Rockrider 20″ MTB Expl 120 Review: An Affordable First Mountain Bike
Finding a capable first geared mountain bike for a child without spending a small fortune is no easy task. Most quality kids’ mountain bikes sit well above the £300 mark, which puts them out of reach for many families.
When the cost of living started to bite, we wanted to find out whether a much cheaper option from Decathlon could genuinely do the job. So, we bought the Rockrider 20" MTB Expl 120 ourselves and sent it out to one of our review families to find out.
6-year-old S and his mum Helen have been riding with it for eight months across country lanes, forestry tracks, pump tracks and the farmland where they live in Cumbria.
This review is their honest verdict of how the Rockrider 20" MTB Expl 120 has performed for them.
Disclosure: Cycle Sprog bought this bike at full retail price. We were not paid to write this review, and all opinions are our own. We do use affiliate links, so may earn a small commission if you choose to buy through links on this page. This funds the website, and we thank you for your support.
Rockrider 20" MTB Expl 120 key data:
Price - £169.99
Weight - 11.5kg
Rider height range - 120cm to 135cm
Age range - 6 to 9 years
Colours - White
Date of review – Summer 2025 to Spring 2026
Pros - A genuinely capable budget rigid fork mountain bike. Good gear range, solid brakes and knobbly tyres that handle mixed terrain well.
Cons - Weight may be an issue for smaller or less strong riders. Small areas of rust appearing after 8 months of use, grip shifters less intuitive than trigger shifters for beginners.
Rockrider 20" MTB Expl 120 scores
Quality - 3/5 - The frame and main components are solid for the price, but small amounts of rust appearing on a bike after eight months of use. The overall build is functional rather than refined.
Weight - 2/5 - At 11.5kg this is one of the heavier bikes in the 20 inch category. While S coped well, it would be a lot of weight for a less confident, or not so strong, child.
Value for money - 5/5 - At under £170 this is one of the most affordable geared 20 inch mountain bikes currently available new. It comes with a reasonable component set, knobbly tyres and child-specific brake levers, and the gear range is more than adequate for varied terrain. The price is the real selling point of this bike.
Customisability - 4/5 - The frame has a bottle cage mount. Decathlon list mudguards, a kickstand, saddle bag and stabilisers as compatible accessories, all available separately.
Resale value - 3/5 - Not a bike you buy for its resale value, but Decathlon's Second Life scheme means they may buy it back from you if you have a store nearby. A well-maintained example should also sell on the second-hand market at a realistic price.
Final verdict - A capable and affordable entry point into geared mountain biking. If budget is the priority and your child is ready to try gears and rougher terrain, the Rockrider Expl 120 can do the job. Just go in with realistic expectations about weight and longevity.
Find out more about the Cycle Sprog review process.
About our reviewer
S was five years and eleven months old when he took on the Rockrider 20" MTB Expl 120 - right at the younger edge of Decathlon's recommended 6 to 9 age range.
He is a confident young rider who before getting the bike had been getting around the family’s Lake District farm, local forestry trails and to school on a 16 inch Islabike Cnoc and was keen to try his first geared bike.
S and his mum Helen are a proper cycling family. Between work on the farm they get out whenever they can, with favourite routes including Grizedale Forest and the West Windermere Way.
They have also done a handful of cycle touring trips together as a family.
Who are Rockrider?
Rockrider is Decathlon's own mountain bike brand, covering everything from first bikes for young children through to adult full suspension electric mountain bikes. The range is designed to make mountain biking accessible and affordable, with bikes built and sold exclusively through Decathlon stores and their website.
The Rockrider 20" MTB Expl 120 is the smallest and cheapest bike in their range, with the aim of encouraging young riders to give mountain biking a try. They have two other models in the 20” wheel size, which we will look at later in this article.
The frame and components are designed with repairability and longevity in mind, which matters at this price point. This is a bike that should last through its full age range and still have second-hand value at the end of it.
Rockrider 20" MTB Expl 120 specifications
The Expl 120 is built around a steel frame and steel fork. It is fitted with 6-speed gearing operated by a grip shifter - you twist the handlebar grip to change gear - alongside a 36 tooth single chainring and 140mm cranks sized for children's legs.
A chainguard protects clothing and legs from the chain.
Braking is handled by V-brakes front and rear. The brake levers are designed specifically for children's smaller hands to make reaching and pulling more manageable – something often missing with bikes at this price point and under.
The 20 inch wheels use aluminium rims with MTB-profile knobbly tyres, designed to work across mixed surfaces.
The handlebar and stem are combined into a single integrated steering block, which means there is no stem length adjustment.
First impressions of the Rockrider 20" MTB Expl 120
S's reaction when he first saw the bike was immediate. He was excited to see there was a bike with gears for him to try and simply declared it looked "really good".
Helen's first thought was that it felt heavy - particularly coming from a smaller, lighter 16 inch wheel Islabike.
