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+--- Thread: 30mm vs 30.4mm Wheels & Tires (/thread-29084.html)
30mm vs 30.4mm Wheels & Tires - tom alphin - 2025-09-21
There are a number of wheels with an outer diameter of around 30mm, but they are very inconsistently described as 30mm or 30.4mm across different websites. I noticed that LDraw is the most consistent here, using 30 (mm) for all four of these parts. (Within MLCad they appear to be exactly the same width.)
1. Are they actually the same size, or are they actually measurably different?
2. Even if they are ever-so-slightly different, is 0.4mm so small that almost nobody cares?
3. Does LDraw prefer technically accurate measurements, or is it generally preferred to nudge measurements ever-so-slightly to make the system work better together?
I would like to make my part naming both technically correct and consistent.
---Tom
LINK TO RELEVANT PARTS: https://brickarchitect.com/parts/category-144?&retired=1&partstyle=1
P.S. Here are dumps of the part names across the relevant sites:
56904 (14mm wide)
- Brick Architect Long: 30mm D. × 14mm Wheel w/ Technic Axle Hole
- Brick Architect Short: 30mm D. × 14mm, Axle
- Bricklink: Wheel 30mm D. x 14mm
- Rebrickable: Wheel 30 x 14
- Brickset / LEGO.com: RIM NARROW Ø 30/14 W/ CROSS
- LDraw: Wheel Rim 14 x 30 with 6 Spokes and No Pegholes
42716 (20mm wide)
- Brick Architect Long: 30.4mm D. × 20mm Wheel w/ Technic Axle Hole, 5-Angled Spokes
- Brick Architect Short: 30.4mm D. × 20mm, Axle, 5-Spokes
- Bricklink: Wheel 30.4mm D. x 20mm with No Pin Holes and 5 Large Spokes
- Rebrickable: Wheel 30 x 20 5-Spoke
- Brickset / LEGO.com: RIM WIDE DIA.30X20 W/ CROSS HOLE, NO. 1
- LDraw: Wheel Rim 20 x 30 "Torq Thrust" with 5 Spokes and External Ribs
66155
- Brick Architect Long: 30.4mm D. × 20mm Wheel w/ Technic Axle Hole, Angled Spokes
- Brick Architect Short: 30.4mm D. × 20mm, Axle
- Bricklink: Wheel 30.4mm D. x 20mm with Center Axle Holes Motorcycle
- Rebrickable: Wheel 30 x 20 with Center Axle Holes
- Brickset / LEGO.com: RIM, WIDE, DIA. 30X20, W/ CROSS HOLE
- LDraw: Wheel Rim 30 x 20 with 3 Dual Angled Spokes and 4L Hub
RE: 30mm vs 30.4mm Wheels & Tires - Rene Rechthaler - 2025-09-21
your rim list of that family misses 44292 (oldest one), 7301 (newest one, Shelby Cobra), 54087 (variant of 56145) and 5650 (Countach)
tyre list misses 44309 (old) and 6620 (Bronco, not on the PT yet)
RE: 30mm vs 30.4mm Wheels & Tires - Magnus Forsberg - 2025-09-21
It looks like our description of 66155 is wrong. The dimensions are reversed.
Rims should always have it in (width) x (diameter) in millimeter. Not including the hub/centre.
RE: 30mm vs 30.4mm Wheels & Tires - Magnus Forsberg - 2025-09-21
(2025-09-21, 9:35)Magnus Forsberg Wrote: It looks like our description of 66155 is wrong. The dimensions are reversed.
Updated, corrected files uploaded to the tracker.
RE: 30mm vs 30.4mm Wheels & Tires - tom alphin - 2025-09-22
(2025-09-21, 8:22)Rene Rechthaler Wrote: your rim list of that family misses 44292 (oldest one), 7301 (newest one, Shelby Cobra), 54087 (variant of 56145) and 5650 (Countach)
tyre list misses 44309 (old) and 6620 (Bronco, not on the PT yet)
Rene,
My guide doesn't aim to be a complete catalogue, but rather to provide an easy-to-use visual guide to the most common current parts (top 2000 current parts). I am also aiming to include the top 2000+ parts of all time, which includes a bunch of common retired parts. (They are hidden by default though.)
That said, I am adding parts to the database all the time, and less common parts show up eventually if they are used in multiple sets.
---tom
RE: 30mm vs 30.4mm Wheels & Tires - tom alphin - 2025-09-22
(2025-09-21, 9:35)Magnus Forsberg Wrote: It looks like our description of 66155 is wrong. The dimensions are reversed.
Rims should always have it in (width) x (diameter) in millimeter. Not including the hub/centre.
I'm glad this thread helped the group in an unexpected way, too :-)
RE: 30mm vs 30.4mm Wheels & Tires - tom alphin - 2025-09-22
Does anyone happen to have any insight into the original question - whether both "30mm" and "30.4mm" wheels are actually the same diameter or not?
RE: 30mm vs 30.4mm Wheels & Tires - Magnus Forsberg - 2025-09-22
(2025-09-22, 5:57)tom alphin Wrote: Does anyone happen to have any insight into the original question - whether both "30mm" and "30.4mm" wheels are actually the same diameter or not?
I think one reason might come from how we choose to describe these.
All ldraw description should be the same. Rim (width) x (diameter) in millimeter. The rim and the tyre share a common size, here it is "30".
They belong together.
If I check all of these in ldu they all measure 76 ldu, which translate to 30.4 mm
And then we round it of to the nearest integer. No decimals, 30mm
RE: 30mm vs 30.4mm Wheels & Tires - tom alphin - 2025-09-22
(2025-09-22, 15:03)Magnus Forsberg Wrote: I think one reason might come from how we choose to describe these.
All ldraw description should be the same. Rim (width) x (diameter) in millimeter. The rim and the tyre share a common size, here it is "30".
They belong together.
If I check all of these in ldu they all measure 76 ldu, which translate to 30.4 mm
And then we round it of to the nearest integer. No decimals, 30mm
That's good enough confirmation for me, thanks.
RE: 30mm vs 30.4mm Wheels & Tires - tom alphin - 2025-12-26
I just wanted to provide a quick update because it is my desire to leverage my experience creating the online and now print versions of The LEGO Parts Guide to feed back to this community and (hopefully) help make LDraw better too.
As you will find, I attached a draft of the chapter for my upcoming book that describes the wheel elements. Of particular interest to this group is the illustration on the bottom of page 84. The additional digit (30.4 vs 30) is actually relevant to the narrative of the book because this is the precision needed to reinforce that most wheels are an even multiple of LEGO units in width. (I define 1 'unit' as the width of a wall, which is 4 LDU. I prefer to use this unit system since it reinforces the core geometry of the brick, plate, and stud.)
LDraw Suggestion: Since the decimal is actually useful in correctly defining the measurements, and since the decimal is often used on other online LEGO resources (bricklink & rebrickable) - my suggestion is to use the decimal on LDraw part names as well. I suspect this would require an official change to the naming conventions for parts.
Please let me know if you have thoughts on my approach in the book, the chapter text itself, or on my proposal to allow one decimal of additional precision for part names.
Sincerely,
—Tom Alphin