Re: x-post: Bike Biz: Wheel ejection theory goes legal



On Thu, 15 Feb 2007 02:11:41 -0600, Ben C <[email protected]> wrote:

>On 2007-02-15, [email protected] <[email protected]> wrote:
>[...]
>> Dear Frank & Ben,
>>
>> To clarify things, disk brakes first appeared on large, heavy street
>> motorcycles--and were originally mounted with high leading calipers by
>> Honda.
>>
>> In ten years or less, the industry (including Honda) had switched to
>> overwhelmingly to low trailing calipers.
>>
>> The only modern leading calipers that I know of are found on trials
>> machines, where the high leading position helps avoid damaging
>> calipers when the bikes are dropped to one side or the other in rocks,
>> a routine hazard.
>>
>> Trials machines are not designed or certified for road use and
>> nowadays weigh under 170 pounds. Their braking can be quite savage at
>> low speeds in stunt-riding maneuvers, with the contact patches at
>> absurd angles.
>>
>> I don't know why all other machines switched to low trailing calipers
>> after the first high leading calipers, but I doubt that it had
>> anything to do with failures. More likely, the lower trailing caliper
>> improves handling at normal speeds--the motorcycle calipers and disks
>> are much larger and heavier than what's found on bicycles.

>
>Possibly there are aerodynamic advantages as well.
>
>[...]
>> Any RBT poster interested in such things should stop by a motorcycle
>> shop or just peek at a parked machine and see how much thicker,
>> heavier, and stronger motorcycle parts are.

>
>I have been getting funny looks recently as I scrutinize the caliper
>placement of every parked vehicle I walk past. All the motorbikes I've
>seen had them low and rear.
>
>Most cars I saw had them front mounted, but some were rear mounted.
>
>I saw a couple of Porsche 911s with cross-drilled discs. Grrr.


Dear Ben,

Alas, motorcycles are so hideously unstreamlined that aerodynamics are
ignored for all practical purposes. Their impressive power-to-weight
ratio tends to conceal aerodynamic characteristics that would shame a
garbage truck.

In the late 1970's, the wretched wind drag of motorcycles was
demonstrated in an amusing experiment.

A well-tuned Yamaha TZ750 racing motorcycle was put through the speed
traps.

Without even a spark plug change, the motorcycle engine was switched
into the modified engine bay of a small racing car--much heavier than
the motorcycle, but also much more streamlined.

The car went 20 mph faster on the same track.

Even well-fendered pavement-racing motorcycles with narrow handlebars
bear a strong resemblance to tumbleweeds in a wind-tunnel.

Here's a view of an ordinary street machine, showing the mirrors, turn
signals, speedometer, tachometer, fender, radiator, headlight, cables,
disk brake lever and master cylinder, and so forth that greet the
wind:

http://www.mccullagh.org/db9/10d-14/honda-919-motorcycle.jpg

For pracitcal riding, the wind drag just doesn't matter much when you
have around 100 real horsepower available at the rear tire to move
about 600 pounds of machine and rider.

Cheers,

Carl Fogel
 
On Thu, 15 Feb 2007 13:26:30 -0700, [email protected] wrote:

>On Thu, 15 Feb 2007 02:11:41 -0600, Ben C <[email protected]> wrote:
>
>>On 2007-02-15, [email protected] <[email protected]> wrote:
>>[...]
>>> Dear Frank & Ben,
>>>
>>> To clarify things, disk brakes first appeared on large, heavy street
>>> motorcycles--and were originally mounted with high leading calipers by
>>> Honda.
>>>
>>> In ten years or less, the industry (including Honda) had switched to
>>> overwhelmingly to low trailing calipers.
>>>
>>> The only modern leading calipers that I know of are found on trials
>>> machines, where the high leading position helps avoid damaging
>>> calipers when the bikes are dropped to one side or the other in rocks,
>>> a routine hazard.
>>>
>>> Trials machines are not designed or certified for road use and
>>> nowadays weigh under 170 pounds. Their braking can be quite savage at
>>> low speeds in stunt-riding maneuvers, with the contact patches at
>>> absurd angles.
>>>
>>> I don't know why all other machines switched to low trailing calipers
>>> after the first high leading calipers, but I doubt that it had
>>> anything to do with failures. More likely, the lower trailing caliper
>>> improves handling at normal speeds--the motorcycle calipers and disks
>>> are much larger and heavier than what's found on bicycles.

