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#661
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Carlton Reid on QR safety
Tony Raven wrote:
Tim McNamara wrote: Those calculations were based on real world numbers, published measurements of pull-out resistance, and minor things like the laws of physics. Your calculations are all fine except that you keep considering pull out forces in the absence of lawyers lips. Now most disk brake forks have lawyers lips. With them the pull out forces are probably at least an order of magnitude higher because you need to physically push them out the way or stretch the skewer so it passes over them. Actually, it's been explained in some detail how a bolt will likely loosen under transverse slipping, and there have been several descriptions of riders who have regularly found their QRs loosened in exactly this way - ie with the lever still closed, but when installed on the LHS, rotated in an anticlockwise direction: "Both my G/f and I have had problems with Hopes. 1) They were done up f@cking tight. 2) Every time they've come loose, the lever has been shut, but instead of being next to the fork leg, its pointing straight down to the floor, implying they've "wound" loose. Spin the lever back through 180° to its usual position and its "normal tight" again." James -- James Annan see web pages for email http://www.ne.jp/asahi/julesandjames/home/ http://julesandjames.blogspot.com/ |
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#662
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Carlton Reid on QR safety
Jay Beattie wrote:
"James Annan" wrote in message ... dvt wrote: Tim McNamara wrote: One thing I wondered about was instantaneous loading versus static loading, if those are the correct terms, which I have no idea how to calculate. I would think- but don't know- that a quick jam on the brake at 25 mph would result in a high sharp force compared to my more static force based on a .6 g deceleration. Would the magnitude of the force be raised with higher speeds, or just the time interval over which the load develops? My understanding of physics suggests the latter. It is possible to generate more than 0.6g deceleration momentarily. Others have written in this NG about the peak forces possible on rough terrain. But I don't think many people ride that hard. The cannondale "tests" measured a peak 235 ft-pounds of braking torque on the front wheel, fromw hich you can work out about 950N deceleration and 3800N ejection force, far in excess of the ballpark estimates I and others have produced based on a steady 0.6g braking. http://www.ne.jp/asahi/julesandjames...annondale.html Do these calculations assume infinite traction and occupant restraint? There is a point at which braking force will eject the ride, and the ejection of the wheel will be irrelevant since the rider already is airborne. I also wonder whether you can generate those super high braking forces when your front wheel is skidding down the road or sliding in soft dirt. In fact, most of the hard braking on an MTB is on the rear wheel. Somebody should rig a real bike with strain gauges or accelerometers (or whatever the instrumentation should be) and find out what the real world forces are. I had a broken frame case where we did that. Very enlightening. -- Jay Beattie. The bit you apparently missed in my post: The cannondale "tests" MEASURED a peak 235 ft-pounds of braking torque If you'd bothered clicking on the link, you'd have seen what they did. James -- James Annan see web pages for email http://www.ne.jp/asahi/julesandjames/home/ http://julesandjames.blogspot.com/ |
#663
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Carlton Reid on QR safety
Jay Beattie writes:
In fact, most of the hard braking on an MTB is on the rear wheel. Dude! Most of you braking should be applied to the front wheel. If you're jamming on the rear brake, you're not slowing you're sliding. Oh and the rest of your post was wrong too. Dude! You are not going to lock up your front wheel on loose dirt because it is really hard, dude, to control a front wheel skid. More back brake than front on loose terrain, dude. That's what I'm getting at. In fact you lock up your rear even on purpose sometimes to carve that big, dirt churning turn, dude. Try that on the front wheel for a little fun sometime, dude. That may be your perception but in fact most heavy braking is done with the front wheel. If traction is miserable, then no heavy braking is done with either wheel. Just the same 1g braking occurs often on descending hard and dry trails with embedded river bottom (rounded) rocks. On such descents the bicycle is barely on the ground between bumps and on bumps maximum braking occurs as the suspension fork absorbs the rock. These are the jarring force reversals that cause axles to move in the dropout and only on the hub brake side, the other end having no ejection forces. Jobst Brandt |
#664
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Carlton Reid on QR safety
Tony Raven writes:
Tim McNamara wrote: Those calculations were based on real world numbers, published measurements of pull-out resistance, and minor things like the laws of physics. Your calculations are all fine except that you keep considering pull out forces in the absence of lawyers lips. Now most disk brake forks have lawyers lips. With them the pull out forces are probably at least an order of magnitude higher because you need to physically push them out the way or stretch the skewer so it passes over them. I considered the lawyer lips in their correct role: they are there in case of *failure* of the QR to retain the axle in the dropout and are not part of the retention system proper. If they lawyer lips are what's holding your axle in the dropouts, that is because of a *failure* of the QR to retain the axle. Relying on the lawyer lips to hold in the wheel is simply horrendous incompetent design. I didn't even have to appeal to the issue of transverse cyclic forces to show that the problem exists. Only because you and everyone else keep ignoring the presence and influence of the lawyers lips. If you include them you have to resort to transverse cyclical forces and QR unscrewing or some other mechanism to allow wheel ejection. That is the critical bit that everyone avoids dealing with and has yet to show clear demonstration of a mechanism preferring instead to rely on the