50/34 x 12/??

aracer

Try dropping a feather and a ball bearing and see which lands first.

Also try reading my second post.

I think you'll find the 10 ton truck running over both the fat and the thin rider (though the fat rider will last a little bit longer) as its aero drag is nowhere near the 150 times as much the drag on the cyclist which it would need to be to counteract the additional gravitational force acting on it.

The mistake you're making is that there is no law saying that the acceleration due to gravity is a constant - that's just what you get by combining F = mg and F = ma. That's fine provided there are no other forces involved, however as I keep repeating, there is - wind resistance, and this is very definitely not proportional to mass.

oldwelshman

simbil1 wrote:

aracer wrote:

You're wrong, oldwelshman. That theory only applies in a vacuum. In the real world we have air resistance, which doesn't tend to increase that much for fatter people (at least not until the gut gets so big you can no longer hold a tuck). Since the pull of gravity has increased more than the drag holding them back they will go faster.

I could be wrong, but that really doesn't sound right.

The acceleration due to gravity is constant, whether I'm fat or thin or a 10 ton truck. That means my velocity will increase at a constant rate irrespective of my mass.
If you add in wind resistance, a fat or thin rider will be roughly the same but the 10 ton truck will get left behind as it has more drag.

The acceleration provided by a slope is g.sin(x); irrespective of mass.
I agree and go back to my original point Acceleration due to gravity is a constant on earth 9.82m/s/s and although air resistance is an issue with a feather, it will not be so extreme between riders, if anything would be worse for a fatty

Wasn't this what Galileo showed when he dropped different masses off the leaning tower of Pisa and they landed at the same time?

simbil1

aracer wrote:

Try dropping a feather and a ball bearing and see which lands first.

Also try reading my second post.

I think you'll find the 10 ton truck running over both the fat and the thin rider (though the fat rider will last a little bit longer) as its aero drag is nowhere near the 150 times as much the drag on the cyclist which it would need to be to counteract the additional gravitational force acting on it.

The mistake you're making is that there is no law saying that the acceleration due to gravity is a constant - that's just what you get by combining F = mg and F = ma. That's fine provided there are no other forces involved, however as I keep repeating, there is - wind resistance, and this is very definitely not proportional to mass.

I've read all your posts, but I still disagree

The feather vs a ball bearing is a bit of a red herring as we are talking about 2 cyclists that have the same wind resistance.
Are you saying that if we tossed the 2 cyclists (with the same wind resistance but different masses) off the leaning tower of Pisa, that the heavy one would hit the ground first - did Galileo get that wrong?

See question 1a and b here and the answers:

http://www.physics247.com/physics-homew ... /slope.php

As wind resistance does not depend on mass for our 2 same sized cyclists, I can't understand what you are getting at.

redddraggon

simbil1

Read this:

http://www.sportsci.org/jour/9804/dps.html

I shared your scepticism, but being a good scientist I did some background research to see if he was right, and it appears he was.........damn mechanics....

dennisn

redddraggon wrote:

simbil1

Read this:

http://www.sportsci.org/jour/9804/dps.html

I shared your scepticism, but being a good scientist I did some background research to see if he was right, and it appears he was.........damn mechanics....

I knew I stayed fat for a reason. Fat is faster.

Dennis Noward

simbil1

Thanks RedDragon.

After reading that, the way I would explain it is that although acceleration due to gravity is constant and resistance due to wind the same for both riders, that force has more of an effect on the lighter cyclist and so he is slowed more.

Makes sense now

oldwelshman

This also prooves my point that you do not really need a 11 as you will not go faster than terminal velocity

doyler78

Now I know why I dropped physics after 3rd year but glad you sorted that one out

I take it that carrying all that fat up the hill in the first place is going to take a lot more time than you will ever make up on the descent? Would it worth filling a rucksack with bricks and picking it up at the top of a mountain climb ready for the descent or would that cause me problems with my balance? Though my descents are usually measured in seconds so maybe not for me after all

feel

simbil1 wrote:

Thanks RedDragon.

