Downhill descent: Can a bike be too light for a particular person?

vonhelmet wrote:

kil0ran wrote:

A heavy rider is likely to descend faster than a light rider, assuming they have the same technique and bravery. I can't think of many out and out climbers who are also good descenders.

How do you figure? A heavy rider will have more frontal area than a light one, so will experience more air resistance. Other things being equal, they’ll be slower as a result.

I'm 4kg heavier than last year and descending measurably faster on all my local hills as a result.

But on sharp corners, I'm not as fast, and sprinting out of the corners is slower.

On balance, more pies = descend most hills faster !

Pantani was a fearless descender - Eurosport covered the 95 World Champs at 2500m in Colombia, and there was a helicopter shot of him descending past loads of riders at least 5mph faster.

vonhelmet wrote:

kil0ran wrote:

A heavy rider is likely to descend faster than a light rider, assuming they have the same technique and bravery. I can't think of many out and out climbers who are also good descenders.

How do you figure? A heavy rider will have more frontal area than a light one, so will experience more air resistance. Other things being equal, they’ll be slower as a result.

However, other things aren't equal.

Extra weight would actually help when going downhill, providing that the weight doesn't hugely increase your frontal area (unlikely to make much difference unless you're carrying dustbin lids or something).

Assuming that you're just coasting and not pedalling, then the forces driving you forwards will be a component of gravity i.e. the steeper the slope, the greater the force. NB. The force increases with increased mass, although in a vaccuum, the acceleration would stay the same as more force is required to accelerate more mass (F = M x A).

The forces acting to slow you down will be mainly the air resistance against you (approximately proportional to your frontal area and the square of your speed) and the rolling resistance. Rolling resistance is proportional to your weight and speed, but is generally quite low compared to the air resistance, so we can forget about it at higher speeds.

So, increased mass will produce a larger downhill force which will be balanced (at terminal velocity) by the air resistance at a higher speed - hence heavier riders will go downhill quicker unless they are bizarrely shaped.

hawkinspeter wrote:

However, other things aren't equal.

Extra weight would actually help when going downhill, providing that the weight doesn't hugely increase your frontal area (unlikely to make much difference unless you're carrying dustbin lids or something).

Assuming that you're just coasting and not pedalling, then the forces driving you forwards will be a component of gravity i.e. the steeper the slope, the greater the force. NB. The force increases with increased mass, although in a vaccuum, the acceleration would stay the same as more force is required to accelerate more mass (F = M x A).

The forces acting to slow you down will be mainly the air resistance against you (approximately proportional to your frontal area and the square of your speed) and the rolling resistance. Rolling resistance is proportional to your weight and speed, but is generally quite low compared to the air resistance, so we can forget about it at higher speeds.

So, increased mass will produce a larger downhill force which will be balanced (at terminal velocity) by the air resistance at a higher speed - hence heavier riders will go downhill quicker unless they are bizarrely shaped.

 

Does this affect how quickly fat and thin squirrels can run up and down trees, too?

StraelGuy wrote:

hawkinspeter wrote:

However, other things aren't equal.

Extra weight would actually help when going downhill, providing that the weight doesn't hugely increase your frontal area (unlikely to make much difference unless you're carrying dustbin lids or something).

Assuming that you're just coasting and not pedalling, then the forces driving you forwards will be a component of gravity i.e. the steeper the slope, the greater the force. NB. The force increases with increased mass, although in a vaccuum, the acceleration would stay the same as more force is required to accelerate more mass (F = M x A).

The forces acting to slow you down will be mainly the air resistance against you (approximately proportional to your frontal area and the square of your speed) and the rolling resistance. Rolling resistance is proportional to your weight and speed, but is generally quite low compared to the air resistance, so we can forget about it at higher speeds.

So, increased mass will produce a larger downhill force which will be balanced (at terminal velocity) by the air resistance at a higher speed - hence heavier riders will go downhill quicker unless they are bizarrely shaped.

 

 

Does this affect how quickly fat and thin squirrels can run up and down trees, too?

