GuyBoden
Guru
- Location
- Warrington
Hopefully, the more I weigh, the slower I will be riding, thus enjoying the scenery more............... 
It Doesn't matters, It's not the bike/kit you have...
- Thread starter itaa
- Start date
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derrick
The Glue that binds us together.
- Location
- On the edge of London
I have a few times, loads of people do.
Not quite 50 but close,https://www.strava.com/activities/1246175983
Ok. Certainly leaves me with zero room to infer whatsoever. The edit kind of backfired it appears. Feels free to unedit accordingly.The 'unusually' was an edit, though quite a quick one, as I thought without it I was being harsh (for the season of good will). Please infer what you prefer.
This was the scenario I focused on (quote from the OP):
That is: on the flat, same aero drag (ie position) - just different weights of bike.
The scenario contradicts itself as a MTB cannot have the same aero drag or be ridden in the same position. We focused on different elements.
Ewwww! Naughty step for ‘trousers!
Randy Butternubs
Über Member
Acceleration due to gravity is independent of mass. You must know that.
Err, only on the moon surely?
On Earth (which has an atmosphere) acceleration due to gravity is tempered by aerodynamic drag. Generally speaking, for heavy/dense objects this drag is small relative to the weight. Which is why on Earth a bowling ball falls faster than a feather but on the moon they fall at the same rate.
Randy Butternubs
Über Member
I chose the bowling ball and feather example as it was famously used during the moon landing...or was it a hammer and a feather? You are right though - it isn't a great example. A better one would be a balloon filled with air vs one filled with water - they both have the same drag coefficient and obviously the one filled with water will accelerate far faster under gravity.
A rider is a bit different since a bigger, heavier rider will generally have greater drag. I think an increase in weight should lead to a comparatively small increase in drag though. For simplicity's sake consider a cube with length l sides - the mass will rise proportionally to l^3 but the frontal area only to l^2.
Anecdotally, when I freewheel down hills (heavy rider, cheap bike) my much lighter brother on a far, far fancier bike has to pedal to keep up.
Randy Butternubs
Über Member
I'm so sorry. No one should have to go to sleep with maths in their head.
- Location
- Inside my skull
In an atmosphere, objects of different masses, but of similar surface areas and profiles, will achieve different terminal velocities.
huwsparky
Über Member
- Location
- Llangrannog
As I understand it the main benefit a heavier cyclist has going downhill is the momentum carried as the road evens out. That's certainly where it's very noticeable to me as I (as a relative lightweight) having to pedal to keep up whereas my heavier riding buddies carry their momentum better coming off the decent.
That's pretty much what Galileo worked out, long before bicycles were invented. If you dropped fat me off the Tower of Pisa together with skinny Chris Froome, we'd hit the ground at the same time - ignoring air resistance. And if you account for air resistance, Froome would get down quicker.
Yes, there's greater momentum going into the flat.
I'm a well-built person who rides steel bikes, but I haven't noticed any speed advantage going downhill.
Randy Butternubs
Über Member
This is a bit sloppy but:
a = acceleration
F = resultant force
m = mass
g = gravitational constant (about 10m/s^2)
D = drag force (it changes based on several factors including velocity but I think for this we can say it is constant)
acceleration = force/mass
a = F/m
Assume that the only significant forces are gravity and aerodynamic drag. Gravitational force = gm. Therefore:
acceleration = (gravitational force - drag force)/mass
a = (gm - D)/m
= g - D/m
acceleration = gravitational constant - (drag force/mass)
So when comparing, say, a 50kg and a 100kg object the 100kg one will accelerate faster unless the drag force acting upon it is double. This seems unlikely thanks to the square-cube law - volume (and so mass) increases far faster than surface area. I don't know anything about aerodynamics though so I'm happy to be corrected. I'm also using free-fall as an example but rolling down a hill shouldn't be fundamentally different.
a = acceleration
F = resultant force
m = mass
g = gravitational constant (about 10m/s^2)
D = drag force (it changes based on several factors including velocity but I think for this we can say it is constant)
acceleration = force/mass
a = F/m
Assume that the only significant forces are gravity and aerodynamic drag. Gravitational force = gm. Therefore:
acceleration = (gravitational force - drag force)/mass
a = (gm - D)/m
= g - D/m
acceleration = gravitational constant - (drag force/mass)
So when comparing, say, a 50kg and a 100kg object the 100kg one will accelerate faster unless the drag force acting upon it is double. This seems unlikely thanks to the square-cube law - volume (and so mass) increases far faster than surface area. I don't know anything about aerodynamics though so I'm happy to be corrected. I'm also using free-fall as an example but rolling down a hill shouldn't be fundamentally different.
Ajax Bay
Guru
- Location
- East Devon
gilespargiter
Veteran
- Location
- N Wales
According to my, admittedly anecdotal - but repeated observations; people seem to be considering completely the wrong factors concerning why a solid person on a steel bike descends more rapidly.
To relate such an anecdote in a definitely tongue in cheek way but including what seems the main key fact.
I find that when approaching the latter parts of an ascent such as say, Pen y pass over yr Swyddfa, one finds that those on lovely carbon cycles wearing super - shall we say- "streamlined" kit tend to look over their shoulders and upon noticing someone like me, with my carradice and steel frame they then go flat out eyes popping - guts busting. Although it is of course obvious that I must have been winding them in for a considerable distance. If they then avoid the ignominy of pretending to take pictures at the top, the next thing I observe is their super light frames flapping and twisting around in such a way that they can scarcely keep hold of them. At this point the main problem I have is giving them a wide enough berth as I streak past like an arrow on my ancient 531 steel.
On hills like that I have no fear of having to repeat the maneuvre as by the bottom I'am seldom less than a mile in front.
Clearly the thing here is the stability of the frame when descending. I have noticed this as an issue on some very high end frames, some of which I have managed to have a much closer inspection of at the bottom.
To relate such an anecdote in a definitely tongue in cheek way but including what seems the main key fact.
I find that when approaching the latter parts of an ascent such as say, Pen y pass over yr Swyddfa, one finds that those on lovely carbon cycles wearing super - shall we say- "streamlined" kit tend to look over their shoulders and upon noticing someone like me, with my carradice and steel frame they then go flat out eyes popping - guts busting. Although it is of course obvious that I must have been winding them in for a considerable distance. If they then avoid the ignominy of pretending to take pictures at the top, the next thing I observe is their super light frames flapping and twisting around in such a way that they can scarcely keep hold of them. At this point the main problem I have is giving them a wide enough berth as I streak past like an arrow on my ancient 531 steel.
On hills like that I have no fear of having to repeat the maneuvre as by the bottom I'am seldom less than a mile in front.
Clearly the thing here is the stability of the frame when descending. I have noticed this as an issue on some very high end frames, some of which I have managed to have a much closer inspection of at the bottom.
