Anyone got a 36t cassette and can help me with the diameter of that cog?

Discuss light weight issues concerning road bikes & parts.
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Joined: Mon Aug 29, 2016 11:19 am

by alcatraz

Hi WW's...

An older and heavier friend is preparing for the Taiwan KOM 3km climb and wants to max out his ultegra 6800 longer cage derailleur. (Bike is a Trek Domane) He's currently on a 32t cassette but visually the upper pulley of the derailleur is a good ~10mm away from the largest cog and the B-screw is barely being used.

Since 34t 11s cassettes are hard to find, and a sram pg1130 36t cassette is available online at a price low enough to risk trying it, I am thinking to try a 36t on there.

Without actually having the cassette to try I'm trying to assess if it could fit.

Does someone have a 36t cog they can help me measure the diameter of so I can see if it could fit?

My 25t red cassette has a 102.4mm diameter which extrapolated to 36t makes 147.5mm and 32t 131.1mm. Diameter difference is around 16.4mm and half of that is 8.2mm. So the derailleur should need around 8.2mm + chain in clearance on a 32t to clear a 36t cog. Does this seem right to you?

by Weenie

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Joined: Wed Jan 22, 2014 11:48 am
Location: Vienna Austria

by Marin

Seems right to me, plus you can calculate it via the chain pitch.

FWIW my mid cage 6800 would shift a 36t cassette - it mainly depends on how long the hanger is. You can get a Wolftooth road link to be on the safe side though.

Posts: 154
Joined: Thu Feb 25, 2016 9:55 pm

by Kurets

It is easy to calculate the diameter of a cssette as measured in the midpoint of the "valleys" that is because the tooth spacing is 1/2 of an inch. So a 36t cassette will have a circumference of 36*0.5*25.4mm which gives a valley diameter of 145.5mm. Add in the height of a tooth which is somewhere around 4mm I'd geuss and you end up close to your number.

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