Running on empty?

[yashicamat]

Given my new super bright front lights, I decided to go for a spirited (but short) cycle this evening around the periphery of the town.

The lights didn't slow me down, so apart from the rather strong wind (which being a circular route, helped me in one direction of course), the hills (ditto to the advantage / disadvantages but I didn't go into the Peak District so avoided the big ones) and the odd set of traffic lights, I had a pretty good run. Or so I thought. I reset the trip meter before I left and over the 9 miles, I only averaged 14.2 mph. Now I'm not obcessed with averages or anything (although this post might suggest otherwise), but that does seem exceedingly slow.

My only thought was that I hadn't eaten anything for the previous 6 hours (although I was well hydrated), but over that distance I would have thought that the glycogen naturally stored would be sufficient?

Or am I totally misunderstanding the concept of glycogen stores.

This was cycling on my Surly LHT, so not a lean racing machine but it's not *that* slow either.

Cheers

Rob

 

[Tollers]

14.2 ave doesn't sound too slow to me (depending on hills/wind). Did you feel weak or generally feel you were doing well?

6 hours without eating should be fine. 9 miles wouldnt impact your glycogen stores that much. As a general rule, i make sure i have carbs on me if i do over 30 miles.

Also worth noting that glycogen stores depends on your overall recent diet and activity, not just your last meal.

Do you do same ride often? what do you usually average?

Tollers

 

[yashicamat]

14.2 ave doesn't sound too slow to me (depending on hills/wind). Did you feel weak or generally feel you were doing well?

6 hours without eating should be fine. 9 miles wouldnt impact your glycogen stores that much. As a general rule, i make sure i have carbs on me if i do over 30 miles.

Also worth noting that glycogen stores depends on your overall recent diet and activity, not just your last meal.

Do you do same ride often? what do you usually average?

Tollers

I felt like I was doing reasonably well, but that said, I felt like I was going much faster than I was, must be an illusion at night.

I very rarely do exactly the same ride, so it's difficult to say, but I've done a not entirely different ride in the past (the same except some of the distance was on flat-ish lanes and avoids one of the hills I did yesterday) and averaged 16.8mph on that.

 

[WimbledonCyclist]

I've done a not entirely different ride in the past (the same except some of the distance was on flat-ish lanes and avoids one of the hills I did yesterday) and averaged 16.8mph on that.

Ah, the explanation may be in the wonders of "averaging" for speed. "Average speed" is effectively a harmonic average, not an arithmetic average. Simple example: cycle 10 miles continuously. The first 5 miles at 10mph, the second half at 20mph. Now do your calculations and discover that your average speed over 10 miles was not 15mph (the arithmetic average), but, surprising to many who do such a calculation for the first time, it was only 13.3mph (the harmonic average).

The "issue" is that many people have an underlying assumption that you can use the arithmetic average to calculate average speed. But it doesn't work that way - if you cycle any given distance of your trip at a significantly slower pace, it will drag your average speed (seemingly) disproportionately down. Seems the hill explains all.

 

[Norm]

Ah, the explanation may be in the wonders of "averaging" for speed. "Average speed" is effectively a harmonic average, not an arithmetic average. Simple example: cycle 10 miles continuously. The first 5 miles at 10mph, the second half at 20mph. Now do your calculations and discover that your average speed over 10 miles was not 15mph (the arithmetic average), but, surprising to many who do such a calculation for the first time, it was only 13.3mph (the harmonic average).

If you base your "averaging" calculations on distance, that is indeed the case. Base it on time, though, and it works.

10 minutes at 10mph covers 6.7 miles, 10 minutes at 20mph covers 13.3 miles. Add them together on a ride and you have a 20 minute ride at 15mph.

I'm sure you already knew that, WC, just writing it to salve the confused minds who, like me, probably needed to take their socks off to figure out why the arithmetic and harmonic averages are different. The clue is in the initials "mph", the simple averaging only works, I understand, if you do the 50:50 trick on the denominator (time) rather than the numerator (distance).

 

[WimbledonCyclist]

Norm - spot on, it's the time factor that's the clue. The logic behind average speeds and their potential deceptiveness becomes even clearer when you use extreme numeric examples.

Imagine that there is a hill in yashicamat's 9-mile ride that is so steep, that speed during that stretch goes down to 1mph. If that hill is a mile long, then the ride will always take more than a hour (compare that to the 32-odd minutes that it takes to cycle 9 miles if your average speed is 16.8mph, as per yashicamat's "reference ride"). Even if the rest of the ride is at the speed of light, the average speed for the ride will fail to reflect that incredible speed - simply because the ride must be over 1 hour, average speed can never be above 9mph.

