CPSC judgement on disk brakes and QR forks



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S

Super Slinky

Guest
Robin Hubert said...

> If the front brake on mtbs is so unimportant, why the need for an expensive, complicated, and
> "powerful" disc brake on the front?

I use my front brake a lot. I always use both brakes unless it is very slippery and only the back
one can be safely used. Disk brakes have better modulation and perform better in wet or muddy
conditions. They also don't wear grooves in your rims, and disc rotors are cheaper and easier to
replace than rims. They are also easier to adjust than rim brakes.
 
J

James Annan

Guest
Doug Taylor wrote:
> [email protected] (James Annan) wrote:
>

>>That may be the way you think things should (or do) work, but it's not what the law says.
>
>
> Now you're a lawyer (excuse me, "solicitor")?
>
> On this side of the pond, if a product is determined by a jury be defective under products
> liability law (in this case, a design defect, making the product unreasonably dangerous to use), a
> manufacturer will be held strictly liable for resulting injuries.

I've read some of the relevant legislation, and it is very clear. If the manufacturers discover a
defect, they have a duty to tell the CPSC imediately, and failure to do so may result in heavy fines
and even jail time.

This is one reason why I do not believe those who say "don't worry, the manufacturers will look into
it and will fix it themselves, eventually". They _can't_ fix it themselves, because they have to
tell the CPSC as soon as they realise there is a problem. They are still denying there is a
problem....

> Granted, there MAY come a time when the liability claims become so numerous, newsworthy, and
> immense (Firestone, Pinto) that it becomes cost effective to recall the damn product and
> start over.

I suggest you have a look at the CPSC web site. On the front page, they have their latest news. Over
100,000 computer monitors have been recalled, because there is a risk of them overheating. From
these 100,000 monitors which cost $370 each ($37 million in total) there have been 7 cases of
overheating monitors, one case of minor property damage and one minor smoke inhalation case. Last
year I had a seatpost recalled. There had been a handful of reported failure and no injuries.

Don't blame me, this is how the system is designed. There have certainly been no substantial
compensation claims made in either of these cases. Your understanding of the system (both how it is
supposed to work, and how it does work in the vast majotiry of cases) is flawed. These relatively
rare cases of massive compensation claims and punitive damages are limited to the occasions where
the manufacturers do not follow the correct procedure and cover up problems.

> Time will tell. So, how much to you want to wager?

My house. I have a small house in the UK, where I am not living at present. It's worth about
£50,000, or $80,000.

James
 
J

James Annan

Guest
Spider wrote:

> While I have seen now several dismissals of this question, maybe somebody could now tell me why
> none of this matters,

You inability to work this out for yourself marks you out as a clueless troll.

James
 
D

Doug Taylor

Guest
"Robin Hubert" <[email protected]> wrote in message
news:%[email protected]...

> If the front brake on mtbs is so unimportant, why the need for an
expensive,
> complicated, and "powerful" disc brake on the front?

Who said front brakes on mt. bikes are "unimportant"? The point made was that rear brakes are "more"
important on mt. bikes than road bikes.

Is that too difficult a concept to grasp?
 
S

Sorni

Guest
"James Annan" <[email protected]> wrote in message news:[email protected]...
> Spider wrote:
>
> > While I have seen now several dismissals of this question, maybe somebody could now tell me why
> > none of this matters,
>
> You inability to work this out for yourself marks you out as a clueless troll.

When formulating a one-sentence flame, it is highly advisable to proofread one's work.

Bill "magic marked in?" S.
 
J

Jobst Brandt

Guest
Joe Riel writes:

>> It has also been demonstrated that many riders, if not a large majority of the public at large,
>> do not tighten QRs properly.

> I don't know about a majority, but agree that there is certainly a reasonable percentage that does
> not tighten quick releases enough. The fork designers/manufacturers should take that into
> consideration. That is why we have the [annoying but apparently useful] lawyer lips.

They are essential if you have a disc brake.

