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

In article ,
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.

On Feb 14, 7:33 pm, jim beam 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, 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 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, wrote:

On Feb 15, 6:46 pm, Tim McNamara 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

On 2007-02-15, jim beam 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.

On 2007-02-15, 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

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

On 2007-02-16, Mike Causer 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?