- Who will get to the bottom first?
Northwind – Member
Probably not me.
Is it as simple as it looks? Heavier rider/bike package will have more rolling resistance won’t they? Deform tyre more.
SHould do, plus other frictions will be greater, and he’s probably bigger, so has more wind resistance.
Or maybe his greater momentum overcomes rolling resistance more easily.Posted 4 years agob45herMember
never realized wheel sucking was regarded as a skill.
anyway back on topic, anyone who has done any amount of road riding will know that heavier riders tend to decent faster in a straight line, add corners and its more the rider with the biggest balls.Posted 4 years ago
riders who ride mountain bikes a lot tend to be much better road descender’s too in my experience.wilko1999Member
Where’s the additional force coming from to make a heavy person go faster then? The force acting on them is the same ie gravity. The same way that two objects of different mass dropped from a high building will hit the ground at the same time, discounting the aerodynamic properties of those objects. It must be down to aerodynamics, rolling resistance, imagination etc 😀Posted 4 years agoscaredypantsSubscriber
I remember my first mega being behind people spinned hard and blowing a huge effort while I pumped & flowed down the trail behind them
Bugger, now I’ve got an erection and I need to stand up in a minute
wind resistance is area, weight is volume (mostly, sort of). If I weigh twice as much, my SA won’t be twice as bigPosted 4 years agothisisnotaspoonMember
discounting the aerodynamic properties of those objects
Pointless thing to discount, you always reach a terminal velocity due to the air resitance, unless the hill is very very short of you run out of bottle and hit the brakes.
On a road, the heavier one, wind resistance goes up aproximately linearly with frontal area
Assuming the rider is modeled by a cube and has a density of 1000kg/m3 and a drag coeficinet (Cd) of 1 to keep the maths simple. Rolling resistance is very small compared to air resistance therefore
0.5pv^2 CdA = Force of drag
where p is density of air and v is velocity.
90kg = 0.09m3 = 0.448m tall = 0.2m2 frontal area
100kg = 0.1m3 = 0.464m tall = 0.215m2 frontal area
Pick a speed and air density that 0.5pv2 = 1 to simplify the equation and drag went up by a 7.5%.
Force is mass x 9.81 / sin(gradient). Make the gradient vertical to simplify sinn(90)=1
Force went up by 11%.
Therefore a gain of 10kg on a 90kg rider means at the same speed he had 7% more drag, but 11% more force. So Mr 100kg is accelerating quicker than the lighter rider.
That pattern will be true for any weights.
It’s a good argument against disks as the heavier riders in the pelaton will always drag their brakes.Posted 4 years agopalmer77Member
Article from Cyclist on this:
The argument began one afternoon in the Cyclist office. Who descends quicker: a skinny rider or a fat one? A skinny rider is more aero; a fat rider has more gravitational energy; a light rider has less rolling resistance… The debate grew, but there was no decisive answer. We put the question to Google, Facebook and Twitter but none of the 100-plus answers resolved the issue. So what’s the answer?
First things first – gravity. Secondary school science lessons taught us that a stone and a feather fall at the same rate in a vacuum. Borut Fonda, a researcher into cycling science at the University of Birmingham, explains why: ‘When freefalling in vacuum conditions, where there is no aerodynamic drag, weight would not make any difference to the speed of two objects that fall in a similar way.’
But the influence of air changes the picture. Tom Compton of cycling science site AnalyticCycling.com says, ‘In a vacuum, yes a feather will fall at the same rate as a stone. Only gravity exerts a force on the objects. In air, the air exerts a force against the force from gravity. Because of the difference in shape, the force from air slows the feather more than it slows the stone.’
So is it all down to aerodynamics? Our skinnier rider has less frontal area and, in principle, less aerodynamic drag. On a downhill run, the effect of aerodynamics is exaggerated because the riders are travelling at a higher speed than normal. But that doesn’t tell the whole story. Aerodynamics and gravity act on objects in different ways, says Compton: ‘The force from gravity would be greater for the heavier object and the speed at terminal velocity would also be greater.’ But there’s still more to it. Terminal velocity is a term thrown around a lot in the debate, but often incorrectly. It’s not fair to say that one shape simply has a higher terminal velocity than another, especially when the shape is as malleable as the human body. As Fonda explains, ‘Terminal velocity is the speed at which aerodynamic drag counters the force of acceleration.’ But as most of us know, aerodynamics can be changed depending on position and frontal area.
