Boost Spacing

Summary

Boost is a mountain-bike hub-spacing standard that uses 110 mm front and 148 mm rear hub widths with thru-axles. The additional width gives hub designers more room to position the spoke flanges, potentially improving wheel bracing angles, while shifting the cassette, rotor, and drivetrain interfaces outward. Boost appeared on 2015-model mountain bikes and became standard across XC, trail, enduro, and e-MTB categories. It affects frames, forks, hubs, wheels, chainline, and axle compatibility but does not by itself guarantee a stiffer frame or stronger wheel.

Quick Facts

Introduced: Announced in 2014 for 2015-model bikes
Category: Hub and frame standard
Developed with: Trek, SRAM, and RockShox
Front standard: 15 × 110 mm
Rear standard: 12 × 148 mm
Earlier MTB standards: 15 × 100 mm front and 12 × 142 mm rear
Original Boost chainline: Commonly 52 mm instead of 49 mm
Primary applications: XC, trail, enduro, downhill-derived platforms, and e-MTBs
Mainstream gravel standard: Usually 12 × 100 and 12 × 142 mm, not Boost

Overview

Boost widens a conventional mountain-bike front hub by 10 mm and a rear hub by 6 mm. The extra width is divided across both sides of the hub:

• Front: approximately 5 mm per side
• Rear: approximately 3 mm per side

This creates room to move the spoke flanges farther from the wheel centerline. A wider flange position increases spoke bracing angle, which can improve lateral wheel stiffness and reduce the compromises involved in building large-diameter wheels.

Boost 148 first gained attention on Trek’s 2015-model 29-inch bikes, announced in 2014, and was supported by SRAM and RockShox components. It quickly spread through the mountain-bike industry as 29-inch wheels, wider tires, 1× drivetrains, and more demanding riding increased wheel and frame-design requirements.

The standard is now common on nearly every mountain-bike category, including modern XC race bikes. Its adoption in conventional gravel bikes is much more limited because most gravel frames continue to use road-derived 12 × 100 and 12 × 142 mm spacing.

What Boost Standardizes

Rear: 12 × 148 mm

A Boost rear hub measures 148 mm across its over-locknut or end-cap surfaces and uses a 12 mm thru-axle. Compared with a 142 mm hub, the cassette and brake rotor positions move outward by approximately 3 mm.

The wider shell gives hub designers additional room to move the flanges outward. The actual flange spacing varies between hub models, so two Boost hubs do not necessarily produce identical spoke angles or wheel stiffness.

Front: 15 × 110 mm

A standard Boost front hub measures 110 mm wide and uses a 15 mm axle. The brake rotor position is approximately 5 mm farther outward than on a 15 × 100 mm hub.

The axle diameter matters. Older downhill hubs also used 110 mm spacing but commonly had 20 mm axles. A 20 × 110 mm hub is not automatically compatible with a 15 × 110 mm Boost fork.

Chainline

The original Boost drivetrain arrangement moved the chainring approximately 3 mm outward, commonly changing MTB chainline from about 49 to 52 mm. On many SRAM direct-mount cranks, this was achieved by changing from a 6 mm-offset ring to a 3 mm-offset ring.

That relationship is no longer universal. Modern 55 mm chainlines, SRAM Transmission, Shimano wide-chainline cranks, Super Boost systems, and frame-specific layouts use other combinations of spindle width and chainring offset.

Boost rear spacing therefore does not identify the correct crank or chainring by itself. The frame and drivetrain manufacturer’s specified chainline remains the controlling measurement.

Why It Exists

Larger wheels place the rim farther from the hub, which can make it harder to produce a laterally stiff wheel without adding spokes, rim material, or spoke tension. Moving the hub flanges outward improves the spoke bracing geometry available to the wheel builder.

The wider rear interface also gives frame designers more packaging space around the cassette, tire, chainring, and chainstays. Depending on the frame, that space may be used for:

• Wider tire clearance
• Different chainstay shapes
• Shorter rear-center dimensions
• Increased pivot or motor clearance
• Greater room for frame reinforcement

These are design opportunities, not automatic Boost features. A Boost frame does not necessarily have shorter stays, wider tire clearance, or greater stiffness than a well-designed 142 mm frame.

Rider Experience

Most riders do not feel hub spacing as an isolated feature. Any difference is experienced through the complete wheel and frame.

Heavier or more aggressive riders may benefit from improved wheel stability, especially with 29-inch wheels. However, rim construction, spoke count, lacing pattern, spoke tension, hub-flange dimensions, and wheel-building quality can influence stiffness and durability more than the Boost label alone.

Boost does not inherently change suspension behavior, steering geometry, or ride comfort. It also does not necessarily increase crank Q-factor because many cranksets achieve the required chainline through chainring offset rather than wider crankarm placement.

Mechanic’s Perspective

Boost-related compatibility problems commonly appear during wheel, fork, crank, and chainring replacements.

