[albertw]

Aerobod, you're just obfuscating.

Why not try to explain a simpler situation, drag racing? If you're going to drag race with a 500 hp car you will do best if you put very sticky tires on. The question is why don't you buy 205s instead of 315s? The narrower tires and wheels would be cheaper for the same rubber compound, and the law of friction says both tires should give the same traction. (And Maynard claims they should have the same contact patch area.) Yet reality says different.

Why?

[dradernh]

Originally Posted by aerobod

I would take that tyre characteristics calculator with a grain of salt without being able to look at their input equations.

It just doesn't pass any reasonable analysis as being accurate, but may overall give a general indication of trends. Most likely the equations used are very basic and not based on real-world materials values and characteristics or dynamic tyre properties.

I'll admit the math (and even some of the concepts, generally) are over my head; the site, however, does claim the following on its home page at https://bndtechsource.wixsite.com/home:

"Our capabilities are derived from many years of working with Global Automotive OEMs and their suppliers."

[aerobod]

Originally Posted by albertw

Aerobod, you're just obfuscating.

Why not try to explain a simpler situation, drag racing? If you're going to drag race with a 500 hp car you will do best if you put very sticky tires on. The question is why don't you buy 205s instead of 315s? The narrower tires and wheels would be cheaper for the same rubber compound, and the law of friction says both tires should give the same traction. (And Maynard claims they should have the same contact patch area.) Yet reality says different.

Why?

The narrower tyres on smaller wheels do work better in many circumstances. My 13" wheels and tyres have less inertia than the 15" wheels. I haven't gone wider in tyres on the front of the Caterham, as it is hard enough to get the 205 wide ones to stay at temperature with the cooling that takes place on the straights. It is all about the right rubber at the right temperature, not the tyre width.

The basic rule is largest brakes needed to lock the wheels at any speed without fade, smallest wheels to fit over those brakes with the right size of tyre that can be kept at the right temperature for optimum grip.

In the drag racing scenario, a 500bhp car would shear a soft rubber compound when the tyre is too narrow, but the question would be why wouldn't you go for an even wider tyre? You reach the limit where the rubber doesn't shear any more, so the grip doesn't change as the load on the tyre decreases, as friction decreases per unit area.

Here is a basic comparison that also shows the contact patch area changing and as the width goes up. The wide rubber just being added at the rear reaches a limit where it slows the car down and in the wet the widest config is the slowest. https://www.tyrereviews.co.uk/Articl...wide-tyres.htm

Fitting the right width of tyre for the car's grip needs and rubber temperature vs friction of the surface are the important things to get right, as opposed to just adding width.

[harma24]

Originally Posted by TajoMan

I have feeling that OP is simply entering a turn little too fast when wet (or any low friction situation)
Especially OP is saying the dry surface is not real issue, I bet it's just too much entry speed.

That was my first thought when I read this post as well. Turning in too fast and scrubbing off speed with the front wheels. Slow down and you'll end up faster in the end.

[XutvJet]

Originally Posted by TajoMan

I have feeling that OP is simply entering a turn little too fast when wet (or any low friction situation)
Especially OP is saying the dry surface is not real issue, I bet it's just too much entry speed.

Exactly. Pushing the car hard in less than ideal conditions and expecting it to exhibit normal handling traits is a bit of a stretch.

Also, traction is quite dependent on tire compound and design in addition to some extent the width of the tire.

A 205/50r17 DWS06 isn't going to offer a lot of lateral grip or feel. The chassis and steering may feel livelier, but overall handling, lateral grip, and overall limits will be noticeably lower than a solid summer only 245/35r18 square setup. Steering with the 245/35r18 setup will feel slower, but way more dialed in. Corning speeds will be way higher too but at the risk of more danger because a lot more speed can be carried.

Put a 255/40r17 square race tire on and the limits go even higher.

If you want to slide around in a controlled fashion at slower speeds and with much lower risk, a narrow tire or rft setup is probably the way to go for most.

