Technical FAQ: Thru-axles, discs, and cranks

Following up on thru-axles

Dear Lennard,
You recently addressed the rationale for front wheel/fork thru-axles when using disc brakes in your column, “Thru-axles and frame stiffness.” I would just like to ask why most frames are transitioning to front and rear thru-axle setups. This requires complete new wheel sets, whereas just replacing the front fork and wheel with thru-axles would be less than half the cost. You made a most eloquent argument for the front wheel change, but why also the rear wheel to thru-axle?
— Matthew

Dear Matthew,
Indeed, in the thru-axle discussion here a couple weeks ago, the majority of the discussion was on the advantages of a front thru-axle. While perhaps less compelling, particularly on road bikes, thru-axles also have advantages over quick releases on the rear of the bike.

I have seen many disc-brake mountain bikes with quick releases where the left rear dropout had become so worn out that the wheel was constantly getting yanked back in the now-widened dropout slot, causing the tire to rub on the chainstay within a short riding distance. I have seen it on bikes with steel dropouts and aluminum dropouts. The stress created by application of the disc brake, combined with insufficient clamping force by the quick release, can really beat up the left dropout.

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Analogous to the discussion of the front disc brake and fork dropout damage, when the brake clamps on the rear rotor, the fulcrum of the imaginary lever connecting the tire’s ground contact patch with the top of the tire now becomes at the top of the rotor, rather than at the center of the wheel, and the angular momentum of the rear wheel drives the axle back in the left dropout. Especially when combined with bouncing around on rough terrain along with one of those aluminum, cam-shaped quick-release levers, which generally are incapable of generating sufficient clamping force, the axle can move — in this case, back against the rear wall of the dropout. Do this enough thousand times while braking and riding hard, and you will have ground down the dropout to the point that the axle can wallow all around in there, changing the angle of the rear wheel in the dropout and allowing the tire to move over against the chainstay. A thru-axle completely eliminates the potential for this problem.

I wonder what you mean by your statement about cost. If you were to switch from rim brakes to disc brakes, you would have to get a new bike anyway, and whether the wheels are quick-release or thru-axle has a minor effect on the price. Or did you mean that you could just replace the fork and front wheel and front brake on an existing rim-brake bike and use a front disc brake with a rear rim brake? In that case (see the next question), you would avoid getting a new rear wheel, and the thru-axle discussion would not come into play, as you would still have a rim brake.

Your wish for a front thru-axle and a rear quick-release axle did come true briefly; there was one model year a couple of years ago where some disc-brake cyclocross bikes were coming that way, and there were many years where mountain bikes generally came with a front thru-axle and a rear quick-release axle. Now, most disc-brake bikes have front and rear thru-axles, and I would bet that will be the new normal for the foreseeable future.
― Lennard

Mixing cantilever and disc brakes

Dear Lennard,
I ride and enjoy my 2011 Specialized S-works Tricross, which is fitted with TRP cantilever brakes. I’ve been happy with the canti brakes for ’cross racing and trail riding, though I do recognize the benefits of disc brakes. However, I’m not planning on buying a new CX bike anytime soon. That brings me to my question: what would you think about replacing the front fork in order to run disc brakes? I’d keep the canti brake in the rear.

I’d like to try the Crusher in the Tushar ride in 2018 and have been reading rider opinions on bike selection and course conditions. Being very comfortable with my CX bike, I plan to stick with what is familiar. The description of the descents on washboard roads favors a MTB, so I thought the front disc brake would make the descents safer at speed (as well as allowing for a slightly larger tire up front).

Would you recommend this modification? If so, what are your thoughts/suggestions?
— Chris

Dear Chris,
I think that is a perfectly reasonable solution, for the reasons you cite. See my answer to the prior question for another reason.

The cheapest way to do this would be with a cable-actuated front disc brake, since you could use your existing left lever. That said, the braking performance of a hydraulic disc brake greatly exceeds that of cable-actuated discs. The latter solution would provide you with better braking at the expense of a lighter wallet and mismatched levers.
― Lennard

What’s that little plastic plate for?

Dear Lennard,
I put a Hollowtech II 9000 crankset on my Kent Eriksen bike last spring when I switched to SRAM eTAP and got it working with an 11-36 cogset — partly with your help to try it, against SRAM guidelines for their WiFly. In Sept and October, I rode the bike in Umbria and Abruzzo, Italy to good effect, considering I’m now 70, and I’ve been concerned about arrhythmias even well before reading “Haywire Heart.” The shifting was fine.

But here’s a more technical gear question that, again, the very small local shop mechanic couldn’t help me with. In taking the crank off the bike in Umbria or putting it back on in my garage here a week ago after changing the BB (it grinds), I seem to have broken that little plastic safety plate between the two bolts securing the left crank on the spindle.

I looked in the Zinn road book that I bought early this year (though now I see that it is the 4th edition, 2013) and found no mention of that plate. Drawings don’t show a plate there. Presumably, Hollowtech II came out later than press time for the 4th edition.

The many amateur YouTube how-to videos about the crankset have various advice about how tight to screw in various parts and whether that little plate needs to be there at all. One site called it a spacer. The part had a projection that maybe dropped into the hole that is in the spindle to keep the crank on there if the plastic pre-tension nut or the two bolts all failed.

How important is that plate? Will the crank slide off without it?
— Eric

Dear Eric,
That plate is only important if you ride with those pinch bolts insufficiently tight. If you tighten them to torque spec and check their torque periodically, the plastic safety plate will never come into play.

The plate has a little tab on it directed radially inward, and it is intended to drop into the little hole drilled radially into the side of the spindle a centimeter or so from its non-drive end. As the hole is drilled parallel to the drive-side crankarm, it is also a visual indicator; without looking at the opposite crank, you know the proper installation orientation of the left arm by putting that hole under the slot. The left arm then ends up at the correct 180 degrees to the right arm.

If your crankarm pinch bolts were loose, that tab on the plastic safety plate would keep the arm from sliding off for a little while; otherwise, the arm would come right off while you pedaled. If you were perceptive enough to notice that your crankarm was flopping around before your continued pedaling eroded away that plastic tab, you would save yourself the indignity and damage of having the crankarm come off while riding. If you were to ignore the wallowing around of the crank, however, it would eventually come off anyway, once that tab became worn.

The way to properly use that safety plate on installation and removal without losing it is to remove one of the pinch bolts so that the safety plate can pivot on the other bolt.
― Lennard