Jobst Brandt [email protected]
 
T

Tim McNamara

Guest
In article <[email protected]>, Doug Taylor
<[email protected]> wrote:

> [email protected] (Spider) wrote:
>
> > In addition, peak forces that might create an endo might be approached, but they (IME) are
> > almost never reached, as most MTBer's use their rear brakes when braking, and shift their weight
> > such that the rear brakes are to be effective. On the road, I could disconnect my rear brake and
> > not notice much. On dirt, I would crash on the first steep descent.
>
> This is an excellent point which many roadies just don't get. And too many of the contributors to
> this thread either: 1) don't ride mountain bikes at all; 2) ride mountain bikes infrequently; or
> 3) ride mountain bikes infrequently without disc brakes.

I won't speak for anyone else here, although I know this applies to many of us in this newsgroup.

I'm older than mountain bikes. I was riding offroad 15 years before mountain bikes or BMX bikes
existed- just like every other kid in my home town. Probably just like every kid in your home town
prior to about 1982. Maybe even just like you!

Whether any one participating in this thread uses disk brakes or not is not germaine to the issue of
the design flaw inherent in current designs. It's another red herring, a tactic many in the
anti-Annan brigade are inordinately fond of. The implication that "you don't ride disk brakes and
therefore you can't understand the ejection force issue" is just silly.

You have also managed to demonstrate that the arrogance of mountain bikers now exceeds the arrogance
of roadies in this forum, as well as it being quite evident out on the trails. Roadies are far more
pleasant. Why, back when I got my first mountain bike you could ride 20 miles uphill both ways on
virgin singletrack without encountering another biker and when you did grumble grumble snort whinge
kvetch moan...
 
S

Spider

Guest
Tim McNamara <[email protected]> wrote in message
news:<[email protected]>...
> In article <[email protected]>, [email protected]
> (Spider) wrote:
>
> > > The standard placement for the rear disk brake does not create problems. As far as design
> > > compromise, all that has to be done is to move the caliper ahead of the fork leg, which will
> > > result in a retention force rather than an ejection force. End of problem. It would cost no
> > > more to make a fork with that mounting than the current design.
> >
> > I agree. All well and good, except: what to do about all the forks already out there? James
> > wants a recall. Not going to happen without much more solid *evidence* (as opposed to force
> > diagrams.)
>
> As far as the forks already out there, it looks like it would be possible to make a bracket which
> would bolt into the existing mounting holes and which would provide new mounting holes in a
> different location. I don't know that this would be possible with every suspension fork, but the
> ones I've looked at would appear to have ample room to accomodate this. This approach would be far
> cheaper and (I think) equally effective to recalling and replacing forks whole.

That seems like one reasonable solution. Jobst has said that conical QR surfaces mated with
like-faced drop outs might also be a solution. I do not know how much "cone" is needed. Maybe too
much fork material would have to be machined.

In any case, this doesn't answer my question about the coefficient of friction between dirt and
knobbies. This one figure could significantly change the vertical force on the axle. This is one of
the things that James continually avoids, and I can't figure it out for the life of me.

Just to prove what I initially said about the responses to my questions being condescending
dismissals, here's what I said:

"While I have seen now several dismissals of this question, maybe somebody could now tell me why
none of this matters..."

To which James replied:

"You [sic] inability to work this out for yourself marks you out as a clueless troll."

I'm sorry, Tim, but this is just the thing that I'm talking about. How am I supposed to "work out"
the mu of knobbies on dirt? Since this figure is crucial for the calculation of the *actual* forces
on the QR, one would think that it would not be swept under the rug.

Doing my freshman physics labs, we had a force-measuring device that was essentially a spring with a
known value of k and a calibrated scale that read in Newtons. If I were to goto my local trail, and
with an assistant, drag my bike along with it's brakes locked, hopefully finding the peak of static
friction before it becomes kinetic friction (a lower number,) then we might have some idea of what
mu might be for a few different types of mtb terrain.

Why is this important? IIRC, most mechanical systems are designed to a "sustained use" threshold.
But most of these systems can withstand peak forces higher than that without failure. Thus, if I
ride three times a week on pavement, and use my brakes to the point of endo once or twice a week,
the QR/drop out/brake system should be able to take that without problems. Now, if I constantly
cycled that in a "peak/off/peak/off" manner, I might be able to get unscrewing. But over the course
of this riding season, cycling the brakes a lot, I have yet to see the QR move even a part of a
millimeter.

