Huh, so I should go for Sm-ma-f203p/pm to update my Shimano rotor from 180 to 203.
It may be a dumb question but isn't that f in the article name meant for front? I couldn't find an r variant. does that F stand for something completely different or should work for front and rear wheel?
Schorsch,
I have posted at the end of this message a screen capture taken from Shimano's website which explains how Shimano goes about numbering their brake caliper adapters.
I will also answer your question regarding the correct caliper adapter you should use for a 203mm rotor below. This will be a detailed response, so you and others will better understand the type and what size caliper adapter you should buy for any size brake rotor or caliper mount.
To start off, the short answer to your question is Yes, the "F" refers to front. However, the use of "F" for a front caliper adapter is not entirely accurate. Caliper adapters with the letter "F" can also be used on either the front or rear of the bike depending upon the brake caliper mounting system the bike manufactuer has used.
Rear Brakes: When disc brake systems first began to appear on mountain bikes in the late 1990's, bicycle manufacturers used a wide assortment of mounting designs in which to mount brake calipers to the rear triangle, i.e. side frame bracket mount, post mount, etc.
Because of this mish mash of designs, a confusing assortment of brake caliper adapters had to be designed. For the consumer, trying to decipher which caliper adapter was required for a given type of brake application was headache inducing.
Front Brakes: Fork manufacturers such as Fox and Rockshox adopted a post mounting system for brake caliper front brakes. Because post mount was the dominant style of brake caliper mounting for forks, Shimano identified it's post mount caliper adapters as front fork use.
HOWEVER......Over the years, mountain bike manufactuerers have gravitated towards using the "Post Mount" style of brake caliper mounting for both the front and rear brakes. Therefore, on bikes which have a post mount rear brake caliper mounting system, you can use brake caliper adapters marked for "F" front fork use also on the rear of the bike.
Unfortunately, bike frame and fork manufacturers have not adopted a universal standard for brake caliper "Post Mount" lug size/height. Nowadays, the majority of bike frame manufacturers use a 180mm post mount lug for the rear triangle brake caliper mount. However, this is not written in stone and some frames may have something other than a 180mm post mount size/height.
In regards to front fork post mount lug size/jheight, it pretty much dpends upon the fork manufacturer and fork size. As an example, pre-2022 Fox 34 forks used to use a 160mm post mount height. Fox has since adopted a 180mm post mount standard across it's line of 34, 36 and 38 forks. On the other hand, Rockshox uses a 180mm post mount on the Pike and Lyrik and a 200mm post mount on the more robust ZEB.
As a general rule, if a mountain bike uses a "Post Mount" lug system for mounting brake calipers to the front fork and rear triangle, it will use a brake caliper mounting adapter which will look similar to this in design:
Which size caliper mounting adapter a person will use depends on two things:
1. Size of the post mount height
2. Size of the brake rotor
a. As an example, a bike came from the factory with a Fox 36 fork and a 180mm Shimano front brake rotor. The Fox 36 uses a post mount height/size of 180mm. Since the 180mm post mount and 180mm rotor are the same size, no brake caliper adapter is required.
b. As an example, a bike owner wants to use a 203mm Shimano brake rotor on his front brake. The bike has a Fox 36 fork. Because the Fox 36 has a post mount size of 180mm, the brake caliper must be raised in height 23mm to accomodate the 203mm brake rotor.
180mm post mount + 23mm = 203mm. Therefore a 23mm post mount brake caliper adapter is required.
c. As an example, a bike owner wants to install 200mm SRAM brake rotors on the font and rear of his bike. The bike has a Rockshox Lyrik front fork which has 180mm post mount. The bike manufacturer has set the bike up with 180mm "Post Mount" on the rear triangle. Because the bike has 180mm front and rear post mount, the owner will need to raise the front and rear brake calipers 20mm in height.
180mm post mount + 20mm = 200mm. Therefore two 20mm post mount brake caliper adapters will be required.
d. As an example, a bike owner wants to install a 220mm Shimano brake rotor on his front brake. The bike has a Fox 36 fork. Because the Fox 36 has a post mount size of 180mm, the brake caliper must be raised in height 40 mm to accomodate the 220mm brake rotor.
180mm post mount + 40mm = 220mm. Therefore a 40mm post mount brake caliper adapter is required.
e. As an example, a bike owner wants to install a 220mm Shimano brake rotor on his front brake. The bike has a Rockshox ZEB. Because the Rockshox ZEB has a post mount size of 200mm, the brake caliper must be raised in height 20 mm to accomodate the 220mm brake rotor.
200mm post mount + 20mm = 220mm. Therefore a 20mm post mount brake caliper adapter is required.
The larger the height of the brake caliper adapter, the longer the bolts must be in order to bolt the caliper and brake caliper adapter to the post mount lugs. Additionally, when you increase the size of the brake rotor, this also increases the rotational stress placed upon the brake caliper adapter mounting bolts which occurs when the brake caliper clamps hard onto the rotating brake rotor.
To lower the stress loads placed upon brake caliper adapter mounting bolts, brake system manufacturers use a different style of "Post Mount" in situations where the caliper is raised 40mm in height, i.e. Fox 36 fork and 220mm brake rotor. The 40mm caliper adapter is seen depicted below and utilizes four caliper mounting bolts instead of the standard two mounting bolts. The purpose of the adapter is to significantly shorten the length of the caliper adapter mounting bolts. The design affords a much stronger brake mount and offers less chance of shearing off a caliper mounting bolt under hard braking.
below is depicted an explanation of how Shimano numbers the brake caliper adapters it produces.
I hope you and others find this information useful.
Be safe,
Rod