Handlebars × compression × clamp: slit, shim, and bar adapter explained

⚡ In short: handlebars need a slit for HIC and IHC.

Shim is a thin metal sleeve that compensates for the difference between standard handlebars (32 mm outer diameter) and an oversized clamp (35 mm). Bar adapter is an insert that lets you use handlebars with a slit on an SCS. The clamp is selected based on the handlebar’s outer diameter, and the handlebar based on its inner diameter where it fits onto the fork.

The tightening sequence is always compression → clamp, with the clamp screws tightened in a crisscross pattern. The notch should face backward toward the rider and be covered by the clamp; if the notch is longer than the clamp, this is not a defect.

🔺 Why are handlebars, compression, and the clamp connected?

The three parts that hold the front of the scooter together must match one another. Fork determines the compression system (SCS, HIC, or IHC; the older ICS system is no longer used). Handlebars are fitted onto the fork, so their inner diameter must match the fork steerer tube. The clamp grips the handlebars from the outside, so its inner diameter must match the outer diameter of the handlebars. Compression then defines the play in the headset (the bearings in which the fork rotates).

Handlebars are available in three size classes. We round the numbers; in product descriptions, you may sometimes come across more precise values, but they mean the same thing:

• Standard steel: 32/28 mm (outer / inner diameter).
• Oversized steel or titanium: 35/32 mm.
• Aluminum: 35/28 mm, meaning oversized on the outside and standard on the inside.

This article addresses only this triangle. You can find an overview of all parts (wheels, brakes, headsets) in the pillar Freestyle Scooter Part Compatibility: What Fits What.

✂️ What is a notch in handlebars, and why do some have one while others don't?

A slit (or slot) is a lengthwise cutout at the lower end of the handlebars, several centimeters long. Its purpose is simple: in HIC and IHC systems, the handlebars are secured to the scooter by the clamp, which tightens them directly onto the fork steerer tube. The tube couldn’t be tightened without a cutout; with the notch, it clamps by a few tenths of a millimeter and grips the post firmly.

At SCS, it works differently. An SCS clamp is a large part with four bolts, into which the fork is inserted from below and the handlebars from above; the clamp holds each of them separately, and the compression system is inside it. So the handlebars are not clamped onto the fork, but sit in the clamp. The notch would serve no purpose here and would also be harmful: the clamp would compress it, the pipe would deform at that point and eventually crack. That’s why handlebars without a slit are bought for SCS.

Notch orientation. The notch should face backward, toward the rider; on some handlebars it is on the side, which is also fine if that is how the manufacturer intended it. It is more important that it is under the clamp. The clamp should cover the notch, because the uncovered part of the notch is a weak point. However, manufacturers often make the slot longer than the clamp so that the handlebars (especially children’s handlebars) can be tightened more easily. So if a small part of the notch sticks out above the sleeve, it is not a defect. Just move the sleeve up as high as possible so that it covers as much as possible.

🧩 What is a shim and when do I need one?

A shim is a thin metal insert in the shape of a split tube that compensates for the difference between two diameters. In practice, you’ll encounter two different parts called shims:

• Shim for the clamp (32 → 35 mm). Used when you have standard handlebars (outer diameter 32 mm) and an oversized clamp or oversized SCS (inner diameter 35 mm). The shim is installed on the handlebars beneath the clamp and fills a 3 mm gap. Many oversized clamps come with it in the package. Without a shim, you can tighten the clamp, but the handlebars will rotate after a few jumps.
• HIC shim on the fork. Part of the HIC system: a metal shim fitted onto the fork steerer tube, which fills the space between the steerer and the inside of oversized handlebars (32 mm inner diameter). It is not an optional add-on; HIC does not work without it. It is usually included with the fork or handlebars.

