Tripod headlamp options

(This article originally appeared in Safety Fast Jan 2015)

I’ve recently been looking at headlamps again in my continuing efforts to improve the Magnette experience as everyday transport - obviously in the summer months the lighting performance (or lack of) isn’t such an issue. My Varitone had its original Lucas P700 tripods with British pre-focus (BPF) tungsten bulbs when I bought it, and although they weren’t brilliant it was only when one of these was accidentally broken several years ago that I felt the need to consider alternatives.

This is a question often discussed on web forums for 50s/60s cars, where the general consensus is that tripod headlamps are inherently worse than some other designs of similar age, and yet others as well as me still grapple with wanting to keep their classic appearance while driving safely. When you see replacement headlamps without the unmistakable three-armed support for the centre badge and bulb shield on a Magnette it somehow doesn’t look quite right.

Original TripodA quick google search will bring up a range of new P700 headlamps, however you’re unlikely to find a genuine Lucas unit manufactured on original tooling unless it’s ‘new old stock’ on ebay. The repros look pretty authentic from a distance, but most don’t have the Lucas name on the centre badge (presumably for copyright reasons) often replacing it with ‘P700’. Some are supposedly exact replicas for BPF bulbs, while others have the bulb holder adapted to take a modern halogen-type base.

There’s several useful web pages on Lucas tripods, including an excellent article by Loz Scott at mgaguru.com, but I have to admit to only studying these recently. At the time I was looking, I decided that reproduction headlamps with halogen H4 bulbs must undoubtedly be superior to the originals, so they were duly fitted with minor modification of the loom connectors.

Relay modification

I should also mention that I fitted relays to provide the main and dip beam switching, although some time after replacing the headlamps, so I didn’t have the opportunity to see their effect with the originals. With halogens I measured nearly 2V drop from the battery to the bulb, which theoretically results in more than 25% reduction in potential lighting power - possibly it would have been a little less with BPF bulbs.

Mostly this drop seems to be at the dashboard and dip switches, with the wiring itself still perfectly up to the job. But the simplest solution as frequently recommended is to take a feed directly from the battery (or just before the unswitched fuse connection in my case) and use the original wiring to activate relays that switch the battery feed. I’ll put up some photos on the website of my relay positioning on the inner wing, in case anyone’s interested in doing the same. When the relays turn on, the drop at the bulb is now an insignificant 0.25V, and the dash and dip switches now have a much easier life without having to take the full load.

Repro performance issues

The halogens do almost certainly put out more light on main beams, since the H4 is 60W compared to a 50W BPF, and also halogen bulbs convert power into light more efficiently with less heat (worth mentioning that I’m still running a dynamo, but the extra couple of amps load hasn’t been an issue). However a headlamp will only be effective if most of the light from the bulb can be focussed onto the road exactly where it’s needed, which is the purpose of the reflector and lens, and I always felt the beam from the repro units was more spread out than it should be, with dipped beams quite effectively lighting objects high up on the side of the road !

The reflector is a standard parabolic shape, which causes a light source at its focal point to be reflected forward in a parallel beam. The main (high) beam uses a filament at this central focal point, while the dip filament is offset upwards and to the right so the beam direction is slightly down and to the left, for right-hand drive at least. Crucially, the focussing was designed to work with the reasonably well-controlled position of BPF filaments, which as shown in the photo are different to those of a halogen equivalent when mounted in the same base position.

From my measurements the H4 main-beam filament is reasonably close, but the dip filament is about 10mm further from the base, and not offset from the axis. Hence the beam does not dip to the left, and also the tripod shield obscures significantly more of the reflector from the filament than intended. so the dip beam becomes just a dimmer and less focussed version of the main.

The headlamp lens also needs to have correct optical properties, as it provides spreading and further shaping of the parallel beam from the reflector. Most likely the lens of a reproduction P700 is manufactured with a mould which was itself produced from a genuine Lucas, inevitably introducing some rounding of edges. It’s only when you put the two side by side that the more crisp detail in the original can be seen, which probably accounts for some of the difference in beam pattern. where the light is scattered much more widely. Some articles on the web by other dissatisfied repro buyers suggest that the glass itself is likely to be of unknown optical quality.