The experiment that I am performing is actually more controlled than all of the anecdotes that James
has compiled thus far. I closed my Shimano knurled QR in the manner I always have, which is to say
that the lever is on the drive side (opposite the disk and caliper side,) opened to about 95 or 100
degrees, and the nut tighted into the dropout while the fork is under a small amount of compression
(me over the handlebars, compressing it about 10mm). Close the lever over, and make fine register
marks in both sides of the QR and on the drop out, such that I will be able to tell if the QR nut or
lever moves at all, including up and down in the drop out itself (without unscrewing.) Then ride
like I always have.

The marks are in register, just as they have been from the first tightening. I check before and
after every ride, and they haven't moved at all.

So, with all of this, I would imagine that those folks who dismiss out of hand what I am asking
can't really ignore it as a "troll", nor dismiss it with the wildly arrogant claim of "if you
understood it, then you wouldn't be questioning it," etc.

These questions aren't going away, and neither am I.

You stated above that the only use for disks was in mud, and that you didn't see any other reason
for them. Let me list the reasons:

Water (when you need brakes, you need them NOW, not some time after the pads have swept the water
off the braking surface)

Modulation. IME, disk brakes are much easier to "feather" than rim brakes. V-brakes often have that
"on/off" feel to them when they are not in perfect adjustment. And Vs do not keep adjustment long,
especially on a long ride.

Rim trueness. Whack your rim on a rock, it goes out of true, sometimes. With Vs, you either have to
mess around with the barrel adjuster or change the wire clamping position. With disks, if the wheel
turns, you can ride.

Heat dissapation. The rim/tire/tube system can build up heat to the point where tires can blow off
the rim. I have friends to whom this has occurred. Blowing your front tire off at speed going
downhill seems to me to be an excellent way to check on your insurance coverage. Disk systems rarely
build enough heat to compromise performance.

Rim life. Because it's not a wear surface, the sidewall will not become thin from braking.

Rim strength. Theoretically, the rim can be made stronger and/or lighter when it only has to serve
the function of holding the tire/tube in place. This is a question for an engineer.

Ease-of-set-up. My mechanical disk brakes are trivial to set up. The center-pull cantis on my wife's
bike are a complete ***** in comparison.

And, as you say, mud.

Finally, there have been several additional comments about "not using the front brake," said in sort
of a joking or hyperbolic fashion. I'm not exactly sure how these comments do anything but make the
poster of the comments look arrogant, but I'll address it. Washing out the front wheel on a narrow
trail with rocks and trees can get you killed. Worst case, of course. So, most MTBers take pains to
NOT wash out the front from brake lockup and use the rear brake extensively to scrub speed. Weight
shift to keep the rear wheel on the ground and as an effective tool for braking is a skill learned
in the first bit of an MTBer's "career," as it were. Again, this is a subtractive number to James'
hypothesis. I, for one, NEVER use only the front brake when I'm on the dirt.

Spider
 
T

Tim McNamara

Guest
In article <[email protected]>, "Mike S." <[email protected]> wrote:

> Its not that you don't understand the ejectoin force issue, its just that as a non-disc riding
> individual, you have no direct experience to say whether its a problem or not!

I disagree with your reasoning, but there's no point in rehashing reasons I've already written many
times. :) One doesn't have to directly experience the problem to know there's a problem.

> I haven't been riding off-road quite as long as you ('88). I've seen the trails eroding quicker,
> more user conflicts, etc. because there's a host of new riders that don't have the skills to
> ride the trails that they're on, but their FS mtn bike allows them to get away with it time, and
> time again.

This is certainly something we agree on.
 
T

Tim McNamara

Guest
In article <[email protected]>, [email protected]
(Spider) wrote:

> Tim McNamara <[email protected]> wrote in message
> news:<[email protected]>...
>
> > As far as the forks already out there, it looks like it would be possible to make a bracket
> > which would bolt into the existing mounting holes and which would provide new mounting holes in
> > a different location. I don't know that this would be possible with every suspension fork, but
> > the ones I've looked at would appear to have ample room to accomodate this. This approach would
> > be far cheaper and (I think) equally effective to recalling and replacing forks whole.
>
> That seems like one reasonable solution. Jobst has said that conical QR surfaces mated with
> like-faced drop outs might also be a solution. I do not know how much "cone" is needed. Maybe too
> much fork material would have to be machined.