[albertw]

Originally Posted by aerobod

Here is a basic comparison that also shows the contact patch area changing and as the width goes up. The wide rubber just being added at the rear reaches a limit where it slows the car down and in the wet the widest config is the slowest. https://www.tyrereviews.co.uk/Articl...wide-tyres.htm

You haven't characterized the video fairly.

First, a simple calculation using the dimensions of the contact patches given by the website shows that the contact area goes up by 12.2% and 22.8% respectively for 225/40 vs 255/35 and vs 285/30. (The photos of the contact patches are deceptive.) That strongly supports the results from the website draderh linked to, and further indicates that area of the contact patch does increase with increasing tire width. (A complication: the narrower tires used much lower tire pressures, whereas to keep the tire distortion the same it would be fairer to use higher pressures in the narrower tires. That would make the change in contact patch with width even larger. I think it is pretty clear that Maynard is wrong about this.)

The host made it clear that the lower wet traction of the wider tires arose because they hydroplane more easily.

Overall the results make it clear that over the range of tire widths commonly used on our cars, wider tires gave better braking and faster lap times on a dry track. Traction at the front was the main determinant of braking and lap times, as the wider tires at the rear were not traction limited.

Going from a staggered setup to a square setup eliminated understeer, which shows that you can actually sense that wider tires at the front give better traction.

[aerobod]

Originally Posted by albertw

You haven't characterized the video fairly.

First, a simple calculation using the dimensions of the contact patches given by the website shows that the contact area goes up by 12.2% and 22.8% respectively for 225/40 vs 255/35 and vs 285/30. (The photos of the contact patches are deceptive.) That strongly supports the results from the website draderh linked to, and further indicates that area of the contact patch does increase with increasing tire width. (A complication: the narrower tires used much lower tire pressures, whereas to keep the tire distortion the same it would be fairer to use higher pressures in the narrower tires. That would make the change in contact patch with width even larger. I think it is pretty clear that Maynard is wrong about this.)

The host made it clear that the lower wet traction of the wider tires arose because they hydroplane more easily.

Overall the results make it clear that over the range of tire widths commonly used on our cars, wider tires gave better braking and faster lap times on a dry track. Traction at the front was the main determinant of braking and lap times, as the wider tires at the rear were not traction limited.

Going from a staggered setup to a square setup eliminated understeer, which shows that you can actually sense that wider tires at the front give better traction.

The higher pressure used in the narrower tyres decreases the contact patch and makes it more oval, reducing the pressure would increase the contact patch area.

The tyre shear is probably not maxed out on the 225 tyres, but seems to be getting close on the 255 tyres on that car, where much beyond isn’t going to make much difference anymore. As I said with a lighter car it is easy to go too wide and start to lose grip when the tyres can’t reach optimum temp, as happens in my Caterham. Tyre temp and compound shear are more important than tyre width to get the optimum grip, at some point increasing the tyre width won’t be beneficial, besides the downside of increased rotational inertia and unsprung mass.

[albertw]

Originally Posted by aerobod

The higher pressure used in the narrower tyres decreases the contact patch and makes it more oval, reducing the pressure would increase the contact patch area.

That's what I meant / thought I said. If they had used higher pressures in the smaller tires or even just the same pressure instead of a lower pressure, the differences in contact patch areas would have been larger than they saw.

Of course there are more important things than tire width in determining traction. If they can be kept constant, then they are irrelevant to the question of the effect of tire width on traction. If they can't be kept constant then the comparison of tire widths is invalid. I don't understand why you keep bringing them up in the context of this discussion.

[aerobod]

Originally Posted by albertw

That's what I meant / thought I said. If they had used higher pressures in the smaller tires or even just the same pressure instead of a lower pressure, the differences in contact patch areas would have been larger than they saw.

Of course there are more important things than tire width in determining traction. If they can be kept constant, then they are irrelevant to the question of the effect of tire width on traction. If they can't be kept constant then the comparison of tire widths is invalid. I don't understand why you keep bringing them up in the context of this discussion.

