Incorrect Information re: Tire Pressure in Efficiency Mods

[marinpa]

Originally Posted by UFO

I agree. I think that point is either at 90% of the potential mileage gain, or the maximum sidewall rating. Going over that is minimal FE gains and reduced margin of safety.

Again... you believe the safety margin is reduced because of... ...
Do we have any definitive, empirical data reflecting that?

How did the manufacturer find that "golden" sidewall max. value? How did they come up with? They won't tell us.. .. "Trade Secret" So, we have to take their word for it.

I am neither a scientist nor a tire designer. So what makes you think you can come here and advocate 5-10 PSI over the sidewall max.?

Well, for one, I am talking from my own experience. 8 years and over 200,000 km driven on 10PSI on multiple vehicles ( see previous posts).

When I first got introduced to this concept, my jaw dropped on the floor.

The thing is, 10 minutes after (being introduced to the idea), I had the opportunity to experience it right away and I was sold by the end of the day.

[Arragonis]

I have no 'empirical evidence' but it seems fairly obvious that what works on one vehicle (an old fashioned, lardy, RWD live axle car) may not work on another (FWD, all independent suspension, lighter) so any universal rule is stretching it a little - people's experience may vary and anyone doing this needs to try it a step at a time for safety.

I did this in 5psi steps since the spring and now I've settled on pressures of 45 (makers rating is 36) which seems like a reasonable compromise - sidewall max is 51 fronts, 54 rears. This has made coasts much better and dry handling is improved. Wet handling is no different but when I tried 50 it did get 'lighter' at the rear, 55 and it was very edgy on roundabouts. Ride quality is always bad so it made no difference.

I did the same on Mrs A's Octavia (also 35ish makers rating) and took them to 40. All OK, tried 45 but the handling certainly felt worse and ride was far more crashy - too bad for Mrs A. I backed it to around 42 and use that now and she has reported better tank to tank MPGs depending on the trips she is doing.

Most roads here are black tarmac and they do get quite a dousing of fuel spills and drips especially from commercial vehicles such as trucks and buses. The first bit of wet weather just after a spell of dry is always dicey. So its worth knowing what difference any changes will make.

Disclaimer - I am not a racing driver or extra special trained and rarely track day. I have driven skinny tyred cars since 1985, and I did get Driver of The Day at Knockhill on my birthday a couple of years ago - in streaming wet weather and on slicks too.

[Arragonis]

PS - One aspect of a margin of safety is if you hit something in the road - ironically probably a piece of someone else's blowout. A lower pressure allows the tyre to absorb some of the impact and a higher pressure will reduce this capability.

Maybe.

[euromodder]

Originally Posted by marinpa

When you say running 10 PSI over the sidewall max it'd downright dangerous, what do you base your affirmations on?

I'm not really saying 10 psi over the rated pressure is dangerous.
I'm saying 60psi on my car would become dangerous - which happens to be 10psi above the tyres' rated pressure.

So :
Increase tyre pressure well beyond car placard rating: Sure !
Stating everyone by default should run 10psi over the sidewall rating : Nope !

Where did you get the information that reflects your statement?

Seat of the pants when trying out something around 54-55psi resulting in a rear-end that gets all too tailhappy when the suspension is upset while cornering.

we have to get to the bottom of this without using personal opinions or beliefs based on popular anecdotes

It's empirical.
Tried it and didn't like the effect, which was significant enough to notice.

If my suspension gets renewed, I'll try increasing the pressure again, and see if the bounciness above sidewall max. goes away.

[euromodder]

Originally Posted by Arragonis

I did this in 5psi steps since the spring and now I've settled on pressures of 45 (makers rating is 36) which seems like a reasonable compromise - sidewall max is 51 fronts, 54 rears. This has made coasts much better and dry handling is improved. Wet handling is no different but when I tried 50 it did get 'lighter' at the rear, 55 and it was very edgy on roundabouts.

You've got an even lighter car, with a heavier engine up front and even less weight in the back.

