Club 80-90 Forums

P.S. a well polished car should get higher mpg too, although this is perfectly safe the effect is minimal.

It would be nice to think so, and is certainly true of wings on aircraft, subsonically anyway.

However, the bulk of drag on a car is not skin friction, neither does it have a large wetted area c.f. its frontal area. This means it is effectively a blunt body, where pessure drag predominates.
As with a smooth golf ball, a boundary layer that is laminar cannot sustain such a large pressure recovery as flow decelerates again over the maximum section and thus has higher pressure drag...
So it's quite possible that enough imperfections (roughness) can create enough turbulence in the boundary layer to move the separation point back enough to easily outweigh any small additional skin friction by a reduction in overal pressure drag.

The best effect is to drive in hot summer air, which is thinner... or heat the bonnet, wings and roof
So a matt black car may well have less drag than a polished white one (in bright sunshine)

A hot golf ball also travels further, dimples or no dimples.

Wow. Its great to see this thread commented on again. I thought it was dead.

What is the best preasure to pump my tyres to if I want to get maximum fuel economy and still maintain an appropriate amount of grip?

Thanks

Llamapup

For dry roads, I suppose the max allowable tyre pressure as marked on the tyre sidewall, though you should be aware of front-rear balance and oversteer/understeer issues as well !

For grip on dry roads too (or really, minimal slip-angle) I remember a Lancia track day at ilverstone where I drove 2.0 Beta Volumx, 1.6 Prisma and Delta Turbo which had just been released...

asking what pressures we had, I was told max permiss, about 50 psi at the front (they were all FWD).

Agree with you Clive. When I was around sports cars on the track we always used max pressure for best handling/control/traction and could wear out a set of street brake pads/shoes in less then 2 hours on the track. Don't quite understand why tire at max because not getting maximum tread on road, but it did improve everything.

--accelerate slowly: We are driving vans they cant accelerate quick
--never exceed the speed limit: Diesels cant go over 25MPH anyway
--anticipate stops by using the brake as little as possible : I use the car in front to stop
--avoid idiling at traffic lights as much as possible: Drive through red lights

I would be very carful about over inflating tyres.
By over inflating the tyre, you reduce the tyre contact patch with the road.
This obviously reduces the total amount of grip available.
The car may subjectively appear to handle steer and brake better, as you have effectively stiffened the tyre carcass giving the tyre walls less flex and reducing the pneumatic trail, so it feels a bit like having had a low profile fitted. Great if you have more than enough contact patch to begin with.
But you don't get something for nothing, the over inflated tyre is working much harder to gererate the same cornering force from a smaller area and will reach its limit of grip sooner and without much notice.
None of this is good for the road as it give a false impression of invinicibility, a suttle change in road surface or some contamination can be enough to switch the tyre from grip to slide.
On the track no problem, there are no walls trees or people to run into. The purpose of over inflating the fronts of a track demo car may be to ensue that when control is lost by over excited drivers, the car is even more prone to understeer, so they run wide and leave the track going forwards not backwards. Despite the fact we all secretly think we have the control skills of the Stig, the average driver is not too good at dealing with sudden or un intentional oversteer, contrary to popular belief even rear wheel drive cars are designed to understeer in normal driving.
Regards,
Mike.

MIke, I agree generally with your sentiments re-over-inflation. Of course was just answering the question

Also...

None of this is good for the road as it give a false impression of invinicibility, a suttle change in road surface or some contamination can be enough to switch the tyre from grip to slide.

Especially in the wet

On the track no problem, there are no walls trees or people to run into.

It's because tracks are smooth tarmac, nothing to do with what you might hit...

The purpose of over inflating the fronts of a track demo car may be to ensue that when control is lost by over excited drivers, the car is even more prone to understeer, so they run wide and leave the track going forwards not backwards.

No, reduced slip angle at the front creates oversteer , which under power (FWD) combats gross understeer (severely limiting lap times and cornering forces). Lift-off and/or hit the brakes and the default 'get-out-of-jail-free' is a spin (slowing the car rapidly but continuing in the same general direction due to momentum).
I imagine that less heat is generated in the tires' sidewalls too at higher pressures.

