Radial Tyre Pressure - How Low Can You Go ?

[KTMsm]

I have a large one off classic kitcar - a big non sporty tourer. It's a V8 and weighs approx 1200kg

I started at 30 psi and it drove terribly shuddering at every road imperfection

I've dropped them to 25 and it's transformed the ride - how much lower can I go ?

It will be used on motorways

It's running 245/70/15 radials

I know the better option would be softer springs / shocks but I'm shocked how much difference 5psi has made and would like to try lower - can I go as low as 20psi

[sixor8]

TVRs usually have a recommendation of 24 psi ish. There are owners who have the front tyres as low as 20 psi. Wide tyres and a (relatively) light car.

[Fastpedeller]

Presented with this issue many years ago I just reduced the pressures on my kitcar tyres until the 'squish' looked correct (based on how production RWD cars look). This seemed to work (running 20 PSI). At the time someone in the car club was working at Dunlop and put the weights and tyre sizes into the computer and it came up with the same figure! So it could be as easy as 'what looks correct'. Of course, no pressure gauge (unless calibrated) will be 'accurate', so once you have the figure for your car (based on even tyre wear - or lack of) always use the same gauge. There may be a tyre pressure calculator on the 'net?

[KTMsm]

I was more concerned about how low can I go before risking it being dangerous - ie the tyre coming off the rim when accelerating etc

[sideways man]

‘Reccomended’ tyre pressures for caterham type vehicles are in the 18-20 psi area, though these are typically half the weight of yours.

[Jadatis]

Found your 245/70R15 standard load, to be loadindex 105/ maxload upto 160kmph/99mph caries 925 kg/2039 lbs AT 36 psi


So check this.

Then official european system says not to go lower then 1.5 bar = 22psi. BUT I have seen car-recomandations going as low as 1.4 bar= 20 psi.
And to my idea if humps on rimm you can even go lower, without tire coming from rimm.

What you need to determine acurately are the GAWR's or whatever it called in your country.
Better even real axleloads in your use, only weighing per axle, in loading you use (also persons) is acurate, estimating is dangerous.

Once you have them, and the tirespecifications I found, are right, you can look axleweights back in list I made, to get cold pressure, with max reserve build in, at wich comfort and gripp is still acceptable.
Main goal of pressure determination is to never overheat any part of tire-material.

I daubt if 1200kg empty would bring heavyest axleload with 2 persons in car, above 900 kg, for wich 18 psi is enaugh.

Axleload/cold psi/ Axleload
1701 lbs/ 15,0 psi / 771 kg
1797 lbs/ 16,0 psi / 815 kg
1892 lbs/ 17,0 psi / 858 kg
1988 lbs/ 18,0 psi / 902 kg
2083 lbs/ 19,0 psi / 945 kg
2178 lbs/ 20,0 psi / 988 kg
2272 lbs/ 21,0 psi / 1031 kg
2367 lbs/ 22,0 psi / 1073 kg
2461 lbs/ 23,0 psi / 1116 kg
2555 lbs/ 24,0 psi / 1159 kg
2649 lbs/ 25,0 psi / 1201 kg
2742 lbs/ 26,0 psi / 1244 kg
2836 lbs/ 27,0 psi / 1286 kg
2929 lbs/ 28,0 psi / 1329 kg
3022 lbs/ 29,0 psi / 1371 kg
3115 lbs/ 30,0 psi / 1413 kg
3208 lbs/ 31,0 psi / 1455 kg
3301 lbs/ 32,0 psi / 1497 kg
3393 lbs/ 33,0 psi / 1539 kg
3485 lbs/ 34,0 psi / 1581 kg
3578 lbs/ 35,0 psi / 1623 kg
3670 lbs/ 36,0 psi / 1665 kg/referencepressure
3762 lbs/ 37,0 psi / 1706 kg
3853 lbs/ 38,0 psi / 1748 kg
3945 lbs/ 39,0 psi / 1789 kg
4037 lbs/ 40,0 psi / 1831 kg
4128 lbs/ 41,0 psi / 1873 kg
4220 lbs/ 42,0 psi / 1914 kg
4311 lbs/ 43,0 psi / 1955 kg
4402 lbs/ 44,0 psi / 1997 kg
4493 lbs/ 45,0 psi / 2038 kg
4584 lbs/ 46,0 psi / 2079 kg
4675 lbs/ 47,0 psi / 2120 kg
4766 lbs/ 48,0 psi / 2162 kg
4856 lbs/ 49,0 psi / 2203 kg
4947 lbs/ 50,0 psi / 2244 kg
5037 lbs/ 51,0 psi / 2285 kg/ often maxcold

Edited by Jadatis on Wednesday 6th March 17:32

[TarquinMX5]

Tvr Chimaeras are 22/24 psi fr/rear, with 205/55/15 and 225/50/16 fr/rear tyres. For high speed 'continental' driving or if fully loaded with luggage, 25/28 psi. Weight 1060kg.

Having said that, if it's the same car as on the other thread and it has a 4.2 XK engine, that's a bit heavier than the RV8, probably about 60kg more.

[KTMsm]

Jadatis said:

Found your 245/70R15 standard load, to be loadindex 105/ maxload upto 160kmph/99mph caries 925 kg/2039 lbs AT 36 psi


So check this.

Then official european system says not to go lower then 1.5 bar = 22psi. BUT I have seen car-recomandations going as low as 1.4 bar= 20 psi.

Thanks - where did you find this / what did you Google ?

[KTMsm]

TarquinMX5 said:

Tvr Chimaeras are 22/24 psi fr/rear, with 205/55/15 and 225/50/16 fr/rear tyres. For high speed 'continental' driving or if fully loaded with luggage, 25/28 psi. Weight 1060kg.

Having said that, if it's the same car as on the other thread and it has a 4.2 XK engine, that's a bit heavier than the RV8, probably about 60kg more.

Were they ???

I've had a few Griff / Chim - but I probably inflated them for high speed

No this is another one - with a 5.7 V8

[TarquinMX5]

I should have read the opening post properly , or remembered what I'd just read

[Jadatis]

KTMsm said:

Thanks - where did you find this / what did you Google ?

The size I googled and found that 105 loadindex, and gave standard load wich nowadays AT 36 psi
Other posibility is XL/ reinforced/extraload AT 42 psi.


That not going liwer then 1,5 bar/22 psi, was written in the pages about tirepressure calculation, I got hold of end 2007, and went running with.
This started the tirepressure story of this " pigheaded Dutch selfdeclared tyrepressure specialist"

If you want to read the desighnersguide of ETRTO, send me a mail at my hotmail. com adres with username jadatis, and I will send it in return.
Like this cripted to prevent spamm machines reading it, combine my mail yourselves.