More Tire Stuff....
 

This is a re-post from the TRS-triathalon website. I did not write or have anything to do with the creating of this article.

http://trstriathlon.com/talking-tire...shua-poertner/

WRITTEN BY JIM GOURLEY JUNE 6, 2016
Talking tires with Josh Poertner – new discoveries about the relationship between pressure and performance indicate new directions for research and development

I rode a 200km brevet just over a week ago and I'd forgot to top off my 28mm tires, usually I run 'em about 80psi and I would guess they were closer to 60. I could see the blob on the back as I was riding but since it was raining I didn't want to stop and get out the mini-pump. I ended up finishing the ride in 9:28 and that's my fastest 200km to date. It's not super fast by rando standards but it seemed pretty easy for me to keep up that pace for most of the ride. It was also noticeably more comfortable. I think I want to keep enough air in my tires to avoid pinch flats but beyond that point I think I will try running them a bit softer from now on.

Science! :thumb:

...I've always run my tire pressure based on my weight (a cool 145lbs since forever) plus the bike's. I read that somewhere... So I typically run 85psi rear and 75 front; maybe I'll drop it another few notches on the gauge.

EDIT: Oh, and thanks for posting, [MENTION=46029]fender1[/MENTION]!

Very, very interesting.

I'd think there is a third factor, which is impact resistance - the tire needs to protect the rim and the tube, when you hit a road irregularity that is much larger than the rest. The killer pothole.

With tubulars, you're not worried about protecting the tube against pinch flats. With deep section carbon rims (and a spare set on the following team car) perhaps you're somewhat less worried about damaging the rim.

But I know that if I run my 25 mm clincher tires at 70 psi, I'm prone to get pinch flats. Of course I weigh a lot more than Cancellara . . .

The other consideration that needs to be considered is cornering. At low enough pressure, the tire starts to feel mushy in turns. At even lower pressure, it comes off the rim. I don't know exactly where that constraint lies.

This brings me to another area where I think bike tires could be greatly improved. Rubber compound. Why should an all-out race tire last longer than the race, plus a safety margin? Design for a 500 km life, and you should be able to use really soft compounds that have terrific grip, even in the wet. Race motorcycles wear tires so sticky that they will pick bits of litter from the pavement. And how about the days when the peloton is racing in near freezing weather, e.g. the high mountain stages of the Giro or some of the spring classics?. Snow tires use rubber compounds that stay soft at low temperatures, extreme condition bicycle racing tires could as well.

Bike tire design seems really focused on resistance (rolling, impedance, aero) but a chute slows you down ore than an extra 1% of resistance.

Another factor in the lower pressures leading to faster times could be rider fatigue. If the rider feels better after riding 100 miles of rough roads, he is also likely to finish stronger.

Interesting reading, thanks for posting this article Fender1. Printed it out so I can read it, at leisure, again and take what Joshua found. I suppose Mr. Poertner is needing to justify the cost of those elegant new Silca floor pumps, wish I could spring for one just to admire the workmanship involved in making them.

Bill

IMO most riders have their tire pressure too high and that article kind of affirms it (though they tested tubulars). With a 25c I have 65f/75r and I weigh 160, my last flat was over 2 years ago. That makes any bike ride smooth. I have 60f/70r in my 1-1/4 Paselas and it's a magic carpet ride. My chipseal roads don't feel like chipseal. I could go lower but then it gets a little too squishy. Everyone is worried about pinch flats, and when they get one they blame it on tire pressure but I've never had an issue with it. I know MTB tires are bigger but I've always ran #25-30 and don't pinch flat with those and they aren't going over pavement.

You'll need to pry my 23mm tires at 140psi from my cold, dead hands.

I think skepticism is healthy, especially when there is a commercial venture involve. IME, I read what Jan Heine had published awhile back on the same subject and was interested but skeptical as well. Rather than just dismiss it all as marketing hype, I bought some Vittoria Hyper tires and ran them at a lower pressure. I found that for me, what had been reported, was pretty much spot on.

I have since purchased the Compass tires (700x38 with the extra light casing) and have taken to running latex tubes in them as well. I find the ride to be fantastic. I am running the tires 55 psi in the rear and 50 psi in the front and I am 6"1" and 185lbs. I have also noticed (fingers crossed) a steep drop off in the number of flats. Could just be luck of the draw or the tire could be less likely to pick up debris as it deforms and rolls over it more easily. For me the most important part is the comfort. I now find it much less enjoyable to ride bikes with skinny tires and higher pressures. YMMV.

Actually, I was just being a smart ass about the pumps (and failing mightily:notamused:, as usual,) I doubt that it was any marketing hype from Josh, that could be related to his acquisition of Silca. I would say that his findings get borne out in my own riding, for both clinchers, and with tubulars. And, I tend to agree with your final statement about what matters to me as far as pressure is concerned is comfort, then it would be the handling/cornering, and finally the speed aspect. Our tire pressure levels do differ, I like to stay up at 85-90 psi with the clinchers, and at 100 psi for tubulars (up to 145 psi is recommended by Continental.)

Bill

Good read. Thanks.