Not sure in the test why they had a 10% higher pressure in the 225 wide tyres, as BMW normally specifies the same pressure for summer tyres when under the sane passenger load, independent of width variation. The 10% higher pressure would reduce the 225 contact patch area by about 10%, if reduced to the same as the 255 or 285 tyres on the same axle, the area would be close, excluding the small difference from tyre sidewall and tread stiffness between the tyres.

As this thread was originally about understeer, I’m not sure why it would just drift towards tyre width, as there are lots of other effects from tyre characteristics, let alone suspension, weight and power distribution effects. More than anything the F22 needs more front camber to reduce understeer.

[albertw]

Originally Posted by aerobod

Not sure in the test why they had a 10% higher pressure in the 225 wide tyres, as BMW normally specifies the same pressure for summer tyres when under the sane passenger load, independent of width variation.

It's not clear what pressures were used during the collection of the contact patch dimensions. The data on the tire patch dimensions do not give the tire pressure used. Both high (rear) and low pressures (front) were used for each tire width during testing (except the 285s which were not used on the front) so I don't think we can assume that either front pressures or rear pressures or a mixture were used for contact patch dimensions. The numbers I gave for the relative patch sizes assumed the same pressure was used when the contact patches were recorded.

[msej449]

I'm with @aerobod in terms of underlining that things like compound chemistry and tyre build have as much, and sometime more of an effect as width and diameter. But because they're less tangible, or even invisible, people tend to focus on the more explicit metrics of width/diameter, which is natural. And all this is often within a fairly narrow range of adjustments available to the average owner i.e. you can't change the suspension design etc. I think people sometimes expect a magic bullet to solve what is an intrinsic limitation of their specific car (or rather, a limitation that would be so expensive to address that they'd do better to buy something else).

My take away from this is that yes, there may be a couple of things you can do if you're experienceing understeer to moderate it, but beyond driving slower, and going for a modest change in tyre spec' it is what it is. If you want significantly different handling then it may be time to change vehicle, but you'd need to be aware that to make you happy, you might be talking about paying for it in terms of ride comfort etc.

Being an ex-Elise owner, one big issue with all mainstream cars is weight. A Cateram weighs typically around 1200 lbs, my Elise weighed around 1500 lbs, but an M240i Coupé weighs 3,406 lbs. You can tinker with all sorts of things on the BMW, and some of the changes will improve things, but you'll still never get close to the handling of a substantially lighter road car.

[Maynard]

I am not a tire engineer, but I am a scientist, so I remain open to this as an empirical question. I'd be foolish to assume that there isn't anything else that could be at play here beyond my limited knowledge. Really 2 questions arise: does contact patch increase meaningfully w/ wider tires? and do wider tires offer better grip? - might actually be different factors at play in grip besides just area. I'd welcome any truly informed opinions or direct evidence.

My position is based upon True Facts - i.e. 'what I was taught in my youth by people I thought knew a lot' but it is supported directly by fairly basic physics - with a thin membrane that does not exert any of it's own properties, the contact patch is determined by pressure and weight, regardless of shape/width - AFAIK, they will have to repeal the laws of physics to change this, but those laws don't say anything specific about tires. I am also relying upon the general (and thus somewhat inaccurate) concept that friction is fairly consistent, so that grip is a direct function of contact patch area, also unchanged by the shape if overall size is constant. For now, and until I see something really convincing, I continue to believe that wider tires on street cars are a marketing ploy or cosmetic device (but with definite advantages on track, up to a point).

I am actively interested in getting a more definitive answer to this, since there are a host of people who swear by the positive effects of even modest width increases. Yet there are also plenty of stories like the one above about the Chumpcar, where changes in suspension geometry produce the same results (and wider tires usually change geometry). This subjective 'evidence' is also suspect due to what we call 'confirmation bias' (people believe that their decisions are correct, and bias their perceptions to support that), coupled with the notorious inaccuracy of the 'butt dyno' - how often have you heard 'if a lap feels fast, it wasn't'. I also can regularly see cars with fairly modest tire setups cleaning the clock of steam-roller-tired megabuilds; the practical advice I got generally matched what was posted above (smallest brake you can lock up, wrapped in the lightest wheel). I will take this as a challenge to find some more reliable sourced info. Any actual tire engineers on the boards who care to weigh in?

