High grip or lower grip tyres for road use?

High grip or lower grip tyres for road use?

Author
Discussion

ddom

6,657 posts

47 months

Sunday 5th July 2020
quotequote all
Depends on the car, depends on the conditions.

I'd rather know something will significant shove will have a good and consistent level of grip. If it's soaking wet, I would choose a tyre for that, not a summer track day tyre.

Why would you want a tyre to give you less grip, as fun as it may be it'll wear off. However, winter tyres on a Caterham are hilarious in the summer.

DonkeyApple

54,923 posts

168 months

Sunday 5th July 2020
quotequote all
300bhp/ton said:
Isn't this true of all traffic at all times though? I drive various vehicles on the roads, some stop very well. Some much less so. And that's in the dry.
Yup. It’s exactly the same for exactly similar scenarios. Here’s the point: you drive a range of vehicles so you know that most modern drivers will maintain the same distance behind you regardless of what you are driving. Regardless of whether it stops like a modern car or like an oil tanker. Gone is the natural understanding of most drivers as to what they need to drop back a bit from or potentially expect to behave slightly anomalously.

camel_landy

4,863 posts

182 months

Monday 13th July 2020
quotequote all
ATM said:
I have been de-tyre-ing all of my cars in the last few years. Best example I can give here is the 996. I bought it with 18 wheels. I then switched to 17 inch. The diameter makes a difference but for anyone reading who is not a Porsche expert - very few people hopefully - the biggest difference is the tyre widths. The fronts on 18 inch are the good old 225 40 18 which is one of the most common tyres sold. The 17 are 205 50 17. It makes such a difference to the feel and feedback through the steering wheel going to a slimmer and lighter wheel and tyre combo. You might look at that little 205 tyre and think no way it can cope with the power and weight of car but it does just fine. The car still has way more grip than I have balls or my skill set will allow and I only drive it on roads.

Now it is difficult wrapping your head around going for the less grippy option because we always believe that more is more but I am starting to believe that less is more. Cheesy I know.

I also had an unfortunate aquaplaning moment recently which has reinforced my belief that slimmer tyres are better. No one tyre is perfect for all conditions so we have to compromise.

My current thought process is not to buy average tyres deliberately but instead to go down a size or 2 and still buy good tyres. Hopefully then I will get good grip and feel and progressive break away but also minimise my chances of more aquaplaning.

^^^ Some very valid points here.

Wider tyre does not = more grip!!

If you're sticking more rubber on the ground, you're going to need more downforce to make best use of it. Without that extra weight/downforce, it's useless. If anything, it'll make driving in the wet worse as you've got a greater risk of aquaplaning.

Tyre choice is ALWAYS a compromise.

I'm another fan of sticking to manufacturer spec of sizes and to not go for the 'Bling' options. The smaller rims mean you get larger side walls and a more comfortable ride. The drop in width also means you're going to benefit when the weather turns...

M

300bhp/ton

Original Poster:

41,030 posts

189 months

Monday 13th July 2020
quotequote all
camel_landy said:
I'm another fan of sticking to manufacturer spec of sizes and to not go for the 'Bling' options. The smaller rims mean you get larger side walls and a more comfortable ride. The drop in width also means you're going to benefit when the weather turns...

M
I think it might depend on the car in question. My TR7 would have had 175 or maybe 165's from new on 13" rims. Not sure I'd really want something that narrow with a 200hp V8 under your right foot. Not to mention 13" rims wouldn't clear the brakes now anyway.

camel_landy

4,863 posts

182 months

Monday 13th July 2020
quotequote all
300bhp/ton said:
camel_landy said:
I'm another fan of sticking to manufacturer spec of sizes and to not go for the 'Bling' options. The smaller rims mean you get larger side walls and a more comfortable ride. The drop in width also means you're going to benefit when the weather turns...

M
I think it might depend on the car in question. My TR7 would have had 175 or maybe 165's from new on 13" rims. Not sure I'd really want something that narrow with a 200hp V8 under your right foot. Not to mention 13" rims wouldn't clear the brakes now anyway.
<sigh> Yes... You're going to have to chose a suitable rim to fit. rolleyes

...but don't forget the TR8 was a production car, so so you probably want to be using the TR8 rather than the TR7 specs.

The point I made earlier is that without any extra weight, you're not going to be able to make use of any extra rubber. You should hopefully get better traction from the modern tyre compounds but you're not going to gain much from going wider. (OK, there's a bit more to it than that but it's a good starting point.)

M

Edited by camel_landy on Monday 13th July 13:03

camel_landy

4,863 posts

182 months

Monday 13th July 2020
quotequote all
Here's a useful explanation from a Pistonheads thread a while back:

carbibles said:
If there's one question guaranteed to promote argument and counter argument, it's this : do wide tyres give me better grip?
Fat tyres look good. In fact they look stonkingly good. In the dry they are mercilessly full of grip. In the wet, you might want to make sure your insurance is paid up, especially if you're in a rear-wheel-drive car. Contrary to what you might think (and to what I used to think), bigger contact patch does not necessarily mean increased grip. Better yet, fatter tyres do not mean bigger contact patch. Confused? Check it out:

Pressure=weight/area.

