Why are RGB frames limited to 180mm rotors?

As you may know, Martin Charrier and Roger Davies are working on an adapter to put a disc brake inside a frame with a Schlumpf hub. The disc can’t be anything else than a 203mm disc.
As the frames need to have the fixations for an inner disc, we can’t use any frame. RGB frames are really great and would be my main choice to use the adapter. However, it is stated on the frames description that “the rotor-size is limited to 180 mm.”. Does anyone know what causes this limitation? Maybe @qu-ax?
I suspect it to be a weakness issue.
(Roger thinks it’s still OK to use the adapter in a RGB frame but I’m curious :stuck_out_tongue: )

There has been at least on 27.5" RGB frame broken. It had a 203mm disk - and in general large disks are associated with breaking forks/frames on bikes and unicycles alike (The RGB frame isn’t the only unicycle frame with a failure like that I’ve seen). Pretty common practice on bikes to limit rotor sizes for that reason, and I guess Qu-ax adopted that.

Of course, depending on your use, you can decide that you can get away with a 203mm disk without breaking a frame (and risk having to sit on the costs if you are wrong). The broken frame I saw was by someone who does rides downhill on a really high level.

Ok, so my guess was right. I fear that with a Schlumpf hub the effort that I’d put on my frame would be too hard to guarantee that the frame wouldn’t break . Thanks!

The flipside of that is: I’m honestly not sure if the QX RGBs are much weaker than other frames (well, probably less strong than their old frames, but those were pretty beefy), or if they are just the only ones who have decided to put a limit on disk size.
I’ve seen similar failures on other brands too.

Yes, we had two riders with issues. One is the one @finnspin talks about - at a pretty high level, the second rider landed 1,5m drops with brake tightened - which is nearly the biggest load a tab can get. In addition, both riders used downhill-4-piston brakes (Saint/Zee, or Trickstuff) and both 203 mm rotors. Also, on both this happened after quite some riding time.

Compared to the total qty - it is really a little count - and certainly due to the really low weight of the frame. It is always a compromise. But as Finnspin says, we prefer to recommend 180 mm rotors max.

On the other hand, you have riders like @Becky98 riding thousands of kms with a freewheel-unicycle on the #rgb frame. But it is clearer to recommend a max. rotor-size than to say “do not land while braking”.

We reinforced this part in future productions - which will be a running change - once through, we will adapt this recommendation - until then, we recommend not using more than 180 mm rotors. This is more clear than silently accepting claims of broken frames - it’s about safety in the end.

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I think riding a Schlumpf inside a RGB frame with the adapter and the 203 mm disc will be not be a problem, since you will not ride huge drops with the Schlumpf hub in anyway.

I am riding a 203 mm disc for the Quaxle Freewheel prototype inside a RGB frame (done about 1300 km with it yet) and I use this frame also on my normal Muni wheel since 2018 with 180 disc. And I have still no sign of weakness.

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Thank you all for your insights! I’m reassured about using a 203mm rotor with a RGB frame for road racing.
BTW, do you know when the reinforced frames will be available? :slight_smile:

as written above, it will be a running change. it will be a long run :slight_smile:

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@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:

  1. the Qu-Ax #RGB frames are designed for general purpose muni. For most people it should be more than sufficient.
  2. 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).

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Thanks for this exhaustive insight! I’ll probably add an RGB 29 frame to my wishlist, then :smiley:

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Hi, since I was the first to design and build these adapters (since 2008) I can tell you why I had to use the larger diameter disc. The clearance needed to fit a caliper between the spokes, frame and line up correctly with the disc. Thats it!

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+1 as someone who installed one of the adapters made by @lobbybopster !

I remember some mails about this issue. As I love having a great brake that’s not an issue for me. I just wanted to be sure not to break such a beautiful frame :heart_eyes:

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Thank you Pierrox, Hope it was worth your trouble. Only have made two more but for first gen. since finding yours in the garage. But I am happy to see that Roger was paying attention, we all can now have disc brakes, oh and disc brakes on non-geared also. Maybe suspension next? I have been inventing/building a couple of new items that may blow you away.

                                                                                                        Lobbybopster (Mark)
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