Technical Q and A with Lennard Zinn

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By Lennard Zinn

Exhibit A: The dropout wear.

Photo: courtesy

I’ve received a lot of mail about our story in the print VeloNews, issue 14, about climbing wheels. It was a great test that revealed a lot about climbing wheels we saw right afterward in the Tour de France. Matt Pacocha rode 50,000 vertical feet on the wheels and performed wind tunnel tests on them, and then we did our own measurements of rotational inertia. For those not understanding what rotational inertia is and how we measured it, please go here.
Lennard

Disc Slippage
Dear Lennard,
Look at the wear pattern on these dropouts on a hardtail steel 29er with disc brakes. The wheel sits straight in the frame when I pull the axle back in place in the dropouts. I can tighten the skewer as tightly as I can, but as soon as I apply the rear brake hard, the axle moves forward on the right, into the notch you see in the dropout, and the tire rubs on the left chainstay. What’s going on here?
JP

Dear JP,
I imagine that the rear skewer was insufficiently tightened at some point for an extended period of riding, which, combined with disc braking, started the process. The loose skewer would have allowed the left end of the axle to slam upward in the left dropout every time you put on the rear brake, and the right end of the axle to move forward every time you stomped on the pedals, particularly with the high torque generated in a low gear. Once it started to happen, it created a taper on the dropout faces, and the wheel will always want to settle into that spot.

Exhibit B: Another view.

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Photo: courtesy

At that spot — forward in the right dropout and upward in the left, the skewer is less tight because the dropout is less thick there, and that is where the axle will always want to return. I’m willing to bet that if you measure the thickness of the dropout with a caliper at different spots, that you will find it to be thinner on the forward part of the flats of the right dropout and perhaps on the upper part of the left dropout as well (it only takes one tapered dropout to loosen the entire skewer, and the upper part of the left dropout is a full semicircle, distributing the grinding action of the moving axle and skewer end faces, so it might be harder to measure the thickness change there).

With the tapered dropout thickness causing the loosened skewer, the interior of the dropouts could be worn away quite rapidly, even though they’re made of steel, because the skewer is no longer preventing the axle’s movement. The wear rate can be considerably higher yet with aluminum dropouts. If you have a bike with a replaceable derailleur hanger, get a replacement one and compare it with the one on your bike. You can see how much this wear has happened by comparing the shape of the axle slots in the two hangers.

With the skewer insufficiently tightened, the only thing standing in the way of the right axle end moving forward and the left axle end moving upward would be the inner wall of the dropout itself; there would be no friction of the axle ends and skewer nut faces to take the load off of it. With a strong rider and especially with big 29er wheels, that kind of repeated action can wear away metal fast. Think how fast you could remove material from the dropout with a file, and then notice that the ends of the axle nuts and the inboard faces of the skewer have teeth much like a file, and this back-and-forth action is going on constantly when riding. On the inboard faces of those dropouts, you can see how much steel that action has pushed forward and up; that material had to come from somewhere, so I’m sure that back part of those dropouts is thicker than where you see the axle end has pushed material forward and up.

To avoid this happening in the first place, keep the skewers really tight on any mountain bike, particularly one with a disc brake, and even more so with a 29er with a disc brake. DT Swiss makes heavy-duty screw-on ratcheting skewers for exactly this purpose — to tighten the rear axle more tightly than a quick-release could. We now supply these with all of our Zinn full-suspension 29ers to avoid the problems you are seeing. The DT ratcheting skewers are almost as fast to use as a quick-release skewer, particularly on the front, where you have to unscrew the QR anyway to get past wheel retention lips.

This is similar to the problem of using a quick-release skewer with front disc-brake wheel. If that skewer were also not tight enough, every time the front disc brake goes on, the axle would be trying to shoot down out of the dropouts. Think about it: where the caliper grabs the rotor now becomes the fulcrum — the fulcrum is no longer at the center of the axle as it would be with a rim brake. Now, all of that rotational inertia of that big, 29-inch wheel is trying to rotate forward in the direction of wheel rotation, except around the point of brake application on the rotor, not around the axle. This will force the axle downward in the fork ends.

Of course, there are “Nader hooks” (a.k.a. “lawyer tabs”, “safety tabs”, etc.) on those fork ends to keep the wheel from coming all of the way out. But each time this process happens, the fork end’s walls become tapered toward the bottom, so that the next time, no matter how tight the skewer, when the front brake goes on hard at high speed, the wheel moves down because the dropout has become less thick there. It will seek the location where the skewer is least tight. After a long downhill with a lot of (disc) braking with a quick-release front wheel, flip open the front skewer; don’t be surprised if the fork were to drop down a bit and clunk back down onto the axle.
It’s a big problem for the industry; here is more on the subject.

Fox’s front-opening fork dropouts for QR wheels, the new 15mm fork through-axles used by Fox, Marzocchi, DT Swiss and others on cross-country forks, and steel screw-on skewers on lightweight mountain bikes are all designed to minimize this problem. 
Lennard

Tough Rubber
Dear Lennard,
I was on a ride a few weeks ago, and I pulled my rear wheel over in the dropouts and did not discover that it had been rubbing the left chainstay for about 15-20 miles. When I re-positioned the wheel, the tire (a Michelin Axial Select) had rubbed a gash in the titanium chainstay to a depth of almost 1mm. The tire is absolutely fine and shows no real wear where it was contacting the frame.

The conditions were dry (i.e. not raining) and the pavement was smooth and free of debris.
It seems amazing to me that a tire could rub a potentially fatal gash in a titanium tube. Do you have any thoughts on what might have happened?
Bruce

Dear Bruce,
Wow. I am surprised for sure, although cutting and machining and filing titanium is hard because it tends to smear rather than cut. In this case, that works against you with slow erosion. A tire is not a lot different from the Scotchbrite rotary wheels we use to polish titanium, which are soft to the touch but which will grind right through the stuff if you are not careful.

Sorry to hear about it. Keep an eye on it for cracks after each ride.
Lennard

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