[dradernh]

Originally Posted by Maynard

I am not a tire engineer, but I am a scientist, so I remain open to this as an empirical question. I'd be foolish to assume that there isn't anything else that could be at play here beyond my limited knowledge. Really 2 questions arise: does contact patch increase meaningfully w/ wider tires? and do wider tires offer better grip? - might actually be different factors at play in grip besides just area. I'd welcome any truly informed opinions or direct evidence.

My position is based upon True Facts - i.e. 'what I was taught in my youth by people I thought knew a lot' but it is supported directly by fairly basic physics - with a thin membrane that does not exert any of it's own properties, the contact patch is determined by pressure and weight, regardless of shape/width - AFAIK, they will have to repeal the laws of physics to change this, but those laws don't say anything specific about tires. I am also relying upon the general (and thus somewhat inaccurate) concept that friction is fairly consistent, so that grip is a direct function of contact patch area, also unchanged by the shape if overall size is constant. For now, and until I see something really convincing, I continue to believe that wider tires on street cars are a marketing ploy or cosmetic device (but with definite advantages on track, up to a point).

I am actively interested in getting a more definitive answer to this, since there are a host of people who swear by the positive effects of even modest width increases. Yet there are also plenty of stories like the one above about the Chumpcar, where changes in suspension geometry produce the same results (and wider tires usually change geometry). This subjective 'evidence' is also suspect due to what we call 'confirmation bias' (people believe that their decisions are correct, and bias their perceptions to support that), coupled with the notorious inaccuracy of the 'butt dyno' - how often have you heard 'if a lap feels fast, it wasn't'. I also can regularly see cars with fairly modest tire setups cleaning the clock of steam-roller-tired megabuilds; the practical advice I got generally matched what was posted above (smallest brake you can lock up, wrapped in the lightest wheel). I will take this as a challenge to find some more reliable sourced info. Any actual tire engineers on the boards who care to weigh in?

I suggest taking a look at Paul Haney's The Racing & High-Performance Tire: Using the Tires to Tune for Grip & Balance, co-published by Haney and SAE.

I've chosen to use the book as the occasional reference, so I can't offer an overall assessment of its content. My take, however, is that it is extremely comprehensive. The book is available from the author via his website: http://www.insideracingtechnology.com/, and from SAE: https://www.sae.org/publications/books/content/r-351/.

Here is the author's section on "Why Wider Tires are Better", a subset of the section entitled "Wide Tires Are Better":

[albertw]

This has been a most informative thread. I've learned more about tire traction here than I had in 20 years of reading other car forums.

To me, the evidence is overwhelming that over the small range of tire widths that can be used on our cars, going to wider tires improves cornering and braking traction. The clearest examples are (i) how a square setup with either 225s front and rear or 245s front and rear eliminates the understeer that you get with a staggered setup of 225 front, 245 rear - that pretty well eliminates every other variable, and (ii) better braking with wider tires on the front.

I had assumed that contact patch area was the main issue. (The balloon analogy applied to tires always struck me as silly. At one extreme, runflat tires don't even need any air pressure to work adequately, so the idea that the tire structure has no effect on how the tire deforms at the contact patch makes no sense to me.) The evidence presented here - all new to me - that contact patch area increases with tire width reinforces that assumption. The problem with this is that the basic law of friction says that contact patch area should make no difference to traction, so it is necessary to invoke some sort of non-ideal behaviour between the rubber and the and the road.

The idea that contact patch shape can be important is brand new to me. (Thanks dradernh!) It makes sense to me if you assume the underlying hypothesis is correct that tread deformation causes the centre of the tread (front to rear) to slide first.

I find this idea plausible because I spent six months intermittently reading and thinking about an engineering textbook on motorcycle dynamics before accepting that tread deformation caused by the "cone effect" is one of the two mechanisms that generate the turning force. (The cone effect itself causes a steering torque that helps to turn the wheel into to corner but does not generate turning force itself, contrary to what you probably have read in motorcycle publications. Tread deformation caused by the wheel being turned into the corner is the other turning force, so it comes down to tread deformation created by two completely different mechanisms.)

