What's the correct wheel circumference for 29" MTB wheels on the Kiox?

IggyBBR

Member
I have read so many threads, FB groups and forums on the topic, but I can get a definite answer.

What's the correct wheel circumference for 29" MTB wheels on the Kiox? Because if I do it, based on the roll-out method, I get a number thats higher than what can be inputted in the Kiox Display. I even brought it up to a few dealers, but they seem to brush it off. The highest size I can set is 91.37", but if I roll-out method I get the 92.52", just like in any tire size calculator.

Is there a chart some where, so it eliminates any guess work.

The bike is 2020 Trek Rail 9.7, with 29" wheels and 2.5" tires. The default Kiox setting was 90.35".
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Alaskan

Well-Known Member
The only way to eliminate guess work and get the actual circumference of your tires at the inflation level you use it to actually measure it.

The easiest and most accurate way to do this is to tie a bit of yarn or a zip tie transversely around the tire between two spokes. Using a piece of electrical tape, mark the spot on the floor where the yarn or zip tie is at the lowest point, centered at the contact patch of the tire. Roll the bike forward one tire rotation till the yarn is straight down again and mark that spot with tape. Measure the distance between the tape marks. For the best precision, place the tape such that the front of the tape is perfectly even with the front of the zip tie or yarn each time and measure from the front of both pieces of tape.

Note that control over the min. and max. available circumference, that the user can enter into the system, is set by the manufacturer of the bike, based on the range of tires they approve for the bike, not by Bosch. In case of my Trek Allant 9.9s I had to bring the bike in when the shop could put it on the diagnostic system and a guy from Trek HQ got in on line and expanded the limit to accommodate my tires. Interestingly the installed limits were shy for the stock tire that the bike came with.
 

IggyBBR

Member
I have done the multiple measuring methods, and came to roughly the same numbers as Cateye's calculator. I guess I need to take another trip to a Trek dealer and see if what they can do?
 

peterh_nz

New Member
Region
New Zealand
I have done the multiple measuring methods, and came to roughly the same numbers as Cateye's calculator. I guess I need to take another trip to a Trek dealer and see if what they can do?
Hello Iggy.

I have successfully used the rolling method for calibration. I found that sitting on the bike while rolling was essential.

The results made sense and were within the range preset by the manufacturer.

The tires were the manufacturer's originals.

Good luck

Peter
 

IggyBBR

Member
Hello Iggy.

I have successfully used the rolling method for calibration. I found that sitting on the bike while rolling was essential.

The results made sense and were within the range preset by the manufacturer.

The tires were the manufacturer's originals.

Good luck

Peter
Thanks Peter, I'll try that in the morning. (Sorry for being a PITA) Quick question, what sizes are your tires and do you have the same limits of 91.37" on your system.
 

peterh_nz

New Member
Region
New Zealand
Thanks Peter, I'll try that in the morning. (Sorry for being a PITA) Quick question, what sizes are your tires and do you have the same limits of 91.37" on your system.

I should have stated the my bike has 27" wheels and is a different make.

That said, the technique worked for me. Putting your full weight on the bike, in normal riding position, will slightly decrease the diameter and more closely represent the effective circumference when riding.

Are you using standard tires for that bike?
 
Last edited:

IggyBBR

Member
I should have stated the my bike has 27" wheels and is a different make.

That said, the technique worked for me. Putting your full weight on the bike, in normal riding position, will slightly decrease the diameter and more closely represent the effective circumference when riding.

Are you using standard tires for that bike?
The original tires that came with the bike were the Bontrager XR5 (29x2.6"). My current set are a set of Maxxis 29"x2.5" (front- Asseai, rear- Aggressor).
I just redid the measure using your recommended method and with my current pressure of 30psi, it rolled out to 89". Thank you, for the information. It is me or this method not mentioned in the Bosch's documentation or any standard tire measurement procedure?
 

Tars Tarkas

Well-Known Member
I agree with Richard and Peter on measuring circumferences. There can be slight differences in your results depending on tire pressure and the weight on the bike, and whether you measure in exactly a straight line. Whether Bosch or anyone else suggests this method, the circumference is a finite number (subject to the variables I just mentioned.) I wonder where you got the numbers you entered into the calculator you used. Whether off the tire or from your own physical measurements, they are bound to be off by enough allow for at least a 2" margin of error, I would guess.

I calculated, and a 2" error would add up to 179 feet every mile, which I agree is significant, especially when multiplied by hundreds or thousands of miles a year. And it's about a mile every 30 miles. For a lot of people and purposes though, that's good enough. Get it as close as you can with reasonable effort and move on would be my advice.

Another method to deal with this would be to ride a measured mile and see how far your odometer is off. Do the math, and make the necessary adjustments.

TT
 

William - Bosch Team

Active Member
Unfortunately, there is not a universal value for any general tire size. The actual circumference will vary based on width, make, and model of tire, as different tires manufactures measure and manufacture different. Some measure width at the casing, others at the tread. Some over estimate, some under estimate. Rim width will also affect the circumference of any given tire. All of this not considering the effects of different amounts of pressure, and the resulting tire sag.

That being said, OEMs using the Bosch System on their eBikes determine the effective circumference they will program in the DU of any given model. Some OEMs use a general, theoretical value, while others use the average of multiple physical, weighted rollouts during testing. Some use a complex value based on factors that are not made public. Whatever the case, all OEMs are working towards the same thing- making the ride experience consistent with their product design.

This factory-programmed value cannot be adjusted at the dealer, which you know, but dealers CAN adjust the displayed speed by +/- 5% (some Bosch displays allow this 5% cosmetic adjustment).

If you feel your programmed value is way off with your stock tires, you can visit your local Certified dealer, and have them contact the OEM to check your bike's programmed circumference against what the OEM has on file for your particular make/model.