The data plate on the door is different to the owners manual for Normal and max load, note *1 normal load is up to two persons. Furthermore the owners manual doesn't differentiate between petrol or diesel. If the diesel weighs 100 kg more than the 1.6 petrol - which it does, you can't help thinking that the extra 100 kg is right over the front wheels. So should diesel owners be running slightly higher pressure in the front even with just 2 people in the car?

[Phil №❶]

Well  surferdude, what do we do 

[Surferdude]

Yep, that's what this is.......^^^^^^

I hadn't noticed the variation between the two sources but it's the same with mine.

Firstly, the placard is there to comply with the Australian Design Rules (ADR's). I'm not sure if other countries have such a thing. There is no reason in my experience as to why the two would vary. I can say that sometimes the tyres on the car from OE have a higher load or speed rating than is shown on the placard, which means you can retrofit to the level on the placard and still stay legal. But I would imagine the placard takes precedence at least in Oz over the handbook.

There are two issues which need addressing here.

Firstly, in answer to Steve R's question, I would certainly be advising drivers of diesel engined cars to run a couple of extra psi. A tyre with a rating of 91H is capable of carrying 615 kgs at its maximum pressure or in the case of one inflated to 32psi - 580 kgs. Multiply x 2 for the axle and you have 1160 kgs. At first glance that would appear to be plenty in either case but you have to take into account the excess weight applied either during heavy braking or whilst negotiating a dip in the road ---- or both at the same time. So there's a margin of safety built in.

The second issue is the practice of putting more pressure in the back than the front for which there are many varied and valid arguments both ways.  My personal preference in relation to my driving style is to run the same pressure all round - in most cars I've had. But this comes from personal experience and driving style. The reason I mention this is that, unlike the standard load example above, many car makers suggest less pressure in the front than the rear (although this may be changing - I haven't paid a lot of attention to it in recent years.

So, I run 36 psi all round under normal conditions. For a trip I just make sure the pressures are at that level before I leave as I reckon that's enough in an i30 hatch. If I had a fully loaded CW and/or was towing something I might run a couple more psi in the rear.

In those cases you need to be aware of how much weight is in the drawbar - and consider that the aforementioned negotiation of dips places an immensely higher load on the rear axle for  short periods of time and the sidewall distortion can be significant. Yo should also be aware that all tyres have a maximum rated psi limit. It should be printed on the sidewall.

Oh. And only check your tyre pressures when they are cold or as near as possible to cold. They'll read higher as they heat up so don't let air out of a hot tyre to bring it back to recommended pressure.

[Phil №❶]

surferdude

Can you explain to me the theory behind Diesel weight and petrol weight. Should the tyre pressures be any different due to different engine weights. After all 32 psi is 32 psi no matter what it is supporting isn't it. 

btw love the pic of the worms, great. 

[Surferdude]

surferdude

Can you explain to me the theory behind Diesel weight and petrol weight. Should the tyre pressures be any different due to different engine weights. After all 32 psi is 32 psi no matter what it is supporting isn't it. 

btw love the pic of the worms, great. 

Yeah. I can. But I don't have a Tyre & Rim Association Standards manual at home with me. Basically the air pressure is what carries the weight. So if 32psi carries 580 kgs, the same tyre with (approx) 40 psi in it will  carry 615 kgs.

The manual above carries page after page of tables by tyre size and pressures with the associated weights. Vehicle manufacturers then work out what their car will be and make a recommendation allowing for a margin of error as described earlier.
So, if there is 100 kgs extra weight in the front of a diesel i30 and the manufacturer has allowed a margin of, say, 30% (which is purely a guess on my part - I have no idea how they do this), then you would add something to the tyre pressures to maintain the same margin of error (Not 30% as there are other factors in play).

Hope that helps.

[Phil №❶]

It does thanks, but doesn't this mean that there would be a different tyre pressure recommendation for diesel / petrol, which was the original question btw 

[Surferdude]

It does thanks, but doesn't this mean that there would be a different tyre pressure recommendation for diesel / petrol, which was the original question btw 

Yes. And my answer was this.

Firstly, in answer to Steve R's question, I would certainly be advising drivers of diesel engined cars to run a couple of extra psi. A tyre with a rating of 91H is capable of carrying 615 kgs at its maximum pressure or in the case of one inflated to 32psi - 580 kgs. Multiply x 2 for the axle and you have 1160 kgs. At first glance that would appear to be plenty in either case but you have to take into account the excess weight applied either during heavy braking or whilst negotiating a dip in the road ---- or both at the same time. So there's a margin of safety built in.
It gets confusing don't it?? But i can't explain why there is no variance on the placard, for either heavy loads or diesel engines.

Surferdude.
Thanks for your input, I certainy appreciate your professional responses to any question I ask. I'm going to try 36 PSI all round.

Note - I do tow a trailer at times, last year 3000 kms in one trip with the boot loaded up too. The outsides of the back tyres were worn substantially more than the front ones after the trip.
I also notice that when I stop on a slight gradient my i30 just sits there.. If I did the same thing in our other car a Getz it would definitely start to roll. I'm thinking the increased pressure will lower rolling resistance a bit.

