dynamo lamp broke 3 times. Luckily I took along 2 spares. However once the third one had gone (3 days in a row) I had to buy a torch and tape it to my handlebars as the bulbs (3W 6V) cannot be bought in cycle shops in the scottish highlands... unless they are ordered.

My commuting bicycle's front lamp uses the same bulbs and I have had no trouble at all with it. My question is a request for kind CTC folk to help me with the technical specifications so that I can order a lamp that is similar to the one on my commuter. My commuter uses a nexus dynamo hub. My tourer (the bicycle which has the dodgy front lamp) uses an inexpensive Basil bottle dynamo.

This is all that I can tell you about the lamp on my commuter... and I wonder if someone can untangle this description and tell me what I need to ask for when ordering something similar? Here goes: It has 'halogen' written on the back, and the characters '9430HR' and 'K20630' embossed on the back. It is mounted on a metal frame that sits between the mudguard and the underside of the forks. The lamp dissociates from the bit into which the bulb fits by twisting it (no pesky screw). Does this information help? It seems pretty generic to me?

The lamp which breaks the bulbs (I am thinking it is due to its frame bouncing up and down on the mudguard (my commuter lamp frame is quite solid and doesn't do this) is made by Soubitez (HS3, CH1 1245, K 10792).

Any comments or suggestions will be welcomed.


Re:front dynamo lamp breaks bulbs

Postby avalanche » 20 Nov 2004, 7:44pm

Can I explain this a little better? I believe that it is the rattle/vibration of the bracket that holds the lamp to the top of the forks which is causing the filament of the bulb (or something) to break.

On my commuting bicycle the front lamp bracket attaches to a hole at the *front* of the top of the forks. The bracket forms part of the the lamp unit, and is quite solid. My touring bicycle (the one which breaks the bulbs) only has a hole at the rear of the top of the forks. So, there is a flimsy bracket which goes from hole in rear of top of forks to the base of the lamp unit. This bracket isn't part of the lamp unit, but a separate component shaped like a 'U' but with one arm very much shorter than the other.

I have searched for a lamp unit which, like the one on my commuter, has a bracket as part of the unit - but can't find any that affix to the rear of the forks (i.e. have that 'U' shape).

I have a Dawes Galaxy (circa 1999). Surely I can't be the only person who's not happy with a rattley dynamo bracket? Surely there is something out there that has a front lamp unit which has a bracket? Surely?


Re:front dynamo lamp breaks bulbs

Postby CJ » 22 Nov 2004, 9:36am

This lack of front hole is a known nuisance with Galaxys of a certain period, but easily solved: just drill it! (Better still: drill it 5mm and tap an M6 thread.) Then you can fit all sorts of lamps – no probs. Well, maybe slight probs: since nice touring bikes often have neater fork crowns than common bikes, so the brackets are sometimes too tall where they but up against the crown, for the narrow gap between mudguard and headset. So you may also need a lump of alloy with a hole in, to space the bracket forward and clear of the headset.

A recommended replacement lamp: Hella Micro – very neat with good vibration damping from plastic bracket.

But it's probably not just the vibration that's killing your bulbs. Most likely the zener diode overload protection has blown in your lamp, so the bulb routinely gets too much current. The thing that most commonly blows that zener is running a front lamp only without swapping the bulb for a 3W one – or not disconnecting the dynamo soon enough after a bulb has blown. And there's no obvious indication of a dead zener – other than your lamp having acquired an expensive bulb habit!


Re:front dynamo lamp breaks bulbs

Postby AHB » 22 Nov 2004, 11:33am

It is a pity the manufacturers won't rate the zener to withstand the full dynamo output:

When making my own from pairs of back to back 1.3Watt BZ85 Zeners (7V5, I just parallel each regulator pair. This is OK because 7.5 Volt zeners have a +ve temp coefficent.

Its even more annoying that expensive LED rear lamps have the same underated regulators(to save a few pence).


Re:front dynamo lamp breaks bulbs

Postby avalanche » 22 Nov 2004, 9:16pm

CJ. Whilst I value your comment regarding it not being vibration which is causing the bulbs to break, I am pretty well convinced it is. I don't believe that it can be the current. I replace the bulb in the morning - test it - it works fine - cycle all day - flick on the dynamo at night and it doesn't work, at all. Obviously what I did in the end was insert the bulb just prior to use... but I was defeated by a bumpy gravel private road... ended up cycling last few miles with a torch between my teeth!. This is a bottle dynamo, so clearly it is only on when I flick it on. The lamp bracket bounces up and down on the top of the front mudguard. I use 3W bulbs.

I take your advice re creating a hole the the front of the forks. I don't have the tools to do this (or the skills), so I'll be going with your suggestion to my favorite bicycle shop.

"A recommended replacement lamp: Hella Micro – very neat with good vibration damping from plastic bracket. "

OK, this sounds good. Thank you for this recommendation.

AHB. Thank you for your contribution. I don't have the technical knowledge to be able to follow it... but I'm sure the folk who do will appreciate it.


Re:front dynamo lamp breaks bulbs

Postby troywinters » 20 Dec 2004, 6:24pm

5w zeners are still available if one searches for them. ... &XPAGENO=1