Garmin Speed sensor

[Stu Smith]

Hi all,

I have a mate who consistantly posts quicker times than I, although we ride together + start and finish together.

He use's the Garmin speed sensor to record speed and distance, where I use the Garmin GPS to record speed and distance.
He as manullay entered the wheel diameter.
Would or could this make a difference?

 

[Milkfloat]

I would suggest that his manual wheel size is most likely the problem. You would only get minor differences between GPS and sensor speed, although that could be worse in urban canyons and under trees.

 

[si_c]

Depends what you mean when you say "posts quicker times", if he's still using a Garmin GPS head unit, then it should still use the GPS trace to calculate the distance travelled - the speed sensor is useful for showing speed in areas where the GPS signal is poor or lost - for example I find it useful in tunnels or under heavy tree cover. Turning the sensor off in the GPS unit shouldn't make any difference to your overall speed and time taken.

At least that's my understanding of it.

 

[Stu Smith]

He says the Garmin defaults to the speed sensor from the start. Which seems to be true when I read the Garmin forums.

If I return with a 16 mph average he will be a least at 17mph

 

[Yellow Saddle]

The GPS does a lot of intrapolation whereas the speed sensor records wheel revolutions and calculates distance from that.
The two will never be the same.
You can fine-tune the speed sensor somewhat by keeping on changing the wheel diameter until the two of you have a closer relationship, display-wise of course.
But at the end of the day, the figures are meaningless. What does it matter?

 

[Stu Smith]

The GPS does a lot of intrapolation whereas the speed sensor records wheel revolutions and calculates distance from that.
The two will never be the same.
You can fine-tune the speed sensor somewhat by keeping on changing the wheel diameter until the two of you have a closer relationship, display-wise of course.
But at the end of the day, the figures are meaningless. What does it matter?

Would you start by changing to a smaller wheel diameter or larger.
Larger diameter in my mind would give a faster than actual speed reading??

 

[Milkfloat]

He should just turn the manual wheel setting off. Garmin do a very good job of automatically setting the wheel sizes via the GPS.

 

[Racing roadkill]

The Garmin wheel speed sensors that look a bit like those cheap lights you can get, and fit on the hub will automatically work out the wheel diameter. Even then there is a disparity between it’s readings and GPS calculated speeds. I find the wheel sensors tend to read high, by a couple of mph, compared to GPS speed.

 

[Stu Smith]

Its the GSC 10 with the magnet attached to the wheel.
I know I shouldn't be overly bothered , but the wry smile I get from him after a ride when he asks me what's your average?
Just need to prove he either as it set up wrong or GPS reads slower.

 

[Yellow Saddle]

Would you start by changing to a smaller wheel diameter or larger.
Larger diameter in my mind would give a faster than actual speed reading??

If I return with a 16 mph average he will be a least at 17mph

You don't say what the two finishing distances are, although you did hint that the total time is the same. For a different average, the displayed distances will thus be different. If all things are what they seem to be, it means that your computer thinks you're doing a shorter distance, which in turn means your entered (in the computer) wheel circumference is too small. Increase the figure in your computer from say 2085 to 2115.

Make it 3000 if you want to really, really beat his average.

 

[Stu Smith]

You don't say what the two finishing distances are, although you did hint that the total time is the same. For a different average, the displayed distances will thus be different. If all things are what they seem to be, it means that your computer thinks you're doing a shorter distance, which in turn means your entered (in the computer) wheel circumference is too small. Increase the figure in your computer from say 2085 to 2115.

Make it 3000 if you want to really, really beat his average.

I use GPS only, no speed sensor.
To be honest I have never checked the distance comparison with him.

 

[screenman]

Its the GSC 10 with the magnet attached to the wheel.
I know I shouldn't be overly bothered , but the wry smile I get from him after a ride when he asks me what's your average?
Just need to prove he either as it set up wrong or GPS reads slower.

Simple, get the same setup and put two magnets on your wheel.

 

[Ajax Bay]

I have a mate who consistently posts quicker times than I, although we ride together + start and finish together. He uses the Garmin speed sensor to record speed and distance, where I use the Garmin GPS to record speed and distance.
He has manually entered the wheel diameter. Would or could this make a difference? [minor typo edits]

Yes.
Pedant corner: If you and your mate "ride together + start and finish together" surely for that ride you are 'posting' identical times - how could his time possibly be 'quicker' than yours?
The Edge 800 and others, with a 'speed sensor' fitted, use the distance determined by received GPS signals to calculate the wheel circumference (by determining how many revolutions the wheel has rotated over a distance ?1000m). This calibration (given it's done over a km or less) cannot be accurate to closer than 0.5% (maths available on request). Once it is thus calibrated most Edges continually use the GPS data captured during rides to check and adjust the value (but again using a relatively short distance). This, for example, will make 'automatic' adjustments for a (slightly) different tyre (height) and a change in the tyre pressure used: we're talking millimetres in a wheel/tyre circumference of 2100mm (say) here: 8mm difference is less than 0.4%. In addition, those Garmins (and others) also let you specify the wheel circumference.
In the context of this thread, Garmin Edges (so enabled with speed sensor/Ant+) allows one to set the speed to the speed sensor calculation rather than the GPS one. Most riders default to GPS. But a regular turbo rider, too lazy to press a couple of buttons when starting a turbo ride, might choose to set the speed to the speed sensor calculation without any GPS involvement. I'm guessing the OP's mate has set their circumference too high. If they want to continue kidding themselves and it makes them happy, that's fine.

He should just turn the manual wheel setting off. Garmin do a very good job of automatically setting the wheel sizes via the GPS.

It's his mate's choice what to do.

If I return with a 16 mph average he will be a least at 17mph
I use GPS only, no speed sensor.
To be honest I have never checked the distance comparison with him.

Well next time you and your mate compare, compare the distance you've cycled together and then ask them why the discrepancy. Personally I would plot the route you've ridden on RidewithGPS and see who's 'closer' (answer: you will be - he will have a distance 6+% too high (100/16)

 

[Stu Smith]

Thank You Ajax.

After much persuasion my mate rode with GPS enabled..
Guess what, as near as damm it is to swearing, we had identical average speeds and distance.

He as now reverted back to the speed sensor.

 

[Ajax Bay]

He as now reverted back to the speed sensor.