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Thread: Any benefit to running low beam lamps on instead of DRLs?

  1. #1

    Default Any benefit to running low beam lamps on instead of DRLs?

    Just curious, is there any benefit to running the actual low beam lights instead of the DRLs? I noticed that in some conditions shadows can obsecure the back of a car during the day. Since low beams activate the taillights as well, is there any benefit to running the low beams during the day instead of DRLs?

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    Moderator Alaric Darconville's Avatar
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    Default Re: Any benefit to running low beam lamps on instead of DRLs?

    Quote Originally Posted by Ls400 View Post
    Just curious, is there any benefit to running the actual low beam lights instead of the DRLs? I noticed that in some conditions shadows can obsecure the back of a car during the day. Since low beams activate the taillights as well, is there any benefit to running the low beams during the day instead of DRLs?
    You use a lot more power, lighting the low beams and all the associated lamps (front/rear side markers, front position, rear position, & registration plate). You'll also increase the wear on all those bulbs. Since headlamp DRLs run at reduced power, you have lower power consumption and less wear on the bulbs.

    In some lighting conditions, the rear position lamps make discerning a stop signal slightly harder because it's easier to discern a lamp going from off to full on than from dim to full brightness. However, in others (like the sun being forward of you (like when driving west at sunset on September 22) the rear position lamps can make the car easier to see for those behind you.

  3. #3

    Default Re: Any benefit to running low beam lamps on instead of DRLs?

    The DRL strips have a beam pattern that is closer to high beam (at lower intensity) than low beams. Low beams will be less visible from a distance. That is why automakers that do not have dedicated DRL use the high beams at half brightness.

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    Moderator Alaric Darconville's Avatar
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    Default Re: Any benefit to running low beam lamps on instead of DRLs?

    Quote Originally Posted by czechopino View Post
    The DRL strips have a beam pattern that is closer to high beam (at lower intensity) than low beams. Low beams will be less visible from a distance.
    Given the technical requirement for a DRL, a low-beam based DRL will meet the requirement. Many turn signals will also meet the requirement.

    That is why automakers that do not have dedicated DRL use the high beams at half brightness.
    About half the *voltage*, or whatever will bring it down to the legal limits for DRLs. 50% is good start but it depends on the photometry of the lamp itself. A bulb rated at 1000lm at 12.8V, run at even 7.2V (about 1/2 charging system voltage) will be at 141lm. That's 14% the original output.

  5. #5

    Default Re: Any benefit to running low beam lamps on instead of DRLs?

    Quote Originally Posted by Alaric Darconville View Post
    Given the technical requirement for a DRL, a low-beam based DRL will meet the requirement. Many turn signals will also meet the requirement..
    Without knowing exact details of what requirements are for DRL perhaps low Beam headlights would meet the requirements. But automakers do not choose that route. They do use high beams and turn signals, as retrofit kits also do, but they do not use low beams or foglamps because the beam cutoffs are too low.

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    Moderator Alaric Darconville's Avatar
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    Default Re: Any benefit to running low beam lamps on instead of DRLs?

    Quote Originally Posted by czechopino View Post
    Without knowing exact details of what requirements are for DRL perhaps low Beam headlights would meet the requirements.
    And because you don't know the exact details of the requirements, you're not qualified to say why a low-beam based DRL would not perform at the distances they are intended to perform.

    But automakers do not choose that route.
    They've chosen it before. It's up to them how they implement them, so long as they meet the standard. A more likely explanation as to why they use high beams instead of low beams is that even at the reduced voltage, low beam DRL usage does eat into the bulb life and may result in more annoyed customers (even attempting to make warranty claims) for burned out low beams. Since high beams don't get used nearly as much as low beams (or even nearly as much as they ought to be used), it makes more sense to use those bulbs-- and the voltage to them is also much much lower, eating less into their lifespan.

    they do not use low beams or foglamps because the beam cutoffs are too low.
    They do use low beams from time to time. My '01 Corolla had them. They don't use fog lamps because fog lamps are not Federally-regulated, and therefore their performance can vary greatly from model-to-model. They can have very low optical precision when the older standards are used (including having a lot of light in the glare zone), high surface luminance (often fog lamps will use the same bulb as a low beam headlamp, but the lamps themselves are much smaller). Fog lamps are also mounted very low compared to headlamps, reducing the distance at which they can be seen.

