Breaking and Making Light Mounts

The Problem

So on my commuting bike I've been running my old faithful Busch + Muller Lyt headlamp for nearly six years now, originally fitted on my old Raleigh Twenty back in Sydney.   The positioning of the headlamp was less than ideal, it protruded slightly above the level of the parcel tray so any large luggage tended to bear its weight on the lamp itself.  Years of this lead to the mount finally cracking which made the wiring go a bit funny...  Anyway, it was time to replace it and I decided to go with the Philips Saferide 40, this is the smaller brother of the Saferide 60 which I fitted to my tandem bike.

This was mounted protruding out the front of my parcel tray, the Saferide 40 is a little longer than the old one so it protrudes about 10cm.  Only after a few weeks whilst going through a gate I smashed the light and the mounting eyelet broke... Thankfully this didn't affect the wiring however, the electronics still worked fine.  I knew all along that really it would be better slung underneath the parcel tray out of harm's way.  No one sold anything like this off the shelf as generally dynamo lights are intended to be mounted on the crown of the fork.  I'm no stranger to making custom light mounts like on Phil, where I fabricated one from old brake parts so what I decided to do was to 3D print one.

3D printing has crossed over into the mainstream, there are many methods of 3D printing and the method I am using constructs the object by breaking it down into many very thin layers of plastic.  These layers are constructed by a head which moves in a 2D plane which streams out liquid plastic.  As these layers are built up a 3D form takes place.

First Prototype

I work as an Architect normally so am quite familiar with 3D software and also we happen to have a 3D printer at my office.  So off I went and I modelled one using Rhinoceros 3D which is a type of 3D modelling software.

The objectives were that it needed to be able to recess the light underneath the parcel tray, enough to prevent it from being knocked about again.  It also had to clear the front tyre and allow for some angle adjustment.  Also it has to be strong enough to last a while, the medium the 3D printer uses is PLA which is Polylactic Acid plastic, this is a biodegradable plastic.  This isn't as strong as ABS plastic which is commonly used in industry however printing ABS requires different and more complex equipment.

So I designed a mount which holds the light in what you call a gimbal mount, it kind of looks like a birdcage, but it is meant to allow the light adjustment up or down and it is set back from underneath the parcel tray to protect the light from collision.

The design took a little while as this was really the first time I had used this modelling software, I have experience using other 3D software so this was a reasonably quick learning experience.  After modelling I sent it to the 3D printer and loaded up the black filament...  2 hours later and I have my first prototype!

The webbing and extra material you see is called bridging, this is meant to be a temporary support for the work piece as it is being printed.  It is easily removed after printing with a knife and some filing.   This was done in a slightly quicker method which has less thickness and material inside, more for testing as I assumed I would need to revise the design.

The first problem I encountered was that the holes were full of webbing and I needed to use a drill to clear them out thouroughly, also a lot of the holes were a little small.  Despite having measured a standard M5 metric bolt which I intended on using, it still came up a little small.  What I was missing was a bit of tolerance space.  The next problem was that the part just behind the top mount protruded too far back and any object on the parcel tray would bear weight on this part of the mount instead of the leather straps.

Secondly the gimbal mount part was a little too narrow to fit the headlight body comfortably and it interfered with the angle adjustment, also the top mount where it fixed to the parcel tray was a little bit too small and the bolt wasn't able to pass through easily.

Despite these problems I managed to mount it and ride with it, although I went over some bumps and I heard a crack sound... the mount snapped under the weight of some cargo bearing down on the back of it.  You can see the construction of the mount as it is quite thin, it was set at 0.8mm outside thickness with not too much fill inside.  PLA plastic is known for being a little bit more brittle than ABS plastic, and this demonstrated this.  There isn't as much deformation before breaking as I would normally be used to with mass manufactured ABS plastic.

Second Try...

Back to the drawing board, this time around I redesigned the mount so there was not so much offset.  Also I scaled up the gimbal mount and bracket by about 10%.  As I was a bit more certain it would fit this time, I increased the wall thickness to 3mm and increased the amount of fill material.  This caused the print time to go up to 4 hours.

This mount turned out a lot better, thankfully the light fit into the gimbal perfectly and the angle adjustment worked with ease.  The parcel tray mount also fit better and there was no interference with the luggage in the parcel tray.

So far so good, hopefully with the light a bit more protected I should be less likely to damage it in use.  It still does protrude out about 5cm however it is much more protected than before, if there is a collision in the future I should be able to revise it again for the third (hopefully final) version.

Thanks for reading.


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