JohnJ80

You're supposed to measure the actual width of the tire not the labeled width. That accounts for a lot of the internal rim width issues. That said, it's a starting point but it's a lot closer than where the OP is currently at.

I will also note that it's been pretty much right on for me on two different bikes with different width tires. It's also been equally accurate for my wife's bike and two other friends - a wide range of weight, tires and road conditions, So I think it's a lot better than guessing and it's a lot better than anecdotal information from other riders who may or may not be anywhere close to the OP.

VicBC_Biker

The 'actual tire width' clarification makes sense. I was wondering why the results I got 'with your numbers' were about 10 psi greater than you say you are running.

There are a few online calculators that I've tried; I find the differences (and similarities) in recommendations interesting.

It's hard to know how good ('accurate') any of the recommendations are, really - I can't do rolling resistance measurements. I guess I'll know when the pressures are much too low when I start getting pinch flats. So far, so good.

JohnJ80

Apparently how Poertner (Silca CEO with an MSEE) did it, was by collecting lots and lots and lots of user data from race teams and users and then applied a little machine learning to it. It's not a calculation, it's experiential based.

Anyhow, it's been surprisingly successful for me. I've found that my sweet spots are about 5psi wide in general.

The other piece that Poertner talks about being a big issue is that pump gauge accuracy is terrible in general. In a blog post about trying to run carbon rims in Paris Roubaix while he was the tech director for Zipp, he talks about how the results were all over the map and they ultimately found that their pumps were off by as much as 12psi when they were trying to tune to just a few psi. It's a fascinating read and I'd recommend it. So that could be why your results don't match with some calculators either. Until I checked my pumps, I found a considerable variance too.

J.

VicBC_Biker

It's not clear what the OP is running - 65/85, 65/70, 65/75 ?
The recommendations have all been pretty close IMO :64/68,65/65,60/65,60/70,60/60,70/65,67/71

I don't think the gauge on my floor pump is more accurate than +/- 2psi - if that. But, how to know? The digital pressure gauge isn't necesarily more accurate. Anyway, I don't check the pressures before every ride, if I'm riding often, so I guess I'm not that concerned about it. It's just nice to be away from the 95-100 psi 23 mm stiff tires on the road bike and the Armadillos @ 80 psi on the hybrid.

JohnJ80

See above - our posts crossed in the ether.

It also depends on how repeatable the pump gauge is. You could have an inaccurate gauge but it was repeatable and as long as you used that pump, you'd get back to where you were. But if it's not repeatable (and a lot of the times mechanical gauges aren't especially cheap ones) then it's a crap shoot.

VicBC_Biker

Yup!
I should go do some work and get away from the keyboard!
Good point.
As long as the tires aren't too soft- with the risk of pinch flats- I'm not extremely worried. If I were a better cyclist I might be able to detect smaller differences, while riding. If I'm going slow, it's very very unlikely that tire pressure is to blame, but it's a handy excuse to 'keep in my back pocket'.

Off topic- (I don't want to drag this down to a tubeless debate..) A couple of friends were telling me to switch to tubeless since they are running lower tire pressures since they switched. Questioning them, it seems they had been mostly running 'the pressure marked on the sidewall' with tubes. And the recommended pressures I was getting from the online calculators for my tubed tires weren't very much higher than they were running with tubeless. So the online calculators are a big plus, for me.

JohnJ80

It’s all tied together one way or another.

I follow Poertner’s podcast Marginal Gains and, if I have this right, the rolling resistance doesn’t change a whole lot until it gets really soft and then it goes up fast. So - simplifying here - you’re trying to optimize patch size and ride quality without destroying your rim.

scottfsmith

My pump gauge is off by 18psi when the pressures are in the 60-70psi actual range. One reason why I tested a lot of lower pressures is I didn't realize how much off my pump was -- accidental testing! I wrote with a marker "-18" on the gauge so every time I pump up I remember to subtract 18 from the reading. It is +-2psi that way, very accurate once calibrated :-)

Digital gauges are WAY more accurate on average than pump gauges. I have a couple of them for different uses and they are within a psi of each other and are also close to what my car's internal tire gauges report.

VicBC_Biker

Thanks for the 'heads up' on the pump gauge readings.
I just pumped up my road bike tires (28 mm) a bit and the Zefal floor pump gauge and the digital gauge were within a couple of pounds of each other in the 75 psi range. Only a couple of readings so definitely not enough to draw any firm conclusions, but still good to know.