>>
>>Possibly there are aerodynamic advantages as well.
>>
>>[...]
>>> Any RBT poster interested in such things should stop by a motorcycle
>>> shop or just peek at a parked machine and see how much thicker,
>>> heavier, and stronger motorcycle parts are.

>>
>>I have been getting funny looks recently as I scrutinize the caliper
>>placement of every parked vehicle I walk past. All the motorbikes I've
>>seen had them low and rear.
>>
>>Most cars I saw had them front mounted, but some were rear mounted.
>>
>>I saw a couple of Porsche 911s with cross-drilled discs. Grrr.

>
>Dear Ben,
>
>Alas, motorcycles are so hideously unstreamlined that aerodynamics are
>ignored for all practical purposes. Their impressive power-to-weight
>ratio tends to conceal aerodynamic characteristics that would shame a
>garbage truck.
>
>In the late 1970's, the wretched wind drag of motorcycles was
>demonstrated in an amusing experiment.
>
>A well-tuned Yamaha TZ750 racing motorcycle was put through the speed
>traps.
>
>Without even a spark plug change, the motorcycle engine was switched
>into the modified engine bay of a small racing car--much heavier than
>the motorcycle, but also much more streamlined.
>
>The car went 20 mph faster on the same track.
>
>Even well-fendered pavement-racing motorcycles with narrow handlebars
>bear a strong resemblance to tumbleweeds in a wind-tunnel.
>
>Here's a view of an ordinary street machine, showing the mirrors, turn
>signals, speedometer, tachometer, fender, radiator, headlight, cables,
>disk brake lever and master cylinder, and so forth that greet the
>wind:
>
>http://www.mccullagh.org/db9/10d-14/honda-919-motorcycle.jpg
>
>For practical riding, the wind drag just doesn't matter much when you
>have around 100 real horsepower available at the rear tire to move
>about 600 pounds of machine and rider.
>
>Cheers,
>
>Carl Fogel


A non-bicycling friend promptly emailed me to insist that I was
thinking of the Yamaha TY750, not a TZ750.

This gave me the opportunity to gloat and score impressive points in
petty one-upmanship by reminding him that the TY series was Yamaha's
line of trials machines, whose engines never exceeded 350 cc's.

He rallied by demanding to know why I hadn't specified the actual
speeds.

I confessed that my fading memory supplied the 20 mph difference, but
not whether it was 130 mph versus 150 mph, or 150 mph versus 170 mph.

He stated that even the girls who present the trophies at the race
track know that the motorcycle went 140 mph, while the car using the
same engine went 160 mph.

He added that he could provide a link to an Alzheimer's support group
if I thought that I would be able to click on it.

It's nice to be reminded that RBT is not that different from the rest
of the world.

CF
 
On 15 Feb 2007 19:06:38 GMT, [email protected]anfordalumni.org wrote:

>Ben Micklem <[email protected]> writes:
>
>>>> Front calipers on bicycles are rare.

>
>>> Front *disk* calipers on bicycles are nonexistant.

>
>> The Cotic Roadrat's roadhog fork has them:

>
> http://www.cotic.co.uk/product/roadrat.html
>
>Interesting. It's an example of a manufacturer who understood not to
>use tapered road forks with hub brakes and that there is a benefit to
>front mounting a disk brake caliper. This doesn't occur accidentally.
>
>Jobst Brandt


Dear Jobst,

I'm not sure, but I think that the manufacturer offers whichever disk
caliper mounting the customer wants, front or rear:

Disc Brakes:

Hope Mono Mini Front (160 disc size, IS or Post Mount) £100
Hope Mono Mini Rear (160/140 disc size, IS or Post Mount) £100

http://www.cotic.co.uk/product/roadrat.html

The picture isn't very clear. It appears to be a rear-caliper front
disk brake, with the angle showing the brake cable visible between the
forks, going down behind the bicycle's left fork leg. (I'm not sure
about this.)

But several posters have indicated that they've seen front-calipers on
this model.

What do "IS" and "Post Mount" mean on that page? Front or rear
calipers for front disk brakes? Or something else unrelated to our
subject?

Cheers,

Carl Fogel
 
E

Ed Pirrero

Guest
On Feb 15, 11:51 am, Ben C <[email protected]> wrote:
> On 2007-02-15, [email protected] <[email protected]> wrote:
>
> > Your mistake is to believe jim beam's assertion that cast aluminum can
> > never be designed to resist tensile fatigue.

