How to determine proper tire pressure for 650b?

[Kevin]

On my 700c bikes it was easy. The chart done by Frank Berto always did me well when touring or commuting. However, having just switched to 650b tubeless, I am finding few available resources. What would be a good starting PSI for a rider plus bike and cargo weight of 225lbs on WTB Horizons on pavement or on gravel and packed dirt? On my last bike, I would have run 50-60psi front and 70-80 psi rear on 700x35c tires depending on the surface.  

[njh...@gmail.com]

There's an Android tyre pressure phone app based on Berto available at https://play.google.com/store/apps/details?id=com.edisongauss.bertotirepressure. It works quite well and allows tyre diameters of 26", 650B, and 700C). You can setup each of your bikes with tyre width and diameter, bike geometry and weight, front and rear loads if any, rider weight, and it will suggest front and rear pressure to use.

I believe the app also exists for IOS.

Nick

[Reed Kennedy]

I’ll second Nick’s recommendation. I use the iOS version of the same app, which is available here:

https://itunes.apple.com/ae/app/bicycle-tire-pressure/id1100803616?mt=8

For what it’s worth, my total rider + gear + bike weight is similar to yours, about 230 pounds. Here’s what I’m currently running on pavement after starting with the app and then experimenting a bit:

650b x 42 Babyshoe Pass: 40 psi front, 45 psi rear

650b x 47 Switchback Hill: 35 front, 40 rear

I’ll drop those each by about 5 psi when riding off road. 

Best,

Reed

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[Ken Freeman]

I haven't tried the apps, but I've used the Berto charts since the '90s, and have read his writing and BQ writing on it.  It and the apps (I assume) are able to give you pressure targets based on tire sag under load, the 15% thing.  I strongly think pavement is assumed, by Berto.  I do not see any tools for adjusting for gravel riding or packed dirt riding.

You may have to interpret (from the printed chart) to find a number for tires as wide as you want to use.  This is a small inaccuracy.  If the apps resolve this problem, that's a significant plus for the apps.

Just to be realistic!

Ken Freeman

Ann Arbor, MI USA

[Scott Stulken]

BikeTinker put together a spreadsheet a few years ago that digests the Berto chart down into a formula that can be extrapolated out to any tire width you care to enter: http://www.biketinker.com/tire-pressure-calculator/  The values come pretty darned close to what I get from the Berto chart, so I think it's on the right track.  That said, there's probably only so far you can extrapolate before things get much less accurate.

Frank Berto also included minimum recommendations for MTB tires (1.5-2.125") in his original "All About Tire Inflation" article, where he acknowledges that off-road riding has some different demands, so that might be useful.  Here's a link:  http://campusebikes.com/documents/Tireinflation.pdf

But that's just data to give you a starting point!

- Scott

On Wednesday, March 21, 2018 at 3:32:15 AM UTC-5, Kevin wrote:

[nash...@gmail.com]

Can you experiment by noticing tire deflection and ride feel?

[John Hawrylak]

Kevin

I suggest using the formula 

Pressure (psig) = 0.0833 * (Weight on Wheel) +8.234,  if the tires you are using are 47mm

This is from F Berto's data (as read from the graphs in the BQ article), calculating the slope and intercept for each tire size from the BQ graph, and then extrapolating the slope and intercept for different tire sizes.  The interecpt is very linear, increasing with tire size.  The slope is not and decreases with increasing tire size (wider tires require less pressure for the same weight).  Berto's data ends at 37mm, so larger tire sizes require extrapolation.  I extrapolated the slope using the change in slope from 32mm to 37mm and adjusting the slope at 37mm for the 47mm width, since it is non-linear and a second order fit has a minimum around 42 mm, making the fit unrealistic

The low slopes offset by a higher intercept, so the pressure is hopefully reasonable.

remember the weight is the Weight on the Wheel, not total weight.  If you assume a 50% frnt and back, the Wheel Weight = 0.5 * Total Weight

Let m know if you have questions

John Hawrylak

Woodstown NJ..

[David Cummings]

#whatpressureyourunning?

David “you can have my pressure gauge when you pry it from my cold dead hands” in MT

[Andrew]

Kevin, the simple answer (and several of the replies have suggested approaches different to that which you have been happy to use) is that for the same width, a 650b tyre has 6 and a bit percent less volume than a 700c, so addding 6% to your tyre pressure will be about right. For 26”, 10%. There will be much different pressures required as you change tyre width, where seemingly small differences change the air volume significantly. And most people don’t appreciate that they need to allow for the thickness of tyre and tube to calculate air volume correctly.

[devinh...@gmail.com]

Around 36psi front, 49psi rear as a starting point to match your preferred pressure with 622x35 to a 584x47, according to my math*.

*For the nerds: I've actually calculated the pressure based off a tire height % drop, approximating the tire as a torus and contact patch as an ellipse. Any wheel and tire size can be input. Curiously, it does match up with the Frank Berto/Compass chart, but only with a 7.5% tire height drop input. I couldn't tell you why. If anyone was interested, I can share the math I've used.

[Kevin]

Thanks everyone!