1D vs 1.5D vs 2D Bends: Centreline Radius (CLR) Explained
Pipe elbows and mandrel bends are sold by centreline radius (centerline radius, or CLR), usually written as 1D, 1.5D or 2D. The catch is that "D" means one thing on a butt-weld pipe elbow and another on mandrel-bent tube. Mix them up and a manifold that fitted on paper ends up in the wrong place. Here is what CLR is, the real numbers in millimetres, and how to choose.
What centreline radius (CLR) means
Picture a line running down the middle of the pipe. Through a bend, that centreline follows an arc, and the radius of that arc is the CLR. The inside of the bend (the throat) has a radius of CLR minus half the OD; the outside (the heel) is CLR plus half the OD.
A 1.5D bend has a CLR of 1.5 × D. The real question is what D is.
What does the D mean? Pipe elbows vs mandrel-bent tube
Butt-weld pipe elbows: D is the nominal pipe size
For butt-weld elbows to ASME B16.9, the radius comes from the nominal pipe size (NPS), not the outside diameter. A long radius (LR) elbow has a CLR of 1.5 × NPS in inches, hence "1.5D". A short radius (SR) elbow is 1 × NPS, or "1D".
| NPS | Pipe OD | SR (1D) CLR | LR (1.5D) CLR | LR CLR ÷ pipe OD |
|---|---|---|---|---|
| 1 | 33.4 mm | 25.4 mm (1") | 38.1 mm (1.5") | 1.14 |
| 1¼ | 42.2 mm | 31.8 mm (1.25") | 47.6 mm (1.875") | 1.13 |
| 1½ | 48.3 mm | 38.1 mm (1.5") | 57.2 mm (2.25") | 1.18 |
| 2 | 60.3 mm | 50.8 mm (2") | 76.2 mm (3") | 1.26 |
| 2½ | 73.0 mm | 63.5 mm (2.5") | 95.3 mm (3.75") | 1.30 |
| 3 | 88.9 mm | 76.2 mm (3") | 114.3 mm (4.5") | 1.29 |
On a 90° elbow the CLR is also the centre-to-end dimension. Catalogues often round to the nearest millimetre (48 mm rather than 47.6 mm).
Small-bore pipe is bigger than its nominal size, so look at the last column. A "1.5D" elbow here is only 1.1 to 1.3 times the real OD, and a short radius elbow's CLR is smaller than the pipe itself.
Mandrel-bent tube: D is the actual OD
Exhaust tube is sized by its real outside diameter, and mandrel bend radii follow suit: 2" tube at 1.5D has a 3" (76.2 mm) CLR. Many suppliers quote CLR as a dimension, so divide it by the tube OD to get the D.
| Tube OD | 1D CLR | 1.5D CLR | 2D CLR |
|---|---|---|---|
| 1.75" (44.45 mm) | 44.45 mm (1.75") | 66.7 mm (2.625") | 88.9 mm (3.5") |
| 2" (50.8 mm) | 50.8 mm (2") | 76.2 mm (3") | 101.6 mm (4") |
| 2.5" (63.5 mm) | 63.5 mm (2.5") | 95.3 mm (3.75") | 127.0 mm (5") |
| 3" (76.2 mm) | 76.2 mm (3") | 114.3 mm (4.5") | 152.4 mm (6") |
Why this catches people out
2.5" tube at 1.5D and an NPS 2½ LR elbow both have a 95.3 mm CLR, because both are 1.5 × 2.5". But the elbow sits on 73.0 mm pipe and the tube is 63.5 mm, so the elbow is much tighter for its size.
It works the other way too. NPS 3 pipe and 3.5" tube are both 88.9 mm OD, but an NPS 3 LR elbow is 114.3 mm CLR and 3.5" tube at 1.5D is 133.4 mm. Same OD, same label, different bend.
Never compare bends by the D number alone. Compare CLR in millimetres. For more on the two sizing systems, see SCH10S pipe vs OD tube sizes.
1D vs 1.5D vs 2D: the trade-offs
A tighter bend takes less room but turns the flow harder. A wider bend is gentler but needs more space.
| 1D | 1.5D | 2D | |
|---|---|---|---|
| Space needed | Least | Moderate | Most |
| Flow | Slightly more restrictive | Good all-rounder | Gentlest |
| Torch access at joints | Tightest | Easier | Easiest |
| Typical use | Compact turbo manifolds | General manifolds and headers | Long primaries, downpipes, show work |
1D turns a corner in the least space, so it suits compact turbo manifolds where primaries must leave the head and tuck under the turbo quickly. Where two tight bends meet, the throat closes in on the joint, leaving less room for a TIG cup and filler rod. On mandrel-bent tube, a tight bend also stretches the heel more, thinning the wall there.
1.5D is the all-rounder. Long radius is the default butt-weld elbow and 1.5D is a widely stocked mandrel bend radius, so it is usually the easiest to buy.
2D gives long, sweeping bends that are gentler on flow and look good, but it needs noticeably more room. On 1.75" tube, the centre-to-end of a 2D 90° is 88.9 mm, twice the 1D figure.
Useful bend geometry
- Arc length at the centreline = CLR × angle in radians. Radians = degrees × π ÷ 180, so a 90° arc is 1.571 × CLR.
- Centre-to-end of a 90° arc = CLR, measured from the face of one end to the centreline of the other, on a bend with no straight legs.
