Tyre pressure solution

Some time back I posted that my S2 V8 was wandering all over the road and I was looking for a solution. I climbed under and over it checking bushes, steering dampers and tie rods.

I had just put new tyres on the back and at the same time fitted a UHF radio to the roof console taking the constant power for the radio from the interior light supply, and I wondered if I might have upset the ACE sensor up there or done something to the suspension.

I hadnt taken it on a high speed run until last Sunday where I almost **** myself trying to control it on a winding road at 100kph (60 mph)

I had my second spare in the back as we were heading outback and I checked its tyre pressure and thought while I was there I would check all the pressures, and low and behold, the new tyres on the back were inflated to 52 psi !!!!! Obviously the tyre bloke had pumped them up high to seal the bead on the rim like they do, and forgotten to reset them properly.

After setting all six tyres to 36 psi, I went for a high speed blast and found my old stable S2 was back and cornering brilliantly again. What a load off!

It would appear that the super hard rears were tracking on every little ridge on the road, squirming around as they followed every little contour, and out here with 100s of tons of road trains all the time, the roads have a lot of little ridges.

So..problem solved... I thought I would share this with you in case anyone was having similar difficulties. I never thought that it would have such a major impact on the handling.

wot didi you expect?? a lot of yer suspension is actually the sidewall of yer tyre... too hard and it takes away the ability to ride the bumps.

Hi Outback,

I had a similar experience but it was actually caused by a considerable change in altitude during the summer (7000 foot change); sure enough when I checked the tires at altitude the pressure had grown by about 15 psi.
This made for some interesting times on the mountain gravel roads until I figured out what had happened and adjusted psi down

The book says for a Series, 25psi all round.

My book shows 26psi front 34psi rear

However, book says you can also lower psi for offroad to 17psi front and 25psi rear (I would recommend this for deep gravel and/or soft surfaces) - speed not to exceed 40kph or 25mph with the soft pressure

Following the advice on this forum (regarding fuel consumption), I have increased my squidgy tyres to 36psi front, 40psi rear. I thing mpg has now increased, but by god it's a harsh ride now onroad.

Might back 'em off to 36 at the back to match the front as a compromise.

My father-in-law says that tyre wear will increase with harder tyre pressures. Surely that's not right? I though softer tyres would wear quicker??!

Here's the politician's answer: yes & no!

If you run the tyres very hard - it means there's less rubber in contact with the road (and the rubber not in contact with the road as heavily will not wear as much), thus less friction = better mpg; however, the part of the tire that's in contact with the road is bearing greater weight per square inch and thus greater wear. Personally, I think its a good tradeoff - I would rather pay money out to BFG or Michelin than frigging Imperial Oil or Exxon (they already have enough of my pay)!

The downside to running your tyres hard is that your traction reduces offroad and, infact, in that scenario can actually deplete your mpg - as you dig further into the soft surfaces (bringing back, again, our old enemy - greater friction). So, this is no big deal - just deflate a little before you hit the offroad or soft road areas and reinflate before getting back on to the motorway.

As for running best on gravel - I am still working on that (depending on the depth of gravel before you hit non-gravel dirt affects things a lot).

Front skids and rear cat-tracks, best of both worlds. Mind the rocks and they won't mind you

wolf said:

Here's the politician's answer: yes & no!

If you run the tyres very hard - it means there's less rubber in contact with the road (and the rubber not in contact with the road as heavily will not wear as much), thus less friction = better mpg; however, the part of the tire that's in contact with the road is bearing greater weight per square inch and thus greater wear. Personally, I think its a good tradeoff - I would rather pay money out to BFG or Michelin than frigging Imperial Oil or Exxon (they already have enough of my pay)!

The downside to running your tyres hard is that your traction reduces offroad and, infact, in that scenario can actually deplete your mpg - as you dig further into the soft surfaces (bringing back, again, our old enemy - greater friction). So, this is no big deal - just deflate a little before you hit the offroad or soft road areas and reinflate before getting back on to the motorway.

As for running best on gravel - I am still working on that (depending on the depth of gravel before you hit non-gravel dirt affects things a lot).

Click to expand...

Great explanation, understand what you're saying!

I agree, if it reduces my monthly fuel bill, I can deal with 1k less mileage on the tyres!

Cheers
James

Buy running the pressures high you will wear the centre of the tread prematurely, reduce you tyre contact patch thereby reducing your grip, causing roadholding problems also braking will be effected and the tyre more likely to skid. I'd rather pay the extra for fuel and get there alive.

Spyderman said:

Buy running the pressures high you will wear the centre of the tread prematurely, reduce you tyre contact patch thereby reducing your grip, causing roadholding problems also braking will be effected and the tyre more likely to skid. I'd rather pay the extra for fuel and get there alive.

Click to expand...

There's a flwa there somewhere.

What if you have a full load? 4 people in the seats and a full boot? Would 36 psi all round then see better tyre contact with the road surface? I ask because I'll be in that situation this Friday, driving across to the Czech Republic with a full load, and whilst I want good fuel economy, I also want to 'be safe', as I have my wife and children travelling with me too!

somebody explain it to him

slob said:

somebody explain it to him

Click to expand...

Slob, which part of my post did you not understand? Perhaps you need to read it again.

I understand that with higher TP's, the tyre centre line will often be the only contacting point with the road surface. However, it stands to reason that with added vehicle weight, more of the tyre width will contact the road surface. The question is, how much load is required to allow better tyre/road surface contact, with a tyre pressure standard of 36PSI? Or, with a full vehicle load, what would the optimum tyre pressure be, with 235/70 R16's fitted?

yes i have read it agin and agin.

right, if you bang 40 psi in em then this is enough to increase mpg, i know cos i ran mine to southern spain and back like this. but on the other hand, 40 psi is not enough to make your tyres so hard that you will lose suspension or just have a 1" strip of rubber in contact with the road. it is not dangerous as some text book readers would have you think, slightly increasing tyre pressure is just good for economy.

yella disco said:

right, if you bang 40 psi in em then this is enough to increase mpg, i know cos i ran mine to southern spain and back like this. but on the other hand, 40 psi is not enough to make your tyres so hard that you will lose suspension or just have a 1" strip of rubber in contact with the road. it is not dangerous as some text book readers would have you think, slightly increasing tyre pressure is just good for economy.

Click to expand...

Southern Spain is a tad different than central Europe in the middle of winter!

I think I'll compromise and put 36 in front and rear. Kids on board and all that........

its up to you, its got nothing to do with weather conditions, if you are thinking that way, deflate them to 20 psi like they do in iceland

Ran my 235's at 38psi with a FULL load all the way to the Czech Republic and back, got good fuel consumption and the ride wasn't TOO bumpy! Mostly motorway driving, and the inside lanes were full of HGV tyre ruts (which made for some interesting wheel pulls!), so I tried to keep to the middle/outside lanes (difficult to do in Germany with cars doing 140mph).

One thing though, the engine was at its quietest and seemed most harmonious at 85mph, which was about 3000rpm ('97 300tdi Auto). I've been in louder petrol BMW's!