Scotland Cairngorms Loop 181 miles : bike ?

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mnichols
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Re: Scotland Cairngorms Loop 181 miles : bike ?

Postby mnichols » 21 Feb 2017, 6:23pm

NATURAL ANKLING wrote:It rolls down hill faster because of weight, but this does not balance the up hill struggle?


Is this a myth? I thought gravity was constant regardless of weight. I don't remember much from Physics at school, but they did teach that if a feather and a bowling ball fell in a vacuum they would both fall at the same rate

rualexander
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Re: Scotland Cairngorms Loop 181 miles : bike ?

Postby rualexander » 21 Feb 2017, 10:12pm

mnichols wrote:
NATURAL ANKLING wrote:It rolls down hill faster because of weight, but this does not balance the up hill struggle?


Is this a myth? I thought gravity was constant regardless of weight. I don't remember much from Physics at school, but they did teach that if a feather and a bowling ball fell in a vacuum they would both fall at the same rate

We don't cycle in a vacuum though.

mnichols
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Re: Scotland Cairngorms Loop 181 miles : bike ?

Postby mnichols » 22 Feb 2017, 7:59am

rualexander wrote:We don't cycle in a vacuum though.


The point of the vacuum bit is to do with wind resistance, so if that's the same and weight doesn't matter then the should descend at the same speed. Which would mean that the reason bikes descend at different speeds would be rolling resistance, aero dynamics, and mechanics (hubs, etc) but not weight..... Just my take, I'm no expert

Am I right or wrong?
Last edited by mnichols on 22 Feb 2017, 1:48pm, edited 1 time in total.

rualexander
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Re: Scotland Cairngorms Loop 181 miles : bike ?

Postby rualexander » 22 Feb 2017, 8:04am

mnichols wrote:
rualexander wrote:We don't cycle in a vacuum though.


The point of the vacuum bit is to do with wind resistance, so if that's the same and weight doesn't matter then the should descend at the same machine. Which would mean that the reason bikes descend at different speeds would be rolling resistance, aero dynamics, and mechanics (hubs, etc) but not weight..... Just my take, I'm no expert

Am I right or wrong?

But heavy people and light people rarely have the same aerodynamic profile, and they would also exert different forces through the tyres affecting rolling resistance.

mnichols
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Re: Scotland Cairngorms Loop 181 miles : bike ?

Postby mnichols » 22 Feb 2017, 9:06am

rualexander wrote:But heavy people and light people rarely have the same aerodynamic profile, and they would also exert different forces through the tyres affecting rolling resistance.


Without wishing to generalise, heavier people are more likely to have a bigger profile and therefore be less aerodynamic. A smaller person would have a smaller profile, be more aerodynamic and therefore descend quicker

rualexander
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Re: Scotland Cairngorms Loop 181 miles : bike ?

Postby rualexander » 22 Feb 2017, 9:44am

mnichols wrote:
rualexander wrote:But heavy people and light people rarely have the same aerodynamic profile, and they would also exert different forces through the tyres affecting rolling resistance.


Without wishing to generalise, heavier people are more likely to have a bigger profile and therefore be less aerodynamic. A smaller person would have a smaller profile, be more aerodynamic and therefore descend quicker


Heavier people have more inertia than light people and therefore are less affected by wind resistance so would descend quicker.
It's not a simple situation once you bring in real world things like air, wind, ground surface, etc.

Mike777
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Re: Scotland Cairngorms Loop 181 miles : bike ?

Postby Mike777 » 22 Feb 2017, 1:47pm

Hi all

Many thanks for all the informative replies.

Really appreciate the help.

Regards

Mike

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andrew_s
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Re: Scotland Cairngorms Loop 181 miles : bike ?

Postby andrew_s » 22 Feb 2017, 3:23pm

mnichols wrote:Which would mean that the reason bikes descend at different speeds would be rolling resistance, aero dynamics, and mechanics (hubs, etc) but not weight
.
If you take an item, and double it in linear size, keeping the shape and everything else identical, the weight increases by a factor of 8 (2x2x2), but the frontal area, and hence aerodynamic drag force, only increases by a factor of 4 (2x2).

