# Bike Computer vs GPS



## hankscorpio (Jun 20, 2012)

I want to get more diligent in tracking my miles this year. I had been running everytrail app on my droid but I feel like it frequently loses signal on my ride and gives me inaccurate mileage read outs. 

I have an ocd quirk/issue about putting a computer on the handlebars. I really just want to avoid it. 

My question is about using a gps watch. Are they more accurate then the phone aps? Can you see mileage as you are riding or do you need to wait until you upload it later. 

I'm debating looking into a gps watch vs a cateye q3 watch with cycling accessories. I'm leary of the gps but that is really just based on my phone use, i'm wondering how accurate a dedicated gps system is.

Thanks for any input or personal experience.


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## jamesh75 (Feb 4, 2011)

A dedicated GPS is going to be much more accurate in my experiences. However, if you mountain bike in tight woods, once there are leaves on the trees, you'll lose a signal periodically even with a dedicated GPS.


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## JoePAz (May 7, 2012)

I have both. I have wireless computer on my bars to give me speed and distance. I also track my rides on my blackberry and then upload the tracks to Strava. I never look at my GPS when riding as it is tucked away.


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## WR304 (Jul 9, 2004)

If your mobile phone supports Bluetooth 4.0 you could use a Wahoo bluetooth wheel speed sensor with your phone to increase the accuracy of tracking how far you've travelled.

http://www.dcrainmaker.com/2012/06/wahoo-fitness-blue-sc-bluetooth-smart.html

Something like a Garmin Edge with GPS will still have problems tracking accurately if you ride a lot in thick woodland that can block the GPS satellite signal. For the best accuracy of tracking distance you'd want to have an additional wheel speed sensor fitted for recording using a Garmin Edge also.


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## Lone Rager (Dec 13, 2013)

My Galaxy S4 uses both GPS and Glonass satellites and is at least as good if not better than my Garmins. Some of the newer Garmins (510 810?) also add Glonass so should also be better than those that do not. A wheel sensor helps for low speed and tortuous tracks as often done on mtbs. 

The S4 can also receive ANT+ and BTLE sensors.


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## bruce219 (Jul 9, 2012)

Same as what Jamesh75 said,i have a garmin edge and it loses signal in the trees,i mounted an ant speed sensor and that keeps track once i lose signal


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## jim98zj (Sep 24, 2007)

Why not get a wireless comp you throw in your pack and just check it to the gps at the end of the day?


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## hankscorpio (Jun 20, 2012)

Thanks for the input. I tried that first with a cheap bell one I have but the range is really barely past the handlebars. It seems like the ones with cadence have further range. I could probably get a good one for like 60 (i know there are cheaper ones with cadence but they seem a little flimsy) but I can get the cat eye watch for 100 with cadence and a heart rate monitor. That could prove to be useful with regular exercise as well. At this point i'm leaning towards the cat eye watch.


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## DethWshBkr (Nov 25, 2010)

My Specialized Sport computer was reading 9.5 miles on this one small course. The official length of the course is 7.4.

I finally just learned about this thing called Strava (this sentance was stated in a very "dumb" tone), and have been using it to help me track my improvements, as I'm coming back to riding after 15 years.
Well, my Droid, and my brother's iPhone app both show about 6.9-7.5 miles. But, my computer, his computer, and my dads computer all read 9.4-9.55 miles.

My brother's one buddy works in GPS, and he told him that if there is a tree within 100' of a standard (non-military or commercial) GPS, it can throw it off a lot. That's woods riding.....


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## shiggy (Dec 19, 1998)

jim98zj said:


> Why not get a wireless comp you throw in your pack and just check it to the gps at the end of the day?


Most wireless computers do not have the range to read from wheel to backpack. The head unit needs to be within 18-24" of the transmitter.


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## Learux (Jun 4, 2012)

No GPS is more reliable then a wired bike computer. Make sure you calibrate it according to instructions.

With that said if you don't ride in thick forest or narrow canyons a good GPS will be just as accurate.

My GPS(phone with GPS and Glonass support) and bike computer are within .5% from each other.


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## bruce219 (Jul 9, 2012)

The problem with the watch HM/cadence is its hard to keep an eye on everything,my mate tried the watch on the bike and he is sorry now,he actually gave up on it.if its mounted in front of you its only a quick glance.I have the cadence on my rd bike and i like to keep a close eye on that,i don't bother with it on mountain bike as its sometimes impossible to try and even keep cadence.


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## hankscorpio (Jun 20, 2012)

That's part of the reason I want it on my watch actually. I found when i had it on my bars i would keep looking at it. If its on my watch i can occasionally glance to see distance (like when i'm taking a break huffing and puffing) It comes with a mount for the bars too if i want to use it on road and looking at cadence. I frequently ride in the woods with a lot of tight twisty trails so i find that the phone gps is often inaccurate


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## Harold (Dec 23, 2003)

IMO, the watch format does not work as well when used on the wrist while biking. The antenna is not optimally located when used that way. Use an adapter to put it on the bars, or get one designed to be used there in the first place. Otherwise gps reception will not be as good as it can be.

