Jennifer, Chicago asked:
My bicycling club has been having a debate. All other things being equal, who goes downhill faster? A fat bicyclist or a skinny bicyclist?
This question was answered by Dr Jos Darling, Dept Engineering, University of Bath.
Thatís a tricky one. Itís been thought about for a long time really because Aristotle was the first bloke that thought about object falling due to gravity. At that time he decided that heavy object fell more quickly than light objects. Later on people like Newton decided that with gravity objects fall at the same rate. Strictly speaking thatís only true if youíre in a vacuum. On a bike youíre far from it. The big issue with the bike is the aerodynamic drag.
If there were no aerodynamic drag then the fat person or a thin person would end up accelerating down a hill at the same rate. The point is that with the fat person, assuming that theyíre not incredible wide, the aerodynamic drag is going to be less significant in terms of their falling down the hill than for the thin person. Ultimately, a thin personís going to end up going slower than a fat person. So if youíre in a race then you want to minimise that aerodynamic drag and of course the downside to being fat is that thereís always going to be a hill on the other side of the downhill, meaning that youíve got to put a lot of work in to get up the other side. Thereís always a catch!
The problem here is the assumption that fat is the same as heavy.
The maximum speed freewheeling downhill is limited by the air resistance.
So the skinny but more muscular person could be about the same weight as the fat person, in which case the greater drag on the latter should make the difference..... opus, Sat, 19th Jan 2008
In a practical test while out cycling with my daughter we stopped on a slope of about 25 degrees then released our brakes at the same time and over a distance of about 200 metres without pedaling I was about 20 metres ahead, I am 12 stone my daughter is 9 stone. Although we are not fat I think that proves that weight wins. remlapwc, Sun, 20th Jan 2008
But were your bikes the same?
I don't think the difference in density is enough for making the skinny win. lightarrow, Mon, 21st Jan 2008
Really? Shame- go skinnies go! opus, Mon, 21st Jan 2008
i have an addition to this - me and a friend used to go rollerblading down this hill near my house, we both had the same rollerblade (Bauer FX1's incase you're interested) and both used to use pledge on them to keep the friction to a minimum...starting at the top we would just freefall down the hill, now, she was around 11 stone, i was around 6and a half, and every time she would hit the bottom first (unless i cheated and pushed myself forward when she wasnt looking)
In 2002 I cycled through Tasmania, and this was the subject of many discussions...
Generally Speaking in the ideal world mass does not have any bearing only gravity and the angle of the slope... in the real world there are other factors as well as gravity....
As a regular cyclist, I can tell you that the tyre-pressure (rolling resistance) has a significant effect on the speed at which I can cycle (higher pressure = faster). Wind resistance is also a huge factor, with cycling being notably harder in a winter jacket and baggy trousers than in summer shorts and T-shirt. Cycling into or away from the wind makes a big difference.
I have a small amount of experimental evidence to add to this discussion. I am a 60-pounds overweight cyclist. When I ride with others of similar fitness and experience, the lighter people pass me easily on the uphills and I zoom past them on the downhills. The only way I can keep up in a fit group of riders is to pass others on the downhills, then they catch up on the uphills etc.
If the cyclist is heavy enough friction within the bike can increase (pressing down on the tires flattens them out and increases drag on the ground)
If you have a short fat cyclist and tall thin one of the same weight, there will be no difference from the tyres as they have the same weight pressing on them (ignoring aerodynamic lift if the shapes of the riders generate different amounts - you'd have to select ones with the same component of lift). The tall thin cyclist should win out every time (unless you add other aspects of shaping to slow the thin one down).
Are there any mathematical calculations that could be used to prove what you have stated in your article? Divij Gala, Mon, 9th Feb 2015
I am an engineer. Sosjay is correct. Air friction will not slow the heavier ride as rapidly as the light rider. Both have the same acceleration due to gravity, g, but the more massive person has a greater force F = mass X g. This greater force is significant in overcoming frictional resistance (counter force) from air. Aerodynamics can help reduce friction but mass will always trump aerodynamics on down hill slopes where air resistance persists (notwithstanding terminal velocity etc.). Rob, Tue, 29th Dec 2015