The terminal speed is where the accelerating force due to weight is balanced by the retarding force of aerodynamic drag. At the terminal speed of the small version, the accelerating force on the large object would be double the retarding force, so its speed increases until aerodynamic drag has increased enough to balance the weight again.

(That's for a falling object - a bike is more complicated due to rolling resistance etc, but the same principle applies)

mnichols
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Re: Scotland Cairngorms Loop 181 miles : bike ?

Postby mnichols » 22 Feb 2017, 5:25pm

andrew_s wrote:If you take an item, and double it in linear size, keeping the shape and everything else identical, the weight increases by a factor of 8 (2x2x2), but the frontal area, and hence aerodynamic drag force, only increases by a factor of 4 (2x2).

The terminal speed is where the accelerating force due to weight is balanced by the retarding force of aerodynamic drag. At the terminal speed of the small version, the accelerating force on the large object would be double the retarding force, so its speed increases until aerodynamic drag has increased enough to balance the weight again.

(That's for a falling object - a bike is more complicated due to rolling resistance etc, but the same principle applies)


but in this case isn't the accelerating force Gravity, which is constant for both objects if the shape is constant.

https://en.wikipedia.org/wiki/Galileo's_Leaning_Tower_of_Pisa_experiment

My point was that weight isn't the determining factor in descending, it's frontal area/aerodynamic drag as well as more complicated factors such as rolling resistance etc.

hamish
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Re: Scotland Cairngorms Loop 181 miles : bike ?

Postby hamish » 22 Feb 2017, 8:17pm

Well the thread wasn't really about fat bikes and speed. But a fat bike climbs well on bumpy stuff as it rolls over bumps and grips well. They aren't that heavy anyway. For bumpy tracks and beyond they are great.

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andrew_s
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Re: Scotland Cairngorms Loop 181 miles : bike ?

Postby andrew_s » 23 Feb 2017, 2:09pm

Fatbikes have their use, which is being taken for an off road play on the back of a car.
If you had to ride to the start from Inverness (say), you would probably pick a different bike.

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andrew_s
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Re: Scotland Cairngorms Loop 181 miles : bike ?

Postby andrew_s » 23 Feb 2017, 2:26pm

mnichols wrote:but in this case isn't the accelerating force Gravity, which is constant for both objects if the shape is constant.
https://en.wikipedia.org/wiki/Galileo's_Leaning_Tower_of_Pisa_experiment
My point was that weight isn't the determining factor in descending.

Gravity is constant, but the force it exerts is proportional to the mass of the object.

In the presence of air, weight does affect speed.
Had Galileo dropped a ping pong ball and a lead ball the same size off the leaning tower of Pisa, he would have found that the lead ball landed first (or alternatively, the Dutch chap in Delft). They are the same size and shape, so the aerodynamic drag is the same.
If you want to try it, table tennis balls are 40mm, and you can get a 40 mm steel ball from here for 46p (plus post etc). I expect a bridge would be easier than a trip to Pisa - just make sure nobody is standing below when you let go.

hamish
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Re: Scotland Cairngorms Loop 181 miles : bike ?

Postby hamish » 23 Feb 2017, 9:12pm

Fatbikes have their use, which is being taken for an off road play on the back of a car.
If you had to ride to the start from Inverness (say), you would probably pick a different bike.


I don't wish to sound like a fat bike evangelist - but I am not sure I agree.

I use mine for touring off road. We have ridden thousands of miles on them over many weeks of touring. When we rode the Cairngorms Loop we arrived by sleeper which was perfect. But they are more capable than "an off road play from the back of a car". They have been ridden from Alaska to Tierra deal Fuego and with good tyres pumped up hard are fine on tarmac. Not ideal I grant you, but on a ride where tracks and moors predominate, a fat bike is a fine choice even if you have substantial Tarmac sections. I would of course use my touring hike for a tour with no off road or where the off road sections are short. The trouble is that off road touring gets addictive because its it is so much fun, gets you away from traffic and allows you to get to remote places.