Tyres And Emissions

[Dr. FrankenRex]

You seem very sure about this, but you are not correct. Yes, Mazda got some things wrong, and for certain parts we recommend buying better, or different, components to Mazda's OE fit. For no components that I can think of do we recommend worse components. You might think a 225/40 is almost the same as a 225/45, but it has over 10% less air in it. And that is crucial in ways that only tyre physicists properly understand.

Describing my view as "crazy", given that I do actually know exactly what I'm talking about, isn't well appreciated. Your continued insistence that an inappropriate tyre will be just fine, despite having no knowledge, experience or training to make such assertions, is also not appreciated. I don't tend to give opinions on here unless I know what I am talking about, and I'm always keen to learn from those that known more than I do about a subject. It's a good philosophy to live by.

Now, I have to go to Barcelona to run the tyre tests for the McLaren F1 team. Perhaps I should ask the Pirelli head of R&D for his view about amateur "experts" on forums making random recommendations on the fitment of inappropriate tyres?

Perhaps my use of the word crazy was inappropriate, so apologies for that.

Ultimately I have run multiple tyres, including ad08r (with a lower load rating than pilot sports) in 225/40r18 on an rx8 to great effect. So whilst I don't have experience of the theory, I do have experience of it to some degree.

I know of many other owners using 225/40 track tyres on rx8s and heavier cars besides.

I don't claim to be an expert, I am giving my opinion on something that I have personal experience of. OP (and everyone else) is obviously free to make their own decisions based on their research, comments from others and the experiences of themselves and others.

[Eddie_r32]

This thread shows one of the things I don't like about this forum and is damaging it, a Mazda know best attitude, let people experiment, a different tyre size won't kill anyone or do any harm, let people experiment and find out what makes the car better or worse, don't jump down their throats for modifying, accept that experience can be more valuable than book learnt knowledge.

[Dazza44]

I wont pretend to know much at all with regards to the tyres,but when i lowered my car and fit the spacers i was advised by many on here to fit 225/40s to avoid any possibility of rubbing the arches,
And on road use and abiding by all the speed limits , ive had no problem with gripping the road etc.

Sent from my SM-G920F using Tapatalk

[goodeggbob]

It is very difficult to put in writing what we are really trying to say. We don't have the use of pauses,facial expressions and hand movements.
This apparent disagreement between 2 people, both with their own experiences, would probably be no where near as confrontational as it appears here ,had they been discussing the subject face to face, but it needs to be said though, that on reading the topic,the original poster is probably left a bit confused and non the wiser,which is not helping anyone.

[350matt]

uniroyal rainsports for me if you want a reasonable tyre thats not too expensive
Michelin are overpriced for what they are

[Lewis_]

Well didn't mean to start up an argument but thanks for all the advice didn't realise there would be so many people willing to help. Also when taking off the exhaust i found a rusted hole in it and i believe that may be the cause of the emissions failure.

Thanks again for all the help

[warpc0il]

Usually a hole in the exhaust system helps emissions, as less of the nasty stuff finds its way to the tailpipe tester.

[13Black]

Not sure I wholly agree with that Mister David - holes also draw in oxygen which throws the sniffer's lambda reading out of spec, that's what it's thar for.

[Django91]

''You seem very sure about this, but you are not correct. Yes, Mazda got some things wrong, and for certain parts we recommend buying better, or different, components to Mazda's OE fit. For no components that I can think of do we recommend worse components. You might think a 225/40 is almost the same as a 225/45, but it has over 10% less air in it. And that is crucial in ways that only tyre physicists properly understand.''

