Bike Computer Wheel Diameter
I have a bike computer and I'm having trouble setting up the correct wheel diameter for an accurate mph speed. My wheel is 700C with a 38mm tire. On the bike computer there is no 700C setting, only 6-34.
I'm trying to decide if a 700C wheel is closer in size to a 27 or 28. I'm thinking 27 but I have read that a 700C 32mm or wider is bigger than 27 and so has to be 28. What do you think? |
Might be helpful if you identify the brand and model of the computer.
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The bike computer is actually a Bafang 500w motor hub with a 800S Display.
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Originally Posted by The Big Wheel
(Post 23226688)
The bike computer is actually a Bafang 500w motor hub with a 800S Display.
If it shows a list of wheel sizes, it’s making unknown assumptions about the tire size. This is kind of a dumb way to set this up. Cycle computers usually let you specify the radius, diameter, or circumference (basically interchangeable) of the whee/tire combination. This lets you do a “roll out” that will take into account the wheel and tire size, what ever pressure you use, and the load (the total weight). What you could do is compare the speed reading from the computer against a phone and pick the next higher or lower setting. (Be careful doing that.) |
Originally Posted by njkayaker
(Post 23226740)
What you could do is compare the speed reading from the computer against a phone and pick the next higher or lower setting. (Be careful doing that.) That's actually a great idea, I also have a Garmin Edge Explore 2 GPS, I can just compare the two and increase or decrease the input number. |
If the setting is by the circumference, then you can estimate it by adding twice the tire width to the BSD of the wheel and multiplying by 3.14. Tire height on the rim is roughly equal to it's width. A 700C wheel will always have a BSD of 622mm.
So that'd give you a circumference of 2191.72. So use 2192. If you find that is doesn't come up true when you ride a measured distance, you can adjust that number to match by just using simple math to figure out the difference. Since you are using a really wide tire on that 700C rim, you might find your answer in the tables for a 29er or 29" tire which also will always have the same 622mm BSD. Up until recently, 700C tires where typically less than 30 mm. So a lot of tables for wheel size on bike computers never went larger. You can also measure it by doing a roll out. Put a piece of tape on the ground and then put the valve stem of the wheel over it. With your weight on the bike, roll the tire one revolution and then measure the distance between in millimeters. Or roll several times keeping track of the number of turns and then measure and divide. |
You might ask to get this moved to the E-bike subsection there are people with some Bafang knowledge.
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Originally Posted by Iride01
(Post 23226803)
If the setting is by the circumference,…
(It should use circumference, since that allows for a more accurate number.) |
The OP mentions that a Garmin Edge Explore is used too. If that's going to be a permanent fixture on the bike to get distance from both. Then that might for various activities be like the proverbial man with two watches.
But it will let the OP find a value that close enough. |
Originally Posted by Iride01
(Post 23227278)
The OP mentions that a Garmin Edge Explore is used too. If that's going to be a permanent fixture on the bike to get distance from both. Then that might for various activities be like the proverbial man with two watches.
But it will let the OP find a value that close enough. The bike is still going to report the speed, etc. What it reports should be as accurate as possible. |
Guys, I got an email back from the seller and get this, he says to input "29" into the computer.
But I'm pretty sure that my 700c 38mm tire equals to a 28" 1.5 inch tire. At any rate I set it to 28 right now and will compare it to the Garmin GPS. |
Well if you set it to 29 you will see the maximum speed it could possibly be so it you can feel really good about your purchase.
Really it's right in between 27 and 28. You might be as much as 1.8% off!!! |
So does it ask for anything else? Or does it seem that you only enter the approximate overall diameter and it figures it out from there.
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Originally Posted by Iride01
(Post 23227614)
So does it ask for anything else? Or does it seem that you only enter the approximate overall diameter and it figures it out from there.
https://support.biktrix.com/kb/guide...1035267,943481 Wheel Diameter The speed display will show Wd. You will not need to change this setting unless you are putting different diameter wheels on your bike than what came with it. Press the UP and DOWN buttons to change between 16/18/20/22/24/26/700C/28/29 inch wheel diameters. |
Originally Posted by njkayaker
(Post 23227711)
Looking up the manual for the display shows that it provides a choice of "16/18/20/22/24/26/700C/28/29 inch wheel diameters". This doesn't match what the OP said exactly (which is why I didn't provide the link earlier). But, yes, all evidence points to only being able to enter the "approximate overall diameter". (I'm not sure why you keep insisting following a path that there is no evidence of existing. I'm also not sure why I'm the only person who appeared to do any research.)
https://support.biktrix.com/kb/guide...1035267,943481 |
Check the Sheldon Brown site. I believe there was a chart as to setting bike computers.