Before ordering parts, verify:

• Front and rear hub width
• Axle diameter
• Axle length and thread pitch
• Fork and frame spacing
• Rotor position
• Freehub and cassette interface
• Required chainline
• Chainring offset
• Hub end-cap type
• Torque Cap compatibility, where applicable

“12 × 148” describes the hub interface but not the complete thru-axle. Frames use different axle lengths, thread lengths, thread pitches, heads, and retention systems. A rear axle from one Boost frame may not fit another.

The same applies to front axles. Fork manufacturers use different thread specifications even when both forks accept 15 × 110 mm hubs.

Torque Caps

RockShox Torque Caps use oversized hub end caps that contact a larger surface inside compatible fork dropouts. They are separate from the Boost standard.

A Boost fork does not automatically require Torque Caps, and a standard 15 × 110 mm hub is commonly usable in a Torque Cap-compatible fork unless the fork manufacturer states otherwise. Torque Caps may improve dropout contact, but they do not change the hub’s 110 mm spacing.

Hub Conversions

Some hubs can be converted between non-Boost and Boost spacing with manufacturer-approved axles or end caps. Others cannot.

Aftermarket conversion kits may add end-cap and rotor spacing, but they do not move the original hub flanges. Such a conversion can make a wheel fit while providing none of the spoke-bracing benefit of a purpose-built Boost shell.

Rear conversions may also require rotor spacers, cassette-position corrections, or wheel redishing. Compatibility must be evaluated for the specific hub rather than assumed from its replaceable end caps.

Maintenance Notes

Boost hubs require the same routine service as other thru-axle hubs:

• Inspect and adjust hub bearings
• Service the freehub mechanism
• Check axle and end-cap condition
• Tighten thru-axles to specification
• Monitor spoke tension and wheel dish
• Inspect rotor and cassette lockring security
• Clean and grease axle threads where specified

After replacing a hub, end cap, or conversion component, verify brake-caliper alignment and rear shifting before riding.

Advantages

• More space for wider hub-flange placement
• Potentially improved spoke bracing angles
• Useful for 29-inch and high-load wheels
• Additional frame-design and tire-clearance options
• Broad modern MTB component availability
• Well established across current mountain bikes

Engineering Trade-Offs

Boost introduced another set of incompatible frames, forks, hubs, cranks, chainrings, and axles. Existing wheels could not normally transfer directly to new Boost bikes.

The wider rear chainline can also increase chain angle in the largest cassette sprocket if the complete drivetrain is not properly matched. More width is not automatically better; chainline must balance tire clearance with drivetrain alignment.

Boost also adds no benefit when a converted hub retains its original flange spacing. In those cases, the wider end caps provide fitment without the wheel-geometry advantage that motivated the standard.

Standards Comparison

The front axle diameter must always be included. A listing that states only “110 mm front” is incomplete.

Boost vs. Super Boost Plus

Super Boost Plus uses a 157 mm rear hub, borrowing the overall width long associated with downhill bikes but repositioning components to support wider flange spacing and a wider drivetrain chainline.

It can provide additional wheel and frame-design space, but its component selection is smaller than Boost 148. A 157 mm downhill hub and a Super Boost Plus hub may share overall width without having identical flange, cassette, or frame-design intentions.

Buying Considerations

Boost is relevant when purchasing a frame, fork, wheel, hub, crankset, chainring, or replacement axle. Confirm the entire specification rather than relying on the word “Boost.”

For riders maintaining a non-Boost bike, hub spacing alone is rarely a reason to replace an otherwise suitable frame or fork. A properly built 100/142 wheel can remain durable and precise. Boost matters most when compatibility is required or when a frame and wheel are designed together to use the wider architecture.

Mainstream gravel buyers should not assume that large tire clearance means Boost spacing. Most gravel wheels and frames still use 12 × 100 and 12 × 142 mm interfaces, even when equipped with MTB-derived cassettes.

Common Questions

Can a Boost wheel fit a non-Boost frame?

Normally, no. A 148 mm rear hub is too wide for a 142 mm frame, and a 110 mm front hub is too wide for a 100 mm fork.

Can a non-Boost hub be converted?

Some can, using manufacturer-approved end caps or axle assemblies. Conversion does not necessarily reproduce the flange spacing of a purpose-built Boost hub.

Does Boost require a special crankset?

It requires the correct chainline, but not always different crankarms. Many systems use the same crank with a different chainring offset.

Does Boost make a bike stiffer?

Not automatically. It can improve wheel bracing geometry and give frame designers more space, but actual stiffness depends on the hub, wheel build, frame structure, and materials.

Is Boost common on gravel bikes?

Not on mainstream gravel bikes. Most use 12 × 100 front and 12 × 142 rear spacing. Boost or Road Boost appears on selected adventure and electric-gravel platforms.

Related Topics

Hub Spacing
Thru-Axle
Chainline
Direct-Mount Chainring
Super Boost Plus
Torque Caps
Wheel Dish
Spoke Bracing Angle

References

SRAM MTB Hub Build Specifications
SRAM MTB Frame-Fit Specifications
SRAM Chainring and Chainline Manual
SRAM Boost Chainring Offset Guidance
DT Swiss/SRAM Hub and Wheel Dimensions
Shimano 148 × 12 MTB Freehub
Shimano 100 × 12 Road/Gravel Front Hub