I don't know if this could be retrofitted to existing forks- Jobst would have a better idea about
this- but it's the same principle as a lug nut. Jobst has had his pedals and cranks modified in a
similar fashion to stop breaking them at the pedal eye.

> In any case, this doesn't answer my question about the coefficient of friction between dirt and
> knobbies. This one figure could significantly change the vertical force on the axle. This is one
> of the things that James continually avoids, and I can't figure it out for the life of me.

Well, hopefully James will address that. I know Jobst has talked about this already to some
extent, anyway.

> You stated above that the only use for disks was in mud, and that you didn't see any other reason
> for them. Let me list the reasons:
>
> Water (when you need brakes, you need them NOW, not some time after the pads have swept the water
> off the braking surface)

Hmm, having ridden and raced in the rain this has never been much of a problem. My Scott/Mathauser
brakes pads on polished aluminum rims are excellent in the wet. More typical OEM brake pads on
anodized rims are horrible, not much better than chromed steel rims, and perhaps that's what you're
comparing to. Another reason I think anodizing is useless- but that's a different thread. ;-)

> Modulation. IME, disk brakes are much easier to "feather" than rim brakes. V-brakes often have
> that "on/off" feel to them when they are not in perfect adjustment. And Vs do not keep adjustment
> long, especially on a long ride.

V-brakes are an execrable design in this regard, I agree. I have two bikes with V-brakes and I would
replace them with cantilevers if it was easy to do. In one case it would be possible, in the other
it is not. While I have not ridden with disk brakes on other people's bikes, I found them less
"modulatable" than properly adjusted cantilever brakes and no more powerful.

> Rim trueness. Whack your rim on a rock, it goes out of true, sometimes. With Vs, you either have
> to mess around with the barrel adjuster or change the wire clamping position. With disks, if the
> wheel turns, you can ride.

You have a point there. As long as the wheel goes through the frame, you can ride it. However,
cantis and single pivot sidepulls provide much more clearance than V-brakes and this is not much of
a problem with them.

> Heat dissapation. The rim/tire/tube system can build up heat to the point where tires can blow off
> the rim. I have friends to whom this has occurred. Blowing your front tire off at speed going
> downhill seems to me to be an excellent way to check on your insurance coverage. Disk systems
> rarely build enough heat to compromise performance.

This is also true, although I would argue that knowing how to descend prevents this problem. I've
never had it happen in 30+ years of riding bike. Now, I have not lived in mountainous terrain and
haven't had much opportunity to do extended descending, so I have to qualify that statement so as to
avoid being misleading. I have ridden in the Alps, however, and had no trouble along these lines
using canti brakes on long technical descents such as the Col de Sarenne. Tandems have traditionally
had more trouble with this, having much more momentum to control with the same brake and rim surface
area as a single bike.

On the other hand, while the rims are not getting hot the disks are. They have less surface area and
get hotter, which vaporizes pad material quicker than would be the case with rim brakes.

> Rim life. Because it's not a wear surface, the sidewall will not become thin from braking.

Can't argue with that.

> Rim strength. Theoretically, the rim can be made stronger and/or lighter when it only has to serve
> the function of holding the tire/tube in place. This is a question for an engineer.

I suppose this is theoretically possible, but I'd have to defer that to someone who knows more about
rim design than I do. Jobst, Willet, et al.

> Ease-of-set-up. My mechanical disk brakes are trivial to set up. The center-pull cantis on my
> wife's bike are a complete ***** in comparison.

I've never had much problem with this, but then I've used fairly simple cantis (Mafac, DiaCompe 986)
that are easy to adjust. Some certainly are not. The brakes I use also stay in adjustment so I only
have to do it once every few years, which I presume is also the case with disk brakes.

> And, as you say, mud.

The obvious reason. The less obvious reason is marketing. Disk brakes are superior to drum brakes in
automotive applications, and it's easy to carry that same thinking over into marketing for bicycles.
In the case of bicycles it is misleading. All rim brakes already are disc brakes- the automotive
industry appropriated the concept from bicycles!

> Washing out the front wheel on a narrow trail with rocks and trees can get you killed.

