if you wanted maximum braking, where would you sit?

On Sun, 11 Jan 2004 09:54:37 GMT, meb
wrote:
Omitted assumtion: the tires are at or near the limits of adhesion.

Hence, the equal load on the front and rear tires, since optimal
traction occurs when both tires are equally loaded.

Okay, but optimal _braking_ traction would have to take into account
how the load distribution changes under hard braking -- the front
tire ends up with [nearly] the complete load on an average upright
bike with acceptably good brakes, for example.
--
Rick Onanian

On Mon, 12 Jan 2004 19:35:04 GMT, Simon Brooke
wrote:
bike. Note that if you are in mid air at the time this may not be
relevent...

Since when was relevance a relevant concern here?
--
Rick Onanian

Rick Onanian wrote:

... In
http://www.ihpva.org/com/PracticalInnovations/weld.html
under the section titled "Trike steering geometry", sub-title
"Ackerman steering compensation", steering is treated as if there
was a live axle with no differential for the front wheels. I think
such trikes are nearly all rear wheel drive, leaving no need for a
live axle; why would there be front wheel scrubbing?

Does your trike use Ackerman steering compensation?

I am aware of only one tadpole [1] trike with front wheel drive (a low
production model made in Russia), so I would estimate that almost all
are rear wheel drive.

If a tadpole trike did not have Ackerman steering, there would be tire
scrub even though the wheels are free to rotate at different speeds.
Since the wheels are traversing arcs of different radii in a turn, the
inner wheel must be turned more sharply than the outer wheel for both
tires to have a zero slip angle [2].

The Ackerman compensation on my trike is visually obvious at large
steering angles.

[1] Two front wheels, one rear wheel
[2] Slip angle is the difference between the direction in which the tire
points and in which it travels.

Tom Sherman - Quad Cities

meb wrote:

Obviously, the designer of your lowracer was not focussing on best
braking as the ultimate design objective to sacrifice all other
attributes in hopes of achieving the optimal braking bike.

Agreed. Moving the seat back farther would result in crank/front wheel
overlap (NOT a good thing) unless the crank bearing was about 18"
(46-cm) above seat level, which would be very bad ergonomically. (I had
a bike with the crank bearing about 13" (33-cm) above seat level, and I
would not want a difference greater than that.)

Tom Sherman - Quad Cities

That's just because you've defined effective as "easily able to stop as
quickly as possible" rather than "easily able to brake so hard I can kill
myself." What on earth are you thinking?

I believe it's because he might not exhibit the kind of control that a
trials rider such as myself might have. Anyone can stop more quickly on
V-brakes with flat bars than a road bike with drop bars, with 10 minutes of
practice and instruction on body positioning.

--
Phil, Squid-in-Training

Dry tarmac coated with dust or gravel? Otherwise I find it hard to
believe. A panic snatch on my bikes will result in me going over the
bars.

Try it sometime on a MTB with good V-brakes or disc brakes. Pull the brake
at the same time that you throw yourself back, extending your arms fully and
putting your butt just over the rear wheel. It's doable.

--
Phil, Squid-in-Training

Rick Onanian wrote:
On Sun, 11 Jan 2004 09:54:37 GMT, meb usenet-
wrote:
Omitted assumtion: the tires are at or near the limits of adhesion.

Hence, the equal load on the front and rear tires, since optimal
traction occurs when both tires are equally loaded.
Okay, but optimal _braking_ traction would have to take into account how
the load distribution changes under hard braking -- the front tire ends
up with [nearly] the complete load on an average upright bike with
acceptably good brakes, for example.
--
Rick Onanian


Exactly. The posting you quoted was my followup response to my
immediately preceding posting. The immediate preceding posting showed
that derivation of the optimal seat position (seat behind front contact
patch as a dependent variable, seat hieght as an independent variable)
with the weight transfer effect included for differing decel rates. I
had omited the relevance to the equal weight distribution from the same
post since I had earlier posted the fact tjhat the optmized seat
position will distribute the load equally over both tires under decel
not static speed conditions. I followed up the posting a few minutes
later since it was an earlier post a few days earlier, I thought it
better to reiiterate for clarity reason for equalizing the weight
distribution on each tire under decel since the earlier posting was so
disjoined in time and lineage to the posting of several days earlier.



--

Tom Schneider wrote:
On Sun, 11 Jan 2004 22:55:58 GMT, meb
wrote: clipped
In fact, the CG location can be determined by the following formula:

Df= h*CF+WB/2

Where, Df is the horizontal distance of the CG from the front contact
patch, h is the CG height above the ground, CF is the coefficient of
friction between the tires and the road, and WB is the distance between
the front and rear contact patches.

Having said all that, it may be true that the drop of the CF for a
bicycle with unequally loaded tires is so small that it is not worth
considering. In that case, the weight must only be shifted back to a
distance of:

Df=h*CF

to achieve maximum braking. “

I made bike and rider cg independent variables.

