Trek CarBack Radar Tail Light

As well as making you visible to other road users approaching from behind, the Trek CarBack Radar Rear Bike Light alerts you to their presence. While the visibility element works well, I found that the radar on other units I've tried works better, especially on winding roads.

If you're just after a rear light, check out our guide to the best.

I'm not going to spend too long talking about what a radar rear light does and whether you need one as I covered that in detail when I reviewed the Garmin Varia RTL515. I use one all the time; I think they are especially useful when I'm out on my own.

The CarBack's unique selling point is that it detects objects travelling at a greater speed than you from behind, from a greater distance, than the competition. Notice I said objects here; it doesn't have to be vehicles. It'll alert you to a cyclist, for example, speeding up to get (back) on your wheel in the same way.

The claimed distance is 240m, a full 100m longer than the Varia. Brilliant, if this claim were to live up to reality.

Distance is not the only important factor, though; there's also the radar beam width. The CarBack's beam width is narrower than the Varia on paper as well as in use. Trek's owner manual has a diagram that shows the beam width is 60m for the nearest 140m and narrows down to 30m at 240m. Garmin's owner manual claims a 40-degree beam width, actually getting wider as the distance increases. If my trig hasn't deserted me, that's a width of over 100m at a distance of 140m.

If you look at road design then, the CarBack detection would work well on straight roads, whereas the Garmin would work better on windy roads, with the wider beam angle.

Alert reliability

And that explains my experience with the CarBack. Most of the time, it'll alert you to an approaching object in good time. However, a significant number of times it'll alert you as a car is actually overtaking you, at the point where you already know it's there and it's too late to take any evasive action if necessary.

This is especially noticeable when riding with other people, and on smaller roads. Our Dave, who reviewed the Magene L508, also noticed that on average the CarBack's alert actually seems to come in later, that is when the approaching object is closer, than he experienced with the Magene.

Both of us mentioned in our reviews of the Varia and Magene respectively that the computer would show a car approaching before we would have heard it, initially thinking it was a false alarm, then looking over our shoulder and seeing that there was in fact something there. After a while we'd trust the radar more and more.

That is not the case with the CarBack. Don't get me wrong, it's still way better than not having radar, but the unit is not reliable in the same way.

Pairing options

The radar doesn't work on its own; it needs pairing with either a bike computer or phone, which it did without a hitch in both cases.

To pair it to your phone, you'll need to download the Trek Accessory App, which gives you more information, though I didn't use it other than checking it works with the light as I prefer to use a computer.

On my Hammerhead Karoo 2, alerting works in exactly the same way as the Magene (which I've also used a lot) and the Varia (shown below). Approaching vehicles appear in a strip down the side of the screen, and you get an indication of the speed differential through the colour of the strip. It bleeps at you every time it detects an object approaching as well, if that's how you've set up the unit.

Rear light

The rear light itself works well. You get four modes: day steady (25 lumens), day flash (90-lumen bursts), night steady (5 lumens) and night flash (5-lumen bursts).

On the Karoo, I can switch on 'autocontrol', which means the light will automatically switch on in the default mode when I start a ride, and switch off when the ride has ended. Default mode options are solid max, solid min, random flash and custom 1. I had mine set to random flash and was perfectly happy with the visibility this gave me to other road users, in daylight or in the dark.

This autocontrol option can be slightly less useful at a cafe stop if you want to switch the light off to conserve battery, or to stop it shining in people's eyes, as the computer will keep switching it back on again. If this bothers you, you can always switch the setting off.

Battery life and charging

Battery life for the CarBack is pretty close to the claimed seven hours in good conditions. I expect it will probably be a bit less when it's chilly out. Compared to the Varia's 16 hours and the Magene's 12 hours for a similar weight and brightness, that is significantly shorter.

There's a four-bar battery status indicator on the side of the light, which is useful. Your paired computer will also give you a low battery warning with about an hour of radar use to go. When the battery drops to less than 5%, the radar function switches off but the light will continue to shine, which is exactly how you'd want it to work.

Charging is via a USB-C port on the bottom and takes around four hours from empty. If you're charging your GPS at the same time as the CarBack, it might keep your light switched on, it says in the manual, and you have to switch it off manually. It's almost like that auto switch-on feature could get in the way of keeping the light charged.

The battery charge port is protected by a rubber flap, which is not as substantial as I'd like. On the first unit we were sent it started separating from the light – Trek told us that this would be covered by warranty; the second unit we were sent didn't have this problem. However, the way the flap is designed means it doesn't have sufficient retention power on the port, especially when it gets a bit dirty; it doesn't seem to stay on to cover the port fully, and after wet rides there was evidence of dirt on the inside of the flap/port protector. The Magene's is more substantial and works much better.

