Wrong Wheel Size?

[GrahamPaul]

No, this is not another rant about 27 vs. 29 wheels!

I've just been comparing the figures off my Garmin Edge against those from the Bosch display.

The distance figure on the Garmin is ±66.8km

On the Bosch display it's ±72km

I've also noticed that the handlebar display consistently shows a speed of a few km/hr above that shown on the Garmin. (I need to find a long flat road to ride at a steady speed to test this better, but the Garmin always seems to read lower).

The ratio of the distances is exactly the same as 27/29.

Now this is where I start to get brain ache. My bike has 29" wheels. If it was set to 27" wheels in the Bosch computer then the number of rotations to achieve 25km/hr cut off would be greater than the number of revolutions for a 29" wheel for the same speed.

To put it another way, for the same number of revolutions of a 27" wheel, a 29" wheel will go further. Therefore a given value of 100km measured on the Garmin should show as 100 x 29/27 = 107km if the wheel size is set as 27" in the Bosch software. Does that make sense?

But that's not what's happening. I'm getting less distance on the Garmin than on the Bosch display. It's almost as though I have 27" wheels installed and the system is set for 29". (I checked! The wheels and tyres are Twentyniners! ).

Any thoughts before I see the LBS on Monday morning?

 

[Zimmerframe]

It's a well known fact that the marketing machine, and @Gary are pushing for 31inch wheels .. Or "Big One's" as the sales/marketing/slang has them pre-emptively named.

You have the latest Bosch and it's default factory setting is 31inch wheels .. sounds like they've forgotten to step it down to 29" ..



Have you got air in the tyres ? As in more than atmospheric pressure ?

 

[Gary]

The distance figure on the Garmin is ±66.8km

On the Bosch display it's ±72km

Neither are correct.

Any thoughts before I see the LBS on Monday morning?

er... Stop caring?
It's not as if you're actually powering the bike yourself for most of it anyway so why waste your local bike shop's valuable time? That's time that could be spent helping genuine cyclists you know

HTH

Alternatively. D
Do you have the garmin speed sensor installed?
If you genuinely care about distance accuracy. buy and fit one (to your front wheel). and make sure it's calibrated EXACTLY to your tyre circumference.
Then have the Bosch software calibrated EXACTLY to the rear tyre circumference.
Oh... and you'll also need to re calibrate circumference every time you change/fit new tyres or wear them down.

Can you really be bothered doing all that for the sake of some pointless data?

 

[GrahamPaul]

Neither are correct.


er... Stop caring?
It's not as if you're actually powering the bike yourself for most of it anyway so why waste your local bike shop's valuable time? That's time that could be spent helping genuine cyclists you know

Oi! Don't you know it's rude to eavesdrop people's conversations?!

I mean, I'm pretty sure that the conversation I was holding with myself while I was pootling along this morning was all in my head. But the trouble with having reached this decrepit old age is that incontinence has set in. Happily, for the moment, this is only the verbal type - but there are those times when I'm sure I thought it until I see the stunned faces around me...

Can you really be bothered doing all that for the sake of some pointless data?

Now you just go wash your mouth out with soap, you young whippersnapper!

Yer talking here to the proud owner of a Shimano Flight Deck who even went to the lengths of tinkering with tyre pressures to tune the circumference measurement - which I then checked against the measured kilometre on a local, straight, smooth, level cycle path, which had been installed for the OCD amongst us to check our cycle computer calibrations. (That's hard, possibly, for a Scot to understand: a 1 kilometre straight path which is also horizontal. As for living in a country where such things are installed for citizens to check their cycle computers - that'll probably have you spitting out yer porridge at the sheer waste of it all!).

However, in the interests of scientific investigation and satisfying my OCD neurosis, I did a bit of road work today and I think I may have an answer. And that answer is "Marketing".

Let me explain: I found that the motor tails off to zero at 27.9km/hr as shown on the Bosch display. On my Garmin gps (I know! I know! The accuracy is not brilliant but it does give a good indication of speed when maintained for a "reasonable period") this motor-off speed was the magic 25km/hr.

When the Bosch displayed 25km/hr the motor was still pulling like a train. I tested the effects on some rises just to make sure that it wasn't the clutch hanging so that my legs were driving the motor.

My thesis: Bosch are overstating the speed on their displays (just like Ford do on their car speedos) as a marketing ploy to make people believe that they are "cheating" the law just that little bit and going that little bit faster than their mates.

I won't be able to test this until Friday when I go on the "Old Gits Coffee Ride", where the others all ride along at exactly 0.0001km/hr under the motor support cut-off speed. I'll soon find out whether I'm keeping up by leg power or not

 

[GrahamPaul]

It's a well known fact that the marketing machine, and @Gary are pushing for 31inch wheels .. Or "Big One's" as the sales/marketing/slang has them pre-emptively named.

You have the latest Bosch and it's default factory setting is 31inch wheels .. sounds like they've forgotten to step it down to 29" ..



Have you got air in the tyres ? As in more than atmospheric pressure ?

The air has definitely stayed in the tyres now that I've changed the rim tape. But I think that I may have added too much tyre sealant. The weight of this has caused the bottom of the tyre to bulge outwards, rather than inwards, from the sheer weight of it all. This bulge has resulted in an extra inch of radius and I now have 31 inch wheels. It's the only possible answer.