anyone know how to figure out Tire Radius/rim radius?

[Joel Stern]

I was asked to get my tire radius and I have no cluehow to do it.

I am riding the 42mm Baby Shoe Pass

I have them on a Velocity Synergy rim, this are the specs. I am running the rear at 40psi and the front at about 37psi.   Any help will be appreciated.

Thanks, Joel

At 23mm wide, with eyelets and a low profile, it's the perfect rim for all those times you want to use a little wider tire. Use it for your 29'er or your commuter. The possibilities are endless.

Item Specifications
Color	Silver
Weight	584g
Width	23mm
Intended Use	Road
Hub Drilling	32spokes
Brake Compatibility	Rim Brake
Valve	Presta
ISO Diameter	584 / 650b / 27.5"
Valve Length	Short 32-40mm
Wheel Size	650b
Tire Type	Clincher
Rim Center Offset	0mm
Rim Material	Alloy
Brake Track	Non-Machined
Defined Color	Silver
Internal Rim Width	17mm

[Jason Fuller]

I go with this simple formula, which may not be exactly right but it's close I think, and good for comparing one size to another. 

26" = 559mm 

650B = 584mm 

700c = 622mm 

Tire outside diameter = rim diameter + (2 x tire diameter)

ie 650b x 42 would be 584 + 42 + 42 = 668mm -> your radius is therefore half that, 334mm

I think this is accurate enough for whomever's asking, but if you have the bike with the wheels and you can just measure, just measure ground to the top of the tire and divide by two

[Joel Stern]

Thanks Jason. 

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[Ted Durant]

On Wednesday, July 15, 2020 at 6:21:55 PM UTC-5 Joel Stern wrote:

I was asked to get my tire radius and I have no cluehow to do it.

Jason gave the straight math version from the rim and tire sizes, which is fine for most purposes.

If you want to get it more precisely, here are two approaches. For both of them, if you want it accurate for when you are on the bike, you'll want some help. Note that the results are tire pressure dependent.

With the bike as close to perpendicular to the ground as you can make it, measure the distance from the ground at the center of the tire contact patch to the center of the axle. Measure both sides several times and take the average of all the measurements (throwing out any oddball outliers if you have them).

The old Avocet Cyclometer prescribed method: Set the front wheel so the valve stem is at the very bottom and mark a line on the ground at the center of the tire contact patch, where the valve stem is. Ride the bike forward in a straight line one revolution of the front wheel, and mark another line where the center of the contact patch (and the valve stem) now are. Measure the distance between the two marks. Divide that distance by 2*Pi, and that's your radius. (For more accuracy, repeat that measurement several times and take the average.). It's possible to put a chalk line on your tire and do this yourself, without help.

[Joel Stern]

Thanks Ted.  

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[Benz Ouyang, Sunnyvale, CA]

On Saturday, July 18, 2020 at 1:00:50 PM UTC-7, Ted Durant wrote:

 

The old Avocet Cyclometer prescribed method: Set the front wheel so the valve stem is at the very bottom and mark a line on the ground at the center of the tire contact patch, where the valve stem is. Ride the bike forward in a straight line one revolution of the front wheel, and mark another line where the center of the contact patch (and the valve stem) now are. Measure the distance between the two marks. Divide that distance by 2*Pi, and that's your radius. (For more accuracy, repeat that measurement several times and take the average.). It's possible to put a chalk line on your tire and do this yourself, without help.

A trick to Ted's method is to mark the tire with a thin line of chalk, so the ground will be marked by the chalk as the tire rolls over the mark. Also, your error will be lower if you do two or more revolutions of the wheel instead of one, and lower the tire pressure such that it approximates the sidewall droop when a nominal load is applied, but that's bordering on OCD.

[Joel Stern]

Thanks Benz. 

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[Matthew P]

https://bikecounterculture.com/2017/08/17/bike-wheel-diameter-calculator/

mouse over the points

On Saturday, July 18, 2020 at 1:35:12 PM UTC-7, Joel Stern wrote:

Thanks Benz. 

On Sat, Jul 18, 2020 at 4:19 PM Benz Ouyang, Sunnyvale, CA <benzo...@gmail.com> wrote:

On Saturday, July 18, 2020 at 1:00:50 PM UTC-7, Ted Durant wrote:

 

The old Avocet Cyclometer prescribed method: Set the front wheel so the valve stem is at the very bottom and mark a line on the ground at the center of the tire contact patch, where the valve stem is. Ride the bike forward in a straight line one revolution of the front wheel, and mark another line where the center of the contact patch (and the valve stem) now are. Measure the distance between the two marks. Divide that distance by 2*Pi, and that's your radius. (For more accuracy, repeat that measurement several times and take the average.). It's possible to put a chalk line on your tire and do this yourself, without help.

A trick to Ted's method is to mark the tire with a thin line of chalk, so the ground will be marked by the chalk as the tire rolls over the mark. Also, your error will be lower if you do two or more revolutions of the wheel instead of one, and lower the tire pressure such that it approximates the sidewall droop when a nominal load is applied, but that's bordering on OCD.

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[Joel Stern]

Thanks Mathew, that is very cool.  

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[Nick Payne]

Stand the wheel upright and measure the distance from the floor to the centre of the axle...

[Joel Stern]

Thanks to all, got it, did it....

On Sat, Jul 18, 2020 at 7:34 PM Nick Payne <njh...@gmail.com> wrote:

Stand the wheel upright and measure the distance from the floor to the centre of the axle...

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