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OR_biker.
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October 23, 2018 at 8:29 am #29072
bikezero
I presume that if going uphill the lighter the bike, the better, but what about downhill? Is it possible that say a very lightweight rider could find he or she goes faster downhill on a bike that is light vs one that is super light?
Sorry if this is a foolish question, but it’s something I’ve often genuinely wondered.
Like probably many new bike riders out there one of my favourite things is going on long downhill descents where you can feel yourself slowly but surely getting faster and faster reaching speeds that seem like they would infinitely grow the longer the descent is.
When it comes to downhill descending and reaching the highest speeds, is there a sweet spot with a bike and a particular person when it comes to the weight of the bike and the weight of the person?
Thanks.
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OR_biker
Plus, since the OP was
Plus, since the OP was actually just talking about the bike and not the person, it’s even more likely that heavy doesn’t equal slower. Especially since most aero bikes are heavier than super lightweight climbing bikes. But even if you had identical tube shapes/sizes, the heavier bike would likely be faster downhill assuming bearings, wheels/tires and such were also the same.
OR_biker
Canyon48 wrote:Long story short, lightweight = lower drag coefficient = faster.Extra weight can overcome some of the aerodynamic drag force – but don’t forget that it’ll take longer to slow down. Extra weight also comes with increased aerodynamic drag, so it’s no good being heavy.
Umm… nope. You may not have noticed some of the other posts, but yeah… you’re wrong on this one. For one, lightweight doesn’t always equal lower drag coefficient (I’m betting a fit/muscular 85kg rider has less flapping in the wind than a rotund 75kg rider). And what has been mentioned earlier, drag coefficient only increases at a fraction of what the increase of volume is. And yes, heavier objects have more inertia and a higher terminal velocity than lighter objects (though that’s probably not too relevant in this case). But basically, unless your body shape is signicantly larger, being heavier is not in itself going to be a detriment on a descent.
Canyon48
Long story short, lightweight
Long story short, lightweight = lower drag coefficient = faster.
Extra weight can overcome some of the aerodynamic drag force – but don’t forget that it’ll take longer to slow down. Extra weight also comes with increased aerodynamic drag, so it’s no good being heavy.
JohnnyRemo
kil0ran 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.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.
Drinfinity
Red squirrels, being lighter
Red squirrels, being lighter than greys, can reach further out on pine branches. This helps them evade pine martens. Consequently the reds do better in forests where pine martens live.Greys, being heavier, are faster downhill, enabling them to escape urban ground based predators. Which is why you see more greys in towns.
OldRidgeback
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.
madcarew
Canyon48 wrote:Weight is a hindrance on a descent.Though having extra mass will give you extra force to overcome aerodynamic drag, extra body mass comes with extra volume.
The bigger you are, the more drag you have, drag increases with the square of speed – so it’s much better just to be as aerodynamic as possible (i.e. as small and low as possible).
The other big problem is, if you weigh more, it’s harder to slow down.
As covered further up in the thread, extra volume doesn’t come close to outweighing (sorry) the advantage of extra mass. As Dave says, he can have 50% extra mass, but only 10% extra frontal area
hawkinspeter
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?
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.
dave atkinson
Canyon48 wrote:Weight is a hindrance on a descent.Though having extra mass will give you extra force to overcome aerodynamic drag, extra body mass comes with extra volume.
The bigger you are, the more drag you have, drag increases with the square of speed – so it’s much better just to be as aerodynamic as possible (i.e. as small and low as possible).
The other big problem is, if you weigh more, it’s harder to slow down.
It’s better to be aero, for sure. But if I’m 92kg and the chap next to me is 60kg, and we’re both in an aero tuck, my frontal area isn’t going to be half as much again as his, not even close.
Weight is a hindrance on a
Weight is a hindrance on a descent.
Though having extra mass will give you extra force to overcome aerodynamic drag, extra body mass comes with extra volume.
The bigger you are, the more drag you have, drag increases with the square of speed – so it’s much better just to be as aerodynamic as possible (i.e. as small and low as possible).
The other big problem is, if you weigh more, it’s harder to slow down.
hawkinspeter
davel wrote:So… Are unroasted or roasted beans faster?Unroasted are heavier, so they help more with downhills.
davel
hawkinspeter wrote:fukawitribe wrote:hawkinspeter wrote:vonhelmet wrote:Mountain bike tyres will have huge resistance compared to road tyres.Yes, that resistance will be proportional to your speed but the air resistance will be proportional to the square of your speed, so the rolling resistance will only be significant at slow speeds.
True, but the rolling resistance can be significant yet still only proportional to speed – it’s the value of the coefficient that can knacker things. You can easily get 20+W per tyre difference* between decent road tyres and decent, but pretty unsuitable, MTB tyres depending on pressure and speed – Kenda Small Block 8s seem popular but particularly shite in this regard. Tubeless and latex tubes seems to be able to help the MTB tyres significantly that said
https://www.bicyclerollingresistance.com/specials/tubeless-latex-butyl-tubes
* quick look-up at 25-30km/h at lowish pressures.
Yes, absolutely. Tyre pressure also makes a surprisingly large difference to rolling resistance, but again, once you get over a certain speed, it’s the aero effects that dominate (it’ll just be at a slower speed than with optimised tyres/pressure).
I thought I’d test out the whole “rucsack of bricks” on my commute home this evening by carrying a 2Kg bag of unroasted coffee beans (along with my usual work clothes, mini computer, squirrel suit etc). However, I wasn’t very scientific about it and unfortunately I overtook a cyclist and then had him on my rear wheel for a couple of miles. That wasn’t a problem except that I usually try to drop people off my rear wheel and end up knackering myself in the process. I think the extra weight might have helped on the downhills, but I think he might have turned off rather than me losing him. Still, I’ll count that as a success.
So… Are unroasted or roasted beans faster?
PRSboy wrote:Is there a point at which mechanical drag also has a bigger influence… so bearing quality, tyre resistance/pressure etc start to have a proportionally bigger impact as speed increases?For example, there is a descent near us where if I am riding my old faithful MTB I struggle to get anything like the same speeds that I will on my road bike, even crouched low in an ‘aero position’ and in normal roadie clothes.
The bike frontal area can only account for so much drag, and the MTB is around 5 KG heavier than my road bike.
In short, no. The mechanical drag remains roughly constant and continuously decreases as a proportion of the total drag, until at 50kph it is less than 1% of drag. At 70 kph it is less than .4% of drag.
On your MTB assuming knobblies you have considerable wind resistance from the tyres, and just generally less aerodynamics in the bike, and it would be very very difficult to get as aero on a MTB as you can on a road bike
kil0ran
vonhelmet wrote:
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.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.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
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