Tech & Wearables

Technical FAQ: rim wear, tire sealant woes, mullet brakes, and fork trail

VeloNews technical expert Lennard Zinn answers your questions about mechanical issues and bike geometry and handling.

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

Dear Lennard,
My bike has rim brakes, and, as a result, I routinely inspect the rim for wear. Is there a reliable method for determining when a rim has worn out and should be replaced? I’m wondering why manufacturers do not put in wear indicators (e.g., dimples in the braking surface) like they do on tires so as to indicate when a rim has worn out. The method I currently use is to place a straight edge across the inferior and superior edges of the brake track to get an idea of how much of the aluminum braking surface has been worn away. I’m wondering how much of a gap — between the lowest point on the track and the straight edge — would be acceptable?
— Marc

Related:

Dear Marc,
With your method, 1mm under the straight edge indicates it’s high time for rim replacement.

Some rims do have wear indicators. Mavic, for example, offers two types, either a hole appears on each braking surface when material has been worn off to reveal it, or a groove cut into the braking surface all the way around. It’s time to replace the rim when the hole appears or the groove disappears because the surrounding material is worn down as deeply as the top of the void or bottom of the groove. The hole appears when 1mm of rim has been worn off, and the groove is also 1mm deep. This is probably a good amount to use as a rule of thumb; if the rim braking surface is worn down by 1mm, replace it. Assuming even wear on both sides, this would mean that the total width of the braking surface has decreased by 2mm.

One method to measure this is to put a small paper clip or something else whose width is narrower than the height of the braking surface of the brake pad and whose thickness is at least 1mm against either side of the braking surface when the rim is new. With a caliper, measure from the outer face of one paper clip across the rim to the outer surface of the other. Record the number. Then check periodically, using the same paper clips centered on the brake tracks on either side, to see how much the combined width of the rim plus the paper clips at the height of the braking surface has decreased. Don’t wait to replace the rim until 2mm of width is lost if wear appears greater on one side than the other, since one side will have exceeded 1mm of wear when total combined width has decreased by 2mm.
― Lennard

Dear Lennard,
I was planning to upgrade from a rim-brake Trek Domane to a disk version, moving my eTap group across and adding mechanical (or TRP’s hybrid) disc brakes. My rationale is that I occasionally get caught in the rain, and rim-brakes on carbon wheels are not great for stopping.

With the pandemic, the year-old and used frame markets are pretty dry, so I’m considering buying a disk-brake fork and running “mullet” brakes until I find the frame. This seems to be a popular setup for off-road but wondered how an expert feels about the idea for a road bike.
— David

Dear David,
I think the mullet is fine. Most of the braking is on the front brake anyway, so running the disc brake only on the front achieves the majority of the braking improvement of disc brakes.

That said, my recommendation is to not even bother with cable disc brakes. The performance is so much lower than hydraulic ones that you might as well continue to use your rim brakes and just stop using your carbon wheels if there is any chance of rain and get aluminum-rim wheels for riding in the rain and perhaps on long, hot, tortuous descents. You’d still likely have a net lower weight gain with those wheels than with switching to disc brakes, and the braking would be better in all conditions than cable disc brakes, except perhaps driving rain.

The TRP HyRd is an acceptable alternative; it offers much of the performance, as well as the automatic piston adjustment with pad wear, of a hydraulic brake, yet it can be used with a cable brake lever. You still have efficiency loss in cable stretch, and if you want that automatic piston adjustment with pad wear, you have to set up the cable length long enough for the link arm to swing through its entire range, meaning that you have to use a lot of lever travel for brake engagement.
― Lennard

If you use a standalone pressure gauge with tubeless tires make sure to not get any sealant in it.

Dear Lennard,
I read all your articles and have a question on the one about pressure gauges. I run tubeless on road, gravel, and MTB. I have had a couple of digital stand-alone pressure gauges, but they have failed, and I think it has something to do with tubeless sealant.

Any thoughts on that, and how to prevent or clean it? Thanks for years of solid advice.
— Dave

Dear Dave,
Good question. Perhaps rotate the wheel so the valve is at 5 or 7 o’clock so sealant drains out and none comes back up from below. Then let some air out first before putting on the tire gauge.
― Lennard

Dear Lennard,
I’m long-legged, with long femurs and big feet. My feet and knees prefer my cleats in a center foot position. Needless to say, I struggle with toe overlap. My question is this: If I keep the trail constant, but increase room for my toes by lowering my headtube angle and increasing my fork rake, will the bike feel the same?

For example, my current front end has a 73.5-degree headtube angle with 45mm fork offset, and a 28mm/700c tire for a trail of 54.7mm. If I were to build a frame with a 72.5 head angle and 51mm fork offset and a 28mm/700c tire, the trail remains 54.7mm, but I gain toe room through both the slacker head angle and increased fork offset. Would the handling feel consistent due to the constant trail or would the longer wheelbase result in a substantially less responsive/more stable feel?
As ever, thank you for sharing your wisdom.
— Zach

Dear Zach,
To get those numbers, it looks like you are using a tire diameter of 686.21mm. Using that, I get that the front-wheel flop goes from 14.9mm in the first example to 15.7mm in the second. The greater the wheel flop, the greater the self-righting capability of the bike and hence the higher stability at medium and high speed and the more squirreliness/weaving at low speeds. That’s what you can expect with those changes to rake and head angle.
― Lennard

Dear Lennard,
One more tip to prevent bottle cage bolts from seizing to the rivnuts: use anti-seize on the bolts.

When a rivnut started to slip on a carbon frame, I used the trick you described, but added a convex washer under the nut to ensure that the clamping force was directed onto the outer surface of the rivnut. Not sure if that was 100 percent necessary, but it worked. Convex washers are commonly used on cantilever and v-brakes for the pads, and with disc brake mounting bolts.
— Dan


Lennard Zinn, our longtime technical writer, joined VeloNews in 1987. He is also a custom frame builder (www.zinncycles.com) and purveyor of non-custom huge bikes (bikeclydesdale.com), a former U.S. national team rider, co-author of “The Haywire Heart,” and author of many bicycle books including “Zinn and the Art of Road Bike Maintenance,” “DVD, as well as “Zinn and the Art of Triathlon Bikes” and “Zinn’s Cycling Primer: Maintenance Tips and Skill Building for Cyclists.” He holds a bachelor’s in physics from Colorado College.

Follow @lennardzinn on Twitter.