Hi Pete, could you explain why the lower profile tyre would a worse choice technically, would it be the same if it has a similar load and speed rating as the OEM fitment as well? Just curious as someone from a technical background

Toby

[PeteH]

could you explain why the lower profile tyre would a worse choice technically, would it be the same if it has a similar load and speed rating as the OEM fitment as well? Just curious as someone from a technical background

Since you asked for the technical answer. A tyre needs a certain amount of "gas mass" to support the load that the car imposes on it. Less gas mass results in more tyre deformation, and that generates more heat, and can ultimately lead to tyre failure (heat accelerates ageing). You can reduce the gas mass by reducing the tyre pressures, and you can reduce the gas mass by choosing a smaller tyre. A lower profile tyre is a smaller tyre (all other things being equal). Therefore a lower profile tyre can carry less load, or it needs a higher tyre pressure to carry the same load. Raising the pressure might seem like a sensible solution, but that brings significant problems. The contact patch will be smaller, and will generate more heat (that heat problem again). More heat always equates to less grip. There is another problem. The increased pressure results in lower cornering stiffness, and it's a significant decrease in cornering stiffness. This means that the tyre needs higher slip angles to achieve the required cornering forces. And this means that the lower profile tyre will feel less direct, and will feel less responsive. This will again increase the temperatures, reducing the grip still further.

The effects are all subtle, and some people would never be able to tell. These plots below are from a rather special tyre model which can reproduce the performance of a tyre with the standard RX-8 profile (the red lines), and a 40 series tyre (the blue lines). You can see that at all normal loads the lower profile tyre has less cornering stiffness (by up to 10%), and at all slip angles the lower profile tyre produces less lateral force. The peak grip is about 1% lower, and this is before the extra heating is taken into account.

Can you recover this lost performance by specifying an XL tyre? No. The basic physics of the tyre behaviour doesn't change with an XL tyre. The only thing that changes is that the construction is stronger, which allows you to run higher pressures, and therefore higher loads. If you use that XL tyre at those higher loads and pressures then its performance will be significantly degraded.

Low profile.jpg

[delta0]

defining a 225/40 Michelin pilot sport as not fit for use compared to a 225/45 Michelin pilot sport is crazy...

It's not what Mazda specify for the car, but we have established they didn't make ALL of the best decisions for the car.

Ultimately it's a case of using sense. If the OP had posted saying "I could buy Michelin for xx or LingLong for yy, should I save the cash?" then yes, Pete's point is right, but that's not what OP is saying.

So yes, regardless of experience, education and profession, I do disagree with Pete's statement above. It's all relative to what the OP is asking

You seem very sure about this, but you are not correct. Yes, Mazda got some things wrong, and for certain parts we recommend buying better, or different, components to Mazda's OE fit. For no components that I can think of do we recommend worse components. You might think a 225/40 is almost the same as a 225/45, but it has over 10% less air in it. And that is crucial in ways that only tyre physicists properly understand.

Describing my view as "crazy", given that I do actually know exactly what I'm talking about, isn't well appreciated. Your continued insistence that an inappropriate tyre will be just fine, despite having no knowledge, experience or training to make such assertions, is also not appreciated. I don't tend to give opinions on here unless I know what I am talking about, and I'm always keen to learn from those that known more than I do about a subject. It's a good philosophy to live by.

Now, I have to go to Barcelona to run the tyre tests for the McLaren F1 team. Perhaps I should ask the Pirelli head of R&D for his view about amateur "experts" on forums making random recommendations on the fitment of inappropriate tyres?

Would you run 235/40 over 225/45?

[mi60o0]

235 40 18 federal rs r
225 40 18 rainsport the two sets of wheels I have

[Dr. FrankenRex]

could you explain why the lower profile tyre would a worse choice technically, would it be the same if it has a similar load and speed rating as the OEM fitment as well? Just curious as someone from a technical background

Since you asked for the technical answer. A tyre needs a certain amount of "gas mass" to support the load that the car imposes on it. Less gas mass results in more tyre deformation, and that generates more heat, and can ultimately lead to tyre failure (heat accelerates ageing). You can reduce the gas mass by reducing the tyre pressures, and you can reduce the gas mass by choosing a smaller tyre. A lower profile tyre is a smaller tyre (all other things being equal). Therefore a lower profile tyre can carry less load, or it needs a higher tyre pressure to carry the same load. Raising the pressure might seem like a sensible solution, but that brings significant problems. The contact patch will be smaller, and will generate more heat (that heat problem again). More heat always equates to less grip. There is another problem. The increased pressure results in lower cornering stiffness, and it's a significant decrease in cornering stiffness. This means that the tyre needs higher slip angles to achieve the required cornering forces. And this means that the lower profile tyre will feel less direct, and will feel less responsive. This will again increase the temperatures, reducing the grip still further.