Most definitely.

The larger-boned squirrel will quite often mistakenly climb onto a branch that isn't quite strong enough, so they end up plummetting to the ground. The more svelte squirrel will choose to clamber down the tree trunk which will be slower.

StraelGuy wrote:

hawkinspeter wrote:

However, other things aren't equal.

Extra weight would actually help when going downhill, providing that the weight doesn't hugely increase your frontal area (unlikely to make much difference unless you're carrying dustbin lids or something).

Assuming that you're just coasting and not pedalling, then the forces driving you forwards will be a component of gravity i.e. the steeper the slope, the greater the force. NB. The force increases with increased mass, although in a vaccuum, the acceleration would stay the same as more force is required to accelerate more mass (F = M x A).

The forces acting to slow you down will be mainly the air resistance against you (approximately proportional to your frontal area and the square of your speed) and the rolling resistance. Rolling resistance is proportional to your weight and speed, but is generally quite low compared to the air resistance, so we can forget about it at higher speeds.

So, increased mass will produce a larger downhill force which will be balanced (at terminal velocity) by the air resistance at a higher speed - hence heavier riders will go downhill quicker unless they are bizarrely shaped.

 

 

Does this affect how quickly fat and thin squirrels can run up and down trees, too?

Thin squirrels are more likely to be younger and less experienced and therefore less likely to run up a tree when a (my) whippet bears down on them at high speed, with a much greater chance of them then being caught. Fat squirrels conversely are more likely to be older and more aware and less likely to end up in the mouth of a whippet (with a high prey drive) and being shaken violently from side to side.

OldRidgeback wrote:

StraelGuy wrote:

hawkinspeter wrote:

However, other things aren't equal.

Extra weight would actually help when going downhill, providing that the weight doesn't hugely increase your frontal area (unlikely to make much difference unless you're carrying dustbin lids or something).

Assuming that you're just coasting and not pedalling, then the forces driving you forwards will be a component of gravity i.e. the steeper the slope, the greater the force. NB. The force increases with increased mass, although in a vaccuum, the acceleration would stay the same as more force is required to accelerate more mass (F = M x A).

The forces acting to slow you down will be mainly the air resistance against you (approximately proportional to your frontal area and the square of your speed) and the rolling resistance. Rolling resistance is proportional to your weight and speed, but is generally quite low compared to the air resistance, so we can forget about it at higher speeds.

So, increased mass will produce a larger downhill force which will be balanced (at terminal velocity) by the air resistance at a higher speed - hence heavier riders will go downhill quicker unless they are bizarrely shaped.

 

 

Does this affect how quickly fat and thin squirrels can run up and down trees, too?

Thin squirrels are more likely to be younger and less experienced and therefore less likely to run up a tree when a (my) whippet bears down on them at high speed, with a much greater chance of them then being caught. Fat squirrels conversely are more likely to be older and more aware and less likely to end up in the mouth of a whippet (with a high prey drive) and being shaken violently from side to side.

So, what about a fat whippet vs a thin squirrel?

hawkinspeter wrote:

OldRidgeback wrote:

StraelGuy wrote:

hawkinspeter wrote:

However, other things aren't equal.

Extra weight would actually help when going downhill, providing that the weight doesn't hugely increase your frontal area (unlikely to make much difference unless you're carrying dustbin lids or something).

Assuming that you're just coasting and not pedalling, then the forces driving you forwards will be a component of gravity i.e. the steeper the slope, the greater the force. NB. The force increases with increased mass, although in a vaccuum, the acceleration would stay the same as more force is required to accelerate more mass (F = M x A).

The forces acting to slow you down will be mainly the air resistance against you (approximately proportional to your frontal area and the square of your speed) and the rolling resistance. Rolling resistance is proportional to your weight and speed, but is generally quite low compared to the air resistance, so we can forget about it at higher speeds.

So, increased mass will produce a larger downhill force which will be balanced (at terminal velocity) by the air resistance at a higher speed - hence heavier riders will go downhill quicker unless they are bizarrely shaped.

