31.8 vs 35mm handlebar clamp: which should you choose?

Walk into any bike shop or browse any handlebar page and you'll see two numbers over and over: 31.8 mm and 35 mm. They refer to the diameter of the bar at the centre, where it's clamped by the stem. This guide explains the 31.8 vs 35mm handlebar clamp question: what the difference is, how it affects ride feel and which one makes sense for you.

What does 31.8 vs 35mm handlebar clamp mean?

The clamp diameter is the outside diameter of the handlebar's centre section. Your stem's faceplate is machined to fit one size, so the two must match. A 35 mm bar won't fit a 31.8 mm stem, and a 31.8 mm bar physically can't be clamped in a 35 mm stem: the bar is too small for the clamp and would simply fall out, so it can't happen by accident.

• 31.8 mm has been the standard mountain bike clamp size for many years. It's still widely used on trail bikes and is compatible with a huge range of stems and bars.
• 35 mm arrived later and has become common on enduro, freeride and downhill bikes, as well as plenty of modern trail bikes.

Neither is "wrong". They're two different standards with slightly different characteristics.

How the clamp size affects ride feel

Stiffness

A larger-diameter tube is generally stiffer for a given wall thickness. That's the main reason 35 mm bars and stems exist: a bigger centre section can make the cockpit feel more direct and precise when you're pushing hard through rough ground or heavy compressions.

Compliance and comfort

Stiffer isn't always better. A bit of flex at the bar can take the edge off trail buzz and reduce hand and arm fatigue on long descents. Many riders find 31.8 mm setups feel a touch more forgiving, although bar design, material, rise and width all play a part too, so clamp size is only one factor.

Weight

Differences in weight depend more on the specific bar and stem than on the clamp standard. Check the actual figures for the parts you're considering rather than assuming one size is lighter.

Which should you choose?

Use these points as a guide:

• Keep your current standard if you're only replacing the bar or only the stem. Matching what you already have is the simplest and cheapest option.
• Consider 35 mm if you ride enduro, bike park or downhill, you're a heavier or very aggressive rider, or you want the most precise front end.
• Consider 31.8 mm if you want a little more comfort on long rides, you're a lighter rider, or you want the widest choice of compatible parts.

Stem length and bar width also affect how the cockpit feels — see our MTB stem length guide and MTB handlebar width guide. If you're replacing both bar and stem together, you can pick either standard freely, as long as the two match.

How to check what you have

1. Look for markings. Most bars and stems have the clamp diameter printed or etched near the centre or on the faceplate.
2. Measure. If there's no marking, measure the bar's diameter right next to the stem with callipers. You'll read close to 31.8 mm or 35 mm.
3. Check the spec. Your bike or component manufacturer's spec sheet will list the clamp size.

PEMBREE GCS bars and stem in both sizes

Our GCS handlebars and GCS stem are both available in 31.8 mm and 35 mm, so you can choose the standard that suits you and build a matching cockpit. Both are certified to EFBE TRI-TEST® GR, the gravity standard covering freeride and downhill use.

The GCS bar is 800 mm wide, with marks for cutting down to 720 mm, and comes in 20 mm or 40 mm rise with 7° backsweep and 5° upsweep. The GCS stem is 35 mm long with 0° rise and a 40 mm stack, and has a wide bar clamping area to hold the bar securely.

Fitting tips

Whichever clamp size you choose:

• Make sure the bar and stem are clean and free of grease in the clamp area, unless the manufacturer says otherwise.
• Centre the bar in the stem. The GCS stem has machined alignment marks on the body and faceplate to help.
• Tighten faceplate bolts gradually and evenly in a cross pattern, keeping the gap even top and bottom where the design calls for it, and always use a torque wrench set to the manufacturer's specified torque. PEMBREE recommends 9 Nm for both the steerer clamp bolts and the faceplate bolts on the GCS stem.
• Recheck bolts after your first ride.

Frequently asked questions

Is 35mm stiffer than 31.8mm?

Generally, yes. A larger-diameter centre section tends to be stiffer, which can make the front end feel more precise. Other factors such as material, wall thickness and design matter too.

Can I put a 31.8mm bar in a 35mm stem?

No. The clamp sizes must match. Using shims to fit a smaller bar in a larger stem isn't something we recommend for mountain bike handlebars.

Is 31.8mm outdated?

No. 31.8 mm is still widely used and well supported. It's a good choice for many trail riders and anyone who wants a slightly more forgiving feel.

Which clamp size are PEMBREE GCS bars and stems?

Both are available in 31.8 mm and 35 mm.

Do I need to change my stem if I change my bars?

Only if the clamp size changes. If your new bar uses the same clamp diameter as your current stem, you can keep the stem.

Build your cockpit with the GCS stem and GCS handlebars.