bike sliding

Quoted post said:

Originally posted by Seekeroftruthan
while standing at the side of bike when you put on the rear brake you can slide the bike forwards
but not rearward similarly when you put on the front brake you can slide the bike rearward but not
forwards,whys that then

When sliding forward you are applying a force to the handlebars. The handlebars are positioned nearly above the front tire. That creates a moment about the front contact patch having a magnitude equal to the forward force applied to the handlebar times the height of the handlebar above the ground. Virtually all of this force is applied to creating the moment till the bike is free to slide. This causes weight transfer from rear to front equal to the moment divided by the wheelbase. With weight transferred from the rear, the is less traction since the traction forces are equal to the coefficient of friction times the downforce on the tire. The weight taken off the rear tire makes it easy to slide.

When going the opposite direction with the opposite brake, the handlebars are not above the contact patch of the rear brake. The portion of the applied force is smaller as only a portion of the force is used in creating the moment about the rear tire patch, with a bigger portion of the force applied to the rearward pushing of the bike. The force applied on the moment is equal to the force applied times the height of the handlebar at which the force is applied times squaroot of (the height of the handle bar squared / (the height of the handlebar squared plus the horizontal distance of the rear patch to the handlbar)). A smaller moment and hence a smaller weight transfer. You need more force applied to the handlebars since a much smaller portion of the force is applied to creating a moment to transfer weight from the front making it easy to slide.

Actual results will vary greatly with differing bikes.

"seekeroftruthandjustice" <[email hidden]> wrote in
message "]news:[email hidden]...

Quoted message said:

while standing at the side of bike when you put on the rear brake you can slide the bike forwards
but not rearward similarly when you put on the front brake you can slide the bike rearward but not
forwards,whys that then

Weight transfer. Also, direction and position of force has an effect as well.

Cheers,

Scott..

anonymous writes sheepishly:

Quoted message said:

While standing at the side of bike when you put on the rear brake you can slide the bike forwards
but not rearward similarly when you put on the front brake you can slide the bike rearward but not
forwards,whys that then.

Try pushing a gadren rake and pulling it. This is not hich-tech!

Jobst Brandt [email hidden]

[email hidden] (seekeroftruthandjustice) wrote in message news:<[email hidden]>...

Quoted message said:

while standing at the side of bike when you put on the rear brake you can slide the bike forwards
but not rearward similarly when you put on the front brake you can slide the bike rearward but not
forwards,whys that then

Dear SOTAJ,

Think of an early airplane's tail-skid.

Now think of a weather-vane.

Like a weather-vane, a trailing skid centers itself.

You can actually push a bicycle with its leading wheel skidding (forward or backward), but you'll
spend most of your time and effort trying to stop the skidding leading wheel from pivoting off to
the side or the other like a weather vane to drag behind.

Your question is not entirely academic. Wretchedly skillful motorcycle trials riders sometimes show
off by locking their front wheels on wet grass and riding for ridiculous distances without falling
down. (When they do fall, it serves them right.)

Carl Fogel

As I suspected this is far more complex than we realise.Nobody has mentioned the tire which I
believe plays the biggest part in this phenomenon.For instance If you do the same thing again but
this time without a tire you can slide the rim in both directions.Transfer of weight only seems to
be half the answer !

seekeroftruthandjustice said:

As I suspected this is far more complex than we realise.

No, it is not. A quick free-body diagram will tell you almost everything you need to know.

Quoted message said:

Nobody has mentioned the tire which I believe plays the biggest part in this phenomenon.

Nobody has mentioned it because it is assumed to be on the wheel. No need to voice that
assumption, IMO.

Quoted message said:

For instance If you do the same thing again but this time without a tire you can slide the rim in
both directions.

Why would you do it without a tire? That changes the coefficient of friction drastically. What
implication does that have on "normal" bicycle use?

Dave dvt at psu dot edu

Quoted post said:

Originally posted by Seekeroftruthan
As I suspected this is far more complex than we realise.Nobody has mentioned the tire which I
believe plays the biggest part in this phenomenon.For instance If you do the same thing again but
this time without a tire you can slide the rim in both directions.Transfer of weight only seems to
be half the answer !

The answer is still the same, only the threshold forces for siding the bike are reduced.
With the lower coefficient of frictions while on the rims, the bike slides at lower thresholds. It still takes less force to slide the bike on its rims forward than backward.