Hitchcocks Forum

Coming back from a run, I heard a metallic clanking, looked down to see the horn dangling, held only by the wires. The horn mounting bracket had fractured just above the engine mounting bolt. Obviously a fatigue fracture, I had been watching the horn oscillating madly with the engine vibrations, and wondering when it would fracture. My question is ; Is it worth welding it, and rubber mounting it, or would it be less hassle just to replace it?

It's probably spring steel so it won't weld well without stress risers.

Enfields kill horns. I usually have to replace mine for every MOT. I've had more than one bracket fracture in the way you describe.

I find using a bit of alloy flat bar is a bit more forgiving as a mounting bracket.

Thanks, Stinkwheel, I did have some doubts about the weldability of that steel, possibly impaired by work hardening, or fatigue related crystalline changes. We have no MOT for motorcycles here so I think I'll hang the horn from the front downtube with a cable tie and some duct tape for the time being.

I bought a metre of mild steel strip and made my own replacement bracket. Possibly one of the easier repair jobs I've done on the bike.

John G wrote: ↑

Tue Aug 03, 2021 10:09 am

Thanks, Stinkwheel, I did have some doubts about the weldability of that steel, possibly impaired by work hardening, or fatigue related crystalline changes. We have no MOT for motorcycles here so I think I'll hang the horn from the front downtube with a cable tie and some duct tape for the time being.

I mean, you could always weld, heat to critical, quench and anneal it... But that seems excessive.

You may find it doesn't work properly unless it's attached to something fairly rigid. The spring steel bracket seems to be part of the functional design that lets the back of the horn vibrate.

The cause of the fracture is the oscillation of the horn, a heavy item atop a longish mounting bracket, which eventually induces a fatigue fracture, as I noted. I think a preventive measure would be to fabricate a stay connecting the horn mounting bracket to the front downtube. This would stop the horn wagging to and fro with the engine vibrations, and should improve the lifespan of the component.

In my experience, rigidly mounting electric horns results in a marked reduction in sound output. As has been mentioned, horns are often mounted on spring steel or laminated steel brackets. These allow the whole horn to vibrate when sounded, increasing the sound level.
Rigid mounting effectively damps the vibration of the horn body and it is only the diaphragm which can then vibrate.
On my old Guzzi, the standard flexible brackets failed, so I mounted the horns on a short piece of angled steel. The steel bracket was fixed in the original position with a Norton Commando rubber exhaust mount bobbin (aka classic Mini exhaust bobbin.)
This worked for years with no problem. Just remember that rubber doesn't conduct electricity so you may need to put a wire across the bobbin studs if the horn earths via the mounting bolt.....

Thanks for that, Allan, I think the stay would be more to reduce the excessive amplitude of the horn vibrations, i.e. allow a bit of flexibility in the movement, rather than the present set up where the horn is vibrating with the engine. Something like a bungee might do it .