After a frantic race and an equally frantic pack down in Val di Sole, the World Cup circus has rolled into the town of Vallnord, nestled away in the deep valleys of the Andorran mountains. Being a back to back round means teams and their mechanics have their work cut out, getting pits setup and prepping bikes for the onslaught of Vallnord's steep track. We've been out for a lap of the pits to see what's being bolted onto the bikes this weekend:
A standard flat out Wednesday for the Fox boys.
The Norco Factory Racing riders have two linkages at their disposal, a harder and softer tune.
AB Devinci Wilsons equipped with an Extreme Racing Coil Shock.
Fresh frames and colorways for Cube's Matt Walker and Max Hartenstern.
Pivot are going to need a bigger rack soon, with each rider having upwards of three bikes each.
Mick Hannah is running Trickstuff's new Maxima brakes which have a claimed 25% more power than the already super powerful Direttissima.
A rack of GT Forces and Sensors.
Laurie Greenland has opted for DT Swiss' EX471 rims over the EX511s that he has run in the past (and teammate Mike Jones, on the right, is continuing to run). His mechanic says the EX471s offer a little more compliance on the steeper and more technical tracks.
Keegan Wright you’re an animal mate, mad props
It feels like walking into a candy store, but you can't eat anything - torture and frustration.
However, those bikes look damn good.
If I had to pick one for myself, it would be a 29" Santa Cruz.
The new frame looks sick and I love the color. It's a pretty and subtile grey.
Oh, wait, it was 26", so un-ridable! I forgot!
If the rotor was infinitely large, the forces at the caliper would be 0 and there would be no deflection. Same reason why you can grab the wheel by hand while it's spinning no problem, but good luck trying to grab the rotor.
Are they up to task do you reckon?
On a more conventional frame, the bolts securing the adapter are loaded in shear when the braking forces are transferred from the caliper to the adapter. This shear force is however equal to the force the rotor exerts onto the caliper. Since the force to lockup the rear wheel goes down as the caliper moves further away from the center (due to more leverage), the shear force on the bolts decreases as well.
It's important to note that in almost all competitive vehicles equipped with disc brakes, the size of the brakes is not determined by strength/power considerations, but rather by thermal properties. Racing brake pads like to operate over a fairly narrow temperature range, while the brake fluid will boil above a certain temperature. The most common cause of overheated brake pads/boiled fluid is the brake rotor being too small to effectively dissipate all the thermal energy it's absorbing. You generally want the smallest rotor that won't overheat your pads/fluid.
To further complicate things, just because less force could be used to slow the wheel at the same rate doesn't mean it will be. The brake caliper is still capable of applying the same clamping force regardless of the mounting position. So even though the rider could achieve the same angular deceleration as with a smaller mount by pulling the brake more gently, they probably won't because that would defeat the purpose of increasing the mount size in the first place.
The torque applied by the brake is (as you point out) limited by the traction of the tire/dirt so assuming sufficient power of the brakes you're right about thermal concerns being paramount. Larger rotors also mean that it takes less lever force to reach the torque limit of the brake (aka, locking up the rear wheel) and since that lever is powered by the rider larger rotors on steep tracks can result in an improved rider experience.
I'm sorry, but I'm going to have to respectfully disagree with some of your assumptions. You are correct that deflection is non linear and increases to the power of 3 with respect to length, however this only applies for a cantilevered beam. Pretty much every frame is designed to minimize the level of cantilevering of the brake mount, which is true of this Commencal as well. The tangential force at the caliper acts to compress that mount a lot more than it tries to bend it. For a beam in compression, the linearity of Hooke's law still applies.
As for the effort required at the lever, all of my statements are based on the peak load that occurs when the tire is at the threshold of locking up. At the lockup point, the force at the lever and subsequently the caliper pistons will be lower for the larger rotor.
I totally agree that you wouldn't actually analyze this system as a simple cantilever. But looking at the geometry and the direction of the applied load, I think it would be foolish to study the system without the inclusion of bending. To me it looks to be the dominant effect, however this is based on my intuition only.
"EX471s offer a little more compliance on the steeper and more technical tracks"
But, as you said, a little dent here and there can actually help save a run instead of cracking/breaking.
"Esto és un negocio loco"
"This is crazy stuff"
(not the most accurate translation)
What am I doing here?
The one above accompanies me.
What if this were the last time?
Chicken or pig?
Does each step take me where I want to go?
Am I present? Am I feeling it?
Where do I come from?
Where am I going?
Simplicity and humility.
Capacity to adapt.
This is a crazy job.
White wolf or black wolf?
Am I ready to pay the price?
Passion, magic, love and winning.
You must believe!
Optimism. Enjoy.
You can.