Aerodynamics and the everyday cyclist

KneesUp

Guru
I set off for work today and remembered quite early on that I'd left my asthma inhaler on the shelf in the kitchen. This is not normally a problem because I don't normally need it, but today I was running very slightly late (because 4 weeks in, I still haven't got the hang of 'school holiday' routine)

My commute is only short (under 3 miles) and about half of it is down one road if you go the quick way* As I waited at the first set of lights, I saw another cyclist join the road, and he seemed quite fast. It was quite windy, so I decided my best bet when the lights changed was to bust a nut catching him and then wheel-suck the rest of way to maximise my getting to work on time/not getting asthma ratio+. Having duly caught up with him (because he got held up by another cyclist - thanks, whoever you are!) I then sat in, and we skittled along quite nicely. I like to think I was far enough away not to bother him, but close enough to get some benefit aerodynamically, and certainly close enough to use him for pacing. All was well for a mile or so in which the road is basically flat, but the last quarter of a mile is downhill and at this point he dropped me completely. It's a straight road, and it's only -2.5% (accroding to bikehike) so it's not some madly technical break-neck alpine pass, but I've noticed this before with other cyclists - they always leave me for dead going downhill, even ones I've wanted to overtake on the flat bit but have been unable to because of traffic.

Is this all aerodynamics, or could it be that my bike needs servicing? It has a slightly noisy rear hub, but otherwise seems ok to me. The chap this morning had a small rucksack to my two panniers, and was in lycra to my jeans and t-shirt, but I can't see that making that much difference, especially given that the weight in the panniers should help me downhill. Has anyone else noticed this? We were both in the hoods, and he didn't seem to be putting much effort into pedalling downhill.


*I always go the quick way in the morning
+I was slightly wheezy when I got to work but otherwise ok. I was 1 min late (but I'm self employed, so I was in at the same time as the boss)
 

Heltor Chasca

Out-riding the Black Dog
What about weight? I am heavy at 73kg. I will leave my lighter comrades behind on descents.

(They soon catch me up on the next hill though)
 

MichaelW2

Veteran
Rolling resistance and aerodynamic drag both contribute. RR is proportional to velocity but AD is proportional to velocity 2. The quicker you ride, the more significant is AD.
RR is mostly tyres ( model, width,tread, pressure). In a badly tuned bike wheel bearings can add to RR. Freewheel plays a minor part.
 
OP
KneesUp

KneesUp

Guru
What about weight? I am heavy at 73kg. I will leave my lighter comrades behind on descents.

(They soon catch me up on the next hill though)
Interestingly I am also 73kg, which is 2kg less than the weight I have been most of my adult life, so I consider that quite light (NHS advice is that I 'should' weigh 60kg to 82kg, so I suppose I'm over the mid-point) The chap this morning looked to be heavier than me, but only had a small rucksack to my two panniers (roughly 10kg between them I'd say) so our weights were probably fairly equal.
 
OP
KneesUp

KneesUp

Guru
Rolling resistance and aerodynamic drag both contribute. RR is proportional to velocity but AD is proportional to velocity 2. The quicker you ride, the more significant is AD.
RR is mostly tyres ( model, width,tread, pressure). In a badly tuned bike wheel bearings can add to RR. Freewheel plays a minor part.
I guess this might be it, then - my wide tyred bike with 36 spoke wheels, it's rack and panniers and lights and reflective bits is less aerodynamic than his thin wheeled bike and his rucksack, and that difference is amplified at a higher speed.
 
I've followed a few heavier riders downhill, normally at a receding distance, pedalling like billy ho to keep up whilst they freewheel away. I gave one chase up at about 30mph and just thought fark it, I'll see him at the bottom.
 

Globalti

Legendary Member
Panniers and jeans & T shirt will not have been helping you at all compared with figure-hugging lycra, even if the other guy's figure wasn't svelte. These will make a much more significant difference than dry wheel bearings. Tyre width will matter a little, as well as pressure. Finally, getting down on the drops earns you a couple of MPH extra speed as well, especially at 20 mph and above. The other bloke's rucsac will have been in the vortex behind his back and might even have helped a little in filling the area of low pressure.
 

ColinJ

Puzzle game developer
It's a straight road, and it's only -2.5% (accroding to bikehike) so it's not some madly technical break-neck alpine pass, but I've noticed this before with other cyclists - they always leave me for dead going downhill, even ones I've wanted to overtake on the flat bit but have been unable to because of traffic.

Is this all aerodynamics...
It will not ALL be down to aerodynamics, but that will play a part. I was out on my singlespeed bike yesterday and coming down a similar road. I don't have a high enough gear to pedal much above 35 km/hr so I freewheel above that speed. With my hands on the hoods my freewheeling speed was 36 km/hr. I got down on the drops and my speed rapidly increased to 40 km/hr. Back up on the hoods, I slowed to 36 km/hr again ...

What about weight? I am heavy at 73kg. I will leave my lighter comrades behind on descents.

(They soon catch me up on the next hill though)
'Heavy' at 73 kgs - ha ha! I used to be 115 kgs, am now 87 kgs and aim to get back down to about 78-79 kgs at which weight I will be fairly slim. It is only 'heavy' for a shortish person; I would be too skinny at that weight!

Weight does make a significant difference downhill though. I lost most of the weight due to illness and was off the bike for 8 months. I noticed the difference on my first descent after starting to cycle again. I was only doing 70 km/hr on downhills that I used to have to brake on to keep my speed below 80 km/hr when fat.
 
OP
KneesUp

KneesUp

Guru
Surprisingly, no one has mentioned fitness yet (unless I missed it).
The other guy might just be far stronger and fitter than you. Happens to me all the time.
I think he probably was - I kept up with him on the flat mile without much effort, but he was taking more wind. However, on the downhill bit neither of us were putting in much effort, as far as I could tell.
 
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