opsmajor

Tyre pressure conundrum

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I started a thread on tyre pressures in the RC300h forum but this actually affects all of us, so hope I'm not transgressing Forum rules by restarting it here.

I asked the Forum: "In nearly all his 'advice' in the DT Honest John [HJ] advises ignoring manufacturer's recommended tyre pressures, e.g. 35/38 psi, and, unless we are doing 100 mph all the time, run on 30/30 (he says all his cars are on that). What do you say?"

This received many responses some supporting others saying go with the 'door pillar' pressures.

Yesterday I asked HJ to clarify his advice and received this reply: "As I incessantly point out, tyre pressures rise by up to 4PSI when the vehicle is used and the air inside the tyres heats up. Obviously if you set the cold pressures at 38PSI, the pressures could rise to 42PSI in use and that could result in a serious crash."

So, if I read him correctly, he is still saying 'ignore the manufacturer's recommendation (door pillar pressures when cold) and drop them down by 4 psi'?

Mike

 

 

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2 hours ago, opsmajor said:

I started a thread on tyre pressures in the RC300h forum but this actually affects all of us, so hope I'm not transgressing Forum rules by restarting it here.

I asked the Forum: "In nearly all his 'advice' in the DT Honest John [HJ] advises ignoring manufacturer's recommended tyre pressures, e.g. 35/38 psi, and, unless we are doing 100 mph all the time, run on 30/30 (he says all his cars are on that). What do you say?"

This received many responses some supporting others saying go with the 'door pillar' pressures.

Yesterday I asked HJ to clarify his advice and received this reply: "As I incessantly point out, tyre pressures rise by up to 4PSI when the vehicle is used and the air inside the tyres heats up. Obviously if you set the cold pressures at 38PSI, the pressures could rise to 42PSI in use and that could result in a serious crash."

So, if I read him correctly, he is still saying 'ignore the manufacturer's recommendation (door pillar pressures when cold) and drop them down by 4 psi'?

Mike

 

 

Honest John`s advice is, I feel, to be taken as a GUIDE not a BIBLE.

It is based upon his own experience and that of the multitude of members of the public with whom he responds.

I am content to receive and accept his advice and guidance on some 80% of what he says.

He is by far and away the UK`s motoring journalist most communicated with and if I was 10 years old again he would be in my World X1 to play Mars !! 

Regards

John 

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The manufacturer's recommended pressures take the heat expansion into account. It would be impossible for Joe Public to inflate their tyres to the operational temperature.

It's well known underinflation is the more dangerous scenario as tyre sidewall flexes excessively and overheats the tyre with possible disastrous consequences. 

Over inflated tyres will simply wear prematurely and give a hard ride. Because they are harder the tyre wall won't flex so much so overheating isn't a problem.

Tyre pressure is a function of axle weight so it is recommended to maintain the front to rear differential or handling could be compromised.

Bottom line, I'll stick with the pressure recommended by the manufacturer and listed in the handbook. It's stood me in good stead this last 40+ years....

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I agree with the above. The only time I would lower the tyre pressures is perhaps in very snowy conditions where I still have to drive. I am talking emergency use. In those circumstances speeds would be 30 mph at most and the tyres are not going to overheat.

John

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He's talking out of his backside. 

As mentioned already, manufacturer tyre pressure ratings take heat into account. Overinflating tyres isn't going to cause a crash unless it goes over the maximum PSI rate on the tyre (usually around 50PSI or so but check on the sidewall). 

If you start putting the RC and IS on 30psi instead of the pressure they should be at, take a look at how flat the sidewall looks

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On 16/01/2017 at 0:31 AM, rayaans said:

He's talking out of his backside. 

As mentioned already, manufacturer tyre pressure ratings take heat into account. Overinflating tyres isn't going to cause a crash unless it goes over the maximum PSI rate on the tyre (usually around 50PSI or so but check on the sidewall). 

If you start putting the RC and IS on 30psi instead of the pressure they should be at, take a look at how flat the sidewall looks

+1

As repeatedly mentioned on the other thread, manufacturer's take pressure rise into account which is why the recommended pressures are always given as "Cold Pressures".

Why HJ persists with his version of reality is the real conundrum here since a 4 psi pressure rise is nowhere near enough to result in a serious crash due solely to pressure effects.  Tyres rely on the tread (not tyre wall) deforming and heating up to generate grip, but many tyres also contain silica compounds which help cold tyre grip, wet weather grip and in the event you slightly over-inflate your tyres, you are perfectly safe.  I respect a lot of HJs reviews, but on this one, he's wrong as wrong can be.

Under inflating to 30psi from, say, 36psi is far more likely to result in an accident as it increases side wall deflection, increases tyre temperatures and wear, and cause more body roll/pitch into corners and some loss of control.  If you drove hard and pushed  to the tyre and vehicle's limits on a corner you know well with properly inflated tyres, then dropped pressures by 6psi, you'd likely need a change of underwear if you tried tackling the same corner at the same speed again with under inflated tyres!  Don't do it folks, you know it makes sense... (best Del boy accent).

I once remember driving home from work when one of my rear tyres had a puncture (possibly a slow puncture earlier on and I hadn't noticed).  By the time I'd noticed, it had generated so much heat that it caught fire and the smoke in my rear view mirror gave it away.  Bizarrely, I didn't notice anything amiss except a slight weave until the smoke appeared!  This is an example of temperature rise with extreme under inflation where the tyre wall deformation was so extreme that it just carried on heating up with the friction as it rolled about and wallowed, eventually slipping on the rim and catching fire.

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On 19/01/2017 at 6:29 PM, GSLV6 said:

+1

As repeatedly mentioned on the other thread, manufacturer's take pressure rise into account which is why the recommended pressures are always given as "Cold Pressures".

Why HJ persists with his version of reality is the real conundrum here since a 4 psi pressure rise is nowhere near enough to result in a serious crash due solely to pressure effects.  Tyres rely on the tread (not tyre wall) deforming and heating up to generate grip, but many tyres also contain silica compounds which help cold tyre grip, wet weather grip and in the event you slightly over-inflate your tyres, you are perfectly safe.  I respect a lot of HJs reviews, but on this one, he's wrong as wrong can be.

Under inflating to 30psi from, say, 36psi is far more likely to result in an accident as it increases side wall deflection, increases tyre temperatures and wear, and cause more body roll/pitch into corners and some loss of control.  If you drove hard and pushed  to the tyre and vehicle's limits on a corner you know well with properly inflated tyres, then dropped pressures by 6psi, you'd likely need a change of underwear if you tried tackling the same corner at the same speed again with under inflated tyres!  Don't do it folks, you know it makes sense... (best Del boy accent).

I once remember driving home from work when one of my rear tyres had a puncture (possibly a slow puncture earlier on and I hadn't noticed).  By the time I'd noticed, it had generated so much heat that it caught fire and the smoke in my rear view mirror gave it away.  Bizarrely, I didn't notice anything amiss except a slight weave until the smoke appeared!  This is an example of temperature rise with extreme under inflation where the tyre wall deformation was so extreme that it just carried on heating up with the friction as it rolled about and wallowed, eventually slipping on the rim and catching fire.

Good to see someone else with accurate knowledge regarding tyre safety. 👍

Big Rat

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Hi all.

I always take notice off what the car manufacturers say they would no best in my view as they have no doubt spent hours testing all these things and found what is best for the car.:wink3:

65mike:yes:

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On 1/16/2017 at 12:31 AM, rayaans said:

He's talking out of his backside.

Thread from the dead but I concur with my learned friend. He also put it a little more politely than I would.

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