[Surferdude]

Surferdude.
Thanks for your input, I certainy appreciate your professional responses to any question I ask. I'm going to try 36 PSI all round.

Note - I do tow a trailer at times, last year 3000 kms in one trip with the boot loaded up too. The outsides of the back tyres were worn substantially more than the front ones after the trip.
I also notice that when I stop on a slight gradient my i30 just sits there.. If I did the same thing in our other car a Getz it would definitely start to roll. I'm thinking the increased pressure will lower rolling resistance a bit.

The car should roll freely at 32psi (IMHO). You've probably considered this but is it possible there's a slightly binding brake somewhere?

[rustynutz]

I run 38psi and find it's a good compromise between handling and ride.....

When I did a defensive driving course earlier this year they recommended running 40psi on the open road and 36psi around town.

I thought about the binding brake yet all wheels turn freely when on the jack with the handbrake off. My old commodore was the same, I thought it was just a weight/tyre issue. Maybe I need a rethink - any other ideas?

[Surferdude]

I thought about the binding brake yet all wheels turn freely when on the jack with the handbrake off. My old commodore was the same, I thought it was just a weight/tyre issue. Maybe I need a rethink - any other ideas?

No.
Except to try what you were suggesting - pump the tyres up a few psi and see if it helps.
I guess you tried the car - on - the  - jack thing while the brakes were hot??

Surferdude. The brakes were cold when I tried them. I like the way you're thinking.
Next time the wife comes home from work, I'll be like pit crew waiting with the jack. I'll let you know the result in a few days.

[Dazzler]

Surferdude. The brakes were cold when I tried them. I like the way you're thinking.
Next time the wife comes home from work, I'll be like pit crew waiting with the jack. I'll let you know the result in a few days.

I'd like to see a video of that 

[Surferdude]

Surferdude. The brakes were cold when I tried them. I like the way you're thinking.
Next time the wife comes home from work, I'll be like pit crew waiting with the jack. I'll let you know the result in a few days.

I'd like to see a video of that 

Me too. Steve's pretty good with a camera. Maybe he can set it up on a tripod and hit "record".

I'd especially like to see his wife's face as she drives in the garage and he's standing there with the jack and rattle gun.

[rustynutz]

Hmmm kinky.....

[rustynutz]

I'd especially like to see his wife's face as she drives in the garage and he's standing there with the jack........

Upping the pressure from 32 PSI to 36PSI has definitely made a difference to the rolling resistance. Stopping at the lights and removing my foot from the brake on a slight incline now allows the car to roll. I must say I'm surprised at how much difference this has made with this "rolling on an incline experiment".
Other than that I can't feel any other difference in handling, maybe a bit more road noise but that might just be my imagination.
I was thinking overnight I may chuff them up to 38PSI but having recapped on this thread I'll leave them at 36PSI.
So I should now get less wear on the outside of the front tyres (which are currently worn more than down the middle) and I should get slightly better economy.
Nice one Surferdude.
Cheers

[Surferdude]

^^^
Good news Steve.
Surprising but good to know.

I've been cruising the web this morning (due to rain preventing me from being outside) just looking for tyre effects on economy etc.
Scroll down to tyre pressure and gas mileage on the link below, this guy is claiming a 30% improvement in fuel economy by increasing from 28 PSI to 40 PSI.
http://www.carbibles.com/tyre_bible_pg3.html
I know it's off the true topic a bit but I have a couple of bicycles that I ride - both 700mm wheels, a hybrid that has about 32mm wide tyres pumped to 50 PSI and a road bike with 23mm wide tyres pumped to 100 PSI. The road bike rolls for a lot longer without pedaling. I'm just thinking that there's more to tyre pressure and economy than I first thought.

[Surferdude]

Yep, there's no doubt that the harder the tyre pressure the less rolling resistance.
But, as with everything to do with tyres, there's a compromise. The balance is different for each individual.
Lower profile, better handling but less comfort.
Higher profile, better comfort but reduced handling.

And so on.............

Welcome to my life for the last 1,2,3,4,............ 
Since 1975.

[Phil №❶]

surferdude

Yes, but you balance your life with surfing etc.   

[Surferdude]

surferdude

Yes, but you balance your life with surfing etc.   

True. And TBH, I enjoy both.
I was just being a bit facetious.

[Phil №❶]

surferdude


I      -      N      -     V      -     U         

[rustynutz]

Lower profile, better handling but less comfort.

Which leads me to ask why don't F1 run low profile tyres? 

[Surferdude]

Lower profile, better handling but less comfort.

Which leads me to ask why don't F1 run low profile tyres?

Not sure Rusty but it might have something to do with shock absorbtion. Even years ago I remember Alan Jones complaining about the lack of movement in F1 springs. I think Frank Williams told him to sit on his wallet.

[rustynutz]

 

Guess it's just another of life's (or racing) little mysteries.....