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Technical FAQ: Sealant in inner tubes; breaking titanium pedal spindles

Will using tire sealant in an inner tube prevent a catastrophic flat tire? Plus, how much force will break a titanium pedal spindle?

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Have a question for Lennard? Please email him at to be included in Technical FAQ.

Dear Lennard,
I have seen that tubeless tires can be virtually impossible to get off some rims when riding, but I am concerned about fast flats on 50+ mph descents. So I would like to consider adding sealant to the tubes on my Conti 5000 700c x 28 mm (actually measure 30 mm on my HED rims).

What tube with a removable core would you recommend? I currently use a 40 mm Presta stem, and what/how much sealant do you recommend?
— Christian

Dear Christian,
First of all, sealant does not eliminate or even reduce the “fast flats” I think you are concerned about when descending, in which the tire goes flat so quickly that you have no time to slow down before it’s flat. Rather, what sealant does is fixes small flats that would have been slow leaks and instead might go completely unnoticed, because they seal up before losing significant pressure. It can save you the hassle of fixing a flat out on the road; it cannot save you from the possibility of serious injury from a blowout or slashed sidewall at high speed.

Given that, if you still want to proceed with injecting sealant into your inner tube, I recommend using a butyl tube with a removable valve stem, since you can use any sealant inside; I know of no sealants that attack butyl. Latex tubes, on the other hand, can be attacked by some sealants. Q-Tubes is one brand of butyl tubes available with removable valve stems that are widely available in the USA, and there are many others.

The volume of sealant depends on the volume of the tire, as well as how much protection you want. For a road tire up to 700 X 32C, 1/8 of a cup is plenty.

Start with the sealant in a squeeze bottle or sealant syringe with a flexible tube on it that fits over the valve stem. If you have more sealant in it than the volume you want to inject, it’s best to have a clear bottle or syringe with volume markings on it so you can meter out the amount you want. Remove the valve core and rotate the wheel so the valve is at the 5 o’clock or 7 o’clock position. This allows the injected sealant to flow out of the valve stem by gravity and then toward the part of the tube at the bottom (at the 6 o’clock position), away from the base of the valve. Squeeze in the amount you want and remove the sealant injector from the valve stem.

Before replacing the valve core, put a pump chuck on the valve stem. Rotate the valve stem further from the bottom (3 o’clock or 9 o’clock position or above) and pump some air into the tube; this will clear the sealant out of the valve stem so that it won’t gum up your valve so soon. The air will of course blow right back out when you pull the chuck off.

Whenever you deflate your tire — don’t do it often, as the more often you blow air in and out of the tube, the sooner the sealant inside will dry out — rotate the valve to the 5 o’clock or 7 o’clock position first. If there is any sealant inside the valve stem, this will let some of it drain out first before you open the valve and let high-pressure air blow past it, thus solidifying it in the valve and making inflating it and deflating it more difficult. If your valve does get gummed up, you can always remove the valve core again, remove the dried-up sealant from it with your fingers, and replace it.
― Lennard

Dear Lennard,
I have a question regarding rider weight limits for titanium pedal spindles. For example, I recall that the Speedplay X-1 pedals had a rider weight limit of 180lbs. This doesn’t make sense to me. I understand of course that too much weight on the pedal could break it, but shouldn’t it be force applied on the pedal that matters? I may exceed 180 lbs. but can only apply 700 watts max, whereas a lightweight sprinter may be able to apply more than double that. Does this make sense?
— Alfie

Dear Alfie,

What would you have the pedal manufacturers do instead? If they know that a strong enough rider can fail their spindles, how would you have them word it? It’s a lot easier (and more enforceable warranty-wise) to have a weight limit than to somehow ask riders to not push any harder on the pedals than some limit they have established for safety.

Secondly, power output is not necessarily equivalent to load on the spindle, which is certainly something that the manufacturer wants to regulate. Power output equals torque times cadence times 2π, so a rider can get to high power outputs with lower load on the pedal on each stroke by doing it at high cadence. Using a longer crank at the same cadence also increases power output without requiring a higher load, since it increases torque without an increase in force. Which is to say that the load you can put on your pedals on a single stroke with your 180+ pounds when you stand up and pull on the bars may far exceed what a smaller rider does, even if he or she is putting out more output than you are; he or she is instead applying a smaller load more times per minute.

Finally, a broken pedal spindle is something you don’t want to have. Unless perhaps you have the bike-handling skills of Mathieu van der Poel, who somehow managed to stay upright when his handlebar snapped off (!!), if your pedal spindle breaks when you’re sprinting out of the saddle, that will be a very painful and dangerous crash. And small riders can damage titanium spindles, too. In the 1981 Coors Classic, I briefly rode the bike of one of my teammates who was much smaller than me. I noticed immediately that his titanium pedals spindles were bent, because my feet were tilting in and out as they went around; he hadn’t noticed it because it had happened gradually over time.

While titanium pedal spindles nowadays are made of a stronger 6Al/4V (6 percent aluminum, 4 percent vanadium) titanium alloy, and at that time they were made out of CP titanium (“commercially pure,” meaning 99 percent titanium), they still are made of a material that is less strong than cromoly steel yet have the same dimensions in order to fit into the same bearings and pedal body. If a pedal manufacturer says you’re too heavy for its titanium spindles, it’s prudent to go with that recommendation for more reasons than just that your warranty would be void.
― Lennard

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