4

I have an old Fuji road bike that I got a garage sale a while back. It's probably from the 70s or 80s. This summer, I've been trying to fix it up into good condition for some longer-than-usual rides (maybe 10+ miles, as opposed to just 4 or so). The old tires (27 x 1/8, hooked rim) were very worn, so I took the bike to a shop where they replaced them with new ones (32-630, 27 x 11/4 to fit schwinn s-6 or k-2 tubular rim).

The new tires say "inflate to 90 psi", which is what I was planning on doing until one of the bike mechanics told me that I was running them way too high. He said the tires and tubes could handle that 90 psi pressure, but that my rims (which are the original aluminum ones I think) couldn't take 90 psi. Instead, he advised that I keep the tires at 60-65 psi. I tried his recommendation, but the tires definitely seemed to bulging out a lot where they contacted the road as I rode it around. I feel like the tires should be more inflated, but I don't want to damage the wheels.

So, I thought I'd get some advice about the matter. I'm 165 lbs, and my rides won't be too serious for now at least. Just riding around mainly on some wide walking/bike paths we have in the city. There may be a few mile-long stretches of sidewalk, however. Any advice is very much appreciated!

7

I can't see your specific rims from here, but plenty of bikes back in the 70s and 80s had rims that width capable of handling 90-110 PSI.

And I definitely wouldn't run a 1-1/8" tire at 65 PSI. I'd imagine that the rims, if designed for tires that narrow, should be able to handle 80 PSI or so.

  • 1
    Running tires that narrow at lower pressure is bad, it will cause more problems than its worth. The tires will wear faster, you will pick up thorns and glass a million times faster. In the long run it would be cheaper and easier to get rims that can handle the higher PSI. – BillyNair Jun 18 '12 at 19:33
3

I will go further and say that what your mechanic told you is likely to be NONSENSE. See below for my analysis of why, but let me also say that people who ride below recommended pressure are more likely to give up and quit riding, because it's so much more tiring and frustrating!!!

See this article for "full details", but for hook-bead clincher tires, 27x whatever are the same bead diameter - so your worn-out 27x1 1/8 and your new 27x1 1/4 should both interface with your rims "successfully". Rims are not run anywhere near their strength limit as far as tire pressure goes - if they were, they'd fail if you hit a bump or curb particularly hard! But even then, instead of breaking they just go out of true but still hold the pressure.

If you want to do a (destructive) test to "prove" it - get an old wheel, a tire you're willing to sacrifice (with a good tube), and an industrial compressor with an accurate gauge. Now slowly inflate the tire until something goes bang. It will, and quite explosively (wear safety glasses). Now, take a look at what broke - see, it's the steel "bead" wire in the tire that snapped, and let the tire slip over the rim. The rim is still OK, and the tube would have held the pressure if the tire hadn't given way! (This experiment will fail more quickly with a very old used tire - the sidewall will explode instead.) For a tire with no "sidewall" issues, this experiment should allow for almost 2x pressure to be applied before the bead gives up the ghost. I honestly do not know what would happen in this experiment with one of those new-fangled kevlar-beaded foldable tires - if anyone tries it, let us know!

  • I worked in a shop in the mid-80s and was told by a mechanic then that if it says 90psi on the side that means they inflated it until it blew off and cut that number in half... That kind of goes along with the experiment you've suggested. I never tried it myself, but I would run 120psi on tires that had 110 stamped on them when riding in crits back then (on dry roads). – Don Nov 23 '16 at 17:10
  • Rims are not run anywhere near their strength limit as far as tire pressure goes Believing that is dangerous. The surface area of a 25 mm tire on a 622/700c rim is pushing 200 sq in. 4 * pi ^ 2 * 12 * .5 is roughly the area of the torus, which is about 240 sq in, so take away some for the rim area. At 100 psi and almost 200 sq inches, that's about 20,000 lb/900 kg of force total trying to blow that rim apart. Keep increasing the pressure and it could very well be the rim that fails. And just wearing safety googles might not be enough to protect you. – Andrew Henle Apr 22 at 13:34
2

there is a lot of bad information here. First off just because a tire says the max psi is 90 or 110 or whatever doesn't mean you need to use that much air, it's a lot like car tires, the psi recommended on cars varies from car to car, and it's the car manufacture that determines the correct psi for each model...not the tire manufacture, the same is true with bicycle tires except the bicycle manufacture has no clue as to how much weight will be on their bikes...but neither does the tire manufacture, so you have to figure how to calculate that.

Now up until recently when wider aero rims came out this was pretty simple to calculate, thank God that today we have on the internet PSI calculators that take all the hard work out. Now these calculators are based on the old tried and true 15% tire deflection; so see this and I will explain what to do, this is for road bike tires:

http://www.dorkypantsr.us/bike-tire-pressure-calculator.html

On this site there are 3 calculators, you only need to concern yourselves with the middle (or second) one. You need to know how much you weigh with your cycling clothes on, and how much your bike weighs fully equipped with even full water bottles, you add those two up and enter the total where it says rider+bike. Then you go to the next box that says 40%/60% (this setting is more for someone either on a touring bike with most of the weight on the rear or riding a bike with a more sit up straight posture), click on the arrow and change that to 45%/55%; moving down to the next box that says front tire width, click on it and select your width of tire and do the same for the rear tire width; when you enter the rear tire width the calculator will automatically display your ideal PSI. Several notes on this. There is no setting for a 27" tire, but 27" is not that much different than a 700c so don't sweat that, secondly all you need to do is to convert your 1 1/4 (or 1 1/8) to MM which for 1 1/4 would be 32mm. Another note, this is for your average everyday street riding, if you are riding in the rain you can drop your psi by 10 on each tire. Another note, if you put in your total weight and find out that the calculator has suggested a psi greater than that on your sidewall then that tells you that you are riding on a tire too narrow for your combined weight, so go up one size and see what the calculator says.

For those of you with MTBs there is this calculator: http://mtb.ubiqyou.com/

On any of these calculators if the tire manufacture gives you a chart for weight and psi follow their chart not the calculator. Also if you live on very rough roads dropping your psi by 5 will help smooth out the ride a bit, also switching to the next size up tire and reducing your psi according to the calculator results for that size tire will help big time. If by some slim chance you ride on very smooth streets you can increase your PSI by 5 and get good results. Of course the calculator is simply a starting point, but while it's accurate for max tire wear and handling, some people want a smoother ride and some people want a firmer ride, so those may adjust their psi accordingly, but the calculator results are pretty much right on the money.

I have not found a calculator yet for the newer wider rims that allow the tires to run at lower psi, so not sure how to calculate that plus not sure if the old 15% sidewall deflection rule is valid for these new rims. As a guess I would say follow the calculator and reduce your psi by 10 should be ok.

1

I have a 1974 Fuji and I run my tires at 110-120 psi. Anything under 100 dosent work well on my bike. but if the tire says 90 psi I would put 90 psi in it.

0

I can't speak to the specifics of what your rim will handle, but if it were me I would be inclined to run the tire between 70-75 pounds. That should improve the ride as well as leave a margin of safety.

0

I agree with @matt B. You need that higher pressure. I am riding a 1970 Schwinn continental with aluminum wheels. I recently replaced by back tire that advises only 60psi. Ive gotten numerous pinch flats. Upon inspecting my front tire I noticed that it is at 90psi & has had no issues.

  • 1
    Welcome to Bicycles @LikeTheRock. We recommend that new members take the tour to make best use of the site, since it's different to a regular chat site. How to Answer will be useful also. – andy256 Mar 2 '17 at 23:48

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.