Tyre Pressures - 15" wheels

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Tyre Pressures - 15" wheels

Post by 8 8US » Mon Feb 27, 2017 10:17 pm

I've just fitted 15" steelies to my bus, first time running 15s and I have 165/60 R15 at the front and 165/80 R15 at the back, what pressures are people running in similar tyres?
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Richard

1960 Single Cab Pickup “Barny” (resto thread - https://www.ssvc.org.uk/phpbb/viewtopic ... 8&t=122082)
1966 Devon “Hetty”

1980 Brasilia gone :(

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Re: Tyre Pressures - 15" wheels

Post by vwJim » Mon Feb 27, 2017 10:18 pm

28-32 usually.
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Re: Tyre Pressures - 15" wheels

Post by ted698 » Tue Feb 28, 2017 10:40 am

I run 35 on mine Rich - 185/80's - any lower and they 'look' low on pressure, and it seems to handle better too at the higher pressure... that trip back from Viccy farm last year in the wind on the M6 made me look at them, as it was like a pig on rollerskates - seems better now at 35psi.

Your tyres are smaller aspect ratio than mine though....
Last edited by ted698 on Tue Feb 28, 2017 10:43 am, edited 1 time in total.

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Re: Tyre Pressures - 15" wheels

Post by readyboy61 » Thu Mar 02, 2017 9:18 am

165/65/15 34psi, up to 36 when fully loaded,

1865/65/15 32psi up to 36 when fully loaded
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Re: Tyre Pressures - 15" wheels

Post by 8 8US » Thu Mar 02, 2017 10:35 am

I've been reading....

If you take the gross weight of vehicle and the max pressure/load rating of tyres at max PSI from the sidewall you can calculate the correct pressure.

So the unladen weight of a Splitty is 1,150kg (I think), max rear load is also 1,150kg, front 950kg so assume split is 55%/45%.

Guess at weight of interior 200kg, plus kit & people another 450kg? so total 1,800kg? giving 990kg at rear, 810 kg at front, fully laden.
Without kit and just running around I guess it is closer to 1,550kg? giving 853kg at rear, 697kg at front.

My front tyres are 165/60 R15, max load of 462kg max pressure 44psi
Fully Laden
Combined max load of 924 / 810 = 1.14
44 (max psi) / 1.14 = 38.5psi
Normal Running
Combined max load of 924 / 697 = 1.326
44 (max psi) / 1.326 = 33psi

Rear tyres are 165/80 R15, max load of 545kg max pressure 44psi
Fully Laden
Combined max load of 1,090 / 990 = 1.1
44 (max psi) / 1.1 = 40psi
Normal Running
Combined max load of 1,090 / 853 = 1.278
44 (max psi) / 1.278 = 34psi

So looks like you are all in the right ballpark and thanks for your responses.

I also found this:-

Top Tip: As a rule of thumb, the inflation pressure of a hot tyre is usually about 4 PSI higher than when cold. If you check your tyre pressures when hot and it is more than 4 PSI higher, then the cold inflation pressure was too low. If the hot inflation pressure is less than 4 PSI difference than the cold pressure, then the cold pressure was too high.

This is derived from: “For every 10 Deg F temp change the tyre pressure changes by 1 PSI”
Richard

1960 Single Cab Pickup “Barny” (resto thread - https://www.ssvc.org.uk/phpbb/viewtopic ... 8&t=122082)
1966 Devon “Hetty”

1980 Brasilia gone :(

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Re: Tyre Pressures - 15" wheels

Post by readyboy61 » Thu Mar 02, 2017 7:41 pm

Interesting stuff, what you will need to do is compare the results with the max psi on the sidewall of the tyre' That number tells you the maximum cold pressure needed for your tire to carry its maximum load.
Last edited by readyboy61 on Thu Mar 02, 2017 7:44 pm, edited 1 time in total.
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Re: Tyre Pressures - 15" wheels

Post by vwJim » Thu Mar 02, 2017 10:45 pm

So I just been to look at the truck tyres.... Fronts are easy to understand but the rears!

My rears, are Firestone Vanhawk 185/75/R14C which have 70psi on the sidewall, which I assume is the max psi, simle. But there is no max load, instead the 102/100R code.

