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How low should I go? Tire pressure that is

Old 06-09-20, 07:46 PM
  #1  
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How low should I go? Tire pressure that is

Vintage guy and vintage bike. Have been riding the old Schwinn with 27” tires and 70-80 lbs for decades. Fancier vintage bikes now with nicer tires. 70-75 seemed nice in some pasela gravel king 32s on an83 Trek. I weigh 165lbs. Newest vintage bike is a wider rimmed tourer that I put panaracer pasela 35s on. I started at about 70 psi. Last ride was with 60 in front and 65 rear. Smooth quite riding tires it seemed. I ride”soft” so wasn’t concerned about pinch flats. Do others go less? Maybe just try and see I guess. I would think at a certain point I would be giving up speed.
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Old 06-09-20, 08:55 PM
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I believe you've answered your question quite nicely. What feels right to you?
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Old 06-10-20, 02:11 AM
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Lower tire pressures have theoretically higher rolling resistance due to increased deformation of the tire casing, but if you raise pressure to the point of harshness and you lose energy to suspension losses. You seem to be in a pretty sensible range for riding pavement, including relatively rough pavement and maybe smooth gravel.

For unpaved roads you'll probably be fastest/most in control somewhere near where pinch flats and unwanted tire suspension become an issue--probably somewhere around 40psi with tubes. I'm around 190 and regularly ride cyclocross 700x33c tires tubeless in the mid 30s on singletrack to improve traction, but a lack of care and you can absolutely hit your rim on rocks at that pressure at my weight.
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Old 06-10-20, 04:30 AM
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It does sound as if I can go a fair bit lower then. I don’t have a speedometer/computer on this bike yet to help me judge speed loss yet. All paved road riding so far, but a couple have some coarse tar chip repaired sections. I may try 50 front 55 rear just to see. The 35s even at 60psi Smoothed out the coarse pavement pretty well. I was pleased how well these tires seemed to roll at 60 even.
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Old 06-10-20, 10:46 AM
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I also weigh 165. With 32mm tires, I experimented with low pressure. Depending on the tire, lower pressure does NOT increase rolling resistance, most particularly when the sidewall is supple. I hit the edge of a pothole and felt the rim bottom out, but I got lucky and didn't pinch the inner tube. If I had been going a tiny bit faster, it would have been bad. I found the pressure was 30 psi. So that's the limit, so now I make sure I keep the pressure above 40 or 50.

But more important than the numbers anyone gives you, do some experiments. It depends on your weight, your speed, your tires, and your terrain. Hit bumps on purpose and note your speed.
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Old 06-10-20, 01:10 PM
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Here's a quick method to determine if tire pressure is too low to prevent pinch flats. Take the bike onto a street and roll the tire up against but not over the curb. Press down hard on the bike (i.e. handlebar for the front wheel) and see/feel how much the tire deforms. If you feel the rim hitting the curb then you know the tire pressure is too low to prevent a pinch flat.

Cheers
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Old 06-10-20, 01:42 PM
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As a kind of overview, check out
https://www.slowtwitch.com/Products/...ance_7406.html

The key thing is that if you ride on a perfectly smooth surface, you can increase your tire pressure to the maximum your tire will hold with some benefit. If you ride on even moderately bumpy roads with super high pressure inflation, the impedance losses (where your rolling energy dissipates into heat due to the impedance caused by a harder tire) start to outweigh the lower rolling resistance. So in real life, on an asphalt road, being 10-20 psi lower than "optimum" might cost you two watts or somesuch. Its interesting, though, that on bumpy roads having a tire that's 10psi too inflated. might cost you 5 or 10 watts!

The other question is, after riding an hour, is half a percent improvement in your bike's energy dissipation worth the pain in your tush from too hard a ride?

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Old 06-10-20, 03:38 PM
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Well informed and well put, @WizardOfBoz.
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Old 06-10-20, 04:07 PM
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Originally Posted by WizardOfBoz View Post
As a kind of overview, check out
https://www.slowtwitch.com/Products/...ance_7406.html

The key thing is that if you ride on a perfectly smooth surface, you can increase your tire pressure to the maximum you tire will hold with some benefit. If you ride on even moderately bumpy roads with super high pressure inflation, the impedance losses (where your rolling energy dissipates into heat due to the impedance caused by a harder tire) start to outweigh the lower rolling resistance. So in real life, on an asphalt road, being 10-20 psi lower than "optimum" might cost you two watts or somesuch. Its interesting, though, that on bumpy roads having a tire that's 10psi too inflated. might cost you 5 or 10 watts!

The other question is, after riding an hour, is half a percent improvement in your bike's energy dissipation worth the pain in your tush from too hard a ride?
While roling resistance is somewhat important, there is another aspect of tires that people fail to consider. Tires also serve a very important role in keeping the wheels from being damage upon impact. One of the worst bits of advice I ever read was to lower the pressure in tubeless tires and take it on jumps. When the tire burps off the rim, add just a little bit more air. But what they failed to take into account is that by that time the rims will probably look like this

Untitled by Stuart Black, on Flickr

Low rolling resistance means nothing if the wheels won’t take a tire.