That's been my experience as well. Even if the lower pressures (15% drop method) result in a little more rolling resistance on smooth ground, I'm so much fresher at the end of a brevet that it's worth it.

I'm scared to go lower than that, but I'm down to 153 lbs, so I'll try it.

One downside to lower pressure is that if you inflate to your ideal pressure, when the air seaps, you are below your ideal pressure, and you are approaching the danger of too-low pressure. It requires more frequent refilling. Maybe I'll stick to 55 front and 60 rear (also with ~38mm tires).

Here is Jan Heine's post based on the same study: Suspension Losses Confirmed

I'm curious (but not willing to do conclusive tests ;)) how low we 150's-lb guys would have to let our 38mm tires get before risking snakebites. I round up a little from what the calculator gives, and that's good for at least a week. Jan Heine lets his 42mm tires get down into the 30's before he worries about pumping them up again.

[MENTION=251447]ThermionicScott[/MENTION], we would need to do large sample tests to determine the level of risk. About a year ago, I took a ride in Manhattan (with terrible streets) on 32mm tires with 30psi. I was slightly careful, i.e. I didn't barrel over monster potholes, but I did hit plenty of bumps. I was surprised that I didn't even feel the tires bottom out. My conclusion has no numbers in it, but we can get away with less pressure than we think.

On the other hand, remember that this forum is full of cyclephiles, and we are different from other people. Most people don't check their tires often enough. We should be very careful before recommending low pressures to some people because of this. My wife is 110 lbs, and she has 35mm tires. I tell her to use 50 or 60 psi, which is far more than she needs, but knowing how easily she forgets or just skips the tire check knowingly, this seems reasonable. The extra pressure won't make the ride harsh, nor will it slow her down appreciably. In fact, I don't think it will slow her down at all. At her weight, any pressure from 25 to 90 will be fine for her. She can't feel the difference!

So let this be our secret among cyclephiles. Everyone else should follow the sidewall instructions.

:thumb:

Pumping up your tires is like your diet -- whatever works for a person is the right one.

Good read, thanks. I have noticed that significantly lower pressures do seem to increase tire wear by a significant amount, but don't have many data points on that yet to get a feel for how big the increase really is. Might just be the tires I'm using, and it might not matter nearly as much for lighter riders. And to me, the benefit is well worth the higher burn rate.

Riding mixed surfaces does require some compromise in 'ideal' tire pressure. One take away from the article could be to compromise in favor of the rough stuff depending on a given ride's percentage on gravel versus pavement. I had been doing the opposite.

Funny to to see a fat tire Rivendell on a tri site.

That's good to see as well, thanks. Jan's June is off to a good start.

I've been running wider tires on several of my bikes and really like the ride characteristics. However, I have not found them to be faster. My average speeds seem to go down in direct correlation to the width/weight of the tires. Perhaps I am running the tires with too high a pressure, but it's hard breaking old roadie habits (and memories of past pinch flats).

I weigh about 175-180 lbs and here are the tire pressures that I typically run with various tire sizes:
- 700x25s: 105 rear, 95 front
- 700x28s: 95 rear, 85 front
- 700x32: 85 rear, 75 front
- 700x35: 70 rear, 60 front

Those pressures look very high, [MENTION=86492]tarwheel[/MENTION]. The tiredrop chart is a good place. Note that the load weights are for each wheel, not the bike, so assume the rear wheel bears 60% of the load (unless you know otherwise) and look up your weights that way.

I'll assume a 35mm tire. Starting with a body weight of 180 lbs, add 30 lbs, and you have 210 lbs of load. The rear will get 126 lbs, which is 57 kg. That gives you a touch more than 50 psi on the rear.

And this is just a starting point. You may have reason to go higher or lower.

I'm #225 and I run my 38 mm Pacenti/Panaracer Pari-Moto and Bontrager CX0 tires at around 50 psi front and 55 psi rear. This seems like a good balanced pressure for mostly pavement. I lower the pressure to around 40/45 for gravel and dirt.

I've run them below 40 psi without pinch flatting, and I don't ride light.
I think you could follow Jan's example at your weight.

I regularly let my commuting bike tires deflate to around 25 psi and it's only the squish that reminds me to put some air in them. At 165 lbs, I've only had one pinch flat, and that was from hitting a significant pothole (covered by a puddle, of course).

I'm lazy at pumping up tires, I usually put in 90# and don't touch them again until they hit 65-70# (27-1 1/4 Paselas on almost everything). I have noticed the bike just feels better before I top them off, but I can't get past the tire deflection at the lower end of that level.

That's a good reference point. Thanks.

But it's also important to note that different tires respond differently to low pressure. We rented some super cheap three-speeds while on vacation. The tires were underinflated, and the hotel didn't have a pump, so we just rode them. The cheap tires were hell to ride while underinflated. Nice tires are not as bad.

You know, we've been talking about graphs and calculators, but in practice, I usually give the front and rear tires a squeeze with my thumb and forefinger before deciding whether to hook up the pump. While not very precise, it's a pretty good first-pass at judging whether the tires are too squishy. Do I contradict myself? :lol:

To be honest, that and deflection is how I choose to pump up, it generally just happens my fingers and eyes are calibrated to about 65-70# :)