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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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  • October 23, 2018 at 6:39 pm #929985
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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.

    I’m 97kg, not as much muscle to fat ratio as I’d like but looking at me you wouldn’t say I look my weight, so I’m not giving a massively larger frontal area and you have to take into account different clothing and actual body position too.

    I’ve always outsripped ‘smaller’ and lighter riders on the downhill when freewheeling, managed this on a flat bar hybrid with 25mm gatorskins, the difference is always noticeable though I can get a decent aero position on it when needs be.

    Managed 46mph coming down the A505 from Luton to Hitchin (about 7-8%) without much bother, again that was on a flat bar.

    As for whether a bike can be too light, nope, handling ability of the bike, your familiarity with it plus your confidence to descend quickly and importantly picking your line/knowing when to sit up/brake are massively important factors when it comes to making progress.

    October 23, 2018 at 6:27 pm #929983
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    hawkinspeter
    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.

    October 23, 2018 at 5:24 pm #929981
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    fukawitribe
    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.

    October 23, 2018 at 2:53 pm #929979
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    StraelGuy

    That’s a similar principle to

    That’s a similar principle to ballistics. If you double the weight of a bullet for a given velocity, you double the energy carried by the bullet. However, if you double the velocity of a bullet for a given bullet weight, you roughly quadruple the energy it carries (sorry, used to be an FAC holder ).

    October 23, 2018 at 2:47 pm #929977
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    hawkinspeter
    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.

    October 23, 2018 at 2:44 pm #929975
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    vonhelmet

    Mountain bike tyres will have

    Mountain bike tyres will have huge resistance compared to road tyres.

    October 23, 2018 at 2:41 pm #929973
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    PRSboy

    Is there a point at which

    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.

     

    October 23, 2018 at 2:24 pm #929971
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    JMcL_Ireland
    StraelGuy wrote:
    So the upshot of this long and quite interesting debate is that if you want to improve your downhill speed, the solution is basically an aero road bike and a rucksack full of half-bricks ?

    Can’t find a reference at the moment, but I remember reading that was Sean Kelly’s training solution to going uphill faster – up this hill in particular https://www.strava.com/segments/623748 (on a 70s era bike with no doubt a horrendous gear ratio)

    October 23, 2018 at 1:08 pm #929969
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    vonhelmet

    Thanks all. I’d not thought

    Thanks all. I’d not thought about the relative impact of air resistance compared to weight, as I was merrily modelling the cyclists as point masses. Damn you A-level physics.

    October 23, 2018 at 1:07 pm #929967
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    bikezero

    …Well, I will spend a few
    …Well, I will spend a few days getting to know the bike first, but following that i won’t be able to wait!

    One thing I did notice with the Triban over my long time riding it is that it seemed to get faster the more I cycled it. I presume that is down to getting to know the bike, subconsciously learning to position myself better and perhaps stronger power in my legs..

    October 23, 2018 at 12:51 pm #929965
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    bikezero

    Thanks for all the
    Thanks for all the interesting, informative (and sometimes humourous!) replies (enjoyed the laughs).

    For me, losing weight or gaining it for that matter is not really an option. I don’t know if I have an underlying health problem of some kind but in my adult life (and I am pushing 40 years old) my weight has never fluctuated more than the tiniest tiniest bit however much I eat or excercise. I am 6’1 and skinny (not super skinny but you could probably say very skinny) have weighed pretty much exactly the same as I did at age 20 as I do today. “I have a superfast metabolism” I suppose is the comforting way of looking at it, though I don’t know if that’s the real reason.

    Anyhow, I have a cheap Btwin Triban 500 bike…the weight is probably about 10.2kg (I made a couple of small modifications to drop a tiny bit of weight from the bike in stock form).
    It’s easy to notice that on flat stretches the bike with me on it isn’t so fast. Cyclist fly by be constantly. However on long downhill descents it seems insanely fast.

    It is going to be interesting next week when I get my recently acquired somewhat lighter Btwin Ultra AF frame fitted with the Triban wheels and new components groupset (105) I have coming. I’m am going to take the bike (once I am sure it is safely set up) on these particular very long steep downhill descents I know of and see if I now seem to go even faster.

    October 23, 2018 at 12:43 pm #929963
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    StraelGuy

    So the upshot of this long

    So the upshot of this long and quite interesting debate is that if you want to improve your downhill speed, the solution is basically an aero road bike and a rucksack full of half-bricks ?

    October 23, 2018 at 12:04 pm #929961
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    hawkinspeter
    alansmurphy wrote:
    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 12:02 pm #929959
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    Griff500

    Back to the OP,which was

    Back to the OP,which was about bike weight, not rider weight, the answer is no. Whilst you could argue, as some have here, that a heavier bike might be marginally faster downhill, and the handling marginally worse due to the higher combined c of g, these effects are trivial.My alloy bike is 3kg heavier than my carbon bike, which makes it 40% heavier, but when all up weight is considered (circa 77kg versus 80kg), the bike only makes a 4.5% difference. That 4.5% equals an extra half a gear uphill where weight matters, but makes sfa difference downhill where drag is dominant.

    October 23, 2018 at 11:49 am #929957
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    madcarew
    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.

    “Other things being equal”… they aren’t, the mass is greater.

     A rider is (kind of) a sphere, so their volume (roughly equivalent to mass) increases as a 3rd power relative to a given dimension, (4/3*pi *r^3) whereas their surface area (think frontal area ) increases relative to the same dimension squared (4*pi *r^2), so a 100kg person all things being equal will only have 30% more surface area than a 70kg person, but 42% more mass. (the difference is accentuated when they both get into an aerodynamic tuck, the larger person’s frontal area is less than 10% greater than the smaller person, but they have 40 % greater ‘motive force’) So their gravitational potential energy (which is converting to kinetic energy) is greater  and will equate to greater speed as they are both losing the same height.

    Fluffy kitten’s tower of Pisa experiment assumes that air resistance is minimal, which it clearly isn’t in the case of a cyclist. So your heavier cyclist will both accelerate faster, and attain a higher final speed. So, if you want to go fast(er) downhill, ride a heavy, aero, bike.

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