650b wheel/tire radius for calculating BB height

[NHAero]

I've always just used ERD + 2xtire size to calculate wheel diameter. Using BikeCad, I notice that the diameter used when I input the tire size is a bit larger than this calculation. I measured three of my 700c bikes with 25, 28, and 37mm tires and the measured diameter was 6-9mm higher than what I calculate, so radius being half that. Once a rider is on the bike there is some "sag" as the tire compresses. I've read 15% is a reasonable place to start, and that's 5mm on a 38mm tire.

What's your experience? Has anyone measured the axle height on their 650b road bike and can report in with tire size, type, and pressure?

Carl Strong is pushing me to a low BB drop, which totally aligns with my own bias. We've tentatively agreed to a BB height of 10 inches (254mm) with the smallest tire I want to be able to run (32mm) but the missing piece of info is actual wheel+tire radius for a 650b road tire. Best case to know is a 650bx38 Gravel King, where I'd be aiming for a 10.25" BB height.

Thanks!

Thanks

[robt57]

What is the actual drop Carl is suggesting??

[Doug Fattic]

Carl's suggestion and my own preference for my framebuilding class students is to aim for a BB height of around 10 1//4" (or 260mm because my fixtures' measuring units are in mms). The is using 170 cranks. My personal bikes I have made, have about a 10" BB height and loved the way they ride compared to higher BBs.

BikeCad's tire diameters seem a bit off compared to real life measurements. I'll go to the shop now and see what measurements I've gotten for 650B wheels. The door to my spray booth has all these markings of actual tires diameters I've measured which serve as my library of tire size information. Of course I have to divide those measurements in half to get a radius to figure BB drop the way I do. I use the formula of desired BB hight - tire radius (smallest tire potentially used) = BB drop (the number to set the fixture).

[NHAero]

We're only on first draft. He sent me two with different drops. I made some tweaks, we discussed, and I look forward to next round designs on Friday. At the moment we're using 70mm drop as a placeholder. Remember this is 650b not 700c.

In 700c my Anderson is 80mm and Firefly is 78mm and I like how both feel. Yet I love riding my Nagasawa fixed gear road trainer, where I measure a BB height of over 11” with 23/25mm tires! But I'm also not riding that bike at 40 mph downhill.

Originally Posted by robt57

What is the actual drop Carl is suggesting??

[PacNW2Ford]

You’d want to use BSD (bead seat diameter = 584 for 650b), not ERD, which is the spoke bed. For reference, my Pari-Moto 42’s have a 13.25” radius. My 650b bike has a 65mm drop, which resulted in a ~10.2” BB height with 32’s. I thought it was kind of low.

[Mark McM]

Originally Posted by PacNW2Ford

You’d want to use BSD (bead seat diameter = 584 for 650b), not ERD, which is the spoke bed. For reference, my Pari-Moto 42’s have a 13.25” radius. My 650b bike has a 65mm drop, which resulted in a ~10.2” BB height with 32’s. I thought it was kind of low.

As I remarked in another thread, BSD is too small also. BSD is the diameter of the tire bead, which seats down inside the rim. The tire actually sits on the tops of the rim sidewalls, so you need to include the rim sidewall height. The actual outer diameter of clincher rim is about 8mm more than the BSD.

Probably a better way to find an approximation of the outer diameter of wheels/tires is to use the wheel circumference chart used to calibrate bike computers. Divide the circumference by 2 pi to get radius.

[Bici-Sonora]

Originally Posted by NHAero

I've always just used ERD + 2xtire size to calculate wheel diameter. Using BikeCad, I notice that the diameter used when I input the tire size is a bit larger than this calculation. I measured three of my 700c bikes with 25, 28, and 37mm tires and the measured diameter was 6-9mm higher than what I calculate, so radius being half that. Once a rider is on the bike there is some "sag" as the tire compresses. I've read 15% is a reasonable place to start, and that's 5mm on a 38mm tire.

What's your experience? Has anyone measured the axle height on their 650b road bike and can report in with tire size, type, and pressure?

Carl Strong is pushing me to a low BB drop, which totally aligns with my own bias. We've tentatively agreed to a BB height of 10 inches (254mm) with the smallest tire I want to be able to run (32mm) but the missing piece of info is actual wheel+tire radius for a 650b road tire. Best case to know is a 650bx38 Gravel King, where I'd be aiming for a 10.25" BB height.

Thanks!

Thanks

On my MAP, with 650b x 42mm and 177mm cranks, bb height measures 10.5" for reference. I run my tires at ~40psi or a little less.

[NHAero]

650bx38
This one
https://www.bikecalc.com/wheel_size_math
and this one
https://www.berkshiresports.org/bike...ze-calculator/
give 660mm diameter, which is 584 + 2x38

and this one
https://www.cateyeamerica.com/tire-size-calculator/
gives 659.2mm

So I haven't made progress yet

Originally Posted by Mark McM

As I remarked in another thread, BSD is too small also. BSD is the diameter of the tire bead, which seats down inside the rim. The tire actually sits on the tops of the rim sidewalls, so you need to include the rim sidewall height. The actual outer diameter of clincher rim is about 8mm more than the BSD.

