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Downhill descent: Can a bike be too light for a particular person?
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.
If you're new please join in and if you have questions pop them below and the forum regulars will answer as best we can.
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42 comments
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.
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.
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.
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.
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
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
).
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-t...
* 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?
Unroasted are heavier, so they help more with downhills.
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
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.
...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..
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.
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)
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.
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!
.
imgID10565531.jpg.gallery.jpg
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!
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).
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)
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.
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