Bicycle Mechanics - MTB Rear Tire - Broken spokes - 70lbs air pressure?

Yesterday I rode a trail and broke two spokes which were on the same side next to each other relative to that side at around the same time after hauling down a trail with some exposed rocks. I usually rode a tire with larger knobbies at less pressure(50ish lbs). This time I rode a tire with much shorter tread and at just over 70lbs of pressure to minimize rolling resistance. This was my first time riding such a high pressure on my back wheel. Thinking about it now, the tire didn't offer as much give. I trued up my wheel alignment and spoke tension a few rides back after breaking a spoke. Over the last three years, I've broken a half dozen spokes on this wheel. After breaking two more yesterday, I'm ready to order a whole new set of spokes/nipples to rebuild the wheel. For my own edification though, would that tire pressure have made any contributions to those spokes breaking? They broke around the same time. I wound them around an adjacent spoke and dropped my tire pressure for the ride back and didn't break any more spokes though I was putting more weight on my front wheel and unloading my rear wheel to be cautious I didn't end up walking. :)

Appreciate any input.

somegeek

somegeek, Hard to say if the tire pressure was the root problem or not, but 70 PSI seems excessive, to me, for rocks and roots. I usually run about 40 PSI off road.

Brad

Yesterday I rode a trail and broke two spokes which were on the same side next to each other relative to that side at around the same time after hauling down a trail with some exposed rocks. I usually rode a tire with larger knobbies at less pressure(50ish lbs). This time I rode a tire with much shorter tread and at just over 70lbs of pressure to minimize rolling resistance. This was my first time riding such a high pressure on my back wheel. Thinking about it now, the tire didn't offer as much give. I trued up my wheel alignment and spoke tension a few rides back after breaking a spoke. Over the last three years, I've broken a half dozen spokes on this wheel. After breaking two more yesterday, I'm ready to order a whole new set of spokes/nipples to rebuild the wheel. For my own edification though, would that tire pressure have made any contributions to those spokes breaking? They broke around the same time. I wound them around an adjacent spoke and dropped my tire pressure for the ride back and didn't break any more spokes though I was putting more weight on my front wheel and unloading my rear wheel to be cautious I didn't end up walking. :)

Appreciate any input.

somegeek

Probably not. Spokes usually break due to metal fatigue, not from load.

You're on the right track by wanting to replace all the spokes. If a couple have broken, that indicates that the wheel was poorly tensioned from the beginning and that all the spokes are fatigued. Rebuild the wheel with all new spokes, use lube on the nipple threads and seats, stress-relieve the elbows, and tension the wheel to the max. I've built wheels this way for many years, and in that time I haven't broken a single spoke that wasn't otherwise damaged.

Probably not. Spokes usually break due to metal fatigue, not from load.

You're on the right track by wanting to replace all the spokes. If a couple have broken, that indicates that the wheel was poorly tensioned from the beginning and that all the spokes are fatigued. Rebuild the wheel with all new spokes, use lube on the nipple threads and seats, stress-relieve the elbows, and tension the wheel to the max. I've built wheels this way for many years, and in that time I haven't broken a single spoke that wasn't otherwise damaged.

Thanks for this. This wheel was hammered pretty good for two years before I bought a truing stand and found it roughly 1/2" out of true. Will be peace of mind to build it with a fresh set of spokes.

How do you stress-relieve the elbows?

Thanks,
somegeek

You stress relieve the spokes by grabbing a parallel pair on each side of the wheel and squeezing the heck out of them.
http://www.sheldonbrown.com/brandt/stress-relieving.html