Speed and Distance variance compared To Garmin 500

[Lobo]

I am hoping you can help me with a few things: Before I proceed let me just say I am very excited about my "Newton". I see endless possibilites to zone in on my training effeciency.

1. There is a speed variance between my Garmin 500 and the Newton. I am currently using my sensors from my Garmin 500 and the Newton recognized them easily. The variance is pretty big. Here are the numbers as recorded on my last ride on my Garmin 500 and the Newton.

2. There is also a distance variance that is probably the result of the speed inaccuracy, can you confirm the issue on this as well.
3. There is also at least one huge spike in power during all of my rides can you explain this and offer a solution.
4. I am sure there is other details that I am not seeing that you may see please include any information you can regarding these issues.

Thank you for your time and help on this.

JM
8/25/12
Garmin Details Dist. Speed Cadence N.Power Avg. Max 20 Min Climbing Hr I. F W/KG TSS Var
Date Power Power Power Indx
25-Aug 7.51 15.7 78 418.00 323 1780 328 82 131 1.27 4.03 70.52 1.29


Newton
Dist: 8.35 mi (0:28:55)
Energy: 397.0 kJ
Cals Burn: 379.6 kcal
Climbing: 8 ft
Braking: -3.3 kJ (-0.8%)
Min Avg Max
Power 0 228.8 2406 W
Aero 0 88.0 997 W
Rolling 0 57.3 209 W
Gravity -887 -2.5 358 W
Speed 0.0 17.2 62.6 mi/h
Wind 0.0 19.5 42.7 mi/h
Elev 1998 2023 2044 ft
Slope -6.3 -0.03 2.8 %
Caden 0 76.3 103 rpm
HR 91 131.3 162 bpm
NP:363W IF:1.11 TSS:59 VI:1.59
CdA: 0.391 m^2; Crr: 0.0067
249 lbs; 8/25/2012 6:54 PM
95 degF; 1008 mbar

[Lobo]

http://connect.garmin.com/activity/214914552

I have posted this, in the event that the previous post is unreadable. Thank you.

JM
8/25/12

[KenS]

The first and most obvious reason is a discrepancy between the wheel circumference recorded in Newton vs Garmin. I'm not familiar with the Garmin 500 but my 705 measures distance and speed using its GPS readings. It only uses the wheel circumference when the GPS is lost. But it also has a feature that calculates the wheel circumference based on a few minutes of riding and using the GPS. I plug that number into the Newton and the two correspond very closely.
If you are using the GT key to send data from Newton to Garmin then you will get power spikes:
- from cal rides and coast downs
- tilt calibration
- if your tilt cal is out then when the Newton automatically fixes the tilt cal after 5 minutes riding

[Velocomp]

If the wheel circumference readings set in your Garmin unit are identical to the wheel circumference readings in your Newton, then speed and distance readings should be identical.

I don't know how to check wheel circumference readings in the Garmin, but in the Newton you can determine your wheel circumference in Setup/Fast Start/Yes/Whl Circ. The default value is 2096mm, which is correct for a 700 x 23c tire.

[Lobo]

Gentelmen:

Thank you for the prompt responses: I did indeed have a variance in wheel circomferance on my Garmin as its default was set at (2083mm), not 2096mm as I previously thought it was. I do have the correct tire size set on my Newton. I have corrected it to the 2096mm that I use on the Garmin. I did a tilt cal, wind cal and 2 mile in and out before my last ride so I believe those should be up to date. I do have a GT Key fyi. I will post the results of my next ride if I have any issues that I may encounter. Thank you again for your time and help.

JM

[racerfern]

I don't know about the Garmin 500, but if the 705 is set to automatic wheel circumference calculation, it calculates every time you start a ride. So I did a roll out for one revolution with me on the bike and got a measurement of 2092. I put that into the iBike and also put that as the forced measurement into the Garmin, obviously shutting off the auto wheel calc option on the 705.

[Zoltan]

If the wheel circumference readings set in your Garmin unit are identical to the wheel circumference readings in your Newton, then speed and distance readings should be identical.

I don't know how to check wheel circumference readings in the Garmin, but in the Newton you can determine your wheel circumference in Setup/Fast Start/Yes/Whl Circ. The default value is 2096mm, which is correct for a 700 x 23c tire.

