Descending with the big guys

phreak

I don't weigh much, which is great going up, but it's a drag when bigger guys can freewheel past me on the descents. The likes of Pantani always seemed to keep up going downhill, is it a case of having to have better technique than the heavier guys?

snickwell

phreak wrote:

I don't weigh much, which is great going up, but it's a drag when bigger guys can freewheel past me on the descents. The likes of Pantani always seemed to keep up going downhill, is it a case of having to have better technique than the heavier guys?

I think so... A really, really tight aero position works well for me! Of course, you'll never beat physics per se, but you can limit losses this way.

twotyred

Acceleration due to gravity is the same regardless of mass so if you can make yourself as aerodynamic as possible you'll go as fast as the big guys.

thecrofter

OH GOD!!!!!!! not this again.

sub55

:!: Been here before

hazychris

twotyred wrote:

Acceleration due to gravity is the same regardless of mass so if you can make yourself as aerodynamic as possible you'll go as fast as the big guys.

Woohoo! In which case the opposite is true and my 105kg of beef should be able to ascend like Pantani!

Maybe not.... although he could probably descend at least as quickly as me through a mixture of technique, balls and drugs.

Buckled_Rims

twotyred wrote:

Acceleration due to gravity is the same regardless of mass so if you can make yourself as aerodynamic as possible you'll go as fast as the big guys.

+1

In fact the big guys have a disadvantage in aero, to that of a thin man. The chances are the big guys are putting more power into the pedals at the start of the descent, building up momentum.

twotyred

Woohoo! In which case the opposite is true and my 105kg of beef should be able to ascend like Pantani!

He he. Not quite. Whilst a cannonball and feather will fall at the same speed if you remove air resistance it will still take considerably more energy to lift the cannonball back up the hill.

stoobydale

All things considered,Fat blokes go faster down hill
True.

tom22

been through all this before on a previous thread and basically....
the fact that they are bigger has no affect at all, its all about aerodynamics, keeping momentum going, and differences in the friction between parts (tyres, bb, hubs, etc.)

i am very light at 60kg and on club rides can descend with (and if not better than) all the bigger guys

Slack

tom22 wrote:

been through all this before on a previous thread and basically....
the fact that they are bigger has no affect at all, its all about aerodynamics, keeping momentum going, and differences in the friction between parts (tyres, bb, hubs, etc.)

i am very light at 60kg and on club rides can descend with (and if not better than) all the bigger guys

Agree with this. If not freewheeling I descend much quicker than my riding partner who is heavier than me. I have heavier rims and tyres with lower rolling resistance.

rick_chasey

Brake less than they do.

phreak

tom22 wrote:

been through all this before on a previous thread and basically....
the fact that they are bigger has no affect at all, its all about aerodynamics, keeping momentum going, and differences in the friction between parts (tyres, bb, hubs, etc.)

i am very light at 60kg and on club rides can descend with (and if not better than) all the bigger guys

Any tips?

Herbsman

The extra weight of the heavier person will cause more friction on their wheel bearings.. So assuming both bikes have identical bearings, if this was the only factor involved, the heavier person would descend more slowly.

If mass made no difference then why would the IOC have weight limits for bobsleighs? Two objects of different mass may accelerate at the same rate and reach the same terminal velocity when in freefall in a vacuum but cyclists don't descend hills in freefall in a vacuum. Assuming identical air resistance, a heavier person will go faster downhill, because in the heavier person there is more mass, hence more force is opposing the air resistance.

tom22

phreak wrote:

tom22 wrote:

been through all this before on a previous thread and basically....
the fact that they are bigger has no affect at all, its all about aerodynamics, keeping momentum going, and differences in the friction between parts (tyres, bb, hubs, etc.)

i am very light at 60kg and on club rides can descend with (and if not better than) all the bigger guys

Any tips?

just try not to brake unless needed and pedal a hard gear until not necessary.
and tuck in (bum slightly off the saddle, knees/thighs pinching the top tube, shoulders low, head up!, hands on drops, elbows tucked in)
just watch some of the other guys, and see what they do

SteveR_100Milers

Herbsman wrote:

The extra weight of the heavier person will cause more friction on their wheel bearings.. So assuming both bikes have identical bearings, if this was the only factor involved, the heavier person would descend more slowly.

If mass made no difference then why would the IOC have weight limits for bobsleighs? Two objects of different mass may accelerate at the same rate and reach the same terminal velocitywhen in freefall in a vacuum but cyclists don't descend hills in freefall in a vacuum. Assuming identical air resistance, a heavier person will go faster downhill, because in the heavier person there is more mass, hence more force is opposing the air resistance.

They do on my wheel!

tom22

Herbsman wrote:

The extra weight of the heavier person will cause more friction on their wheel bearings.. So assuming both bikes have identical bearings, if this was the only factor involved, the heavier person would descend more slowly.

