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Bikes disc brake calipers and adapters explained(Matching calipers with rotors)

Disc brakes have not been standardized for all bikes and that is the cause of alot of confusion when you had to want to buy new rotors or calipers or even adapters it can get very confusing very quickly and there isn’t an easy-to-digest guide online either so I decided to jump in this puddle and share some insights on the issue and try to solve or make a resource that you can use as a reference for better understanding of things.

I did alot of research on this topic as I was in need of changing my brake parts as well, I will start off with a simple introduction of Brake calipers and the concept of adapters and then I will go into deeper details on the topic so you have a complete resource for a reference and at least have some basic understanding of things

These are the topics of these articles so you can easily find and maybe they are comparable or valuable to you in some way and you can directly scroll to those topics

Types of brake calipers

Can you convert IS system to a Post mount system 

What are brake adapters and why do you need them

Which adapters would you need according to the rotor sizes

Do different brand calipers or adapters work with other brands of rotors

Can you use Road bike calipers on mountain bikes and vice versa

These are all the topics that I would discuss in this article and hopefully, you can use all these references to adjust your brakes or provide you a source for a better understanding of how things are standardized

Types/mounting positions of disc  brake calipers

There are currently three standards of mounting disc brakes and some are specific to road bikes while others are specific to mountain bikes

  1. IS (international standard)
  2. Post Mount
  3. Flat mount

IS or international standard used to be the most common standard for bikes in general (road and mountain bikes) but in recent years Post mount standard is widely used right now and there is a good chance that you have the post mount systems on your frames and forks as well if your bike is little old then it will have an IS system on it here are examples of Post mount and IS mount

POST Mount system

picture shows post mount system for disk brakes on mountain bikes

This is a post-mount system there are already spaces made in the forks for it to accept the caliper

Difference between the both bolt pockets on a post mount system is 75 mm and the thread holes go into the forks and they are not sticking on the sides or anything this is called a post mount system

IS standard mount system

picture shows IS mount system on MTB

As you can see this is IS system the screws go through the frame, not into it, ignore the adapter for now


IS mount system on forks (front wheel)

an example of IS mount system on forks

The difference between both bolt mounts on an IS standard system is around 54 mm and the thread holes are not embedded in the frame they are just 2 hollow holes through the frame and not into the frame like you see above on the forks in the post mount picture

And you can also see the post mount system on your forks and IS system on the rear wheels.

These are the major differences between these standards and calipers are designed according to the position of these bolts and are standardized on basic rotor acceptance for example the current standard is a 160mm rotor for mountain bikes and for road bikes current standard is a 140mm rotor

If you still dont get it then let me clarify it more current standard accepts 160mm rotor as a minimum for mountain bikes and 140mm for road bikes so you cant typically go lower than these standards for example if you had like to equip 140 mm rotors on a frame specified for 160 mm rotor minimum that would not be possible but the opposite is possible

for example, if you had like to go beyond the standard of 160 mm and equip a 185 or a 285 rotor then that is possible with the help of adapters.

So if you had a question like how can you match what caliper you need for your brakes then here are all the answers they are standardized and based on those standards you can get the calipers it also clarifies the question of whether disc brake calipers are universal or not and the answer is they are universal

this is just me saying for your understanding there are further standards as well but I will get into them a little bit later

Now, these are very good examples for understanding post mount and IS standard systems both are still in use and you can still find the parts, especially in Shimano brakes they still offer IS mount system adapters and calipers.

A flat mount system has been introduced on the road bikes recently and you may find this on newer road bikes but mostly you will see the Post mount system on both the mountain bikes and the road bikes and at least right now it is the most common system out there.

Forks or frames of a bike are specifically designed to accept a certain amount of rotors for example a 140mm fork will not accept a rotor of 203 mm it’s not that it can’t accept it rather it will damage the fork and the frame if you forced it on that system

Calipers can accept the rotors of sizes whatever you may force on them with the help of adapters or by other means the problem occurs when you exceed the specified limit on forks or frames because the force will far exceed what a specified fork or a frame can handle and under stress, the fork may fail and even the frame may bend or crack.