The effects are all subtle, and some people would never be able to tell. These plots below are from a rather special tyre model which can reproduce the performance of a tyre with the standard RX-8 profile (the red lines), and a 40 series tyre (the blue lines). You can see that at all normal loads the lower profile tyre has less cornering stiffness (by up to 10%), and at all slip angles the lower profile tyre produces less lateral force. The peak grip is about 1% lower, and this is before the extra heating is taken into account.

Can you recover this lost performance by specifying an XL tyre? No. The basic physics of the tyre behaviour doesn't change with an XL tyre. The only thing that changes is that the construction is stronger, which allows you to run higher pressures, and therefore higher loads. If you use that XL tyre at those higher loads and pressures then its performance will be significantly degraded.

Low profile.jpg

Not intending to be facetious here, especially given the way my replies earlier in the thread were taken, but you're talking about the absolute limit of the tyre. Ignoring track driving for a second, if anyone is getting close to pushing a tyre to that kind of limit on the road then there are more likely reasons for them to hurt themselves than running a lower profile tyre.

Also, they probably won't get a tyre hot enough to produce anywhere near peak grip, regardless of the profile. As such a 10% reduction in cornering stiffness and a 1% loss in peak grip is hardly worth worrying about on the road in my opinion.

I fully get you're utilising tyre calculations from an F1 car tyre and applying them to road car tyres - probably very accurately - but at what point is someone on the road utilising 99% of their tyre's capabilities such that the 1% differential in peak grip and the 10% deficit in cornering stiffness causes them to crash?

I also understand your point on the tyre degrading faster because of the additional heat generated, but most people wear a tyre out before degradation becomes an issue...

[Conan]

defining a 225/40 Michelin pilot sport as not fit for use compared to a 225/45 Michelin pilot sport is crazy...

It's not what Mazda specify for the car, but we have established they didn't make ALL of the best decisions for the car.

Ultimately it's a case of using sense. If the OP had posted saying "I could buy Michelin for xx or LingLong for yy, should I save the cash?" then yes, Pete's point is right, but that's not what OP is saying.

So yes, regardless of experience, education and profession, I do disagree with Pete's statement above. It's all relative to what the OP is asking

You seem very sure about this, but you are not correct. Yes, Mazda got some things wrong, and for certain parts we recommend buying better, or different, components to Mazda's OE fit. For no components that I can think of do we recommend worse components. You might think a 225/40 is almost the same as a 225/45, but it has over 10% less air in it. And that is crucial in ways that only tyre physicists properly understand.

Describing my view as "crazy", given that I do actually know exactly what I'm talking about, isn't well appreciated. Your continued insistence that an inappropriate tyre will be just fine, despite having no knowledge, experience or training to make such assertions, is also not appreciated. I don't tend to give opinions on here unless I know what I am talking about, and I'm always keen to learn from those that known more than I do about a subject. It's a good philosophy to live by.

Now, I have to go to Barcelona to run the tyre tests for the McLaren F1 team. Perhaps I should ask the Pirelli head of R&D for his view about amateur "experts" on forums making random recommendations on the fitment of inappropriate tyres?

Would you run 235/40 over 225/45?

Absolutely 235x40x18 every time I wouldn’t recommend 225x40 to anyone at all
I did some tyre tests back in 2011 and recommend the 235 x40x18 size which makes the car feel like a sports car.
I believe someone then said that it was a sports option from Mazda from new.
Pete

[Ranjan]

Can I just second the use of 235/40/18 and the avoidance of 225/40/18.
I also did some testing between fitting 225/45 against 235/40 and 245/40 and 225/40; the 225/40 was the worst of the lot, made the car feel very uncomfortable mid corner and skittish.