Mounting options

The CarBack's clamp is mounted with a rubber ladder type strap and a hook. The clamp itself is a proprietary affair that works on a round seatpost, but not so well on aero-shaped seatposts.

The catch on the clamp is a bit fiddly to undo; if you're wearing gloves you'll probably have to take them off to get the light off the clamp.

Both the Magene and the Varia use the Garmin type system which is much easier to get on and off, and works on a range of seatposts. I'd have preferred this system as there's a range of aftermarket mount options available for mounting on saddles, mudguards and racks. The CarBack will work with a Trek Blendr saddle mount, with compatible saddles.

Value

The CarBack costs £169.99, which is the same as Garmin's Varia RTL515, while other options are cheaper: Magene's L508 costs £99.99 on its Amazon store, and Bryton's Gardia R300L is £119.99.

I have used both the Magene and the Varia extensively and found them better in almost every way. Reading George's review of the Bryton, it looks like that's a better option too.

Conclusion

The thing I like best about Trek's CarBack is its name; it's what you shout when you're out with your mates and there's a... car back.

It doesn't reliably deliver on the promise of alerting you to cars that are 240m away in real-life conditions, and in the pursuit of that goal it seems that the narrower radar beam width actually gets in the way of alerting you to the presence of all cars in all circumstances.

The mount is OK, but not as versatile or as easy-to-use as what competing units use.

Given that it costs the same as the Garmin, which is the best one I've used, and that there are cheaper alternatives available – all with a better battery life – I struggle to recommend buying this one.

Right to reply

We put Iwein's findings to Trek, to give the company the right to reply:

Re: "Battery life for the CarBack is pretty close to the claimed seven hours in good conditions. I expect it will probably be a bit less when it's chilly out. Compared to the Varia's 16 hours and the Magene's 12 hours for a similar weight and brightness, that is significantly shorter."

Trek says: "We stated minimum battery life on the package, and generally the rider will experience far longer run times – in line with Magene's claimed battery life in most instances. Radar affects battery life more than the light itself - the CarBack light will run for over 20 hours in high steady mode in absence of cars to trigger the radar. CarBack differs from the competition in that it uses a more modern 77Ghz radar, through tuned antennas consisting of 4 transmitting channels and 3 receiving channels – the 77GHz radar is the current standard used for automobile radars."

Re: "The way the flap is designed means it doesn't have sufficient retention power on the port, especially when it gets a bit dirty; it doesn't seem to stay on to cover the port fully, and after wet rides there was evidence of dirt on the inside of the flap/port protector. The Magene's is more substantial and works much better."

Trek: "CarBack is rated as IP67. We have not gotten feedback on any closing issues on the charging port tab or experienced any issues with it firsthand, through test rider, retailer, or customer feedback. We just have not seen any ingress related issues that would affect performance – warranty claims are non-existent. We balanced ease of use for charging and closing the port with egress performance for an all-around best experience.

There are many elements of some of the competitor lights that are done well, and kudos to Magene on their port. That said, looking at each small component in isolation without weighting the items that really matter runs the risk of guiding readers to make buying decisions based on the wrong priority."

In response, Iwein says: "I agree that the port protector should not be a top priority for a buying decision, but if I were to pay the RRP, I'd be disapppointed with the port protector."

Re: "The clamp itself is a proprietary affair that works on a round seatpost, but not so well on aero-shaped seatposts. The catch on the clamp is a bit fiddly to undo; if you're wearing gloves you'll probably have to take them off to get the light off the clamp. Both the Magene and the Varia use the Garmin type system which is much easier to get on and off, and works on a range of seatposts. I'd have preferred this system as there's a range of aftermarket mount options available for mounting on saddles, mudguards and racks."

Trek: "We choose to keep the light mount in line with our Flare light family to ensure they work together and with our Blendr system. There are Blendr options for an integrated saddle mount as well as hard mounting a mount to an existing fender mount from your frame if you desire a hard mount. Garmin's mount is good, but does add extra complexity, especially if switching the radar light across multiple bikes."

Re: "Given that it costs the same as the Garmin, which is the best one I've used, and that there are cheaper alternatives available – all with a better battery life – I struggle to recommend buying this one."

Trek: "When we were developing CarBack, early on we established Garmin as the clear best in class – agree with your assessment there. It was the best at consistent detection of approaching cars and minimizing false positives and false negatives. It had to work every time – which was also our priority in developing CarBack, and we were proud to match the performance of Best in Class through all of our testing and rider feedback. In our experience the same cannot be said for the performance of the other competitors."

Verdict

Radar rear light that works, but not quite as well as the alternatives out there