This is a difference between trail riding and road riding. Repack riders have talked about
descending on their coaster brake klunkers
vs. MTBs with rim brakes, and said that with coaster brakes they didn't have to worry about washing
the front wheel out since there was no front brake.
 
M

Mike S.

Guest
> That seems like one reasonable solution. Jobst has said that conical QR surfaces mated with
> like-faced drop outs might also be a solution. I do not know how much "cone" is needed. Maybe too
> much fork material would have to be machined.

I saw an alternative over on www.mtbr.com when I posted the question we're debating here. One
indivual recommended a Skraxle-type arrangement. The axle could be used as a QR through the fork
dropouts ala Specialized's Skraxle hubs. Not a bad compromise since the bottom of the dropouts are
sealed, but the XC guys can still get the hub out relatively quickly.

Mike
 
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James Annan

Guest
Spider wrote:

> That seems like one reasonable solution. Jobst has said that conical QR surfaces mated with
> like-faced drop outs might also be a solution. I do not know how much "cone" is needed. Maybe too
> much fork material would have to be machined.

I agree that any sort of retrofitted solution (so long as it works) would be fine. I only wish the
manufacturers would put the weight of their engineering talent behind fixing the problem, rather
than continuing to claim that it does not exist.

> In any case, this doesn't answer my question about the coefficient of friction between dirt and
> knobbies. This one figure could significantly change the vertical force on the axle. This is one
> of the things that James continually avoids, and I can't figure it out for the life of me.

Ok, I'll type this veeerrrrrryyyyyyy sloooooowwwwwwlllllyyyyyy.

1. Use a smaller number if you prefer. There will still be a significant outward pull which is
completely unacceptable from a design point of view.

2. The British Standard demands that the front brake can generate 0.41g, and that's for any cheap
junk let alone a high performance mountain bike.

3. The notion that because people don't _usually_ brake so hard, it's ok to have a system that may
fail when people _do_ brake hard, is a complete nonsense anyway. Sometimes, mu will certainly be
above 0.6 which is all that is required for the higher figure. Just like the ridiculous claim
made earlier that it was the fault of the rider for riding downhill, this is really scraping the
barrel as far as excuses go.

The description of the failure mechanism has been in the public domain for 5 months now. There is a
lesson to be learnt in the fact that the best 'criticism' you can find of it is utterly trivial,
irrelevant and wrong. Whether you choose to learn this lesson, is another matter.

James
 
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Tim McNamara

Guest
In article <[email protected]>, "Mike S." <[email protected]> wrote:

> > That seems like one reasonable solution. Jobst has said that conical QR surfaces mated with
> > like-faced drop outs might also be a solution. I do not know how much "cone" is needed. Maybe
> > too much fork material would have to be machined.
>
> I saw an alternative over on www.mtbr.com when I posted the question we're debating here. One
> indivual recommended a Skraxle-type arrangement. The axle could be used as a QR through the fork
> dropouts ala Specialized's Skraxle hubs. Not a bad compromise since the bottom of the dropouts are
> sealed, but the XC guys can still get the hub out relatively quickly.

Being too lazy to go to a Specialized dealer and not finding a picture of this on the Web, whazzit
look like and how does it work?

I did notice that a lot of Web sites I found through Google using Skraxle as the search term kept
mentioning that this design enhances steering, improves fork rigidity, ete. Sounds like independent
leg movement is still a problem with suspension forks. Or is that just more marketing palaver?
 
D

David Damerell

Guest
VinceA <[email protected]> wrote:
>"Mike S." <[email protected]> wrote in message news:<[email protected]>...
>>Yeah, and I'll bet that if you did the same with cantis or V-brakes, it'd do the same thing. Since
>>I haven't tried it, I don't know. Anyone willing to try?
>It has been done by one of the major bike manufacturers and it does NOT happen with V-brakes
>or Cantis.

Forget the major manufacturers - if you have a bike with rim brakes, checking this yourself
is trivial.
--
David Damerell <[email protected]> Distortion Field!
 
D

David Damerell

Guest
Spider <[email protected]> wrote:
>James Annan <[email protected]> wrote in message news:<[email protected]>...
>>2. The British Standard demands that the front brake can generate 0.41g, and that's for any cheap
>> junk let alone a high performance mountain bike.
>constants of the equation not one tiny bit. I have no doubt that disk brakes COULD generate more
>than 1.0g

Except for the trivial consideration that that's going to send you flying, yes.

