beezaur

I have a Specialized Tricross with disc brakes, new last summer, with 2700 road miles on it. It's my commuter.

I'm riding along today, looking down at the wheel and notice it's to the left maybe 5 mm on top. So I stop, open the quick release, and "thunk" the wheel seats all the way into the dropouts. That looks better.

Watching, I notice the top of the wheel pulls to the left with moderate braking pressure. No idea how long this has been going on.

The quick release is as tight as I can get it. Brake is stock BB5 with 160 mm disc.

Is there some issue with this bike, or some other attachment method I should install to make it more secure?

mechBgon

From what you're describing, it's a generic QR clamping problem.

One possibility is that your axle is protruding from the locknut too far. In that scenario, the QR ends up clamping against the end of the axle, not the fork's dropout, so no amount of QR force will actually clamp the wheel to the fork. Check that your axle end is recessed into the fork dropout, leaving room for the QR springs to compact. And make sure your springs are big-end-outwards.

I've also seen QRs get so bindy from lack of lubrication that most of the user's force is being wasted on friction in the lever pivot and the cam. If that's the case, lubricating them with some light oil will improve the amount of clamping you get. Switching to a quality all-metal QR (Shimano for example) can also be a big improvement over typical OEM QRs with plastic cam followers.

prathmann

When the disc brake caliper is mounted behind the axle then applying the brake results in an upward force exerted on the caliper and a corresponding downward force on the axle thereby pushing it down if the QR isn't very tight. I agree with mechBgon's suggestions on how to check that your current QR is working as well as possible and to use a conventional QR, such as Shimano or Campy.

beezaur

Everything seems fine and in good working order, a little dirty from the winter's riding though.

The QR has an eccentric cylinder that the lever sticks into, both aluminum. The part that engages the dropout is aluminum too. However there is a plastic bushing (?) in between the cam and the part that touches the dropout, I would assume to alleviate Al-on-Al galling and friction.

Would that plastic part maybe be too "smashy" (elastic modulus too low) and cause the whole thing to slip under braking? It's pretty tight though . . .

bobotech

The lawyer lips present should prevent the wheel from dropping.

And you do know how to properly use a QR? You don't tighten a quick release by rotating it, its done only via the cam action when you close the QR arm.

mechBgon

...except that it's slipping and trying to kill you Can you post a photo of your axle in the dropout with the QR absent, so we can make sure the end of the axle is recessed? Also verify that the axle locknuts are firmly jammed against the cones, and oil your QR's pivot and cam follower if you didn't already.

Another possibility is that the entire dropout has unbonded from the fork and is moving with the wheel. Easy to check, just grab the dropout in the jaws of an adjustable wrench and make sure it doesn't move when you apply a light twisting force to it.

beezaur

Right. I'm getting the nut on the other side as tight as it will go and still be able to lever the thing closed. The lever goes through about 90 degrees of rotation between contact and all the way closed.

beezaur

Here is the left side front dropout:


and the QR:

cobba

Info on QR skewers: https://sheldonbrown.com/skewers.html

Scroll down the following page and read about the ROADHOG Fork.

https://www.cotic.co.uk/geek/

beezaur

Thanks for the links.

Would a non-QR skewer be safer?

prathmann

A traditional enclosed cam QR skewer (such as those by Shimano and Campy) would also be safer.

beezaur

Safer, but is that safe enough?

Reading through the likes of this it would seem I've been flirting with disaster more than I have realized. I come in to town at over 30 mph, and brake at the bottom of the hill to turn into my grocery store.

And here I was thinking my increased braking power in wet weather was a good thing.

reptilezs

that skewer sucks, get a shimano

HillRider

Not worded strongly enough. A traditional enclosed cam QR skewer (such as those by Shimano and Campy) is essential.

Retro Grouch 

Are you lucky! It's seldom that a problem this serious can be solved this easily and this cheaply.

Not to be condescending but it's also important to use the quick release correctly and most people probably don't. Hold the lever out parallel with the axle and tighten the nut just finger tight. Now push the lever in so it's parallel with the bike. It should be hard to push. Don't push it up against the fork because, if you did it right, you won't be able to get a finger under it when it's time to release it.

bobotech

I agree with others, get a Shimano/Campy enclosed skewer after reading Sheldon's article.

Now the link about the Roadhog fork, I take it that is the fork that the OP has since it was linked here? If so, is it possible that the OP has his fork facing the wrong way since if he is using a Roadhog fork, the front disc should be on the right side of the bike? It would look odd at first and odd enough so that someone would think that they are doing the right thing by reversing the thread around.