Probably a better way to find an approximation of the outer diameter of wheels/tires is to use the wheel circumference chart used to calibrate bike computers. Divide the circumference by 2 pi to get radius.

[NHAero]

Thank you.
What would be super helpful is the axle height on that wheel/tire combo, and to know what the tire is too.

Originally Posted by Bici-Sonora

On my MAP, with 650b x 42mm and 177mm cranks, bb height measures 10.5" for reference. I run my tires at ~40psi or a little less.

[weisan]

Sorry Aero pal, out celebrating and enjoying my 650b bike, can't do it right away... when you are done with your Ph.D research, do you mind publishing it for posterity sake?

[Doug Fattic]

So I went to my shop and on my spray booth door, I have personally measured four 650B X 42 tires. The smallest radius was 332 (Rene Herse Baby Shoe Pass), another was 334 (Gravel King), the 3rd was 336 (a Panaracer T serv) and the biggest I only identified as a Panaracer was 340.

I don't have a measured 650B X 38. The Cateye tire circumference chart (which I've found usually to be pretty accurate) says a 38 has a circumference of 2105 which using math comes out to be a radius of 335. So whatever tire Cateye used as a reference is bigger than some of my tire measurements.

Actually rolling out a tire mounted on a bike for maybe 3 or 4 revolutions and establishing a circumference is the most accurate method but that # represents sitting on a bike compressing the tire at your body + bike weight.

332 - 260 would calculate to a 72mm drop.

[NHAero]

Thanks so much. Data!
Using the 584+2xtire size, your measurements encompass 42mm tires from 40 to 48mm when input into this formula. So one smaller than 42, one on the money, and two larger.

The Cateye value of 2105 divided by Pi is 670, which translates a 38 tire into 43mm.

Originally Posted by Doug Fattic

So I went to my shop and on my spray booth door, I have personally measured four 650B X 42 tires. The smallest radius was 332 (Rene Herse Baby Shoe Pass), another was 334 (Gravel King), the 3rd was 336 (a Panaracer T serv) and the biggest I only identified as a Panaracer was 340.

I don't have a measured 650B X 38. The Cateye tire circumference chart (which I've found usually to be pretty accurate) says a 38 has a circumference of 2105 which using math comes out to be a radius of 335. So whatever tire Cateye used as a reference is bigger than some of my tire measurements.

Actually rolling out a tire mounted on a bike for maybe 3 or 4 revolutions and establishing a circumference is the most accurate method but that # represents sitting on a bike compressing the tire at your body + bike weight.

332 - 260 would calculate to a 72mm drop.

[Doug Fattic]

Custom builders like myself usually spec a lower BB height than production or even semi-custom frames. Lawyers don't know the pedaling-through-corner-style or crank length of their company's customers. They like their clients to be conservative in their specs. Getting sued because someone got hurt clipping the ground with their pedal can be expensive.,

Typical BB heights were established in the era of toe clips and straps when pedals had cages that stuck out with a strap guide so it could comfortably get by the foot on its way back to the clip. Clipless pedals allow for more cornering clearance.

When companies publish their specs they often use BB drop as their reference. Of course drop by itself is meaningless without knowing the tire radius being used. However it is the comparison number used on forums because finding the actual BB height (which is the truly useful number) is more abstract. For example what is the pressure of the tire when you measured it and were you compressing the tire or not?

My recommendation is to go with whatever drop Carl says after telling him what tire and crank length you primarily want to use and your cornering style.

[robt57]

Originally Posted by NHAero

At the moment we're using 70mm drop as a placeholder. Remember this is 650b not 700c.

1st: Enjoy your Strong experience, I can't recommend Carl highly enough 3 frames later. [two to stay for ever]

I'd say go more drop only with low Q factor cranks. If you are choosing big Q, like MTB for example, I'd lean less drop.

I am running a 66mm drop Lynskey with 47*650b with very low Q chainset and MTB pedals. I'd not go both higher Q and smaller [less tall] tire myself.

My Strong is 8CM drop and I thought I could do both 700 and 650b with it. I can manage 47x650b with 170 MTB double cranks, but it is pushing it. And I use single sided Ultergra SPDs on this when lots of pave and smooth tires where I am more likely to get cocky on lean angles.

Strong allroad lived like this for 5 years until recently getting 175 cranks and 38x700 and dual sided pedals and used 50/50 pave/loose.

Note: I am a pedal thru kinda guy.

[Mark McM]

Originally Posted by NHAero

650bx38
This one
https://www.bikecalc.com/wheel_size_math
and this one
https://www.berkshiresports.org/bike...ze-calculator/
give 660mm diameter, which is 584 + 2x38

and this one
https://www.cateyeamerica.com/tire-size-calculator/
gives 659.2mm

So I haven't made progress yet

I didn't say use a wheel size calculator, I said use a wheel circumference chart (which are usually based on rollout). The calculators appear to use BSD for the rim size (which we know is too small) instead of actual rim size. Even Cateye should know their wheel size calculator is wrong - their circumference chart gives different numbers than their calculator

https://www.cateye.com/data/resource...ENG_151106.pdf

For example, the Cateye calculator gives a wheel circumference of 2071mm for a 650x38b tire, but their circumference chart gives a circumference of 2100 for a slightly smaller ERTRO 37x584 tire. The chart is far closer to reality.