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LATEST UPDATE OCTOBER 23rd 2025
HOW DO WE REACH THE OVERALL RATINGS FOR PRODUCTS WE REVIEW?
Gaciron Cetus 1700 Underneath Mounted Bike Light
269g £59.99
The Gaciron Cetus 1700 Underneath Mounted Bike Light is a six-mode design capable of producing a very useable, pure arc of light with sufficient clout for the darkest backroads, yet practical enough for town and around, thanks to the sensible spread of modes and run times. I’m particularly fond of the “breathe” and daytime flash, which are frugal and incredibly extrovert - perfect for overcast days and town work, where it’s about being conspicuous. Build quality is generally very good and the package represents excellent value for money. However, while perfectly adequate, the mount felt less robust than some (resulting in the light’s ejection on one occasion) and again, though very welcome, the wireless remote also felt lower rent and built to a price.
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Pros: Impressive output, sensible spread of modes, reasonable charge and run times, IPX6 waterproofing, wireless remote, high specification for the money.
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Cons: Mount and remote merit refinement, USB C charge cable not supplied.
Specification
Comparisons with the Magicshine Evo 1700 Underneath Mounted Bike are perhaps inevitable and they are, in my view, obvious rivals, but contrary to claims, no “badge engineering” here. Starting with the aluminium alloy/nylon housing, its shape is apparently influenced by whales. Apparently, engineers have also designed it with aerodynamics in mind and if their engineers are to be believed, it boasts a drag co-efficient of 0.35CD, supposedly half that of a traditional bike light and the equivalent of dropping 500g of weight. Ours was black, but there’s the option of black/white, or black/pink for the last word in colour coordination.
Measuring 47.6*52.5*71mm, its slightly bigger than the Magicshine, yet still very compact and it also meets IPX6 for weatherproofing. Behind the curved polycarbonate lens, which employs double layer light calibration technology, theoretically ensuring an even spread of light, in both high and low beam.
I was surprised to discover the diode was not a CREE unit, rather domestic units. I was also slightly surprised to discover its fuelled by a 400mAh lithium-polymer cell, capable of withstanding up to 500 charge cycles before losing 20% of its capacity.
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This fuels via a USB C cable (not supplied), the port located underneath, concealed by a well-fitting cover. Thermal cut-outs are standard these days, but this was seemingly very reliable and responsive, so diodes and switchgear should live long and happy lives. There’s also a very accurate auto stop/start sensor. If the light senses no movement for two minutes, it will put itself to sleep, awakening when it senses movement. Super convenient at rest stops, or if you’ve forgotten to pop it off when you’ve reached your destination.
Mount 2.75/5
This again shares similarities with the Masgicshine Evo 1700 Underneath Mounted Bike Light. Both lights mount beneath, courtesy of a Go-Pro style bracket while there’s a Garmin platform above for computers and other tech. Both are composite and up to the job, although both seem slightly stressed when you’ve an action camera, or similar riding atop. There’s only one shim, which proved fully compatible with bars between 31.8 and 26.4. However, narrower diameter, or quirky shapes may call for some rummaging through the shim box. The 3mm Allen screw benefits from a lick of grease or drop of wet lube and torqued to 2.5nm. The light slides into the channels while you depress the lip. While secure, there’s no audible “click” to let you know its home, so double check before scooting off. I got caught out on our third outing, resulting in the light’s ejection over some washboard tarmac 300 yards from home. I should also point out this was an isolated incident and there’s been no issues sans tarmac, either.
Modes & Switch 3.5/5
The switch cum battery charge indicator is mounted on the left-hand side and employs the traditional traffic light system. Blue denotes 100-61%, Yellow 60-21% and red flashing, 20-1%. The charge light indicator is quite small, and positioning means it’s tricky to see from the saddle but easy enough if you’re powering up/down at the start, close or ride rest stop. I’ll discuss the wireless remote in a minute-in my view, it’s what makes the unit so user-friendly.
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The main switch is very positive, so combined with the locking function (which stops unwanted power ups when it’s bouncing around in a rucksack, or pannier). It requires five distinct presses to release and will need doing, straight from the box. To engage, press the switch for five seconds, whereupon the light will flash rapidly three times. The switch communicates presses with an audible beep, which I’ve also found welcome.
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The Cetus defaults to the steady modes, which consist of Ultra High Steady 1700lumens, High Steady 900 lumens, Medium Steady (low beam) 600 lumens, Low steady (low beam) 300 lumens. A sustained double press unlocks the two flashing modes “breathe”, which is a 50-100 lumen3 and a 600 lumens daytime flash.
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It’s possible to switch between modes mid ride using the main switch, but it’s tricky, especially if the environment suddenly changes, say leaving town and entering a very dark stretch of road, or even trail. Enter the wireless remote. This attaches via a long Velcro strip with silicone backing.
The big round switch also incorporates a battery life indicator, which employs the same traffic light system. Fuelled by a CR2032 cell, there’s no weather sealing and the cover is a little fiddly to reinstate.
I’ve had no issues with water ingress, or corroded contacts, but a quick, precautionary lick of Vaseline, or shot of (PTFE free) maintenance spray is a wise investment. Unlike higher end models, it won’t switch the light on, but the soft, rubberised pad is very reliable and unexpectedly positive-even wearing full finger gloves. Official run times are cited as 2hours (1700 lumens), 3.5 hours (900 lumens) 4 hours (600 lumens), 6 hours (300 lumens), breathe and daytime flash 20 and 24hours respectively.
