How to measure a bike stem so a phone mount clamps without slipping
Fit a phone mount by measuring two things first: the stem's clamp bore, usually 28.6mm (1-1/8 inch) on modern bikes, and the stem length from clamp centre to clamp centre. Then buy a mount whose clamp grips that bore, and pack the gap so it cannot turn.
On this page (11 sections)
- Remember this
- How to measure a bike stem in one pass
- Identifying 1-1/8 inch and 1.5 inch stems under spacers
- Which mount clamp size matches which stem
- Fitting the mount so it cannot rotate or rattle
- Swapping the stem when the mount needs a different diameter
- Why phone mounts slip, creak and rotate
- Rounded clamp face and undersized bore
- Brake lever and cable clearance at full lock
- Phone and camera clearance from rider and bars
- FAQ
Part of our guide on car mount holding phone in thick case
Fast facts
- Common stem bore
- 28.6 mm (1-1/8 in)
- Larger stem bore
- 31.8 mm (1.5 in)
- Stem length measured
- Steerer clamp to bar clamp centre
- Ideal clamp fit
- 0-0.5 mm under the stem bore
- Steerer cap torque
- 2-5 Nm
- Faceplate bolt torque
- 4-5 Nm each
- Clamp interface
- Foam or bar tape, 1-2 wraps
Remember this
- Measure the steerer bore and the stem's centre-to-centre length before choosing a mount
- 28.6mm (1-1/8 inch) is the common stem bore; 31.8mm (1.5 inch) needs a larger clamp or an adapter
- A rubber-lined clamp that grips slightly undersize stops rotation better than a snug metal clamp
- Wrap the stem with foam tape or a strip of bar tape so the clamp bites something with grip
- Steerer cap bolts want roughly 2-5 Nm; faceplate bolts usually sit around 4-5 Nm
- Check brake cable and lever clearance with the bars slammed before you ride
How to measure a bike stem in one pass
Two numbers decide whether a phone mount stays put: the bore of the stem's steerer clamp and the distance from that clamp to the bar. Both are readable off the bike in a few minutes, and both are worth getting right before you spend anything.
The bore is the inside diameter of the ring that grips the steerer tube. Easiest read comes off the steerer itself: undo the top cap, lift the spacers off, and measure the outside diameter of the exposed steerer with a caliper. That number matches the stem bore exactly, because the stem has to slide over it. A 28.6mm reading is a 1-1/8 inch stem. 31.8mm is a 1.5 inch stem.
Stem length runs from the centre of the steerer clamp to the centre of the bar clamp. Lay a ruler between the two holes and read between the hole centres, not between the outer edges of the stem. A stem marked 90mm puts the bar 90mm forward of the steerer axis. A 60-80mm stem is a stubby stem that crowds the head tube.
If your stem has a removable faceplate and a pinch bolt, take the whole stem off the bike first. Working with it in your hands beats fighting a bolt in a tight space between the head tube and the brake cables. Photograph the spacer order before anything comes apart. We cover tamper-proof bolt kit for replacing a lost phone mount bolt in its own article.
Identifying 1-1/8 inch and 1.5 inch stems under spacers
Most stems carry a marking somewhere near the clamp: a stamped number such as 31.8, a fraction like 1-1/8, or a size on the back of the clamp body. Some manufacturers print a model code that includes the diameter. Check the underside of the clamp and the flat behind the faceplate, where the stamping lands on many designs.
When a top cap and a stack of spacers hide the steerer, the marking is your first route and the caliper is your second. Take the top cap bolt out, note the order and thickness of the spacers as they come off, and slide them down the tube. With10-20mm of bare steerer exposed, a caliper across the tube reads the diameter straight off.
Two clues confirm a 1.5 inch steerer even without measuring. The headset bearing seat will look visibly wider, and the stem's lower clamp will have a deeper, more squared body than the slim 1-1/8 inch clamp. Older steel frames with a 25.4mm steerer are rare now, but a few touring and cargo bikes still use them. It helps to understand iphone 13 rear seat mount before going further.
Read the marking before you remove anything. If the stem says 1-1/8 inch, you have your answer in ten seconds and no reason to strip the headset. The measurement pass only matters when the marking is missing, worn off or hidden.