After reading that, the way I would explain it is that although acceleration due to gravity is constant and resistance due to wind the same for both riders, that force has more of an effect on the lighter cyclist and so he is slowed more.

Makes sense now

Terminal velocity is reached when the forces balance. (downward force and the air resistance) A heavier rider will have more force [F=Mg] so will accelerate for longer so reaching a higher terminal velocity. air resistance is proportional to speed so the resistive force increases as you speed up eventually balancing the down force.

musto_skiff

So a 27 is OK then :?

aracer

simbil1 wrote:

Are you saying that if we tossed the 2 cyclists (with the same wind resistance but different masses) off the leaning tower of Pisa, that the heavy one would hit the ground first

Yes. Galileo's experiment largely avoided wind resistance issues by having objects with orders of magnitude less drag area relative to their mass than a cyclist - hence to a first approximation they did accelerate at a rate of g. The cyclists are actually more like feathers than ball bearings.

simbil1 wrote:

although acceleration due to gravity is constant

Although you seem to have got it now, I thought I should just point out that as I said before, this is the fallacy which has resulted in your misunderstanding. The acceleration due to gravity isn't constant. That just comes about from combining the two equations F=ma and F=mg, which are equations for two completely different things. g is a gravitational force constant, not an acceleration. As soon as any other force starts to act (eg wind resistance, bearing drag, rolling resistance) then you can no longer simply say that the F in F=ma is the F in F=mg, hence a doesn't equal g, and acceleration isn't constant (otherwise why would you reach a terminal velocity where you're not accelerating?)

feel

aracer wrote:

[ The acceleration due to gravity isn't constant. g is a gravitational force constant, not an acceleration.

I know what you are saying and i largely agree, but my pedantic nature gets in the way
Acceleration due to gravity (equivalent to gravitational field strength) is usually given the letter g and at the Earth's surface is pretty much a constant, hovering around 9.8 ms-2 (N/Kg) depending on where you actually are on the Earth's surface. But yes the acceleration of a falling object, in an atmosphere, caused by gravity will not be constant. It will be constantly diminishing until it reaches zero and terminal velocity is reached.

simbil1

aracer wrote:

Although you seem to have got it now, I thought I should just point out that as I said before, this is the fallacy which has resulted in your misunderstanding. The acceleration due to gravity isn't constant. That just comes about from combining the two equations F=ma and F=mg, which are equations for two completely different things. g is a gravitational force constant, not an acceleration. As soon as any other force starts to act (eg wind resistance, bearing drag, rolling resistance) then you can no longer simply say that the F in F=ma is the F in F=mg, hence a doesn't equal g, and acceleration isn't constant (otherwise why would you reach a terminal velocity where you're not accelerating?)

Yes you're right, I was thinking of 2 objects at rest as they are dropped - the initial acceleration would be g but soon wind resistance would effect it when they are cyclist sized rather than ball bearing sized.
I'm glad we did not get as far as trying an experiment with that 10 ton truck and some cyclists I mentioned earlier - would have got messy

bikerboy1000

First post guys !

I'm running a compact (50/34) with a 12-23 rear and find I'm running out of gears on up hills. I thinking of changing the rear cass to a 12-25 but am worried that the 25 won't be big enough and don't really want to go up to a 12-27 and I aim to use the bike for TT as well. Has anyone had experience of going from a 12-23 to a 12-25 ? Is there a big difference?

Do I have to buy a complete new cass or can you change rings (if people still do that).

Or should I stop being a fool and get the 12-27, in this case would I need to add any chain links?

Any help etc would be great. Thanks all.

MartinJ

Or should I stop being a fool and get the 12-27, in this case would I need to add any chain links?

I would get the 12 - 27 - that way you're effectively gaining 2 gears for the hills. 50-34 with 12 -27 works really well - spend most time in the big ring and use the 34 for climbs / easy riding.

I've just done a couple of time trials with this combination - I certainly didn't run out of gears. It would probably not be high enough on a TT bike though.

I think you would have to add chain links for the larger cassette

Eat My Dust

bikerboy1000 wrote:

First post guys !