Output 4/5
Quality of output is impressive across the board, thanks to the optics. In common with the Magicshine EVO 1700 (and their Hori 1300) it’s a mix of flood and spot with a very broad, car-type arc, which, true to claims gives an accurate sense of perspective, most notably when tackling windy rural backroads at speed. Though not marketed as a trail light, I’ve also found the 1700 lumen high beam makes useful transition to gravel roads, green lanes and bridlepaths.
As you’d hope, the full 1700 means speed-wise, I was only limited by legs and gearing. I’ve been able to tackle descents at 30mph plus with ample warning of holes and other hassles to around 25 metres or so. You’ll still want to nudge down (as you would in a car) to the 900lumens to avoid dazzling oncoming traffic, but this is a very simple intuitive prod of the wireless remote.
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Off road, I’ve been able to maintain 20-23mph with similar sweep, keeping me aware of malevolent muntjac deer, bewildered badgers and similar woodland dwellers. I’ve also found the 1700 useful on some very foggy dawn outings, but with the prod-down narrative.
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If I didn’t know better, I’d say the 900 lumens mode was closer to 1000, but that’s the old thing about optics, not just firepower. I was able to navigate my local lanes (that typically require 1300) at a respectable 18mph with good view of conditions and seemingly without cause to nudge down when meeting oncoming traffic. This said, the beam isn’t potent enough for ventures beyond tarmac, since it “only” projects a carpet of light at the ground.
Practical enough for those occasions where you need to conserve reserves. No issues with being seen with either the medium or low steady along the backroads. Otherwise, I’d save the 600 for dusk and semi-rural sections, the 300 lumens for town work, assuming you didn’t fancy the flashing options.
I’ve defaulted to the 600 as a booster to Denise’s Exposure Revo dynamo, which pumps out 800l umens, thus giving me 1400 lumens for extended night rides.
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Daytime Flash also feels brighter than 600 lumens suggested and I’ve found it really effective during overcast days and dusk. It’s still very good in strong autumnal sunlight and still sits the right side of assertive in town centres and rush hour traffic. Along the open road, oncoming vehicles seemed to take note at 100 metres, this dropped to 60 or so through built-up areas and when tackling bigger roundabouts but really helpful when entering the flow of traffic. Breathe is 50-100 lumens, although again, feels brighter thanks to the tempo. Fine for well-lit town centres, or if reserves are tanking, although generally speaking, I’d be more inclined toward cycle and shared use paths.
Charge/Run Times 3.25/5
Charge times have, true to claims, been 2 hours from the mains and using a USB C tablet charger.
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Tested in temperatures between 6 and 15 degrees, I have returned within seven minutes of those cited and the charge indicator seems similarly reliable, so the likelihood of being plunged into darkness is more likely due to rider oversight (is there an auto kick down?) The sensored stop/start function doesn’t seem to have any pronounced effect on run times and on balance, is a very welcome feature, even for the uber disciplined.
Care/Durability 3.5/5
Difficult to speculate long-term. That said, The Cetus 1700 has stood up well to the usual everyday carelessness, persistent heavy rain, regular sudsy bucket washes, the odd direct hit front stones thrown up by the front tyre and being ejected onto tarmac at 16mph.
The latter resulted in some minor blemishes, but nothing you’d write home about. Otherwise, the usual rules apply - don’t let the battery deplete too far, too regularly, ensure the port cover’s snugged down, ditto the mounting hardware.
Value 4/5
Taking everything into account £59.34 (reduced to £47.99 at time of writing) for the specification and overall performance is pretty impressive. Its £20 cheaper than the Magicshine Evo 1700 Underneath Mounted Bike Light which impressed me, although with similar reservations regarding the remote switch and omission of auto kickdown (bailout mode) when the battery’s seriously depleted. On balance, the Gaciron Cetus’ beam pattern is a notch better, too. If you didn’t need 1700lumens, Magicshine Hori 1300 is another capable model with sensible modes, wireless rechargeable remote, can be synced with some of the brands’ rear lights and meets IPX6 for weatherproofing. I would prefer to see a more robust bracket and though undeniably very good value, at £69.99 its also notably dearer than the Cetus 1700
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You can also pay a good deal more. If you weren’t set on an under bar design, Oxford Ultratorch CL1600 (£69.99) offers a maximum of 1600 lumens, CNC machined body, the ability to extend run times via power bank is another welcome feature. However, there’s no remote, it cannot be paired with other lights, only meets IPX4 for weatherproofing and the mount lacks the sleek lines and versatility. I remain very fond of the Ravemen LR1600 . It has excellent beam quality, accurate sensored technology, solid build quality and a user-friendly wireless remote. There’s also an under bar remote, but this is aftermarket and The LR1600 is still considerably dearer. Sigma Buster 1600 (review to follow) produces 1600lumens and has five other modes catering for most road biased riding contexts. However, there’s no wireless remote and it only complies with IPX4.
Summary
Ultimately, the Cetus 1700 delivers phenomenal bang for very modest buck. While the mount and remote merit some refinements and are arguably, where money has been saved, they are genuinely reliable and for the most part, pleasant to use. While USB C charge cables are increasingly ubiquitous - most of us have a couple for tablets and phones, the omission also seems a little odd.
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Crucially, the light itself offers an excellent beam pattern and purity, a good spread of modes, sensible run and practical charge times. While it should be regarded as a road light, first and foremost, there’s scope for more moderate ventures sans asphalt, though I’d point you elsewhere if dirt was your primary diet. Without wind tunnel facilities, it’s difficult to comment on the light’s aerodynamic prowess. However, mounted beneath the bars and against the stem, coupled with a Garmin plate ensures a sleek, organised aesthetic.
Verdict: 3.5/5 Impressive budget light but mount and remote merit minor refinements.
Michael Stenning
CETUS-1700 Underneath Mounted Bike Light
PUBLISHED OCTOBER 2025