Can you convert IS system to a Post mount system and vice versa:

Yes you can if you have an IS system on your forks or frames and you had like to convert it to a post mount system then you would simply need a IS to post mount adaptor to make the change possible and mount the post mount caliper on that adapter

But again you would be changing your rotor size with that as well so if you were using an IS system 140mm rotor on your bike and you are trying to mount the post mount system adapter on it for it to accept the post mount caliper then that standard will apply to you meaning you would need a 160 mm rotor because that’s the current post mount system standard.

Other than that the conversion is very much possible no matter what standard the IS standard was based on like 140,160 or 180 it can be converted to a post mount with no problem.

What are brake adapters and when do you need them

The brake adapter’s main use is to make it possible to upgrade to bigger rotor sizes for example the standard rotor size on any bike for disc brakes is 160 mm for mountain and 140 mm for road bikes and if they had like and their frame and forks can take that type of stress they can upgrade to bigger rotors of even 203 again if their frames and forks can take that kind of stress.

Which adapters would you need according to the rotor sizes

Now that you know when you would need an adapter there are use cases and do you even need them in the first place or not let’s move on to if you need an adapter then how would you decide which adapter you would need?

As I have explained above as well adapters are only needed if you are trying to mount bigger rotors on your brakes they have no other use what so ever but to clarify the question if you need an adapter in the first place let’s discuss some standards beforehand so come off you may not get confused.

the 160 mm rotor standard is quite common but it’s not the only standard out there times we even see on some bikes that they are equipped with 185 mm rotors with no adapters and those are standardized based on their use cases for example on a DH mountain bike there will no adapter placed on it only the caliper and a rotor of 185 mm or bigger now that things are clear for you here are the standards that you might see and get confused but you shouldn’t be

  • On road bikes/XC, post mount system the standard starts just like on mountain bikes it starts with 160 mm rotors in front and 140 mm IS mount on the back of the new road bike system as I mentioned above the flat mount system both can be found but the standards are the same meaning 160 + 140
  • On mountain bikes, things change a bit and the change in standards varies for example the usual standard on most bikes is still we see 160 mm or 180 mm on XC/ Enduro bikes while on DH bikes like on some forks the standard changes, and you can mount 200mm or 203 rotors without any adaptor

so this is the summary of the whole standards and if for example, the mounts did not or could not accept the 200mm rotors but it is a DH bike then you can make the conversion possible with the help of adapters.

Which adapters would you need according to the rotor sizes?

Now that you have a better understanding of things here’s the very simple formula for determining what adapter size you would need

the formula is very simple and I will explain that with examples as well for a better understanding

Rotor size (203 mm) – Standardized mount size (160 mm) = 43 mm adapter (post or IS mount) depending on your mount

similarly rotor size (203) -Standardized mount (180 mm) = 23 mm adapter

and the list goes on this way this is the simple math formula that you can apply to choose what size adapter you may need to keep in mind that beforehand make sure what kind of mount system you have IS or PS or Flat mount and then buy the adapters that are compatible with that mount.

Do different brand calipers or adapters work with other brands of rotors

different brand rotors can work with different brand adapters and vice versa but there can be differences like the rotor thickness or pad thickness you have to be careful around these types of things other than that both are compatible with each other in any way you might need to make some changes but that’s all there is to its standards are different as a whole but on brands level all brands follow the same standard

Can you use Road bike calipers on mountain bikes and vice versa?

for the simple understanding of things, brakes are universal and brake standards are universal for all types of cycling including road bikes mountain bikes, BMX and all kinds cycling standards are the same everywhere but there can be differences with the styles they are mounted on any bike

For instance if a new road bike has flat mounts on them and you had like to take that caliper and you had want to fit it on a mountain bike which has a P.S mount system than you would just need a adapter to make it compatible with the road bike or the mountain bike other than that there are no differences in calipers and they dont care how big or how small of a rotor passes through them

Ofcourse there is the preference of the 4 piston or 2 piston calipers but those are preferences not standards in any way

Conclusion :

Conclusion of this article is that all kinds of upgrades are possible ofcrouse adapters do put some weight on your bike but the most important protect that you should always keep in your mind is that can your bike sustain those changes or not

Because as i have explained all sorts of changes and upgrades are possible but you should always be careful around can your forks and bike sustain those changes and do you even need bigger brakes.