Technical FAQ: Headset spacers, disc brakes, carbon rims

Questions answered about steerer tube spacers, disc brake maintenance, and carbon rim brake track wear.

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

Dear Lennard,
Can having too many spacers on a long steerer tube cause issues with the headset coming loose? Are fewer, larger spacers better?

Dear Ryan,
As long as the entire stack of spacers is snugged down by the top cap against the stem and hence against the headset top cup until the headset is in perfect adjustment, it shouldn’t make a difference how thick each individual spacer is.

It’s critical that the top spacer has enough overlap above the top of the steering tube that the convex bottom surface of the top cap doesn’t bottom out on the steering tube before it has pushed the spacer down enough to bring the headset into adjustment. If your top spacer doesn’t overlap the top of the steering tube by at least 2mm — preferably more than 3mm — it is almost certain that when you tighten down the top cap it will be flush against the top of the steerer tube, rather than pushing down on the stack of spacers, and it hence won’t take “play” out of the headset.
― Lennard

Dear Lennard,
I have two bikes. Both have hydraulic disc brakes, and I wouldn’t have it any other way.

The rear disc on my MTB may have become contaminated due to a less than fastidious chain and cassette cleaning. The rear brake does not have the power to lock the rear wheel on tarmac. (It is not my goal to lock the rear wheel under normal riding circumstances.)

I changed the pads to new, and still, the brake cannot lock the rear wheel. Is there a recommended way to clean the rotor? Is there some other condition I might need to inspect to restore the braking power? The brakes make a howling sound as the bike slows on descents with the brakes engaged.
— David

Dear David,
Making the assumption that your rear brake, with the same type of pads as you replaced the old ones with, used to offer you the wheel-stopping performance you’re using as a gauge of brake power, then I’d surmise that you are dealing with pads contaminated with oil. Presumably, they were contaminated by an oily rotor.

Cleaning the rotor is easy enough—wipe it thoroughly with rubbing alcohol and a clean rag.

Cleaning the pads is going to be key, and that can only be done by removing the entire top layer of the friction surface of each pad. Remove them and clean them by sanding them clean by rubbing them face down on a piece of drywall-sanding screen sitting on a flat surface.

Howling is due to vibration, which can reverberate through the frame. To eliminate any sources of vibration that may have developed as well as contaminated pads and rotor, check that nothing is loose in the rear end of your bike. Make sure that the caliper bolts and the rotor bolts are tight, and, if your bike has full suspension, check the tightness of all of your suspension pivots.
― Lennard

Dear Lennard,
I recently (finally!?!) obtained a set of tall-profile carbon-fiber wheels, Zipp 404s, which will now serve as the primary set for my Calfee. I’ve also installed carbon-appropriate brake pads to mate with the Zipps’ carbon brake tracks. I have several sets of standard wheels — with alloy rims — which I also may want to use on this bike (due to varying gearing needs, weather conditions, etc.), from time to time. Given that swapping and re-aligning brake pads can be modestly cumbersome, how critical is it that I run the appropriate pads with the corresponding rim material? If I’m careful, may I occasionally run my alloy wheels with the carbon pads (or vice versa)? Or, am I destined to be a recurring pad-swapper?

— John

Dear John,
The issue you are trying to avoid is getting aluminum embedded into the brake pads that you will use on your carbon rims. If you use the pads that you run on your aluminum rims on your carbon rims, then, unless you dug each little bit out, it is likely there is aluminum embedded in the pads. That embedded aluminum will tear up the carbon rims.

Yes, I think you’re destined to be a recurring pad-swapper to extend the life of your new carbon rims.
― Lennard

Dear Lennard,
I had a question for myself about carbon wheelsets. I’ve been using Farsports wheels for quite some time and rate them highly, but I’ve recently been seeing deposits of short fibers on the rims and brake pads. Cleaning aggravates this fiber shedding, as I believe a layer of brake pad material is deposited on the rims that slows wear. This, as you may imagine, also reduces stopping power. Conversely, when shedding short fibers, braking performance is excellent, at least in the dry.

I’m OK with cleaning the braking surfaces every now and then, but should I be worried about the fibers? How can I tell for sure how much the wheels are worn? There are no wear indicators nor any bulging/concaveness on the brake track. I had measured the width of the wheelset and the thickness of the rim walls and any reduction in thickness is beyond what I can measure with a Vernier caliper.
— Efe

Dear Efe,
I don’t know anybody at Farsports to ask about this. I did check with knowledgeable staff at Zipp, Enve, and Alto, and I got the following responses that apply generally to all carbon rim braking surfaces.
― Lennard

From Zipp:
The Zipp user manual gives clear indications about brake track wear on carbon rims. We show renders of worn and new rims. If the users are seeing wear signs, then the rim is compromised and should be retired.The user manual can be found on the Zipp web site, and the section related to brake track wear is on page 11 – 12.
– Michael Zellmann, SRAM/Zipp marketing directorFrom Enve:
Because we have a molded-in brake track texture, it is pretty easy to tell when an ENVE rim is getting on in years as it relates to brake track wear. We would say that when the texture is gone, and visible/palpable cupping exists on the brake track, it would be time to consider replacing your rim. There is no predefined mileage or period for which a brake track is expected to last as brake track longevity is heavily determined by the rider’s style, ability level, region, weather, etc…In this case, it sounds like the brake track is done…
– Jake Pantone, Enve VP product and consumer experienceFrom Alto Cycling:
It sounds like your reader experienced a bit of delamination due to heat stress, which can cause the clear coat on the brake track to fail and expose open fibers from the 3k weave (or whichever weave they’ve chosen for the outer layer of the brake track). This is typically due to instantaneous failure during descending, as opposed to chronic fatigue. Under normal circumstances (without heat transfer related issues) the brake track will wear and lose material over time, similar to what you’d see in an aluminum brake track. Over time, the rim will start to cup where the brake pad is wearing a groove into the track. Each rim has different wall thicknesses, but for Alto rims, we designate a safety limit up to 0.6mm of material removal before the rim should be replaced. This can be measured with a digital caliper, but always use your best judgment when inspecting your carbon rims. Look for any warping/grooving/delamination that could be of concern.
Bobby Sweeting, founder, Alto Cycling

Dear Lennard,
Re: Your response to Jernaj concerning flat mount to post mount brake mismatch. I am using an adapter from AS solutions to mount a flat-mount Ultegra R8000 caliper to a post-mount fork.
It takes a few days to cross the border, but it was worth the wait.
— Mike

Dear Mike,
Here’s a photo of a Campagnolo caliper, a great brake which without this adapter only those with flat-mount frames and forks could enjoy.

I wish I’d known about this when building up my e-bike, which has IS mounts on the seatstay and flat mounts on the fork. We can’t put flat mounts on the chainstay of our titanium e-bikes because our jig system for holding the chainstay while machining it for flat mounts and while welding on the mounts indexes off of the bottom bracket shell. The jig requires the left chainstay to first be welded to a bottom bracket shell, and e-bikes don’t have a bottom bracket shell.
― Lennard

Lennard Zinn, our longtime technical writer, joined VeloNews in 1987. He is also a custom frame builder ( and purveyor of non-custom huge bikes (, 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

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