Is my cycle computer wrong?

[RSV_Ecosse]

I've got a Bikemate cycle computer, one of those Lidl jobs from last year.

I thought it was reasonably accurate but I'm not sure now.

It has been telling me that my usual commute to work is @15 miles each way. However I was on Bikehike today and mapped out my route exactly as it is. Bikehike is telling me that my commute is just under 20 miles.

That's a bit of a difference, innit??!!!.

What's going on there?. How accurate is the Bikehike route planner thingy?.

 

[HLaB]

Did you set your wheel circumference in the computer?

Bike Hike is probably a bit like bikely and I find its reasonably accurate (certainly more accurate than +5 miles) it tends to be out by about 1 mile in 100 or less (my last ride it was out by 0.7miles in 110.5 miles) as it takes routes straight down the road which I don't always do

 

[RSV_Ecosse]

Yeah, set up the Bikemate as per instructions I read/asked about on here, circumference included.

I was just rather shocked to find the difference of @5 miles between the two.

 

[fossyant]

Yup - you've probably not set the computer properly...

adjust accordingly...

 

[RSV_Ecosse]

I recall asking about the circumference in this thread :-

http://www.cyclechat.co.uk/forums/showthread.php?t=18816&highlight=computer

It's been set at 2095 since I got it.

 

[RSV_Ecosse]

Hmmm. According to Sheldon, it should be 2097 not 2095 :-

http://www.sheldonbrown.com/cyclecomputer-calibration.html

Would a difference of 2mm make any huge odds?.

I'm going to adjust it to 2097mm and try it out on the way to work tomorrow to see what happens.

 

[HLaB]

My cat eye says 2096 but that's not going to make a difference of 5 miles. My computer tend to mis read like that when the battery is near dud and it doesn't pick up every circumference of the wheel. My cheap wireless is particularly prone to this and interference. A sign is when I glance down at it says I'm going 6mph when I'm freewheeling down a steep hill.

 

[Angelfishsolo]

The best way to get the correct circumference measurement is as follows. Place a chalk mark or piece of tape on the tyre wall and floor and line up the two. Push the bike forward under the mark on the wheel is back on the floor again. Mark that point. Measure the point to point distance in mm. That is the wheels circumference. It is almost certainly be different to figures given in the set up guide.

That said, my friend bought one of the Lidi cycle computers and within weeks was getting erroneous readings from it so maybe they do have some inherent issues???

 

[HJ]

Hmmm. According to Sheldon, it should be 2097 not 2095 :-

http://www.sheldonbrown.com/cyclecomputer-calibration.html

Would a difference of 2mm make any huge odds?

Given that it is about 16,000 revolutions of the wheel yes...

 

[Angelfishsolo]

Given that what is 16,000 revolutions of the wheel??

By my calculations a 2mm variance in circumference would introduce a <0.8m discrepancy / mile.

Given that it is about 16,000 revolutions of the wheel yes...

 

[Banjo]

I did a measured mile with the car to check my Halfords computer. The two agreed exactly .Also my sons have computers so we see if theres any discrepancy between us now and then.

Re setting up the circumference measurement,I think the chalk mark measurement should be done with you sitting on the bike as your weight will be flattening the tire a mm or 2 which will reduce the wheels effective diameter a bit.

 

[Hont]

The best way to get the correct circumference measurement is as follows. Place a chalk mark or piece of tape on the tyre wall and floor and line up the two. Push the bike forward under the mark on the wheel is back on the floor again. Mark that point. Measure the point to point distance in mm. That is the wheels circumference.

+1

For greater accuracy push the bike forward until the wheel has done several revolutions then divide the total distance by the number of revolutions for your circumference.

 

[Downward]

So say I had mine set up as 2155 for ages when it should be 2168 what would 10 miles = on each of the above ?

My Lidl computer tells me its 6.5 miles each way but before I accidently reset it it came through as 6.9 miles each way

 

[jimboalee]

Do Sheldon's 'Roll out' test.

 

[Angelfishsolo]

There would be a 45 meter discrepancy over 10 miles (I think)

So say I had mine set up as 2155 for ages when it should be 2168 what would 10 miles = on each of the above ?

My Lidl computer tells me its 6.5 miles each way but before I accidently reset it it came through as 6.9 miles each way