Tech FAQ: Road disc guards, frame design, more motors

Lennard Zinn answers questions about frame design, hidden motors, chainrings, anti-seize treatments, and the ongoing disc brake debate.

Dear Lennard,
Here is a question that I am having a hard time locating impartial information about: the pluses and minuses of sloping vs. traditional frame design. Do you have a preference both in terms of performance as well as comfort? I also assume that there is no difference in durability.
— Bill

Dear Bill,
For the same top tube length and frame size, a sloping top tube makes the seat tube, seat stays, and top tube shorter. That means that, with the same tubing utilized, the frame is lighter and torsionally stiffer.

This may not be as obvious with the top tube, but I think it is particularly important and illustrative. For the same horizontal effective top tube length, the more the top tube angles up until it becomes perpendicular to the seat tube and head tube, the shorter the actual length of the top tube becomes. This certainly gives it more torsional rigidity and also resistance to bowing. Or, a lighter (thinner wall, or smaller diameter) tube can be used and offer the same stiffness and strength as the original tube built horizontally into the frame with even lower weight yet.

Of course, a sloped frame depends more on the seatpost than does one with a horizontal top tube. The seatpost must be longer, stronger, and stiffer on a sloping frame.

Comfort, due to the flex of the long seatpost, may be higher on the sloping bike.
— Lennard

Dear Lennard,
I am a mechanical engineering student and collegiate racer. I couldn’t help but take action after reading the stories across the web about “motorized doping.”

Cycling has a hard enough time holding a clear image with the public and spreading speculation about cheating isn’t going to help. I could be completely wrong, but at least for now I ask VeloNews to stop feeding the rumor mill and display all sides of the argument.

Almost every rider is using an electronic groupset — there is the distinct possibility that the heat signature is merely the seatpost battery (Campy/Shimano) discharging from repeated shifts.
— Ben

Dear Ben,
In the few Forward Looking Infrared (FLIR) photos I’ve seen, the eyebrow-raising heat signature is in the bottom of the seat tube. It is certainly possible to drop a Shimano Di2 or Campagnolo EPS battery all of the way down there, and it would indeed give off some heat during repeated shifting. Normally, however, the battery is much higher up in the seat tube: either stuffed inside the seatpost, or, in the case of Campy, it is bolted in place through the seat-tube water-bottle bosses.

It would be interesting to examine more of these FLIR photos to see what the heat signature of the battery in electronic-shift bikes generally looks like. In the photo labeled “02:06” with the hotspot at the bottom of the seat tube, there also appears to be a relatively warm area where the bottom of the seatpost is as well; that might well be the shifter battery.
― Lennard

Dear Lennard,
I have a previous-generation 10-speed SRAM Force crankset and chainrings. In the near future, I’d like to move to SRAM 22 and put 11-speed chainrings on my Force crank. However, the bolt pattern has changed with the transition to 11-speed — one bolt is now hidden behind the crank arm. This would be a 36-degree shift in chainring mount orientation. Is it likely to cause shifting problems?
— Matt

Dear Matt,
The shift pins and ramps will not be ideally located, and the chain will not shift as well on that ring as on your old chainring.

Furthermore, your chain-drop pin will not be behind the crankarm and thus won’t be able to prevent the chain from jamming between the arm and the spider, should you throw your chain off to the outside.

When you need new chainrings, you’d be better off just replacing your rings with 10-speed rings. They will work better on your 10-speed crank with the 11-speed drivetrain than will SRAM 11-speed chainrings made for removable spiders or for the Red crank with one spider arm integrated with the crankarm.
― Lennard

Dear Lennard,
I always apply lithium grease to bolts before torqueing unless the part comes with instructions to use Loctite, and I’ve always greased stem bolts. Recently a titanium stem bolt broke while torqueing and the replacement bolt from the manufacturer (Deda) came with a strip of dry blue Loctite. What gives? Should stem bolts be greased or ‘Loctite-ed’ before torqueing? Do you have any guiding principles on when to Loctite and when to grease bolts on high end bikes?
— David

Dear David,
Titanium bolts in particular need to be treated differently than steel bolts. Titanium has a tendency to gall, seize, and tear. The issue is worst when dry, and grease can also be insufficient to prevent this from happening.

What you need on a titanium bolt is anti-seize compound specifically designed for titanium threads. Finish Line Anti-Seize Assembly Lubricant is the ticket for this. This is grease with copper flakes in it to ensure that the threads continue to slide on each other as the tightening torque increases.

Deda could not have shipped the bolts with anti-seize grease on it, so it did the next best thing, providing dried threadlock compound on them. This will also be fairly effective at preventing galling and seizing. Furthermore, you can still put Finish Line Anti-Seize Assembly Lubricant on those bolts; there is no reason to remove the dry threadlocker.
― Lennard

Regarding road disc brakes last week:

Dear Lennard,
I’ve worked in and out of the industry for a number of years and still it seems odd to see discs on road bikes.

None the less, they are here, perfect or not. Yesterday I saw Chris D. from Specialized on what seemed to be a prototype disc cover. Please see the photo [above].
— Patrick

Dear Lennard,
The current hub-bub has reminded of this article.

Safety first!
— Joe

Dear Joe,
Yes! Thanks for reminding me of that article; I loved it back when it came out in 2014!
― Lennard

Dear Lennard,
There is a third option no one mentions concerning disc brakes on road bikes. That option is a disc only on the front. Considering the large differential in the work a front brake does versus rear, putting a disc on the front would give a rider almost all of the stronger braking benefits of a pair of discs. But the risk of injury would be greatly reduced. One would assume that the risk would be cut in half, but I think that is not true. In crashes, the front wheel rarely is elevated and freely rotating. Whereas the rear wheel often is. The risk should be well less than half — something to consider. (Campy has long understood that the front brake needs more power than the rear with its “differential” brakeset.)

BTW, and regarding MTB, I sure wish I had the option to save some weight and run a front disc paired with a rim brake.
— Will

Regarding thumb pain from two weeks ago:

Dear Lennard,
You posted a thread discussing thumb pain. One man sent you a picture of his custom-fabricated thumb splint. The splint was well made and the man was pleased with the result achieved. However, had his therapist known the man was going to use the splint primarily for cycling, he/she would have likely fabricated a dorsal version of the splint for him. His picture shows a palm-based splint, which uses the palm as a foundation for thumb immobilization. One can get the same result with the foundation provided over the back (dorsum) of the hand. I believe a dorsal version would be more comfortable while weight bearing through the arms on handlebars. In addition, a palmar based splint might cause a small arm length discrepancy, which may lead to postural problems, neck pain, etc. Another option, his therapist could fit him with a neoprene or elastic splint with a rigid aluminum insert to support the thumb. If the soft splint works as well as the hard one, it would be much more comfortable than the rigid version.
— Tom