That's about as simple a physics equation as you can get. For the general case of most car tyres travelling on a road, it works pretty well. Let me explain. Let's say you've got some regular tyres, as supplied with your car. They're inflated to 30psi and your car weighs 1500Kg. Roughly speaking, each tyre is taking about a quarter of your car's weight - in this case 375Kg. In metric, 30psi is about 2.11Kg/cm².
By that formula, the area of your contact patch is going to be roughly 375 / 2.11 = 177.7cm² (weight divided by pressure)
Let's say your standard tyres are 185/65R14 - a good middle-ground, factory-fit tyre. That means the tread width is 18.5cm side to side. So your contact patch with all these variables is going to be about 177.7cm² / 18.5, which is 9.8cm. Your contact patch is a rectangle 18.5cm across the width of the tyre by 9.8cm front-to-back where it sits 'flat' on the road.
Still with me? Great. You've taken your car to the tyre dealer and with the help of my tyre calculator, figured out that you can get some swanky 225/50R15 tyres. You polish up the 15inch rims, get the tyres fitted and drive off. Let's look at the equation again. The weight of your car bearing down on the wheels hasn't changed. The PSI in the tyres is going to be about the same. If those two variables haven't changed, then your contact patch is still going to be the same : 177.7cm²
However you now have wider tyres - the tread width is now 22.5cm instead of 18.5cm. The same contact patch but with wider tyres means a narrower contact area front-to-back. In this example, it becomes 177.7cm² / 22.5, which is 7.8cm.

Imagine driving on to a glass road and looking up underneath your tyres. This is the example contact patch (in red) for the situation I explained above. The narrower tyre has a longer, thinner contact patch. The fatter tyre has a shorter, wider contact patch, but the area is the same on both.

And there is your 'eureka' moment. Overall, the area of your contact patch has remained more or less the same. But by putting wider tyres on, the shape of the contact patch has changed. Actually, the contact patch is really a squashed oval rather than a rectangle, but for the sake of simplicity on this site, I've illustrated it as a rectangle - it makes the concept a little easier to understand. So has the penny dropped? I'll assume it has. So now you understand that it makes no difference to the contact patch, this leads us on nicely to the sticky topic of grip.

The area of the contact patch does not affect the actual grip of the tyre. The things that do affect grip are the coefficient of friction of the rubber compound and the load on the tyre. As far as friction is concerned, the formula is relatively simple - F=uN, where F is the frictional force, N is the Normal force for the surfaces being pressed together and u is the coefficient of friction. In the case of a tyre, the Normal force basically stays the same - mass of the car multiplied by gravity. The coefficient of friction also remains unchanged because it's dependent on the two surfaces - in this case the road and the tyre's rubber.
The coefficient of friction is in part determined by the rubber compound's ability to 'key' with the road surface at a microscopic level.

This explains why you can slide in a corner if you change road surface - for example going from a rough road to a smooth road, or a road surface covered in rain and diesel (a motorcyclist's pet peeve). The slide happens because the coefficient of friction has changed.

So do wider tyres give better grip?
If the contact patch remains the same size and the coefficient of friction and frictional force remain the same, then surely there is no difference in performance between narrow and wide tyres? Well there is but it has a lot to do with heat transfer. With a narrow tyre, the contact patch takes up more of the circumference of the tyre so for any given rotation, the sidewall has to compress more to get the contact patch on to the road. Deforming the tyre creates heat. With a longer contact patch and more sidewall deformation, the tyre spends proportionately less time cooling off than a wider tyre which has a shorter contact patch and less sidewall deformation. Why does this matter? Well because the narrower tyre has less capacity for cooling off, it needs to be made of a harder rubber compound in order to better resist heating in the first place. The harder compound has less mechanical keying and a lower coefficient of friction. The wider tyres are typically made of softer compounds with greater mechanical keying and a higher coefficient of friction. And voila - wider tyres = better grip. But not for the reasons we all thought.

What about lateral force in cornering?
In terms of the lateral force applied to a tyre during cornering, you eventually come to a point where slip angle becomes important. The plot below shows an example of normalised lateral force (in Kg) versus slip angle (in degrees). Slip angle is best described as the difference between the angle of the tyres that you've set by steering, and the direction in which the tyres actually want to travel. As you corner the lateral force increases on your tyres, and at some point, the lateral force is going to overcome the mechanical grip of the tyres and that point is defined by the peak slip angle, as shown in the graph. ie. there comes a point at which no matter how much vertical load is applied to the tyre (from the vehicle weight), it's going to be overcome by the lateral force and 'break away' and slip. So why do wider tyres perform better when cornering? Well apart from the softer rubber compound giving better mechanical keying and a higher coefficient of friction, they have lower profile sidewalls. This makes them more resistant to deforming under lateral load, resulting in a more predictable and stable contact patch. In other words, you can get to a higher lateral load before reaching the peak slip angle.
M

anonymous-user

53 months

Monday 13th July 2020
quotequote all
camel_landy said:
do wide tyres give me better grip?