Worked example: 2" tube at 1.5D
CLR = 1.5 × 50.8 = 76.2 mm. Half the OD is 25.4 mm.
- Centreline length of a 90°: 76.2 × 1.571 = 119.7 mm.
- Throat (inside): (76.2 − 25.4) × 1.571 = 79.8 mm.
- Heel (outside): (76.2 + 25.4) × 1.571 = 159.6 mm.
- Centre-to-end: 76.2 mm each way.
A 45° piece of the same bend is 59.8 mm along the centreline, and a 30° piece is 39.9 mm.
This matters when balancing primary lengths. Measured to the corner along both legs, that 90° looks like 152.4 mm. Along the centreline it is 119.7 mm, 32.7 mm shorter. Count the arc, not the corner.
Cutting smaller angles from a 90° or 180° bend
Headers and manifolds are mostly built from pieces: 15°, 30°, 45°. Taking several angles out of one 180° bend wastes less than buying a bend for each. But mandrel bends are expensive, and cutting one badly wastes the whole piece.
Why the cut must be square to the tangent
Each cut has to run along a line through the bend's centre point, which puts it square to the tangent at that spot. Cut like that and the end is a true circle that matches the next piece.
Cut at a slant and the end comes out oval. You get a step on one side and a gap on the other, and the piece is no longer the angle you wanted. If the cut also leans out of the plane of the bend, the next piece comes off twisted.
Marking this by eye is hard: there is no flat face for a square, and the heel and throat are different lengths.
Allow for the blade too. On 2" tube at 1.5D, every 1 mm of kerf costs about 0.75° of arc (1 ÷ 76.2 = 0.013 rad), so two 45s cut from one 90° each come out slightly short. If both must be exact, start with more bend.
Wear eye protection when cutting, and use fume extraction when welding stainless.
Our mandrel bend marking guide is a clamshell that clamps over the bend, with magnets holding the halves together, so you can mark the full 360°. It is marked every 7.5°, covering 15°, 30°, 45°, 60° and 90°, and the marks keep the cut square to the tangent. It sits against the end of the bend, so the angles are counted from the end face.
How to choose a radius
- Packaging first. Mock up the tight spots: head flange, turbo flange, wastegate, chassis rail, steering shaft. Use 1D where a wider bend will not clear, not everywhere.
- Then flow. Where there is room, use 1.5D or 2D.
- Check availability. Make sure you can buy the radius in your size, wall and material. On pipe, LR is the standard elbow. On tube, check the supplier's CLR in millimetres. If nobody sells the radius you need, pie cut one from straight tube.
- Match your mockup to your metal. A mockup only tells the truth if its bends have the same CLR as the ones you will weld. A route mocked in 1.5D and built in 1D ends up somewhere else.
Bendlabs mockup blocks follow the same conventions as the metal. The 1.75" tube blocks use the OD: 1D is 44.45 mm, 1.5D is 66.7 mm and 2D is 88.9 mm. The SCH10S pipe blocks follow the nominal size, like B16.9 elbows, so 1.25" NPS blocks are 31.8 mm (1D) and 47.6 mm (1.5D), and 1.5" NPS blocks are 38.1 mm (1D) and 57.2 mm (1.5D), the same as standard short and long radius elbows.
The block angles (30°, 45°, 90°) are all multiples of 7.5°, so a 1D or 1.5D tube design can be marked out on real bends with the marking guide. For the full process, see how to mock up a turbo manifold.
Tools that help
- Mandrel bend angle marking guide. For OD tube from 1.5" to 4.5", in 1D, 1.2D and 1.5D CLR, where D is the tube OD. Order the size and radius that match your bends; 2D isn't available. On a 180° bend, mark the first 90°, then move the guide along to mark the second. It also works on bends with straight legs.
- Mockup block add-on packs. 30°, 45° and 90° bends in packs of 5, 10 or 20. SCH10S packs (1.25" and 1.5" NPS) come in 1D and 1.5D; 1.75" tube packs in 1D, 1.5D and 2D. Top up a kit with the radius you plan to buy.
- Variable mitre jig for 1.5D elbows. For 1.5D mandrel bends in OD tube, 1.5" to 4", where D is the tube OD, so it won't fit SCH10S weld elbows. Two magnetic halves clamp round the bend. Set the removable angle key anywhere on the 10° to 80° scale and it acts as a stop to butt the tube against, so every mitre (miter) comes out at the same angle.
FAQ
What does 1.5D mean on a bend?
The CLR is 1.5 times D. On mandrel-bent tube, D is the OD, so 2" tube at 1.5D has a 76.2 mm CLR. On a butt-weld elbow, D is the nominal pipe size, so an NPS 2 LR elbow is also 76.2 mm, even though the pipe is 60.3 mm OD.
What is the difference between a short radius and long radius elbow?
A long radius elbow has a CLR of 1.5 × NPS; a short radius elbow is 1 × NPS. SR takes less room, LR is gentler and is the default.
How do I measure the CLR of a bend?
Stand it on one end on a flat bench, measure to the top of the other end and subtract half the OD. On a pure 90° arc, that is the CLR. Subtract any straight leg on the end it stands on.
Is 1D or 1.5D better for a turbo manifold?
Usually a mix. 1D gets primaries off the head and round the turbo; 1.5D flows a little better on open sections. Use the tightest radius only where the space demands it.