>
> I don't think he asserted that. Or if he did I don't remember.


He didn't. Frank has a bit of a tendancy to lie to attempt to improve
his position on a subject.

Or try and impugn someone else's.

Because of his weak thinking and position on issues, he creates
strawmen instead of discussing the actual issues. What the heck?
He's retired and has nothing but time on his hands.


> > jim beam's idea is that one can never load a casting in tensile
> > fatigue. Logically, it requires only one counterexample to prove it
> > wrong.

>
> Again, I don't remember him making such an extravagant claim.


Read Frank's stuff long enough and you'll detect a pattern. Actually,
I think you've read enough, and can see the pattern...

E.P.
 
J

James Thomson

Guest
<[email protected]> a écrit:

On 15 Feb 2007 19:06:38 GMT, [email protected] wrote:

>> Interesting. It's an example of a manufacturer who understood not to
>> use tapered road forks with hub brakes and that there is a benefit to
>> front mounting a disk brake caliper. This doesn't occur accidentally.


Cotic is a one-man operation. The proprietor reads the
Singletrackworld.co.uk forum - a community which became very aware of the
wheel-ejection issue when one of its number broke his back a few years ago.

> I'm not sure, but I think that the manufacturer offers whichever
> disk caliper mounting the customer wants, front or rear:


> Disc Brakes:
>
> Hope Mono Mini Front (160 disc size, IS or Post Mount) £100
> Hope Mono Mini Rear (160/140 disc size, IS or Post Mount) £100


I think that says that they'll supply the brakes in either format if bought
with a frame. The frames are of the IS type. They're batch-produced in
Taiwan, and there are no custom options.

ROADHOG Fork Specifications

Bladed profile.

Butted Cromo Legs.

700c V-brake bosses.

Disc mount positioned on front right ; brake forces always go
into the dropout and there's no interference with mudguards.
Max rotor size : 160mm.

400mm a-c

45mm offset.

250mm steerer length uncut - about 40mm more than the
minimum required for a large frame.

Clearance for up to 46c tyre.

> http://www.cotic.co.uk/product/roadrat.html


> The picture isn't very clear. It appears to be a rear-caliper
> front disk brake, with the angle showing the brake cable
> visible between the forks, going down behind the bicycle's
> left fork leg. (I'm not sure about this.)


> But several posters have indicated that they've seen front-
> calipers on this model.


http://homepage.mac.com/spittingcat/cotic/product/interface/roadrat/gallery/mainpic_03.jpg

http://www.legslarry.beerdrinkers.co.uk/misc/Depravo_2.htm

It's possible that the main picture is of a prototype with an alternative
brake mounting. Hard to tell.

> What do "IS" and "Post Mount" mean on that page? Front or rear
> calipers for front disk brakes? Or something else unrelated to our
> subject?


Post Mount calipers bolt directly into attachments that are rather like
water bottle bosses on the rear of the fork leg. On IS type calipers the
bolts enter from the side.

http://www.performancebike.com/shop/sizecharts/allaboutdiscbrakes.cfm

James Thomson
 
T

Tim McNamara

Guest
In article <C1FA3598.2312C%[email protected]>,
Ben Micklem <[email protected]> wrote:

> in article [email protected], Tim
> McNamara at [email protected] wrote on 15/2/07 15:22:
>
> > In article <[email protected]>,
> > Ben C <[email protected]> wrote:
> >> Front calipers on bicycles are rare.

> >
> > Front *disk* calipers on bicycles are nonexistant.

>
> The Cotic Roadrat's roadhog fork has them:
> http://www.cotic.co.uk/product/roadrat.html


Hey! Lookitdat! Thanks for pointing that out. There's something I am
quite happy to be wrong about.
 
T

Tim McNamara

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

> Ben Micklem <[email protected]> writes:
>
> >>> Front calipers on bicycles are rare.

>
> >> Front *disk* calipers on bicycles are nonexistant.

>
> > The Cotic Roadrat's roadhog fork has them:

>
> http://www.cotic.co.uk/product/roadrat.html
>
> Interesting. It's an example of a manufacturer who understood not to
> use tapered road forks with hub brakes and that there is a benefit to
> front mounting a disk brake caliper. This doesn't occur
> accidentally.


Maybe they read rec.bicycles.tech and understood the concept.
 
