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Old 03-18-08, 04:10 AM   #1
bgm1961
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Minimum safe tire pressure - newbie question

Hi all,

I tried searching for the answer to this question elsewhere on these forums... couldn't find exactly what I was looking for...

I haven't ridden in 30 years... just got a Trek 7.5 FX... awesome bike... however the only thing I found disconcerting about my first ride was how jarring it was, as I hit every bump or rut on the local paved bike path. When my LBS set the bike up, the tech mentioned he pumped the tires to 105 PSI. Since riding the bike 4 more times, I've discoverd that I get a very comfortable ride with the tires at 75 PSI.

So the question is, how low can I go on a 110 max psi tire, inflated to only 75 lbs, before problems arise? Most everything I've read on these forums suggest reducing the tire pressure by only 5 or 10 lbs, yet the "Ideal Inflation" table found at: http://www.precisiontandems.com/phot...tirechartx.jpg says I should be riding at 72-75 lbs... pretty much right where I like it now.

Here's all the data:
Bike: Trek 7.5 FX
Tires: 700 x 32c Bontrager Race Lite Hardcase (actual width 30.5mm)
Max pressure: 110 lbs
Wheels: Bontrager SSR (don't know anything about these as I can't find them on the Bontrager website)
My weight + bike: 230 lbs
My comfortable speed right now: 13-18 MPH

Thanks in advance for your comments!
Barry

Last edited by bgm1961; 03-19-08 at 04:21 AM.
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Old 03-18-08, 05:41 AM   #2
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Just keep the tyres inflated enough that you don't get pinch flats and you'll be fine.
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Old 03-18-08, 06:45 AM   #3
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That must have been a rough ride.

See Sheldon Brown's tire pressure page.

From psimet2001's tire pressure formulas: (see this thread - lots of good information)

Tire Width=32: Pressure(psi) = 0.17 * Rider Weight in lbs + 41.67

So a 210 lb rider would calculate to 77 psi for the rear tire, less 10% for the front =70 psi.

Your tires are slightly narrower, so you might want a bit more pressure in the back. Pinch flats happen when the tire bottoms out against the rim when you hit something big, like a pothole. So pressures aren't as critical if you are a careful rider.

I calculate 107 psi for my 23 tires, just about right, I use 105-110 in the back, 95-100 in the front.

Last edited by rm -rf; 03-18-08 at 07:01 AM.
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Old 03-18-08, 06:56 AM   #4
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The issue is "pinch flats". If you don't have enough air pressure in your tires, when you hit a bump the tire will compress and the rim will make two little parallel slits in your inner tube.

Bike shops will generally inflate tires a bit above the optimum level because it will make the bike ride a little faster (customers' like that) and not pinch flat (customers' don't like flat tires). Bike tires also lose air pressure over time so, by over inflating the tires a bit, the bike shop is buying themselves a little time before the dreaded pinch flat can occur.

If you are happy and not pinch flatting at 75psi, that's a good air pressure for you. Just keep in mind that you might not have a lot of safety cushion so you need to check your air pressure frequently.
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Old 03-18-08, 01:33 PM   #5
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I generally run my 32's (which measure 29mm on Mavic Open Pros) at 75-80 psi rear and 70-75 psi front. I weight 155 lbs., the bike is 25 lbs., and I generally carry 15-20 lbs. of gear on the commute. I ride gravel roads, ride off curbs, blow off potholes, and hop up curbs all the time complete with a rack and panniers and have yet to pinch flat. I think you're fine if you're careful but I'd go up 5-10 lbs. if you're going to ride aggressively.
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Old 03-18-08, 01:45 PM   #6
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I run 120 to 140 psi in my 23's as the normal pressure- but that is put in with a track pump . Out on a ride and punctures occur. I pump as hard as I can with the pump and that is it. On checking back home and that pressure was around 80psi. Mountain bike pumps on road tyres don't get you very high- but then I am only 150lbs and ride carefully avoiding lumps in the road.

Now got a Topeak road morph pump and can get back to 120 psi and then I get bored with pumping.
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Old 03-19-08, 10:30 AM   #7
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Quote:
Originally Posted by bgm1961 View Post
Hi all,

I tried searching for the answer to this question elsewhere on these forums... couldn't find exactly what I was looking for...

I haven't ridden in 30 years... just got a Trek 7.5 FX... awesome bike... however the only thing I found disconcerting about my first ride was how jarring it was, as I hit every bump or rut on the local paved bike path. When my LBS set the bike up, the tech mentioned he pumped the tires to 105 PSI. Since riding the bike 4 more times, I've discoverd that I get a very comfortable ride with the tires at 75 PSI.

So the question is, how low can I go on a 110 max psi tire, inflated to only 75 lbs, before problems arise? Most everything I've read on these forums suggest reducing the tire pressure by only 5 or 10 lbs, yet the "Ideal Inflation" table found at: http://www.precisiontandems.com/phot...tirechartx.jpg says I should be riding at 72-75 lbs... pretty much right where I like it now.

Here's all the data:
Bike: Trek 7.5 FX
Tires: 700 x 32c Bontrager Race Lite Hardcase (actual width 30.5mm)
Max pressure: 110 lbs
Wheels: Bontrager SSR (don't know anything about these as I can't find them on the Bontrager website)
My weight + bike: 230 lbs
My comfortable speed right now: 13-18 MPH

Thanks in advance for your comments!
Barry
Read the tire sidwall info for tire PSI and adjust accordingly to your comfort level. Be sure
to watch for tube pinching.
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Old 03-19-08, 10:44 AM   #8
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That bike also comes with higher pressure road type tires. Maybe if you want such a low tire pressure you should try out some larger tires or even hybrid tires.
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Old 03-19-08, 02:03 PM   #9
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I weigh a bit more than you do (240 without the bike), and I run 700x35 Paselas on my Atlantis at 75 psi with no problems ever. I'm sure I could go at least 5 psi lower without pinch flatting on decent pavement, but there's not a lot of that around here...
I think your best move would be to go up a size or two. I rode for years on 25s and 28s before switching to 32s and 35s (Grant Petersen recommended it for riders my size when I bought the Atlantis), and the larger tires made a big difference. Paselas (which I love, and have put on all three of my road bikes) run very true to the labeled sizes--my 35s measure 34.5--and come up to 700x37. If you can't find them anywhere else, look here: http://www.rivbike.com/products/list...product=10-071
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Old 03-21-08, 04:09 AM   #10
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All,

I sincerely appreciate all the awesome comments. Now I feel much better about what I'm riding on, and all the rest of your advice is not lost on me. Now I'm excited about experimenting with different pressures- especially the advice to keep the rear a little higher than the front.

Again, thanks to all for your time!

Barry
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Old 03-21-08, 10:33 AM   #11
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Mind you, once you get used to the jolts, you will get a much faster ride when you pump the tires up to 100. And it will be much, much easier to ride.
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