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Technical Q&A with Lennard Zinn – Hangin’ out

Dear Lennard,I always wondered about hanging my mountain bikes vertically too but not really worried about the shocks but about the hydraulic brakes and what that might do to the lines. Is it fine?Paul Dear Paul, That’s a good question. If the bike is hanging vertically with the front wheel up, there would be no problem, because any air bubbles would gravitate toward the lever, which would be where you’d want them. But how about with the rear wheel up?

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By Lennard Zinn

Can we hang this one by the rear wheel?

Can we hang this one by the rear wheel?

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Dear Lennard,
I always wondered about hanging my mountain bikes vertically too but not really worried about the shocks but about the hydraulic brakes and what that might do to the lines. Is it fine?
Paul

Dear Paul,
That’s a good question. If the bike is hanging vertically with the front wheel up, there would be no problem, because any air bubbles would gravitate toward the lever, which would be where you’d want them. But how about with the rear wheel up?

Some systems are designed to have all air evacuated completely from the entire closes system when bled properly, and with such a system, it would be irrelevant if it’s hanging upside down.

More commonly, though, many modern systems do have air above the fluid in the lever’s reservoir, and as long as there is sufficient fluid in the system, there is no problem. If air gets in the system while in use, it rises up to the lever, out through the metering hole in the master cylinder, and up to the reservoir, where it sits above the fluid and causes no problem. With such a system, if you crash arse-over-teakettle, you can find that you have no braking for a while once you get going again, because air from above the fluid in the reservoir passed through the metering hole ahead of the piston and into the brake lines.

The metering hole is ahead of the piston so that fluid can flow down from the reservoir into the master cylinder until the piston is pushed in far enough that it closes off the hole, at which point it can now build pressure through the system and push the pistons in the slave cylinders to force the pads against the rotor. It would seem that hanging the bike upside down (rear wheel up) could conceivably create the geometry such that the air bubble in the reservoir is lined up with the metering hole, in which case air could bubble up into the brake lines.

(For those to whom this is new, air in hydraulic brake lines, whether on your bike or in your car, is a bad thing, because air, unlike liquid, is compressible. So when you pull your brake lever or stomp down on your brake pedal, if there is air in the brake lines, the pressure can simply compress the air, rather than pushing the brake pads, and you won’t be able to stop.)

In practice, I’ve hung a lot of bikes with Shimano and SRAM hydraulic brakes (both of which have reservoirs in the lever that can have air above the liquid) upside down without noticing a loss in braking performance afterward. I would have to guess that normally the angle of the reservoir when upside down is such that the air bubble does not come in contact with the tiny metering hole because of the angle of the lever on the handlebar in three dimensions relative to horizontal. But if air were indeed to travel into the cylinder and into the brake lines, pumping the lever a few times once the bike was again upright should squeeze those bubbles, and, with the help of gravity, move them rapidly up to the master cylinder and up through the metering hole and out into the reservoir, at which point the brake would perform correctly again. I doubt that, in a static situation like hanging a bike, a bubble could make its way all of the way into one of the wheel (slave) cylinders, due to the various bends in the hydraulic tubes.

Air bubbles generally are unimpeded coming back up to the master cylinder under repeated braking unless they get stuck in corners of a slave cylinder.

So, in answer to your question, hanging the bike by the front wheel will most certainly not compromise your hydraulic braking, but hanging it by the rear wheel could, although I suspect that it is unlikely. And in most cases, the problem should clear itself up again soon with some vigorous pumping of the levers.
Lennard



Dear Lennard,
My wife bought me this wicked TT bike for Christmas and I’ve been riding it a bunch on the trainer (winter and all…).

It’s developed a noticeable groove in the middle of the Hutchinson Fusion 2. Now, I know I should have bought one of those garish yellow trainer tires, but I didn’t. So, my question is, when do I need to be alarmed about this wear compromising the tire integrity when the rubber actually does hit the road?
Illya

Dear Illya,
You can continue to ride that tire on the trainer until the rubber is worn through to the casing. You don’t need much puncture resistance on a trainer. But it would be foolhardy to then take that same tire out on the road, even though I have seen a number of people riding with the casing showing through the center rubber.

However, I think that the issue really is a question of cornering as much as it is of durability. Once you create that flat spot, when you lean the tire over in a turn, especially at high pressure, the edges of that flat center section do not give you a smooth transition from center to sidewall. That abrupt transition at the sharp edge disrupts your cornering. In some situations, this could be a significant safety concern, especially in the case of minimal traction due to water or sand on the road, or in the case of an emergency maneuver.

Once it’s gotten that flat on top, I’d get a different tire for riding on the road.
Lennard



Dear Lennard,
I would like to put a set of Mavic Ksyrium Elite wheels that I used on my 9-speed Cannondale on my new Specialized Tarmac 10-speed bike. Will they be compatible? I wasn’t sure about rear dropout width or what spacers might be required behind the cassette. My components are Shimano 105.
Jim

Dear Jim,
Yes, it will be completely compatible. Dropout overlock dimension is the same, and the freehub body is the same. It may require a thin spacer behind the largest cog.
Lennard



Dear Lennard,
It’s been a while since you bailed me out big time when you suggested I use a beer can shim to hold my Look Ergopost in my KG451 ? it’s still solid ? much appreciated.

I happened to notice how very thin the seat stays are on the latest Cervélo frames, reminiscent of the older Scapin frames. Then I see the beefy seat stays on Marcus Storck rides like the Absolutist… what is the school of thought?

Is one a better solution than the other?
Ron

Dear Ron,
The thinner seat stays would generally provide more vertical compliance on rough roads than the larger diameter seat stays. And no, one is not necessarily a better solution than the other – it depends on what kind of ride quality you want to achieve.
Lennard


A formula for failure: Dirt, grime, time and rim brakes.

A formula for failure: Dirt, grime, time and rim brakes.

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Dear Lennard,
Here are some great photos of what happens when you don’t replace your rims frequently enough. I think these will support your article regarding rim replacement.
Dave

There won’t be a column from me next week; I’m hiking into the
Grand Canyon this morning and floating out of it in a week and a half. Enjoy the beginning of April.
Lennard


Technical writer Lennard Zinn is a frame builder (www.zinncycles.com), a former U.S. national team rider and author of numerous books on bikes and bike maintenance including the pair of successful maintenance guides “Zinn and the Art of Mountain Bike Maintenance” – now available also on DVD, and “Zinn and the Art of Road Bike Maintenance,” as well as “Zinn and the Art of Triathlon Bikes” and “Zinn’s Cycling Primer: Maintenance Tips and Skill Building for Cyclists.”

Zinn’s regular column is devoted to addressing readers’ technical questions about bikes, their care and feeding and how we as riders can use them as comfortably and efficiently as possible. Readers can send brief technical questions directly to Zinn. Zinn’s column appears here each Tuesday.

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