how to caibrate bike computers

big manc

Got myself a wireless computer from decathlon this weekend (sigma bc1606l) with cadence kit . Fitted it to my bike as per sketchy instructions and all works fine. How do i know if it is showing the right speeds as the instructions don't say where to put magnet and the receiver on spoke and fork it just has a picture and no measurements. Could my local lbs calibrate it for me or is this a specialist job

Any thoughts Cheers

Zendog1

The magnet can go anywhere on the wheel - the computer just counts the number of times the wheel goes round. It works out the speed by multipling this by the circumference of your wheel. You have to input this as part of the setup. There should be a table of values in the instructions (varies with wheel diametet and tire size).

Defintately no need for the lbs

Cheers
Steve

big manc

Cheers for that. I did put the wheel and tyre sizes in the computer my thoughts on the magnet position were that putting it close to the hub it would spin faster than putting it near to the rim. But hey the wonders of modern technology . thanks again

Cunobelin

Easiest way...................

Get a piece of chalk. and a tape measure.......
Place bike on a flat piece of pavement or road, withthe valve at the lowest point.
Mark position with chalk
Walk bike forward in a straight line until the valve is in the same position
Mark position

Measure the distance and this si the circumference of your rim, tyre and setup.

(For increased accuracy, do two rotations and divide by 2)

Siechotic

I did put the wheel and tyre sizes in the computer my thoughts on the magnet position were that putting it close to the hub it would spin faster than putting it near to the rim.

The magnet spins at the same revolutions regardless of where it is placed on the wheel.

The center of your wheel spins at the same number of revolutions as the outside.

big manc

Thanks everyone for the tips and advice

Bronzie

big manc wrote:

my thoughts on the magnet position were that putting it close to the hub it would spin faster than putting it near to the rim

You were asleep during physics weren't you

big manc

I never claimed to be scientist of the year. I asked a question which was answered for me before your little reply bronzie. :roll:

drenkrom

The chalk tip is great, but a friend of mine took it a step further:

- "Borrow' your girlfriend's lipstick (DO NOT tell her you'll be using it on a bike tyre)
- Apply a thin yet thick line onto your tyre, perpendicular to its rotation axis.
- Take your bike onto the kitchen floor (again, the girlfriend should NOT be around)
- Roll your bike for two wheel rotations.
- You should have lipstick marks on the floor, separated by the exact circumference of your wheel.
- "Borrow" your girlfriend's makeup remover.

Gives very precise measures.

CrookedCucumber

I would suggest calibrating against a gps unit over reasonable distance. That is, cycle (say) a mile on a reasonably straight road with the computer and a GPS, and compare the readings. Adjust the computer calibration to bring the two readings into agreement.

The problem is that the rolling circumference of a bike wheel is tricky to work out from its radius, even using the lipstick tip above. When your weight is on the bike and you're moving at speed, the tyre will deform, changing the effective rolling circumference.

Still, any kind of calibration will be better than relying on the `typical' figures supplied in the instructions.

p.ilchard

I would suggest calibrating against a gps unit over reasonable distance. That is, cycle (say) a mile on a reasonably straight road with the computer and a GPS, and compare the readings. Adjust the computer calibration to bring the two readings into agreement.

But make sure it's a level road - GPS systems give false readings on hills.

anthonyc

p.ilchard wrote:

But make sure it's a level road - GPS systems give false readings on hills.

If you are referring to the difference between the slant distance and the horizontal distance, the effect is quite small.

On a 4% hill,the difference between the horizontal difference travelled (as generally reported by a GPS) and the sloping distance is approximately 1 part in 1250.

CrookedCucumber

anthonyc wrote:

p.ilchard wrote:

But make sure it's a level road - GPS systems give false readings on hills.

If you are referring to the difference between the slant distance and the horizontal distance, the effect is quite small.

On a 4% hill,the difference between the horizontal difference travelled (as generally reported by a GPS) and the sloping distance is approximately 1 part in 1250.

There are all sorts of reasons why your GPS might give a distance figure that is not perfectly accurate. As you say, I don't think I'd be worried about gradient unless the terrain were very hilly. But it's worthing checking the sampling rate and any positional smoothing, if these things are adjustable on your unit.

