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Technical FAQ: Dropper posts on road bikes, brake bleed after pad change, road bike low gearing

Is the pro peloton finally catching up to Lennard Zinn's predictions about dropper posts? More disc brake maintenance tips.

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

Dear Lennard,
Mohorič won Milan-San Remo with a dropper post! You suggested this more than a year ago! Bravo!
— Frank

Dear Frank,
Thanks! I’ve been amazed that I hadn’t heard of anyone using them in the pro peloton before this. I really thought we’d see them in last year’s Tour. Now everyone wanting to descend faster will get one, unless the UCI steps in on that one, too.
― Lennard

Mohorič won Milan-San Remo on a road bike equipped with a mechanically actuated dropper post.

Dear Lennard,
I have very little experience with the 11-speed flat-mount Shimano brakes, so I’m hoping you can help. When a friend changes the pads on his 2-year-old Canyon, he said he was doing a bleed each time, which has not been my experience with my R785 brakes. And this also recently happened to another of my tribe, who has a Domane that is also relatively new. The Domane rider used his LBS, so I assume they did a full bleed.

In both cases, the brakes were fine after they were bled, but neither rider noticed any leakage over the years, so both are wondering what might have happened. Any idea? I’ve only changed my rear pads once on my 2018 Roubaix, and they pumped right back up, just like my R785 brakes on my 2016 Roubaix.
— Steve

Dear Steve,
I have flat-mount brakes on my bike and have never had that problem for more than four years with them. Since we only build new bikes, and since most of our customers are not local, we typically don’t see the bike again after it goes out. So, we tend not to be the ones changing their brake pads, and no customers have said anything to me about this happening to them.

It’s also strange that replacing the pads would have created this problem. Yes, when you push the pistons back in their bores to make space for the new (thicker) pads, the brake will be soft for the first series of lever pulls until the fluid fills back in behind the pistons enough that the spacing between the rotor and each pad goes back to its tiny, normal distance. (Ideally, before pushing the pistons back in their bores, you first cleaned the protruding sides of each piston with a cotton swab dipped in the same brake fluid that’s in the brake. This includes also holding one pad in place and pushing the opposite pad out a bit more by carefully squeezing the lever to be able to clean further back along the sides of that piston, and vice versa for the other piston as well.) But once you have squeezed the levers enough times to bring the pads back to their normal spacing from the rotor, if anything, one would think that the brake might be tighter rather than squishier, because the pistons are returning to a place slightly further back in their cylinders, thus requiring slightly less fluid volume between the master cylinder (the lever) and the caliper.

However, if it has happened, I would like to address the use of a full brake bleeding to fix the problem; it may not have been necessary to go to the time and expense of a full bleed or of even buying a bleed kit. Your tribemates might have only needed to obtain some of the correct fluid for their brakes.

If the brake has become a bit softer due to air in the system (the lever comes back closer to the bar), in this case apparently after changing pads, the more you squeeze the lever, the more the air bubbles will migrate up to the lever. You can increase this likelihood by strapping (or tightly rubber-banding) the lever to the handlebar and leaving it overnight, which puts pressure on the air bubbles and drives them up more rapidly. Ensure that the hose is trending upward the whole way from the caliper to the lever; this may require hanging the bike by the front wheel or propping up the front wheel on a bench or something. Tapping the caliper with the handle of a screwdriver can release air bubbles trapped there. (When doing a full bleed, you would also unscrew the caliper and allow it to dangle from the chainstay, with a spacer between the pistons, to better ensure upward hose routing the whole way from the caliper, but this also requires re-centering the caliper after bolting it back on. As that can be time-consuming, and this is intended to give you a quick fix, I’m not suggesting you do that.)

The next day, you may be able to eliminate the air quickly and easily with only a drip bottle of the brake fluid for your brake. After removing the strap or rubber band from the lever, remove the bleed screw from atop the lever and add a little fluid into the bleed port. Gently pull the lever slightly and release it a few times with the bleed port open while tapping the lever with the handle of a screwdriver to let out any bubbles trapped in the lever. Finish by topping off fluid at the port and replacing the bleed screw.

If this has not completely brought the brake back to the feel of the other brake, letting it sit overnight with the bleed screw off can sometimes vent a bit more air. Don’t strap the lever to the bar in this case, as that would close off the hole between the master cylinder and the fluid reservoir. The next day, top off fluid at the bleed port, replace the bleed screw, and try the brakes again.

You can often tighten up a brake just fine this way without a full bleed job. Make sure you use the recommended fluid for the brake. It’s a BIG no-no to put DOT fluid into a mineral-oil brake or vice versa! If you were trying to save time and money, that would be counterproductive!
― Lennard

absoluteBLACK oval chainring. (Photo: absoluteBlack)

Dear Lennard,
FYI, there’s yet another option (for small chainrings in addition to the AbsoluteBLACK elliptical sub compact rings mentioned). bikinGreen make 46×30 and 48×31 rings to fit both traditional 5-bolt and Shimano 4-bolt 110 BCD cranks. They use special bolts (as do the Absolute Black rings) and shift the chainline inboard slightly. They can be found on eBay, Amazon, etc.
—Stephen

Dear Lennard,
In your recent Tech FAQ, a reader Bob stated, “I am thinking of moving to a triple front to get a lower gear for loaded touring on a tandem” and “I don’t think there are 3×11 STI shifters”.

As this topic of “looking for lower gears” seems to come up often, I would like to point out the option of the Shimano XTR Di2 triple 11 speed, which I believe provides the widest range of just about any derailleur system. The remarkable benefits were sadly overlooked in the marketplace, but I hope Shimano will see the value to continue that option for people like Bob, and maybe he can find it still available somewhere.

Consider that the low of 40×22 and high gear of 40×11 provide a gearing range of 6.6 times; that’s quite good, and the 11-40 cassette that goes with the system is a very nice progression for touring. With just a 168 q-factor, it’s quite manageable. I installed this on my touring/gravel bike in 2015, when the first Di2 levers (785 series) would allow that XTR combination on a gravel/tour bike with drop bars, and upgraded to Ultegra 8070 when that came out. Brilliant! No dropped chain or missed shift in years of riding. So nice for touring to have that low gear…conditions like soft surface, steep grade, headwind, tired, end of the day sort of benefits.

If that 40×11 is not enough for Bob’s tandem (spin out at 30mph?) then I think it could be possible to experiment with a larger triple, say up to 46 or something. Maybe if enough people try, Shimano will see the value and try this again; they could easily offer both a gravel tour low range like the XTR, 40/30/22, and a slightly greater tandem road triple.
—Tim


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 Bikesand 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.