Alas there some cycling CPUs which are not able to that simple maths. So if somebody will turn up and say that my iBike distance is different from eg, that calculated by my Cycleops Joule 2.0 one cannot help. Even if the circumference is identical, even if the guy did not stop during the ride, so the whole issue cannot be the result of the different speed threshold of autopause or wake up mechanism. It is my experience with Joule vs other tools, for some misterious reasons Joule overestimates the distance vs my Garmin 60csx, unless there is a wobbling tracklog, thus it is a caveat also for Garmin 500. Who knows?

On the other side the perfect circumference test is when seated on the bike with tire inflated to the typical pressure. If the bike is just simply rolled to get the circumference it has a distance overestimation of some 0.2-0.5% depending on the bike type and on where the speed is measured, at front wheel or at rear wheel.

Just to make everybody crazy let me mention that there are other biasing effects like standing out from the saddle, frequent sliding bike due to heavy braking if the speed is measured at the rear wheel, or (and it is my favourite) riding on one wheel if the speed is measured at the front wheel.

I guess that the threshold of targeted accuracy should be not less than 0.1% even if tires are inflated to the same pressure very frequently.

[Zoltan]

Gentelmen:

Thank you for the prompt responses: I did indeed have a variance in wheel circomferance on my Garmin as its default was set at (2083mm), not 2096mm as I previously thought it was. I do have the correct tire size set on my Newton. I have corrected it to the 2096mm that I use on the Garmin. I did a tilt cal, wind cal and 2 mile in and out before my last ride so I believe those should be up to date. I do have a GT Key fyi. I will post the results of my next ride if I have any issues that I may encounter. Thank you again for your time and help.

JM

It wont make you happy to increase from 2083mm to 2096mm meaning a "pump up" of a mere 0.6-0.7% , because the distance discrepancy that you cited was like 10-11% (7.51 mi vs 8.35 mi) depending on which of the two data you use for 100%. Sorry for disappointing you.

Since you are using the same speed sensor I have no idea from where any malfunction can come, generally I would check whether the magnet and/or one of the speed sensors are loose?

Just an idea for a proper checking. Since the distance difference was 10-11%, check the two displayed speed data during the ride whether there is a constant speed difference of 10-11% or not. Since there can be some speed smoothing over a couple of seconds, try to hold on the same speed at differen levels like 5 mph, 10 mph, 15 mph, 20 mph. You can focus on the two distance covered data, too, but it makes no sense to keep on checking both speed and distance, because the difference is the same, around 10%, so you can exclude any calculation error of distance from speed or vice versa. Or there are the same for iBike and Garmin 500.

You will be more clever after the ride. Let us know.

[racerfern]

Be careful about the differences. First, the Garmin does an auto calc each time. So it can easily come up with different numbers each time depending on the quality of the GPS signal. Also, my 705 still insists on using GPS speed even though I told it not to. It uses the wheel magnet to determine if you are actually moving. I don't know if the 705 and 500 share the same attributes.

So... the best check would be to do a long ride on rollers with the GPS shut off on the 500. Make sure you have the same circ. setting in both units and you'll see they match up. Be very careful to have your magnet properly aligned. The iBike is very sensitive to missing speed signals (as it should be) whereas the Garmin units are more forgiving as they do filtering that may mask errors.

[KenS]

The max speed of 62.6 mph on the Newton is a pointer to something strange. Could be the speed magnet is not properly set up and you are getting a doubling up of the reading. With the Garmin reading speed from the GPS a couple of minutes of the Newton reading double your speed (60 odd mph instead of 30) would give the sort of difference you are seeing.

[Zoltan]

The max speed of 62.6 mph on the Newton is a pointer to something strange. Could be the speed magnet is not properly set up and you are getting a doubling up of the reading. With the Garmin reading speed from the GPS a couple of minutes of the Newton reading double your speed (60 odd mph instead of 30) would give the sort of difference you are seeing.

Good point, but what about some dangerous close drafting after a truck on the slope of -6%?

Apart from joking more than 45 mph is not really possible on a descent of 6%. appr. 120 RPM in 53-11 is needed for 45mph and -6% is not enough to speed over 45mph. So again a good idea.