Jim’s Tech Talk
By Jim Langley
Several good technical questions were dropped in the comments to Techs Talks of the past few weeks. Here they are with my answers.
Shift cables and housing issues
About the story on Tips for Internal Brake and Shift Cable Routing Adrian Hobbs asked:
For the first time I changed the cable myself yesterday. I found the cable had an internal sheath that had jammed tight onto the cable where it turns under the bottom bracket – presumably from repeated stretching and rubbing. Because of this the sheath was moving backwards and forwards with gear changes, and the top end where it exits the frame before going to the gear levers was jamming up against the end of the normal outer cable. As a result the end was heavily wrinkled and further jammed the inner cable. Gear changes became very hard, probably leading to the inner cable breaking yesterday.
My questions is:-
1a. Should the inner sheath remain on the inner cable through the carbon frame? or should it be removed after getting the new inner cable installed? (Is this make/frame specific?)
1b. If I leave the inner sheath on the cable inside the frame, does it need to be a particular type of sheath? e.g. Teflon perhaps.
As well as the problems I can see advantages of leaving this sheath in position:- reducing the sound of cable slapping the inside of the down tube, and possibly providing a reduced friction turn at the bottom-bracket.
My Answer
Congratulations on changing your cables, Adrian. Without seeing the cables and housing you have on your bike and also the path through the frame I can only give you general guidelines on cable setup and routing. But you should be able to use these tips next time. I would say that it is possible that the wrinkled sheath was causing the shifting difficulty. If the cable broke at the point then it contributed to that too.
Cables and housings differ a lot. There are bare cables and ones coated with Teflon or plastic or covered with a thin Teflon sheath. Similarly, there’s housing with and without a lining inside.
The best practice is to only use the cable and housing designed for your components. So, if you break a shift cable, you should replace it with the same exact cable. That way you will know that it will fit and work properly inside the housing on your bike (assuming no one has ever changed the housing).
If you don’t know what housing and cables are on your bike, the safest approach is to buy a cable and housing set to replace your cable and housing with. That way you’re sure to get a cable and housing made to work together.
Covered Cables
You asked about the sheath on the cable and if it should remain on the cable inside the frame. This depends on what’s going on inside the frame. If the cable passes through a tube inside the frame then the sheath should work fine. But if the cable runs over something that can snag it (like a carbon edge/transition), then ideally there would be a piece of housing inside the frame for the shift cable to pass through. That would protect the cable and prevent the sheath and cable wearing over time.
The thin sheaths covering some shift cables are more fragile than the Teflon coating on cables. But, you have to be careful with both. If the housing sections have Teflon/plastic liners inside some coated or sheathed cables might be too tight a fit to function well.
That’s why the easiest approach when replacing cables and housings that are causing problems is to start fresh with a cable and housing set you know is made to work together and designed for your specific components. I mostly ride Shimano components and I use their cable and housing sets.
Through the BB
When routing cables and housing through a BB it’s a matter of looking at what’s going on inside. If the frame is designed well there should be a dedicated cable path and even a secure way for the housing and cable to stay in place and not suffer any wear and tear.
Bottom brackets often have sleeves/dust and moisture seals that cover the spindle (the spinning part), so the cable and housing should not be able to touch that and be worn by it. But, if I found one that was, I’d either install a BB sleeve (any home made sleeve will work – even a rolled up piece of plastic) or route the housing and or cable to be well above the spindle. Kind of a fun thing to solve.
Here’s Park Tool’s latest video on shift cable and housing installation
Gearing for Mt. Lemmon
About the article Tackling an Epic Climb: Mt Lemmon Tucson, Arizona Jerry Brick asked:
I plan to take the Mt. Lemmon challenge next year, but was wondering what bike to take. I have an 18 lb road bike with a 53/39 crankset and 12-25 cassette. I also have a 25 lb gravel/touring bike with a 42/28 crankset and a 11/32 cassette. Would the heavier but lower geared bike be the smart option?
My Answer
I mentioned in my article Jerry that I saw about 50 riders when I made the ascent a few weeks back. I was the only one on a gravel bike. Everyone else was on standard road bikes like your 18-pounder.
That argues for riding the lightweight road rocket. But, consider that it’s possible those 50 riders live at elevation in and around Tucson. If so, the long ramp to the 7 to 9K top probably affects them less than someone who lives at sea level like this roadie. It’s also possible they were all fitter and younger than I am. I’d go further and say it’s possible that they’re all natural climbers or at least love to climb (which means they likely spend lots of time going vertical, which equals legs & lungs ready to handle climbs).