But she also noted it looked well made, the brakes felt solid and the knobbly tyres caught her eye. "We've never had new bikes before, so it looked very shiny," she told us.
Sizing of the Rockrider 20" MTB Expl 120
Decathlon recommend the Expl 120 for children aged 6 to 9 and between 120 and 135cm tall.
S was not quite six when he got the bike, and at around average height for his age he sat at the lower end of that height range.
Helen set the saddle at almost its lowest position for S, who was initially on his tiptoes to reach the floor, and it took a little confidence for him to get going. Once he did, he settled into riding around the farmyard quickly and with a big grin on his face.
By the end of the eight-month review period S had grown to be 123cm tall with a 53cm inside leg, and nearing his 7th birthday. Helen had raised the saddle to around the middle setting, and the bike was still fitting S very well.
Brakes on the Rockrider 20" MTB Expl 120
Decathlon have designed the brake levers on the Expl 120 specifically for children's hands, positioned closer to the handlebar than a standard adult lever to make them easier to reach and pull. This was one of the first things S noticed when he tried the bike, commenting that he did not have to reach far for the brakes.
In use the brakes performed adequately. Helen felt they held up well in wet weather and on muddy tracks. S rode the pump track after heavy rain without any issues with braking confidence or performance.
Gears on the Rockrider 20" MTB Expl 120
The Expl 120 uses a grip shifter – the rider rotates the handlebar grip to move between the six gears - rather than a trigger shifter, where they push a lever with their finger or thumb.
Decathlon describe this as an easier and safer system because children's hands stay on the handlebars while changing gear.
S took to gears more quickly than Helen expected, given that this was his first geared bike and he is not always the most patient when trying something new. He learned to change gear while moving rather than stopping, and to move through gears one at a time.
However, Helen did notice that when S had a chance to try friends' bikes with trigger shifters, something clicked for him in a way that grip shifting had not quite managed. "I would say trigger shifters were maybe better for his learning," she told us. "Having got the hang of changing gears on his friend’s bike, he then found it easier to use the grip shift."
There was occasional sticking or grinding when changing gear. Helen's view was that this was likely down to S still getting to grips with technique rather than a mechanical issue with the bike, and it happened infrequently.
The gear range itself was well suited to the riding S was doing. Low enough to get him up some fairly steep hills around their home, and high enough to cruise at speed along flat tracks and forestry roads.
Tyres and handling of the 20" MTB Expl 120
S rode the Expl 120 on country lanes, pump tracks, forestry tracks and packed gravel, in dry, wet and muddy conditions. The furthest he rode it in a single go was around 12 miles. The knobbly tyres handled all of these surfaces without issue.
Helen did not notice a significant difference in grip compared to S's previous bike. On the pump track after heavy rain, S rode with full confidence.
As there were no punctures during the review period, we haven’t tested how easy it is to remove a rear tyre.
Weight of the Rockrider 20" MTB Expl 120
At 11.5kg the Rockrider Expl 120 is heavier than the more expensive options we usually review on Cycle Sprog. It is also nearly double the weight of the Islabikes Cnoc 16 S was riding previously.
The additional kilos are due to the steel frame and fork, which keeps the price down, but adds additional weight compared to a more expensive aluminium framed bike.
It turns out that for S the weight of the bike was not a significant issue in practice. He can lift the bike independently if it falls over, and the weight did not seem to affect his enjoyment or confidence on the bike. Loading it onto the family's rear car rack was manageable too.
Helen reflects that her concern about the weight was largely based on comparison with S's smaller previous bike. "S didn't seem so bothered and it doesn't seem to affect his riding," she told us.
That said, Helen did wonder whether the weight would be more of a barrier for a less confident. or less strong, child attempting rougher terrain for the first time.
This is worth considering if your child is on the less confident end of the spectrum or is not particularly strong physically.
How the Rockrider 20" MTB Expl 120 performs as a mountain bike
The Expl 120 is marketed by Decathlon as a first mountain bike for children wanting to try trail riding. Whilst S used it primarily on forestry roads, pump tracks and farm tracks rather than dedicated trail centres, he did tackle some lumpier downhill sections on their rides without any issues.
Helen felt the bike performed well in that context and seemed robust.
The bike itself attracted plenty of positive attention while S was out on it. Helen noted that a lot of people commented that it looked like a cool bike.
We asked Helen whether she would buy the Rockrider if she wasn’t on our review panel. Her response is really honest and reflects the pros and cons of buying a mountain bike for the type of mixed terrain riding they are doing.
"Having seen S trying out other friends' bikes, I think I would look for a second-hand Frog hybrid bike or similar instead, mainly because we do quite a bit of road and trail cycling and they feel lighter."
Their plan for now is to start using the Rockrider to explore the local trail centres, and S has also riding it up some steep hills to school.
Wear and tear on the Rockrider 20" MTB Expl 120
After eight months of regular use, including through a winter, the bike developed a little bit of rust. This includes where the bottle cage bosses meet the frame, around the cable fittings and the headset bolts.
