Why Are Shimano Rotors 203mm Instead of 200mm?!

In a world where 200mm rotor mounts are becoming the norm on trail bike frames and suspension forks, Shimano still insists on making 203mm rotors... but why? I went down the rabbit hole to answer a very niche yet surprisingly interesting question:

➡️ Is there a performance reason for the 3mm difference?
➡️ Is this just an industry quirk leftover from the imperial-to-metric transition?
➡️ Or is it just… bikes loving new standards?

Follow my imperial tape measures and metric mayhem as I break down where the 203mm brake rotor standard came from, why SRAM and Shimano don’t agree, and whether the difference actually matters for your riding.

Spoiler: It’s not that deep — but also kinda fascinating. 😅

🔧 Topics covered:
• Shimano vs SRAM rotor sizing
• Metric vs imperial in the bike industry
• What happens if you mix 200mm callipers and 203mm rotors
• Why we love to argue about bike standards

👍 Like the video if you want small questions answered more often.
🛠️ Subscribe for more deep dives into things that maybe shouldn’t matter (but totally do).

#Shimano203mm #RotorSizes #BikeNerdStuff #MTBTech #DiscBrakes #ShimanoVsSRAM #MTBStandards #MountainBiking #BikeTechExplained #It’sJustBikes

[valciobanu]

You have to rise them 1.5 Not 3mm the shim is 1, 5 because we’re talking about the radius not the diameter.

[philipcooper]

"These rotors go to 203. Well, it's three mm bigger, isn't it?" - Spinal Tap.

[GuyKesTV]

Fun fact. SRAM actually made 170mm rotors for a short period when 27.5in wheels were introduced. A couple of brands figured that if 160mm rotors were for 26in bikes and 180mm rotors were for 29in bikes then they needed an in between size for the new in-between wheels. Thankfully they canned the idea pretty soon.

[litwanlazerdimfan]

so you got your own channel now?
im liking the change ✌️

[jamesrussell]

Everyone (including Avid who became SRAM) used 203mm until SRAM decided to be difficult.

[florianstaiger]

Hey Anna, thank you so much for this video.
I'm an an automotive engineer for 28 years now, doing MTB nearly as long.
And I was always wondering about these fancy 203mm.
100% clear for many many years, the performance increase answers are total bullsh...
8 inches, due to some historic imperial/metric mix, that's a crystal clear answer.
Thumbs up!

[karomac]

Hi Anna, I missed you at GMBN - and now it's great to find you on your own channel!

[MaplSyrup]

I did an apprenticeship where I designed and built mountain bikes. There are so many standards that are just metric and imperial conversions. I got really good at converting units in my head

[SnootchieBootchies]

There was also 185mm for a while, which became annoying to get adapters for

[adnanmajeed]

Loving your own channel, wishing you all the success, pretty sure you’ll rock it!

[ChrisCapoccia]

8 inch = 203 mm … but if we're really going back to old measures, let's settle on span, shaftment & cubits!

[joshportelli]

I was literally wondering this same thing last week! Great timing Anna! Thanks for the knowledge and history!

[ianfurqueron]

As someone who raced DH back in the late 90s, I remember all of this. At least here in the US we usually just referred to them as "Eight inch rotors".
I seem to recall Hope offered a somewhat dizzying array of rotor sizes back then. I think partly because they sold the C2 caliper to fit almost every fork fitment that was around back then - IS 51, Formula 48mm, and that slightly off 51mm mount the original Boxxer has. Caliper adapters were a challenge back then.

[tomwoggle]

I still have a bike with two of those really high tech fancy 711mm rotors they made back in 1997. It even has the tires mounted directly onto them. Yup, bigger is better they say.

[That_Stealth_Guy]

It isn't just Shimano who makes 203 mm rotors, Magura, TRP, and Galfer all make their "Large" rotor in 203 not 200 mm. 220 MM rotors are the next big thing, with TRP even making a 223 . Seems like SRAM is the odd one out.

[OverPoweredOperator]

anna, i am so happy you have your own channel congrats
the reason is very simple, 203mm is 8 inch dead on
every other size is rounding down, if u think of lift or travel 1 inch is 25mm rounded down
at 8 inches, its 203 dead on motorcycle rotors and suspension are sold in inches

[twentydeck]

Great job Anna, good luck with the new Chanel!

[XEvil]

That's also quite a pain, if you use "160-180" mm adapter on fork with 180 mm disc mount you end up having issue installing 203 mm rotor, as it's a bit bigger, so you need to use the washers under caliper, it's not always a case with every caliper, but still possible

[FozzyS]

Good investigating there, Anna! I wouldn't have thought it.

[cedriclynch]

The 203mm disc is a piece of "soft metrication". It is 8 inches in diameter but is described in metric units. To be really pedantic it is probably 203.2mm.