@Maxence, within the mountainbiking world you see this question pop up quite often as well.
On one hand the rotor size could be limited to the clearance it has in the frame or fork and on the other hand it can be due to the forces that can be applied with bigger rotors.
Just as I mentioned in this thread:
And this is exactly where choices are being made by the manufacturer. A 6061 frame is easier to create and therefore less expensive. A 7005 can be stronger, if properly designed and built.
For most general purpose muni a brake is used only to slow down (not to stop) the wheel.
This doesn’t require that much force to do. As you move to the more extreme forms of biking or unicycling, the material must be adjusted to it as well.
A decent aluminum downhill bike weighs an average of 17-18kg whereas a decent aluminum XC bike weighs an average of 11-12kg.
This is due to the beefer frame, the stronger wheelset, heavier tires, etc.
If you would take an XC bike to a serious downhill track, there’s a big chance something will break as well. With a muni it’s no different since the forces applied and the materials used are similar.
The techniques explained by @qu-ax as used by those 2 riders are exactly the techniques I am used to using with my street trial bike. Hence I’ve also setup a strong (Shimano Zee) brake on my muni.
When I started street trial riding I rode a 24" bike with 180mm rotors. Since the torque generated by 24" wheels isn’t that big, the 180mm rotor with 4 piston brakes is sufficient to force a complete stop of the wheel, even when dropping into the brake as described by @qu-ax.
I switched to a 26-er street trial bike, and here the 180mm is barely sufficient to create the same torque. Therefore I’ve placed a 203mm rotor on my rear brake as well.
Therefore to be able to stop a 27,5" wheel in the same way, or even a 29-er, you would need even more stopping power and with that the corresponding strength in the frame.
Since this style of riding (both on bikes and on uni’s) isn’t performed by the general public, it doesn’t make sense for a company like @qu-ax to create frames (which will automatically be heavier as well) which are overkill for their purpose.
Hence the 180mm rotor limit makes a lot of sense for them.
What is not taken into acount in this yet is the rider’s weight. Although @Becky98 has awesome uni skills and rides a lot of km’s / miles, she wouldn’t be the best person to perform durability /strength tests on parts since she weighs a lot less than most guys. I think I weigh at least 1,5 and maybe close to 2 times as much as her. That means that my body weight is causing a lot more stress on the parts than hers if I would perform the same riding.
As a result she could most likely get away with a 203mm rotor on a #RGB frame and perform 1,5m drops with the brake locked without ever having to worry about the frame strength.
Yet if I would do that same drop using the same technique, there’s a good chance I would break the frame too.
For me it was an obvious choice to choose the stronger 7005 aluminum M41 frame in combination with my Shimano Zee brake, since I need that extra strength in the uni if I want to perform stunts like that.
But if you are riding “normally” and do not attempt anything extreme like those drops there would be no issue at all to install a 203mm rotor (or even bigger).
Long story, but in short:
- the Qu-Ax #RGB frames are designed for general purpose muni. For most people it should be more than sufficient.
- The amount of force you can apply on your frame with your brake is due to various factors:
- wheel size
- rotor size
- caliper / brake pad strength
- rider weight
- technique used
With regular riding you will never be able to break your frame with a 203mm rotor just by braking at max (your tire grip will break sooner than your frame).
Only when doing drops (or jumps at speed) where you come to a complete stop at a certain angle so that your tire grip is maximized (since your “weight” is increased by the downwards movement), that’s when the frame will be put under so much stress that you could brake it (if it wasn’t designed for it).