Garmin Edge 705 - How is speed measured?

schweiz

amaferanga wrote:

antfly wrote:

GARMIN KNOWLEDGE BASE .....

Someone should tell the Garmin Knowledge Base that the 605 isn't compatible with the GSC-10.

hmmmm good point!

Can the GKB be trusted for accuracy of information?

:shock: :?

I give up, I'm just gonna ride. The distances measured tally close enough both with road distances in the car and with various mapping programmes. Accuracy of time is not in question so the speeds will be more than accurate enough.

rich_e

schweiz wrote:

Alleluia! Thanks for finding that Antfly

Now, a new question..how close do you all run the speed sensor arm to the wheel magnet. On both the BMC and the VN, it isn't possible to tuck the arm in behind the chainstay due to the clearance so the arm has to stick up. It's as close as I dare have it, especially on the BMC which has rectangular stays and therefore the GSC-10 isn't seated well.

My 705 is in auto wheel size as I use it on all my bikes and with different wheels and I don't bother to switch the bike profile when I switch bikes. Occasionally I get a 'Wheel Size Calculated' message after a km or so but more often than not I don't. I'm starting to think that I'm losing data although speeds and distances always seem to tally +/- 1% with other non-garmin computers on club rides.

Surely the simplest answer is to set up a profile for each of your bikes?
Well for three of them at least, if you have more.

It only takes a couple of minutes to setup a profile.

schweiz

Rich_E wrote:

schweiz wrote:

Alleluia! Thanks for finding that Antfly

Now, a new question..how close do you all run the speed sensor arm to the wheel magnet. On both the BMC and the VN, it isn't possible to tuck the arm in behind the chainstay due to the clearance so the arm has to stick up. It's as close as I dare have it, especially on the BMC which has rectangular stays and therefore the GSC-10 isn't seated well.

My 705 is in auto wheel size as I use it on all my bikes and with different wheels and I don't bother to switch the bike profile when I switch bikes. Occasionally I get a 'Wheel Size Calculated' message after a km or so but more often than not I don't. I'm starting to think that I'm losing data although speeds and distances always seem to tally +/- 1% with other non-garmin computers on club rides.

Surely the simplest answer is to set up a profile for each of your bikes?
Well for three of them at least, if you have more.

It only takes a couple of minutes to setup a profile.

I have profiles set up, but I always ended up forgetting to switch them, so like I said in my previous post, I just don't bother anymore. Also If I switch wheels between bikes, there's a (admittedly small) difference which is why I just leave everything on auto and just ride. I was more interested if other users get more frequent 'wheel size calculated' messages, which could mean that I'm getting my speed from GPS as the sensor is not calibrated (maybe because there are too many drop outs on the wheel sensor - hence my question re. positioning) The battery in the GSC-10 is ok as I get cadence no problem.

amaferanga

Auto calibration seems to work ok most of the time, but will occasionally throw a wobbly which is why the general advice is to always input wheel circumference manually.

schweiz

amaferanga wrote:

Auto calibration seems to work ok most of the time, but will occasionally throw a wobbly which is why the general advice is to always input wheel circumference manually.

Please can any 705 users who use auto wheel size calculation confirm whether or not they get a 'wheel size calculated' message every ride?

Bronzie

schweiz wrote:

amaferanga wrote:

Auto calibration seems to work ok most of the time, but will occasionally throw a wobbly which is why the general advice is to always input wheel circumference manually.

Please can any 705 users who use auto wheel size calculation confirm whether or not they get a 'wheel size calculated' message every ride?

Yes, after about 10 mins riding. But I switched it off because it threw a wobbly when used on the turbo.

antfly

I don't have the 705 but the 305 and 500 and if you have profiles set up but don't switch it to the correct one for the bike it won't pick up the sensor at all. If you are not on a profile it calibrates it at the start of every ride. I don't see how it can auto-calibrate on the turbo when you're not moving.

schweiz

Bronzie wrote:

schweiz wrote:

amaferanga wrote:

Auto calibration seems to work ok most of the time, but will occasionally throw a wobbly which is why the general advice is to always input wheel circumference manually.

Please can any 705 users who use auto wheel size calculation confirm whether or not they get a 'wheel size calculated' message every ride?

