Wheel Size Adjustment

[Peaky Rider]

A few months ago I bought a Rail 7, which Rail experts will know comes with 29" wheels and a Purion display, with a Bosch motor. I haven't ridden it much until recently but I did swap out the Purion display for a Kiox quite soon after the initial purchase.

Now that I have started riding the Rail I noticed the distance recorded on the Kiox was reading some 10% lower than my Garmin. My Garmin agreed with others on the rides.

No problem I thought, the Kiox allows you to alter the wheel circumference,within certain limitations.

However, when attempting to do this I noticed my bike was set to a default setting which corresponded to that of a 27.5" wheel and the largest circumference I could set it to was 91.7" where as my 29" wheel and tyre has a circumference of 94".

Can anyone explain to me what is happening and how, in theory, is this affecting the actual speed of my Rail?

Surely, if the setting is for a smaller dimension wheel that would give an inflated mileage reading? I could be wrong on this but my brain is starting to hurt.

Also posted in Bosch motor forum.

 

[steve_sordy]

Is the Garmjn accurate?

Have you measured the circumference of your rear wheel, loaded and unloaded? I chalk a line on the pavement, line up the valve and with me on the bike, roll it forward until the valve is at the bottom again. Mark with chalk and measure.

 

[Peaky Rider]

As I mentioned in my post, my Garmin is above suspicion as it agreed with others on the ride Steve. I measured the wheel loaded and no, I didn't try and wrap my wifes sewing tape measure around it.

Since starting this thread I have googled the 'naked' circumference of a 29" rim and it falls just within the upper limit of adjustent allowed on the Kiox, so I guess the adjustments must be based on an unloaded/tyreless/naked wheel.

I find that surprising as I thought the adjustment was to account for different tyre sizes but maybe not.

I will just fiddle with it on a few test rides now to get it reading somewhere near the Garmin readout.

 

[Mikerb]

are you aware that a supposed 29 inch wheel actually measures 28 inches!!

 

[Peaky Rider]

are you aware that a supposed 29 inch wheel actually measures 28 inches!!

Have you been talking to my wife?

 

[steve_sordy]

are you aware that a supposed 29 inch wheel actually measures 28 inches!!

It all depends upon the tyre fitted.

Quite a few years ago, my mate changed from a bike with 26" wheels to one with the latest 27.5". Riding with him soon afterwards I asked him if he could tell any difference. When he said he couldn't, I measured his wheel diameter and then compared it to mine. I had 26x2.2" Conti TKs on, and he had 27.5 x 2.1 Rocket Rons or something similar. His wheels measured 27.25"overall diameter, mine measured 26.75". No wonder he couldn't tell the difference.

 

[Mikerb]

The ETRTO standard specifies the rim diameter
27.5 = 584mm =23 inches
29 = 622mm =24.5 inches
26 = 559mm =22 inches

 

[Peaky Rider]

The ETRTO standard specifies the rim diameter
27.5 = 584mm =23 inches
29 = 622mm =24.5 inches
26 = 559mm =22 inches

I think that has cracked it Mikerb. I've just measured my 29" wheel and it came out at 24.75".
So, the settings in the Kiox/motor look like they are based on the dimensions of the wheel, not the wheel dressed in a tyre.
Seems strange to me but Bosch must have their reasons.

 

[steve_sordy]

......................
So, the settings in the Kiox/motor look like they are based on the dimensions of the wheel, not the wheel dressed in a tyre.
Seems strange to me but Bosch must have their reasons.

VERY strange indeed. Apart from Danny MacAskill, who rides a bike without tyres on the rims?

 

[Mikerb]

It makes sense that the software would be adjusted for wheel size only since it cannot know what profile of tyre is fitted or indeed whether the tyre is fully inflated or run soft. The speed sensor software presumably makes an allowance for an average tyre..possibly 2.3 or 2.4.

 

[Peaky Rider]

i just checked mine and it is 86.61 and my wheels are 27.5

I see you have a Bosch motor with a Kiox display so can I ask what range of settings you have when you go into "wheel adjustment settings" in the Kiox?