It sounds obvious that a heavy rider will descend quicker than a light one. After all, gravity is your friend when barrelling downhill... but how much difference does it make? In an attempt to find out, we strapped 8kg to ourselves and set off to do some garage science!
Heavier riders descend quicker right? It's been ingrained in us since secondary school physics lessons. Well, that's what we thought, but look at recent descending masterclasses and it's the light riders often leading the way. Take Tom Pidcock for example, both one of the lightest and fastest descenders in the peloton, and it kind of upsets this theory.
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Does this mean that weight has a far smaller impact on descending than we might have previously thought?
Whilst watching Ed struggle home from the supermarket with 8kg of water, an idea popped into my head. Firstly, I wondered if I could beat him up a hill if he was 8kg heavier, and secondly I wondered if it could make him beat me down one...
To find out once and for all how much difference weight makes when descending, we found a suitable hill and devised a plan for two tests. Test number one would involve timed runs down a steep hill with and without the additional water, and the second included a roll back up the other side of the tip until Ed ground to halt. In the first test the lowest average time would win, and in test two the winner would be the one who travelled the farthest distance.
We decided that strapping the weight to Ed's front would have less of an impact on his aerodynamics than if it was on his back. It also better simulates a larger/heavier rider as this does tend to be where many of us hold that additional weight.
Results, part 1 (the steep descent)
We start off on the timed runs down the steeper of the two descents. According to Strava this had an average gradient of -10% and lasted around 40 seconds.
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To make the results as reliable as possible, Ed wasn't allowed to pedal at any point during the descents. He got a TT-style start to avoid having to push off the line, and took the same line through the one corner each time. He also tried as hard as possible to maintain the same position throughout the testing, and we did it on the stillest day possible to try and avoid any wind gusts that could throw the results.
For the first test, Ed did a total of eight attempts, each three-quarters of a kilometre long, four with the 8kg of water and four without. Overall he was quicker (drum roll, please)... WITH the additional weight; but not by as much as you might think.
That means that the additional 8kg made Ed on average 1.5 seconds faster on our descent. That might not sound like a lot, but it is over 3% difference.
It is worth noting, however, that the weight would have less of an effect on a shallower descent, or indeed if there were more corners where braking and acceleration were required.
Results, part 2 (rolling to a halt)
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For the second test, we set Ed off down the same descent, but instead of stopping at the bottom he carried on rolling up the other side of the dip until he ground to a halt. We then measured the distance he reached each time, again four times with the 8kg of additional weight and four without.
To Ed's surprise, this time he performed better without the water strapped to him, stopping on average two meters further than when bottled up.
Why is this?
There are two main forces acting on a rider and their bike when cycling downhill. As Ed isn't pedalling, any forward acceleration is due to gravity. As you’ll remember from back in school F=ma where:
Force = mass x acceleration
Therefore, when we make Ed heavier the m in the equation gets bigger, so the force pushing him forwards gets bigger too.
However, the second major force to consider is air resistance, which opposes a rider's motion. The faster you go or the less aerodynamic you are the bigger this force gets.
As we strapped the water to Ed's front and given the results, it's unlikely that we changed Ed's drag coefficient significantly. This is why on the timed descent, Ed was quicker when carrying the additional weight.
However, when we then threw in the ascent on the other side it’s clear that the weight has more of a negative impact going up the hill than going down it, otherwise he would have travelled further.
Because air resistance has a non-linear relationship with speed, the additional weight has a far bigger impact when travelling at lower speeds, i.e uphill. Unfortunately, for many of us, this is the area we want to improve and is also where many races (both professional and on the club run) get decided.
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It’s for this reason that nearly all the successful general classification riders look in such depth at improving their power-to-weight ratios, and are seriously lean for target events.
So, does a heavier rider descend quicker? Well, yes, in a straight line at least. However, as we previously discussed your weight pales into insignificance when compared to aerodynamic drag. Therefore, if you want to get faster at descending, our advice is not to hit the all-you-can-eat buffet but rather work on your position and ditch the flappy clothing.
It's also worth noting that any advantage on the descents that you do get from being heavier will have far more of a negative impact when the road heads upwards! If you want a better idea of how much weight can slow you down, when climbing then you can check out our analysis of Tadej Pogacar's fateful climb from last year's Tour De France below.
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Let us know what garage science you’d like us to do next time in the comments section below!
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