Asymmetric Rim Spoke Offset: What It Means and How to Measure It

Asymmetric rim spoke offset is the sideways distance, in millimetres, between the rim centerline and the line of the spoke holes. An asymmetric rim moves its spoke holes toward one side of the rim. That makes the spoke angles on the two sides of the wheel more equal, which reduces the tension difference between left and right spokes in dished wheels.

The offset is not printed on the tire. The tire sidewall gives you the bead seat diameter, which tells you which rim size the tire fits. Spoke length depends on two different rim dimensions: the effective rim diameter (ERD) and the asymmetric offset. Both come from the rim specification or from your own measurement. This article starts with the tire numbers, then explains how the rim dimensions that set spoke length relate to them.

What do the numbers on a bicycle tire sidewall mean?

A bicycle tire sidewall usually carries one or more size designations and several ratings. The size numbers are dimensions in millimetres or inches. Pressure figures are performance limits. Thread counts and tubeless labels describe construction. Only the size numbers relate to wheel geometry.

ETRTO size: width and bead seat diameter

The ETRTO (ISO) designation is the most precise size marking. It has the form width-bead seat diameter, both in millimetres. In 37-622, 37 mm is the nominal tire width and 622 mm is the bead seat diameter (BSD). BSD is the diameter of the rim shelf where the tire bead sits. A tire fits a rim only if their BSDs match. Common BSDs include 622 mm (700C and 29-inch), 584 mm (650B and 27.5-inch), 559 mm (26-inch mountain bike) and 406 mm (20-inch BMX/folding).

Inch and French size markings

Traditional size names are labels, not exact measurements.

• French sizes such as 700x35C give an approximate overall diameter in millimetres (700) and a nominal width (35 mm). The letter was originally a width class. Today 700C simply means a 622 mm BSD.
• Decimal inch sizes such as 29x2.20 give an approximate outer diameter class (29 inches) and a nominal width in inches (2.20 in, about 56 mm).
• Fractional inch sizes such as 28 x 1 5/8 x 1 3/8 come from older systems. They can refer to different BSDs in different regions. Check the ETRTO number before buying.

Two tires with different inch names can share a BSD. Two tires with similar-looking inch names can have different BSDs. The ETRTO number settles the question.

Ratings and construction markings

Several markings describe how the tire may be used, not its size.

• Pressure range is printed in psi, bar or kPa. It gives the minimum and maximum inflation the tire maker approves. Your rim may set a lower maximum, especially a hookless rim.
• TPI (threads per inch) describes how dense the casing fabric is.
• Tubeless Ready, TLR or similar labels show that the tire is designed to be used without an inner tube.
• Rotation arrows show the intended rolling direction.
• Some e-bike tires carry an additional approval marking with a speed figure for higher-speed use.

How do width and rim diameter change tire height, diameter and circumference?

For bicycle tires, overall diameter is roughly the bead seat diameter plus twice the tire's section height. Circumference is that diameter multiplied by π (about 3.1416).

Bicycle tires do not print an aspect ratio the way car tires do. As a rough working assumption, the section height of a typical bicycle tire is close to its width. Actual height varies with the tire's design and with the rim it is mounted on.

Simple calculation steps

1. Read the ETRTO size, for example 37-622.
2. Estimate section height as roughly equal to width: about 37 mm.
3. Add twice the height to the BSD: 622 + 2 x 37 = 696 mm overall diameter.
4. Multiply by π: 696 x 3.1416 ≈ 2187 mm circumference.

The table shows two useful patterns. On the same 622 mm rim, going from 28 mm to 37 mm adds about 18 mm of diameter and about 57 mm of circumference. A wide tire on a smaller rim can also come out close to a narrower tire on a larger rim: 55-584 and 37-622 give nearly the same calculated diameter.

Calculated geometry versus actual size and fitment

These figures are estimates, not measurements. A tire's real width and height depend on rim internal width, inflation pressure, casing construction and wear. To get an accurate circumference for a bike computer, measure the rollout: mark the tire and the floor, roll the bike one full revolution with the rider seated, then measure the distance.

Fitment depends on frame and fork clearance, brake type and the rim's approved tire widths. Those are separate checks. A matching BSD guarantees only that the tire bead and the rim shelf are the same diameter.

Which sidewall number matters for spoke length?

No tire number sets spoke length directly. The BSD tells you which rim diameter family you are working with. Spoke length depends on the rim's effective rim diameter (ERD) and, for asymmetric rims, its spoke offset.

ERD is the diameter measured at the points where the spoke nipples seat inside the rim. Two 622 mm rims can have very different ERDs: a deep rim has a much smaller ERD than a shallow one, so it needs shorter spokes. Changing tire width or tire model never changes the spokes you need. Changing the rim always requires you to check them again.

To measure ERD, thread a nipple onto each of two spokes. Place the spokes through holes directly opposite each other, seat the nipples as they will sit in the built wheel, and measure across the rim between known reference points on the spokes. Then add back the lengths you did not measure. Use the rim maker's ERD if it is published and you trust it. Otherwise measure it.

What is asymmetric rim spoke offset?

Asymmetric rim spoke offset is the lateral distance, in millimetres, from the rim centerline to the plane of the spoke holes. On a symmetric rim the holes sit on the centerline and the offset is zero. On an asymmetric rim the spoke bed is moved to one side, creating an offset spoke bed.