T

Tim McNamara

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

> Even well-fendered pavement-racing motorcycles with narrow handlebars
> bear a strong resemblance to tumbleweeds in a wind-tunnel.


Carl, that may be the most entertaining visual image you have ever
conjured up in this newsgroup.
 
J

Jay Beattie

Guest
On Feb 14, 7:33 pm, jim beam <[email protected]> wrote:

<big snip>

> question: in a situation where there is a known risk, but another known
> benefit with a certain design, then what happens? example: most modern
> cars have negative scrub radius for their steering. it's "safe" because
> it tends to counteract the steering effect of a front tire flat at speed
> thereby allowing an unskilled driver more chance of retaining control
> and bringing the vehicle to a halt in a controlled manner. but the
> danger is that it makes steering feel light [and therefore "safe"] when
> braking /through/ corners, thereby encouraging that unsafe habit.
> physics will tell you that braking through corners is a bad thing, and
> positive scrub makes braking through feel heavy and uncertain, thereby
> discouraging that kind of bad behavior. how might a legal test be
> applied to that?


This example highlights my point about the risk-utility test being
used when a product is so complicated or arcane that a reasonable
consumer (me, for example) has no expectation of safety. Not that I
don't expect everything to be safe, but I have no knowledge or
expectation concerning the safety of scrub versus no scrub, both of
which are are apparently acceptable design choices. Even in Oregon,
which is supposed to be a pure "consumer expectation" test state, the
courts will admit risk-utility evidence in cases like this even if
they do not expressly instruct on the risk-utility test. Under the
risk-utility test, the plaintiff usually must prove the existence of a
safer alternative design. In your case, it would be a battle of the
experts -- plaintiff's expert offering evidence of a safer alternative
design that is economically feasible, and the defendant offering
evidence that the alternative design was unworkable, more dangerous
and prohibitively expensive. Also, in cases like this where there may
be no better design, plaintiffs often claim that the manufacturer
should have warned of the unavoidably dangerous characterists of the
vehicle. In your case, the manufacturer should have indicated in the
owner's manual, or on the visor, dash, seat, ashtray, floormat, etc.,
etc. that "this car nails the corners, so you may go too fast through
corners and be temped to brake hard which may result in serious injury
or death. It is recommended that you leave this car parked in a
garage at all times." Something like that. That is why owner's
manuals are so thick.

Sadly, the outcome in these types of cases (assuming they go to trial)
often depends on whose expert is more charismatic. It is also an
example of how juries can end up designing products. -- Jay Beattie.
 
On Feb 15, 2:02 pm, [email protected] wrote:
>
> I'm not sure, but I think that the manufacturer offers whichever disk
> caliper mounting the customer wants, front or rear:
>
> Disc Brakes:
>
> Hope Mono Mini Front (160 disc size, IS or Post Mount) £100
> Hope Mono Mini Rear (160/140 disc size, IS or Post Mount) £100
>
> http://www.cotic.co.uk/product/roadrat.html


That refers to whether you want a disc brake on the
front or rear wheel (the alternative being V-brakes).
It doesn't suggest that they will weld up a different
position brake mount for you.

> The picture isn't very clear. It appears to be a rear-caliper front
> disk brake, with the angle showing the brake cable visible between the
> forks, going down behind the bicycle's left fork leg. (I'm not sure
> about this.)
>
> But several posters have indicated that they've seen front-calipers on
> this model.
>
> What do "IS" and "Post Mount" mean on that page? Front or rear
> calipers for front disk brakes? Or something else unrelated to our
> subject?


If you click on the little thumbnails over on the left,
there is a detail picture of the fork which shows that
the legs are much less tapered than a typical road fork
and the disc mount is ahead of the fork (i.e.
front-mounted on front wheel). This is what Jobst
was referring to. Note that the manufacturer warns
you not to use a >160mm disc on the fork.

Also from that page:

: Disc mount positioned on front right ; brake forces
: always go into the dropout and there's no interference
: with mudguards. Max rotor size : 160mm.

So clearly they're thinking about it. Hey, it's a
new market niche.

Ben
 
On Feb 15, 6:46 pm, Tim McNamara <[email protected]> wrote:
>
> Under ideal circumstances, jim's assessment that the retention provided
> by the QR should exceed the ejection force is probably reasonable. But
> non-ideal circumstances are all too readily encountered in real life.


Jim seems to think a retention force three times the ejection force
(IF everything is done perfectly) is obviously adequate.

