Short answer: almost every modern road and mountain handlebar uses a 31.8mm clamp, and almost every stem clamps to a 28.6mm steerer. The number that catches people out is width, because Deda measures its drop bars outside to outside while most brands measure centre to centre, so a Deda 42 is roughly a 40 from anyone else.
Contents
- Handlebar clamp diameter
- Steerer diameter
- Bar width, and how it is measured
- Reach and drop on drop bars
- Stem length
- Stem angle and flipping
- Integrated cockpits
- Torque and carbon
- Frequently asked questions
Handlebar clamp diameter
This is the measurement at the centre of the bar where the stem grips it. It must match the stem exactly. There is no adapting, other than shims to fit a smaller bar into a larger clamp.
| Diameter | Found on |
|---|---|
| 31.8mm | The current standard. Road, gravel and most MTB. |
| 35mm | Modern mountain bikes, mostly trail and enduro |
| 25.4mm | Older MTB, many flat bar hybrids and city bikes |
| 26.0mm | Older road bars, pre-2000s |
One quirk worth knowing: Deda specifies 31.7mm rather than 31.8mm on its bars. This is not a separate standard and Deda bars work with 31.8mm stems in practice. Do not go hunting for a 31.7mm stem.
If you are unsure what you have, measure the clamp area of the bar with a vernier caliper, not the section where the tape sits, which is narrower.
Steerer diameter
The other end of the stem clamps to the fork's steerer tube. Here there is far less variety than people expect.
| Steerer | Stem clamp | Notes |
|---|---|---|
| 1 1/8 inch straight | 28.6mm | The standard on virtually every modern bike |
| Tapered, 1.5 to 1 1/8 inch | 28.6mm | Wider at the crown only, so the stem is unaffected |
| 1 inch | 25.4mm | Older bikes, often threaded with a quill stem |
The practical takeaway is that a tapered fork does not need a special stem. The taper is at the bottom, inside the head tube, and the top of the steerer is the same 28.6mm as a straight one. Our bottom bracket standards guide covers the same kind of confusion at the other end of the frame.
Bar width, and how it is measured
This is where compatibility becomes genuinely confusing, because two brands can print different numbers on physically identical bars.
- Centre to centre, measured between the middle of each drop. Used by most brands.
- Outside to outside, measured across the outer edges. Used by Deda, among others.
The gap is usually around 2cm. A Deda 42 is roughly equivalent to a 40 measured centre to centre. If you order by the number on your current bar without checking which convention each brand uses, you can easily end up a full size out.
Choosing a width
The traditional guidance is to match your shoulder width, measured across the bony points at the front of each shoulder. Most adult riders land between 38cm and 44cm on the road.
| Type | Typical widths |
|---|---|
| Road drop bar | 38 to 44cm |
| Gravel drop bar | 40 to 46cm, often flared |
| Flat bar, hybrid | 660 to 700mm |
| MTB riser bar | 720 to 780mm |
Road bars have been getting narrower, because a narrower position is measurably more aerodynamic. Do not follow that trend past the point of comfort: a bar that restricts your breathing or makes the bike twitchy at speed costs more than it saves. The range on offer runs from 380mm upward, and the full selection is in handlebars.
Reach and drop on drop bars
Two numbers that change how a bar feels more than width does, and that most people never look at.
- Reach is how far forward the bar extends from the clamp to the furthest point of the bend. Typically 70 to 85mm on modern bars.
- Drop is the vertical distance from the top of the bar to the lowest point of the drops. Typically 120 to 145mm.
A compact bar, meaning short reach and shallow drop, makes the drops usable for riders who are not especially flexible. A classic deep bend puts you lower but is unusable for many people, which defeats the purpose.
Reach interacts directly with stem length. Moving from a long-reach bar to a compact one shortens your effective reach by 15mm or so without changing the stem at all. Take this into account before buying a shorter stem to fix a stretched position.
Stem length
Measured centre of steerer to centre of bar clamp. Common road lengths run 70mm to 130mm, with 100mm the most frequently fitted.
| Change | Effect |
|---|---|
| Shorter stem | Less reach, more upright, quicker and more nervous steering |
| Longer stem | More reach, lower position, calmer and more stable steering |
A 10mm change is clearly noticeable and 20mm is transformative. Because stems are inexpensive relative to the difference they make, changing one is the single most cost-effective fit adjustment available. If you are working through a comfort problem, our guide to the signs your position is wrong covers what to change and in what order.
Avoid going shorter than about 70mm or longer than about 130mm on a road bike. Outside that range the handling becomes noticeably odd, and it is a sign the frame size is wrong rather than the stem. Options are in stems, and an adjustable-angle stem is a useful way to find your position before committing to a fixed one.