Part of the reason I don’t like tubeless is because they tell you nothing about tire inflation until something catastrophic happens. A pinch flat at least tells you to put some more air in a tire. I’d rather fix or replace a tube than replace a rim.

Finally, in response to the “smooth and drama free” comment in the Slow Twitch article, I’ve ridden with low pressure tires. I don’t feel smooth and in control on corners when the tire is doing its best to fold off the rim.
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Old 06-10-20, 04:27 PM
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Originally Posted by Miele Man View Post
Here's a quick method to determine if tire pressure is too low to prevent pinch flats. Take the bike onto a street and roll the tire up against but not over the curb. Press down hard on the bike (i.e. handlebar for the front wheel) and see/feel how much the tire deforms. If you feel the rim hitting the curb then you know the tire pressure is too low to prevent a pinch flat.

Cheers
That could be too low, depending on bike speeds. I don't get pinch flats from potholes or bad railroad track crossings. I have gotten a few from hitting an unseen sharp edged piece of gravel, out on an otherwise smooth road. Each time, I was going downhill, 25 mph or higher. Those same rocks I can ride right over at 15 mph can pinch flat at fast speeds. So watch the downhills!

Pressures: at 170 pounds, I go as low as 60 psi front and 70 psi rear on 28mm tires. But I usually run 68 to 70 front and 80 to 85 rear. Either set of pressures feels fast and efficient. I like the (slightly / moderately?) reduced chance of pinch flats with the higher pressures.

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Old 06-10-20, 04:53 PM
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Originally Posted by rm -rf View Post
That could be too low, depending on bike speeds. I don't get pinch flats from potholes or bad railroad track crossings. I have gotten a few from hitting an unseen sharp edged piece of gravel, out on an otherwise smooth road. Each time, I was going downhill, 25 mph or higher. Those same rocks I can ride right over at 15 mph can pinch flat at fast speeds. So watch the downhills!

Pressures: at 170 pounds, I go as low as 60 psi front and 70 psi rear on 28mm tires. But I usually run 68 to 70 front and 80 to 85 rear. Either set of pressures feels fast and efficient. I like the (slightly / moderately?) reduced chance of pinch flats with the higher pressures.
I'm not saying it's the pressure you should ride the tires at; it just gives a good idea as to what is too low to avoid a pinch flat.

Cheers
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Old 06-10-20, 06:29 PM
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near match

Originally Posted by sd5782 View Post
Vintage guy and vintage bike. Have been riding the old Schwinn with 27” tires and 70-80 lbs for decades. Fancier vintage bikes now with nicer tires. 70-75 seemed nice in some pasela gravel king 32s on an83 Trek. I weigh 165lbs. Newest vintage bike is a wider rimmed tourer that I put panaracer pasela 35s on. I started at about 70 psi. Last ride was with 60 in front and 65 rear. Smooth quite riding tires it seemed. I ride”soft” so wasn’t concerned about pinch flats. Do others go less? Maybe just try and see I guess. I would think at a certain point I would be giving up speed.
Four of my six bikes are vintage and I've gone to 28s on everything but my modern KHS 223 which wears 35c panaracers. I weigh 170 and run all the 28s at 70/75 and the 35c at 65/70. I experimented a LOT to arrive at those numbers and I think you will find that both speed and handling (goes "soft" on high speed curves) as pressures go below 60.
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Old 06-10-20, 07:22 PM
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Thanks all

Lots of useful advice here. I am probably at the sweet spot now on those 35s. I have to laugh when I hear of handling on fast downhill curves though. Flatlands here and straight roads when I travel 5 miles of mixed city/suburbs and hit some country. Even a touring bike gives up nothing since handling is not at a premium and a serene ride prevails. Probably hard for people to even understand.
I think perhaps 65/70 on my 32s and 70/75 on my 28s makes sense too. Reason for the post was I am seeing how the more comfortable ride can still feel fast. The 35s were new for me too so was seeing other opinions although different tires and conditions will not be the same.
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Old 06-10-20, 07:36 PM
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cycommute, we probably agree more than disagree. What I was arguing about was folks who fill up their 26mm road tires to 116psi because that's the maximum specified on the tire, with the expectation of "reducing rolling friction". If you read some of the tire pressure recommendations, I should be running 28s at 116. I'm running 26s at 100R and 90F. Not exactly "low pressure".

Just did a 16 mile jaunt this evening, and I was thinking "Golly, this is way more comfortable than pumping everything up to 116.

But I agree, if you you lower the pressure, do it gradually (not 116 to 50, more like 116 to 100) and see how you like it. Anything that makes you feel unsafe, note the pressure, add 10psi, and retest.

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