Our round-up of the best mountain bike stems in both 31.8mm and 35mm clamp sizes.
Mountain bike stems are an important factor when it comes to the control of your mountain bike, but which one should you choose?
Of the stems we’ve tested here, half accept regular 31.8mm diameter handlebars, and the rest are designed for the newer, oversized 35mm option.
It is possible to measure differences in weight between models of stem, and if you’re really in tune with your bike, there are subtle differences in stiffness to evaluate.
It’s not easy, but keep all the components the same, and ride the same trail over and over again with different stems, and the nuances begin to become apparent.
Equally, get up close and it becomes obvious which stems are really well thought-out and have the best combination of construction, material choice, geometry and even the fasteners — something that’s important on a component that needs to handle big forces.
Watch: The complete guide to mountain bike geometry
Know your mountain bike stems
4mm and 5mm Allen bolts are most common on mtb stems, with the heavier, larger-diameter bolts typically enabling higher torques and subsequent clamping forces.
Cheaper bolts round off more easily, and anodised bolts can lose colour. Stainless steel or titanium bolts maintain looks and are more resistant to corrosion.
The vertical real estate the stem demands on the steerer tube.
It can be an issue if your fork steerer is too short, and can also relate to the handlebar height achievable and overall stiffness of the stem.
30mm is about as short as you can go on a standard stem.
Stubby stems are something we’re big advocates of here, since reducing the distance the bar extends beyond the steerer will sharpen the steering response, provided your bike is roomy enough.
Zero gap fastening
More and more stems are now featuring a zero-gap bar clamp design, whereby one pair of clamp bolts fasten fully, and the remaining bolts tighten to lock the bar in place.
The design prevents uneven tensioning and reduces the number of bolts you need to fix to a precise torque setting, making set-up easier.
Rise is the difference in height between the clamps.
Most stems are zero-rise, which means if you flip them over they don’t change the bar height.
However, a six-degree rise stem allows you to effect a plus or minus six-degree change in bar height by simply turning the stem upside down.
CNC v Forged
Stems are either 3D forged or CNC machined from an aluminium billet (basically a chunk of metal).
Forging sees aluminium squeezed under immense pressures into (close to) the stem’s final shape.
The technique compresses the material around high stress areas, and it’s argued produces a stiffer product with a better-aligned, denser grain structure.
The initial forging cost is expensive and the finish is sometimes less angular, although you do see stems that are forged initially and then tidied up on a CNC machine.
Burgtec Enduro MK2
Made in the UK from 6061 T6-series aluminium, the Enduro MK2 is Burgtec’s second-generation trail and all-mountain stem accommodating both bar sizes. Overall stiffness and resistance to twisting is decent, but it’s not quite as rock-solid as some other stems.
Gamut is better known for chain devices, but this minimal, matt-anodised stem shows another side to the US manufacturer. Stripped back almost to its bare bones, the skeletal Cillos shape results from being squeezed densely by a forging process in a custom mould, followed by a quick cycle though a CNC machine.
Being machined out of a solid billet of 2014 T6-series aluminium is exactly what you’d expect from the Lancashire component giant. It works flawlessly and some will love the finish, but it’s not quite as solid and responsive as one or two here other stems when leaning hard into corners.
Ragley’s Stubbing stem comes in a multitude of colours, all with a two-tone shot-peened/shiny finish. The smaller size doesn’t result in a below average weight, but there’s a less solid, direct feel at the bars and the construction quality is a little less precise.
Syntace Megaforce 2
The Megaforce 2 is mega expensive, but comes with a 10-year guarantee and, if money were no object, would be our first choice. Syntace really has ticked all the boxes.
Deserved winner of the 2017 MBR stem grouptest. It’s not the lightest, but it’s still perfectly acceptable for trail riding, or even XC, and the price is excellent considering the quality.
Azonic The Rock Fat35
The design proved stiff enough and the unusual 45mm reach may also be useful if you’re between lengths. Solidity and steering response is good, but it’s let down by poor finishing.
Chromag BZA 35mm
The BZA is a massive chunk of metal — designed for 35mm bars — that looks perfectly in keeping with the company’s no-nonsense attitude and aggressive riding ethic. It works great, but doesn’t provide anything over a stem that’s half the money, or 50g lighter, especially when you factor in the cost.
Gravity Grid Stem
There’s nothing to complain about here. Gravity’s price is fair, the finish is good and performance is right up there in terms of stiffness and bar response.
The Ether’s ride feel is great, with good stiffness and connection to the bars, without being jarring and transferring too many vibrations. For a first effort this stem is very good, and if 50mm is short enough, also offers fantastic value for money.
Race Face Aeffect 35
Race Face’s Aeffect is a no-nonsense, forged and CNC finished 6061 aluminium stem at a bargain price. It’s only available down to 50mm, though, so if you’re after a super-stubby model you will need to look elsewhere.
Renthal’s Apex is designed to handle the extra twist of modern, wide bars and is available right down to 31mm in length. At 110g it’s the lightest 35mm option we’ve tested and that helps justify the cost.
In previous stem tests we’ve always had a few howlers — examples that were too flexy, or didn’t clamp tightly enough — thankfully, this time round, they offered increased stiffness for less weight, and the evolution in clamp designs has mostly weeded out any poor function.
Modern stems generally have wider bar clamp zones too, and that has improved the steering precision and integration, especially when using wider bars.
Performance isn’t entirely level between brands, though, and within the construction techniques used, the forged stems generally feel stiffer and tighter than ones entirely machined from aluminium.
Ironically, angular and stylised CNC stems can often cost more money too, since machine time is expensive.
The upshot is, machined stems look nicer, but forged stems still have the edge when it comes to performance.
Our favourite, money-no-object, stem for 31.8mm bars is the Syntace Megaforce 2.
It has a solidity and ride feel that is hard to beat, and the brand has extensively tuned every parameter of stiffness and detail — from the titanium bolts, with rubber retainer rings, to the shot-peened finish — is functional and well thought out.
The strong and solid Truvativ Descendant may be a more logical choice if you want near equivalent performance for half the cost.
Of the 35mm stems, Race Face’s Aeffect offers good performance for a bargain price. It’s not that light, but it’s stiff and durable, and were there a 30mm length in the range, would have easily taken top honours.
The only issue we have is the lack of usable lengths, which is where the Renthal Apex scores.
Yes, we don’t see the need for a 90mm stem in this day and age, but with options at 10mm increments from 30-90mm, there are few stem manufacturers offering this level of cockpit tuning.
How we test
With 12 stems to evaluate, we initially spread them between a mix of test bikes and trusted riding pals to put the early miles in and assess long-term durability.
The testing process was topped off with a final round of direct back-to-back analysis on our test track.
The six 31.8mm stems were tested on a 160mm Giant Reign with a full-width 800mm Spank Vibracore handlebar, and the 35mm stems with an aluminium Renthal Fatbar, also at 800mm, to generate maximum leverage and torsional forces.
The test track blended man-made g-out berms with technical, twisty, rock gardens that demanded plenty of body language to stabilise the bike.
All bolts were greased as necessary, and tightened to the recommended torque settings using a torque wrench.