Tech Mailbag #6: How do I know if my chainrings are worn?

Also: how to tackle mystery flats and freehub body swaps, and a follow-up on tubeless wheels and maximum inflation pressures.

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Welcome to the CyclingTips Mailbag column, where you send us your tech questions, and our team of nerds gives you answers. Got a question about wheel and tyre standards? Want to know how to diagnose that weird shifting issue? Wondering where that darn ticking sound is coming from?! Post your question on the CyclingTips Forum using the #askanerd hashtag, or send it to tech@cyclingtips.com to be featured in an upcoming CT Mailbag column.

In this week’s Mailbag, we discuss mystery flats, potentially worn-out chainrings, and the frustration of swapping freehub bodies. 

From the CyclingTips forum:

Like most roadies, I get the occasional puncture out on the road. I’m running Conti GP5000 and Pirelli P-Zeros with (quality brand only) tubes. However, much more common is for me to go to take the bike out of the garage, and find the tyre flat. Often I can’t find a hole in the tube; it may even hold air when pumped up, but I prefer to replace it anyway. Other times, the tube has a hole, yet I’d ridden home and parked the bike up a few days earlier with no indication of a problem. What’s the story?!

 — Toon

Hi Toon,

This is a bit of speculation on my part since I can’t inspect your tires and tubes in person, but my guess is this is simply a matter of time. In other words, the punctures you’re getting out on the road are likely more obvious because the holes are bigger and the air is escaping more quickly. When you have a much smaller puncture — say from a metal sliver or a tiny shard of glass — you may not notice the slow loss in pressure while you’re on the road. But when your bike is hanging up in the garage, there’s far more time for that air to leak out, which would explain why it seems like the tire has magically gone flat with no apparent cause.

I always have the same advice in these situations regardless of how big the hole may be. Remove the tube, but leave half of the tire still mounted to the rim. Inflate the tube enough so that the rubber is slightly stretched, and then slowly cycle the tube past your face and/or ear to locate the hole (the skin on your face is more sensitive to tiny rushes of air than your fingers). If you still can’t find the hole, fill a sink or basin with some water, and then slowly dunk sections of the tube in there until you find a bubble. 

It may take a while, but I assure you that if your tire was flat while your bike was hanging up in the garage, there’s a hole in that tube somewhere! Once you’ve located it, hold the tube next to the wheel and align the valve stem with the valve hole, then carefully search the inside of the tire casing in the area where you found the hole in the tube. Chances are good that there’s a tiny bit of something sticking in there that caused your puncture. 

One last note: I know it’s easier to just install a new one, but consider patching the tube for later use instead. A properly patched tube is just as good as new (as well as cheaper and less wasteful). 

 — James


Dear CyclingTips,

I’m one of those people who got into cycling over the pandemic and am trying my hand at being a home mechanic. I’m going to attempt to swap the freehub body on two sets of rims. One is a DT Swiss 370 hub with a SRAM XDR body; the other is a Scribe hub with Shimano 10/11. After 10 minutes of YouTubing and Googling, it looks pretty easy. Am I being reasonable here? I feel like I’m missing something about different sizes or interfaces between the hub and freehub body. At the same time, any particular grease(s) you recommend using?

 — Matt Barletta

Hi Matt,

Welcome to cycling! We hope you like it here and will stick around for a while :)

Unfortunately, hubs are far from universal in terms of parts compatibility. The drive systems that DT Swiss uses are more common than most — particularly since they do contract manufacturing for so many other wheel brands — and while the system that Scribe uses is similar in concept, it’s different enough that you won’t be able to interchange the two. 

That said, it’s never a bad idea to pull things apart if you’ve got the right tools on hand and just want to better understand how everything works. DT Swiss has its own recommended ratchet lubricant, and my guess is Scribe might have one of its own. However, I’ve generally found that Dumonde Tech Freehub Grease does a great job, too.

Keep that curiosity going!

— James

Hi CyclingTips Nerds!

I am wondering if you can give any advice on assessing chainring wear on my Dura-Ace 9100 chainrings. The chainrings have roughly 20,000 km of road use over about three years, plus quite a few hours (guessing at around 150?) of indoor training, which I can’t quantify accurately due to having a “dumb” trainer that doesn’t record my indoor kilometres. 

The teeth don’t look to be too pointy or have sharks’ teeth, and I regularly change my chain to keep below 0.5% elongation. Also, Dave Rome will be pleased to know I do my best to keep it clean (although I haven’t committed to waxing just yet). Recently, I feel as though the drivetrain is getting noisier, and am wondering if this might be a sign the chainrings are coming to the end of their life. Any advice would be much appreciated. I love the podcast(s); keep up the great work!