In the end. however, I would expect that unless your GPS is very badly set up, you're still going to get a more accurate measure of distance from a GPS than just about anything else that most people have access to.

Before GPS I used to calibrate my computers by measuring out a few miles in my car, and then pedalling the same route. But I didn't realize at that time that car odometers typically over-read. It's surprising how difficult it is to get a really reliable measure of distance.

ColinJ

My bike computer packed up and I didn't bother replacing it. I find every day riding more pleasurable without bothering about speed and distance all the time.

If I'm riding an event, I use my Etrex GPS which can tell me everything I want to know and more. How about: speed. average speed, average speed when moving, elapsed time, ETA, altitude, distance covered, distance remaining, total distance covered since GPS last reset...? Oh, and OS grid reference accurate to 10 metres or so, plus sunrise and sunset times! Not to mention the tracklog which can be reviewed later to show me exactly what I was doing all the way through the ride.

I found using a bike computer for audax events a very frustrating experience. Even if my computer was well-calibrated, it didn't necessarily match the event organiser's. Over the course of a 200, the figures started to drift apart and I found myself having to add or subtract an increasing offest as I was riding along. I did one ride that I can hardly remember because I spent so much time looking at the route sheet and computer and doing mental arithmetic. I also ended up getting lost a few times, which seems to happen to most of my audax-riding pals too.

Since getting the Etrex, I've ridden about 10 200s and the worst navigational mistake I've made is to overshoot a turn by 100 metres because I was busy talking to someone. I've had people with computers and routesheets swearing to me that they are going the right way, but I just follow the GPS instructions and ignore them. Those riders usually come past me about 20 minutes later having gone round in circles to get back on course.

CrookedCucumber

I agree on all counts. I never give much though to my speed when I'm riding, and I'm not overly fussed about it after I've stopped.

But if you are going to use a speedometer, you might as well have it read the correct speed

And not everybody can afford an eTrex. I have one because I use it for hiking, where decent navigation can quite literally mean the difference between life and death. But I doubt I could justify the expense just for cycling.

Bronzie

big manc wrote:

I never claimed to be scientist of the year. I asked a question which was answered for me before your little reply bronzie. :roll:

Don't take it the wrong way - just having a giggle...............visions of rims rotating at a different rpm than the hubs would give an interesting spoke pattern I think

dennisn

Bronzie wrote:

big manc wrote:

I ............visions of rims rotating at a different rpm than the hubs would give an interesting spoke pattern I think

It's a concept whose time has come or maybe not. In any case if you pursue this I'm
sure it will become the next big thing. You just have to iron out a few problems and
the world is yours.

Dennis Noward

antfly

CrookedCucumber wrote:

I would suggest calibrating against a gps unit over reasonable distance. That is, cycle (say) a mile on a reasonably straight road with the computer and a GPS, and compare the readings. Adjust the computer calibration to bring the two readings into agreement.

The problem is that the rolling circumference of a bike wheel is tricky to work out from its radius, even using the lipstick tip above. When your weight is on the bike and you're moving at speed, the tyre will deform, changing the effective rolling circumference.

Still, any kind of calibration will be better than relying on the `typical' figures supplied in the instructions.

Do the same method whilst sitting on the bike then, it will be more accurate than a GPS which is why Garmins use the speed sensor to measure speed rather than the GPS.
If you can`t be bothered measuring just get the figures off Sheldon Brown or the computer instructions, they will all give you the same measurement within a couple of mms. 2096mm for a 23mm tyre I think.