Looking up the R = 106mph speed rating, but why 102/100? Off the Tyre Load Rating tables thats 850kg/800kg?? What figures do you use? Why the variation???
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Re: Tyre Pressures - 15" wheels

Post by 8 8US » Thu Mar 02, 2017 11:03 pm

Jim

It's because that tyre can be used as a single or twin, hence the different ratings.

See here http://www.motuk.com/mot_manual/4-2.asp

So it's the 102 rating, 850kg you use.

Richard
Last edited by 8 8US on Thu Mar 02, 2017 11:06 pm, edited 1 time in total.
Richard

1960 Single Cab Pickup “Barny” (resto thread - https://www.ssvc.org.uk/phpbb/viewtopic ... 8&t=122082)
1966 Devon “Hetty”

1980 Brasilia gone :(

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Re: Tyre Pressures - 15" wheels

Post by vwJim » Thu Mar 02, 2017 11:23 pm

Unladen weight of a Split Pickup (with a tilt & bows fitted) is listed as 1,120 kg, so assume same ratio of 55%/45%.

With just the cover fitted, full of fuel & driver 1,230 kg, giving 554 kg at front & 677 kg at rear.

Max payload is 765 kg, giving 848 kg at front & 1,037 kg at rear.


My front tyres are 165/70 R14, max load of 515 kg max pressure 50 psi

Normal Running
Combined max load of 1,030 / 554 = 1.86
50 (max psi) / 1.86 = 27 psi

Fully Laden
Combined max load of 1030 / 848 = 1.21
50 (max psi) / 1.21 = 41 psi


My rear tyres are 185/75 R14C, max load of 850 kg max pressure 70 psi

Normal Running
Combined max load of 1,700 / 677 = 2.5
70 (max psi) / 2.5 = 28 psi

Fully Laden
Combined max load of 1700 / 1,037 = 1.64
70 (max psi) / 1.64 = 43 psi


Have I worked that out correctly?
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Re: Tyre Pressures - 15" wheels

Post by vwJim » Thu Mar 02, 2017 11:24 pm

8 8US wrote:Jim

It's because that tyre can be used as a single or twin, hence the different ratings.

See here http://www.motuk.com/mot_manual/4-2.asp

So it's the 102 rating, 850kg you use.

Richard

Great, thanks.
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Re: Tyre Pressures - 15" wheels

Post by 8 8US » Fri Mar 03, 2017 11:42 am

Looks right to me, I spoke to someone at Goodyear and he said this calculation can only be a guide and that I should speak to the manufacturer as tyre pressures affect the steeering geometry, suspension, handling etc, I then pointed out it was a 50odd year old vehicle which had been lowered, running different wheels and spec of tyres and he said oh, yes the calculation is as good as anything then!
Richard

1960 Single Cab Pickup “Barny” (resto thread - https://www.ssvc.org.uk/phpbb/viewtopic ... 8&t=122082)
1966 Devon “Hetty”

1980 Brasilia gone :(

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Re: Tyre Pressures - 15" wheels

Post by vwJim » Fri Mar 03, 2017 12:06 pm

Great, well done.
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Re: Tyre Pressures - 15" wheels

Post by Who.Me? » Sat May 27, 2017 2:08 pm

Hi

I've just tried the calculation above for my tyres and get silly results. What am I doing wrong. Single cab, so I'm using Jim's 554 & 667 front/rear unladen weights.

The tyres are Minerva 195R15C with max load ratings for single tyres of 950kg and max pressure of 65psi

2x950 = 1900 max axle load for the pair of tyres

Front
1900/554 = 3.43
65/3.43 = 19psi (seems really low!)

Rear
1900/677 = 2.8
65/2.8 = 23 psi (again, really low)

Where am I going wrong?

Thanks

Andy

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Re: Tyre Pressures - 15" wheels

Post by 8 8US » Mon May 29, 2017 9:30 am

The calculation looks right to me Andy, but as you say the pressures seem very low, what do the tyres look like with that pressure in?
Richard

1960 Single Cab Pickup “Barny” (resto thread - https://www.ssvc.org.uk/phpbb/viewtopic ... 8&t=122082)
1966 Devon “Hetty”

1980 Brasilia gone :(

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Re: Tyre Pressures - 15" wheels

Post by Who.Me? » Mon May 29, 2017 9:52 am

8 8US wrote:The calculation looks right to me Andy, but as you say the pressures seem very low, what do the tyres look like with that pressure in?
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