Now I think I need to get of copy of dradernh's reference book.

[msej449]

As briefly mentioned, don't forget about aquaplaning (hydroplaning) - Most of these discussions assume a dry surface. Yes, the 'wider-is-better' principle will be true for a warm, dry surface, but once it rains heavily or you hit standing water there's a price to pay for the wider tyre: its greater tendency to aquaplane.

This is something of a zero sum game, especially for a performance road car. And even if you go for narrower winters principally to address this issue in wet winter weather (as I do), it's not as if you're going to escape any rain whatsoever in the summer (well, not in England, anyway). So you have to choose a summer tyre understanding that any extra width will buy you traction in the dry but also rob traction from under you in the wet.

Unless you are fortunate enough to live in a region guaranteeing no summer rain, then most owners have to decide on a compromise between

the widest tyre (best dry grip but more prone to aquaplane)
and at the other extreme
the narrowest tyre (most resistant to aquaplaning but worse dry grip)

for their vehicle (all other things being equal). This is quite difficult to do because there's usually very little empirical data to go on. You may understand the principles but just how do you choose between the various widths/diameters available? Moreover, once you start looking at wet-weather performance, you'll start to see some big differences between brands even of the same specification.

[dradernh]

Originally Posted by msej449

As briefly mentioned, don't forget about aquaplaning (hydroplaning) - Most of these discussions assume a dry surface. Yes, the 'wider-is-better' principle will be true for a warm, dry surface, but once it rains heavily or you hit standing water there's a price to pay for the wider tyre: its greater tendency to aquaplane.

This is something of a zero sum game, especially for a performance road car. And even if you go for narrower winters principally to address this issue in wet winter weather (as I do), it's not as if you're going to escape any rain whatsoever in the summer (well, not in England, anyway). So you have to choose a summer tyre understanding that any extra width will buy you traction in the dry but also rob traction from under you in the wet.

Unless you are fortunate enough to live in a region guaranteeing no summer rain, then most owners have to decide on a compromise between

the widest tyre (best dry grip but more prone to aquaplane)
and at the other extreme
the narrowest tyre (most resistant to aquaplaning but worse dry grip)

for their vehicle (all other things being equal). This is quite difficult to do because there's usually very little empirical data to go on. You may understand the principles but just how do you choose between the various widths/diameters available? Moreover, once you start looking at wet-weather performance, you'll start to see some big differences between brands even of the same specification.

This can be ameliorated by over-inflating the tire. Drivers running Hoosier Wets at the track choose both a narrower tread width and increased inflation pressure. I chose to run with +4 psi when there was standing water on the track.

For a street car, tire pressure can be quickly and easily adjusted with something like this Dewalt cordless air inflator: https://www.dewalt.com/products/stor...lator/dcc020ib.
I don't think it will turn a 245 into a 225, but it might turn it into a 237.5. You'd have to test to be certain of the result being produced.

I think slowing down is still the best approach when hydroplaning is a risk.

[msej449]

What I took from the video was that the two wider tyres only gave him a +2% faster lap in the dry each, but in the wet it gave him a 16% slower lap from narrowest to widest, thanks to having to throttle it back because of hydroplaning. So it's worse than a zero-sum: every increment you widen the tyre and get a unit of extra grip, you lose at least 4x that much in wet grip (if I read the test results right).

I'm not saying it's wrong to widen the tyres, but I am saying it's important to understand what it costs you, as well as what you gain.

Where I live, there are many summer days when the weather is uncertain and I can easily depart home in the dry but find myself in the wet later in the day. And vice-versa. Long stretches of warm and dry days are less common than 'changeable' days that may mix the conditions. So I don't really have the option to inflate/deflate to match.

And of course, yes, if it's raining hard I try and adjust my driving. But on unfamiliar roads or busy but fast major routes, there's always the unexpected. And in that event, in heavy rain, I may be glad that I opted to trade a bit of warm traction for better resistance to aquaplaning.