Frankly, if you want a computer of any kind, it needs to be placed with care to receive signals from all its sensors the best way it can. For anything with gps, that location is on the bars or stem. Anything else will degrade your results.

And, it needs 1sec recording for mtb use.


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## fishymamba (Oct 19, 2013)

I could not afford to purchase a Garmin cycling computer, so I got this: https://www.starlite-intl.com/Detail.asp?pid=3081#.Ut9JGRDTmM8

For the past ~2 years it has worked great for me. It came with a cadence sensor, but I had to purchase a heart rate strap separately(ANT+).

I have not had a problem with the signal cutting out, but I live in California and there are not too many trees on the trails around me.


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## Gasp4Air (Jun 5, 2009)

I started riding my hardtail on rail trails last summer and was curious about mileage and speed, so I got a computer. I found that setting the wheel size was critical to accuracy. Mtn bike tires are far bigger than road tires, and are generally ridden at much lower pressures, even on a rail trail. The effective diameter of the wheel can be changed significantly by the amount of deformation at the contact patch - that is, how much does it squish when you're on the bike. Before I discovered this, my mileages varied considerably, and I thought the computer was defective. Even after I compensated as best I could for the squish factor, I never got really consistent mileages over the same sections.

If accuracy and consistency is important, GPS is the way to go. If close is good enough, be sure to pay attention to consistent tire pressure, and factor in the loss of tire height when you're sitting on the bike.


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## hankscorpio (Jun 20, 2012)

well riding in the woods on windy trails gps isn't very accurate. 

Another thing I'm not sure of with computers is exactly where you place the sensor? I know programing the tire size is important but wouldn't the distance of the magnet from the hub also effect it? None of the directions i've seen specify exactly where it needs to be? I'm not sure how it would account for the difference in tire size if say the magnet were exactly the same distance from the hub on a 29er or 26?


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## Learux (Jun 4, 2012)

All it does is it counts revolutions. 

Once you enter the correct wheel circumference it computes distance traveled by multiplying revolutions with circumference.

As long as sensor does not miss a pass, you wheel have very accurate information.


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## hankscorpio (Jun 20, 2012)

hmm.

There is a bike path near my house that posts its mileage so I usually check for accuracy there.


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## Learux (Jun 4, 2012)

Who says that the mileage posted there is correct?


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## hankscorpio (Jun 20, 2012)

the mileage police


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## dirtdawg21892 (Jul 20, 2009)

after resisting for 2 years I finally broke down and got strava. I wanted a bike computer to track distance and a ride mapping app, but strava was cheaper than either and does both. go figure.


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## WR304 (Jul 9, 2004)

Gasp4Air said:


> I started riding my hardtail on rail trails last summer and was curious about mileage and speed, so I got a computer. I found that setting the wheel size was critical to accuracy. Mtn bike tires are far bigger than road tires, and are generally ridden at much lower pressures, even on a rail trail. The effective diameter of the wheel can be changed significantly by the amount of deformation at the contact patch - that is, how much does it squish when you're on the bike. Before I discovered this, my mileages varied considerably, and I thought the computer was defective. Even after I compensated as best I could for the squish factor, I never got really consistent mileages over the same sections.
> 
> If accuracy and consistency is important, GPS is the way to go. If close is good enough, be sure to pay attention to consistent tire pressure, and factor in the loss of tire height when you're sitting on the bike.


Because an offroad tyre at low pressure compresses so much when you sit on the bike a measured wheel circumference roll out with no weight on the bike is going to be inaccurate (too large a circumference). I typically have a few favourite tyre types that I use most of the time and use a track pump so they always start off at the same pressure for consistency.

For each tyre at the air pressure that I intend to use I measure the moving wheel circumference whilst actually riding by putting a small blob of white grease on the centre of the tyre tread (something that is easy to identify, will leave a mark and show up on the ground) and then ride along on the bike for a few metres sitting in the saddle as I would normally. The front and rear tyres compress different amounts so you need to measure the one that your wheel magnet and sensor will be attached to.

Using a tape measure you then go back and measure the distance between two of the white blobs left on the ground as the wheel rotated. That distance is your actual moving wheel circumference to enter into the computer settings for that particular tyre. That ought to give you accurate distance measurements.


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## Learux (Jun 4, 2012)

^^That is absolutely the way to do it. You can also use the valve stem as a marker.

Start with the valve stem in 6 o' clock position. Ride for as many revolution as you feel comfortable(more is better) stop at 6 o'clock position.

Measure distance divide by number of rotations and enter correct number in computer.

Your bike computer will be extremely accurate this way.

This is a mileage police approved procedure btw.


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## hankscorpio (Jun 20, 2012)

Thanks for all the info


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## mr_chrome (Jan 17, 2005)

I ran both on trails last year - a Garmin 450 and a good cycling computer......the Garmin didn't track distance quite so well due to how often it measures your movement but the bike computer is constantly counting rotations.......I compared both when I was on a greenway trail with the mileage posted every tenth of a mile and the computer won, hands down.......my Garmin is good, possibly better than their bike counterparts, but it has its limitations........


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