cycle computer problem

[Kevin Hawkins]

I’m 71 years old and have just returned to mountain biking after a three year break.

My son, a keen road and MTB biker, has put me on to the Strava Ap on my iphone.  I also have a cycle computer fitted to the h/bars.

After a couple of rides I noticed quite a discrepancy between the two, particularly distance travelled.  A couple of days ago I rode a circuit on tarseal with straight stretches of road and basically all flat.  I also ran the same circuit in my late model Honda Civic.

Strava distance read 19.7 kilometres

Cycle computer read 21.6 kilometres

Honda Civic trip read 19.5 kilometres.

These readings suggest the cycle computer is about 2 kilometres out over this short ride.

One thing I did notice is the cycle computer speed fluctuates on a regular basis.  I will be riding about 18kph and it will leap from 18kph to 45kph, then back to 18, then within a minute or two it may leap to 67 kph for no more than a second.  This afternoon it leaped to 87kph!

Is this normal?

I should point out I have calibrated the cycle computer accurately ( in my opinion) and double checked by pumping tyres to 60PSI, marking a line on flat area of smooth concrete, lining up the valve and mark on the tyre with the line on the concrete, and whilst sitting on the seat, moved one complete revolution and marked the ground.  Measured the line mark to mark, and input this figure in to the computer.

While accuracy is not important for my type of riding, I am a fussy old bugger and would like to know where the problem is, and how to fix it.

In anticipation, thanks

[Jeff Barber]

I’ve actually done some accuracy tests between smartphones, GPS devices, and a cyclocomputer:

GPS Accuracy Test: GPS vs. Smartphone vs. Cyclocomputer (Round 2)

Like you, I found the cyclocomputer overestimated the distance, while the Strava and Endomondo apps got pretty darn close. In another test on a twisting singletrack trail, I found the cyclocomputer distance was double the GPS app distance. In that case, I suspect the cyclocomputer was actually the more accurate one due to the way GPS calculates distances between points.

Bottom line: no device is perfect, they all just give estimates. Not only that, one device may be more accurate for a particular ride, and not as accurate for a different ride.

[stumpyfsr ✓ supporter]

Hi! I’m not an expert but that issue isn’t normal. There could be many reasons for your computer acting weird. First I’d check magnet and sensor alignment and make sure theres correct distance between two.  If that doesn’t help, clean contacts and battery with alcohol. And as the last resort, replace the battery. If all these steps doesn’t help, then you’re dealing with defective unit.

Hope it will help.

[Jeff Barber]

You’re right @stumpyfsr, the fact that the unit is giving out funky speeds and is off by 10% isn’t normal.

[stumpyfsr ✓ supporter]

@Jeff, 10% difference in distance wouldn’t bother me. Mountain bike’s tire pressure changes almost every ride and it will affect distance. But such a rapid speed change says about something wrong besides wheel circumstance. So I guessed what could be done to find a problem.

I personally stopped using computer a while ago

[Kevin Hawkins]

Thanks Jeff and Stumpy,

Tomorrow, weather permitting, I intend to go down to Athletic Park ( located in Blenheim, New Zealand ) and check both the Strava ap, and my cycle computer on a 400 metre running track.  (Thanks for the idea Jeff)

I suspect after reading comments and Jeff’s article, that the problem will probably be with the sensor and magnet alignment.  When fitting the wireless cycle computer, I had problems aligning the sensor with the magnet.  The gap was too big so I was forced to turn the sensor at a 45 degree angle to the direction of travel, so it was the required distance from the magnet.

It’s not a big issue, but as I previously stated, I am a fussy bugger and like getting things right if I can.

Perhaps I need to loosen up, forget about the distance travelled, and just enjoy the MTB tracks around Marlborough.

It’s a tough life in retirement, deciding whether to play golf, go for a ride, or go fishing!!!  No good staying home, my beloved will find me jobs to do!!

 

Thanks for the help team.

[Author]

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