Jim Kukula

Here is a diagram similar to Berto's but with my formula:

Carbonfiberboy 

I don't know what your formula is because a notation of "**(-1.5)" doesn't mean anything to me. If this is a formula involving an exponent "^2" is the usual way of writing a power of 2, etc. However that may be, it doesn't work. If I'm reading it right, your graph shows a pressure requirement of ~180 psi for a 25c tire and a wheel weight of 175 lbs. As I've mentioned above, on our tandem we run 25c at 120 lbs. very satisfactorily with a 175 lb. per wheel weight. We also run 23c at 140 and 28c at 115. A useful formula must work at the extremes.

On my singles I also run higher pressures than shown on Berto's graph. I think it's a lower limit thing. On our tandem, we run the pressures I've indicated and do not pinch flat. We do get more bottom-of-the-sidewall damage than I do on my singles, though.

Jim Kukula

That's right, W**(-1.5) is Fortran-style exponentiation. W^(-1.5) is Excel-style. Could be fun to do an analysis of linguistic families in mathematical notation, pre- and post-computers, etc. In a different forum, maybe.

My formula is a reasonable fit to the Schwalbe pressure recommendations:

https://www.schwalbetires.com/tech_in...ation_pressure

They've got 100 psi for a 25 mm tire and a wheel load of maybe 95 lb. My formula give 106. Raise that proportionally for 175 lb load and yeah that's up like 195 lb.

Seems like if you want the wheel drop to be constant then pressure should be proportional to load!

fietsbob

only if you want to.. your bike fit might suck, but I cannot help you , I'm just a keyboard making words.

visit a bike shop , they can see you, I cannot.

Jim Kukula

This formula was fit to the Schwalbe recommendation. Berto's charts are a bit lower. A reasonable fit to Berto would be:

P = 120 * L * W**(-1.5)

Carbonfiberboy 

That's all well and good, however one also doesn't want one's rims to explode while riding at 50 down some mountain pass. A simple formula which approximates rim limits is that (tire in mm)*psi = ~3000. This formula approximates the maximum pressures written on the sidewall of high quality tires. Oddly enough these maximum pressures are approximately the same as those published by rim manufacturers for the various tire widths. Thus any formula for tire pressures must take into account the limits of the tires and rims which are available to us. The pressures you suggest could definitely explode a clincher rim and/or blow the tire off the rim. One wouldn't want to find out which happens first. Abide by the manufacturers' recommendations! This becomes especially important for heavier tourists with heavy loads who want to run wide tires.

Jim Kukula

You're absolutely right, my formula totally ignores rim limits, and that is a serious fault. I have read that wider rims can handle higher pressures... a tire won't bulge out so much so the pull of the tire won't be so much parallel to the axis of the wheel, i.e. tending to pull the rim apart. Of course a narrower tire won't stress the rim as much either!

Somehow the load capacity of a wheel gets limited by the need for higher pressure to avoid pinch flats but then the rim and tire have pressure limits. It's strange because e.g. Schwalbe's Tandem Dureme tire lists a load capacity of 145 kg but then a max pressure of 80 for a 40 mm tire. Even with my lower Berto factor of 120, I would say that 80 psi means a load of about 170 pounds for a 40 mm tire. Hmmm, maybe that Schwalbe load limit is the total for both wheels.

I am surely not claiming to have any ultimate answers! Just trying to gain an incremental step in understanding!

Carbonfiberboy 

The world of cycling is so empirical. We try to reason backwards from what we know to work. I see very little mention of published theoretical work.

Bekologist

it's going to be tight at the brakes. Can't tell if those are standard or long reach.

Some shops are cool enough to try and mount a 28, and if it don't fit, they'll sell you a 25.

Find your local.

seeker333

Looks like you have room for a 28, if the current (pictured) tire is actually 25mm. Maybe even can fit a 32, since some of them measure closer to 28mm.

There's no way to know for sure without actually trying, unless you have the tire in hand, in which case you can lay it flat, measure casing width from bead to bead, add inside width of rim, then divide by 3.14 to determine a close approximation of mounted diameter.

Jim Kukula

Here is a nice data point - my formula gives about 11 psi for the 102 mm Pugsley tire. Pretty much spot on!