I'm running a compact (50/34) with a 12-23 rear and find I'm running out of gears on up hills. I thinking of changing the rear cass to a 12-25 but am worried that the 25 won't be big enough and don't really want to go up to a 12-27 and I aim to use the bike for TT as well. Has anyone had experience of going from a 12-23 to a 12-25 ? Is there a big difference?

Do I have to buy a complete new cass or can you change rings (if people still do that).

Or should I stop being a fool and get the 12-27, in this case would I need to add any chain links?

Any help etc would be great. Thanks all.

I've just changed from a 12-27 to a 12 - 25 and to be honest I've not really noticed the difference between the 2.

saif

I have been riding on my summer bike which has 50/34 front and 12-25 rear for the last few weeks , initially I found it a bit difficult the change over from 52/39 Front and 11-24 rear on my winter bike. But probably that was due the difference in saddle set up and minor differences in the geometry of the two bikes. However , now that I have done about 150 miles on my summer bike , I think I'm settling down well with it. The Saturday run was my usual circuit in Kent , which I would consider hilly . Not sure how other people would rate this circuit and it's hill. I've collected some data on my Nokia N95, which also gives the hill profile. The 12-25 was good enough for me climb all these hills, although I did feel that I have compromised on speed a bit by have a 50 instead of 52 as my big chain ring, but the advantage I felt was peddalling on flat(ish) was relatively easy.

http://sportstracker.nokia.com/nts/work ... ?id=158695

Do you think I could improve my performance on hills if I had 12-27 instead ( assuming all the other factors remain constant) ? How do you rate the hills on this route ?

doyler78

saif wrote:

http://sportstracker.nokia.com/nts/workoutdetail/index.do?id=158695

Do you think I could improve my performance on hills if I had 12-27 instead ( assuming all the other factors remain constant) ? How do you rate the hills on this route ?

Can't see the profile though doubt I could be much help as I don't really do hills - was just being nosey

redddraggon

saif wrote:

I have been riding on my summer bike which has 50/34 front and 12-25 rear for the last few weeks , initially I found it a bit difficult the change over from 52/39 Front and 11-24 rear on my winter bike. But probably that was due the difference in saddle set up and minor differences in the geometry of the two bikes. However , now that I have done about 150 miles on my summer bike , I think I'm settling down well with it. The Saturday run was my usual circuit in Kent , which I would consider hilly . Not sure how other people would rate this circuit and it's hill. I've collected some data on my Nokia N95, which also gives the hill profile. The 12-25 was good enough for me climb all these hills, although I did feel that I have compromised on speed a bit by have a 50 instead of 52 as my big chain ring, but the advantage I felt was peddalling on flat(ish) was relatively easy.

http://sportstracker.nokia.com/nts/work ... ?id=158695

Do you think I could improve my performance on hills if I had 12-27 instead ( assuming all the other factors remain constant) ? How do you rate the hills on this route ?

Kent has hills?

saif

Redddraggon ... how high and long does it have to be classified as a hill ?

Doyler .. I'm ading the link again .. see if you can access tis time

http://sportstracker.nokia.com/nts/workoutdetail/index.do?id=158695

doyler78

saif wrote:

Redddraggon ... how high and long does it have to be classified as a hill ?

Doyler .. I'm ading the link again .. see if you can access tis time

http://sportstracker.nokia.com/nts/workoutdetail/index.do?id=158695

Nah its a permissions things. Access denied. You are not authorised to see this page.

Are you sure you are able to share if so then you probably need to enable something to make them publicly available.

bikerboy100

hmm, still not sure what to change to. Looking at the graph below it looks a waste of time changing to a 12-25 for only one gear...anyone have a comment on this?

12-13-14-15-16-17-18-19-21-23T
12-13-14-15-16-17-19-21-23-25T
12-13-14-15-16-17-19-21-24-27T

itisaboutthebike

You could look at the SRAM Rival 11-28................

saif

a colleague of mine has got his changed from 12-25 to 12-27 , and he has done 155 miles of riding on it over the weekend, he seems to be rally pleased with this change.