Bluechip

We had beautiful weather today and I had to go for a ride. On the way home I was relaxing and riding no handed and looked down at my computer (Polar S720i) and my speed had doubled to 32 mph. I put my hands back on the bars and it went back down to the correct speed. It did this every time I took my hands off the bars. It would slowly pick up speed up to 32 and stay there until I replaced my hands on the bars. The tires were properly inflated to around 105 lbs so I don't think there was that much differance in tire size because of the lightend weight on the bars but I can't think of any thing else. The bike I ride the most is diffucult to ride no handed so I have not noticed it before. Has this happend to any one else?

the beef

Wa-wa-wee-wa! Nice!

Ride no handed more often!

Just kidding. I don't think it has something to do with the tire inflation - it's more likely an issue with a loose something-or-other in the mounting bracket or wireless deal slash cable deal. Check your sensor and magnet, too.

Bluechip

All of the mounting points are secure. I have 3 bikes with Polar sensors and switch out the head unit from bike to bike. I will try it out on the other bikes and report back later.

FrankBattle 

do you have a watch and/or strap on when this happens?

rodrigaj

I have a 720i. The speed sensor is extremely sensitive to placement between the monitor and wheel. Try moving the watch on the handle bar to the right 1 cm. Also make sure that the speed sensor and watch are on the same side. This would usually mean the right side if you are right handed.

My only complaint with the 720i is the sensitivity issue. For some reason, the cadence does not have these problems. I've meant to experiment with the auto on feature to see if this is the reason.

The only explanation to your problem is that the heart rate transmitter is affecting the speed sensor. I have never experienced this. I have only experience the distance speed monitor inexplicably shutting off and not turning on again. Once positioned diffenently on the handle bar, all of my problems went away.

Bluechip

I was not wearing the heart belt at the time. The head unit is mounted on the stem about 13" away from the speed sensor. It works perfectly with my hands on the bars. I can't think of anything that might have caused any interference. I will try shifting the head unit around a bit and see if it still does it.

jrennie

My biggest complaint about the 7XX series is that is very sensitive to vibration, I would get similar results (or none at all) if the road was really rough. Maybe the lighter front end without your body weight to dampen the vibration had something to do with it?

cuda2k

[QUOTE=jrennie]My biggest complaint about the 7XX series is that is very sensitive to vibration, I would get similar results (or none at all) if the road was really rough. Maybe the lighter front end without your body weight to dampen the vibration had something to do with it?[/QUOTE]

That was my initial thought as well. Vibration on the contacts causing the erroronous readings.

DocRay

I've had similar issues with my CS200CAD since new, side-by-side with another rider over a ride, the distance is off by 10-15% some days, the same on others. The speed sensor reads 0 at times while just trolling along. The HRM works well, but it's a mediocre bike computer.

I've given up, I've got a Garmin 305 on order, hopefully, it's designed better.

Bluechip

After hearing all the ideas the vibration one sounded the best so I conducted a few tests this morning. It's a wireless system so I not sure how vibrtation would affect the unit but it was worth a shot. Just riding up and down my block on smooth pavement the results were the same. If I took my hands off the bars the speed would slowly go up until it reached around double the actuall speed. Back on the bars it would return to normal. The mounts are differant on my two main bikes so I moved it over to the other bike and tested it. It worked fine both with hands on bars and without. I swapped front wheels and both bikes performed normally. I swapped back and the problem continued. I played with the magnet on the wheel and went out again. It works fine now. So a slight adjustment in the setup with the wheel magnet was the cure. I still don't see how taking my hands off the bars affected anything but problem solved. Thanks.

spdrcr5

you answered your own question. By placing your hands on the bars it eliminated or lessened the vibration enough that the magnet didn't move around as much. Once you removed your hands the vibration carried right through the entire bike to the magnet.