36" wheels, are dual discs necessary?

After a thread on singletrackworld where I asked if anyone had seen such a device for dual discs. It was recommended that I use a 4 pot hydro brake system with a very large rotor that is either from a tandem or custom laser cut (which I am uncomfortable with without a large amount of calculations). The single disc also proves potential problems with weird reaction torques through the hub shell (to the spoke flanges), only applying torque to one stanchion therefore making it fatigue sooner and the reaction from the axle (if a quick release is used) could eject the wheel from the forks. All of this is seems to me like a pretty bad idea for something that could be going up to 50mph.

The solution (in my mind anyway) is using two discs with one on the left hand stanchion in the standard orientation and then one on the right hand stanchion on the front as seen on Cotics road hog forks then this would have no reaction forces going through the axle so a QR could be used if required (although unlikely as stiffness will dictate a through axle should be used), would put near equal forces on the stanchions and there would be no torque through the hub shell.

So it would seem that dual discs were a good idea in these respects but in terms of stopping power (if you read the thread) it was questioned if it was really necessary in terms of stopping power, I tried to reason that it was as the energy of the rotating mass is larger on a 36" wheel and all the mass is further away from the centre, so requires a lot more torque. There were many nay sayers including Ben Cooper (a custom frame builder with a large number of interesting projects under his belt) so I thought I must have been wrong, so I did some calculations (outcome of 36" and 26" energy comparison seen below). After these calculations it was found that for a 180mm rotor on a 26" wheel I needed a 240mm rotor on a 36" wheel (or 2x120mm rotors in this case) but after talking it over with a few people I decided that dual 180mm discs were the way to go. You can see the force required at the tyre contact patch to deccelerate a 26" and a 36" wheel along with the force at the contact patch with dual 180mm rotors on a 36" wheel.

Kinetic energy against velocity, assumptions made of wheels taken to be hoops with all the weight taken at the rim bed.

Force at the contact patch, assumptions of no slip, front wheel only braking.

Weirdly it didn't make as much of a difference as I had expected on the energy front (about 5% greater over most speeds) but seen as there are other large benefits as listed above its going to be carried on with.

Unfortunately for me there has only been one dual disc system made and its about as rare as hens teeth so buying used is out of the question. Buying new is also out of the question as no one has a desire to make such a system due to the current systems being so good.

For the actual braking system the assembly will be pretty easy, two shimano SLX calipers will be used with a Zee lever which is designed for a 4 pot caliper so the stoke should move enough to provide ample pad/disc clearance and as they are easy to bleed it should be a simple process.

Strangely no one makes a dual disc hub, so I set about to design one using hope end caps and a hope bearing kit for easy rebuilds and spares sourcing. The spoke flanges had to be designed to fit the only rim available in the world, which is 36H and I did want to build it 4 cross but I would have to custom cut my own spokes (the wheel build comes out at ~£800 for this) and that means I would have to have a 3 cross build (which comes out a lot less money!). So this no longer has any bearing on the hub. The drawing for the hub can be seen below. Although the flange thickness's will probably change along with a few of the key dimensions.

So are dual discs nessacery? Yes, just because.