Hi Roy,

A lot depends on what you want to use the tandem for, all-up weight, as well as where you want to ride it. With disc rotors on tandems, generally speaking, the larger the better, coupled with careful consideration of how the brakes are used.

For example, our experience with 203mm Shimano XT rotors was just about okay in the UK, but very poor in the Alps, with warped rotors despite taking all efforts to keep them cool, and that was with a light Cannondale RT2 tandem.

More recently, on our Moustache e-tandem with its twin batteries and much greater weight, I replaced the stock 203mm Magura rotors with their larger 220mm MDR-P rotors designed for e-bikes (and hence ideal for tandems), coupled to Magura 8.S pads. This combination has proven to be extremely tolerant of tandem-grade abuse and in fact to prove to myself that they were up to the job, we descended the 2000-odd metre Croix-de-Fer using only the rear brake to see if I could get it to fade. I couldn't. I'm really happy with that combination and it's proven to be extremely reliable on Alpine descents, British 1:3s and everything in between. And the rotors make a fantastic ticking sound when cooling down!
 

I've since fitted the 203mm version of the Magura MDR-P rotors to the Cannondale, because that's the largest size that will fit, but I doubt we'll ever take that to the Alps again, otherwise I'll incur the wrath of my cardiologist...

I hope our experience helps. 

Thanks for your input guys.

You have given me an idea of what I need.

I'll see what fits the frame and go from there,

Ill' be mostly using it in the UK to do Lands End to John O Groats next year.

Cheers

Roy

Photo of frame attached

Your foremost concern is not to exceed the max rotor size for your fork, which appears to be carbon, the manufacturer will only recommend ot warranty it up to a certain size.

Search the forum, lots of correspondence about disks, I started using then in the late 70s or early 80s and have made most of the mistakes re fitting and usage.  I now have two tandems each with 220mm floating rear rotors, both excellent but not as good as a main stopper as the 254mm 10 inch ones we had on a Santana.  I have 10inch 6 bolt fit rotors if you want one.

If going hydraulic then use 4, 6 or 8 piston calipers, the extra pad length and fluid volume helps cope with over heating.

I fitted Hope V4 with twin vented disks to our Orbit after brake fade riding in Wales a couple of years ago. If you can do this at least on the rear (front too if your forks will take it) you can more or less forget about them. Only time we've decided to give them a rest is on really big descents fully loaded such as Col du Rousset where we stopped a couple of times!

Hi Roy, 

Firstly, congratulations on the frame build, that must have been both fun and very satisfying 

I think the general advice of the bigger the better is some but staying within discussion is very important.  As mentioned above, forks may be rated you a certain rotor size - both my travels are rated for 203mm at the forks - well my new fork which is currently with USPS in Los Angeles, is very security rated for 203mm.

Was a pint of reference for you, we are a 130kg team and ride fast road and Audax unladen, I have 2 bikes, terrain is pennies and Peak District - always going up and down hill

Dolan - 180mm rear (biggest that will fit), 203mm front, Shimano Deore calipers with vented Shimano pads, hope rotors.  These brakes are excellent and really don't see any need to upgrade and we do punish them on fast descents 

Cannondale T2 203mm front and rear, 4 pot calipers, hope rotors, again excellent. 

Both bikes are hydraulic, far superior to my first bike with Spyre cable pulls.

Shimano XTR finned disks warped easily, find the Hopes (once trued) more robust

This is a fast road setup with 2 light weight riders. Also our riding style is too decent quickly and use the brakes for corners of needed and stopping at junctions etc, we generally don't use them to control our speed on decent unless terrain (danger) dictates.

Hope this helps