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(See postcript at the end of this article)
One way of making a lighter wheel is to make the rims thinner, which has led to an increasing number of riders experiencing a disintegrating rim, often during a ride, as shown in this photo. One of our riders was left stranded last week when her rim suffered this fate. Hence the photos and this article.
CTC’s Senior Technical Officer, Chris Juden, described this improvised tool (see second photo), made by bending a 2mm spoke (my cheap and well worn vernier gauge shows 3.6mm at the tip where I did the measurement). You can see from the third photo that I have a rim thickness of just 1mm. I happen to have in the shed a brand new Shimano RS31 wheel, which has a rim thickness of 1.3mm. I think my 2-year old Ultegra rim started with about the same thickness. That’s 0.3mm of wear in 2 years.
Should I scrap my Ultegra wheel now? The wear hole punched onto the rim opposite the valve is no longer visible, so Shimano is advising me to stop using the wheel. But of course, Shimano must play safe and it is in their interest for us to buy new wheels as often as possible. If you google it, you will find a range of opinions, some suggesting you could go as thin as just 0.5mm. CTC’s advice, however, which I intend to follow, suggests that anything less than 1mm means a new rim in the not too distant future (in my case, probably a whole new wheel). An experienced bike mechanic can feel how much dish there is on the braking surface (or simply use a straight edge to see the amount of wear), but without knowing the original thickness, this is of limited use and might lead to scrapping a serviceable wheel.
Here’s my own experience, which may be of use: I am not great about keeping the rims and pads clean, but it appears that I get about 0.1mm of wear for 2,500 miles. Given the number of variables – weight of rider, cleanliness of braking surfaces, hardness of brake pads and rim surfaces – the best plan seems to be: accept that lighter (non-touring) wheels are unlikely to last more than 10,000 miles and get your LBS to have a look if in doubt. You could also buy a cheap vernier caliper (one less than £10 will be accurate enough) and follow the advice of CTC’s Senior Technical Officer.
Postscript – I bought one of these (see fourth photo) last week. It’s much easier to use and a lot simpler than bending a 2mm spoke. And cheaper too. Writing this addendum I have added one further point, illustrated by the fifth photo: whether I use a vernier or the dental caliper, it is possible to measure only the top part of the braking surface, the rest being inside the box section and therefore inaccessible. One would like to hope that this part is the same thickness.
And finally (last photo), is it really worth reducing the thickness of the braking surface just to save 70g?