Too much Oversteer

[bensale]

The braking distances were in a book my Dad has, I'll see if I can find it.

I have anouther book that has some good stuff about pressures and aquaplaning.

A tyre with 1.5mm (legal minimum) of tread in 1.5 mm of water will aquaplane with a tyre pressure of...

16 psi at 67kph
24 psi at 82 kph
32 psi at 94 kph
36 psi at 105kph

Big differences...

[JBT]

Interestingly, I found >32psi cold in the old NA8 made it skittish over any road irregularities and ride like a dray. OTOH, the car's handling seemed much better at 30-32psi cold.....and the ride was more bearable.

The NC seems to work fine with 32-34psi cold on the street or the track.

[GP]

Please explain why you are so shocked, GP...

Higher pressures = less grip normally.
How come Mazda recommend 26psi?

[Boags]

Please explain why you are so shocked, GP...

Higher pressures = less grip normally.
How come Mazda recommend 26psi?

Because they want you to be comfortable and buy replacement panels/radiators/lights etc. after you've been unable to avoid a collision.

You can;t say Higher pressures = less grip. It's not that simple.

[JBT]

But I can say pressure too high = greased pig handling.

[GP]

Because they want you to be comfortable and buy replacement panels/radiators/lights etc. after you've been unable to avoid a collision.

I can't say I've heard it all now, Must be coming close though

[Mactype]

I can't say I've heard it all now, Must be coming close though

I must say its causing a few chuckles in our office....

[Garry]

Interestingly, I found >32psi cold in the old NA8 made it skittish over any road irregularities and ride like a dray. OTOH, the car's handling seemed much better at 30-32psi cold.....and the ride was more bearable.

I agree with JBT here. 28 to 30psi in my NA seemed to be the sweet spot, 32 to 33psi in my NB. The guy that did my alignment last time pumped my tyres up to 39psi hot, somewhere close to 36psi cold I guess and on the last club run to Jenolan Caves the car was skittish over corrugations and bounced over bumps rather than soaking them up. 36psi cold might be fine for a small car with a fixed roof, but from my experience it doesn't work very well in an older model MX5. The NC's being less flexy can cope with higher pressures.

[Boags]

The oldest trick in the book for mechanics is to drop tyre pressures at a service; when you leave you notice the difference in ride quality and think "wow, that feels much better than before".

JBT has said many times before, a major factor in him running the pressure he does is because it's too harsh at 36+ PSI.

Not sure if that's news to you GP, but it's not in the "now I've heard it all" category.

Ben posted some info before; whether or not you believe it without seeing the source is a different matter. You can see that at a given speed you will be less likely to aquaplane with 36PSI (as opposed to lower pressures). So you're less likely to lose control and hit something in this circumstance. I'd say you'd find similar figures for most areas of driving (dry cornering, emergency braking, brake and avoid etc.), but I won't go any further because figures haven't been posted in the thread.

If you hit something you have to buy replacement parts. Simple. Manufacturers KNOW that they will have to sell replacement parts for people who have prangs. The number of parts is dependent on the safety of the car, its handling characteristics..... and the grip of OEM tyres at a given pressure. Up the recommended pressure by 1 PSI and the number of replacement parts you sell will change. It's fairly logical.

They also have to look at things like: how many people will be killed or permanently disabled if I use a seat-belt clip made in Vietnam as opposed to Germany. Is the cost of the parts worth those extra lives?

So, perhaps you'd like to point out what is so surprising to you? Maybe you just haven't thought about it before. That's your fault, not mine.

[Boags]

I'd be interested to see MX5 specific testing; I still think you'd see better results from higher pressures, although it doesn't "feel" as good.

I run 38PSI cold and don't have a problem with the car being skittish. Isn't your suspension supposed to soak up the bumps? Maybe that's because I'm on standard suspension.

Anyway. It's the interwebs; I don't mind being wrong, just getting my thoughts out there.

[Garry]

Does having more than the minimum tread depth make any difference to wet stopping distances? I'm guessing as tread depth increases then tyre pressure would make less difference.

[GP]

So, perhaps you'd like to point out what is so surprising to you? Maybe you just haven't thought about it before. That's your fault, not mine.

Now the Insults are starting

[Ted]

No real generic rule for tyre pressures, comes down to tyre construction, weight of vehicle and distribution of weight (and balance of car). I would agree that too much is better than too little in tyre pressure, but too much is not as good as just right. Working out just right for your car on your tyres is the key. The MX5 is a fairly light car which is well balanced, and relatively low profile tyres (later cars especially). I would simply start with the placard number as the starting point, and then play around from there and see what suits. For my MX5, which is on 195/50/15 RE001s, I like 33psi cold but when I had A539s I liked 35psi cold. On a 3MPS, the placard says something lie 34/32psi whereas on the 6MPS with the same tyre, same size, same sized rim it says 38psi. I use 39psi cold on my 6MPS, but I wouldn't do that on the 3MPS. I would also expect a manufacturer trying to sell cars have to try and strike a balance. Look at the MX5, fairly light car, fairly small dimensions on 205/40/17s with a 7 inch rim or something like that. Add relatively small amounts of suspension travel, and you could end up with a fairly uncomfortable ride. On the NC, I have noticed 2 different tyres on new cars (Michelin and Bridgestone) and I find that I like more pressure in the Bridgestone.

However, then other factors come into play, like human preferences. It could be adjusting to suit your driving style (trail brake a lot, harsh on and off the pedals, abrupt steering, smooth) and tyre pressures are a cheap way of customising it slightly. And the better the chassis balance of the car, the more I notice the effects of tyre pressures. I'll give you a quick example, I drove the same RX8 for 4 days in Perth this year at AHG. On day 1, it was bucketing and so we just drove the car as we got it from Mazda. Felt great. On day 2, we just drove it and it was a rocketship and handled brilliantly. Too well, as we started to scrub out the outside tyres. So for day 3, a bit of tyre preservation went in and we stuck an extra 4psi in and equalized. Car became an understeering pig with snap oversteer when you invariably had to back off mid corner, and turn in was woeful. Dropped the pressure by the end of the day, and the car was a neutral rocketship again.

[Mactype]

Does having more than the minimum tread depth make any difference to wet stopping distances? I'm guessing as tread depth increases then tyre pressure would make less difference.

Boags is just looking at it a bit simplistically.

Adding pressure will decrease aquaplaning because your making the tyres contact patch less and will also make braking better on a wet road to a point because of this smaller contact patch. but when the road dries up you'll get better braking with a larger contact patch so its swings and roundabouts...

What you can't say is that x pressure will work on all vehicles, your not looking at vehicle weight, tyre width, compound and construction, etc etc.

[Boags]

So, perhaps you'd like to point out what is so surprising to you? Maybe you just haven't thought about it before. That's your fault, not mine.

Now the Insults are starting

No insult there; I used the same amount of cheek that you did in your previous post. No hard feelings; as I said, it's the interwebs...

Mactype; yes I'm looking simplistically but IMO with lower pressures the tyre is more likely to deform under load (braking or cornering) giving worse results.

Of course, tyre construction plays a massive part - semi-slick motorsport tyres are meant to be run at 22-30PSI. They perform brilliantly, so I agree it's all swings and roundabouts.

Thanks for your input Ted; always good to hear from Driver Trainers - something to cut through the BS.