    Canada allowed fog lamps as DRLs but (I'd have to check to be sure) it's either going away or already has.
    Last edited by Alaric Darconville; 09-24-2018 at 11:54 AM.

  7. #7

    Default Re: Any benefit to running low beam lamps on instead of DRLs?

    Quote Originally Posted by czechopino View Post
    Without knowing exact details of what requirements are for DRL
    That does seem to be the problem at the moment. :-)

    perhaps low Beam headlights would meet the requirements. But automakers do not choose that route.
    Yes, many of them do. In North America, full-intensity low beams are permitted as DRLs if (and only if) all the rest of the lights usually lit with the headlamps also come on (parking lights, tail lights, side marker lights, license plate lights, dashboard lights), or full-intensity low beams by themselves if the car has an an ambient light sensor to turn on the parking/tail/etc when it begins to get dark out. Low beams by themselves at 75% to 92% of regular intensity are permitted as DRLs, without regard to the parking/tail/etc.

    they do not use low beams or foglamps because the beam cutoffs are too low.
    That's not correct. Fog lamps have never been allowed as DRLs in the US, but in Canada they've been allowed all along, and will remain legal until next year. And low beams, as detailed above, have always been (and still remain) legal as DRLs.

    With that said, many low beams (at any voltage) are not very good DRLs. The answer to the original poster's question is no low beams are not better daytime lights than the car's built-in DRLs.
    Last edited by -Virgil-; 09-25-2018 at 02:11 PM.

  8. #8

    Default Re: Any benefit to running low beam lamps on instead of DRLs?

    Thank you for the informative responses. My main point is that there is a reason automakers use high beams and turn signals for DRL when there is not a dedicated DRL strip. They're may be a couple exceptions but almost all cars in the USA do not use low beams as DRL. Regardless of the laws, low beams are not better suited as DRL because they are designed to not shine into oncoming drivers eyes. High beams and turn signals are designed to be seen, not to see with.

  9. #9

    Default Re: Any benefit to running low beam lamps on instead of DRLs?

    Quote Originally Posted by czechopino View Post
    Thank you for the informative responses. My main point is that there is a reason automakers use high beams and turn signals for DRL when there is not a dedicated DRL strip.
    Not really -- not the way you seem to have in mind.

    They're may be a couple exceptions but almost all cars in the USA do not use low beams as DRL.
    That is not correct. There are (and have been) many, many vehicles with low beam DRLs in the USA. Including some very high-volume cars, trucks, SUVs, vans, and buses.

    Regardless of the laws, low beams are not better suited as DRL because they are designed to not shine into oncoming drivers eyes.
    That's generally true, but they're equally as legal, and "what makes the more effective DRL" is far down the list of why automakers choose what kind of DRL to put on any given vehicle.
    Last edited by Alaric Darconville; 09-25-2018 at 02:56 PM. Reason: fixed quote tag

  10. #10

    Default Re: Any benefit to running low beam lamps on instead of DRLs?

    Quote Originally Posted by -Virgil- View Post

    That is not correct. There are (and have been) many, many vehicles with low beam DRLs in the USA. Including some very high-volume cars, trucks, SUVs, vans, and buses.

    Interesting. Which current year model car has low Beam DRL? I haven't seen any.

  11. #11

    Default Re: Any benefit to running low beam lamps on instead of DRLs?

    Quote Originally Posted by czechopino View Post
    automakers that do not have dedicated DRL use the high beams at half brightness.
    No, actually high beam DRLs are run at 50% voltage to put out about 10% of the normal high beam intensity (legal max = 7000 candela for high beam DRL...75,000 candela for high beam).

    Which current year model car has low Beam DRL?
    You're moving the goalposts, and I have paying work to do. Keep your eyes open; you'll see 'em.
    Last edited by -Virgil-; 09-25-2018 at 04:34 PM.

  12. #12

    Default Re: Any benefit to running low beam lamps on instead of DRLs?

    Quote Originally Posted by -Virgil- View Post
    No, actually high beam DRLs are run at 50% voltage to put out about 10% of the normal high beam intensity (legal max = 7000 candela for high beam DRL...75,000 candela for high beam).



