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I typically do ~100km rides (almost always alone). In summer, I manage average speeds of 30km/h or more. This is the first (of only three in total anyhow) winter in which I keep riding somewhat regularly in winter.

I did > 1000 km per month in the summer months, and around 700km (some of which indoors) in the winter months (but extra running etc.). So I would expect my form and fitness to be somewhat lower, but not dramatically so. For instance, average watts on the indoor trainer as well as times when going running are fairly constant. (My PM also shows fairly constant wattage on the bike.)

Yet, average speeds are more like 27-28 km/h at the moment.

(In my case, although I hope the question could be of more general interest, I also ride a new bike which I had hoped would be faster although it is an endurance bike, simply because it is more high level in terms of the frame than my previous fairly entry level alu one.)

My question: Is that an expected decrease in speed? If so, is it possible to break down this difference into different components (next to, possibly, less fitness)? I think of

  • typically more windy conditions, which hurts average speed
  • more layers of and less aerodynamic clothes
  • wider tyres (28mm insted of 25mm for me, also lower quality ones; also, the endurance bike maybe has somewhat less agressive geometry)
  • more rolling resistance on wetter surfaces
  • more careful riding style on wet surfaces (surely hard to quantify, but definitely applies to me)
  • I heard something about colder air implying more resistance
  • more energy needed to keep the body warm (although I rather overdress, I think)
  • psychology?
  • anything else?
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    When cycling, the body does not need to expend heat to keep warm. It simply uses the waste heat from the muscles and distributes it throughout the body with the blood stream. To estimate how over/underdressed you are, look at how you feel after 10 kilometers: If you are still shivering, you are seriously underdressed, if you feel hot, you are overdressed. The goal should be to feel positively comfortable or slightly warm at the 10 kilometer mark and after. Feb 6 at 16:08
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    @Chris, almost only daylight, but indeed, darkness is yet another factor! Feb 6 at 16:10
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    @Criggie, of coure a good tailwind helps, but on balance, a ride in windy conditions (say, 50km with 25 km/h headwind and 50km with 25 km/h tailwind) will be slower than one with 100km without wind, no? Simply because the parts of the ride with headwind that you do riding more slowly will take longer than those in which you ride more quickly benefiting from tailwind, so that the former contributes more to average speed, which will hence be lower overall. Feb 7 at 10:13
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    @ChristophHanck sure - but some people commute or ride for transport, and don't return to the start in the one ride. I even had one glorious ride where the wind turned around at the far end, and I got a tailwind both ways on the same road. Normally is the opposite :)
    – Criggie
    Feb 7 at 10:38
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    As an aside - for a whole host of reasons, some of which you mentioned, it's often best to stop measuring rides in terms of average speed, especially if you have a power meter, and even moreso if you're planning on building fitness from one year to the next.
    – Altom
    Feb 8 at 15:44
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This source goes into quite a bite of detail on the subject.

The three main reasons they go into are higher air resistance in the cold because cold air is more dense. Increased rolling resistance as cold tires cannot deform as much as warm ones. They also mention the bulkier clothing causing more air resistance.

One advantage of colder weather is that you are less likely to overheat. This can increase performance. I have felt this effect of being able to maintain my efforts longer on cool fall days, while cycling on my trainer is often more difficult as there is no air flow to carry the heat away. Also, cycling on hot summer days can also be exhausting.

Just be careful not to overdress for the cold as having too many layers could also make you overheat. It's ok to start off cold as you will warm up during the ride.

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    If the start is downhill or stop-start, it's easy to be cold enough to shiver early on (wasting energy), and take a while to warm up, but still to overheat later on. Descents later on can have the same effect
    – Chris H
    Feb 6 at 16:11
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I think you’ve listed all the possible reasons. Though I have one more detail regarding the body and cold: From personal experience it at least feels like blood flow is worse when you are cold.

You’d need a power meter to find out if it’s actually your power output which is lower or if it’s your equipment, road conditions etc.

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I've noticed the same, and largely blame the wind and tyres (same bike, but I've changed my tyres for ones that better handle mud on the road).

However there are 2 closely related factors that may play a part: nutrition and hydration.

