Determining the correct rotor size on a disc brake

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Permalink Reply by Thunder Snow on September 30, 2011 at 2:32pm

Interesting question.  My Trek Soho came with a front disk brake premounted, but I have no idea how you'd determine which size to use if you're adding your own.

 

Checked my Park Tools Blue repair book: and found nada on speccing sizes, some on maintenence/repair.

 

Next, a quick Google:

http://www.bikeforums.net/showthread.php/268701-Choosing-a-Rotor-Size

 

"Disc Rotor Size - How big the disc is.

Max Rotor Size is determined by the frame/fork manufacturer. Frames and forks must have extra clearance designed into them to take oversized rotors. If you are interested in oversized rotors and unsure of the maximum rotor size for your equipment, contact your bike or fork manufacturer.

:: 160mm (or 165mm) 6 inch standard for Cross-Country and most riding.

:: 180mm (or 185mm) ideal for larger riders, riders who see a lot of tough conditions or steep down hill sections and Freeride. This size is very popular for the front disc paired with a 160mm rear.

:: 203mm Suited for Downhill use and large riders who see tough conditions. Usually requires a frame or fork designed for oversized rotors.

It is not uncommon to spec a larger diameter disc rotor for the front and a standard 6 inch rotor for the back. Most braking power is supplied by the front brake, so you can maximize performance and reduce expense/weight by running a more standard 6 inch rotor in the rear."

"160 in rear and 170 in front"

 

From this, I think you need to check in with Trek to see what the frame and fork can support in clearance.

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Permalink Reply by Darius on September 30, 2011 at 3:58pm

Hey there.  I had the same Q as my city bike is an XC full suspension bike that came with rim brakes and I decided to give disk a shot.

 

I contacted Giant (maker of my bike) and Avid:  their suggestion is 160mm.

Assuming you're not riding down any mountains where the rotor might get hot from prolonged, applied heavy use, you won't need the extra surface area for a city bike riding stop sign to stop sign (assuming that is, that you don't ride right through!).  Weight is negligible, but a consideration too with the larger rotors.

 

Good luck and post up your results pls.

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Permalink Reply by Doug Haynes on September 30, 2011 at 3:59pm

Brakes often come with two different brackets for the different sizes of rotors unless your frame/fork is set for one specific size of rotor via a direct mount.

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Permalink Reply by Gecko on September 30, 2011 at 4:18pm

You should be fine with 160mm rotors.  They're standard on almost all mountain bikes unless they're for extreme use like downhill racing.  Also, I'm pretty sure the BB7s come with 160mm rotors as standard, the upgrades being 180 and 203.  As far as performance goes, I have 160s on my mtb and they work really well for pretty much anything, even pretty gnarly mountain biking in Tennessee.  One note about the frame though, depending on the model year of 4500 you have, you may need an extra adapter for the rear brake.  back in the early to mid 2000's, Trek had a unique caliper mount that you needed a special adapter to convert to international standard (IS) sizing.  From about 2005 on, they started following trends and making their frames with standard size mounts.  If you take the frame to a good Trek shop, they should be able to tell you at a glance which type it is, and possibly find an adapter for you if your frame needs it.

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Permalink Reply by Duppie on September 30, 2011 at 8:59pm

Thanks for all the replies.

I did stop by Village Cycle center (where I bought my bike) and asked them. They confirmed that it would have come with a 160 mm rotor.

 

I was on a different bike, but I think that my Trek has different braze-ons than the current Treks have. My Trek is either 2004 or 2005. So El Gecko's point is valid and it may require an adapter. I'll stop by there to confirm the next time I ride my Trek.

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