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Re:What tyre psi you running???

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01 Aug 2020 23:46 #225959 by gilburton
My standard 2008 with 215 tyres have always been kept at 23 front 26 rear as stated in the manual and on the door. Fully loaded it's supposed to be 30 psi on the rear but even towing a caravan I've never increased them.
It has to be remembered that these tyres are part of the suspension settings and need a fairly soft ride unlike low profile tyres where the tyres are hard and the suspension is made accordingly.
Tyre pressures for vehicles are determined by manufacturers and are nothing to do with the pressures on the side of the tyres as that is just a max figure by the TYRE manufacturer for safety.
Yes garages just inflate them to 30/32 these days as most low profile tyres are around there.
Back in the day I was a tyre fitter and if the chart on the wall didn't have the car listed then we used to put in 24/25psi as that was the average back then.
I've had Halfords do it and also a Suzuki dealer lol
My wife went out after a tyre change and she said it was all over the road and when I checked they had only inflated them to 2.3 and 2.6 atmospheres instead of psi!!

I started off high and worked my way down ever since :-)

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02 Aug 2020 06:26 #225963 by Lambert
Car manufacturers talk to their tyre manufacturers to come up with a deal for X numbers of tyres at Y specification for Z cost per unit. Having established numbers and cost, the basic specification of the tyre in size speed and load index is then determined to the nearest industry standard with a factor of safety applied. The vehicle manufacturers then provide a pressure chart based on the pro rata maximum safe limits of the tyre when used with the specific model of vehicle. Hence when I used the maximum weight and pressure information on the side of my standard specification road tyres along with the rated load of the axles on my car the numbers correlated with the specification on the door plate. When you alter the specification of the tyre you then have to recalculate your pressure accordingly. Going from a 96 or 97 load index to a 100 or 102 or more is going to have an effect otherwise the whole concept of load and speed index rating of tyres is meaningless.

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02 Aug 2020 07:38 #225965 by Busta

Grima wrote: Index - jimny chat - page20 - topic - tyre pressure for 215 75 15?
Lambert & Busta you both commented in this thread by B3HULK.
Have a read, It will back up what i said.


Grima, you can use whatever tyre pressures work for you! It's your car, do as you please. You will notice that Lambert and my advice is consistent because it is based on our own experience. Looking at that picture, BFGoodrich recommend anything from 19 to 30psi depending on speed and conditions, so it is not exactly definitive. A big difference that shouldn't be ignored is that they won't have actually driven a Jimny on those tyres in those conditions, but the people giving advice on here have.
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02 Aug 2020 09:03 #225966 by Roger Fairclough
Tyre pressures are all about maintaining the maximum amount of tread in contact with the road. To much pressure, the tyre balloons, the contact width is reduced and you have less tread on the road.and the inner tread wears faster. To little pressure, the inner buckles and the outer wears faster.

Try this.

Drive the car onto a flat board. Mark the tyre across with a 50mm wide band of chalk. Push the car so that the chalked band rolls over the board. The chalk imprinted onto the board should be perfectly equal both in width and density.

Roger

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02 Aug 2020 10:36 #225972 by Lambert
Don't forget that with alterations in pressure you not only affect the width of the tyres contact patch but also and more significantly you are altering the length of the contact patch. This is why true 4x4 vehicles have high profile tyres that are relatively narrow compared to their overall height. It means that as you lower the pressure you get a very significant increase in surface area touching the ground compared to what is possible for something like an SUV with big wheels and low profile tyres. This in turn has led to the evolution of systems like Terrain Response where computers have to try to manipulate the available grip to best effect instead of just mechanically increasing it. That's one of the reasons things like unimogs and humvees with on the fly ctis systems are so very much more capable than a Range Rover in technical off road conditions.

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02 Aug 2020 12:30 #225977 by Roger Fairclough
The correct tyre pressure is obtained when the chalk imprint on the board has parallel sides. If it balloons in the middle then pressure is to low and vice versa. remember, this is to ascertain sealed road surface pressures.You may find that there is a small variation in pressures that fit the profile in which case I would choose the average. Terrain response does not alter tyre pressures, it will use traction control linked to throttle control to obviate driver control. The driver just operates the throttle and the computer selects gear and adjusts the power to go where there is traction. Ultimately the car will high centre or sink in soft mud and the system will fail. Then the driver is helpless due to lack of knowledge.

Many years ago The Rover co. invited selected Range Rover drivers who p/e their RR for new ones at regular intervals to a lunch and factory tour at Lode Lane. Each driver was asked "have you ever locked the centre diff".

They all answered "pardon".

And that is why Rover fitted a viscous coupling.

Roger

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02 Aug 2020 13:55 #225979 by Lambert
That's what I said. Terrain Response etc monitor wheel speed at the most basic level and use computer control over the drive line to attempt to maintain forward progress. Ctis affords control of the tyre pressure to maintain traction in a mechanical fashion. These are both off road systems designed to maintain control in situations of limited availability of grip. Running super low tyre pressure on a high traction sealed surface will overheat tyres i very short order. Hence we are having a discussion about the correct and safe pressure to set the tyre. The operating pressure that a tyre will sustain without failure is a function of the load and speed it is subject to. Too much in any given parameter will have some combination of negative effects be that uneven wear, erratic handling, undue heat build up, lack of traction or whatever its a long list. Because of this the tyre manufacturers are obliged under law to provide the necessary information to determine whether the choice of tyre is suitable for the proposed application. Because it would be impractical to have a specific tyre for every different vehicle tyres have reasonable operating ranges so that with some simple calculations of pressure relative to vehicle weight it is possible to use the same size and spec of tyre on significantly different vehicles for example a Jimny and a VW transporter.

I'm sorry if I'm labouring the point but there isn't one single pressure for one type and size of tyre to use in all situations and vehicle types. The weight of the vehicle, the type of ground it is covering and the speed at which it is doing so are all meaningful parameters for calculating the most suitable pressure for the conditions at hand.

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02 Aug 2020 13:58 #225980 by CC Baxter
Whatever happened to kicking the tyres and saying "that looks about right"?
Chris

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02 Aug 2020 14:08 #225981 by Lambert
You mean my preferred method of pressure assessment? Dunno.

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02 Aug 2020 19:44 #225994 by kirkynut
Wow! I've just read this thread. Now I'm tired! Get it? Tired?

No pressure or anything but it's an easy thing to understand!

Kirkynut

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02 Aug 2020 19:52 #225996 by Lambert
Nice!

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03 Aug 2020 20:42 #226042 by Phattaff
Sorry,what was my question again?????? :-)
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