Sy Reene

really? average speed = distance/time. I do suppose if a rider starts at the back of the peloton and stays there, he'll lose because the clock starts well before he gets to the starting line.

rubiksoval

As computed by the race clock, yes, a fact.

As computed by a rider's clock who takes a free lap or a wheel or whatever, then...eh.

Only the former actually matters.

wphamilton

Because, as was pointed out to me by a racing type when I opined something similar, sometimes people who take the shortest line in corners win while someone close behind them is swinging wide. The guy swinging wide is going further and it *might* be enough to have a higher average speed. I'm skeptical that there's enough difference in path to make that happen, but not being a "racing type" myself I have to take their word for it.

Sy Reene

Yeah, I guess variances like this exist. I just find it curious though that there's a somewhat non-unique mention from time to to time, that average speed is a worthless metric.

canklecat

GPS sampling and interpolation error. Very common.

And apps can vary significantly in interpolation. When I've run Strava, Wahoo Fitness, Cyclemeter and others, all showed differences. Not enough to matter much to me, but enough to snag a new PR or miss it by a hair.

I usually record with Wahoo Fitness but occasionally run Strava simultaneously on the same or another phone. They'll be very close in overall speed, time and distance, and on most segments. But occasionally there will be significant discrepancies.

A few weeks ago Strava showed my time on the final sprint to be the fastest, and the ride-by feature on the website showed me in the lead. Far from accurate. I was middle of the pack. The difference was only a few yards, but that's enough to show the relative inaccuracy of GPS.

Another factor is the individual rider's approach to a segment. In groups often one or two riders will try for a new PR or KOM. Sometimes when they take off I'll follow. If I close the gap Strava will show my time as faster than theirs, even though I was behind. That's because technically my time was faster because I was closing the gap, even though I sprinted late.

Same with climbs. I'm not really any stronger than I was a couple of years ago but I've dropped 15 lbs so my climbing has improved a bit. I tend to lurk in the back of the pack on groups when I'm not familiar with some riders. I'll often pass a few folks along the climb and close the gap on the riders ahead. So occasionally Strava shows my time as significantly faster due to GPS sampling and interpolation error, when my speed was probably only 1/2 to 1 mph faster.

On Sunday morning's group ride Strava logs showed anywhere from 15.5 mph to 17.8 mph average over 33 miles, even though the group wasn't separated by much.

I used to run wired and wireless sensors to check against GPS data but haven't bothered for a few months. I'm mostly interested in my fitness trends, not so much in short segments along a ride.

colnago62

I have heard about this before. 😂😂

colnago62

Or gets dropped on a climb then descends back into the pack but gets beat in the sprint.

Sy Reene

Then he's covering the same overall distance but took more overall time to do so, so his average speed was less and he lost. It's a fairly simple notion.

Leinster

If he gets dropped on a climb, then no. Because he’ll always be moving, so his higher average on the downhill will be balanced by his lower average on the uphill.

If he gets a flat on the climb, stops to change a wheel, then chases back on and loses the sprint, then yes, his average moving speed in his ride data will be faster than the winner (presuming the winner didn’t also have a mechanical during the race).


As pointed out above, though, the only true metric for average speed in a race is Official Race Distance (as determined by the organizers) divided by Official Race Time (as determined by race clock and timekeeping officials). And the highest average speed by this analysis is always the winner. No point showing up after the race and telling the officials “but my Elemnt Bolt says I was 0.2km/hr faster than the guy who crossed the line first!”

frankenmike 

Agree. If I go 20 miles in an hour, I’ve averaged 20, but my computer always reads lower, so it is horribly inaccurate, and therefore worthless.

bruce19

I have a car which I have compared my speedo to those roadside radar readouts. It's dead on. So, I think 40 mph is actually 40 mph. The other day I went out in the car and took my Garmin Edge 200 (about 4 years old). When the car was at 40 mph the Garmin was reading around 38.5 mph. Not exactly sure if this means anything.