In other words, he's satisfied with a design factor (or safety factor)
of three.

But in mechanical design, all common parts are always designed with a
safety factor. That is, parts are not designed to be precisely as
strong as necessary. If that were done, then any tiny error in
evaluation of force, material property, manufacturing technique, or
operating condition would cause failure.

For ordinary industrial design using well-known materials, well
understood loads, ordinary controlled conditions of operation, etc.
safety factors of three are quite common.

For situations where adjustments have to be made in the field by
operators using their own feel and judgement, where loads are
difficult to assess, and where failure can result in serious bodily
harm, a safety factor of three is entirely inadequate.

> Since the fork can be readily redesigned to eliminate the ejection
> force, there is no reason not to do it- and is already being done to a
> degree by the fork makers.


Exactly.

- Frank Krygowski
 
On 15 Feb 2007 20:04:07 -0800, [email protected] wrote:

>On Feb 15, 6:46 pm, Tim McNamara <[email protected]> wrote:
>>
>> Under ideal circumstances, jim's assessment that the retention provided
>> by the QR should exceed the ejection force is probably reasonable. But
>> non-ideal circumstances are all too readily encountered in real life.

>
>Jim seems to think a retention force three times the ejection force
>(IF everything is done perfectly) is obviously adequate.
>
>In other words, he's satisfied with a design factor (or safety factor)
>of three.
>
>But in mechanical design, all common parts are always designed with a
>safety factor. That is, parts are not designed to be precisely as
>strong as necessary. If that were done, then any tiny error in
>evaluation of force, material property, manufacturing technique, or
>operating condition would cause failure.
>
>For ordinary industrial design using well-known materials, well
>understood loads, ordinary controlled conditions of operation, etc.
>safety factors of three are quite common.
>
>For situations where adjustments have to be made in the field by
>operators using their own feel and judgement, where loads are
>difficult to assess, and where failure can result in serious bodily
>harm, a safety factor of three is entirely inadequate.
>
>> Since the fork can be readily redesigned to eliminate the ejection
>> force, there is no reason not to do it- and is already being done to a
>> degree by the fork makers.

>
>Exactly.
>
>- Frank Krygowski


Dear Frank,

What is an entirely adequate safety factor "for situations where
adjustments have to be made in the field by operators using their own
feel and judgement, where loads are difficult to assess, and where
failure can result in serious bodily harm"?

Possibly rock climbing equipment will give some insight?

Cheers,

Carl Fogel
 
B

Ben C

Guest
On 2007-02-15, jim beam <[email protected]> wrote:
[...]
> it's about flats. in positive scrub, a flat will drag you off to the
> side of the flat, possibly into oncoming traffic if you're not alert.
> negative, and if you think about it this makes sense, has the drag force
> in the steering acting counter to the drag force on that corner of the car.


There's also a relationship with whether you split the dual brake
circuit front/rear or diagonally.

The Mini, with centre-pivot steering, had a front/rear split. Most other
FWD cars I've seen had a diagonal-split. With negative scrub you can
brake in a fairly straight line with a front left and rear right brake,
which I would think would be more effective than rear brakes only on a
front/rear split system with a failed front.

So maybe that's part of the reason too.
 
B

Ben C

Guest
On 2007-02-15, [email protected] <[email protected]> wrote:
[...]
> The purpose of the scrub radius is to take actual and elastic slack
> out of steering linkage. Positive radius puts the linkage (ball
> joints) in tension. The dimension being in the range of +- an inch.
> It has nothing to do with handling except in FWD where it is generally
> negative (offset to the inside)


Isn't negative offset to the outside? I mean contact patch further to
the outside of the car than the projection of the steering pivot? We may
just be using different terminology.

> and comes into play when one wheel loses traction, causing steering to
> have a bias to that side. Positive radius does the same if braking on
> snow or ice for instance.
>
> If you look at the jacked-up pickup truck/SUV syndrome with wheels
> extended beyond the side of the vehicle, you see a guy who needs shock
> absorbers on this drag-links. The scrub radius of these vehicles is
> nearer to eight inches and puts a hell of a jolt on the steering wheel
> when one wheels hits a sharp bump. Hey, but it looks cool!


It is possible that the steering pivots are angled enough that the
centre of the tyre is at the projection of the pivot-- i.e. zero scrub.
So I think you can't say for sure what the scrub radius is just by
observing how far the wheels stick out.