Since the OP was interested in the maximum braking position, I
calculated optimal seat position premised upon the even weight
distribution under decel, independent of whether deviations from that
position might be small or large. The non-linear coefficient of
friction not trivial with automobile, although I’m not sure whether the
round top profile of the bike tire reduces or enhances this effect
relative the flat top profile of a car tire or if the lighter weight of
the bike tire would place the function of coefficient of friction as a
function of weight in a flatter curve.


Obviously, positioning a seat for maximum braking means ignoring ideal
handling positioning, so this is a braking purpose only solution
equation, and except for the low cg recumbents, is so poorly weight
distributed outside of decel as to be a very poor handling bike. A road
bike optimized for a coefficient of friction of 1, will be doomed to
wheelies when seated and not braking.



Changing the decel rate to 1G, gives the following seat positions for
the previously posted bikes:

Road bike 77.11 inches behind patch Second lowracer (9” seat height)
35.38 behind patch (that puts us in the rear tire). LWB recumbent 53.28
“behind patch (that also puts us in the rear tire). MWB recumbent
56.14” behind patch (above the rear, 2” back of axle, so no
wheelies.) Carbon Prone 61.11” behind patch.

CG is approximately at the navel I've read, so the lowracer with 9 inch
would have a CG at 16 to 18 inches high or 16-18 inches behind the
contact patch for 1 G braking. Practically, the front wheel can do 100%
of the braking without changing the CF much, using automobiles (front or
rear engine) as an example.
Tom Schneider


Ton Schneider wrote

“CG is approximately at the navel I've read, so the lowracer with 9 inc
would have a CG at 16 to 18 inches high

My independent variable RhcgvsS reflects that vertical difference in th
CG. My mention of the subsequently added offset: “Rider center o
gravity for back/forward leaning added later since it is a simpl
negative offset insertable with the solution above.” Reflecting on som
bikes the rider is leaning forward or backwards

I plugged into the equation a rider cg height of 5” vertical above th
seat and 4” horizontal behind the seat on both the 12” high and 9
high examples

That is a seat position 39 degrees back from vertical

Tom further wrote: “or 16-18 inches behind the contact patch for 1
braking. Practically, the front wheel can do 100% of the braking withou
changing the CF much, using automobiles

The object of the OP was to find the max (or peak) braking performanc
seat positon rather than a braking performance plateau edge

Automobiles are a bad example for a pro plateau argument

Rear engined cars outbrake midengined cars which outbreak front engine
cars. Ever notice how well the rear engined Porsches and Volkswagon
braked relative to similar sized and equipped cars of the same era. O
compare the low tech ‘87/88 midengined Fiero with no ABS to a high tec
front engined ‘87/88 Corvette with ABS.

Handling optimization, is also related to that same nonlinearity on
coefficient of friction as a function of weight on tires as braking,
just left right instead. A front engined car generally handles poorer
than a rear car which in turn is poorer handling than a mid engine car
because the front/rear weight distribution is poorer on the front
engined car.

Many automobile oval racing bodies limit maximum static weight
distribution on the left side tires.

There is even an interesting story of one driver’s attempt to circumvent
this limitation. At a local NASCAR event in Bakersfield's Mesa Merin
Speedway, one ingenious driver developed a system with left-right frame
rails slightly higher on the left than the right. The tubes were filled
with mercury. When on the high 36 degree banks on a slow pace lap, the
driver opened the valve, the mercury flowed to the left, the valves were
closed locking in the weight on the left side of the car. After the
race, the valve was opened, and on level ground the mercury flowed back
to the right to comply with the weight distribution load requirement.

One race they spotted water on the track. The safety crew got out to
sweep away the water and found for some strange reason the water
wouldn’t sweep away. On further inspection, it wasn’t water but mercury.
Now you know why NASCAR banned closed framerails after the multimillion
dollar cleanup.



--

wrote:

That's just because you've defined effective as "easily able to stop as
quickly as possible" rather than "easily able to brake so hard I can
kill myself." What on earth are you thinking?

I believe it's because he might not exhibit the kind of control that a
trials rider such as myself might have. Anyone can stop more quickly on
V-brakes with flat bars than a road bike with drop bars, with 10 minutes
of practice and instruction on body positioning.

As has been continually pointed out, those of us on road bikes with drop
bars can easily brake hard enough to raise the rear wheel. You can't stop
more quickly than that -- at least not by braking harder.

--
Benjamin Lewis

Now is the time for all good men to come to.
-- Walt Kelly

RE/
As has been continually pointed out, those of us on road bikes with drop
bars can easily brake hard enough to raise the rear wheel. You can't stop
more quickly than that -- at least not by braking harder.

Even with somebody's butt hanging back over the rear axle?
--
PeteCresswell