Jon T

Are they wireless or hard-wired units? If wireless they're "stepping" on each other and fouling up the readings.
Jon

merlinextraligh

Worthless is an overstatement. The point, that bears repeating, particularly for newer riders is that average speed, for most purposes, is not a very useful metric.

More importantly, focusing just on average speed is counterproductive to actually getting faster.

Doing intervals will result in lower average speed than just going out and trying to ride consistently hard. But the former approach makes you faster, and a steady diet of the later leads to a quick plateau, and you being slower than you could be.

eduskator

Speed = distance / time. They have different distances and times recorded on their devices. Don't overthink this one it's pure physics!

caloso

Truth. The fastest riders I know don't give a fig about average speed, except maybe TT'ers where average speed is literally the metric that matters. And they don't even talk about it in training or riding around.

Sy Reene

Though while riding as a group, it's speed that the group is using to set a pace and to stay together. It's also usually speed that sets the group levels for clubs. For a group to team to stay together and trading places at the lead, they're not basing this on cadence, HR or watts/kg, or are they?

Leinster

Anticipated average speed is useful information for planning a ride and letting prospective riders know what kind of group they might be joining. Achieved average speed is a relatively unimportant data point for post-ride analysis.

WhyFi

Stated group rides speeds are a result, not a target; they're expectations based on previous rides.

caloso

Cadence and HR are personal, wattage depends on whether you're on the front or sitting in. If I'm on a group ride and it's announced that we all stay together, then it'll be "everyone keep it to a nice steady 20-21" or something like that. But I think that's understood to be a target cruising speed, not average speed.

And then there are the group rides where the average speed is determined by who shows up and the wind speed and direction. I'm thinking of my local Saturday morning world championship. The average speed of the entire ride might be 21, but if you can't hold a wheel at 28 you're probably going to get dropped.

woodcraft

What is this "walking the bike" that you speak of?

GlennR

I sometimes take a break and go into a rather large supermarket to use the bathroom, refill my water bottle and buy a banana. Since I don't carry a lock, I bring the bike in.

woodcraft

Oh, I see. You might not be wanted back if you rode to that bathroom...

Cypress

There can also be a difference between someone running an actual speed sensor mounted to the bike vs just using a GPS headunit for speed.

I'll address the average speed thing, which I wish would just die off already.

I plucked three solo rides from my data that show three different average speeds in increasing order. A newer rider might think 15.8 to be easy, 19.5 would be iffy and 22.2 to be out of their reach. The thing is, the normalized power data is almost identical for two of the rides (261W/260W), but the ride with the lowest NP (245W) was the hardest. I don't mean to toot my own horn, but a newer rider wouldn't have been able to keep up on any of these rides, especially the 15.8 mph average.





If someone rode the same route every day for a year they'd notice that on the same route with the same windspeed, a headwind on the wrong sections can pull 1-2 mph off of an average. Even the difference in air density due to temperature can knock 0.5 mph off of an average. When I get asked "what's your average?" when someone is considering riding with me I'll just say "I can go whatever speed you want. Let's ride."

caloso

AA? Is that on the road to happy destiny?

autonomy

I'm assuming you're talking about GPS-based units. You can't really compare those with magnet-based wheel units.

Haven't seen anyone mention this, but do you know the GPS sampling rate and accuracy? Not everybody is going to be sampling at 1Hz. As an exaggerated example, think about going on a road with lots of switchbacks but taking a measurement every minute except every second. Your reported path will look very different.
Not everyone's computed positions will be close to one another - at every measurement your reported position will be within a circle with a ~4m radius (in the best case!), centered at your true position. Of course, it depends on how devices process the data, but simply speaking, at each measurement you accumulate and propagate the local error into your total distance. Also, what kinds of GPS units are people using? Smartphone, smartwatch, cycling computer? Are smartphones using any sort of cell-tower correction? GLONASS/other satellites? Different antennas in all these devices... see, lots of variables.

You should also compare vertical displacement (feet climbed) - I think you'll find that this is even more inconsistent on group rides. GPS is worse at altitude than lat/lon estimates.