Well-controlled tire pressure experiment

[Catalyzt]

The other issue that factors into the equation is the grades that you ride and how powerful your motor is.

I ride at 50 PSI on 2.1s that are designed for 60 max because the hills here are so steep and my motor is so weak. If I rode somewhere more normal, I'd probably run them at 38 or low 40s.

The tires are tubeless, they do leak slowly over the course of 50 miles or so. And when they are below 40 PSI, I tire much more easily, and my range drops noticeably. On a 20 minute fitness ride, I don't care, I'll let 'em get down to 35 if I'm just riding around the neighborhood.

I am acutely aware of when they are at 50 PSI, and adjust my riding style accordingly, staying looser in the saddle and on the bars so my hands don't get hammered with the extra vibration. What I can't figure out is why the traction is better on loose Sierra sand at 50 PSI than at 40. I've only made it up Hell Hill once, which is really slippery dirt, and then was at 50 PSI. It should spin out more, but it doesn't.

THICK sand is just going to suck on 2.1s no matter what pressure I'm running. I let my buddy get way ahead on his Serial 1, and I'm not embarrassed that he has to wait for me, and he doesn't care, either. I don't want to dump it. Sure, it would be a little better, probably, at 40 PSI, but there are so many long, uphill climbs over cracked, broken pavement that I'd rather just run 50 PSI for the whole ride.

Next time I tackle the Verdugos, it will be at 50 PSI on the way up.... and I might let out 5 or 10 PSI for the way down.

 

[Jeremy McCreary]

I run the tubeless 38 mm Pathfinder Pros on my Vado SL 1 at 38-40 psi. The sidewall says 50-80 psi. My LBS manager has run them at 30 psi on gravel rides.

I do this for ride quality, gravel traction, and cadence. If there's a rolling resistance penalty, I don't notice it, and I often ride in OFF. But I do notice when the pressure gets down to ~35 psi.

And how on earth does tire pressure affect cadence? Well, my own power maxes out at around 92 rpm these days, but only when I can keep the pedaling smooth.

Easily done above 90 rpm when the tires are below ~45 psi. But at higher pressures, the rear end of the bike starts resonating with my pedal strokes at these high cadences. Smooth pedaling then goes out the window, and power drops.

Weird, I know, but it's a totally reproducible phenomenon confirmed by the bike's display.

 

[Stefan Mikes]

I don’t know if I’ll go lower as I don’t see the point for asphalt. If I go on a gravel ride I may go lower. I think I’ll stay at 50' though.

Please try 45 psi at least once for an experiment. The tyres are the only bike component that is able to absorb rapid road vibration. Asphalt is rarely as smooth as a table

 

[mschwett]

I run the tubeless 38 mm Pathfinder Pros on my Vado SL 1 at 38-40 psi. The sidewall says 50-80 psi. My LBS manager has run them at 30 psi on gravel rides.

I do this for both ride quality and cadence. If there's a rolling resistance penalty, I don't notice it, and I often ride in OFF. But I do notice when the pressure gets down to ~35 psi.

And how on earth does tire pressure affect cadence? Well, my own power maxes out at around 92 rpm these days, but only when I can keep the pedaling smooth.

Easily done above 90 rpm when the tires are below ~45 psi. But at higher pressures, the rear end of the bike starts resonating with my pedal strokes at these high cadences. Smooth pedaling then goes out the window, and power drops.

View attachment 193105
Weird, I know, but it's a totally reproducible phenomenon confirmed by the bike's display.

that’s interesting, are you talking about resonance in the bike frame itself … or bouncing/oscillation of the rider on the seat which transfers into the frame?

the latter is usually from the seat being not quite at the right height, or the bike is helping you out too much and there isn’t isn’t load on the pedals! does this happen on hills?

in any case 90rpm is pretty fast, too fast for me

 

[Jeremy McCreary]

that’s interesting, are you talking about resonance in the bike frame itself … or bouncing/oscillation of the rider on the seat which transfers into the frame?

The latter, but I can pedal smoothly at up to 100 rpm without bouncing when I don't have an overly bouncy rear tire amplifying the seat inputs. The "resonance" might well go away at higher pressures than I've tried (say, 60+ psi), but those are out on compliance and gravel traction grounds.

the latter is usually from the seat being not quite at the right height, or the bike is helping you out too much and there isn’t isn’t load on the pedals!

Current seat height feels like a very good compromise between knee extension and safety at stops. Since I can damp out the boucing with lower tire pressures that still work quite well on this SL, no desire to mess with the seat.

Will explore the assist connection, but I usually ride the SL at low assist, and the resonance also happens in OFF.

does this happen on hills?

Yes, definitely on lesser grades, but I'll check for a grade cut-off.

in any case 90rpm is pretty fast, too fast for me

My legs have always preferred high cadences, and my bum knees now demand them. Kinda surprised that I can still do it at this age. Even more surpised that preferred cadence (currently ~88 rpm) and max power cadence (currently ~92 rpm) are still going up without trying.

As always, many thanks for your expertise!

 

[mschwett]

The latter, but I can pedal smoothly at up to 100 rpm without bouncing when I don't have an overly bouncy rear tire amplifying the seat inputs. The "resonance" might well go away at higher pressures than I've tried (say, 60+ psi), but those are out on compliance and gravel traction grounds.


Current seat height feels like a very good compromise between knee extension and safety at stops. Since I can damp out the boucing with lower tire pressures that still work quite well on this SL, no desire to mess with the seat.

Will explore the assist connection, but I usually ride the SL at low assist, and the resonance also happens in OFF.


Yes, definitely on lesser grades, but I'll check for a grade cut-off.


My legs have always preferred high cadences, and my bum knees now demand them. Kinda surprised that I can still do it at this age. Even more surpised that preferred cadence (currently ~88 rpm) and max power cadence (currently ~92 rpm) are still going up without trying.

As always, many thanks for your expertise!

that’s all very interesting, i hadn’t observed that correlation before. i’m not really a spinner and tend to bounce a bit in the saddle at those higher speeds, but it goes away out of the saddle, under heavy load, or at my more comfortable cadences. i did a ride yesterday that accidentally was at a much lower rear pressure than usual and it felt a bit draggy.

from a pure efficiency standpoint, if significantly lower pressures are absorbing the energy of the bouncing, that’s obviously energy being used for something other than forward motion! i wonder how much energy it takes to compress a pathfinder 90 times a minute?

 

[Jeremy McCreary]

from a pure efficiency standpoint, if significantly lower pressures are absorbing the energy of the bouncing, that’s obviously energy being used for something other than forward motion! i wonder how much energy it takes to compress a pathfinder 90 times a minute?

The last 2 editions of Bicycling Science have extensive sections on the evils of vibration in cycling, and they totally back up your statement. Vibration here includes big oscillations like seat bounce. The losses can be huge. Worse yet, vibration's hard on both bike and rider and in severe cases like cobble, exhausting for the latter.

Obviously lots of tricky trade-offs here. Many of my rides involve some offroad, and nearly all involve some riding in OFF. Given cadence, compliance, and offroad traction goals, my own best play seems to be to let the tires absorb a little seat bounce in order to avoid a lot of power-sapping bounce at useful cadences. The added tire resistance doesn't bother me, even in OFF.

No doubt a cringe-worthy solution for a serious road guy like you.

 

[PedalUma]

My understanding is that a round contact patch is ideal. At too high a pressure the contact patch is an elongated oval front to rear. At too low a pressure it is also an elongated oval side to side.