Tyre pressure

[Banjo]

That's what I've done. Tyre says 50-75 so I've gone for 60.
A friend decided to go to max pressure as marked at 80 on his tyres and the rear exploded in his hallway that evening!

On these cold winter mornings I have wondered what the temperature difference does to the pressure? If your mate pumped in 80 outside in the cold then lent the bike near the radiator would the increased temp send the tyre pressure up ?

 

[swee'pea99]

On these cold winter mornings I have wondered what the temperature difference does to the pressure? If your mate pumped in 80 outside in the cold then lent the bike near the radiator would the increased temp send the tyre pressure up ?

Yes, it would. But the tyre should have enough margin built in to cope with that. A tyre that explodes at its maximum , well, something's wrong. It could be old/a bit perished? I'd definitely go to the max, and only reduce if something goes pear shaped.

 

[Debian]

A 20 deg C rise in temperature would give approximately a 4 psi increase in pressure so if you inflate to 65 psi (normal MTB tyre max pressure) at a 0 deg C ambient temperature then when it warms up to 20 deg C later in the day it will be at ~69 psi. In Theory.

However, the very act of inflating a tyre increases the temperature of the air within the tyre so in reality you'd probably find that if you inflate to 65 at zero ambient and then check again later (at the same ambient temp) that the pressure had decreased. Later when the ambient temp increases the tyre will probably increase in pressure to what you thought it was when you first inflated it.

So, in a nutshell, and IMHO, unless you put your bike in a Sauna after you've inflated the tyres, if you inflate to max pressure at any ambient temp then it'll probably stay at around max pressure. Any tyre that explodes at max pressure is faulty - unless your pressure gauge was faulty in the first place

 

[Hilldodger]

The other crucial factor is saddle height. Should be on tiptoe sitting on the saddle with feet on the ground. The saddle should be level.

bugger, my legs need to grow quite a bit, then

 

[Chrisc]

A 20 deg C rise in temperature would give approximately a 4 psi increase in pressure so if you inflate to 65 psi (normal MTB tyre max pressure) at a 0 deg C ambient temperature then when it warms up to 20 deg C later in the day it will be at ~69 psi. In Theory.

However, the very act of inflating a tyre increases the temperature of the air within the tyre so in reality you'd probably find that if you inflate to 65 at zero ambient and then check again later (at the same ambient temp) that the pressure had decreased. Later when the ambient temp increases the tyre will probably increase in pressure to what you thought it was when you first inflated it.

So, in a nutshell, and IMHO, unless you put your bike in a Sauna after you've inflated the tyres, if you inflate to max pressure at any ambient temp then it'll probably stay at around max pressure. Any tyre that explodes at max pressure is faulty - unless your pressure gauge was faulty in the first place

Sorry it was the tube that popped and yes, he tells me it was next to the radiator. Nice 5 inch rip in the tube, blew the tyre off the rim and bent it.
Gauge was our calibrated workshop gauge and they were very cold when inflated, no heating in the back workshop, about -4 that afternoon so a big temp jump from out to in.

 

[orienteer]

My earlier comment on saddle height was aimed at beginners. When teaching children, it is not safe to take them on the road if they can't put their feet on the ground. Most turn up with the saddle far too low. Worst of all are the BMXs!

Occasionally one turns up with a bike too large for them, and they can't control the bike.

If you are managing to ride a bike with a high bottom bracket, then I would assume you are not a beginner!

 

[tyred]

Can't say i agree with that. on modern day road bikes (racers and hybrids a like) and some mountain bikes, you shouldn't be able to put your foot on the ground whilst in the saddle. otherwise your saddle is too low.

Some bikes may allow you to do this due to their geometry and low bb.

Indeed. I found it interesting to note that the Irish Highway code states that a cycle should have it's saddle adjusted so both feet can comfortably touch the ground. I can only guess that who ever wrote it has never tried to ride a bike up a hill.

 

[marinbearvalley]

Seems I've been unnecessarily flogging myself riding on tyres that I thought were inflated... Trying to use the little ('get you home' as I am now aware) pump supplied with the bike was getting me nowhere so after talking to my bike shop I bought a track pump and now understand just how much air ought to be in there.
Having gone from 25psi to 60psi has made a world of difference as no doubt, you experienced riders would know! Upshot is that this week out on the hills I've climbed every one that previously beat me without having to stop for a rest! Talk about performance enhancers, a bit of air has made me feel like Wiggo!

