AlphaDogg

My bike has Freedom Bicycle tires--specifically the Ryder model. The PSI limit on the sidewalls are 80psi, but today I went to one of my LBS's, as the SLiME in my tires blocked the Schrader valve. The service technician extracted the valve core, cleaned it, reinserted it, and inflated it to what felt like 100ish psi. I rode on them for about 8 miles until I got home. My front tire was a bit low, so I inflated it to about 80psi (55psi according to my pressure gauge, which is incorrect). As my rear tire was above the recommended limit, I took some of the air out. The only tire pressure gauge I have is a car tire pressure gauge. When it happened before, I took it to a different LBS who inflated the tires to 80psi. I know it was exactly 80psi because he had a bike tire pressure gauge. When I got home, I checked the pressure with my car tire pressure gauge, and it read 60psi on both tires (so there's no way both tires had a leak of the same speed).

After I got home from the LBS today, I checked the tire pressure with my car tire gauge, and the rear tire (the one he worked on) read 75psi. This means it was closer to 95psi. How safe is it to ride on 90-95psi in tires rated for 80psi?

qmsdc15

It's safe. There are a lot of threads here about tire pressure. I used to run 10 psi over the max, but based on what I've learned here, currently running recommended pressure.

Over inflating makes the ride a bit harsh, but I believe it will allow you to go faster, at least on paved surfaces. There is a lot of debate on this. Experiment and find what works best for you.

AlphaDogg

Safe to go how many psi over the limit?

Doohickie

Oh young padawan, there is much for you to learn. I fear much knowledge will come the hard way.

armada120

By the hard way do you mean by over inflating the tires and see what happens?

AlphaDogg

I'm pretty sure that's exactly what he means. I don't plan on doing that, though. I put them at (what my car tire gauge says) is 75psi. This is more like 95psi.

Doohickie

Pump it up a little more. It will make for an interesting thread.

AlphaDogg

It seems that my car tire pressure gauge was pretty accurate, because I pumped it up with a floor pump that has a gauge (a pretty accurate one at that) until it read 80psi. I then checked it with my car tire gauge, and it read 70psi.

10 Wheels

Get a bicycle floor pump with a gauge on it.

AlphaDogg

Read the post above yours one more time please.

leob1

The psi rating on the tires is very conservative. I wouldn't worry about over inflation. Under inflation will cause snake bite flats. Rolling resistance is a whole 'nuther topic, like chain lube.

Doohickie

I've had different experience.

khutch

Unless you are a Clyde or a racer there is no point to exceeding the rated pressure. You always decrease the smooth surface rolling resistance by increasing the inflation pressure but beyond a point that is typically well below the rated tire pressure the decrease becomes inconsequential. The actual pressure at which the resistance decrease starts to flatten out depends on the weight on the tire, that is why a Clyde may benefit from a pressure that an average rider would not. If your tire deflects less than 15% when your weight is applied to it you have reached the point of diminishing returns. Do a web search on "Berto pressure" and you should turn up some links that discuss this in detail.

On rough surfaces pressures above the Berto recommendation have been demonstrated to increase, not decrease, the rolling resistance.

In terms of safety I think it is unlikely that you will kill yourself or seriously injure yourself by over-inflating a bicycle tire even if you blow it out while doing so. There is a high danger of soiling your pants when it blows!! However, riding a tire that is grossly over-inflated could produce handling issues that could kill or injure you and a grossly over-inflated tire could easily fail catastrophically due to a road hazard at a time when that would lead to death or serious injury. All tires have a safety margin built into the rated max pressure so all tires can safely be inflated over that pressure when new. No one but the manufacturer knows what the margin is though and as the tire wears and suffers the slings and arrows of outrageous fortune that margin diminishes. It is not uncommon for the safety margin to diminish to below zero: well worn tires do blow out in the real world when run at and below rated pressure.

I did not vote in your poll then because the question has no certain answer.

Ken

late

Sigh.

When I was in Italy, an amateur racer asked what it was about Americans and high pressure.

It made no sense to him.

Actually, it simply makes no sense.

Basically, there is a range of pressure, based on the tire and the weight.
So what you do is find the chart (Sheldon Brown's website) and then use
that. It's very conservative, so once you do it, you can ignore it for about
a month (if you have good tubes and tires).

