TYRE PRESSURES

A friend has just turned up with some new tyres for my hobby car. They are 315.35.17 toyo's. I need your best guess at tyre pressures (with a reason why) for these on a car weighing about 850 kg. many thanksBill
PS road use only.

You really have a car that has tyres that size and only weighs 850kg!
I'm thinking a small Daihatsu on silly large wheels, unless you've made a typo.
If it's not a typo, then it's some exotic or home-built thing, which if you own such a specialist vehicle, you should probably have a better idea of what the correct tyre pressure should be than anyone here is likely to be able to tell you.

Bill is it a pure track day only car or is it for road use. Tyre warmers being used or not!

check the last line of my query. road use

I'll take a crack at that, somewhere between 26-35 PSI, pure guess, and based cars on slicks at the track will start at 24~psi cold generally, anymore might risk balloon'ing the center of the tire out decreasing contact patch!

Possibly a road legal drag tyre! From memory about 26-30psi depending on how they wear. If they start chewing the centres drop the pressure a bit. If they chew the shoulders go up a tad.

Bill---for road use with such a light car HMM. I just don't know if I'd go there. My concern would be getting severe case of train tracking with such wide tyres
I'd probably start at 28psi but do tyre reading pretty quickly to see if I'm on track or not. -Problem is I just don't know what issues running low pressures in those tyres on the roadwould bring about. But then over high and the darn tyres not gonna get to temp

the weight is about right, (my best over estimate) all my playing with fast cars and this size tyres was 25+years ago and they were crossplies. These are radials and I have no experience to go on. this is the reason I ask.

Cobra! Would have to be fiber panels to be anywhere near that weight though.

My guess then is 25psi +/- 5psi.No experience/qualification at all on that, except working as race mechanic on open-wheelers on slicks, where you made some guesses, and the driver gave his opinion, then you argued.I suspect that 30psi will make it feel and drive like the wheels are made of wood.It's 1/2 the weight of practically anything else you'd expect to have tyres that size.
Getting front/rear balance right will be your problem.

My guess then is 25psi +/- 3psi.No experience/qualification at all on that, except working as race mechanic on open-wheelers on slicks, where you made some guesses, and the driver gave his opinion, then you argued.I suspect that 30psi will make it feel and drive like the wheels are made of wood.It's 1/2 the weight of practically anything else you'd expect to have tyres that size.
Getting front/rear balance right will be your problem.

I have a car which weighs in empty at 605 kg and takes 2 pax and the tyres are 205 55 15s and I run 22 rear and 18 front . We tried higher pressures and it was dangerous , its is very slippery in the wet on this size so would imagine yours with wider tyres will be really dangerous What sort of car have you !

Anadd on, sorry, these will be fitted on 12 inch rims. the old cross plies that were used on race cars back when had 16 PSI hot as a normal pressure. variations up and down allowed but not by more than 2 PSI.
mm12345 what class of car did you work on and where in the world!

Just Formula Ford - in the days when they ran on slicks - many years ago. It was, to be frank, a pain in the backside (wallet area in particular).You could do whatever you liked, with all the skill in the world, and be "quite" competitive - which should have been fun.But for podium finishes, you needed new slicks for race day.You needed to pay more for a camshaft than most of the others would pay for an engine.You had to know when to listen to, and when to ignore the clever old farts who would hang around in the pits commenting on wind changes and top gear ratios 5 minutes before starts, winding up the anxious driver who is always much more concerned that he'll have the wrong ratio, than the distinct possibility that in a rush job, you'd have a gearbox p1ssing oil out on the track, or with no reverse gear, or both.We (driver and crew) were generally pretty hopeless.Driver's ego was much too large, budget much too small, crew with not enough experience/confidence to call the shots when needed.Yes - we were hopeless.

