8/20/14 10:34 AM

I imagine that some of you have already seen this, since it's several years old, but I thought I'd post it anyway. It's about the best tire pressure guide I've seen anywhere:

http://www.bikequarterly.com/images/TireDrop.pdf

It tracks quite closely with my own experience.

8/20/14 11:03 AM

I have seen this. But will say since I got the Pave tires it moved away for me and my experience.

Meaning; I run on the Scott Conti GP4 Chilis 23mm-fr/25mm-rear @ 85/90ish. Up from what I started early in the year. As I started pushing out of the saddle efforts once my fitness improved the rear was a bit too soft @ 75/80 I was running in 25mm on front and back. And as the rides got more spirited with a few of us jumping and playing etc.

Roubaix; I put on the 27mm Paves and find myself running them 90/95 to feel the way I like. Started out 75/80 figuring bigger tire less air. It had 25mm Michelins prior.

In fact I like the feel so much of the Paves I started riding the Roubaix almost exclusively. Until recently where the Scott is back to about 1/2 the rides. I am less tired at the end of ride if we screw around, jump, sprint, bang up hills etc. [on the Scott]

We talk about frame flex etc., the Roubaix feels better and less spongy [to the extent IMO it feels spongy] with more air, and the Paves ride so well little is lost in that dept if any at the higher pressure compared to the Conti, yada.

8/20/14 11:25 AM

I had seen this about the same time:

http://www.biketinker.com/tire-pressure-calculator/

8/20/14 3:35 PM

figures right along with what I have found out on my own.

8/20/14 8:21 PM

For the last 20 years, I've tried to set pressures near to where an occasional pinch-flat might occur, but obviously trying to avoid that.
I felt that lower pressures made the bike ride better, since our semi-rural roads have a rough-textured surface and lots of cracks, etc.
Lower pressures also makes the tread last longer, and tires can be safely used for more years if the pressure is kept lower, since they are less likely to suffer outer-ply failure and possible subsequent blowout.
I have several bikes with somewhat-aged tires, so run a 23mm tire at only 90-95psi, that's on an older, heavier bike but with my mere 145lb on board.
I run the 27x1-1/4" tires at between 60 and 72psi, depending on the actual width, the rim width and whether road or off-road use.
The 28mm tires measure only 26mm, so I run those at 75-82psi.
I don't use more than a 5psi difference between front and rear tires, based on my actual experience with pinch-flat frequency. Yes, front tires can pinch-flat almost as easily as rear tires run at similar pressures. You run over a leaf, but it was really a rock, etc...

8/20/14 9:00 PM

rear pressures are bang-on what i use, but fronts are like 10-15 psi lower.

8/21/14 6:38 AM

My 23mm tires actually measure out to be 25mm when inflated. According to that chart, assuming 40/60 front/rear weight distribution, I should be running just north of 70 psi up front and 100+ psi in back. That's a pretty drastic difference.

8/21/14 7:55 AM

I'd like practical tips on how to measure actual tire drop. I'm not completely sure I trust these tables; I'd prefer to measure it myself.

Fatter tires have a different shaped flat zone than thin tires. Because of this, at the _same_ pressure, they have less rolling resistance. The actual area of the flat zone is the same. Tire contact patch can be seen and measured (I've done it) using some water and paper.

I have measured (in the past), F/R weight distribution, and it doesn't change much at all with changes in saddle/stem. But people swear it feels different.

With regard to pinch flatting, it typically happens to me on a descent when I fail to stop the pothole. And it's typically the front tire only. The weight distribution then is different from the static one. (Well, I've flatted spotted both tires at the same time but only a few times.) So I'd say, you need to make sure you have enough safety margin at the front.

YMMV,

Sandiway

8/21/14 10:43 AM

i eyeball how much "tire bulge" i can see past the rim when i'm seated and riding. i pump up rear tire to taste, get that little visual bulge, then release front pressure to match visually.

it's not scientific, but it is a method and yields reasonably repeatable results!

Last edited by walter on 8/21/14 6:55 PM; edited 1 time in total

8/21/14 10:56 AM

Vol. 5, No. 4 (Summer 2007) is the origin of the file.

It begs the question if wider rims make a difference and what difference. I wonder if the data points are valid for the contact patch difference of wider rims. What do you folks think regarding this?

Certainly there were wide rims for a long time, and road tires [not all as big as 35C] for touring use with said rims. The wheels on the Trek 620 I had have 24-5mm wide rims.

8/21/14 11:10 AM

The one and only time I pinch flatted a tire, it was a relatively low speed one: <10mph!

It's the shape of the hole that really matters. Sharp leading edges contacting the tire at a certain angle. I knew it a fractino of a second before contact. Too late to avoid the hole but not too late to prepare for the result of the flat... lost of control.

I've hit many holes are much higher speed and no flats.

So I just keep the pressue at a level that's comfortable and relatively efficient rolling.

8/21/14 11:22 AM

Tire sizes are all over the place. I have a set of Paselas that are labeled 28mm and actually ARE 28mm on a given (fairly narrow--VO PBP) rim. On the same rim, Vittoria Randonneurs are barely 25mm, though labeled as 28mm.

I know, what's the deal, 3mm. Hoo-haw.

8/21/14 11:30 AM

"tire size all over the place"

No doubt about that.

And some grow more than others too.

8/21/14 11:50 AM

And we probably shouldn't talk about Continental's little secrets...lighter tires through fibbing. I decided to try their 25mm tires (forget which) on one bike and they were skinnier than the 23s I had been using previously.

8/21/14 1:26 PM

Conti seems the worst at that... They just want each size to be the lightest as compared to the same size as competitors. Only they all seem smaller than marked/sold as every Conti I have had. 42C Speeds are 37.5, that is a big difference. But 450 grams for a 42 sounds better than 450 grams for a 38??

8/21/14 6:52 PM

was out tooling on the grinder, reflecting on tire grip on motorcycles where braking traction is all about loading up the tire: no load = no grip, and no braking. grabbing a brake on a moto is bad because the brakes grip before weight transfers to the front to increase the contact patch pressure and provide increased grip. grabbing leads to front wheel skids. so the idea is all about braking firmly/intentionally, but not grabbing.

and the linking of weight transfer -> grip got me thinking about tyre pressures on bikes, which brought me back to this thread.

the numbers suggested by this approach may be valid for front wheels while seated, but i think they're meaningfully low for when you're standing on the pedals with your weight shifted to the front. there's no way i want a tire to be squishing or moving around excessively when i'm in the drops, standing on the pedals, puttin it down.

to me, that explains completely the 10-15lb front tyre pressure differential between this approach and the PSI numbers i use.

8/23/14 11:41 AM

As Sparky said, the 42c CycloXKings are 37-38mm on a standard 21mm road rim. However, the 28c GP 4 Seasons measure 26mm on the same rim and the 25c GP4000s actually measures 26mm on a narrower 20mm rim. Go figure...

8/23/14 2:40 PM

Hard to believe, but I have a relatively new GP4000S tire here, mounted on a 20.5mm rim, and the 700x23 tire is a full 25.95mm wide, measured at several points around the wheel at 105psi.

No typo's in this post!

Indeed, Conti's are all over the map.