Higher pressure is slower and slightly more dangerous.

IOW, dumb.

himespau 

I'd say +/- 5 psi probably won't cause too much problem as the manufacturers probably build in a little leeway to account for gauge variations, but I'd say just get a gauge that works and keep it within the recommended range. They recommend that range for a reason.

Sixty Fiver

I have often overinflated tyres well beyond their posted limit to get them to seat properly just as you do with an automotive tyre... we are talking 140-150 psi in a tyre rated for 120 psi. I figure that the posted maximum can be exceeded by at least 50% as long as the tube can handle it but this is not a riding pressure.

I normally run below the posted max on most tyres as I am pretty light and follow Berto's advice on tyre deflection under load... this is sound advice if you want to maximize tyre performance.

The posted limits are very conservative but with wider tyres you have to be conscious of the fact that at higher pressures they exert greater outward forces on the rim and this is one of the reasons that are typically rated for lower pressures and still deliver a firm ride at pressures in the 70-80 range rather than the 100 plus psi ranges narrower road tyres are run at.

There is a lot of research on how tyre width and pressures affect performance and it is not quite the debate that the one on chain lube is as it is based on a lot of research... I often ride on 26 inch slicks at 75-80 psi and people are stunned at how fast I can go but it just comes down to having wider high performance tyres that run out very quickly at their suggested pressure.

Am thinking you are riding a 700c tyre in the 35 range... it is really worth it to follow the 15% deflection rule... it is very easy to see as when you get on your bike your tyres should compress and show a little deflection and not be rock hard. Your tyres also serve as your suspension and this deflection factor allows them to conform to irregularities on the road.

Many a rider has washed out in a corner because they have been running their tyres at too high a pressure and had a tyre skip off an object rather than absorb the impact and when a tyre skips and loses contact in a turn that is something that will end a ride in the worst possible way.

People who come into the shop are often amazed when we dial down their tyre pressure a little and they come back saying that their bicycle's ride quality and performance increased noticeably... their is this notion that running at the maximum pressure equals faster when you really just need to run your tyres at the right pressure.

Sixty Fiver

The Italian was right...

Some years ago I went out to do an old fashioned time trial on my 1955 Raleigh fixed gear... it runs 26 by 1 1/4 EA1 tyres which are 597mm and bigger than a modern 26 inch metric wheel (559mm) and the tyres are Dunlops that run at a mere 65 psi.

The English used these wheels and tyres on many of their club bikes until the later 50's and people raced on them and were very successful with them.

Covered 40 km in 59 minutes on a flat route with no discernible wind and this pretty much convinced me that you don't need 120 psi slicks to go really fast and your wheels don't have to be 700c either.

Figure that with a few tweaks my folder with it's high performance 406 wheels will also be capable of knocking down a sub hour 40 too... also have to tweak the engine a little more.

javal

If speed is vital, pump´em hard. The ride might get a bit more bumpy and uncomfy, its a matter of taste and preference.

himespau 

If speed is vital, don't pump them too hard, or you'll be spending more time changing flats and that'll surely slow you down.

yosarian

I haven't try on tires, but I have applied ~300psi to plastic tubing and never exploted (and the tubing has narrower walls), then I guess you can go to 100-120 without trouble.

Capecodder

It takes 2 to 3 times the rated psi to blow the tire off the rim. I was in the tire business for 10 years, and my brother has been in the business for 40+ years. You either know the answer or you do not, so please do not guess. If the tire blows of the rim before reaching 2-3 time the rated amount, you have a faulty bead/tire or you do not know how to mount a tire and seat a bead.

qmsdc15

If the braking surfaces of your rims are badly worn, higher tire pressure will increase the chance of the rim splitting.

AlphaDogg

The rims (and the bike) are only 2 and a half weeks old, so the rims are hardly worn.

qmsdc15

I doubt you will ride enough to wear out your rims. I especially doubt you will ride enough in the gritty wet conditions that wear rims more quickly.

I did not post that info for your benefit. It was in response to the notion that is perfectly safe to put two or three times the recommended pressure into your tires (which in my opinion is very dangerous advice) and for people who ride enough to wear out rims.

AlphaDogg

Don't be so sure. I am on track to go 100mi per week.

Anyway, I will be sure not to go over 90psi.