Technical FAQ: Hydraulic hoses, eTap, Sagan’s wrenching

This week, Lennard Zinn addresses reader questions about a host of issues.

Hydraulic brake hose quick disconnect

Dear Lennard,
You mentioned in a follow-up that a hydraulic hose disconnect might become available, but I still see nothing. Any further info?

I see units for motorcycles, so it is certainly feasible (and must pass the “lawyer test”).
— Steve

Dear Steve,
I too am waiting for such a thing but fear that it may be in vain. I had high hopes for the SRAM Connectamajig being a quick disconnect like an air hose fitting. But while it is nice in that it prevents fluid loss from the lever after disconnect and is a massive improvement over first-generation SRAM HRR and HRD brakes in that you no longer have to disassemble the lever to disconnect the hose, you still have to unscrew the fitting. And that has to be done right at the base of the lever where the handlebar tape has to be unwrapped to get at it.

I’m still hoping for something like a motorcycle quick-disconnect fitting, which is really what is required for ease of travel with a hydraulic disc brake-equipped bike. I suspect that the light weight required on a high-end bicycle is an insurmountable barrier at this point in time. The thing still has to be so secure that it doesn’t attract ambulance-chasers (personal injury lawyers).

Nate Newton, SRAM’s marketing technical rep for road products, had this to say on the matter:

“The major benefit of the Stealthamajig is that it allows us to assemble the lever and caliper/hose separately and give them a proper bleed at our factory. A bike assembly factory can then route the hose and connect without the need for a bleed, ensuring better bleed quality when the bike hits a showroom floor. It is not intended as a quick disconnect.

“Since bicycle hydraulic systems are so lightweight and low volume, an air hose fitting type of disconnect is not an easy thing. The amount of fluid that can be lost before a rebleed is required is very small. The Connectamajig technology on HydroR does allow for 2 (tooled) disconnects before a rebleed is required, but like Stealthamajig, it’s intended to make bike assembly easier, not for repeated travel use.”

― Lennard [related title=”More Tech FAQ” align=”right” tag=”Technical-FAQ”]

SRAM eTap and compact cranksets

Dear Lennard,
I tried putting SRAM eTap on my wife’s Look 586 frame, but the frame hanger for the front derailleur was set too high for her compact (50-34) crankset. Lowering the derailleur as far as the tab would allow still left a large gap between the derailleur cage and the chainring teeth: much more than the 1-2mm that eTap requires. I went ahead anyway, and on the first ride the chain jammed and tore the derailleur tab off the frame. 

I haven’t found any frame manufacturers that give front derailleur tab height in the technical specifications. How can I find a frame that will accept eTap with a compact crankset, without going full custom?
— Geoffrey

Dear Geoffrey,
The best way is to find a frame without a derailleur tab so that you can just use a band clamp and put it at whatever height you want. Another solution is to talk your wife into a mid-compact 52-36 instead of a 50-34 combo. Then the tab meant for a racing bike with a 53-39 crankset would work.
― Lennard

Follow up on descending speed based on body size

Dear Lennard,
Yes, the larger bodies have an advantage on a cold day on a flat course. However, on a hot day with a finish at the top of a pass, a smaller and lighter body may have an advantage. On a long steep climb on a hot day, the max power is controlled by the cooling limit. A small body has an advantage as its area per pound and thus its power is larger.

If h is height: (1/h^2) / (1/h^3) = 1/h

The smaller the body, the better it is cooled.

Long, skinny body types like Chris Froome’s have an advantage too.
— Pierre

Dear Pierre,
Like Chris Froome, I have a high surface area-to-volume ratio and often experienced that to be the case in the heat.
― Lennard

Follow up on Di2 gear position control

Dear Lennard,
Your discussion on the newer Di2 fail safe for short chain stays was very enlightening. I am new to Di2 and have an R8070 setup with a disc brake, 12-142 thru-axle, and a 34-50 compact crank on a 68mm T47 bottom bracket shell. However, the bike is a gravel bike with 430mm chain stays. I did wonder about what I now know is “gear position control” preventing me from dropping into the smallest cog and chainring combo.

To make removing the rear wheel easier, is it possible and feasible to override GPC if one’s chain stays are long enough? The eTube Studio shows me that GPC is enabled and doesn’t offer any way to disable it. Is there a magical incantation that could disable it, and would that be acceptable given the longer rear triangle geometry?
— Eric

Dear Eric,
I don’t think there is a way to override the GPC.
― Lennard

Dear Lennard,
Regarding Don’s problem, I feel like I have heard of Di2 junction boxes going bad, so I wonder if that’s the issue. If it is, one way to get back to manual mode is just to change something, anything, in the settings with the iPad or phone app. I remember being surprised the first time to find the app switches the Di2 back to manual mode on exit. That’s where my Garmin 520 was nice. When that happened, I was on a ride and briefly very confused, but I was able to cycle (no pun intended) through and see what mode I was in. I’ve been using a Wahoo Elemnt Bolt the past six months. It has a lot of virtues over my 520, but the one thing I do miss is the killer support my 520 gave me with my Di2, particularly when I was just figuring out the synchro shifting. The Bolt only tells me what gear I’m in and my battery level.
— Steve

Follow up to Sagan’s stem tightening during Paris-Roubaix

Dear Lennard,
What if Peter Sagan is just using a new spin on the old “magic spanner” trick? You can’t take a push from the mechanic hanging out the team car window anymore, but you can still take a tow from the group if you do your own wrenching.
— Andrew

Dear Andrew,
Could be, but he sure did turn those bolts a long way!
― Lennard