Which mount clamp size matches which stem
Match the mount's clamp to the stem bore, and the bore is the number that came off your caliper. Most phone mounts ship with a rubber-lined clamp that swells to grip, commonly advertised across roughly 25-35mm, which swallows a 28.6mm stem comfortably. A mount with a hard 31.8mm clamp on a 28.6mm stem will rattle and will not hold position.
Two reader situations give different answers here. If you ride a modern aluminium or carbon road bike with a standard 1-1/8 inch steerer, a rubber-lined clamp adjustable from 25 to 35mm fits and grips. If you run a 1.5 inch stem, you need either a clamp that opens past 31.8mm or a cheap reducer adapter that brings the steerer down to 28.6mm before the stem goes on. Before you commit to anything, it is worth looking at how magsafe works through cases.
Short stems create their own problem. On a 60-80mm stem the clamp sits close to the head tube, so the phone holder has little room to clear the headset spacers and the brake lever clamp. Check the gap between the mount body and the top spacer. If they touch, the mount belongs on the handlebar instead of the stem.
Adjustable stems deserve a closer look because the angle changes what your phone faces. Once the clamp is fixed to the stem, the phone cradle rotates on its own ball joint, so an adjustable stem does not fight the mount. It still matters for cable clearance, since a stem with a high rise pushes the levers further forward.
| Stem bore | Equivalent | Mount clamp to buy | Notes |
|---|---|---|---|
| 25.4 mm | 1 inch | Rubber-lined 25-35mm clamp | Old steel frames, rare |
| 28.6 mm | 1-1/8 inch | Rubber-lined 25-35mm clamp | Most modern road and MTB stems |
| 31.8 mm | 1.5 inch | Clamp opening over 32mm, or a 28.6mm adapter | Check for bar and lever clearance |
Most slipping mounts are the right size; they are a split clamp biting a rounded face instead of a machined flat.
Fitting the mount so it cannot rotate or rattle
The clamp grips a machined flat, and most stems are round at the top with a small flat machined for the pinch bolt. That flat is often barely6-8mm wide and it is where the mount should sit, because a split clamp squeezing a round tube will happily turn.
Wrap the stem first. One or two wraps of self-adhesive foam tape or a strip of handlebar tape around the steerer clamp area gives the rubber liner something with grip to bite, and it kills the rattle at the same time. Do it before the mount goes on, because once the clamp is around the stem you cannot get the wrap under it.
Position matters as much as tightness. Slide the mount to the front face of the stem clamp, not the rear, so it sits between you and the steerer. Leave5-10mm of clearance from the top cap and any spacers. Then tighten the clamp bolt in stages, checking that the phone stays where you put it.
Brake cables are the other clearance problem. Look down the front of the bike with the bars turned full lock each way. A mount fitted to a slammed stem can pull a cable housing into the bar or put the phone straight in the path of the front brake lever.
- Slide the stem off and wipe the clamp area clean of grease and grit
- Wrap the stem clamp area with one or two wraps of foam tape or bar tape
- Fit the mount's clamp over the stem, seated against the machined flat
- Position the phone cradle and rotate it to the riding angle
- Tighten the clamp bolt in stages, checking rotation by hand after each turn
- Turn the bars full lock both ways and re-check lever and cable clearance
Swapping the stem when the mount needs a different diameter
If the only mount you can find for your 1.5 inch steerer is a 28.6mm clamp, or you want a stem whose shape suits the mount, changing the stem is a twenty-minute job with the right tools. The work happens at the steerer, above the headset, and the headset itself stays where it is.
Start with the top cap. Back the bolt out evenly, several quarter turns rather than one big heave, so the cap does not tilt and the preload does not drag sideways. Lift the cap and spacers off and note the order. The stem is then held by its own clamp bolts, two or four of them depending on the design, and they need loosening in a criss-cross pattern.
Torque is where carbon steerers get damaged. Steerer clamp bolts on a carbon fork want roughly 4-5 Nm, and the top cap bolt usually sits in the 2-5 Nm range. Follow the stem maker's figure when you have it, and treat any number here as a starting point. A torque wrench in the 1-10 Nm range is what makes this repeatable rather than a guessing game.