I can easily imagine it getting edgy .

[UFO]

Originally Posted by marinpa

Again... you believe the safety margin is reduced because of... ...
Do we have any definitive, empirical data reflecting that?

It should be obvious to all that there is a bursting pressure, and higher pressure is closer to it. That's all I meant, don't read any more into it.

Originally Posted by marinpa

How did the manufacturer find that "golden" sidewall max. value? How did they come up with? They won't tell us.. .. "Trade Secret" So, we have to take their word for it.

It seems obvious to me that the sidewall pressure is the highest pressure that the manufacturer is comfortable specifying. I have no special tire knowledge so I have to rely on the manufacturer.

Originally Posted by marinpa

I am neither a scientist nor a tire designer. So what makes you think you can come here and advocate 5-10 PSI over the sidewall max.?

Well, for one, I am talking from my own experience. 8 years and over 200,000 km driven on 10PSI on multiple vehicles ( see previous posts).

When I first got introduced to this concept, my jaw dropped on the floor.

The thing is, 10 minutes after (being introduced to the idea), I had the opportunity to experience it right away and I was sold by the end of the day.

Good for you. You have more faith than I, even though you profess none on the other hand. As I stated before, your fuel efficiency gains are minimal after exceeding the sidewall maximum, and you are running with less margin of safety. Since you are not a tire designer or a scientist, why can you not just leave it at that?

[marinpa]

Originally Posted by euromodder

Increase tyre pressure well beyond car placard rating: Sure !
Stating everyone by default should run 10psi over the sidewall rating : Nope !

I never stated that you should, have to, must or need to...do anything
I didn't even say you have to try 10 PSI over the sidewall max.

I am just sharing my experience and the little knowledge I have about cars.
Advocating does not mean enforcing or imposing.

[Ladogaboy]

To refocus back on the original topic, braking alone does not comprise all of the aspects of wet traction. Theoretically, a larger contact patch would always lead to better braking and traction; however, in wet conditions, that might not always be the case. The biggest factor in wet traction is water evacuation, which is dictated by the tread pattern.

Think of it this way (and without taking into account tire pressure), a track slick has a massive contact patch in comparison to an all-season tire, but which tire actually performs better in rainy, wet conditions?

I don't necessarily want to take sides on this, since I don't have any empirical evidence either way. Over inflating might or might not improve wet traction (imo, it would have a minimal effect at best); however, under inflating is definitely the wrong thing to do (unless you're off-roading/rock crawling).

[CapriRacer]

Originally Posted by Ladogaboy

To refocus back on the original topic......

Think of it this way (and without taking into account tire pressure), a track slick has a massive contact patch in comparison to an all-season tire, but which tire actually performs better in rainy, wet conditions?.....

If we follow that analogy, then race cars ought to use extremely narrow tires in the wet - but they don't. Obviously the grooves provide a place for the water to go - and that is important for wet traction.

Originally Posted by Ladogaboy

........I don't necessarily want to take sides on this, since I don't have any empirical evidence either way. Over inflating might or might not improve wet traction (imo, it would have a minimal effect at best); however, under inflating is definitely the wrong thing to do (unless you're off-roading/rock crawling).

What evidence we have is reports of wet traction issues at elevated inflation pressures. Clearly these are anecdotal, but it is also clear there must be something there.

Does wet traction fall off linearly from placard pressure? Is the wet traction curve fairly flat near placard pressure, then fall off suddenly at some elevated pressure? We don't know.

Caution should be observed!

[Ladogaboy]

Originally Posted by CapriRacer

If we follow that analogy, then race cars ought to use extremely narrow tires in the wet - but they don't. Obviously the grooves provide a place for the water to go - and that is important for wet traction.

That's what I was attempting to relay with my comment about tread pattern. The most important factor is water evacuation. Obviously, you still want as much of the tire contact the surface as possible, but if you cannot evacuate the water from between the tire and surface, no amount of contact patch or tire width will help. You'll still have a film of water between your tire and the surface (hydroplaning).