CarleyB

Oooh i'm glad someone brought this up. i set one up but can't get it to accept mileage. All the other bits work..time, etc

I put the magnet thing (technical term ) it doesn't show speed or mileage. I set it up as per instructions. Help - cos the hubster isn't much more electtronically literate than me

Give me some ideas why it won't work...

dukegledhill

CarleyB wrote:

Oooh i'm glad someone brought this up. i set one up but can't get it to accept mileage. All the other bits work..time, etc

I put the magnet thing (technical term ) it doesn't show speed or mileage. I set it up as per instructions. Help - cos the hubster isn't much more electtronically literate than me

Give me some ideas why it won't work...

are you certain the magnet is triggering the sensor? might be too weak (it does happen). take magnet off wheel and wave it about right next to the sensor and watch the speedo... anything happen? if so it's possible the magnet wasn't close enough or out of alignment with the sensor. if nothing happens maybe a loose connection somewhere? or if wireless the sensor bit might have a flat battery

hope that helps
Duke

CarleyB

Oooh thats a good suggestion. thank you

CiB

On some the magnet has to line up with a mark on the sensor, and not [necessarily] the centre of the sensor.

As per the RPM discussion, I always put the magent / sensor nearer to the hub than the rim. Although the RPM is the same in both locations the effective speed of the magent passing the sensor is much slower nearer the hub. Logical - the magnet doesn't have to travel so far in the same time that it would out near the rim. At high road speeds there's less chance of the sensor not being triggered by the shorter time that the magnet takes to pass it. It also means that there's less surplus cable to lose between sensor and head unit.

And mark out the cirumference of your wheel & tyre, rather than relying on typical figures. You want to be measuring your bike, not an arbitrary mean. Go for the valve + chalk method. It's the easiest, and plenty accurate.

antfly

Using the suggested figures is even easier and plenty accurate,I did the roll out method using paint on the tyre whilst sitting on the bike and the measurement was exactly the same as the suggested figure.

Lillywhite

antfly wrote:

Using the suggested figures is even easier and plenty accurate,I did the roll out method using paint on the tyre whilst sitting on the bike and the measurement was exactly the same as the suggested figure.

I've been using and calibrating cycling computers since 1988 and I always did the roll out test.
For the last 5 years I havn't bothered and use the 2096 figure for my 23mm tyres because in the end analysis it makes very little difference.

ilm_zero7

drenkrom wrote:

- "Borrow' your girlfriend's lipstick (DO NOT tell her you'll be using it on a bike tyre)
-
- "Borrow" your girlfriend's makeup remover.

] personally I would ask to borrow the Lippy and remover - the consequences of being found with it inexplicably are worse!

DubaiNeil

antfly wrote:

Do the same method whilst sitting on the bike then, it will be more accurate than a GPS which is why Garmins use the speed sensor to measure speed rather than the GPS.

My Garmin Forerunner 305 only uses the wheel sensor if the GPS unit is unable to communicate with the satellites for any reason. In normal operation the Garmins will alwaysuse the GPS signal, which is why they don't come with the speed sensor in the original packaging.

I only got the speed sensor when I bought the optional cadence unit, which happens to measure both the cadence and speed.

The speed sensor is mostly intended for use on turbo trainers or rollers where (hopefully) you are not actually moving anywhere....

Neil

antfly

DubaiNeil wrote:

antfly wrote:

Do the same method whilst sitting on the bike then, it will be more accurate than a GPS which is why Garmins use the speed sensor to measure speed rather than the GPS.

My Garmin Forerunner 305 only uses the wheel sensor if the GPS unit is unable to communicate with the satellites for any reason. In normal operation the Garmins will alwaysuse the GPS signal, which is why they don't come with the speed sensor in the original packaging.

I only got the speed sensor when I bought the optional cadence unit, which happens to measure both the cadence and speed.

The speed sensor is mostly intended for use on turbo trainers or rollers where (hopefully) you are not actually moving anywhere....

Neil

Not true of the latest cycling specific Garmins, they don`t come with a speed sensor but if you have one then that is what it uses for speed all the time by default.

DubaiNeil

antfly wrote:

Not true of the latest cycling specific Garmins, they don`t come with a speed sensor but if you have one then that is what it uses for speed all the time by default.

How most very strange, a complete 180 degree shift by Gamin then - that'll teach me to use a bloo*y runners watch!

If I end up having to use the speed sensor due to lack of GPS signal then I also miss out on more advanced functionality as well such as auto-lap based upon passing the same position for a second (or third, fourth etc) time.

Neil

antfly

I don`t think you miss out on any functionality with the cycling ones cos the GPS is still on {as long as you`ve got a signal} it`s just not measuring speed.