So how do weight, frontal area and speed alter the equation? Compton argues that weight is the decisive factor in reaching a higher velocity. He cites the example of a tandem bicycle. ‘It would have double the mass and about the same air resistance. Its terminal speed would be close to 70mph versus a single bike in the low 40mph range.’
‘In principle, a lighter rider will accelerate faster under the same pedalling force’
There seems to be a funny imbalance, then, between an increased frontal area and increased weight. A fatter person, we must assume, has a larger area to force through the air than his skinny counterpart. The point seems to be that the more weight you have, the more drag force you’re able to overcome. But why does weight win the battle? ‘When you increase the mass the speed increases by cubic function, whereas if you increase the aerodynamic drag the speed decreases by a square function. Hence why cyclists who are heavier can go faster,’ Fonda says.
So for the heavier rider the pull of gravity is greater than the air resistance, because the difference in weight between the two will be cubed while the difference in surface area is squared. The latter will generally be smaller. The other big player, rolling resistance, is relatively constant at any speed, so will play an increasingly minimal role at higher speeds where normally it would penalise a heavier rider.
Just to complicate matters…Posted 4 years ago
Freewheeling down a long, straight descent, a heavier rider wins, but there are other factors to consider. Is cornering, both leaning into the corner and accelerating out of it, weight related? ‘Yes,’ Fonda says. ‘The centrifugal force that pushes you into a corner is related to weight. So a bigger rider will be given an advantage when taking corners.’
There are two schools of thought on accelerating out of the corner. In principle, a lighter mass will accelerate at a faster rate under the same pedalling force. Yet, at the same time, a heavier rider will have more gravitational force and more momentum preserving their speed. Fonda says, ‘That’s more practical than theoretical, as when you have to accelerate out of a corner, sprinters who have more power accelerate faster.’
Gravity is the major player and even pedalling is an afterthought, says Compton: ‘Pedalling adds force and makes the rider go a little faster. However, at terminal velocity, the force from pedalling is not sufficient to make the rider go fast enough to gap a competitor, especially one sitting on a wheel.’
But there’s still one niggling assumption underpinning this whole argument that’s worth addressing – that a skinnier rider is more aerodynamic than a heavy rider. Compton is sceptical: ‘John Cobb did a lot of wind-tunnel testing for Lance Armstrong. Cobb said you had to measure drag because you couldn’t tell by looking at a position if it was aero or not. You can only truly tell in a wind-tunnel.’ In theory, a fat rider could be more aero.
So there you have it. The fat rider wins… for once. That said, there’s more than gravitational science to descending. ‘The fastest riders are the ones who take risks,’ says Compton. ‘Vincenzo Nibali is one of the fastest. He’s not particularly heavy, he’s not exceptionally powerful – he’s just crazy.’b45herMember
i know the MTBer think is a regular with the commentators but it’s true, i ride with a mixed group, some dedicated roadies and some who do both and the MTBers are generally the better descenders, its the same with road motorcyclists too riders who have years of moto x experience tend to be incedibly fast, i think its to do with your mind and body being more accepting of the bike weaving and moving around at speed.Posted 4 years agorusty90Member
gravity is constant but force exerted is related to mass
Probably out of my depth, but didn’t Galileo demonstrate that acceleration due to gravity is independent of mass? A heavy ball and a light ball rolled down an inclined plane will both reach the bottom at the same time?Posted 4 years agothisisnotaspoonMember
Probably out of my depth, but didn’t Galileo demonstrate that acceleration due to gravity is independent of mass? A heavy ball and a light ball rolled down an inclined plane will both reach the bottom at the same time?
Yes, but only if air resistance isn’t there, i.e. your bal has to roll very slowly or in a vaccum. Hence why a feather and a bowling ball don’t actualy fall at the same speed.Posted 4 years ago
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