Measurements and comparison

Having decided to give genuine Lucas tripods another try, I was very fortunate that Peter Martin found me a good used example to match my remaining original. In fact both of them show slight deterioration of the silvering at the bottom of the reflector where the steel behind has started corroding, but this is probably inevitable unless you can find unused old stock.


Before and after the changeover I used a light meter to measure the lux level at the brightest part of the beam. It’s a fairly cheap handheld unit so absolute accuracy isn’t guaranteed, but it at least allows relative difference to be judged objectively. For the main beam I measured at a distance of 1.5m:

  • Repro H4 without relay - 1600 lux

  • Repro H4 with relay - 2800 lux

  • Original BPF with relay - 2500 lux

So the biggest improvement in peak level is gained from the relay eliminating voltage drop, and there’s little difference between repro and original. The real test of course is how they perform on the road, but it’s not easy to make direct comparisons in close succession with different headlamps. I do drive a particular route fairly regularly at night on unlit country roads, and I’ve found the more tightly focussed main beams of the original Lucas units to be preferable to the fairly diffused repro halogens, even if the total illumination is less. On dipped there’s not so much difference, but when passing an oncoming car it seems easier to make out the edge of the road, which is obviously important.

I suspect many people will fit repros (and quite likely extra-bright halogen bulbs) and judge them a success by the increased and much whiter glow from the headlamps when viewed from outside. The repro lens scattering and lack of focus can also give an impression of much better lighting immediately in front of the car when driving, which perhaps feels reassuring in some situations.

I suppose it depends on the conditions you drive in, but on a dark road I’d rather have lighting for as much distance ahead as possible, so I’m sticking with genuine Lucas, and with the relay modification these are performing much better than when I first acquired the car. I have just come across a possible source of halogen BPF replacements that claim to preserve the original filament geometry and I’ve ordered a pair. I’ll update this article (or on the technical support website) when I’ve had a chance to evaluate them. There is another bulb type very similar to H4 but with a slightly different base to fit the BPF holder, however it has the same issues with filament position, and generally won’t fit a tripod anyway because it’s long enough to hit the shield.

Why the tripod ?

One question which doesn’t seem to be answered conclusively in my research is why Lucas came up with the tripod and associated bulb shield in the first place. It was generally offered only on upmarket and sporting models, isn’t found in other headlamp designs, and almost certainly reduces the amount of light produced.

Best explanation is probably that the shield sharpens the vertical cutoff of the beam and allows the headlamp to be aimed higher (and hence further) without dazzling - but let me know if you have a definitive answer ! To further complicate things, early tripods as on the Magnette have the shield set back from the lens and deliberately blackened inside, while later versions and the PL700 used an elongated shield without blackening.

In conclusion

It does have to be said that if you really want lighting anywhere near modern standards then it may be best to ditch the Lucas tripod design and fit a different 7” headlamp unit. From what I can gather on forums, Cibie and Hella are both well regarded, and likely to fit the standard headlamp bowl. But tripods are such an integral part of the Magnette’s appearance, in common with many other 50s sports cars, that for the moment I’m prepared to drive within the safety limitations of the light output in order to keep the style. As for the reproduction tripods, we should probably have realistic expectations since they’re generally manufactured in the Far East at low cost, mainly perhaps for classic car enthusiasts looking for appearance rather than performance, but if you still have original Lucas headlamps in reasonable condition I would strongly suggest keeping them, with the addition of relays to extract the maximum amount of lighting.

 

 

Comments  

#1 David Harrison 2016-03-16 17:29
This is very interesting. Is there a wiring diagram showing how / where to wire up the relays? To me it seems to be a problem to install the relays for main & dip before the loom splits to the L & R headlights. I understand these relays should be fitted between the dip switch and where the wires emerge from the loom onto the inner wings. Your photos would be a help!
David
#2 Mike Laflin 2016-05-31 15:45
I have fitted a pair of LED dipping bulbs to my original P700 tripods. Much brighter and excellent beam pattern even on dip. Direct replacement for original BPF bulbs. Available from Dynamoregulator conversions of Lincoln. Great improvement.
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