>>Sometimes, mu will certainly be above 0.6 which is all that is required for the higher figure.
>I am going to assume you mean an acceleration of 0.6g, not the coefficient of friction mu.

Of course he doesn't. Perhaps it has not occured to you that the maximum deceleration before the
front wheel skids is equal to the coefficient of friction between it and the surface?
--
David Damerell <[email protected]> Distortion Field!
 
S

Spider

Guest
James Annan <[email protected]> wrote in message news:<[email protected]>...
> Spider wrote:
>
> > That seems like one reasonable solution. Jobst has said that conical QR surfaces mated with
> > like-faced drop outs might also be a solution. I do not know how much "cone" is needed. Maybe
> > too much fork material would have to be machined.
>
> I agree that any sort of retrofitted solution (so long as it works) would be fine.

I think that it would be interesting to explore this "lugnut style" fix first.

> I only wish the manufacturers would put the weight of their engineering talent behind fixing the
> problem, rather than continuing to claim that it does not exist.

This, of course, assumes a real problem, rather than just a perceived one.

> > In any case, this doesn't answer my question about the coefficient of friction between dirt and
> > knobbies. This one figure could significantly change the vertical force on the axle. This is one
> > of the things that James continually avoids, and I can't figure it out for the life of me.
>
> Ok, I'll type this veeerrrrrryyyyyyy sloooooowwwwwwlllllyyyyyy.

Typing speed makes absolutely no difference in the logic required. It is exactly this condescending
attitude which hardens opinion *against* your position. If you have something of value to impart,
then condescension is not required. I am baffled as to what good you think it does.

> 1. Use a smaller number if you prefer. There will still be a significant outward pull which is
> completely unacceptable from a design point of view.

The "completely unacceptable" portion of this is merely your opinion. One which I do not share at
this time. Using a smaller number proportionally reduces the magnitude of the downaward force, and
also reduces the likelihood of QR movement in the drop out.

> 2. The British Standard demands that the front brake can generate 0.41g, and that's for any cheap
> junk let alone a high performance mountain bike.

Non sequitur.

What some standard body says or does not say changes the physical constants of the equation not one
tiny bit. I have no doubt that disk brakes COULD generate more than 1.0g - how much more is not
really important.

> 3. The notion that because people don't _usually_ brake so hard, it's ok to have a system that may
> fail when people _do_ brake hard, is a complete nonsense anyway.

Your unscrewing mechanism relies on large forces to overcome the clamping forceand thus unscrew to
disengage the wheel. If these forces are not routinely experienced, or in fact very rarely
experienced, then the "problem" of wheel disengagement becomes of the same type as frame failure,
wheel failure, seatpost failure, etc, all of which happen on MTBs, sometimes with catastrophic
results. Low spoke count wheels, understrength frames, etc, can and do lead to serious injuries due
to user error or just plain design compromise.

> Sometimes, mu will certainly be above 0.6 which is all that is required for the higher figure.

I am going to assume you mean an acceleration of 0.6g, not the coefficient of friction mu. Since
this mu is a direct, multiplicative figure, a mu of 0.6 means that your disengagement force becomes
suddenly 40% smaller.

> Just like the ridiculous claim made earlier that it was the fault of the rider for riding
> downhill, this is really scraping the barrel as far as excuses go.

I believe it is because you do not understand the relevant distinction between riding a MTB down a
hill, and downhill (DH) MTBing. A crosscountry (XC) MTB might weigh between 10 and 14kg, and is
compromised in strength and/or durability to reduce weight. A DH MTB might weigh between 19 and
25kg, and is overbuilt for everything except bombing straight down a hill as fast as possible.
Riding an XC bike down a DH course is begging for a part failure. Probably wheel or frame damage.

This is, of course, assuming you are not being obtuse on purpose.

> The description of the failure mechanism has been in the public domain for 5 months now.

Yes - the hypothesis has been around for some time. Time will tell on what the German group finds as
the conditions required for your hypothesis to hold true in all cases.