In day-to-day cycling things are not that extreme, but the underlying reasoning remains the same: cycling slowly for a longish part of the ride will add so much time to your ride that you'd have to cycle at pretty fast speeds for the rest of the ride to get anywhere near the speed you'd get if you kept speeds up throughout the trip.

Hence the discrepancy between the average speed and yashicamat's recollection of it all: "I felt like I was going much faster than I was, must be an illusion at night." Speeds on the flat would have been pretty fast, but the low pace on the hill added so much time that the average speed dropped big time.

 

[jimboalee]

When I ride an Audax DIY 100 and I wish to stop at the first control ( 25km ) to achieve an average of 20kmh, what speed should I ride at?

 

[Norm]

In day-to-day cycling things are not that extreme

Indeed not. I can guarantee that I am slower than the speed of light by a number very close to 299,792,458 m/s.

When I ride an Audax DIY 100 and I wish to stop at the first control ( 25km ) to achieve an average of 20kmh, what speed should I ride at?

Whatever speed gets you there in 75 minutes.

 

[WimbledonCyclist]

When I ride an Audax DIY 100 and I wish to stop at the first control ( 25km ) to achieve an average of 20kmh, what speed should I ride at?

I'll take the bite. You probably think it's 20 km/h, but if you want a scientific as well as an impressive physical challenge, assume the ride begins with a 2km-stretch of hill, and you cycle uphill at 1km/h. By now, you've been on the bike 45 minutes too long to reach the 25km-point with the 20 km/h target speed. Or so it seems. Can you complete the remaining 23 km at the truly impressive speed of -30.7 km/h? When did you last try your reverse gear?

 

[yashicamat]

Cheers WimbledonCyclist and Norm. Interesting stuff, although I was already aware of the differences on calculating average speeds using known speeds and distances; although I used the cycle computer to tell me my average, I measured the distance on the map and divided it by the time it took to verify and got the same average speed result (i.e., working out the overall average speed).

One thing that would be interesting (and I don't know if anyone has done this) would be to know the overall payoff of a hill.

E.g.

A hill of 50m height on an otherwise flat road. The road rises over a 500m to length to this 50m spot height, then descends at the same rate. Assuming there are no controlled losses (e.g., braking), I wonder how much energy would be lost compared to cycling the same distance on the flat?
The only major thorn I can see is air resistance, as I seem to recall that in order to increase speed by 10%, something in the order of an increase in effort by 30% is required. Effort could be looked at (ish) in the case of this hill as gravitational potential energy and equated to normal power output on the flat, so the energy stored as GPE during the climb is virtually all available (i.e., the "effort" isn't lost at the top of the hill), it's just that by increasing the speed some energy is lost as air resistance, so the overall average downhill speed isn't as high as it theoretically would be, hence overall average drops . . .

Does that make any sense? Often pondered this thought . . .

 

[Norm]

Does that make any sense? Often pondered this thought . . .

I think the theory is sound.

The main thing that renders the theory useless in practice is that the roads I ride on have a junction at the bottom of every hill. The long downhill on the A355 from Beaconsfield to Amersham, for instance, is about 2/3 of a mile dead straight and 1:10 gradient, it would be lovely but for the roundabouts at the bottom. Every major downhill on my commute would be the same.

 

[jimboalee]

A challenge...

A cyclist knows he can ride round a weather protected cycle track 25km in 50 minutes.

How long will it take that cyclist to climb the 500m long, 10% hill?

 

[montage]

A challenge...

A cyclist knows he can ride round a weather protected cycle track 25km in 50 minutes.

How long will it take that cyclist to climb the 500m long, 10% hill?

30 minutes....but how long was the cake stop half way up that hill?




Once you start going vertical, Gravity comes out to play, which means the mass of the cyclist needs to be known.

 

[Bill Gates]

..... over the 9 miles, I only averaged 14.2 mph. Now I'm not obcessed with averages or anything (although this post might suggest otherwise), but that does seem exceedingly slow.

Rob

My maths makes the ride 38 minutes duration. That's a short ride by most people's standards. Now if you take into account that your body won't be able to get going properly until you've warmed up then you would need to allow 10 minutes or so to be at optimum capacity to ride hard.

That's why looking at averages is misleading.

 

[lukesdad]

especially averge speed.