post hoc fallacy that because a wheel was lost it must have been ejected by the brakes. When the mechanism can be demonstrated I will be persuaded but at present it's the elephant in the room everyone is pretending is not there. Given how often lawyer lips have come up in this and the previous threads on the topic, there is no chance that anyone has forgotten the lawyer lips. The intent of laywer lips is not to provide axle retention for disk brakes, it is to prevent people who don't know how to use QRs from losing their front wheels. The elephant is actually the size of a cat, and we all know it's there. Those of us advocating change are just prudent enough not to rely on the cat to save us. Going back to the first principles of the discussion: there is a reactive force that results from using disk brakes. That force has a specific direction dictated by the location of the brak caliper. With many if not most current designs which mount the front brake on the trailing side of the fork, the direction of that reactive force is towards the opening of the dropouts. This force attempts to push the axle out of the dropout, hence it is called an "ejection force." The magnitude of the force is large- per Cannondale's numbers up to 854 pounds. Even using my more conservative numbers, as corrected by dvt, the net ejection force can easily be over 400 lbs acting on one dropout. That's more than the pull-out resistance measured by Howat et al for quite a few skewers. |
#665
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Carlton Reid on QR safety
In article
.com, "David Martin" wrote: Mike Causer wrote: On Thu, 16 Feb 2006 12:36:31 -0800, Jay Beattie wrote: The more important point was that from a wheel ejection/disc brake standpoint, if your are sliding either wheel, then you are not getting the ejection forces theorized in this thread which, apparently, assume infinite traction. Yes and no. Infinite traction does not give infinite braking capability. Once above a coefficient of friction better than 0.6 - 0.7 any extra does no good because the rider will either be over the bars or using superior skill to pull a "stoppie". In either case the actual "G" is going to be 0.6 - 0.7 So assume infinite traction if you wish, it makes no difference to the forces we are discussing. What does make a difference is if the force is acting in rotating the wheel the other way. Ie landing on a rock with the back part of the front wheel. Given that any rotation is prevented by the rear wheel, a much larger force can be applied than 0.6g. An applied brake caliper will act as a pivot and provide an ejection force. (simple visualisation: Pick up a pen, hold it a short distance from one end with the long end towards the table at an angle. Push down. The pen rotates around your grip, pushing the free end down harder. There is an ejection force that is proportional to the downward force, ie the greater the effective weight of the rider, the greater the ejection force. The weight of the rider acts on the brake caliper, rather than the axle. As my mechanics is a little rusty, I can only provice a qualitative analysis right now. A quantitative analysis can be performed by someone more qualified. We have already had good analyses. The excess of brake reaction at the axle increases with rider weight. The brake reaction force at the axle is a multiple of the rider weight; around 1.4-1.6. A 140 lb rider might see an ejection force that the quick release can hold against where a 190 lb rider braking in the same circumstances as the lighter rider will see an ejection force against which the QR cannot hold. For both riders the bumps and braking act to loosen the QR nut, though the acting does not always succeed in actually loosening the nut. -- Michael Press |
#666
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Carlton Reid on QR safety
A Muzi a écrit :
-Rolled thread on brake bolts?- jim beam wrote: eh? like this? http://www.flickr.com/photos/38636024@N00/99524699/ The Shimano bolt referenced is indeed rolled. That is not at all common to our industry. As I wrote yesterday, the first three examples of caliper brakes that came to hand appear to have rolled threads on a stepped shaft. I've just found a fourth - a modern Shimano BR-A550 57mm dual pivot. The bolt is of exactly the same form as the Shimano 600 bolt pictured, with one rolled thread on the narrower, swaged portion of the shaft, and a second rolled thread on the broader portion. I pulled one of those Shimano bolts and a loupe. It is also the only brake bolt here without a center pip for turning. All three of the Shimano brakes that I have on had (1050, 6208, A550) have an irregular, concave end, like the end of an ordinary rolled M6 screw. The Superbe Pro BRS shows signs of milling along the shaft and on both ends. The step in the shaft of the Suntour looks cut, while the Shimanos look swaged. But all the threads look rolled. James Thomson |
#667
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Carlton Reid on QR safety
Tim McNamara wrote:
I considered the lawyer lips in their correct role: they are there in case of *failure* of the QR to retain the axle in the dropout and are not part of the retention system proper. If they lawyer lips are what's holding your axle in the dropouts, that is because of a *failure* of the QR to retain the axle. Relying on the lawyer lips to hold in the wheel is simply horrendous incompetent design. Its called fail safe design. If the primary retention device fails, the secondary takes over preferably in a way that ensures it is clear to the user that there has been a primary system failure. For an unsafe condition to occur both the primary and secondary systems have to have failed. All the calculations you are doing only show how a primary failure might occur by the QR slipping. What they have failed to address other than by speculation is how the secondary system also fails which is a necessary condition for the wheel to be ejected. Given how often lawyer lips have come up in this and the previous threads on the topic, there is no chance that anyone has forgotten the lawyer lips. The intent of laywer lips is not to provide axle retention for disk brakes, it is to prevent people who don't know how to use QRs from losing their front wheels. Correct but by default it should prevent a QR from exiting the fork for forces many times the highest calculated here for an ejection force. Unless you can demonstrate rather than speculate the mechanism by which this also fails you have failed to show the two necessary conditions for wheel ejection to occur - a slipping QR and a verified mechanism for the QR to pass the lawyers lips. -- Tony "The best way I know of to win an argument is to start by being in the right." - Lord Hailsham |