The order of best to worse was:
First - 235/40
Second - 225/45
Third - 245/40
Fourth - 225/40

Ranjan.

[zippyonline]

More heat always equates to less grip.

I'm guessing there's a temperature envelope in which the tyres work at their optimum? And then different tyres have different operational temperature envelopes depending on clever chemistry and compound stuff. What's the general rules / physics I can start researching to understand this a bit better? Obvs too hot = bad, too cold = bad, but for different reasons. Tyre pressure that goes with the temperature...

In my head I know from experience that in a go kart, the tyres are useless on the first lap when they're cold...then they start to get better, and on the road, I can feel when my tyres are cold and when they're at a more grip friendly temperature (don't ask how ). So this equates to me that warm tyres = good...

[ChrisHolmes]

cold tyres = not good
warm tyres = nice
hot tyres = take care
overheated tyres = bad news

[Dr. FrankenRex]

cold tyres = not good
warm tyres = nice
hot tyres = take care
overheated tyres = bad news

but who here has actually overheated tyres on the road? I'd wager there aren't *that* many that have even overheated them on track to be honest.

[Eddie_r32]

Road tyres are easy to overheat on track, getting enough heat to the point where grip suffers on the road is pretty much impossible unless your on police interceptors with Pat and Carl following you

[PeteH]

Wow, there's plenty to get my teeth into here! Just for info, I'm not going to go into as much technical detail as some of you may want me to. I've no idea who might read this forum....

Would you run 235/40 over 225/45?

Yes, and I do. My 8" wheels have 235/40 tyres. My 9" wheels have 245/40 tyres. But my car is a lot lighter than most, and my tyre pressures are very different. Every case has to be taken individually, on its merits, and what works in one situation may not work in another. But generally, 235/40 is an upgrade over the 225/45.

Dr. Frankenrex. I hope this doesn't read like I'm taking apart your comments. Your questions are valid, and I'm dealing with them one by one, trying to be constructive.

you're talking about the absolute limit of the tyre.

I'm not. The cornering stiffness is what you feel all the time, and that is the thing that is most adversely affected. Cornering stiffness is a rather difficult concept to describe, but it's basically how responsive the car is to your inputs. Put crudely, a 225/45 is 10% more responsive than a 225/40. Focusing on the 1% grip loss is focusing on the wrong bit. I included that for info (and for the fact that it equates to around half a second on most race tracks).

they probably won't get a tyre hot enough to produce anywhere near peak grip

You will. Road car tyres have what is called a "low working range". They produce peak grip at temperatures which are quite normal on the road. A road tyre which is driven at motorways speeds for 15 minutes will be in its operating range, and able to produce peak grip. Only slicks and serious semi-slicks need to be driven hard to get them into their working range.

you're utilising tyre calculations from an F1 car tyre and applying them to road car tyres

(But I'm not going into details)

causes them to crash?

We are not talking about catastrophic differences. We are talking about which tyres are appropriate, and "feel best". I'm giving you the science behind the reason why the 225/45 is better than the 225/40 on the rex. A 225/40 isn't hideous, and I have said many times that many people wouldn't notice the difference. But I would, Mazda engineers would, and you might.

I also understand your point on the tyre degrading faster because of the additional heat generated, but most people wear a tyre out before degradation becomes an issue...

Hmmm. That isn't quite what I said, and would require another essay on the mechanism of degradation. I didn't say that a 225/40 would degrade before it wears out. It wouldn't. But if you use an inappropriately small tyre, at low pressure, and with high loads, then you can structurally fail that tyre due to sustained high temperatures and fatigue. Mazda specified the 225/45 knowing that it would have a big safety margin. The 225/40 has a smaller safety margin.

What's the general rules / physics I can start researching to understand this a bit better?

It's a fair question. But there is no-one out there publishing any useful technical information on this subject. It's what I do for a living, and I'm not going into any more detail either. Sorry! (your basic assumptions about working envelopes, etc. are quite correct, however).

