Emerson

My wife is considering a bike with disc brakes. While poking around on-line I found this website:

https://www.ne.jp/asahi/julesandjames...quick_release/

It details how disc brakes can actually cause a front wheel to suddenly come out of the drop-outs and cause a serious crash.

His reasoning looks OK, but there are thousands of bikes out there with discs. Part of me thinks that if this were a serious problem, we'd hear more about it.

Should we steer clear of discs?

Thanks

Retro Grouch 

1. The issue is real.

2. I'm not ready to say "Steer clear of disc brakes yet."

3. If you have disc brakes, do everything right. Don't file the lawyer tabs off of your fork. If you have a quick release, be sure that you use it correctly. Avoid quick releases that have an external cam mechanism that you can see.

4. Thru axles on mountain bike forks. Why do you suppose they suddenly have become so popular?

amaferanga

If the front drop-outs face slightly forward then its impossible for the wheel to pop out. My forks were designed that way, but I'm not sure standard forks are.

HillRider

If the bike is a recent design and designed to use disc brakes it is likely the front dropouts are angled properly to prevent wheel ejection. Ask the dealer about this before buying.

Also, as recommended DO NOT fill off the lawyers lips and be sure the quick release is properly tightened.

Francis Buxton

Disc brakes are not dangerous. They actually work extremely well. While the writer of the article has somewhat of a valid argument, it's no where near as big of a "sky is falling" scenario as he makes it seem.

The odds say the dropouts won't face forward. It's okay. I will agree with Retro in that you should use an internal-cam skewer (like the kind Shimano makes), and make sure you get them on tight.

I've been riding a custom 29"er with a rigid fork (and no lawyer tabs) and disc brakes for quite some time with no problems.

nodnerb

I have feeling that people with disc brakes (me included) tend to bust many more spokes than someone running V's as well. The forces put on the spokes and rim are tremendous compared to rim brakes in which the spokes only have to support the riders weight. At the same time, it probably keeps the rim so tight and rigid during braking it may even be stronger during. Not sure.

Ophidian

Don't forget people should check the quick releases before every ride. I don't think you'll have any problems if you make sure the quick release tight.

Rev.Chuck

If you set up a rim brake incorrectly or let it get very worn out it can move into the spokes stopping the wheel, and throwing the rider. Are rim brakes dangerous?

edzo

the title of this thread should ask

are poorly set up bicycles dangerous ?

yes they are.


disc brakes are not dangerous.

DannoXYZ 

The force on the QR is exactly the same with disc-brakes as it is with rim-brakes; they both push the hub & axle backwards while the fork & frame tries to continue forward. This real problem is people not tightening their quick-releases correctly. An age-old problem with bikes and people not knowing how to use their equipment properly.

Heck, I've worked in a bike-shop for 10-years and I made an idiotic mistake this last weekend. I swapped in a new wheel and it turned out the rim was 2mm smaller in diameter than the Mavic rim I had on there before (Mavics tend to be oversized). So... I didn't check my brake-pads and they were now riding 1mm too high... Guess what? In less than 30-minutes later, I had a flat-tyre... Am I gonna blame the shop that sold me the new rim? Or Mavic for making their rims slightly too large? Or sue Shimano for making brakes that don't automatically adjust themselves to center the pads on the rims?

jhota

i'm not dead yet.

+1

whatever happened to checking one's equipment out before using it, whether a bike or car or whatever?

i check any quick releases before riding, no matter what. i also walk around my car before leaving home. maybe i'm just an anal-retentive schmuck, but like i said, i'm not dead yet.

PM7771

Are disk brakes dangerous?

well that makes me think of my grandfather; he had low blood pressure;
which he accepted as better than having none at all.

Peter

LWaB

Not quite correct, disc brakes (as commonly positioned on the fork) tend to push the axle downwards, as noted in the earlier link and unlike rim brakes.

Retro Grouch 

I think so too. I think that the wheel tries to rotate around the caliper when you put on the brakes.

Knudsen

This is true of either brake design. As viewed from the left side of the bike, the wheel will want to rotate CCW around the point where the pad grips the rim or disk. The rim brakes are also disk brakes, folks, using the rim as the disk. So if you move the pad (caliper) about, you can get the wheel to want to rotate up down, for or aft. It's a product of the position of the pad, not the brake type. Geeze, didn't you guyz have spiralgraphs?

grolby

Rim brakes and discs grip the disc (be it the rotor or the rim) at different radial points, and disc brakes exert a much stronger force than rim brakes. Rim brakes are not capable of producing enough force to eject a wheel and don't force the axle downward. Discs are and do.

That doesn't mean that it's a big risk if you take proper precautions. According to the linked article, disc brakes certainly ARE capable of ejecting a wheel even with a properly-tightened QR. Still, the liklihood of this happening is very small if you follow the proper precautions. The issue is real, but I don't think you're taking your life in your hands with discs, though, especially for road use.

古強者死神

Seems like a logical solution is a slightly redesigned disk break where the pistons are on the other side of the radial axis, so instead of applying downward force to the wheel it will have an upward force... thus making it even harder to lose a wheel.

DannoXYZ 

That's true, depending upon the location of the caliper itself.

Yup, personally, I would mount it right behind or in front of the fork-tube like a motorcycle, rather than low and 90-degrees to it. This would tend to push the axle longitudinally rearward instead of down.

Knudsen

Funny they didn't do it that way right off the bat. Being a victom of the wheelie of death as a kid, watching that front wheel roll away as I rode a wheelie, I have a great deal of respect for front wheel security! I'd still be riding that wheelie if I could. I almost cleared the handlebars. I remember my brothers laughing as they drug me off the street by my feet, all racked up. Bastards.

vw addict

This is more of a problem when you put bigger rotors on bikes with QR wheels. That's why you see so many thru-axle 8" rotor combo's.