#668
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Carlton Reid on QR safety
In article , Tony Raven
wrote: Tim McNamara wrote: I considered the lawyer lips in their correct role: they are there in case of *failure* of the QR to retain the axle in the dropout and are not part of the retention system proper. If they lawyer lips are what's holding your axle in the dropouts, that is because of a *failure* of the QR to retain the axle. Relying on the lawyer lips to hold in the wheel is simply horrendous incompetent design. Its called fail safe design. If the primary retention device fails, the secondary takes over preferably in a way that ensures it is clear to the user that there has been a primary system failure. The primary device, unlike the secondary, does not simply retain, it secures. The difference between these two functions is the difference between having and losing control of the bike. For an unsafe condition to occur both the primary and secondary systems have to have failed. snip That's an amusing contradiction. The secondary retention device, 'preferably' takes over in such a manner as to clearly inform that the QRs have failed to secure the wheel (what if the Lawyers' Lips cite client privilege, keeping mum?), which in of itself constitutes an unsafe condition, but is not considered so until the secondary retention system fails also. Luke |
#669
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Carlton Reid on QR safety
Luke wrote:
The primary device, unlike the secondary, does not simply retain, it secures. The difference between these two functions is the difference between having and losing control of the bike. I thought everyone here was was arguing against me that a loose QR was not detectable by the rider until it got to the point the wheel was ejected. Now you are trying to argue that a loose QR will be more than detectable - it will loose you control of the bike. Which is it? That's an amusing contradiction. The secondary retention device, 'preferably' takes over in such a manner as to clearly inform that the QRs have failed to secure the wheel (what if the Lawyers' Lips cite client privilege, keeping mum?), which in of itself constitutes an unsafe condition, but is not considered so until the secondary retention system fails also. Its a standard analysis for single fault tolerant safety critical equipment: a single fault shall not create an unsafe condition nor go undetected. Otherwise an undetected failure can continue obscured by the secondary system until the secondary system fails and makes the original fault visible in a potentially unsafe way. Having a loose but retained front wheel is not of itself unsafe. I've ridden that way with disk brakes through my mistake a number of times although unlike many here I pretty quickly knew something was wrong by the feel of the steering and the knocking sound from the front wheel as the QR hit the lawyers lips. -- Tony "The best way I know of to win an argument is to start by being in the right." - Lord Hailsham |
#670
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Carlton Reid on QR safety
Tony Raven writes:
Tim McNamara wrote: I considered the lawyer lips in their correct role: they are there in case of *failure* of the QR to retain the axle in the dropout and are not part of the retention system proper. If they lawyer lips are what's holding your axle in the dropouts, that is because of a *failure* of the QR to retain the axle. Relying on the lawyer lips to hold in the wheel is simply horrendous incompetent design. Its called fail safe design. If the primary retention device fails, the secondary takes over preferably in a way that ensures it is clear to the user that there has been a primary system failure. For an unsafe condition to occur both the primary and secondary systems have to have failed. All the calculations you are doing only show how a primary failure might occur by the QR slipping. What they have failed to address other than by speculation is how the secondary system also fails which is a necessary condition for the wheel to be ejected. That's where we disagree, Tony. You seem to think that it is acceptable for the QR to fail under the reaction force resulting from disk brakes, because the lawyer lips are there to save you. I don't think that it is acceptable for the design to be such that it makes it possible that the QR will fail. No other brake design does this, just disk brakes with the caliper mounted on the trailing side. The laywer lips exist to prevent an *abnormal* occurrence- losing the front wheel because the rider failed to properly close the QR. And even then, the lawyer lips may very well never come into play during riding, unless the rider tries to lift the front wheel. With disk brakes, the ejection force is a normal occurrence and happens every time the rider brakes. The primary retention system- the QR- must be able to retain the wheel 100% of the time, no exceptions. Given how often lawyer lips have come up in this and the previous threads on the topic, there is no chance that anyone has forgotten the lawyer lips. The intent of laywer lips is not to provide axle retention for disk brakes, it is to prevent people who don't know how to use QRs from losing their front wheels. Correct but by default it should prevent a QR from exiting the fork for forces many times the highest calculated here for an ejection force. Unless you can demonstrate rather than speculate the mechanism by which this also fails you have failed to show the two necessary conditions for wheel ejection to occur - a slipping QR and a verified mechanism for the QR to pass the lawyers lips. We are apparently never going to agree on this. By default the lawyer lips should not be required to keep the brakes from pushing the front wheel out of the dropout. Period. There should be no way whatsoever for the brake to create and ejection force on the axle. Period. As a result I will never own a bike with disk brakes of this design. |
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