UKC

From the Froome thread, someone came up with the idea of sky giving Froome a bidon full of lead shot to make him quicker on the descents. 

For the sake of a good argument, I’m not going to explain the reasons this does make him quicker. I just wonder why the teams don’t do this? 

Is it just because they rarely plan to attack on a descent?

In reply to ablackett:

It was I who suggested it, mostly in jest. 

Descending speed is very dependent on the level of skill and experience of the rider. Some can attack on downhills and Froome made up most of the time while descending on that day. I think skill, experience  and courage count more than weight, in the end. 

In reply to ablackett:

You're right it would increase descending speed due to greater weight for same air drag.

On the straight bits (already 85mph !!) between corners the extra weight would help when they're going too fast to pedal.

However that terminal velocity is not the winning factor on a twisty mountain road.

The lead shot would mean braking earlier/harder and pedalling out of the corners would be harder.

 

 

In reply to Rigid Raider:

There goes my only advantage. 

 

In reply to ablackett:

intresting physics  mind . If froome was racing , oh I don't know, let's pick John dunn , down a big hill , then Dunn may well win . Drop them both from a helicopter and they hit the ground at the same time . 

In reply to mike123:

Really?

In reply to mike123:

Only if you did it in a vacuum ie no atmosphere.

In reply to elsewhere:

Count yourself lucky for not pushing it so much. About 15km from home I frequently pass the memorial of Fabio Casartelli - a TdF rider who lost it on one of those demanding switchback descents. Mind you, it was a bit silly not to have worn a helmet :o

https://en.wikipedia.org/wiki/Fabio_Casartelli

In reply to elsewhere: won't the extra weight help with acceleration out of corners?

 

In reply to keith-ratcliffe:

Which makes the helicopter bit tricky too  

In reply to wbo:

acceleration of greater weight == more power == more calories and sweat == more fatigue ??

In reply to wbo:

The energy comes from effort of pedalling out of the corners so no

 

 

In reply to ablackett:

The reason a rider is a good descender is because they brake late, corner fast, and accelerate quick, the detrimental effect on your cornering of having a ridiculously heavy bag hanging off you would more than cancel out any possible benefits. 

In reply to ablackett:

Well it wouldn't though, would it? The weight of the lead would be going downwards through the frame no matter what the angle of the bike so you wouldn't notice anything. 

The most beneficial thing I ever did for my road cycling was some sessions at the Steven Burke Hub near Nelson, which is shaped like a banana with a circle at each end. The radii of the curves are such that you can continue to pedal all the way round, which takes some faith at first but becomes easier as you gain confidence because most tyres will out-stick your own mental limit.  I learned to trust my tyres and this confidence has proved beneficial many times since when riding competitively with friends, especially down hills. A flexible sidewall, soft-compound tyre like a Veloflex Open Corsa sticks like glue to the tarmac and gives even greater confidence.  

In reply to elsewhere:

But more momentum coming out of corners so not clear cut.

In reply to Rigid Raider:

If that's the case you wouldn't notice the effect of gravity going uphill either. 

The road is inclined so there is a component of weight (vertically down) parallel to the road surface (not horizontal, not 90 degrees to the weight).

In reply to ablackett:

I don't know how true it is but I was once told that many years ago when cyclists had there water bottle in a cage on the front of the bars that TDF riders were given bottles filled with lead as they went over the summit to speed there descent.

 

If you can trust wikipedia:

https://en.wikipedia.org/wiki/Jean_Robic

 

In reply to Graham6664:

At the start of a big Alpine climb, Coppi would take his bidon from his bike and put it in his back pocket to make his bike lighter

W

In reply to ablackett:

I find a motor on the hill and a lot of weight on the descent is working quite well. I’m somehow managing to stay in heart zones 3-4, too. 

In reply to ablackett:

imo there is a shocking lack of understanding of gcse level physics on this thread.

 

acceleration d/t gravity = gM/r^2 where M is the mass of the Earth. Therefore to achieve any additional acceleration down the hill you'd have to change the density of the Earth (and hence the term M/r^2) . This is actually possible due to local inconsistencies in density (https://physics.info/falling/grace.jpg) but you'd have to move the race.

It has been suggested that this law only holds in a vacuum. Drag does indeed retard the force of gravity by

Fdrag=p(v^2)CA/2 where p is the density of the air, v is the velocity of froome, C is a dimensionless constant and A cross sectional area. Therefore, as changing the weight of the bidon has absolutely no effect on the drag, and consequently does not effect acceleration due to gravity.

In the turns however, the physics is complicated, gravity, inertia, traction and balance s involved. Its described (incompletely) here https://physics.info/falling/grace.jpg but the amount of understeer needed depends on the height of the centre of gravity. I wonder then whether lowering the centre of gravity with a lead bidon would lessen or enlarge the amount of understeer needed for the perfect corner line. It might well explain why moving the bidon up to a jersey pocket has an (apparently positive) effect on cornering speed.

 

 

In reply to mutt:

Didn't get a word of that, just tell me whether to fill my bidon with helium or plutonium

In reply to mutt:

True, as demonstrated by your post.

The simplest way of thinking about this is to consider the terminal velocity of two bodies of same shape, but different masses. At the terminal velocity, the force due to drag balances the force of gravity, i.e., mg=p(v^2)CA/2 resulting in v being proportional to sqrt(m). Heavier bodies (of the same shape) have higher terminal velocity than lighter bodies. For a cyclist there obviously a further major factor that (hopefully) they're going down a sloping road rather than being in free-fall (adds a factor of sin(slope-of-road) to the left hand side of the equation), but that make no difference to the conclusion.

So a bidon of lead is useful as long as the rider gets it at the point where downhill pedaling ceases to effective (tricky, it needs to be moving at his speed at this point) and ditches it before the slope levels off (easier, but dangerous for bystanders).