Yes, after about 10 mins riding. But I switched it off because it threw a wobbly when used on the turbo.

hmmm, need to do some investigating then. Also I've not had any problems on the rollers. I'll have a look this weekend.

schweiz

antfly wrote:

I don't have the 705 but the 305 and 500 and if you have profiles set up but don't switch it to the correct one for the bike it won't pick up the sensor at all. If you are not on a profile it calibrates it at the start of every ride. It's obviously not going to work on a turbo without a profile set up.

Hmmm, doesn't seem to be like that for me. I have profiles set up, with each of them set to auto, but like I said earlier, I don't switch between profiles, but one will be active. I just put the 705 on either bike with a GSC-10. I get 2 messages...HR monitor detected and Cadence sensor detected and if the wheel is fitted, power meter detected. I zero the power meter, then I just ride and data gets recorded. I've even resorted to leaving it set to 1 second recording as I kept forgetting to switch between smart recording and per second recording for the powertap.

antfly

When you get on your bike is the other bike nearby?

schweiz

No, the BMC lives in the garage with the Cube which has no GSC-10. The VN lives in the spare room.

However, the other week, I took the GSC-10 off the VN and put it on the other half's bike so she could used the turbo trainer. I paired the GSC-10 with my old 305 which she now uses. I bought a new GSC-10 but don't ever remember doing a manual pairing with the 705, but it does get picked up (I've been using it on the rollers). The first time we were both training together in the same room, both the 305 and the 705 brought up a message saying that there are multiple cadence sensors. I just took my bike back into the spare room to until the 705 was happy it could only pick up one sensor. It paired and after that it was fine.

antfly

That's all a bit confusing but the spare room and the garage could well be close enough to pick up a signal. The first time I paired my second bike I had to take it outside and around the side of the house. Profiles would solve the problem of picking up multiple sensors, if you want to train near her that is.

schweiz

antfly wrote:

That's all a bit confusing but the spare room and the garage could well be close enough to pick up a signal. The first time I paired my second bike I had to take it outside and around the side of the house. Profiles would solve the problem of picking up multiple sensors, if you want to train near her that is.

doubt it. I live in a second floor apartment and my garage is 2 floors underground.

antfly

Nevertheless...presumably you have to come overground to go for a ride.

schweiz

With the BMC I go straight out of the garage door which is up two ramps to ground level. With the VN I usually I go out of the front door at ground level, still 2 more floors to go to the garage (and -1 is a reinforced concrete nuclear bomb shelter like every post WWII swiss house!) so there's no chance that the bike in the garage will interfere with the one kept in the apartment or vice versa.

antfly

So where do you turn on the GPS?

schweiz

usually in the apartment

....but I've just done a quick test.

it's doesn't sync with the GSC-10 on the VN unless I turn the crank or spin the rear wheel so that the GSC-10 is transmitting. The 705 is still showing that is has not detected a cadence sensor (the crank symbol is greyed out and flashing). Then when I turned the crank in the normal direction (so the wheel spun too) I got a 'Cadence Sensor Found' message.

I turned off the 705, left it 5 minutes, turned it back on again, went into the spare room and the sensor on the VN still wasn't detected. Then I went down into the garage, gave the back wheel on the BMC a spin and got the cadence sensor found message without manually re-pairing

Then I turned the 705 off again in the garage, came back to the apartment, turned on the 705 and spun the back wheel of the VN and got the 'Cadence sensor found' message, again without a manual re-pair.

To top it all, I've just checked which profile was set (I have BMC, VN and Cube as my 3) and it was set to Cube, and has been for the last 5643.9 km!! Both the Cadence and Power boxes are unchecked.

I have firmware version 2.80 installed which was probably updated about 18 months ago.

antfly

OK, I reckon it works because you haven't got a GPS signal but my theory is that when you get a GPS signal it will use that instead.

schweiz

That sounds like it could be the answer

So by not ticking the sensor box on the profile, it works like a 305 i.e. only uses the GSC-10 when no GPS is available or GPS is switched off (i.e. on the rollers)

By ticking the box, it always uses the GSC-10.

Now I just need to work out why sometimes I get the wheel size computed alert but more often not. I've just done a test of both sensors and the light flashes for each wheel revolution at slow speed, but it's hard to tell at high speed (LED just shimmers). Maybe the reed switch doesn't always switch at higher speeds? One day, I'll understand exactly how this thing works!