Dan Burkhart 

An interesting way to deal with front wheel ejection, which is a well known and documented issue with front disc brakes.
In my opinion, the through axle is probably the best solution, although quality closed cam skewers and lawyer lips have probably prevented millions of catastrophies.

cobba

No, the OP doesn't have this fork, this is the bike & fork the OP has.
I linked that as it has a bit about the forces a disc brake puts on a axle, the problem Cotic found when they were testing the original Roadrat fork with the usual back left leg disc brake mount and the simple way they fixed the problem.

beezaur

Yes, that's the bike.

So, a through axle would be safer than a Shimano skewer?

mechBgon

Through-axle setups are different than a skewer without a handle. Quick visual:



Your axle looks adequately recessed. It would be good to confirm that the axle locknut is jammed against the cone, then switch to a better QR.

FBinNY 

Imagine if you went to a surgeon who decided to do a simple, relatively safe operation without first taking a medical history and finding out you were on blood thinner (anti-clotting) medication.

This is comparable to why your wheel shifts when braking. As Prathmann pointed out disc brakes impart a downward reaction force on the axle that rim breaks don't. Unfortunately folks spec'ing bikes, dropouts hubs, and QRs didn't consider that this effect would increase the required holding force, and didn't factor it into the specs.

A similar thing happened when changing from horizontal to vertical dropouts lowered the holding force of rear QRs. That was OK, but allowed the elimination of the steel biting faces on axle end and QR faces. All well and good until a modern (low hold) QR is used with a horizontal dropout, or as you now know a disc brake hub.

The solution is a better QR of the type designed for bikes before vertical dropouts, but a better solution would include axle faces with serrations which can bite into the dropout and stay put against high sliding forces. Do whatever you can to ensure that the hub reliably stays put, and until you do, inspect it often or do not ride it.

beezaur

That's sort of what I'm trying to figure out - what to do about the problem.

I haven't done the calculations yet, but there is potentially an issue with fatigue or some similar phenomenon with a skewer that is too tight.

One thing to be careful of with machines in general is that, when you see a fastener, often times everything about that fastener is designed to handle only so much stress. Things like overtightening can have unexpected consequences.

In this case, it might be possible that making a standard skewer tight enough to hold the wheel in place might put too much stress on the skewer. Things with too much tension stress can undergo fatigue failure.

That would be the "pong" sound of a snapping skewer immediately prior to wheel ejection at speed.

If that is in fact the risk (and I don't know as I have not done the engineering calculations), then "fixing" the problem with a "better" skewer would actually make the problem worse: failure would mean a broken skewer and wheel ejection instead of just bearing on the lawyer lips. You change a benign failure mode into a catastrophic one.

One thing that I have learned in being an engineer is that you have to be careful how you "fix" load-bearing components.

Right now this whole thing is a mass of red flags to me. I'm sort of pissed that Specialized put out a product that is even capable of doing this.

beezaur

So, I did the calculation . . .

My lever is 2.0" long, and the cam surface is 0.3" from its axis. So the force that I apply on the lever (conservatively) is magnified by a factor of ~6.7.

If I apply 75 lbs (after friction losses) that means the skewer is under 500 lbs of tension.

Measuring the thread root diameter (just eyeballing it) I get 0.145". So its cross sectional area is 0.0165 in^2.

That puts the tension stress in the range of 30,000 psi.

If the skewer was mild steel, that would be yield stress.

Without knowing the details of the material specification for the skewer, this stress probably is in the range of where you could see fatigue failure.

Fatigue failure is where the material develops microcracks over time, then suddenly the part breaks. It can take thousands of cycles to develop, and that depends on the exact nature of the loading as well as the material. So you could use a skewer for years even and then have it snap when you brake.

I am a civil engineer, so my fatigue training has to do with steel structures; I am not a mechanical engineer. However it does give me that "not so fresh" feeling based on what I do know.

My gut feeling is that a solid axle is the only safe solution. For myself I would want something that I know is strong enough.

HillRider

First a good quality steel skewer won't be mild steel, it will be a high strength alloy. Second, fatigue failure occurs after repetitive stress and, below the elastic limit, that stress has to be applied and relaxed thousands or millions of times. How often do you expect to remove and replace that wheel? Finally, internal cam skewers like Shimano and Campy provide are routinely tightened to and above the load you will need to retain your front wheel and skewer breakage is an extreme rarity.

You are overthinking and overanalyzing this problem. Discard the skewer you now have, buy a proper one, install it correctly and ride with confidence.

reptilezs

did you forget that the nylon bushing just deforms after a certain amount of load and loosens the qr? a quality skewer like a shimano closed cam would be my next step. some have aluminum or steel serrations depending on which you get. i would keep the acorn nut on the disc side for its steel faces