The bike has been stored inside a farm building where S's previous bikes have not had any rust issues. However they have been top of the range Islabikes, which come at a significantly higher price point and this detail in quality of components, (along with the weight of the bike), help explain the price difference between different brands.
The tyres, brakes and handlebar grips all remain in good condition, despite regular use.
How does the Rockrider 20" MTB Expl 120 compare to other Rockrider 20" mountain bikes?
If you are considering a Rockrider, it is worth knowing that Decathlon offer three 20-inch mountain bikes at different price points, making it straightforward to match your budget to your child's needs and ambitions.
The Expl 120 at £169.99 is the entry point in the range. It is a solid, functional first geared mountain bike for a child ready to try mixed terrain for the first time.
For £30 more, the Expl 500 at £199.99 adds a front suspension fork, which in theory softens the ride on rougher ground. However, suspension forks on bikes at this price point tend to be quite stiff, and the trade-off is an extra kilo of weight, taking it to 12.5kg.
For most children riding forestry tracks, pump tracks and country lanes, we think the lighter Expl 120 is probably the better choice. But some children will have set their heart on a mountain bike with a suspension fork, so you may have no choice but to go for this option.
The more significant step up is the Rockrider 20" Expl 900R at £329.99. We have reviewed this bike separately and it is an excellent option if your budget stretches that far.
It has an aluminium frame rather than steel, which brings the weight down considerably. It also has 8 speeds rather than 6, a trigger shifter instead of a grip shifter, and mechanical disc brakes rather than V-brakes. Those are meaningful upgrades for a child who is going to be riding regularly on more varied terrain. But note, it is a rigid fork bike, so no suspension (if that is your child’s only requirement).
If the 900R is within your budget, it is definitely worth the extra investment. If £169.99 is your ceiling for a new bike, the Expl 120 is a great choice.
How does the Rockrider 20" MTB Expl 120 compare to other bikes at this price?
At £169.99 the Rockrider Expl 120 sits in a price bracket where there are a handful of other options available from major retailers. Two you may come across are the Apollo Chaos from Halfords at £180 and the Spike 20 from Argos at £160.
Weight matters so much when it comes to kids’ bikes. A heavy bike is harder to control and less fun to ride and a very heavy bike may even put a child off cycling altogether.
At this price point, weight is where the Rockrider pulls ahead.
At 11.5kg it is lighter than both the Apollo Chaos at 13kg and the Spike at 12.2kg. That is a big difference when the rider is six years old.
We have not reviewed either the Apollo or the Spike ourselves, so we cannot give you a first-hand verdict on how they ride in practice.
What we can say is that on the specifications alone, the Rockrider compares favourably, and Decathlon's in-store support and Second Life buyback scheme add value alongside the price tag.
If you are shopping for a 20 inch mountain bike for £180 or under, the Rockrider is the only one that has been through our review process - ridden hard, in real conditions, by a real family!
Thoughts from the Sprog
I really like the tyres, they're big, and it's good at going over bumps. It's quite hard to get going uphill if I have to stop. It's got a good bell.
Overall verdict
The Rockrider 20" MTB Expl 120 does exactly what it promises: it gets a child onto their first geared mountain bike for well under £200. If you want a new bike for that budget, that's a great choice.
Our reviewer took to it quickly, and eight months of country lanes, forestry tracks and a muddy pump track haven't found any serious flaws. The big tyres and confident bump handling are what S himself picked out as his favourite thing about the bike.
His one gripe, that it's hard work getting going again uphill after stopping, is a fair reflection of the gear shifter and weight rather than a fault. It's worth bearing in mind if your child will be doing a lot of stop-start climbing.
The bike isn't without compromise. At 11.5kg it's noticeably heavier than pricier alternatives, and while that didn't hold S back, it's worth thinking about if your child is smaller or less confident.
A little surface rust after one winter, stored inside, is also worth knowing about, even if it hasn't affected how the bike rides.
If a lighter bike matters more to you than buying new, it's worth having a look at the second-hand market too.
A well looked after second-hand bike from a premium brand can sometimes be found for similar money to a new budget bike, with the benefit of better components and a lighter weight.
But the Rockrider Expl 120 is a great choice if a new bike is what you're after. It does the job well, and S has really enjoyed riding it on a very regular basis.
Should I buy a Rockrider 20" MTB Expl 120 second hand?
If you are buying the Rockrider 20" MTB Expl 120 secondhand rather than new, do check the condition carefully.
A bike that has had one careful owner and been properly maintained is a reasonable buy.
One that has been through several children, or pushed to its limit on mountain bike trails, or regularly left outside is a different proposition entirely.
As with any budget bike, the resale value is a reflection of how well it has been looked after.
Where to buy the Rockrider 20" MTB Expl 120
The Rockrider 20" MTB Expl 120 is available from Decathlon, both online and in store. At the time of writing it is priced at £169.99. Decathlon stores offer a click and collect service and home delivery.
Comments