UKC

I set my bike computer up carefully following the presets for my wheel/tyre size. So on getting home from my epic (for me at least) solo ride on Saturday evening I noticed that whilst my bike computer said 190 kms and change, endomondo on my phone had logged the ride as 185.32 kms. Are the US government trying to belittle my cycling efforts or should I trust the GPS tracking on my phone over the bike computer?

I know percentage wise it's not a big difference, but these things matter matter on a personal level!

In reply to TobyA: As much as I'd like to blame the American infidel I'd guess the margin for error over that distance for a bike computer would equate to that sort of percentage. The squash in a loaded tyre should affect the effective diameter so it would over-read. Good distance yours, I did a 100m TT on Sunday which was quite epic

In reply to woolsack: That's sort of what I was thinking, actually looking at the GPS track you could see things like where I went 10 metres past a turning and had to back track, so I figured it couldn't be shaving corners off or something. Actually I pumped my tires up to almost the max at the start and the first 50 kms or so I got that funny tingling in my hands from vibration, but that had gone by the end and I guess the tyres soften up a little over a long ride so the tyre squash must be come into play.

100 mile time trial sounds hellish. I actually got off my bike I think four times and sat down for a little break. Just being on your bike for that long is hardcore. I guess with a TT your are riding on your own?

In reply to TobyA: H has a few times logged runs recorded through the gps on her phone, and when she looked at the results afterward noticed that it quite severely cut some corners. I guess the issue is sampling rate - how often it logs a position record. If the course is straight and open it may not make much difference, but errors could add up on twisty routes.

You could have a look at the route plot to see if this is likely to be a factor.

In reply to TobyA:
Also I'm not sure how well Endo deals with going up and down hills. You will lose distance if it measures hills as less steep than they are. You have to imagine the hill as the hypotenuse on a right angle triangle. As you flatten out the hypotenuse you'll lose distance. Just my guess though

In reply to Wide_Mouth_Frog: It records your altitude and will give you a profile at the end, so I imagine that is taken into account?

In reply to TobyA: I had to get off the bike once to fix a puncture. Once everything went numb it wasn't so bad! There were a load of fast guy that started behind me so the first 30 miles was being overtaken then the last 30 my turn to overtake some. No-one says much though. I might have a bash at a 12 hour in September if the circulation has returned and I can straighten my posture out

In reply to TobyA: endo can be a bit wishy/washy on distances. I'd go with the bike pooter. It frequently will mis judge a route i've done before but i'm still happy with the overall functionality, tracking, online etc it provides.

In reply to woolsack: I was pretty chuffed, no lower back pain at all this time. I carried some ibuprofen with me after sore backs doing sportives, but this ride was great in terms of not joint or back pain during or after. Just tired leg muscles!

How long did it take you? I did my first 100 kms at very nearly 30 kmph which for me is really good. But slowed down a bit in the last 60 odd kms - as the wind rose as forecast (headwind of course!) and I s'pose I was just getting knackered. Over all I was in the saddle for 6.5 hrs, but as I said I had a few shortish breaks.

In reply to gingerdave13: Yeah, it seems to work well. I was told there are similar that use a lot less memory on the phone (my HTC Desire is forever running out of space), but I've never investigated further.

In reply to TobyA: Gps elevation tracking is notouriously inaccurate. I have a garmin 305 which uses gps elevation tracking and an edge 800 that uses barometric pressure. The latter is much more accurate when counting ascent, the former is often 30% different. That with cutting corners could easily account for the difference. In fact for the distance you rode I am surprised at how closely matched they are! Also different software does things in different ways. I can upload a file from the edge to different sites and apps (strava, sporttracks, connect) and get different values from each. They use different correction and smoothing algorithms, all this added to differences in wheel sizes, tyre pressure, how many pies etc! Could add in a lot over that distance.

In reply to Escher: Excellent. So I rode over 190 kms then and I won't hear anyone say different.

In reply to TobyA: There is the elevation error as mentioned above. There's also the question of how many data points. Your bike computer will record a data point every turn of the wheel (about 2mtrs ?). My (ancient) GPS only records what it thinks are significant changes in direction and assumes a straight line between the points. In effect, the GPS will short-cut all those small deviations from the straight and narrow.

In reply to TobyA: I got round in 4hrs 52, I lost 5-10 minutes with the flat. There was a 15mph SW wind and it was two laps of a circular course on the flat with little cover so there was plenty of opportunity to curse the head wind. Being tucked into the TT position certainly makes a difference. The winning time was 3hrs 57 which I still find quite daunting.

I was riding a road bike with tri bars so it was much better than a few guys on road bikes without tri bars and one brave soul on a tricycle. He didn't last the distance in that wind.

I was surprised how effective the energy gels are once you can get into them

In reply to TobyA:

The altitude is much more accurate (and significantly greater, which is obviously important!) from the bike computer but I can calibrate this by knowing how high this house is, and how high Stanage is, and it is usually within 5m when I get there. A GPS Altitude ride sometimes only generates about 50% of the gain that the computer registers.

I tend to take my Ride Time from the bike computer since it stops more quickly when I stop.

Distance is dodgy though and I tend to think that the GPS in the iPhone is more accurate, although I am not sure about this. So average speed becomes iPhone Distance/computer time.

You can of course use UKC to check it (in the UK accurately) at least by using your imported GPS data to create a Map My Route:

http://www.ukhillwalking.com/logbook/adddiary.html

Then do the same procedure manually using our trace on map function and compare the results.

I just did this for a ride I did on Sunday and got 42.5km manually, and 41.43km for the GPS reading. Curiously on this occasion my computer said 42.7 which is closer, so there you go!

Alan

In reply to woolsack:


Once you dribble them out of your mouth whilst trying to steer, change podcasts and suck down a gel at the same time, they do leave really embarrasing looking stains on your shorts though!

In reply to TobyA:

I normally sit on the bike at the correct tyre preasure, mark the wheel, roll it one revolution, measure this and use it on the bike computer as it's more accurate than just the number on the tyre.

Cheers

Toby

In reply to Toby_W:

was just about to say the same thing... get a friend to help, it's a lot simplier that way

In reply to TobyA:

There's a few free Android apps you can try e.g. Strava and Sportstracker. Some of these will allow you to configure how they record the track e.g. Fixed time interval. As others have said errors can arise when interpolating between recorded points.

Over the last month I have been recording all my rides using both my Garmin GPS and Sportstracker on the Android, to see how the compare. However, haven't got round to analysing the results yet.