Technical FAQ: Disc brake dangers, broken chains, and more

Lennard Zinn addresses reader questions about injuries caused by disc brakes, breaking chains while riding, and other topics.

Disc brake injuries

Dear Lennard,
I was reading your article about the disc brake injury of Fran Ventoso. Last week I lost the end of my middle finger down to the knuckle by a Shimano XTR rotor. The circumstances were unusual. In a very remote area on the Maah Daah Hey trail in North Dakota, I was carrying the bike during a difficult river crossing. I am an old Bicycle industry person, mechanic, etc., (I make Bicycle Art now) and had become concerned about the sharp edge injury potential of a Disc rotor. Now I would like to do what I can to help prevent this from happening to others.

I am going to write about it. I am not looking for retribution. I only hope to educate and influence the bike industry to remedy this unnecessary and dangerous situation. Maybe you can assist me in getting the word out.
— Ben

Dear Ben,
That is terrible injury and must have come as a great shock. So sorry that happened to you, and I hope your efforts in getting the word out prevent it from happening to somebody else.
― Lennard

Broken chain

Dear Lennard,
I broke my bike chain this morning while starting uphill from a dead stop at a traffic light. I clicked in and immediately stood up out of the saddle, and the chain instantly snapped.

My local bike shop recommended me not to be aggressive from a dead stop. They told me it was an issue of too much torque. While I appreciate their expertise, I have never seen Peter Sagan break a chain.

Do you recommend any specific bike chain for a guy who is 6-foot-7 and 250-260 pounds?
— Glenn

Dear Glenn,
Do you still have that chain? Did you notice which link opened? I would be very interested to know if it was at one of the normal links or if it was at the connecting pin. Certainly an improper connecting job with the Shimano connecting pin can create a weak spot waiting for the right opportunity to snap.

Chains can also fail at any link. They do so by a link opening when a pin comes free of an outer chain plate. 11-speed chains are more prone to this happening than wider chains due to their narrow outer width combined with their 3/32-inch inner width, the same inner width that all derailleur chains have, even 5-speed chains.

The best ways I know of to cause a properly-assembled chain to fail is to either apply high pedaling force in low gear from a standstill like you did, or to shift the front derailleur to a bigger chainring while under power. And even if it doesn’t fail at that instant, every time you shift the front derailleur under power, you weaken the chain by partially prying the outer chain plate off of the end of a chain pin and thus making it more likely to happen when next starting up from a standstill on a hill.

Here’s David Millar breaking an 11-speed chain in an uphill sprint. The lower gear when he started his sprint increased the chances of it happening due to the increased torque. I am constantly wondering if we will see Peter Sagan, Mark Cavendish, Marcel Kittel, John Degenkolb, or other powerful sprinters breaking chains in bunch sprints, bringing down piles of riders with them. The fact that it hasn’t happened is testament to the sufficient margin of safety being built into 11-speed chains, as well as careful installation by team mechanics. Also, the least likely time to break a chain in a sprint is when in high gear and already at high speed, as in a flat, bunch sprint, since the torque and the sudden increase in chain tension is lowest of any high-power-output situation.

As for a suggestion of which chain to buy for a rider your size, I don’t have one. I don’t know which 11-speed chain is the strongest, because I haven’t seen any independent test results on them; only claims by the manufacturers themselves.
― Lennard

Sealant buildup in tubeless valves

Dear Lennard,
I enjoy your FAQ column a great deal and have a question that I don’t believe I’ve seen answered. With the rise of tubeless tires, I see the buildup of sealant in the valve core and the valve itself. This can make it hard to inflate the tire with a hand pump and render CO2 cartridges completely ineffective. This happens because the pressure gradient across the valve is greater than the pressure in the CO2 cartridge. For the same reason (pressure gradient across the mucked up valve core), hand pumps work slowly and with great effort.

Do you know of any way to reduce this buildup? I’ve tried peeling it off but it doesn’t work well. I’ve considered soaking the valve cores in a solvent, but thought I’d get your opinion first.
— Jeff

Dear Jeff,
I remove the valve core and run a spoke or piece of welding rod up and down through the valve stem to clear it out. I then replace the valve core with a new one, if I have another core handy (which I generally make sure to do have; whenever I dispose of a tire or tube with a removable valve core, I save the core and toss it in a drawer; you can also buy Presta valve cores individually). If I don’t have a spare valve core, I’ve had some success with just rolling the core around between my fingers to peel the hardened sealant off. The major issue is the big chunk of it stuck in the valve stem, not the stuff on the valve core.

Whenever I pump (or vent air from) a tire that has sealant in it, I first stand the wheel up for a while with the valve stem at the top, so that sealant flows to the opposite side of the tire. Then I rotate the wheel so the valve is at the 5-o’clock or 7-o’clock position and let it stand for a while like that as well; this allows the sealant to flow back down out of the valve. Then I rotate the valve to the top and pump or vent the tire in that position. It’s not foolproof, but it does reduce the frequency of having to clear the valve of hardened sealant.
― Lennard

Outfitting old bikes with disc forks

Dear Lennard,
Seth wrote to you looking to put a disc fork on his old Trek, and you cited Wound-Up as his only likely choice. Ritchey makes a straight steerer disc fork, too. Thought you would like to know.
— Tim

Dear Tim,
I would indeed! Thanks!
― Lennard