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

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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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Viewing 12 replies - 31 through 42 (of 42 total)
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  • October 23, 2018 at 11:44 am #929955
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    alansmurphy

    These are treemendous

    These are treemendous insights but I need a root and branch review of the data in the absence of graphs, or we could just leaf it!

    October 23, 2018 at 11:41 am #929953
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    hawkinspeter
    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.

     

    October 23, 2018 at 11:37 am #929951
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    StraelGuy
    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?

    October 23, 2018 at 11:32 am #929949
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    alansmurphy

    I think it’s fair to say that

    I think it’s fair to say that as an amateur cyclist the best gains are to lose weight and increase power to get you up the hill quicker, train at descents including cornering, aero position etc. and enjoy yourself. As for the bike, n+1 iinit!

    October 23, 2018 at 11:17 am #929947
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    Mungecrundle

    I used to think think this.
    I used to think think this. You know the experiment: drop a feather and a lump hammer in a vacuum and they will fall at the same rate, reaching the ground at the same time. Do the same in air and you get complications due to air resistance. Do the same on a bicycle on a slope and it gets even more complicated.

    Upshot is that there are plenty of explanations via your search engine of choice as to why a heavier rider will accelerate faster downhill than a lighter rider (with all other variables being controlled).

    October 23, 2018 at 11:14 am #929945
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    hawkinspeter
    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.

     

    October 23, 2018 at 11:14 am #929943
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    jaysa
    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.

    October 23, 2018 at 10:55 am #929941
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    Anonymous
    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.

    This doesn’t seem to play out quite as expected in my experience. My bigger, heavier mate seems to be able to pull away from me on freewheeling descents, despite having more frontal area, wider tyres and being on a flat bar bike. This is bend free as well.

     

    October 23, 2018 at 10:52 am #929939
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    FluffyKittenofTindalos

    Surely this is relevant?

    Surely this is relevant?

     

    https://en.wikipedia.org/wiki/Galileo%27s_Leaning_Tower_of_Pisa_experiment

     

    If you dropped a light bike and a heavy bike (plus light and heavy riders…say Clarkson and Hammond…yeah, definitely make it those two) off the tower of Pisa at the same time, other than differences due to air resistance they would hit the ground at the same time.

     

    (To be sure, should probably repeat the experiment with Eric Pickles and Matthew Parris)

    October 23, 2018 at 10:48 am #929937
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    vonhelmet

    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.
    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.

    October 23, 2018 at 10:44 am #929935
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    kil0ran

    A heavy rider is likely to
    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.

    October 23, 2018 at 9:56 am #929933
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    dave atkinson

    as you go faster aero is much

    as you go faster aero is much, much more important than weight. look at downhill speed record bikes: they’re all industrial components and big fairings. i wouldn’t suggest something like that, but light weight doesn’t help going downhill. my fastest downhill speeds have been on aero road bikes, which I doubt is a coincidence.

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