Thanks,

Proud VeloClub member Neil Hamilton

Hi Neil,

First off, thanks for being a VC member! Happy to help with this one.

Based on your description, my guess is that either your cassette or chainrings are indeed worn. If your drivetrain is getting noisier, that could be an indication that the spacing of your chain rollers is no longer as in-sync with the teeth on your chainrings and/or cassette. When that happens, the pressure from the chain rollers isn’t distributed as evenly across multiple teeth as they should be, which would translate into more friction and more noise.

While there are a variety of objective ways to measure chain wear, evaluating chainring wear is unfortunately a fuzzier science. Dave and I both use the same method. With a new (or at least non-worn) chain installed, grab the chain at the 3 o’clock position on the chainring and then pull it away from the crankset. If the chain stays firmly engaged on the chainring teeth, you should be good to go. But if the chain pulls away from the teeth, that’s a good sign some new chainrings are in your near future. 

 — James


And here’s a bonus follow-up to a question in last week’s Tech Mailbag asking about inflation pressure limits with tubeless road setups. This reply comes from Luisa Grappone, the engineering and product manager for Hunt Wheels.

“Talking about pressures and stresses on a rim, as many 25mm tyres have a minimum pressure of 70-72.5 psi, we do recommend to use at least a 28 mm tyre with the 32 Aerodynamicist, to be compliant with the ISO regulation on max pressure on hookless rim. In general, when moving to a wider tyre, as the air volume increases, it is highly recommended to decrease the pressure in order not to overstress the rim and its sidewalls. Now, for example, if 80 psi is considered to be ok with a 25 mm tyre, when switching to a 28 mm tyre, to keep the same level of stresses on the rim we must lower the pressure by 10.7%, therefore down to 71.4 psi (calculated value). This is a theoretical value, however many tyre manufacturers recommend a range of pressures of 62-68 psi when considering 28 mm tyre with a rider weight of around 85-90 kg.

Again, in terms of stresses on sidewalls if, for example, a rider used a 25 mm tyre inflated at 90 psi on a 17 mm internal hooked rim, he would need to reduce the tyre pressure to at least 77.5 psi when using the same 25 mm tyre on a 21 mm internal hooked rim to keep the same stresses on the sidewall as by enlarging the internal rim width of 4 mm, the tyre air volume increases massively. While on the contrary, by keeping 90 psi on both rims, we would see 1,125 N/m2 of stress for the 17C rim and 1305 N/m2 for the 21C, which is 16% higher.

Hence, 77.5 psi would be the right pressure for a hooked 21C rim but if we considered a 21C hookless, the tyre air volume would increase even more and the pressure should be reduced even further. A hook normally is around 1 mm thick, so a hookless 21C would widen a further 2 mm compared to the 17C hooked, and this is why 72.5 psi (72.58 psi is the value derived from the formula) is the recommended pressure for a 25 mm tyre on hookless rim.

At Hunt, we have not done any specific tyre air volume/weight calculations as lots involve the kind of carcass, surface, riding conditions, etc. However, I can tell you that on this specific wheel, we run our standard fatigue test which, according to the ISO standards, needs to consider a weight of 65.26 kg at 25 km/h for a period to provide 750,000 hits at a pressure of 90% max tyre pressure, which in our case is 65.25 psi as, with hookless rims, max pressure is 72.5 psi, and the test passed with no issues. Furthermore, we decided to extend this test to failure, but after 2,885,840 hits, we stopped the test as the entire system was not showing any damage and the test could have gone on and on for days. The tyre was still perfectly sitting on the rim, with no issue on rim and spokes. The fatigue test we perform is a rear wheel only test and 65 kg of load applied would correspond to ~60% of the overall rider + bike weight, that is 108.35 kg.”

Hunt road and triathlon brand manager Ken Rodriguez-Clisham chimed in as well.

“In summation, we do agree that for this wheelset, a 25 mm tire is a bit narrow but with a 28 mm tire, theoretically with a tire pressure between 62-68 psi, an equal amount of forces would be enacted on the rim and one could have the same general feeling. Having been huge fans of tubeless technology for a while now, with launching our first road tubeless wheelset in 2017, we believe that running wider tires and lower pressures leads to a more efficient and thus faster setup thanks to lower rolling resistance and loss of power thanks to road vibrations. So no, these wheels aren’t built for only lightweight riders if one is willing to adopt wider tires.”

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