Speed Difference
- Thread starter pauls
- Start date
Count yourself lucky you don't have to pay the dealers to ship the bike back to Germany to get the wheelsize changed.
uk dealers can change the wheel size a bit but you cant use a motor with 27.5 wheels with a bike with 29inc wheels.
bosch will not change this and no dealer can do it either so if you need a 2nd hand motor make sure it is for your wheel size first
It's no different to the speedometer on a car, it's set based on the original specifications so when you change wheel or tyre size this will give a false reading.
I've used a Garmin GPS and my phone's GPS to check my Bosch intuvia and all give slightly different readings for speed and distance. The variable is also different from one ride to the next. I've previously used the Garmin for checking map navigation during mountain leaders training and the accuracy is inconsistent. This in part is down to the number of satalites it's able to lock onto and can be influenced by atmospheric conditions. Mobile phone GPS is even less accurate and I have found it to be typically 3m to 5m out on actual grid position.
Just to add, I haver noticed the exact same issue (display giving a higher speed / distance compared to GPS, appx + 1 Mph) with my new Bosch powered ebike and Intuvia screen. My bike is due a free first service later this month so I will mention it when I take it in and see what they say.
My Shimano Step 2 display reads a greater distance travelled than my Garmin watch does. I believe the watch over the display.
As an example: A Marathon Plus tyre with its' added 5mm puncture-proof band has a taller profile / a physical diameter visibly larger than an equivalent standard/ordinary tyre.
Since speed and distance is calculated on outer tyre diameter, it stands to reason there will be some discrepancy based on what tyres you're running. My two e-bike displays consistently show 0.5 to 1mph less than my GPS once I'd changed those standard factory fitted tyres to same-technical-size but physically larger diameter MP's - where speed/distance was identical before.
On a standard cycle computer you can precisely set that outer tyre diameter: On an e-bike you can't, so you just have to accept there will be some slight error (against GPS anyway, which in my case is far more precise).
Since speed and distance is calculated on outer tyre diameter, it stands to reason there will be some discrepancy based on what tyres you're running. My two e-bike displays consistently show 0.5 to 1mph less than my GPS once I'd changed those standard factory fitted tyres to same-technical-size but physically larger diameter MP's - where speed/distance was identical before.
On a standard cycle computer you can precisely set that outer tyre diameter: On an e-bike you can't, so you just have to accept there will be some slight error (against GPS anyway, which in my case is far more precise).
The difference is, with a car, extremely few people will ever fit it with different tyre sizes than it was sold with. With bikes, they could be fitted with all sorts of different tyre sizes, which vary from one manufacturer to the next. Also, a 1mph inaccuracy in a car means nothing much, whereas to a bike it makes a significant difference.
Basically I agree. Difference is proportional though, so same error in car tyre size would give several mph error.
Car speedo's are made to a legal standard and every car will show a slight overspeed between 1 to 3 mph at a steady 30mph. Generally car drivers don't care.
Bike computers and inbuilt systems are not made to any legal standard in this country, which is why the police can't prosecute a cyclist for speeding. It is also why different systems show slight overspeeds depending on country of manufacture and individual company policy.
Bosch appear to set their systems to show about 1 mph too high, If you have a Purion you can't do much about it. If you have an Intuvio or higher you can reduce the wheel outer circumference to the minimum (about 2.5% lower) and that will largely tune out the margin of error Bosch build in.
The question is, how much do you actually care.
Darren
Bike computers and inbuilt systems are not made to any legal standard in this country, which is why the police can't prosecute a cyclist for speeding. It is also why different systems show slight overspeeds depending on country of manufacture and individual company policy.
Bosch appear to set their systems to show about 1 mph too high, If you have a Purion you can't do much about it. If you have an Intuvio or higher you can reduce the wheel outer circumference to the minimum (about 2.5% lower) and that will largely tune out the margin of error Bosch build in.
The question is, how much do you actually care.
Darren
No, the police can't prosecute because speeding isn't illegal on a regular pushbike or pedalec.
If it were an offence it wouldn't be an excuse that you didn't know you were speeding.
In a car, having a wrongly calibrated speedometer is a separate offence from speeding itself.
The GPS system is USA controlled and at the moment Donald Trump is in charge - I think I'll stick with my Cateye until he's gone ......
My gpsmap 64 can use other constellations too, not that I trust the owners or operators of those any further than Donald Duck.The GPS system is USA controlled and at the moment Donald Trump is in charge - I think I'll stick with my Cateye until he's gone ......
If you bought your bike from a decent dealer he won’t charge for small tweaks in the software.
It takes about 30 seconds after plunging the usb cable in.
Years ago used to be obsessed about the correct distance so did the following, using a Cateye computer. line up a chalk mark on the tyre with one on the floor. With the tyre at your normal pressure and you sat on the bike move forward one revolution and put another mark on the floor. Measure between the marks and input as the circumference. Mark out a measured distance on an OS map and ride it. I would then compare the Cateye and OS distance over several rides up to 30 miles and plus or minus the percentage difference in the Cateye.
I have since got a life and now, if I’m tired, I’ve gone too far.
When I got my trek bike the wheel circumference was set to the default, which was exaxctly in the middle of the range. I had to set it myself. I have the kiox display, so I don't know where it would be on the purion, but you are looking for a setting called 'wheel circumference'.
There are various wheel circumference calculators on line - you need to know your rim and tyre size. Alternatively, just wheel your bike in a straight line for one revolution. Mark the start point and end point and measure the distance. Easiest to do if you start with the valve at the bottom of the wheel.
you cant change it with a purion display so only a dealer can change it, tho you can do it with the intuvia one.
Another simple way is to sit on the bike and place a spirit level on the centre of the top of front tyre with the bubble showing level. Then the distance from the level to the ground is the diameter, that times Pi (3.14) is the circumference.
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