Emerson

OK, so we got the Novara Safari my wife was interested in. Couldn't see one in person, ordering was the only way to get one. The fork has vertical drop-outs, so it looks like Novara didn't design the fork to be safer with disc brakes.

That said, what I am taking from most of the replies is that if the front quick-release is checked regularly then it shouldn't be a problem.

Would putting a zip tie or hose-clamp on the quick release further increase the safety? It wouldn't be able to loosen much if the lever is basically locked against the fork. Does this sound reasonable? Thanks

phoebeisis

Just how much downward force is generated?Anyone have any idea? If I pull the lever with 5 lbs of force, does that mean 30 lbs of brake force. I not sure what the mechanical advantage is.30 lbs on a 6" disc is just 15lbft-no way will that lift 1/2 me and the bike(100 lbs). Pretty sure it would lock the wheel and you would skid anyway.
I just completely loosened the QR on my Manitou forked NRS and did a bit of violent braking.No F_ _ _ing way is that wheel escaping downward. There is about 100 lbs of bike and me holding the fork down on the wheel axel. Even if the opening was straight down(it isn't, it is canted about 10-15 degrees forward) there is no way it was moving downward.
Now since this is a suspended bike, and I was eyeballing the front wheel, it doesn't tell me much about the rear. The weight shift increases force on the front,and decreases it on the rear. Of course, this also makes the wheel more likely to lose traction and lockup.
I maybe could see a rear wheel jumping out if the QR was completely loose, and you were heading downhill.I still think you would lose traction and skid-lockup the wheel 1st.
The caliper is actually holding the wheel in place to some extent. To jump out the wheel has to pivot against the clamping force of the caliper-rotor-even if the "hole" is straight down.
Now a wheel can always "bounce" out of a soft QR ,but that is unrelated to braking.
Why doesn't someone turn their bike upside down, loosen the rear QR,spin the wheel, and hit the brakes??Hmmm-just noticed that my rear caliper is above the rotor-not much downward force there.
This sounds like BS to me. The front wheels are bouncing out.I don't know if any rear wheels have the caliper positioned so that the force would be downward??
Disc brakes aren't dangerous-other than the increased risk of lockup in poor traction.Thanks,Charlie

Emerson

take a look at the website in the initial post, he gives some ideas about the forces generated. There seems to be a lot of evidence that disk brakes can and do cause the front axle to slip--the question is how much and how frequently and is there anything that can be done with completely vertical drop-outs. I agree that it seems like it shouldn't have enough force, but apparently it does. And even if the caliper is holding it in place, on the next bump it could, as you say, pop out if the QR is loose.

phoebeisis

I read and eyeballed his math. He is missing something, but I'm not sure what. His claim is that for a ~200 lb bike and rider(90kg) the ejections forces are 800 lbs more than the "stay in forces"(weight of bike rider). He said the net ejection force was about 1800 Newtons(I think that is about 400 lbs or so) per dropout, or 800 lbs total for a .6 G braking(stopping at about a rate of 18ft/sec/sec0 or going from 18ft/sec to Zero in one second(this is about 12 miles/hr to zero in one second). I did about that with a completely loose QR, and I felt no "lift" at all while riding the bike.
I just flipped my bike over. With the QR completely off, if I spin the wheel about 12mph(3 rps) and give a brisk squeeze, it will eject the wheel, pushing it maybe 5" up. If I give a really violent quick squeeze, it will push it upward about 3/4th of the way out of the dropout before it is seized in place.
If the QR are on at all they tend to stop it before it jumps out because they are in the countersunk "bores" and the "catch" on the way up and out.
I put a glove on and tried to get a feel for the ejection force by putting my hand on the top of the rotor-I really couldn't get much feel for it. Now an 800lb push/impULse should push the wheel up farther than 5-6 inches.
My dropout is pretty much straight up and down.
He might be underestimating/undercalculating the force/wt of the rider/bike since it will actually be moving-have some momentum- because of the wt shift of the suspension bike.I'm also pretty sure he is ignoring the "clamping" effect. Luck,Charlie

Al.canoe

My Fox fork owners manual requires that the thickness of the drop-outs be measured every six months or 100 hours. That's to measure the wear on the lips that surround the skewer contact areas. They require that you replace the lower tubes if the lips get too thin. Smart those Fox folks. My wife's 7 year old RockShok has the same lip design.

In something like 4 years of using Avid mechanicals on three bikes, I've never had a skewer, external cam or internal cam, unscrew one iota. I ride a lot on steep single-track in N Georgia, E Tenn., and western N Carolina and some in Moab. After each ride I remove the front wheels to haul the bikes, so I would notice. Also, long before the skewer would release enough to lose a wheel, the wheel would have to wobble in the drop-outs. You'd notice it. Those lips are very pronounced.

These lips are very beefy and are obviously designed to counter the force from the disc brakes. Those of us who remove our wheels to transport the bike might be wise to loosen the Skewers a few extra turns to reduce the wear on the lips due to the frequent wheel removal/installation. Also measuring the drop-out thickness is a very good idea. The lower fork material on Fox forks is very soft and wears quickly.

Though the article claims that the fork manufacturers have ignored the problem, that appears to not be the case. Also, those pictures showing the bent fork just demonstrates that the fork was too weak in the first place to support a disc brake set-up. Poor design.

I have a ti cyclocross frame/fork for my road bike. The fork is designed for disc's and it's massive carbon fiber. It weighs at least twice what a typical road fork weighs and it's twice as thick. I run V's on it for weight savings.

A final point on loosening skewers-- I brake the Ned Overend way. I stay off the brakes until i
the need to scrub off a lot of speed than brake hard. That should loosen those skewers if they they are prone to do that and they haven't yet.

Al