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-   -   Changes in tire pressure - highway driving (https://www.team-bhp.com/forum/road-safety/94456-changes-tire-pressure-highway-driving.html)

If normal tire pressure for my Fiesta is 30psi, I would raise it to 36 at the rear wheels when 4-5 people are traveling with luggage.

My question is this :
During highway driving with consistent speeds of 100-110 and (some times touching around 130-140) there exists significant heat build up.

What would the subsequent rise in psi be? (from 36psi originally) 42-43 psi or more?
and how much can tires take ? (Let stake 14" Yoko ES100s 185/60s)

The rise is typically 2-4 psi in Radials. It can be up to 10psi in cross ply tyres.

Tires usually don't heat up that much when driving in a straight line at 100-120kmph. After a 6hr drive, my tires were barely warm. Compare this to a 15min run on track, where i couldn't even touch the tires.

Tires usually hold upto 44-46psi. Some might have a higher rating.

Just make sure the tires have the right pressure and that the wheels are aligned properly.

Shan2nu

I personally inflate 32 PSI for my car, while recommended pressure is 30 PSI. Don't go more than 2 PSI above recommendation.

I also observe people inflating the tyres while 5 people are sitting inside. In such conditions the actual pressure will be less than recommended IMO.

No it should be 3-4psi more. Reducing pressure will increase the load on the sidewalls, causing them to fail.

Shan2nu

hi guys, i still dont understand the idea behind increasing the tyre pressure for highways. Why should we increase it?

On the other hand, reducing it makes more sense in my opinion, for bad Indian roads as it gives a better ride, as well as acts as a cushion between the roads and the suspensions.
For good roads, it still helps to have better control as the area of contact is slightly more.
So, does it not make more sense to reduce the pressure for highways instead of increasing?

How so? If the gauge shows 30 PSI with five on board, surely if everyone gets down and you measure again, it will be < 30 PSI?

I never check the pressure when there is someone in the car. It tends to give inaccurate readings.

So increase the pressure when the vehicle is not loaded. This way, you don't have to take any chances by guessing.

Shan2nu

I remember the UK-AA guide in the 70's. Said increase a max 2psi in radials for high speed loaded conditions. 100kph is not high speed by any chance!

You dont need to increase psi for highways.
Only when there is an additional load on the car.
(To clarify there arent 4-5 people in the car or any luggage while i'm in the city )

I never increase more than 2 psi even if the car is fully loaded.
Moreover, the extent of over-inflation will vary depending on the wear out of the tyre. I would be worried about any tinkering with air pressure in a well worn-out tyre.

Tyre pressure increases as tyres get hot when driven, 1 psi for every 5 minutes till 4 psi then insignificantly thereafter. Manufactures allow for this thus tyre pressure is specified "cold". A very useful site for tyre pressure vs load calculations is here Tire Tech Information - Air Pressure/Load Adjustment for High Speed Driving Here you can see that for a W rated tyre no restrictions are required till speed >190 KMPH.

I am bit confused reading the above all, if it needs to be increased when car has more weight load or for long, highs-peed drivings. Though I guess it would be for both.

So, is it necessary to increase the pressure if it is a nitrogen filled tyre?

The increase in pressure is recommended bcoz it reduces tyre flexing and makes them more resistant to external forces.

The advantage that nitrogen has over air is that it does not expand as much as normal air (under similar driving conditions).

But 30 psi of air or nitrogen is not enough to support the tyre at higher speed/load. So irrespective of what gas you inflate the tyres with, you must increase the pressure as the speed/load rises.

Shan2nu

+1 to that, you are supposed to increase the tyre pressure as per the load requirement and not as per the terrain. Stick to the manufacturers specifications for the psi settings and you should be doing good.