When a shim won't help. The shim only addresses the difference between the handlebars and the clamp on the outside. It doesn’t account for the difference between handlebars and forks on the inside: if the handlebars have an inner diameter of 32 mm and the fork uses IHC (a narrower steerer for an inner diameter of 28 mm), no shim will connect them reliably. Oversized 35/32 steel handlebars do not belong on an IHC fork.

🔄 What is a bar adapter for SCS?

A bar adapter (SCS adapter, sometimes called a “sleeve”) is an insert used when you have slit handlebars and an SCS clamp. It is fitted to the underside of the handlebars and fills the slot so that the clamp cannot compress it. Many HIC/IHC handlebars come with an adapter included, because the manufacturer expects someone to use them with SCS. A bar adapter is the only proper way to connect handlebars with a slit to an SCS; cutting off the slit shortens the handlebars and voids the warranty.

Shims, adapters, and compression parts can be found in the compression and shims category, standard clamps in the clamps category, and SCS clamps in the SCS category.

🔩 Which clamp should you choose: 32 or 35 mm?

A clamp is a clamp that holds the handlebars on the fork. It is selected based on two things:

• Inner diameter = handlebar outer diameter. The standard clamp has a diameter of 32 mm; the oversized clamp has a diameter of 35 mm. A 35 mm clamp can also be used on standard handlebars with a shim; a 32 mm clamp cannot be fitted to oversized or aluminum handlebars at all.
• Type by compression. For SCS, you buy an SCS clamp, which has the compression built in. For HIC and IHC, you buy a standard clamp; compression on these systems is handled by a bolt at the top of the handlebars.

The number of clamp screws (2, 3, or 4) is not important. For handlebars with a slot, the clamp should cover as much of the slot as possible. Category clamps.

⚙️ In what order should you tighten them?

Always compression first, then the sleeve. Compression (the bolt under the cap at the top of the handlebars, inside the clamp on SCS systems) takes up headset play; tighten it a quarter turn at a time and find the sweet spot: the handlebars should have no play when rocked back and forth, but should still turn completely smoothly. Don’t overdo it. Only then align the handlebars with the front wheel and tighten the clamp: tighten the bolts gradually in a cross pattern (alternate between the two bolts, or, with four bolts, in the order 1, 3, 2, 4), a little at a time, until they are all equally tight.

If you tighten the clamp first, you’ll lock the handlebars and fork in a position with play, and the compression bolt won’t be able to adjust it afterward. The scooter keeps knocking, no matter how much you tighten it. A detailed procedure for each system is in the article How to Tighten the Compression and Handlebars.

📋 I have X handlebars and Y compression: what do I need?

Find your handlebars in the row and your compression (according to the fork) in the column.

The old ICS system (a bolt underneath the fork, handlebars with an inner diameter of 28 mm, and a star nut inside) is no longer used today; you’ll only find it on old scooters. A shortcut for parents: IHC = bars with a 28 mm inner diameter only (standard steel or aluminum). HIC = oversized steel 35/32 only. SCS = any, but without a notch. The guide How to Choose Handlebars covers choosing specific handlebars (height, width, material), and How to Choose Compression Systems covers all systems.

⚠️ Most common mistakes

• Using handlebars with a notch in an SCS without an adapter. The clamp crushes the handlebars at the notch, causing them to deform and crack. Always a bar adapter.
• Standard handlebars in an oversized clamp without a shim. It lasts a week, then it starts spinning and the sleeve gets damaged.
• Oversized steel handlebars on an IHC fork. It won't work, don't buy them unless you have an HIC or SCS fork.
• Aluminum handlebars for HIC. The outer diameter fits, but the inner one doesn’t. Aluminum belongs in the IHC or SCS.
• Collar tightened before compression. The scooter is knocking, and the compression bolt does nothing to fix it.
• Notch facing forward. Turn the handlebars so that the notch faces the rider (or sideways if that is how they are designed).
• Cutting or trimming the notch at home. Crooked cut, sharp corners, shorter handlebars, no warranty.

❓ FAQ