But if these are "customized" vehicles, they may just be forcing the
huge wheels on with spacers and other bodges in which case they will be
increasing the radius from the manufacturer's intended setting.
 
Ben C? writes:

> [...]


>> The purpose of the scrub radius is to take actual and elastic slack
>> out of steering linkage. Positive radius puts the linkage (ball
>> joints) in tension. The dimension being in the range of +- an
>> inch. It has nothing to do with handling except in FWD where it is
>> generally negative (offset to the inside)


> Isn't negative offset to the outside? I mean contact patch further
> to the outside of the car than the projection of the steering pivot?
> We may just be using different terminology.


Positive is out side of the "king pin axis" and negative is inside.
Positive puts the steering linkage in tension (by tire rolling
resistance) and negative puts it in compression. Neutral is not
desired and neither do anything for flat tires.

>> and comes into play when one wheel loses traction, causing steering
>> to have a bias to that side. Positive radius does the same if
>> braking on snow or ice for instance.


>> If you look at the jacked-up pickup truck/SUV syndrome with wheels
>> extended beyond the side of the vehicle, you see a guy who needs
>> shock absorbers on this drag-links. The scrub radius of these
>> vehicles is nearer to eight inches and puts a hell of a jolt on the
>> steering wheel when one wheels hits a sharp bump. Hey, but it
>> looks cool!


> It is possible that the steering pivots are angled enough that the
> centre of the tyre is at the projection of the pivot-- i.e. zero
> scrub. So I think you can't say for sure what the scrub radius is
> just by observing how far the wheels stick out.


"It's possible" how about not spreading rumors.

> But if these are "customized" vehicles, they may just be forcing the
> huge wheels on with spacers and other bodges in which case they will
> be increasing the radius from the manufacturer's intended setting.


The people who customize them don't care. Having an 8 inch scrub
radius is stupid! As you can see, the ones who actually go off road
usually have horizontal shock absorbers on the drag-link.

Jobst Brandt
 
M

Mike Causer

Guest
On Thu, 15 Feb 2007 03:49:43 -0600, Ben C wrote:

> I used to drive a Mini which I believe had a zero scrub radius.


If you mean the Issigonis/Moulton Mini, no it definitely did not have zero
scrub radius. At the time the Mini was introduced I can only think of one
car that might have had zero, and that's the Citroen with fully powered,
no feed-back, steering.


> I read in a book at the time that negative scrub radius made steering
> slightly vague around the centre position, but made motorway/freeway
> driving easier as constant steering correction was not required. I
> thought that was the purpose of it.


IIRC -ve was introduced by Audi in the 1972. No effect on steering feel,
but it added stability when going through water with one wheel, where the
+ve cars would turn /into/ the water the -ve would turn away.


> The Mini was designed in 1959 (before motorways), had very precise
> steering (low-geared, light, and the steering wheel was connected
> directly to the rack with no UJs in between), but did require more
> correction when cruising on motorways than the average 1980s shopping
> car.


Designed 56-58, introduced 1959, and the first section of the M1 (the UK's
first "long distance" motorway was opened in 1959. A short stretch of
motorway, the Preston Bypass (now part of the M6) was opened in 1958. The
steering of a Mini is absolutely stable, provided it has an open
differential. For anyone brought up on the deliberately vague, and
probably powered, steering of modern front-wheel-drives an original Mini
is possibly too sensitive at first, which might make people think they're
unstable.


Mike
 
B

Ben C

Guest
On 2007-02-16, Mike Causer <[email protected]> wrote:
> On Thu, 15 Feb 2007 03:49:43 -0600, Ben C wrote:
>
>> I used to drive a Mini which I believe had a zero scrub radius.

>
> If you mean the Issigonis/Moulton Mini


Of course, I mean the Mini.

> , no it definitely did not have zero scrub radius.


http://www.minimania.com/web/DisplayID/1084/SCatagory/SUSPENSION/DisplayType/Calver's%20Corner/ArticleV.cfm

or http://tinyurl.com/yr7ka7

"King Pin Inclination (KPI) – the angle is described by a line drawn
down through the top and bottom ball-joint (swivel pin) centres and
vertical viewed from the front. Extended to ground level, the
distance from here to the wheel/tyre centre-line at ground level is
the ‘King Pin Offset’ [aka scrub radius]. Ideally the lines should
intersect at ground level. This will give both lightness of steering
‘feel’ and virtually no kick back through the steering wheel when
hitting bumps – known as ‘centre-point steering’."