Looks like I might be in the same boat. Have been riding for many years but never used a gauge. I have always pumped up until the tire does not bulge too much when I push down as hard as I can, standing by the bike.

Have just fixed a puncture and noted the psi, having pumped up by my normal method - less that 20 psi !!!! My tires say 35-80 psi.

Can it really be right that I have been under-inflating by at least half????? Or is my gauge (which I have also been using for car) completely out.

 

[marinbearvalley]

Can it really be right that I have been under-inflating by at least half????? Or is my gauge (which I have also been using for car) completely out.

I am now set at 50 rear, 45 front and will see how I get on. Can't wait to see the difference! I do a combination of road and single country lanes, lots of gravel and sometimes mud, on a mountain bike. Always hilly.

 

[marinbearvalley]

I am now set at 50 rear, 45 front and will see how I get on. Can't wait to see the difference! I do a combination of road and single country lanes, lots of gravel and sometimes mud, on a mountain bike. Always hilly.

Well the weather was good, so decided to try out the new higher pressures. Started off well nice and fast/smooth on the road. Was very rough going down the track through the park, was giving my leg muscles a work out without pedalling - bouncing up and down. Had to stop and adjust the front shocks.

Then started going very fast down a single track lane - probably about 35mph. Had to brake sharp when I saw a car coming and moved to the side of the road (lots of gravel) to go past. Got back on the road and heard a VERY loud POP. Rear flat. Never had a puncture go POP befpre!

Got home and found a 3 inch spilt down the seam of the (specialized thin) innertube.

I had swapped both tube and tyre, front and back after the previous puncture.

So not sure what to think about the higher pressures??? Doubt this would have happened as they were. Maybe the tyres/tubes were just getting old and couldn't take it??? Any thoughts?

Will put my road tyres on for now with their tubes. Hopefully these are ok although they have been hanging up for a few years.

Thanks to the two guys who stopped to offer help but I was nearly home by then and not much you can do to fix a three inch split in a tube!!

 

[Moodyman]

Here's the rule:

Higher pressure = good for road riding.

Lower pressure = good for off road.

Higher pressure on the road helps keep out road debris, glass, etc.

But on rough surface you need to lower the pressure because the tyre cannot give. For the tyre to ride over rough surfaces the rubber needs to flex. High pressure does not allow this.

So I think you're off-road p*nct*re was probably the result of the higher pressure.

 

[marinbearvalley]

Here's the rule:

ok, thanks. Change of plan then. I will stick with the off-road tyres and fit one of the spare (Kenda) inner tubes I had with the road tyres. The tubes are the same size and feel thicker than the Specialized. Maybe the Specialized were lightweights as they have THIN written on them?

I will reduce the psi's to about 35 which is still more than I am used to but should suit my terrain. Will also check with another gauge because even at 15psi on my foot pump, the tyres feel hard and only flex a couple of mm's.

 

[marinbearvalley]

I will reduce the psi's to about 35 which is still more than I am used to but should suit my terrain. Will also check with another gauge because even at 15psi on my foot pump, the tyres feel hard and only flex a couple of mm's.

erm, just heard an explosion, 10 minutes after leaving the rear newly inflated to 35. FLAT. There is a one inch square blown out of the tube and it has destroyed the tire by ripping several inches away from the reinforcing rim.

 

[swee'pea99]

A thought: I refer you to your own query at the tailend of Msg 23. Might be worth getting a second opinion...? (Especially given your ref to 'under 20 psi'...that really is awful low - hard to believe you've been riding tyres at that pressure.)

 

[marinbearvalley]

Could well be then. I have put my road tyres on and used the remaining spare tube. I used my old fashioned pump (you'll know the type). Pumped up until it felt right - can't squeeze very much and doesn't flex much when I put all my weight on the bike. Connected to my foot pump - 20psi. Sidewall of these tyres have 40-65psi.

It's a Polco foot pump only a few months old.