Disc wheel Lacing
A special concern when building a rear bicycle wheel is the choice of lacing pattern for rim brakes or for hub brakes. In the rear, braking forces far exceed 10x driving forces so that a wheel designed soley for driving forces won't be as strong as needed, especially in crucial braking situations where the wheel may be also experiencing a very hard hit.
What motorcycles do
I have build spoked motorcycle wheels in the past. They are really a pain in the ass since the spokes do not bend quite like bicycle wheels do.
The key to a rear motorcycle wheel is that the spokes are laced from one side to the other with the direction of the spokes changing with each round. Go into any motorcycle dealer and take a look.
Shimano and Magura both confirm the motorcycle technique.
Shimano recomends a specific lace rotation for each side of each wheel in their Hydraulic Disc Brake - BR-M765 Service Instructions. One issue most bicycle wheelbuilders will face when working with this pattern is that lacing the rear wheel it is done from one side to the other (like a motorcycle wheel), rather than from the inside to the outside as has been standard practic with bicycles for years. The goal of this pattern is to keep the outside spokes on each respective side in full tension while brakeing and only the drive side of the rear wheel being changed. See below from Shimano instructions:
Notice that the Shimano lacing pattern is very particular about lacing for direction and sides on both front and rear wheels. The rear is laced like a motorcycle wheel. Any wheel lacing pattern must be designed around braking, not necessarily driving. Breaking forces are tenfold driving forces.
The Shimano pattern does specify that the outside spokes are tensioned and the inside spokes are compressed under breaking. Only on the drive side of the rear wheel is this not the case. Now, Shimano has begun saying that the non-breaking side of the front wheel (when using a HB-M965) can be radial. A poor choice in my opinion.
This is how a Shimano rear wheel lace goes together.
The inner spokes of the rear wheel will be place on each side of the valve hole. At this point, the hub label should be pointing directly at the valve hole.
This is how a Shimano front wheel lace goes together.
DT Swiss Recomendations
"Only tangential or crossed spoking should be used where drive or braking forces (disc brankes) are transmitted from the hub to the rim. The greater the tangnent of the spokes to the hub flange, the more directly drive and braking forces are transmitted. For this reason, radial spoking should not be used on front wheels with disk brakes or on rear wheels regardless of the type of brakes used."
From Mavic UST Owners Instruction Manual:
Chris King Recomendations
Chris King Instructions specify:
"The front ISO Disc should be laced 3-or-more-cross with the rotor (left) side pulling spokes (relative to braking direction) heads out/elbows in (when laced 3-cross). The final cross of the pulling spoke must be on the outside so that as braking force is applied, increased pulling spoke tension will pull the crossed spokes towards the center of the hub and away from the caliper. Lace the wheel symmetrically."
Note that this disagrees with the Shimano in regards to the front wheel, but does not refer to the rear wheel. Since there is a discrepancy, Shimano prevails. Shimano actually employs real engineers, makes far more designs of every kind that meet every price point and performance level. They truly know what they are doing. King makes very nice hubs and headsets, but the devil is in the low end. The high end is easy, the low end is hard.
One problem that king does have is that they still make reference to using BullShot grease. A product no longer manufactured.
Magura Instructions specify:
"No radial lacing with disc brake wheels! Head-inside-spokes (=arc outside spokes) have to be pulled, i.e. these spokes point forward on the front wheel; on the back wheel these spokes point forward on the rotor side and backwards on the drive side."
This agrees with Shimano.
"Attention: disc brakes are an exception, as the highest stress occurs while braking (Downhill - MTB)."
Recomendations from books
The Art of Wheelbuilding - A Bench Reference Manual, Gerd Schraner. Recomends Shimano pattern for lacing disc wheels. The Art of Wheelbuilding
A flip-flop hub should be laced as per the Shimano technique regardless of weather it uses rim or disk brakes. Since the wheel can be put on in either configuration a Shimano pattern will ensure that when driving or braking, the spokes are being loaded properly.
The traditional 32 hole 3-Cross Spoke pattern is hard to beat. It spreads the load around the rim and hub while resisting wind-up under power and braking. A 4-Cross pattern is almost totally unnessesary due to the selection of downhill rims available. I prefer using 32 hole rims on all my mountain bikes since I may destroy a rim at anytime and most shops do not stock a selection of 36 hole rims.
Very light and non-aggresive riders can benifit from a 28 hole 2-Cross pattern. I built a set of road wheels like this for my wife. They are light and strong enough for what she puts them through. Rims will always be a special order item when running 28 or 24 hole patterns.