So you have to consider your altitude aptitude and your overall cycling fitness with age being a consideration, too.
Since you are riding a road bike with 53/39 chainrings and a 12/25 cassette, I’m willing to go out on a limb (based on my own experience) and assume you’re in your 50s and pretty fit. (Although if it’s pancake flat where you live, that throws that theory out the window).
It’s very hilly where I live and I was riding the same gearing as you up until I turned 60 at which point I went to a 50/34 crankset (compact gearing. Soon after that I switched to an 11-28 cassette. On my gravel bike I have a 48/32 crankset and an 11-34 cassette and I felt I needed it on Lemmon. That’s partly because I have bad knees too.
But, if you do well at altitude and are fit and in your 50s, I think you could stick with your 53/39. But you might want to put on a 11-28 or 32 just for insurance that you get to the top. Keep in mind too that even professional roadies these days are running low gearing on tough climbs, some using 36 tooth inner rings and 32 tooth rear cogs.
The other thing to check is your tires. I found the road rough and sketchy in places (seams, sand or gravel, snow and ice at the top). Climbing the mountain you’re not moving too quickly so you can avoid most of the bad stuff. But on the descent you’re flying and there can be a lot of traffic. I hit enough bad patches that I was really happy to be on 40mm wide gravel tires. So I recommend going with at least 28c tires or if they won’t fit, go with durable 25c tires that can take a licking.
Hope these tips are helpful and good luck with the climb Jerry! Please let us know how it goes and what bike and gearing you decided to take. And don’t miss the store with the plate-sized chocolate chip cookies at the village at the top like I did!
Pump Problem
About the article Tips for Tires that Won’t Accept Air Mangesh Dahale asked:
I’m trying to inflate my bike tire with the help of a bicycle pump… but when I pump it down…it bounces back toward me. The tire has a Shraeder valve and the pump connector is properly connected but it fails to inflate the tire. What is the reason behind this? And how can I solve this problem?
My Answer
Great question, Mangesh. It’s actually not a tire or valve problem, it’s a problem with the pump. All bicycle pumps, whether they’re “floor pumps” made for home use or portable pumps for use on rides, should have a check valve inside the pump that prevents blowback, i.e. air going into the pump from the tire which causes the pump plunger to rise if the check valve is faulty.
The plunger shooting back at you like this can actually be dangerous because it’s unexpected. If you don’t see it it could hit you in the face. I’ve seen plastic frame pumps break from the force and the plunger go shooting across the road!
But that was years ago before there were so many quality pumps on the market. So my best guess is that you have a pump with a broken or non-working check valve. Depending on the make and model you might be able to find them online and ask if it’s fixable.
In most cases though, you’re probably going to need a new pump. If you buy it from a bicycle shop you can test it before purchase to be sure it works to your liking. Also the pump should come with a guarantee should anything go wrong and the shop might sell replacement parts for it too.
You can test some pumps for potential blowback by blowing into the head. You shouldn’t be able to make the plunger expand/rise.
Your Turn
If you have any advice for Adrian, Jerry and/or Mangesh I’m sure they’d appreciate your tips. Please leave them in a comment. Thanks!
Jim Langley is RBR’s Technical Editor. A pro mechanic & cycling writer for more than 40 years, he’s the author of Your Home Bicycle Workshop in the RBR eBookstore. Tune in to Jim’s popular YouTube channel for wheel building & bike repair how-to’s. Jim’s also known for his cycling streak that ended in February 2022 with a total of 10,269 consecutive daily rides (28 years, 1 month and 11 days of never missing a ride). Click to read Jim’s full bio.
Check valves, etc… Purchase a high quality pump at a bike shop, there is a good probability the shop will also have replacement check valves, plunger gaskets, etc. In the case of my Topeak Joe Blow pump leaking badly, the shop mech pointed to a yellow knob at the bottom of the barrel (which was the check valve attachment). In 5 seconds the pump was like new (no charge, but I was prepared to buy a new pump).
“I’ve seen plastic frame pumps break from the force and the plunger go shooting across the road!” 🙂 Those of us of a certain age will recognize this as the classic Silca Impero. While it has its “quirks”, it still has a place on my vintage bikes. However, I’ve learned to not loan it to anyone!
After climbing Mt Lemmon last year, I switched to an 11-30 in the rear. I rode it again a week ago and it was much more enjoyable on the few steeper sections.
Never seen a bike with too many gears. I have a tripple w/ 28, 36, 50 chain rings & an11x32 10 spd. cluster. Wished for lower gears in UpState NY. Of course I was 74 at the time.