The offset exists to solve a dish problem. On a rear wheel, the cassette pushes the drive-side hub flange closer to the wheel's center than the non-drive-side flange. On a front disc wheel, the rotor does the same to the disc-side flange. To keep the rim centered between the locknuts, the spokes on the short side run at a steeper angle and must carry much more tension than the spokes on the other side.

Moving the spoke holes toward the far side lengthens the effective flange-to-center distance on the steep side and shortens it on the other. This makes the bracing angles more equal and narrows the tension gap. Published offsets are usually a few millimetres. Use the figure from your rim's specification or your own measurement.

Asymmetric profile versus alternating offset drilling

Two different features are sometimes both called rim offset.

• Asymmetric profile: every spoke hole is moved toward the same side of the rim. This is the asymmetric rim spoke offset discussed here.
• Alternating offset drilling: holes alternate slightly left and right of center so that each spoke points toward its own flange. This improves the spoke line at the nipple. It does not shift the whole spoke bed to one side.

Spoke length calculators may have a field for either feature, and sign conventions vary. Read what the calculator's offset field actually expects before you enter a value.

How do you measure asymmetric rim spoke offset?

To find the offset, measure from each outer edge of the rim to the center of a spoke hole, then halve the difference. A vernier or digital caliper reading in millimetres is enough.

1. Clean the rim and choose a spoke hole away from the valve hole and the rim joint.
2. Measure from one outer rim edge (sidewall or bead hook edge) to the center of the spoke hole. Call this distance A.
3. Measure from the opposite outer rim edge to the center of the same hole. Call this distance B.
4. Calculate the offset: offset = (A − B) / 2. The holes are shifted toward the side with the smaller measurement.
5. Repeat on several holes around the rim and average the results to reduce measurement error.
6. Confirm: A + B should equal the rim's outer width.

Worked example with illustrative numbers: on a rim with an outer width of 21 mm, A = 13.5 mm and B = 7.5 mm. The offset is (13.5 − 7.5) / 2 = 3 mm. The spoke holes sit 3 mm from the centerline, toward side B.

Measure to the center of the hole, not its edge. On rims with nipple washers or angled drilling, measure on the inner spoke bed where the nipple seats if you can reach it. That is the point that matters for spoke geometry.

How does offset change left and right spoke lengths?

Spoke offset changes left and right spoke lengths by changing the effective flange-to-center distance on each side. On the side the holes move away from, the effective distance increases by the offset. On the side they move toward, it decreases by the offset.

The standard spoke length formula is:

L = √(W² + r_f² + r_e² − 2 · r_f · r_e · cos(360° · X / (N/2))) − h/2

• W is the flange-to-center distance, adjusted for offset.
• r_f is half the flange spoke-hole circle diameter.
• r_e is half the ERD.
• X is the number of crosses.
• N is the total spoke count.
• h is the flange hole diameter.

For a rear wheel with holes offset toward the non-drive side:

• Effective drive-side distance = drive-side flange-to-center + offset
• Effective non-drive-side distance = non-drive-side flange-to-center − offset

The length change is small, because W is small compared with the rim radius. As an illustration, take spokes about 290 mm long, a drive-side distance of 18 mm, a non-drive-side distance of 36 mm and a 3 mm offset. The drive-side spokes become about 0.2 mm longer and the non-drive-side spokes about 0.4 mm shorter. Small as they are, these changes can decide which side of a rounding step you land on. Enter the offset in your calculation instead of ignoring it.

Some calculators apply an asymmetric rim offset automatically from a dedicated field. Others expect you to adjust the flange distances yourself. Do one or the other, never both, or the offset will be counted twice.

How does offset affect spoke tension balance and wheel dish?

Offset matters much more for spoke tension balance than for spoke length. In a dished wheel, the sideways pulls of the two sides must cancel. Each side's sideways pull is roughly its tension multiplied by W / L. So the tension ratio between the low-tension side and the high-tension side is close to the ratio of the effective flange-to-center distances.

These numbers show the principle, not the specification of any particular hub. A higher ratio means the low-tension spokes stay tighter for the same maximum tension on the high-tension side. That can help them stay tensioned under load.

An asymmetric rim does not remove the need to dish the wheel. The rim must still sit centered between the hub locknuts. Dish gauges and truing stands reference the rim's outer edges, not the spoke holes, so you check dish the same way as on a symmetric rim. The offset changes how hard the spokes on each side must pull to hold the rim in that centered position.

Which way should an asymmetric rim face?

Install an asymmetric rim with the spoke holes offset away from the steeply angled side of the wheel.

• Rear wheel: holes offset toward the non-drive side.
• Front disc wheel: holes offset toward the side without the rotor.

Fitting the rim the wrong way round makes the tension imbalance worse, not better. It also invalidates spoke lengths calculated for the correct orientation. Confirm the direction with a caliper measurement before lacing. Rim decals may not reliably indicate orientation.

What asymmetric offset numbers can and cannot tell you

The offset figure tells you how far the spoke bed sits from the rim centerline. Combined with ERD and hub dimensions, it lets you calculate left and right spoke lengths and estimate the tension balance.

It does not tell you:

• which tires fit (that is the BSD and the rim's approved width range)
• the tire's actual diameter
• the maximum spoke tension the rim tolerates
• whether the finished wheel will be strong enough for its use

Calculated spoke lengths and tension ratios are geometric estimates. Final tension should be measured with a tension meter and set to the rim and spoke makers' limits.