If mass made no difference then why would the IOC have weight limits for bobsleighs? Two objects of different mass may accelerate at the same rate and reach the same terminal velocity when in freefall in a vacuum but cyclists don't descend hills in freefall in a vacuum. Assuming identical air resistance, a heavier person will go faster downhill, because in the heavier person there is more mass, hence more force is opposing the air resistance.

here comes the physics.....
anything in motion can be described by the formula "force = mass x acceleration"
so therefore if re-arranged "acceleration = force / mass"
so if mass of an object increases, and the same force is applied (pedaling), then acceleration will be smaller.
but to achieve the same acceleration with the increased mass the force applied (pedaling) must be increased by the same factor.

and the reason there is the weight restriction in bobsleighs is due to the way ice skates work (what they use to run on)
ice skates use friction to melt the ice under them and they then run on the small layer of water... if you make a bobsleigh heavier, the friction increases which means that the amount of ice melted increases by a small amount, which means the bobsleigh can reach much greater speeds over the whole bobsleigh run.

Herbsman

tom22 wrote:

Herbsman wrote:

The extra weight of the heavier person will cause more friction on their wheel bearings.. So assuming both bikes have identical bearings, if this was the only factor involved, the heavier person would descend more slowly.

If mass made no difference then why would the IOC have weight limits for bobsleighs? Two objects of different mass may accelerate at the same rate and reach the same terminal velocity when in freefall in a vacuum but cyclists don't descend hills in freefall in a vacuum. Assuming identical air resistance, a heavier person will go faster downhill, because in the heavier person there is more mass, hence more force is opposing the air resistance.

here comes the physics.....
anything in motion can be described by the formula "force = mass x acceleration"
so therefore if re-arranged "acceleration = force / mass"
so if mass of an object increases, and the same force is applied (pedaling), then acceleration will be smaller.
but to achieve the same acceleration with the increased mass the force applied (pedaling) must be increased by the same factor.

But we're not talking about acceleration due to pedaling here. We're talking about cycling downhill, i.e. acceleration due to gravity.

Gravitational force acts upon mass at about 9.81 Newtons per kilogram. So a heavier object, for example a man with a full stomach riding a heavy steel bike, has a greater force acting upon it when rolling down a hill. The same man with an empty stomach, riding on an identical but lightweight carbon fibre bike will have less gravitational force acting upon him.

When it comes to drag, F = -bv where F = force, b is a constant depending on the medium being travelled through (air) and the size of the object (man & bike) and v is velocity. This can be rearranged to v = F/-b. The man with the full stomach and heavy bike has a greater force acting upon him due to his greater mass, but b remains constant because he and his bike are the same size and shape as when he has an empty stomach and his carbon bike. Hence v must be greater; his downhill speed will be higher.

Tom Butcher

It's mostly about bottle and technique anyway isn't it - i mean there aren't that many descents where you can off the brakes in an aero tuck all the way down - and if you are good enough for that just tuck in behind the big guy til the road goes up again.

tom22

Herbsman wrote:

tom22 wrote:

Herbsman wrote:

The extra weight of the heavier person will cause more friction on their wheel bearings.. So assuming both bikes have identical bearings, if this was the only factor involved, the heavier person would descend more slowly.

If mass made no difference then why would the IOC have weight limits for bobsleighs? Two objects of different mass may accelerate at the same rate and reach the same terminal velocity when in freefall in a vacuum but cyclists don't descend hills in freefall in a vacuum. Assuming identical air resistance, a heavier person will go faster downhill, because in the heavier person there is more mass, hence more force is opposing the air resistance.

here comes the physics.....
anything in motion can be described by the formula "force = mass x acceleration"
so therefore if re-arranged "acceleration = force / mass"
so if mass of an object increases, and the same force is applied (pedaling), then acceleration will be smaller.
but to achieve the same acceleration with the increased mass the force applied (pedaling) must be increased by the same factor.

But we're not talking about acceleration due to pedaling here. We're talking about cycling downhill, i.e. acceleration due to gravity.

Gravitational force acts upon mass at about 9.81 Newtons per kilogram. So a heavier object, for example a man with a full stomach riding a heavy steel bike, has a greater force acting upon it when rolling down a hill. The same man with an empty stomach, riding on an identical but lightweight carbon fibre bike will have less gravitational force acting upon him.

When it comes to drag, F = -bv where F = force, b is a constant depending on the medium being travelled through (air) and the size of the object (man & bike) and v is velocity. This can be rearranged to v = F/-b. The man with the full stomach and heavy bike has a greater force acting upon him due to his greater mass, but b remains constant because he and his bike are the same size and shape as when he has an empty stomach and his carbon bike. Hence v must be greater; his downhill speed will be higher.

velocity is not affected because, acceleration due to gravity acts vertically (with a component of the force acting perpendicular into the road and a component acting down parallel to the road, down the slope) the added force (of a heavier rider) applied into the ground means that friction is increased by a greater force than the component down the slope... unless the gradient of the slope is greater than 45%, which is highly unlikely.

so therefore having a heavier rider riding down a slope of less than 45%, he/she will be subject to more friction on the road, and not benefit from the increased acceleration due to gravity.

oldwelshman

Heavier aero riders descend faster that skinny guys, fact.
Just go in a race to find out and see.