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Research was not needed because OP is not confused at all about the way his computer works. He just wanted opinions whether to pick 27 or 28.
27 is clearly the only possible correct answer. <- this is an opinion |
Originally Posted by Iride01
(Post 23227778)
I'm just asking questions out of curiosity mostly and mainly expected that my comments and questions were for the OP.
Originally Posted by Iride01
(Post 23227778)
I did one try to search for the manual and came up empty.
Originally Posted by Iride01
(Post 23227778)
I wasn't expecting you to fill me in, but I appreciate that you did.
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Originally Posted by Darth Lefty
(Post 23227781)
27 is clearly the only possible correct answer. <- this is an opinion
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Originally Posted by njkayaker
(Post 23227820)
It was established that the computer (likely) didn't use circumference and you tried to drag it back to that. You are giving the impression that you aren't paying attention (to the OP or other posts). Doesn't make sense.
Just think of me as ADHD without the H and oncoming dementia from old age. <grin> |
Originally Posted by Iride01
(Post 23228237)
Does it have to?
Continuing to talk about the circumference is communicating that it might matter to the OP. When, as far as other indications show, it isn't relevant at all. That is, it looks like you are trying to provide an answer but you aren't really. |
Originally Posted by njkayaker
(Post 23228317)
Sure, it matters. Continuing to talk about the circumference is communicating that it might matter to the OP. When, as far as other indications show, it isn't relevant. That is, it looks like you are trying to provide an answer but you aren't really.
So does it ask for anything else? Or does it seem that you only enter the approximate overall diameter and it figures it out from there. Maybe I'm not the only one with some type of dementia! <grin> |
Originally Posted by Iride01
(Post 23228327)
Originally Posted by Iride01
(Post 23227614)
So does it ask for anything else? Or does it seem that you only enter the approximate overall diameter and it figures it out from there.
This quote is suggesting the OP missed something. And that suggests that the answer still could be the circumference nonsense (it keeps this open as a possible solution).
Originally Posted by Iride01
(Post 23228327)
It only looks to me like I'm being inquisitive. And that's all it is.
Originally Posted by Iride01
(Post 23228327)
I didn't want to do the legwork to search for a manual.
Originally Posted by Iride01
(Post 23228327)
And was just trying to somewhat understand what this was using to determine the wheel circumference to determine distance and speed.
Originally Posted by Iride01
(Post 23228327)
Maybe I'm not the only one with some type of dementia! <grin>
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Originally Posted by njkayaker
(Post 23228331)
It's the other stuff about the irrelevant circumference that looked like you were providing an answer.
This quote is suggesting the OP missed something. No, it looks like you aren't paying attention. Since the answer was basically provided earlier. And kind of lazy. But was fine with a longish (and irrelevant) post about how to do a roll-out. (You said you tried to search earlier. Here, you suggest you didn't do a search at all.) If you read the posts carefully, the answer was there. Yet you kept asking pointless questions (questions the OP isn't likely to want to answer). No. If you actually have dementia, it would certainly explain things here/ I don't see any issues with my question. My prior statements before that don't matter anymore for that question as the OP's comment about entering a simple number that I wasn't sure was a wheel diameter or wheel size name. And I was curious if it'd also ask for something else such as width of the wheel which would have put the accuracy to which the calculation for distance and speed to maybe 2% instead of what might just be 5% or more based on what I currently understand. |
Originally Posted by Iride01
(Post 23228354)
I don't read everyone's comments in a lot of threads.
You should have read more of the comments in this thread instead of spending the time posting the long (and irrelevant) roll-out post (which indicated you didn't understand what this thread was about).
Originally Posted by Iride01
(Post 23228354)
And I was curious if it'd also ask for something else such as width of the wheel which would have put the accuracy to which the calculation for distance and speed to maybe 2% instead of what might just be 5% or more based on what I currently understand.
Originally Posted by Iride01
(Post 23228237)
Originally Posted by Iride01
(Post 23228354)
I don't see any issues with my question.
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