For years, running rotors that measured 200 or 203mm in diameter was the norm for downhill bikes, while 160-180mm was the typical trail bike rotor size. Those dimension are still common, but we're starting to see a shift towards even larger rotors, especially on the World Cup DH circuit. It's partially due to the arrival of 29ers – those big wheels take more power to slow down, especially when the course is super steep.
Just how big can rotors get? Well, SRAM's athletes have had access to 220mm rotors for the last few seasons, and companies like Galfer, Trickstuff, and TRP, among others, have rotors in that size range as well. It's not just the diameter that's increasing – in many cases the actual rotor thickness has been increased in order to improve heat dissipation, and to reduce the likelihood of the rotor getting warped.
Bumping up a rotor size creates more power, which means that riders don't need to expend as much energy pulling on their brake levers. That's an important factor, especially on those near-vertical tracks – arm pump and overall fatigue are directly related to how hard, and how often, a rider needs to grab those levers. Of course, there are limits, and it'd be silly to put a gigantic rotor on an ultralight XC bike, or on a bike that's not ever going to see steep terrain – it's all a matter of picking the right tool for the job. Plus, fork manufacturers typically have a maximum recommended rotor size that's worth keeping in mind.
All that being said, if 220mm rotors work well, why not go even bigger? That seems to be the route Galfer are taking - Baptiste Pierron was rocking a prototype super-sized 246mm front rotor last week in Vallnord. He's since downsized for Les Gets, where the real challenge is going to be resisting the urge to grab a little brake on the ridiculously high speed, open sections of the course. Val di Sole is the next race on the calendar, a rugged track full of boulders, roots, and plenty of sections where as much stopping power as possible will come in handy – it'll be interesting to see what solutions are employed to find the ideal balance between speed and control.
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motociclo.endrakor.com/buell-front-brake
The original Marzicchi Z1 had twin disc mounts, remember those?
You could run twin 160mm discs and have a symmetrical front wheel. You do need a different master cylinder though and double brake lines. Probably a bigger ball ache than just machining a new spacer and putting a bigger disc in. It would be cool to see some bespoke double discs though.
How about it @hopetech
Mx bikes have only single front discs, but they also have big fat back tyres with a lot of weight on them to slow down.
more material to spread the heat through. Because the lever arm (rotor edge to wheel edge) is smaller with a bigger rotor, less force on the rotor is needed to get the same torque at the rim. That translates to less force needed on the pads, which leads to less lever force needed, which again helps with heat, and greatly helps with arm\hand fatigue.
Tyre grip is a separate issue, relating to surface area and friction.
Better brakes= less efforts at the lever, more heat resistance, but you can't have a more powerful brake than one making you do an OTB.
Any brake has to stop a 100kg mass with 1G deceleration, even V brakes, to be alowed in europe, so any brake can generate enough power if you pull enough the lever. more than 1G is enough for an OTB in many case, 2Gs is the maximum you can obtain with a really good braking technique, during less than a second.
However, if you increase the distance (rotor size) torque will increase in a linear relationship to the increase in rotor size, ie, 20% larger rotor will roughly equal 20% more braking torque - if you have the same level of force (friction) further from the rotation of axis, you have more braking torque.
Its exactly the same reason why riders are using bigger rotors in this article on 29'ers, more mass further from the axis of rotation requires more torque to decelerate it, hence a larger rotor does this as it provides greater torque with all other factors remaining the same.
Peer reviewed engineering paper highlighting rotor size and brake power.
www.sensorprod.com/news/white-papers/2010-03_ctb/wp_ctb-2010-03.pdf
Larger the rotor, the greater the braking force.
Larger rotors (203mm) will also provide the same braking force as a 160mm rotor at less than half less lever pressure.
You don't understand.
If you brake harder than 1G (9.81m/s) more than a few tenth of a second, you crash. It's the ultimate limit of physics you can't best. The first graph of your link shows accelerations under 10m/s except for a small peak.
More powerful brake = you pull less hard to obtain the same braking force. you won't ever have more braking force.
If a brake can't generate the torque for a 1G deceleation of a 100kg mass, it can't be sold in europe Norm should be around the same values in the rest of the world. Any brake you can buy is powerfull enough if you pull the lever enough, to make an OTB at every corners. Only difference is lever feel and consistency.
What I don't get in the article is the "bigger wheels need bigger rotors"
"Answer: By decreasing the distance between the Effort and Load Points."
axle = fulcrum
brake = effort
tire = load
bigger rotor = shorter distance between brake (effort) and load (tire)
Rim brakes are almost 1:1, since the brake is very close to the tire, making the effort to load distance very small. That's why that even though the mechanical advantage of hydraulics isn't there*, Rim brakes don't require super human lever forces (if the rim is clean, but that's another story).
A 160mm XC brake can certainly stop any rider, it's just a matter of lever input, and with the mechanical advantage of the hydraulics almost anyone can squeeze even something like a SRAM Level hard enough to lock the wheel. The problem would be that pulling the lever that hard for long periods of time would cause enormous arm pump, and the smaller rotor area would heat up faster and cool down slower. A bigger rotor (shorter effort to load distance) requires less brake force (AKA effort), which leads to less lever force needed, which means less arm pump from braking. And larger rotor area can better handle the heat.
* (except of course with hydraulic rim brakes, which any trials rider would tell you are still the most powerful type of brakes as long as the rim is clean)
** And yes, rotational inertia of the wheel itself is greater, but that's still a relatively small bit of the total inertia of the entire bike and rider.
Wait until Stam hears about it. They will dub the 203.9 rotor size as the new trend on stiffness and braking power.
We are doomed....
Actually have to hand it to SRAM here for just switching to even 20mm jumps (though there also was 170mm for a hot minute) across the range: 140, 160, 180, 200, 220; and just ignoring the old inch-approxiamations. And not making a big fanfair about it.
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www.peterverdone.com/disc-brake-mounting-systems
Anyone do the math? How much additional brake power does 220mm and 240mm offer over 203mm?
re-inventing V-brakes (this time oil-runned calliper surrounding the tyre) but insted of rubber we will use resin or metal discs/rim pads... in this case the disc diameter will be astonishing 29' or 27.5'
Otherwise, good idea before experimenting with Shigura combinations or shelling out $$$ for a Trickstuff brake.
The situation is further complicated by some companies that make rotors that they call "floating" but which have very little play between the inner spider and outer friction ring.
Only now with the higher rotational momentum (don't know if that is the right term) of the 29er wheels with heavy tires I found the 225mm rotor to be beneficial.
The Shimano riders seems to don't need bigger Rotors