> There is a lesson to be learnt in the fact that the best 'criticism' you can find of it is utterly
> trivial, irrelevant and wrong.

Since you cannot answer the question of how your calculations are effected, and by how much, due to
a very important consideration during actual use, labelling it irrelevant is very convenient. Since
you are trying to prove an assertion, I would suggest that it is up to you to *prove* it's relevance
or lack thereof, rather than attack the questioner.

> Whether you choose to learn this lesson, is another matter.

Your ad hominem commentary doesn't make your point clearer, and doesn't win your POV any converts. I
suggest you drop this tactic, as it is not bearing fruit for you.

Spider
 
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Tim McNamara

Guest
In article <[email protected]>, [email protected]
(Spider) wrote:

> > > In any case, this doesn't answer my question about the coefficient of friction between dirt
> > > and knobbies. This one figure could significantly change the vertical force on the axle.
> > > This is one of the things that James continually avoids, and I can't figure it out for the
> > > life of me.
> >
> > Ok, I'll type this veeerrrrrryyyyyyy sloooooowwwwwwlllllyyyyyy.
>
> Typing speed makes absolutely no difference in the logic required. It is exactly this
> condescending attitude which hardens opinion *against* your position. If you have something
> of value to impart, then condescension is not required. I am baffled as to what good you
> think it does.

He's had the same personal attacks repeated over and over by a series of new posters for neartly six
months. Unfortunately the personal attacks have outweighed the reasoned criticism by a wide margin.
It gets old, I suppose, and makes one irritable. Much of this thread has been just a rehash of prior
threads, which could be avoided by just searching the newsgroup via Google.

As far as I can recall, only your mu argument is new to this discussion in this thread and the rest
is reruns. And that's really only new by virtue of introducing the term "mu" to the conversation,
the issue itself has been discussed.

My frame of reference for my is perhaps different than yours: "What is Buddha-nature?" "Mu."
 
D

David Damerell

Guest
Tim McNamara <[email protected]> wrote:
>As far as I can recall, only your mu argument is new to this discussion in this thread and the rest
>is reruns. And that's really only new by virtue of introducing the term "mu" to the conversation,
>the issue itself has been discussed.

I don't think the issue per se has been discussed, because I suspect Spider has just enough
knowledge to confuse himself; observe the assertion that on a surface with a coefficient of friction
of 0.6 the ejection force is 40% less!
--
David Damerell <[email protected]> Distortion Field!
 
J

Jobst Brandt

Guest
David Damerell writes:

>> As far as I can recall, only your mu argument is new to this discussion in this thread and the
>> rest is reruns. And that's really only new by virtue of introducing the term "mu" to the
>> conversation, the issue itself has been discussed.

> I don't think the issue per se has been discussed, because I suspect Spider has just enough
> knowledge to confuse himself; observe the assertion that on a surface with a coefficient of
> friction of 0.6 the ejection force is 40% less!

I think my comment on the 0.6g or 0.6mu, was missed by some. However, It it possible to repeat that:

That the 0.6g number entered the discussion is unfortunate, because it only ads a diversion,
being valuable only in computing maximum disengagement force for a specific rider. Because one
cannot predict where riders will position their CG when braking, which depends on rider skill
and aggressiveness, the g-force can lie in a fairly broad range, 0.6 being at the
conservatively low end.

The important parameter is that the axle disengaging force when skidding on solid ground is about
three times the downward force of the fork on the axle. Note that this force is concentrated on one
end of the axle and is therefore closer to twice that large. For pavement, where disc brakes are
also used, both on road bicycles as well as MTB's, the coefficient of friction is about one as can
be seen from racing pictures with riders leaning at about 45 degrees to the road surface. Because
brake discs are as small as 1/4 the wheel, they can produce a 4:1 ratio to the skidding force.

I mention "solid ground" at this time of year, but when wet, knobby tires can have traction
coefficients greater than one on impressionable clay like surfaces.

Jobst Brandt [email protected]
 
D

Doug Taylor

Guest
Tim McNamara <[email protected]> wrote:

>My frame of reference for my is perhaps different than yours: "What is Buddha-nature?" "Mu."

Ah. A redeeming quality. But how someone cultivating mindfulness can't appreciate disc brakes
escapes me ;-) --dt
 
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