HarryMann wrote:

The purpose of over inflating the fronts of a track demo car may be to ensue that when control is lost by over excited drivers, the car is even more prone to understeer, so they run wide and leave the track going forwards not backwards.

No, reduced slip angle at the front creates oversteer , which under power (FWD) combats gross understeer (severely limiting lap times and cornering forces). Lift-off and/or hit the brakes and the default 'get-out-of-jail-free' is a spin (slowing the car rapidly but continuing in the same general direction due to momentum).
I imagine that less heat is generated in the tires' sidewalls too at higher pressures.

Clive, don't take offence nothing was meant as critacism just my opinion, I know you were answering the question.
Yes I am aware that a proportional increase of front tyre pressure increases oversteer, I was pointing at the danger of vastly over inflating the tyres, which I think would be necesary to gain any significant fuel ecconomy? My understanding is that there is an optimum pressure for max "grip". Above this pressure the actual level of "grip" starts to fall away again, unless some other factor is changed such as increased load. Certainly at such a high pressure the transitional period between grip and tyre stall is much shorter, so the driver gets less warning its about to slide.
I'm sure you are right about control on track speeds, which may be a subtle ploy by track day organisers who have a responsibility to control safety? I was just guessing, i didn't think race teams intentionally cap their cornering speeds.

My concern to anyone trying to save money by over inflating tyres is that it can have a serious detrimental effect on the way a vehicle behaves. On a road when control is lost there is rarely a "get out of jail free' spin. We have no control over the road conditions or what is around the next corner or what the weather does next, regardless of the differences between FWD/RWD or theoretical dynamics etc one way or another the final result of a spin often ends in the same thing with a car in the hedge or worse the opposite carriageway. In these circumstances any benefits would be self defeating to say the least. In the event of an injury or worse then a court and your insurance may consider the tyres to be contributory and in a dangerous condition and as a result have som2 penalty in mind.

It would be nice to see most people actually set the pressures to what they are supposed to be... (I mean anybody.. not just on here/hypermilers)

Really... for most reading this.... ask yourself when was the last time you checked the pressures... I dare bet for most the honest answer is... "last service/MOT/when someone told me it looked flat....." and that would have been weeks, even months ago.....

Those 4 palm sized (in most cases) pieces of rubber in contact with the road have to steer, stop and keep you and your family safe in all kinds of situations... So show them a little respect...

Oh... I am not looking for answers.... just asking you to think about it...

Yes, in the habit of checking mine pretty freqently, but that's come about on Syncros because you just need to.. a lot of damage occurs and slows or lows are not uncommon, in fact, to be expected almost.

On the Micra, 4 new tyres at Costco inflated with Nitrogen, means after a quick check a few days later to see what pressures they used, have pretty well forgotten them... a quick look, kick and off we go

PS. A kick is a VERY crude check & can be VERY misleading! Measuring cold 1st thing in the morning is of course, the preferred method

Well you know if you don't instinctly know how to drive economically then in a small way you lack 100% qualification to drive the vehicle. What I mean to say is that the controls basically connect the driver and his brain to the various bits that ultimalelly connect the fuel tank to the rest of the world. You can get on just fine without this deep knowlwedge and most including me on a bad day do just that. "Hypermiling" will also add tens of thousands of miles to the life of your car and posibly prevent speeding points etc. The same principles are involved in limping home after a mechanical failure as a classic example being my driving 30 non-motorway miles with no clutch or driving 125 miles from Dorset to Hertfordshire with just the exhast manifold remaining - this invoving passing two police cars at speed without mishap or on one less sulubrious occasion doing that same journey on sidelights at night by "mingling at motorway speeds. It is not too difficult for any half decent engineer to work out what needs to be done but of course if you cannot visualise the whole set up in your head you are at a disadvantage. I did have a go at getting a tow from lorries but I do agree that it needs constant attention and does upset the recipient lorry. Sort of thing you may see when the world get to the Mad Max stage - preferably assisted by a bonnet mounted harpoon gun.
Chill out pepe's
Wolfie
Well look at some of the stuff the politicians and boys at the top get away with!