Stem angle and flipping
Stems are made at an angle to the steerer, most commonly plus or minus 6 or 7 degrees, and they are symmetrical, so the same stem can be fitted either way up.
Fitted with the angle pointing down, the bars sit lower. Flipped so it points up, the bars rise. On a 100mm stem, flipping a 6 degree stem raises the bar by roughly 20mm, which is a meaningful change and costs nothing.
This is the first thing to try if your bars feel too low, along with moving headset spacers from above the stem to below it. Both are free, reversible in ten minutes, and worth exhausting before buying anything.
The limit is the steerer. Once it has been cut, you cannot raise the stack beyond the spacers you have, which is precisely why we cut steerers as part of a build rather than leaving them long and guessing.
Integrated cockpits
One-piece carbon bar and stem units are now common on aero road bikes, and they change the rules.
They are specified as a combination, written as bar width by stem length. A 375-90 is a 375mm bar with a 90mm stem, and the two cannot be separated. Changing either dimension means replacing the entire unit and re-routing the cables and hoses through it, which is a workshop job rather than a ten minute swap.
Two consequences worth planning around. Get your fit right before buying a bike with an integrated cockpit, because correcting it afterwards is expensive. And note that many integrated systems are frame specific, since the bar has to mate with a particular headset and cable routing. A cockpit designed for one frame will often not fit another.
Torque and carbon
Stem bolts are among the most commonly overtightened fasteners on a bicycle, and the consequences on carbon are severe.
- Use the printed value. Stems and bars carry a maximum torque figure, usually 5 to 6 Nm for faceplate bolts. Use a torque wrench, not judgement.
- Tighten the faceplate evenly. Work diagonally in small increments so the gaps at the top and bottom of the faceplate stay equal. An uneven gap creates a stress point.
- Use carbon assembly paste on carbon bars, not grease. The paste contains fine particles that increase friction, so the clamp holds at a lower torque.
- Never clamp a carbon bar in a workstand, and never clamp a carbon steerer.
- Inspect after any crash. Carbon damage is often invisible from outside.
While the bar is off, it is the natural moment to replace bar tape or grips.
Frequently asked questions
Are narrower handlebars actually faster?
Yes, measurably, because your arms and shoulders are the largest source of aerodynamic drag on a bicycle and narrowing them reduces frontal area. Wind tunnel testing consistently shows a gain from narrower bars. The caution is that the gain is small at ordinary speeds and the costs are real: reduced leverage when climbing out of the saddle, twitchier handling, and restricted breathing if you go far enough. Dropping one size from a bar that fits you is usually harmless. Dropping three because professionals do is not.
Can I cut a carbon handlebar to width?
Only if the manufacturer explicitly permits it, and most drop bars do not. Carbon bars are laid up with specific fibre orientation, and the ends often carry reinforcement that cutting removes. Many flat and riser MTB bars do have printed cut lines and can be shortened within them, using a carbon-specific blade and finishing the edge properly. Never cut past a marked line, and never cut a bar with no marking. Aluminium bars are far more forgiving, though the same rule about clamp area applies.
Should I flip my stem to raise the handlebars?
Yes, it is a legitimate adjustment rather than a bodge, and it is free. Stems are symmetrical by design and both orientations are equally strong. Flipping a 6 degree stem of typical length raises the bars by roughly 20mm. The only real objection is appearance, and a comfortable position matters more than a slammed stem. Try this and moving spacers below the stem before spending money on a different component.
What torque should stem bolts be, and do I need carbon assembly paste?
Follow the figure printed on the stem, commonly 5 to 6 Nm for the faceplate and steerer bolts, and use a torque wrench rather than feel, because the difference between correct and damaging is small. Carbon assembly paste is genuinely worth using with any carbon bar or steerer: it raises friction so the clamp grips securely at a lower torque, which is exactly what carbon needs. Do not substitute grease, which does the opposite and encourages overtightening.
How do I measure the stem length and bar width I already have?
For the stem, measure from the centre of the steerer to the centre of the bar clamp, which is easiest done along the top of the stem with a steel rule. Many stems also have the length printed underneath. For bar width, measure between the centres of the two drops at their widest point, and note that you are then working in centre to centre, so compare against brands using the same convention. Send us the numbers and we will tell you what will fit.
Not sure what you need?
Send us a photo of your stem and bars, or your current measurements, on WhatsApp and we will confirm what fits before you order. If the position itself is the question rather than the parts, a bike fit at our Kondapur store will answer it properly.
