Dashboard lamps and headlight aim - for MOT

My mechanic, when fitting my new clocks to the bike on Friday kindly forgot to check if the instrument lights were working or not.

Due to not being able to ride the bike til yesterday, I didn't spot this until last night. Great.

Anyway- as my MOT is tomorrow, is it something (if spotted) that will fail the test? Or is it a borderline?

Handy, as the only place I can try now is Halfords.

Also, the beam height of the headlights is looking a little low- I've just realised that I increased the preload on the shocks by 1 turn earlier this year- is this likely to have made enough difference to fail the MOT?

Last year the MOT tester said they were at the top of the legal limit, but still within it.

I don't have the space to do the test properly as shown by the Haynes manual.

I'll probably give the adjuster screws a little turn anyway, just t raise it a little.

Right. All I could get from Halfords was 12v 1.2w lamps. The proper spec is 14v 0.84w lamps.

Before I take the front end apart, am I likely to blow the bulbs straight away, or just reduce their life?

[ophic]

I wouldn't think they'd blow straight away, although I'd be more worried that they'll get too hot and melt something.

Well- they didn't blow. I'm not sure there's too much to melt there- the plastic surround to the lamps is fairly thick, so shouldn't affect the circuitboard.

I'll keep them as a temporary fix, then put the correct lamps in after xmas. And we're only talking 2v and 0.36w here...

[ophic]

Originally Posted by thedonal

And we're only talking 2v and 0.36w here...

well... 0.36w is almost a 50% greater output. And thats rated at 12V. Add on the extra 17% voltage and you're looking at potentially 60% hotter.

But in fact I agree with you, they should be fine - especially in this weather.

[yorkie_chris]

The bike supplies the voltage not the bulb.

Power is stated at 14v, so running it at 13.2 or whatever will give less power. The 12v bulbs in there previously will have been putting out more than 0.84w unless you don't have a charging circuit...

Basically f##k it it's close enough. So long as the fitting is correct.



Headlight aim is easy to adjust there and then, if your MOT man fails it on that then he's a c0ck.

Cheers Chris. I've had a play with the headlights- seems to be a little better now and the dashboard well lit now- very well lit!

[ophic]

Originally Posted by yorkie_chris

The bike supplies the voltage not the bulb.

Power is stated at 14v, so running it at 13.2 or whatever will give less power. The 12v bulbs in there previously will have been putting out more than 0.84w unless you don't have a charging circuit...

Basically f##k it it's close enough. So long as the fitting is correct.

bulb A rated at 0.84W at 14V:
application of P=IV gives us a current figure of 60mA
V=IR gives us an impedance of 233ohms
so at 13.2V we've got a current of 54mA and therefore a power consumption of 0.75W

bulb B rated at 1.2W at 12V:
current is 100mA, impedance 120ohms
at 13.2V: current now 110mA and power 1.45W

which happens to be 193% of the original power dissipation. 93% hotter. Should be very noticeably brighter as well.

Yes i've assumed the impedance of an incandescent filament stays the same at different input voltages - i know it doesn't. But thats just going too far

[yorkie_chris]

I bow down to you having more time on your hands

Is it on fire yet?

Nope.

Did a run to boxhill and back- 25 mins each way. No fire. No smoke. Nada. I do have to say, though- the speedo clock was very clear. Much better than the dim illumination given from the old clocks (standard lamps).

The ride down to my folks on Weds will be the test- about 1.5-2hours, depending on traffic.