Square brackets mine.

> At the time the Mini was introduced I can only think of one
> car that might have had zero, and that's the Citroen with fully powered,
> no feed-back, steering.


Ah, that's zero caster you're thinking of, not zero scrub. Some Alfas
were said to have that (the Sud or Sud Sprint?). You could apparently
set the steering to the desired angle, remove your hands from the wheel,
and keep going round in circles. I never drove one. Fancy boutique
Italian stuff.

Caster is called "trail" on bikes: it's the distance from the projection
of the steering pivot to the contact patch in the forwards axis (not the
sideways axis, which is scrub).

It's what makes the steering self-centre on cars.

[...]
>> The Mini was designed in 1959 (before motorways), had very precise
>> steering (low-geared, light, and the steering wheel was connected
>> directly to the rack with no UJs in between), but did require more
>> correction when cruising on motorways than the average 1980s shopping
>> car.

>
> Designed 56-58, introduced 1959, and the first section of the M1 (the UK's
> first "long distance" motorway was opened in 1959. A short stretch of
> motorway, the Preston Bypass (now part of the M6) was opened in 1958. The
> steering of a Mini is absolutely stable, provided it has an open
> differential.


And provided you haven't crashed it too many times. What's an "open
differential"? Do you mean one that's not limited slip?
 

dabac

Well-Known Member
Sep 16, 2003
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288
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Ben C said:
Caster is called "trail" on bikes: it's the distance from the projection
of the steering pivot to the contact patch in the forwards axis (not the
sideways axis, which is scrub).

Is it really? Wouldn't caster for a car correspond to head tube angle for a bike?
 
B

Ben C

Guest
On 2007-02-16, dabac <[email protected]> wrote:
>
> Ben C Wrote:
>>
>> Caster is called "trail" on bikes: it's the distance from the
>> projection
>> of the steering pivot to the contact patch in the forwards axis (not
>> the
>> sideways axis, which is scrub).

>
> Is it really? Wouldn't caster for a car correspond to head tube angle
> for a bike?


This is something I've never been 100% clear about.

People talk about trail on bikes, not caster. AIUI trail is the offset
from the projection of the head tube (which is the steering pivot) onto
the ground back to the contact point of the tyre.

Trail is a function of head tube angle, but also of fork rake. Increase
fork rake and you reduce trail for a given head tube angle.
 
On Feb 16, 12:41 am, [email protected] wrote:
> On 15 Feb 2007 20:04:07 -0800, [email protected] wrote:
>
>
>
> >On Feb 15, 6:46 pm, Tim McNamara <[email protected]> wrote:

>
> >> Under ideal circumstances, jim's assessment that the retention provided
> >> by the QR should exceed the ejection force is probably reasonable. But
> >> non-ideal circumstances are all too readily encountered in real life.

>
> >Jim seems to think a retention force three times the ejection force
> >(IF everything is done perfectly) is obviously adequate.

>
> >In other words, he's satisfied with a design factor (or safety factor)
> >of three.

>
> >But in mechanical design, all common parts are always designed with a
> >safety factor. That is, parts are not designed to be precisely as
> >strong as necessary. If that were done, then any tiny error in
> >evaluation of force, material property, manufacturing technique, or
> >operating condition would cause failure.

>
> >For ordinary industrial design using well-known materials, well
> >understood loads, ordinary controlled conditions of operation, etc.
> >safety factors of three are quite common.

>
> >For situations where adjustments have to be made in the field by
> >operators using their own feel and judgement, where loads are
> >difficult to assess, and where failure can result in serious bodily
> >harm, a safety factor of three is entirely inadequate.

>
> >> Since the fork can be readily redesigned to eliminate the ejection
> >> force, there is no reason not to do it- and is already being done to a
> >> degree by the fork makers.

>
> >Exactly.

>
> >- Frank Krygowski

>
> Dear Frank,
>
> What is an entirely adequate safety factor "for situations where
> adjustments have to be made in the field by operators using their own
> feel and judgement, where loads are difficult to assess, and where
> failure can result in serious bodily harm"?
>
> Possibly rock climbing equipment will give some insight?


Try http://www.usrigging.com/SafetySnapHooks.pdf for one example.

And note that with these, there's no fiddling with a screw adjustment
to achieve proper strength. There's no chance of a partial
engagement, by design. But there's still a safety factor of ten.

- Frank Krygowski