From: jbrandt@hpl.hp.com (Jobst Brandt)
Newsgroups: rec.bicycles.misc
Subject: Re: Front tire lifting while climbing
Date: 5 Feb 2001 21:28:56 GMT

John Henderson writes:

>> You can compensate for this failing a bit by spreading your torque
>> pulse more evenly through he pedal stroke but not a lot.  I chose
>> to have a frame built with 17.5" chainstays, the longest tubes
>> commonly available.  You could once get TREK touring frames with
>> these.

> 45cm chainstays can still be found, at least on some hybrids.  I've
> had two Giant hybrids, both of which had chainstays of this length.
> The lenght of stays, when combined with drop bars and bar-end
> shifters, required me to use tandem-length shift cables for the rear
> derailer.

So why should one have to buy a "hybrid," road bicycles should be such
that we can comfortably ride and race them on roads.  I've listened to
the pitch in bicycle stores.  "This bike is exceptionally close
coupled for quick and responsive cornering.  It's tight clearances,
the way the fork crown literally hugs the tire, make it especially
aerodynamic..." and so on.  Customers eat it with a spoon and love
every moment of it.  Of course most of them never ride these racers
for anything but a parade tour.  The rest are brow beaten into
believing in poor riding style if the bicycle seems to interfere with
their hill climbing or braking on descents.

Jobst Brandt      <jbrandt@hpl.hp.com>

From: jbrandt@hpl.hp.com (Jobst Brandt)
Newsgroups: rec.bicycles.misc
Subject: Re: Front tire lifting while climbing
Date: 7 Feb 2001 21:47:41 GMT

Jon Isaacs writes:

>> I see we need a free body diagram here.  It's a pulling oneself up
>> by one's bootstraps sort of thing.  All this pulling and shoving
>> has no effect whatsoever.  What causes wheel lift is forward
>> acceleration about the rear wheel contact patch.> If the rider is
>> sitting too far back, over the rear wheel, the bicycle must raise
>> the front wheel no matter where one places the hands on the bars or
>> how long a stem is used, if your ass is on the saddle.

> Of course this is a bit more complicated because first, the bikes
> acceleration is due to the forces applied to the pedals by the
> rider.

That may be, but telling the rider he must pedal carefully and in a
constrained way is putting the cart before the horse.  Bicycles should
be made to be ridden by humans, rather than a test of skill that
hampers the efforts of the rider, although that is what it appears to
be from the advice given in this thread.  Get a real bicycle.

> However I believe the determining the critical condition for the
> front wheel lifting is best served by summing the moments around the
> contact patch.

That's a complicated way of saying the rear wheel is too far forward,
under the rider.

> The moment which tends to flip the bike is supplied by the rider and
> can either be applied in an even manner or an uneven manner with
> sharp peaks which can momentarily lift the front wheel.

Pedaling is by its nature uneven but even at that, it is not greatly
pulsating, considering that most (racing) bicycles cannot be ridden
sitting ON the saddle on a 20% grade.  That's how close the whole idea
is to tipping over.  20% is about 11.3 degrees, which moves a saddle,
typically 42" above the ground, 8.24" rearwards with respect to the
vertical.  That puts it behind the rear tire contact for most bicycles.

> The moment which resists this is supplied by the bike and the rider
> and is a function of the position of the center of mass of the
> system taken in the horizontal direction.  Thus a steeper hill will
> result in a shorter moment arm thus reducing the resisting force.
> And of course the steeper the hill, the greater the force required
> to over come gravity.

The main effect, however, is that it takes a substantial push on the
downward pedal to move forward and this is the tipping moment.  Done
in a similar gear on a lever road, such forceful pedaling will also
raise the front wheel.

> I suggest that there are several ways to increase the moment that
> resists the overturning or lifting front wheel of the bicycle.  One
> is to certainly use longer chain stays.  Another is for the rider to
> be positioned or position him/herself further forward from the
> contact patch.  This can be done while seated by leaning forward and
> thus moving the weight of ones upper body forward.  And of course an
> longer stem assists moving ones body forward.

If sitting on the saddle, the 8" of a 20% grade cannot be countered
and such moves reduce pedal force needed to climb.  Therefore, if you
plan on riding mountain roads much, get a better bicycle.  I did not
put up with all the "good" advice on how to keep the front wheel down
on a short bicycle when I first came upon the problem years ago.  I
had a bicycle built with longer chainstays.

> In the case of the limiting hill, all these factors are relevant,
> longer chainstays, smooth application of pedal forces, and
> positioning of the body as far forward as is optimal.

Fortunately, climbing for significant length on more than 10% grades
is best done standing so the problem resolves itself for such hills.

Jobst Brandt      <jbrandt@hpl.hp.com>

From: jbrandt@hpl.hp.com (Jobst Brandt)
Newsgroups: rec.bicycles.misc
Subject: Re: Front tire lifting while climbing
Date: 8 Feb 2001 22:22:21 GMT

Jon Isaacs writes:

>> Exactly what difference could a centimeter or two make in the
>> handling of a bicycle?  A slightly smaller turning radius?  What
>> crit courses have you been on that gave an advantage to a bike that
>> can turn a tighter radius?

Well then we could shorten the chainstay a centimeter, and after that
another and another since none of them make a perceptible difference.
Finally when the tire touches the seat tube we could put a dent in the
tube (as has been done) and shorten some more, at which point the tire
hits the bottom bracket.  Even these have been dented in that pursuit.
And to what advantage?

> Not riding crits I could not really comment on this.  However
> shorter bikes are quicker handling.

Wait a minute, no one said "Simon says".  Let's go back to that
statement and show some reason why "shorter bikes are quicker
handling."  Not only does this fail the "one centimeter makes no
perceptible difference" test, but fails riding a tandem downhill on a
winding road test.  The only reason we don't see tandems regularly
beating others downhill, is that most are ridden by people who are not
willing to go faster especially with a person on the back who has no
control over the action.  However, their stability in curves and on
rough pavement is strikingly better than a single, especially a short
one.

> And as you point out, what difference could a centimeter or two
> really make.

Individually it doesn't but taken to the limit it does and that IS
perceptible over a reasonably dimensioned frame.  Those who are taller
than average are more aware of this than others.  The saddle rising at
a 73 degree angle soon resides over the rear wheel making road shock a
1:1 endeavor and the in-plane tilt of the frame a greater than 1:1
effect, causing the saddle to move fore and aft more than the height of
a bump in the road.

> What I can't figure out is what is the big deal here.

Yes?  Why do riders like short bicycles that are demonstrably less
manageable and comfortable than longer frames?

> If the front wheel wants to come up when you are climbing, just put
> a bit more weight up there, pedal a bit more smoothly and all is
> well.  There is no need to make a major issue of this, it is just
> simple technique.

Oh?  You mean riders should develop techniques to overcome the
shortfall of their bicycles that are too short, rather than get
bicycles that are reasonably dimensioned.  Compensating for
inappropriate design by riding technique sounds odd to me.  As I said,
this approach did not appeal to me from the start and I had my frames
built with longer stays with perceptible improvement.

What is it about this that you don't understand?

Jobst Brandt      <jbrandt@hpl.hp.com>