Nutrition first: You're quicker than me, and I don't stop to eat so I assume you don't either. In winter gloves it's harder to get out and open packets while riding. This is even true for opening gels with one hand and teeth. The result is that delaying a feed until a good bit of road is more likely (compounded by puddles, fallen leaves, darkness etc. making some bits of road worse). I may end up having half an energy bar because I stuff it back in my pocket/toptube bag to free up my hands when I see a poor surface coming up, then don't go back to it. In fingerless gloves I'd be more likely to hold the snack on the bars.

Hydration: I always drink as much as I want and riding conditions allow, and the latter may be affected by winter conditions similarly to food. But to some extent I feel like cold suppresses thirst, or at least counteracts it. This may be because of vasoconstriction leading to a full bladder, but still needing to drink, or it may be because consuming large quantities of cold water when I'm only just warm enough lacks appeal, so I have smaller drinks. Much of your water intake is needed for digestion, and the hills are still sweat-inducing on the way up.

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    These are indeed two additional relevant points - although I often use gloves which are so clumsy (or I am) that I do have to stop for eating. This does not affect your argument, though: stopping in cold temperatures and cooling down is no pleasant thought, so that I probably eat too little. Feb 8 at 7:00
  • I assume you're quoting moving speed; I always think in terms of door-to-door speed. Even with moving speed, if you're stopping to eat, you lose a little time to slowing down and speeding up (especially if you stop in a little valley so the climb out warms you, but you can't carry speed into the hill). You may also go a bit slower looking out for somewhere to stop, perhaps on the hoods looking around, instead of on the drops.
    – Chris H
    Feb 8 at 9:46
  • I'd also like to explore vasoconstriction more, but that will take some reading
    – Chris H
    Feb 8 at 9:46
  • In the cold, you should still aim to drink a normal amount, even if you don’t feel thirsty.
    – MaplePanda
    Feb 8 at 19:40
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    @MaplePanda OK, but define normal. On 100km+ rides normal for me is anything from just under 10ml/km fairly cold to 60ml/km hot. Drinking to thirst can work well, but if there are competing urges or distractions it's easy to get behind. On longer rides in normal times stopping for hot drinks helps.
    – Chris H
    Feb 8 at 20:10
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I have created a simulator for cyclist speed. Some results from that:

  • Continental Grand Prix 5000, the best overall tire, gives 28.5 km/h speed whereas Continental UltraGatorSkin gives only 26.3 km/h, at the same rider effort. Thus, variations in performance of slick tires alone can account for 2.2 km/h. If one of the tires has a tread pattern, the difference would be larger.

  • Air density has a smaller effect. At 20 degrees Celsius, performance is 28.5 km/h whereas at 5 degrees Celsius, performance is 28.0 km/h.

Wind and less aerodynamic clothes could also explain some of the difference, but I think if you are using a different endurance frame geometry for winter, the difference from the frame geometry alone would be large.

Try sometime using your summer bike in the winter, with the summer tires and more aggressive frame geometry. As long as there is no snow or ice on the roads, it's perfectly safe and would tell you whether the difference is in clothes/wind or in the bike.

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  • how reliable is your simulator? (is it peer-reviewed? is it open source?)
    – njzk2
    Feb 7 at 19:25
  • "As long as there is no snow on the roads" ice is far worse than snow on any normal tyres, and much harder to spot
    – Chris H
    Feb 7 at 20:56
  • Now it is open source: bicycles.stackexchange.com/questions/74952/… -- the physical equations it is based on have been peer-reviewed probably centuries ago.
    – juhist
    Feb 8 at 14:45
  • @ChrisH Thanks for the improvement suggestion! Usually at the time of year ice is prevalent, snow is prevalent too, but I have now modified the answer to explicitly specify ice. In theory it might be possible to have ice but no snow.
    – juhist
    Feb 8 at 14:46
  • @juhist that's common here - when winter is wet enough for precipitation, it's rarely below freezing; when below freezing it's normally dry but residual water can freeze or rain can fall on frozen surfaces. My latitude is similar to the OP's, but closer to the sea (and Gulf Stream) and slightly lower, so this mild/wet vs. cold/dry winter pattern may be more marked for me than for the OP (or you)
    – Chris H
    Feb 8 at 15:25

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