Seattle Forrest

This. Especially since they've been selling a wheel with those stats for that size tire since 2016, it's become their best selling wheelset, and people aren't blowing tires off them.

Abe_Froman

Those charts DO make me wonder a bit if I'm running far too much pressure.

I've got 28mm GP4000s which run a tad large, on 17mm internal rims. Been using 70psi for a long time...might try dropping it some.

smashndash

Most people probably are. Your fastest pressure depends on tire construction, tire volume (tire size + rim width) your weight, road conditions, and weather.

a 28mm GP4k on a 17mm rim is a bit lightbulby. So you’re more likely to experience a floppy tire than if you had a 25mm internal width rim, if you choose to drop pressures.

That being said, I have a smaller 28mm tire on a 20mm rim, and at 67kg + 9-10kg bike I find that 50 psi is the upper limit for the rear (which is where most of my weight is). After 50, the bike feels like it’s hitting every square edge in the road. I’ve run as low as 35psi in technical, wet crits and not really felt slower (though the tire does feel floppier when out of the saddle).

Sy Reene

And has anybody yet done the same type of deflection/inflation tests with tubeless as have been around for a long time for tire/tube setups (Jan Heine et. al)? Yet, I don't recall too many tubed tire people complaining about rough riding.

Does 50psi in your 28mm tubeless feel any harsher than 50psi in a tire+tube configuration, because 50psi would be well under the recommendation for a tubed setup, assuming a rider weight even at 140lbs.

Abe_Froman

I'm 85kg, so I think 50kg would be low even for normal riding for me. I'm not sure I'd want to go under 70 too much during a crit, because as you mentioned it is a bit light bulby as it is, though still handles fine. I might knock it down to 60psi for a few commutes/group rides and see how it goes.

smashndash

I’d recommend getting a wider rim if you’d like to fully exploit the advantages of wider tires and lower pressures. There’s a massive difference in tire profile between a 17mm rim and 25mm rim. In my opinion, 17mm rims are too narrow for even normal 28mm tires, let alone 4ks.

When experimenting with tire pressure, remember that the “rumbling” you get with traditional setups will be gone. It took me a while to stop associating that rumble with speed. You need to really believe in “smoother is faster” or maybe do a roll down test on a rough road to convince yourself you’re not losing speed.

smashndash

Probably should have mentioned this - I’m using tubed tires. Specifically Pirelli P zeros, which are about as supple as non-cotton tires get. I run orange sealant in my tires and have yet to experience a single puncture (besides a staple that shredded my tube over the course of 10 miles). I have slammed my rim against square edges multiple times and never pinch flatted. I’ve never felt the tire collapse in corners (though I’m not the hardest of chargers).

Unfortunately, while I am switching to tubeless soon, it will probably be with different tires and wheels, so I can’t be of much help when it comes to comparing low psi in tubed vs tubeless. But I highly recommend trying sealant in tubes. Though it may be slower than tubeless setups or clinchers with no sealant, it gives me an excellent compromise.

Seattle Forrest

I'm riding nominal 28 mm tires on 25 mm internal rims and agree with all of this.

rm -rf

Depending on the tire, it might be the equivalent of 32mm.

My 25mm GP4000S on a HED 20.5 mm internal width rim measure 29mm. I use 65 psi front, 80 psi rear with tubes.

The 32mm tires have 20% more air volume than 29mm, so I'd probably try 52F / 60R on my wheels. (The volume is based on the radius squared, so it goes up fast. That's why 25mm can be 10 psi lower, even though they are just 2mm wider.)

~~~

With the threat of pinch flats minimized, I suppose riders get used to the smooth ride at lower pressures and a "normal" road bike ride seems harsh.

I sometimes use 38mm Compass smooth lightweights on my other bike. It's quite amazing to float over chipseal and expansion bumps.

Abe_Froman

But, air volume itself is pretty irrelevant though. What matters is how far off the ground the rim is. Which is going to depend more on the actual width of the tire casing than the volume within it.

Abe_Froman

Have you had similar size tires on more traditionally sized rims? Is there a noticeable difference ?

Seattle Forrest

I had to change my line today because somebody on a lime bike served in front of me. I hit a pothole hard enough to be sure I must have knocked the wheels out of true. But they're still perfect. I can't see how.

I've had 28 mm tires on a cross bike back in the day. Mechanical discs even. Everything he said seems true in my experience. But it's one for the anecdotal record, the whole bike was different, it's not hard data.