I am hoping that a mechanical engineer can give an educated answer to my question here. I am concerned about this because I have always had some trouble with spokes pulling through the rim on the rear wheel of my touring bike. (I use a 36 spoke wheel.)
Consider this: In general, a rear tire will wear at least 3 times faster than the front because of the forces applied to the rear wheel from pedaling. And for this same reason, we have far more problems with rims and spokes on the rear wheel than the front.
I am trying to make the point that the relatively small forces associated with pedaling, all of which are transmitted through the spokes, already cause the rear wheel and tire to fail much faster the front wheel and tire.
Now consider that the rear brake alone can cause the bike to decelerate at a rate of about 5 times that which we can accelerate with the pedals, implying about 5 times the force to the wheel.
When a traditional brake is applied, the forces are transmitted from the brake to the ground, primarily through the circumference of the rim. While there will certainly be some additional forces on the spokes, I don't believe it is substantial.
With a disc brake however, all the braking forces must be transmitted through the spokes. Isn't it logical then to assume that disc brakes will cause even more problems with the rear wheel and spokes than a traditional braking system?
I asked a bike dealer about this, and he insisted that this wasn't a problem. He then went on to show me the new Surly Disc Long Haul Trucker. Curiously, this bike has mounting brackets for carrying 2 spare spokes.
I would suggest that this is either a really dumb marketing gimick, or Surly has a significant amount of trouble with broken spokes on this bike and does this to assuage customer complaints. In my mind, one is just as likely as the other.
To repeat, do disc brakes aggravate the problem with broken spokes or spokes pulling through the rim?
Edit: After reading the comments, I would like to explain that I am trying to understand the differences between a disc brake and a traditional braking system in the way the braking forces are transmitted from the ground to the bike frame.
I tried to analyze the problem myself using force vectors, but lack the ability to reason through the problem. I suspect that disc brakes cause a great deal of additional stress on the spokes and rim, but simply do not know. That is why why I posted the question.
2nd Edit I didn't intend to imply that I have a significant amount of wheel problems. I've been riding for over 30 years and at one time or another, I have been stranded at the side of the road for busting virtually every part on the bike, except the wheels. (Seat rails twice, seat post, frame, rear axle, bottom bracket, chain, pedals twice). I was only asking whether disc brakes aggravate wheel problems, of which we all have some degree of trouble with.