 

 

Does this affect how quickly fat and thin squirrels can run up and down trees, too?

Thin squirrels are more likely to be younger and less experienced and therefore less likely to run up a tree when a (my) whippet bears down on them at high speed, with a much greater chance of them then being caught. Fat squirrels conversely are more likely to be older and more aware and less likely to end up in the mouth of a whippet (with a high prey drive) and being shaken violently from side to side.

So, what about a fat whippet vs a thin squirrel?

Aha, a fat whippet may not have the speed to catch a  thin squirrel. Note too that thin whippets can't turn as quickly as either thin or fat squirrels. Some whippets do have superior squirrel catching abilities and seem to use predictive behavioural techniques.
 

vonhelmet wrote:

kil0ran wrote:

A heavy rider is likely to descend faster than a light rider, assuming they have the same technique and bravery. I can't think of many out and out climbers who are also good descenders.

How do you figure? A heavy rider will have more frontal area than a light one, so will experience more air resistance. Other things being equal, they’ll be slower as a result.

"Other things being equal"... they aren't, the mass is greater.

 A rider is (kind of) a sphere, so their volume (roughly equivalent to mass) increases as a 3rd power relative to a given dimension, (4/3*pi *r^3) whereas their surface area (think frontal area ) increases relative to the same dimension squared (4*pi *r^2), so a 100kg person all things being equal will only have 30% more surface area than a 70kg person, but 42% more mass. (the difference is accentuated when they both get into an aerodynamic tuck, the larger person's frontal area is less than 10% greater than the smaller person, but they have 40 % greater 'motive force') So their gravitational potential energy (which is converting to kinetic energy) is greater  and will equate to greater speed as they are both losing the same height.

Fluffy kitten's tower of Pisa experiment assumes that air resistance is minimal, which it clearly isn't in the case of a cyclist. So your heavier cyclist will both accelerate faster, and attain a higher final speed. So, if you want to go fast(er) downhill, ride a heavy, aero, bike.

vonhelmet wrote:

kil0ran wrote:

A heavy rider is likely to descend faster than a light rider, assuming they have the same technique and bravery. I can't think of many out and out climbers who are also good descenders.

How do you figure? A heavy rider will have more frontal area than a light one, so will experience more air resistance. Other things being equal, they’ll be slower as a result.

I'm 97kg, not as much muscle to fat ratio as I'd like but looking at me you wouldn't say I look my weight, so I'm not giving a massively larger frontal area and you have to take into account different clothing and actual body position too.

I've always outsripped 'smaller' and lighter riders on the downhill when freewheeling, managed this on a flat bar hybrid with 25mm gatorskins, the difference is always noticeable though I can get a decent aero position on it when needs be.

Managed 46mph coming down the A505 from Luton to Hitchin (about 7-8%) without much bother, again that was on a flat bar.

As for whether a bike can be too light, nope, handling ability of the bike, your familiarity with it plus your confidence to descend quickly and importantly picking your line/knowing when to sit up/brake are massively important factors when it comes to making progress.

vonhelmet wrote:

kil0ran wrote:

A heavy rider is likely to descend faster than a light rider, assuming they have the same technique and bravery. I can't think of many out and out climbers who are also good descenders.

How do you figure? A heavy rider will have more frontal area than a light one, so will experience more air resistance. Other things being equal, they’ll be slower as a result.

Personal experience. As a larger rider I overtake the climbing whippets on descents, and I don't consider myself particularly brave. Sadly I'm left for dead on the climbs so it all evens out

kil0ran wrote:

A heavy rider is likely to descend faster than a light rider, assuming they have the same technique and bravery. I can't think of many out and out climbers who are also good descenders.

Nibali, (see his descents of the Poggio) Savoldelli (nicknamed "Il Falco"), Valverde is a great descender, Froome himself can go down pretty fast, the tiny Pantani also rapid downhill. Though we do tend to focus on those at the front of the peloton in the mountains. Alledgedly some of the best descenders are the big guys in the autobus trying to make the time cut.