The Big Wheel

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?

cranky old road 

Might be helpful if you identify the brand and model of the computer.

The Big Wheel

The bike computer is actually a Bafang 500w motor hub with a 800S Display.

njkayaker

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.)

The Big Wheel

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.

Iride01 

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.

veganbikes

You might ask to get this moved to the E-bike subsection there are people with some Bafang knowledge.

njkayaker

What the OP said and doing a search indicates it’s a list of wheel sizes. I mentioned circumference (why it was better)and the OP didn’t come back that circumference was an option. Seems safe to conclude it doesn’t use circumference.

(It should use circumference, since that allows for a more accurate number.)

Iride01 

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.

njkayaker

This isn’t an actual real problem. It’s irrelevant anyway. (It's not that uncommon to have two speedometers and people appear to have no difficulty with it.)

The bike is still going to report the speed, etc. What it reports should be as accurate as possible.

The Big Wheel

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.

Darth Lefty 

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!!!

Iride01 

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.

njkayaker

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

Iride01 

I'm just asking questions out of curiosity mostly and mainly expected that my comments and questions were for the OP. I did one try to search for the manual and came up empty. I wasn't expecting you to fill me in, but I appreciate that you did.

bktourer1

Check the Sheldon Brown site. I believe there was a chart as to setting bike computers.

Darth Lefty 

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

njkayaker

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.

"Bafang 800S" was enough to get hits. Maybe, you used too many search terms?

It wasn't quite clear what was going on. So I did a search. It was easy to provide those results.

njkayaker

42 is the answer <- not an opinion.

Iride01 

Does it have to?

Just think of me as ADHD without the H and oncoming dementia from old age. <grin>

njkayaker

Yes, obviously. Things should generally make sense. Having people wonder if a post of yours is one of the ones that "doesn't make sense" seems like an unreasonable ask.

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.

Iride01 

Really? This looks like I'm trying to provide an answer?

It only looks to me like I'm being inquisitive. And that's all it is. I didn't want to do the legwork to search for a manual. And was just trying to somewhat understand what this was using to determine the wheel circumference to determine distance and speed.

Maybe I'm not the only one with some type of dementia! <grin>

njkayaker

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. And that suggests that the answer still could be the circumference nonsense (it keeps this open as a possible solution).

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. Though, if you actually have dementia, it would certainly explain your posts here.

Iride01 

I don't read everyone's comments in a lot of threads. The OP's comment that I ask a question to didn't seem to indicate any prior reply had a solution in it. So why would I go back and look for that?

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.

njkayaker

It doesn't seem like you read the OP (very carefully).

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).

The question the OP asked was an indication that this wasn't an option.

Not surprising.