P_Tucker

tom22 wrote:

velocity is not affected because, acceleration due to gravity acts vertically (with a component of the force acting perpendicular into the road and a component acting down parallel to the road, down the slope) the added force (of a heavier rider) applied into the ground means that friction is increased by a greater force than the component down the slope... unless the gradient of the slope is greater than 45%, which is highly unlikely.

so therefore having a heavier rider riding down a slope of less than 45%, he/she will be subject to more friction on the road, and not benefit from the increased acceleration due to gravity.

So, what you're saying is that fatter riders descend more slowly? Crikey.

Please show the figures and workings you used for friction to derive your remarkably round 45 degree tipping point. Its been a while since I did A-level physics, but I'm already preparing my bus to drive through your response.

genki

Is it not the case that mass is related to volume, whereas wind resistance is related to area? A rider at twice the weight will have less than twice the area presented the wind. But his momentum is related to his mass.

Experience tells me a fatty can freewheel faster than a skinny if they both hold the same position. The solution if you're skinny is to find a fat arse to shelter behind.

tom22

P_Tucker wrote:

Please show the figures and workings you used for friction to derive your remarkably round 45 degree tipping point. Its been a while since I did A-level physics, but I'm already preparing my bus to drive through your response.

because it is only on a slope greater than 45% that the component of the increased mass going down the slope will be greater than the component perpendicular to the road, therefore over-riding friction and being helpful to the rider.

and i am saying that bigger guys will descend will a slower acceleration... which is what you need to go downhill fast, because of accelerating out of corners, as its rare for a descent to be dead straight.

just look professionals, the better riders on the descents are lighter riders like nibali.

P_Tucker

tom22 wrote:

P_Tucker wrote:

Please show the figures and workings you used for friction to derive your remarkably round 45 degree tipping point. Its been a while since I did A-level physics, but I'm already preparing my bus to drive through your response.

because it is only on a slope greater than 45% that the component of the increased mass going down the slope will be greater than the component perpendicular to the road, therefore over-riding friction and being helpful to the rider.

and i am saying that bigger guys will descend will a slower acceleration... which is what you need to go downhill fast, because of accelerating out of corners, as its rare for a descent to be dead straight.

just look professionals, the better riders on the descents are lighter riders like nibali.

I'm pretty sure your reasoning is wrong, but I can't remember enough physics to prove it. Dammit.

Nonetheless, I'm going to try. You seem to be assuming that the coefficient of friction, u, is 1, in which case at 45 degrees umcos(theta) = msin(theta) as you say. However, I reckon its much less, in which case the tipping point is much lower.

Someone with physics degree please sort this out FFS

genki

genki wrote:

Is it not the case that mass is related to volume, whereas wind resistance is related to area? A rider at twice the weight will have less than twice the area presented the wind. But his momentum is related to his mass.

Experience tells me a fatty can freewheel faster than a skinny if they both hold the same position. The solution if you're skinny is to find a fat ars* to shelter behind.

Here we go...

from this website
http://www.sportsci.org/jour/9804/dps.html

Terminal velocity can be solved for in the cycling equation above by setting power at 0. If one assumes the rolling resistance term is also 0, and that there is no wind blowing (v = s), then the equation becomes:

kaAs3 = -giMs
or s = (-giM/kaA)1/2

Thus, the terminal velocity is roughly proportional to the square root of the ratio of M/A. Scaling reveals that larger cyclists have a greater ratio of mass to frontal area. They therefore descend hills faster as a consequence of purely physical, not physiological, laws. Since the larger cyclist has a greater mass, gravity acts on him or her with a greater force than it does on a smaller cyclist. (Note: A common misconception is to note the equal acceleration of two different sized objects in free fall in a vacuum, and assume that the force of gravity on both is equal. The force on the more massive object is greater, being exactly proportional to mass, which is why the more massive object is accelerated at the same rate as the less massive one.) While the larger cyclist also has a greater absolute frontal area than the smaller cyclist, the difference is not as great as that for their masses. Thus, the larger cyclist will attain a greater s3 before a balance of forces results in terminal velocity.

P_Tucker

YES! Science, it works bitches

oldwelshman

So it is true a fat arse descends faster as his fronal area is still as small as a skinny guy
Tow of the better descneders in the peloton are Nibali and Chinny, I would not describe them as skinny compared to dwarf climbers !!! It is quite rare to see a petite climber who is a reall fast descender.

phreak

So with that in mind, what can the light guy do to stop all of their hard work going up being lost by Mr Pie Eater on the way down?

ut_och_cykla

phreak wrote:

So with that in mind, what can the light guy do to stop all of their hard work going up being lost by Mr Pie Eater on the way down?

get super aero and a blinking big top gear?

Tom Butcher

According to WIki Nibali weighs 10st 1lb, Voeckler is very very fast down hill - according to wiki he's 10,4 - even for pro cyclists these are by no means heavy riders - so I still don't think weight is a significant limiter - there may be a very very slight advantage to be heavy but as a skinny bloke you should be out of sight of the fat boys when you go over the top anyway.