LED Cluster Bulbs in STT lights

I'm often asked what bulb should be used for an an existing STT (Stop Tail and Turn) light which has an 1157 bulb in it. My answer is always, "there aren't any". In a rare situation a cluster bulb will work and give good light out-put, but that is the acceptation and not the rule. In those rare situations the end user put foil or foil tape to increase the light by reflecting off of the foil and the end result was an  acceptable amount of light.

Primarily, the reason why someone wants to use LEDs on the outside of any vehicle is safety. The reason why they are safer is two fold. If the light is designed correctly the result is more light and thus you are more visible on the road. That is not to say that your lights should be so bright that you are a hazard, by blinding your fellow travelers, but just enough for you have good visibility when you are turning or stopping. The other reason why LEDs are a good choice is it takes many of them to make up the amount of light that is needed. If you have a failure you still have lots of light. In an incandescent bulb you have one filament and if you have a failure you have NO light.

One of the main reasons why cluster bulbs don't work well is they product about 1/3 as much light if the cluster bulb is about the same size as the incandescent bulb your replacing. The other reason is most of the STT fixtures are designed to refract and reflect  light from one single location.

Below are two pictures which have a STT light and some markings that illustrate what happens when you use a cluster bulb in some STT lights.

 The little blue marks on the red lens are to show how the lens is designed to refract light.

On the next picture ( below) is what a cluster bulb looks like ( as crude as it is :-) in the same light fixture.

Many points of light being refracted and reflected  has a result of the light going in many different directions. In the event that your cluster bulb was to produce more light (they seldom do) than the incandescent bulb, it would not be as effective ( and safe) because the light does not all show up at the intended target, the guy behind you.

One of the first projects LED4RV started was finding a fix for the 4.25 Monarch tail light. ( pictured above). We had so many people contact us who wanted a brighter light. They did not care if it was LED or not. They just wanted greater visibility on the road at night. One guy called  us and he said he was going to find a fix or sell his Airstream.  At the time we had a large red pad LED-kit1 that glued onto the lens. It worked very well and did make folks safer while on their travels.

As a rule of thumb, LED pads (which have all the LEDs facing outward) give more light and are a much better choice. If you notice, all the new cars have a pad style set-up that point in the same direction and thus they give more light.


At the present time we have discontented  that first light kit and have another sealed light insert that is much easier to install and gives great light. LED-kit2


The purpose of this blog post is not to promote the kit as much as it is to steer folks in the right direction, of what to look for, when trying to retrofit their older units with LED light.