Tech & Wearables

Technical FAQ: Bottom bracket alignment, pump accuracy, cable housing, and gravel frame geometry

Our technical guru answers questions about bottom brackets, floor pump gauges, older Campy style cable housing, and frame geometry.

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

Dear Lennard,
How can I determine if the bottom bracket bores of my new carbon road frame are in alignment with each other? I’ve measured the bores using dial calipers in several places on each side, and they were within spec and round. But, the brand-new SRAM bottom bracket seems to have more drag, even after a 1000 miles or so, than my previous cracked frame from the same company that came with the same type of SRAM bottom bracket. With the cranksets removed, all the bottom bracket bearings seem fine on both bikes. Perhaps the shop that installed a new bottom bracket in the new frame, didn’t press it in exactly flush with the frame? I’ve redone the lateral preload adjustment and removed and reinstalled the crankset several times. I am using a SRAM RED 22 crankset which came from my previous, 3-year-old frame.

I’ve built wheels, bikes, etc. since I started racing in 1970s while getting my BSME. Perhaps I should buy the Park PF30 tool and remove the bottom bracket and reinstall it myself. Can the PF30 bottom bracket be reused?
— Jeff

Related:

Dear Jeff,
It certainly sounds as if the bearings were not aligned parallel with each other, and I have no way of knowing if that’s because they weren’t pressed in straight or because the frame won’t allow them to be. I don’t know how to check if the bores are parallel without equipment that only a machinist would have.

I would try to press that BB in again; yes, you can reuse it as long as the bearings feel smooth to turn in your hand, and a standard headset press will do the job. The Park BB30 removal tool is relatively inexpensive.

If that doesn’t work, I’d install one of these Wheels Manufacturing bottom brackets instead. Since it threads together on itself, it keeps everything in alignment.
― Lennard

Dear Lennard,
As a lighter rider, I have been a lower pressure/larger tire disciple for many years now. I finally came to realize that while we are riding and talking, we think we are talking pressure but are really about talking about different pumps.

All three of my floor pumps read different pressures, and I’m sure other people’s pumps are wildly inaccurate too, so I use a Schwalbe Digital Gauge, which I am inclined to believe more than various pump pressure gauges, but I wonder if all that does is give me a fourth reading. Which one should I believe? How can I reliably read my pumps’ gauges?

Should I split the difference or average them, can I add a fudge factor, or can I calibrate the gauges?
Would the discrepancy be linear; that is, if the pump reads 10 over at 100 psi will it be 5 over at 50 psi?
— Doug

Dear Doug,
I run into this constantly. I believe that a pump gauge, since it is actually measuring the pressure in the hose, a long way from the valve, is going to be inherently inaccurate. I have lots of different digital gauges, and they all seem to read within 1psi of each other. That is plenty good enough for me. I recommend you believe your digital gauge and not any of your pumps.

I don’t know if the discrepancy is linear or not, and I suspect it depends on the gauge. Some gauges seem to have an initial offset, like if the spring inside the gauge is bent, in which case the discrepancy certainly would not be linear.

I don’t know if there is a standard to calibrate against that is easily obtainable. Don’t overthink it. Just use your digital gauge.
― Lennard

Campagnolo cable housing.
A Klein Rascal about to be restored for riding.

Dear Lennard,
At age 67, I’m probably about your age, but my passion for bikes is still strong. I recently acquired a 1992 Klein Rascal frame/fork/mission-control headset and I am in the process of building it up. My question is this:

I really like older Campy-style cabling, the stainless coils kind. I have used it before and would like your advice as to the quality of this type of cable housing. Although I have some vintage XT thumb shifters, I am planning to just use the friction mode, at least for now, so I won’t need the rigidity of indexed shifting cables and housing.
— Pete

Dear Pete,
Wow, you’ve kept that little Rascal in good condition all of these years! If you mean the unlined little piece of coil made of stainless steel round wire (as opposed to flat wire) that Campagnolo used to supply for the last little stretch from the chainstay cable stop to the rear derailleur, I would not recommend it for long cable runs, because it has no liner and will consequently create more friction.

The flat coil in the photo of your white bike looks like standard lined brake cable housing with a clear cover. That could be used with frictional shifters. And you obviously do understand that indexed shifting will not work with coil housing because the compression of the housing results in more cable pull per shift required than the indexed lever pulls on each click.
― Lennard

Dear Lennard,
Why are top tube lengths on gravel bikes the length that they are?

Some gravel bikes are touted as having new revised geometry, compared with road bike geometry. From what I can tell they have simply extended the top tube by 1cm (size 56). Big deal, one whopping cm! What happens to handling when the top tube gets longer, and the stem gets correspondingly shorter? Does a 1cm increase in top tube length make a tangible difference?

Pro road riders use long stems. Would they be better off if they make top tubes longer and stems shorter? Would Caleb Ewan’s rear wheel skitter around less during sprints? Would riders catapult over their handlebars less often? Would there be riots in the streets proclaiming the dangers of disc brakes… I mean long top tubes?

Back to gravel bikes and longer top tubes. I have short legs. My gravel bike has a 130mm stem. Having done a lot of mountain bike racing in the past, I feel way too far over the front wheel when the road gets rough or loose. Specialized came out with the new Diverge EVO. I am going to get a size medium (60cm top tube) and put a drop bar on it with a 90mm or 100mm stem. I’m thinking this will result in a better handling gravel bike? Any thoughts?

If you tell me that long stems slow down handling, then how do mountain bikes, which need to make much tighter turns, get away with such long top tubes?
— John

Dear John,
Some of the same rationale is at work with modern gravel bikes as with modern mountain bikes. On mountain bikes, as top tubes have gotten longer and stems shorter, handlebars have become longer. A lot longer. Yes, on the mountain bikes of the 1980s and early 1990s with 560mm long handlebars, the long stem, sometimes as long as 150mm, slowed down the handling by requiring more hand movement to rotate the steering tube through a given number of degrees. Fast forward to today, where the handlebar is 800mm long and the stem is perhaps 40mm long. That long bar is having a similar effect to the long stems of yesteryear, again requiring more hand movement to rotate the steering tube through a given number of degrees.

The first generation of gravel bikes tended to have the same handlebar, as well as the same top tube and stem length as road bikes. Now, the handlebars are flared out and wider, and the stem has become correspondingly shorter to increase the steering quickness lost with the increase in bar width. The top tube length had to increase to pair with the shorter stem. And, in a similar manner to what has happened with the march of time to mountain bikes, the longer top tube has moved the front wheel further forward relative to the feet, hands, and butt, something you will undoubtedly appreciate.
― Lennard


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 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 on Twitter.