Use the supplied spacer hardware and replace any washers that came off. If the stem sits proud of the spacers, the steerer clamp will load the top spacer and the bearing preload drops as you tighten. Add spacers to fill the gap and the preload stays where you set it.
Why phone mounts slip, creak and rotate
Slipping has four causes, and they leave different clues. A rounded or tapered clamp face lets the mount turn under acceleration and braking, because the split bolts pull the two halves into a wedge that pushes the phone rearwards. An undersized bore does the same thing more slowly. Overtightening an aluminium stem gives you a dented stem, a creak on every bump, and a mount that still rotates.
Brake and lever clearance is its own failure. On a slammed stem the levers sit close to the head tube and the cables run in tight arcs. Add a mount and the cable housing can rub the mount body, which wears a flat into the housing and then into the lever. Check the lever throw with the bars at full lock before the first ride.
The phone or camera also has to clear the rider and the bars. With the steering locked at full lock the bars swing toward the rider, and a phone mounted on a long stem can end up in front of a knee or shin. For a camera-style mount running a wide lens, check that the housing is clear of the stem clamp and the bar end at full lock.
Rounded clamp face and undersized bore
A split clamp grips by friction, and friction comes from the rubber liner biting into the tape. When the stem has no flat to grip, the clamp sits on a round surface and rotates. Wrapping the stem solves it in minutes. If the bore itself is undersize by more than about a millimetre, the liner has nothing to compress against and the mount works loose over a ride.
If the mount still turns after wrapping, check the clamp's rated diameter. A clamp that stops just below your stem bore was made for a different bar size and is the wrong part.
Brake lever and cable clearance at full lock
Slam the bars to the stop and look down at the levers. The lever arm should not touch the mount or the phone. Cables should have a smooth arc with no sharp bends where they meet the mount body. A mount that fouls the housing at full lock will be quiet on a straight road and noisy in every corner.
The fix is usually a shorter stem or a mount that sits further back on the steerer clamp, not a tighter bolt.
Phone and camera clearance from rider and bars
At full lock the bar sweeps toward the rider. On a 90mm or 100mm stem, a phone can end up over the front wheel and out of your line of sight, which is the opposite of what a mount is for. On a 60-80mm stem the phone sits close to the head tube and clears the knee more comfortably.
For a camera mount, check the lens clearance at both extremes of steering. A wide lens on a high-rise stem can clip the stem clamp itself when the bars turn.
Measure the steerer bore and the stem length first; a rubber-lined 25-35mm clamp over a foam-wrapped stem is the combination that stops a phone mount slipping.
FAQ
Can I fit a phone mount to the handlebar instead of the stem?
You can, and many riders do, but the bar rotates with the steering, so the phone swings into the cables on every turn and your footage shakes in corners. A stem mount keeps the phone aligned with the bike. If the stem is too short or the clamp diameter does not match, a bar mount is a workable fallback on flat-bar bikes.
Do I need a special adapter for a 1.5 inch steerer?
Only if the mount clamp you want does not open past 31.8mm. A reducer spacer drops the steerer to 28.6mm so a standard 1-1/8 inch clamp fits. It must sit under the stem clamp, not above it, and the spacer has to be the thickness that keeps the stem flush with the headset spacers.
How tight should the mount clamp bolt be?
Tight enough that the phone holds its angle under firm pressure, then no more. If you need more grip, add foam tape or a strip of bar tape under the clamp rather than turning the bolt harder. Over-tightening dents alloy stems and still lets the mount rotate on a rounded clamp face.
Will a mount on the stem fall off over rough roads?
Not if the bore is right and the interface is wrapped. Most reported drops come from a clamp sized for a smaller bar, from riding without the foam or tape layer, or from a mount that fouled the bars and got levered sideways. Check the interface once a month on rough riding.
Does the stem length affect filming quality?
It affects vibration and framing. A short stem near the head tube transmits more front-end buzz and can shake a wide lens. A long stem steadies the view but moves the phone into the rider's knees at full lock and puts it in the way of the brake lever.
