Tire pressure

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Barrybooner
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Tire pressure

Post by Barrybooner » Sun May 12, 2019 5:34 pm

What tyre pressures are you guys running on your 235 35 r19 setups?

Bionicman
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Re: Tire pressure

Post by Bionicman » Sun May 12, 2019 6:08 pm

I think the tyre pressures should be whatever is on the tyre pressure sticker on the drivers door post regardless of whatever size tyres you have on the vehicle. It does not matter what size tyres you have the pressure inside will be the same after all.

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Beaker2135
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Re: Tire pressure

Post by Beaker2135 » Sun May 12, 2019 7:16 pm

Yep the pressure is related to weight not volume
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bob86
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Re: Tire pressure

Post by bob86 » Sun May 12, 2019 8:18 pm

do people set the pressure with load on - IE fitted, or off the van? my old company car was always done at kwik fit and they always did them off the car to the factory pressure, which then isn't the same once you fit them

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PaulC61
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Re: Tire pressure

Post by PaulC61 » Sun May 12, 2019 8:28 pm

I'm sure that somewhere on the drivers side door frame is the tyre pressure settings for the type of load you have shown as the number of people.

As an edit- For my tyre size (only got small alloys) 2 people and a suitcase is 39psi front and 39 psi rear 4 people and 3 suitcases is 42 psi front and 46 psi rear.
Last edited by PaulC61 on Sun May 12, 2019 8:32 pm, edited 1 time in total.
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skinnyd
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Re: Tire pressure

Post by skinnyd » Sun May 12, 2019 8:29 pm

I always thought that the lower the volume of space in a tyre the higher the pressure.

Like my bicycle tyres are 3bar!


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Skully
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Re: Tire pressure

Post by Skully » Sun May 12, 2019 9:58 pm

skinnyd wrote:
Sun May 12, 2019 8:29 pm
I always thought that the lower the volume of space in a tyre the higher the pressure.

Like my bicycle tyres are 3bar!


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3 Bar in your bicycle tyres = 3 Bar in a 19 inch car tyre = 3 Bar in a huge opencast mine truck tyre.
Volume makes no difference to pressure in a tyre.
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Doc
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Re: Tire pressure

Post by Doc » Mon May 13, 2019 12:38 am

I run 33 front and 38 rear in my caddy.
That's measured on the van with the normal load and a full tank of fuel on cold tyres as close to 8-10°C as possible for summer.
These figures I have tested to great extent, many miles and many sets of tyres.

My winter 225/40R18 I run 30 front and 38 rear. That's set at 0-4°C on cold tyres.

I have live tyre pressure on my instrument cluster so watching what goes on while on a run the expected and assumed normal 3-5psi increase is seen as the tyres get warm and the air inside expands.
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Re: Tire pressure

Post by Drew2k » Mon May 13, 2019 7:04 am

Doc, what size wheels and Tyres are you running when your not on your Winters?

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Re: Tire pressure

Post by MartinR » Mon May 13, 2019 8:33 am

skinnyd wrote:
Sun May 12, 2019 8:29 pm
I always thought that the lower the volume of space in a tyre the higher the pressure.

Like my bicycle tyres are 3bar!
The pressure governs the area of tyre in contact with the ground - the volume of the tyre is not related (but see compromises later). If you work in PSI it makes the maths more obvious... 3 bar is about 44psi so lets assume for the sake of easy maths that you plus bike weigh 176 pounds or 12 and half stone. To balance that weight you will have 4 square inches of rubber on the road : 176 pounds divided by 44 pounds per square inch = 4 square inches, or 2 square inches per tyre. On a 1 inch wide tyre that means a flat bit 2 inches long in contact with the road.

An empty Caddy is about 1.7t or 3750 pounds; at 44PSI that's 85 square inches of contact, or about 21 per tyre. If you've got 205 tyres, they're about 8 inches wide, so the flat bit will be about 2.5 inches long. That sounds about right without getting a ruler out :-)

If you lower the pressures, the flat bit gets bigger; if you increase them it gets smaller. Similarly if you increase the load in the car, the flat bit gets bigger. Caddy payload is about 600kg or 1320 pounds; still at 44PSI that's another 30 square inches of contact patch, so it's now about 3.5 inches long on each tyre.

That's all just physics. Now the engineering or even the art comes in: a bigger contact patch will give you better grip but the tyre has to change shape more as it flattens out. That stresses the tyre, and increasing the rolling resistance. But soft tyres soak up the bumps better. So on your race bike you will have rock hard tyres for low rolling resistance, but your tourer might have softer tyres to save your backside a little pain, and your mountain bike will have fat soft tyres to mould to and grip the rocks (but you'll feel them sticking to the tarmac too!). The compromises (and the art) just get worse from there - 'taller' tyres deform more on cornering, low profile tyres on alloys can have lower unsprung mass, and we haven't talked about tread yet...

It's all about compromises and what's on the door post is a starting point based on OEM wheels and tyres and the manufacturer's expectation of usage - move away from that and you may need to move away from the posted pressures.

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Doc
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Re: Tire pressure

Post by Doc » Mon May 13, 2019 2:10 pm

Drew2k wrote:Doc, what size wheels and Tyres are you running when your not on your Winters?
Sorry, I should have said but I was replying to the question original posted which was for 235/35R19.
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Re: Tire pressure

Post by Barrybooner » Mon May 13, 2019 5:16 pm

Cheers guys, for me it’s all about minimising the risk of a cracked 19” rim because I’m paranoid, and from what you guys have said I should run a slightly lower psi to dampen the ride and put less stress on my alloys? For reference I went with 38 front and rear just to see what it’s like but am going to change to 36 front and 37 rear, just to be pedantic🕺🏿 And see how that works out

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Re: Tire pressure

Post by Drew2k » Mon May 13, 2019 10:07 pm

@Doc nice one fella. I’m going to try running the same in mine on my maxi for a bit as I have the same Tyre size all round. I was running 39psi all round and the front two both worn the middles right down as if over inflated!! :(

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Doc
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Re: Tire pressure

Post by Doc » Tue May 14, 2019 1:16 am

39psi is definitely too much on the front, even the 2.0TDI with DSG wouldn't need that much pressure.
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Re: Tire pressure

Post by Drew2k » Tue May 14, 2019 2:16 am

Thanks Doc

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