    You're moving the goalposts, and I have paying work to do. Keep your eyes open; you'll see 'em.
    Interesting that going from 50-100% voltage increases intensity by 90%

    Leta go back 10 years then. I'd love to know what cars use low beams for DRL. My 1996 Passat had headlights and parking lights that never turned off. It was the dumbest thing.

  13. #13

    Default Re: Any benefit to running low beam lamps on instead of DRLs?

    A few examples: Toyota Corolla 2014 and some number of subsequent years (which right there is kind of enough of an answer, because they sell zillions of them), Chevrolet-GMC C/K pickup trucks and Suburban-Yukon SUVs '95ish to '00 and then '09ish to a couple of years ago. Chevrolet-GMC full-size vans changed from turn signal DRL to low beam DRL in...might have been '08 or so. Chevrolet Astro - GMC Safari from '95 or so. Recent/present model Ford F-series pickups (another very high volume vehicle) except in the very high-spec fancy trims. Almost every '95+ Volkswagen and Volvo until they changed to dedicated DRLs a few years ago. Every Saab '95ish til the bitter, sad end except that silly rebadged Saturn SUV and maybe one other model, possibly the rebadged Subaru (9-2x). All or almost all Subaru models starting in '00 until '05ish.

    The European-style white LED DRL is becoming much more prevalent in the USA, but there are still plenty of headlight DRLs (or too many, depending on your perspective) and a fair proportion of them are low beam.

    Why does 50% voltage get 10% intensity? Filament bulb operating characteristics react exponentially to voltage change, not linearly. The exponents to approximate the changes are:

    1.6 for power (wattage)
    3.4 for output (lumens or candlepower)
    -13 for life (hours -- yes, that is a "minus" before the 13!)

    In the US rules, headlight bulbs are rated for output and wattage at 12.8v, and life at 14.0v So let's make the math easy and say we're starting with a 9006 bulb that produces 1000 lumens at 12.8v and has a 1000-hour lifespan at 14v. We want to figure out what it will do when it is fed 6.4v.

    The math works like this:

    [(operating voltage) ÷ (rated voltage)]^exp

    So let's try it, first putting in the 3.4 exponent for lumen output:

    [(6.4) ÷ (12.8)]^3.4

    [0.5]^3.4

    gives us 0.095. That's what we have to multiply the rated output by, to get the output at the new voltage. As you can see, we will be multiplying by a little under...ten percent!

    We can also check what this 6.4v feed will do to the wattage:

    [(0.5)]^1.6

    gives us a multiplier of 0.33, so our wattage goes from a nominal 55 to about 18.

    And for life:

    [(6.4) ÷ (14)]^-13

    [(0.457)^-13

    gives 26262: yes, that really means the life of the filament will theoretically be 26,262 times longer than if it is run at 14.0v. That doesn't actually happen because of other factors that affect bulb life, but you can see the very steep effect on life of changing voltage.

  14. #14
    Moderator Alaric Darconville's Avatar
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    Default Re: Any benefit to running low beam lamps on instead of DRLs?

    Quote Originally Posted by czechopino View Post
    Interesting that going from 50-100% voltage increases intensity by 90%
    Going from 50% to 100% voltage increases intensity by about 1052%. A bulb rated at 1000lm, run at 50% voltage, will put out 95 lumens. So, we start at 95lm at 50% voltage and get back to the 1000lm at full voltage. That's just about 1052%. If going from 50% to 100% voltage increased intensity by 90%, then that'd just be 180.5lm. Obviously there are no 180lm headlamp bulbs (even the 6V, 32cp bulbs of the '30s or so were higher than that).

    Leta go back 10 years then. I'd love to know what cars use low beams for DRL.
    Is there a point to this? My '01 Corolla had low-beam based DRLs (disabled now, perhaps one day I'll move the function to the turn signals with the DRL-1). The 2014 Corolla uses them. Some F-150s in recent years. Sales volume of the Corolla and F-150 account for quite a few low-beam DRLs on the road. Just because you aren't familiar with the wide variety of makes and models with low beam DRLs doesn't mean they don't exist.

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