Road tyres are easy to overheat on track

[Conan]

Thanks Pete
That clarifys things a little
Tyres are a-never ending subject ( Which is why we have a Zillion posts on them )
It’s not a question of one size fits all
Pete

[ChrisHolmes]

And then there is me running 245/35 for no reason other than I think the wider tyre looks good and at the time I bought them I thought the stiffer low profile wall would be an improvement. Now having read PeteH's various posts on the forum I am not sure I have the best size, but if I changed would I notice giving my average driving skills?

[MightyCondor]

And then there is me running 245/35 for no reason other than I think the wider tyre looks good and at the time I bought them I thought the stiffer low profile wall would be an improvement. Now having read PeteH's various posts on the forum I am not sure I have the best size, but if I changed would I notice giving my average driving skills?

I was also on 245/35s today. Odd size and the diameter is significantly smaller than my 225/45 Rainsport road tyres. Can't complain though, as they were a bargain eBay win.

Sent from my Pixel XL using Tapatalk

[Django91]

could you explain why the lower profile tyre would a worse choice technically, would it be the same if it has a similar load and speed rating as the OEM fitment as well? Just curious as someone from a technical background

Since you asked for the technical answer. A tyre needs a certain amount of "gas mass" to support the load that the car imposes on it. Less gas mass results in more tyre deformation, and that generates more heat, and can ultimately lead to tyre failure (heat accelerates ageing). You can reduce the gas mass by reducing the tyre pressures, and you can reduce the gas mass by choosing a smaller tyre. A lower profile tyre is a smaller tyre (all other things being equal). Therefore a lower profile tyre can carry less load, or it needs a higher tyre pressure to carry the same load. Raising the pressure might seem like a sensible solution, but that brings significant problems. The contact patch will be smaller, and will generate more heat (that heat problem again). More heat always equates to less grip. There is another problem. The increased pressure results in lower cornering stiffness, and it's a significant decrease in cornering stiffness. This means that the tyre needs higher slip angles to achieve the required cornering forces. And this means that the lower profile tyre will feel less direct, and will feel less responsive. This will again increase the temperatures, reducing the grip still further.

The effects are all subtle, and some people would never be able to tell. These plots below are from a rather special tyre model which can reproduce the performance of a tyre with the standard RX-8 profile (the red lines), and a 40 series tyre (the blue lines). You can see that at all normal loads the lower profile tyre has less cornering stiffness (by up to 10%), and at all slip angles the lower profile tyre produces less lateral force. The peak grip is about 1% lower, and this is before the extra heating is taken into account.

Can you recover this lost performance by specifying an XL tyre? No. The basic physics of the tyre behaviour doesn't change with an XL tyre. The only thing that changes is that the construction is stronger, which allows you to run higher pressures, and therefore higher loads. If you use that XL tyre at those higher loads and pressures then its performance will be significantly degraded.

Low profile.jpg

Thanks for this Pete
What is your opinion on running 20 mm spacers all around? Is it right to think, that change in offset would not affect the geometry/handling characteristics of the car?

I currently have eibach prokit springs, and intend to run them with the spacers and a RB antiroll bar, with the right tyre size in a view of improving on the rx8.

Toby

[Django91]

20mm spacers definitely impacts the geometry/handling of the car, and (almost) all in a bad way.

Moving the contact patch changes the steering dynamics.
Increasing the distance between the suspension pivots and the centreline of the tread increases the leverage on the springs, effectively reducing the spring rate and making the dampers have to work much harder.
Increasing the unsprung weight also makes the dampers work harder and makes it more difficult to maintain grip on rippled surfaces.
The increased leverage makes tramlining more likely and more obvious, while also increase the strain on the steering joints and rack.

The increased track could improve body roll, if that had been caused by too narrow a wheelbase, but that's not the case with the RX-8.
Body roll with spacers will actually increase, thanks to the spring-rate impacts.

In short, on an otherwise standard car, wheel spacers are only justified on aesthetic grounds and you should be prepared for the handling compromises.

That said, I run aftermarket alloys with a 34mm offset, which puts the tyres in the same place as OEM wheels with 16mm spacers, only without the weight penalty.