Just Formula Ford - in the days when they ran on slicks - many years ago. It was, to be frank, a pain in the backside (wallet area in particular).You could do whatever you liked, with all the skill in the world, and be "quite" competitive - which should have been fun.But for podium finishes, you needed new slicks for race day.You needed to pay more for a camshaft than most of the others would pay for an engine.You had to know when to listen to, and when to ignore the clever old farts who would hang around in the pits commenting on wind changes and top gear ratios 5 minutes before starts, winding up the anxious driver who is always much more concerned that he'll have the wrong ratio, than the distinct possibility that in a rush job, you'd have a gearbox p1ssing oil out on the track, or with no reverse gear, or both.We (driver and crew) were generally pretty hopeless.Driver's ego was much too large, budget much too small, crew with not enough experience/confidence to call the shots when needed.Yes - we were hopeless.

Edit to say that for your tire pressures, you're just going to have to try and see how it goes, but I'll bet that 30PSI will be way too high, 20PSI probably too low, check temps after a run and tweak, and be bloody careful until you know what's right.Martin11 (above) seems to be on to it - lighter car, 20psi +/- 2 FR balance.

I'd be running 38psi in something with that small of a sidewall.

what has side wall height have to do with tyre pressure in a light car!

how much suspension movement have you got !say 220 kg at each corner = 484lbs,. if the contact patch is 12" x 1 1/2"= 18 sq "thats about 27 psi. as you know bill there are so many variables that only testing will do. BUT the less profile you have, the more suspension you need.

how come then, if I let all the air out of one of these tyres, when on the car it only settles about 1/2 inch! The wheel rim does not touch the ground.Low sidwall tyres are quite stiff in the sidewall in my limited experience.There is a lot of difference between 38 PSI and the 27 PSI you calculate.

yes I agree about the side walls, as u know some tyres can run with NO internal pressure, at pretty high speed in an emergency. How scientific my calculation is, is really opento question, but my logics says i'm not far from a starting point. for a street application you want, durability / fuel consumption / ride quality/ etc, to be in the ball park . the best overall combination will probably see different pressures at each cnr of the vehicle. Most logging trucks that haul out right on those metalled roads can adjust their tyre pressures from the cab to suit conditions. I run the lada 185/70/13 at 50 psi, but i don't drive like fangio, fuel consumption is minimized, the ride isn't too harsh and the tyres DON'T wear out in the middle !but if you get on to a metal road she's like a cat on a hot tin roof esp those corrugations you can get on the cnrs. but i'm not on a rally, so I accept it

I bet your lada is wonderful in the wet. If it is 4WD pumping the tyres up is a waste of time for economy. All the drag of the FWD will ecceed the minimal gains. Let the tyres down and get a bit of comfort and safety.

as an add on to the above post I don't think the word Lada and fuel economy should be used in the same sentence.

no its 2wd, and how much influence tyre pressures have on these tyres in the wet,! really have to do back to back testing. my thinking is compound and tread pattern/depth and speed are the real factors for wets , please let us fellas know what pressures you end with !just out of interest, don't those honda s2000 have a different tyre at each cnr !

Don't know about the hondas but I have found out that the minimum pressure that can be run is17PSI.according to a Toyo race sight.
All dependant on the wheel weight. With my car I get down to the lowest recommended setting.Great news comfort, grip, and tyre life (within reason)
Many thanks all for the information

Stiff sidewall means to low a pressure will cause the centre of the tyre to not have as much contact/force on the ground as the outer edges and will chew through the edges first before the centre of the tyre. Of course too high a pressure will chew out the centre.

I'd start at 36-38psi and monitor wear patterns. Basically all cars these days on 17" and higher and low profile tyres have a manufacturers recommended pressure of around 34-40psi. I'd say they have spent a lot of R&D to come to that conclusion!

315 is a large contact area. You need a good amount of pressure in there to make sure all that contact area is filled and on the road!

Being a road car, pressure is going to really only affect tyre wear and minimally grip (obviously too extreme low (under 20psi) or too high (over 45psi) will affect grip but anywhere in between you won't notice much on the road.

TYRE PRESSURES

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