Home Forum Bike Forum Who will get to the bottom first?

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• Who will get to the bottom first?
• sputnik
Member

Boozecruiser, that pie was amazing.
Couple more and I’m heading for a PR down Pebble Hill !

sugdenr
Member

Which begs the questions – what exactly is gravity i.e. what causes it?

And, which pies are best for going downhill faster?

missnotax
Member

Big pies 🙂

Which begs the questions – what exactly is gravity i.e. what causes it?

It’s actualy very weak, but beyond that I can’t remember enough A-level physics to remember the mechanics of it.

sugdenr
Member

It’s actualy very weak

a little less weak on the big pie eaters though 😉

burnie
Member

Which begs the questions – what exactly is gravity i.e. what causes it?

Gravity is an attractive force which acts between objects with mass.
Its strength can be calculated using F=(GMm)/{4pi(r squared)}. But what causes it? AFAIK nobody knows, however with the recent discovery of the Higgs boson scientist are getting close.

julianwilson
Member

Slight tangent, but a few months ago someone on here posted a chart of various popular roadie tyres rolling resistance (actually it was watts required to maintain a particular speed given same bike rider and tyre pressures) and the differences were quite remarkable including between some quite high end tyres. See also the difference between cartridge bearings and lightly greased and properly set up shimano hubs 😉 .

sugdenr
Member

OK so……..if the pie eater is the fastest down, does that mean he is slower on the up?? 😕

psling
Subscriber

Gravity is an attractive force which acts between objects with mass.

So, a bit like speed dating? The guy/gal that eats all the pies should get the win then…

pdw
Member

Lot’s of wrong on this thread.

Gravity is a force, and is very much dependent on mass – as per burnie’s equation. Of course, the acceleration that results from a force is also dependent on mass (a = F/m) , so in the absence of other forces, the resulting acceleration will be the same.

Of course there is another force – air resistance, and for “similar build” let’s assume they’re the same.

a = F / m

F = F(g) – F(a)

where F(g) = force of gravity = g * m
and F(a) = force due to air resistance which is independent of mass (for “similar build”)

This gives:

a = (g * m – F(a)) / m = g – F(a)/m

A higher mass reduces the impact of the air resistance, and so will accelerate faster.

EDIT – of course it’s not really g * m except on a vertical slope, it’s some fraction of it, but it makes no difference to the conclusion.

porlus
Member

Im 5’8″ and weigh less than 11 stone. My mate is around 6 foot and probaby closer to 14 stone than 13.
Both on road bikes. I have always pulled away from him on a freewheeling descent.
Actually we were just talking about this lastnight. Think his theory was he’s a little nervous and I tend to go with it more.

landcruiser
Member

OK so……..if the pie eater is the fastest down, does that mean he is slower on the up?

YES ! 8)

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