Tire Pressures Calcs

[John Dewey]

Y'all, I'm having a good time with my first 650b experience. Many smiles.

https://www.flickr.com/photos/146626768@N06/52948716296/in/dateposted-public/

Playing around with several of the tire calcs (Silca etc), two of them give me F/R pressures in mid/upper 40s PSI (38mm Pari Moto / Velocity rims) but the RH calc is significantly lower @ + 37 lbs. I know Jan and his guys are on it, but wondering what you think about the significant spread between the two.

Thanks for input.

BEST / Jock Dewey 

[Chris Cullum]

IME, 37psi is *way* too low for 38mm tires. I weigh 165lbs for reference. I run 42mm tires quite a bit higher than that.

Thanks for input.

BEST / Jock Dewey 

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[Harry Travis]

You don’t say which Velocity rims and which internal widths. A wider rim could increase effective volume enough to permit lower tire pressure (for the same rebound) by several psi. 

BQ absorbs that Q by asking for MEASURED mounted  tire width.

And it spits out 2 recommended pressures, depending on road surface for that tire mounted on a these-days slim Velocity A23 rim and 37-38mm mounted tire width:

42psi and 48psi +- 2 psi for my memory error.

Love those tires, especially the original ones w discreet lettering.

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Harry P Travis

16.5

[John Dewey]

Thanks for comments, Harry. Indeed, rim matters but same data input in the calcs returns significantly different results. The two I trust/use most are Josh Poertner’s Silca and Jan’s. 

[njh...@gmail.com]

I weigh 145, and run my 38mm Pari-Motos (mounted on 25mm internal width rims, so they actually measure at 40mm wide) at 35-37 front and 43-45 rear. That's with a medium Carradice saddlebag holding spare tube, raincoat, leg warmers, etc, so maybe another half a dozen pounds hanging off the saddle.

I usually figure out the front tyre pressure by gradually lowering it until I can feel that it is starting to affect the steering (with the Pari-Motos this happens between 30-33psi), and then adding a few psi for the front and a few more for the rear.

Nick

[John Dewey]

There’s a kind of TED TALK podcast with Josh Poertner and Jan Heine, nerdy and technical and most interesting. Josh worked as a consultant for a Dutch team. In short, they sent their guys out to ride a timed loop. When they finished, they’d lower pressure and send ‘em out again. The guys groused about riding slower but their times over repeats at lower pressures were always faster. It’s a fascinating discussion. The take away at the end was fit your bike with the widest tires possible at lowest pressures and you’ll go faster. For most of us, faster isn’t so important but if you can move along with less effort it seems to me you’ll also maximize your smile time. And who among us doesn’t like a delightful tail wind…at least until you have to turn around. I’d say 99% of the good balls I meet out on the road pump their tires hard as if solid rubber and they think I’m totally nuts. Actually I guess better that they don’t know, makes it easier to keep up. 

[Will Boericke]

Interesting to hear the wide variations in pressure.  I find Silca's calculator very conservative - for every tire size I input, the pressures it gives me are way too high (from my personal experience).  Rene Herse's calculator gets close to my lived experience - for 650x38 Gravelkings (on Velocity A23), at my weight of 147ish + 20# of bike, I get 35 psi.  I frequently air down from there on the road, honestly, so I'll wind up at 32 psi.

I also think there's a difference dictated by suppleness of tire.  My 650x38 are Gravelking, on the stiff side of tires.  I'd likely need to run Loup-Loups or Pari-Motos at a higher pressure because they are more supple.

[Chris Cullum]

On Fri, Jun 9, 2023, 15:42 John Dewey <john...@gmail.com> wrote:

There’s a kind of TED TALK podcast with Josh Poertner and Jan Heine, nerdy and technical and most interesting. Josh worked as a consultant for a Dutch team. In short, they sent their guys out to ride a timed loop. When they finished, they’d lower pressure and send ‘em out again. The guys groused about riding slower but their times over repeats at lower pressures were always faster. It’s a fascinating discussion. The take away at the end was fit your bike with the widest tires possible at lowest pressures and you’ll go faster.

I'm pretty sure that timed loop was over cobbles which are pretty extreme roads. Most of us don't ride cobbles for long distances.

For most of us, faster isn’t so important but if you can move along with less effort it seems to me you’ll also maximize your smile time. And who among us doesn’t like a delightful tail wind…at least until you have to turn around. I’d say 99% of the good balls I meet out on the road pump their tires hard as if solid rubber and they think I’m totally nuts. Actually I guess better that they don’t know, makes it easier to keep up. 

On Fri, Jun 9, 2023 at 11:21 AM John Dewey <john...@gmail.com> wrote:

Thanks for comments, Harry. Indeed, rim matters but same data input in the calcs returns significantly different results. The two I trust/use most are Josh Poertner’s Silca and Jan’s. 

On Fri, Jun 9, 2023 at 10:56 AM Harry Travis <travis...@gmail.com> wrote:

You don’t say which Velocity rims and which internal widths. A wider rim could increase effective volume enough to permit lower tire pressure (for the same rebound) by several psi. 

BQ absorbs that Q by asking for MEASURED mounted  tire width.

And it spits out 2 recommended pressures, depending on road surface for that tire mounted on a these-days slim Velocity A23 rim and 37-38mm mounted tire width:

42psi and 48psi +- 2 psi for my memory error.

Love those tires, especially the original ones w discreet lettering.

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Harry P Travis

16.5

On Jun 9, 2023, at 8:17 AM, John Dewey <john...@gmail.com> wrote:

Y'all, I'm having a good time with my first 650b experience. Many smiles.

https://www.flickr.com/photos/146626768

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[Jeff Bertolet in Raleigh, NC]

My experience lines up with the RH calculator. Bike+rider is ~200 lbs, 650x42 tires, 20-30 psi for gravel, ~30 psi for mixed, 40 psi for road.

[John Dewey]

Uh, oops…”goof” balls…

[Harry Travis]

For those NOT on >=47mm tires.

Softest pressures suggested here are not sufficient to protect rims (or tubes) from, say, the edge of a typical steel plate edge used for temporary cover of an under-roadbed utility repair if the edge hasn’t been tapered by asphalt and you hit it @ >6mph /10mph. 

Not the rear rim and tube either. 

In this era of celebrating comfort at seemingly no loss in regular use, that function of pressure as protection from full-compression is underestimated.

I’m quick to add that the very slow leaks from tubes that lower tire pressure between rides and which happen less when tubeless may lead to the failure I described above.

It shouldn’t require more than HS calculus to estimate the degree to which the larger tire offers greater protection to the rim. 

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Harry P Travis

Portland, OR, USA

16.5

On Jun 9, 2023, at 7:59 PM, John Dewey <john...@gmail.com> wrote:



[Will Boericke]

Can confirm that steel plate + 17mph + 28mm tire at 75psi + inadequately timed bunny hop = dented rim :)

Luckily, 32mm tires on that rim mostly hide the slight hop in the rim.  

Will

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