In the dry they are mercilessly full of grip. In the wet, you might want to make sure your insurance is paid up, especially if you're in a rear-wheel-drive car.
Yup, I want "wet grip" and decent tyres cost real money.

300bhp/ton

Original Poster:

41,030 posts

189 months

Monday 13th July 2020
quotequote all
camel_landy said:
<sigh> Yes... You're going to have to chose a suitable rim to fit. rolleyes

...but don't forget the TR8 was a production car, so so you probably want to be using the TR8 rather than the TR7 specs.

The point I made earlier is that without any extra weight, you're not going to be able to make use of any extra rubber. You should hopefully get better traction from the modern tyre compounds but you're not going to gain much from going wider. (OK, there's a bit more to it than that but it's a good starting point.)

M

Edited by camel_landy on Monday 13th July 13:03
TR8 also has 13" rims and low CR 137hp motor. Think they were 185's.... hardly a real improvement in traction. And also very different gearing to what I'm running.

The works rally cars used much wider tyres however, I bet they gripped a lot better than a standard car on standard sized tyres did. But who knows, maybe they should have come to you for advice wink

300bhp/ton

Original Poster:

41,030 posts

189 months

Monday 13th July 2020
quotequote all
camel_landy said:
Here's a useful explanation from a Pistonheads thread a while back:

carbibles said:
If there's one question guaranteed to promote argument and counter argument, it's this : do wide tyres give me better grip?
Fat tyres look good. In fact they look stonkingly good. In the dry they are mercilessly full of grip. In the wet, you might want to make sure your insurance is paid up, especially if you're in a rear-wheel-drive car. Contrary to what you might think (and to what I used to think), bigger contact patch does not necessarily mean increased grip. Better yet, fatter tyres do not mean bigger contact patch. Confused? Check it out:

Pressure=weight/area.

That's about as simple a physics equation as you can get. For the general case of most car tyres travelling on a road, it works pretty well. Let me explain. Let's say you've got some regular tyres, as supplied with your car. They're inflated to 30psi and your car weighs 1500Kg. Roughly speaking, each tyre is taking about a quarter of your car's weight - in this case 375Kg. In metric, 30psi is about 2.11Kg/cm².
By that formula, the area of your contact patch is going to be roughly 375 / 2.11 = 177.7cm² (weight divided by pressure)
Let's say your standard tyres are 185/65R14 - a good middle-ground, factory-fit tyre. That means the tread width is 18.5cm side to side. So your contact patch with all these variables is going to be about 177.7cm² / 18.5, which is 9.8cm. Your contact patch is a rectangle 18.5cm across the width of the tyre by 9.8cm front-to-back where it sits 'flat' on the road.
Still with me? Great. You've taken your car to the tyre dealer and with the help of my tyre calculator, figured out that you can get some swanky 225/50R15 tyres. You polish up the 15inch rims, get the tyres fitted and drive off. Let's look at the equation again. The weight of your car bearing down on the wheels hasn't changed. The PSI in the tyres is going to be about the same. If those two variables haven't changed, then your contact patch is still going to be the same : 177.7cm²
However you now have wider tyres - the tread width is now 22.5cm instead of 18.5cm. The same contact patch but with wider tyres means a narrower contact area front-to-back. In this example, it becomes 177.7cm² / 22.5, which is 7.8cm.

Imagine driving on to a glass road and looking up underneath your tyres. This is the example contact patch (in red) for the situation I explained above. The narrower tyre has a longer, thinner contact patch. The fatter tyre has a shorter, wider contact patch, but the area is the same on both.

And there is your 'eureka' moment. Overall, the area of your contact patch has remained more or less the same. But by putting wider tyres on, the shape of the contact patch has changed. Actually, the contact patch is really a squashed oval rather than a rectangle, but for the sake of simplicity on this site, I've illustrated it as a rectangle - it makes the concept a little easier to understand. So has the penny dropped? I'll assume it has. So now you understand that it makes no difference to the contact patch, this leads us on nicely to the sticky topic of grip.

The area of the contact patch does not affect the actual grip of the tyre. The things that do affect grip are the coefficient of friction of the rubber compound and the load on the tyre. As far as friction is concerned, the formula is relatively simple - F=uN, where F is the frictional force, N is the Normal force for the surfaces being pressed together and u is the coefficient of friction. In the case of a tyre, the Normal force basically stays the same - mass of the car multiplied by gravity. The coefficient of friction also remains unchanged because it's dependent on the two surfaces - in this case the road and the tyre's rubber.
The coefficient of friction is in part determined by the rubber compound's ability to 'key' with the road surface at a microscopic level.

This explains why you can slide in a corner if you change road surface - for example going from a rough road to a smooth road, or a road surface covered in rain and diesel (a motorcyclist's pet peeve). The slide happens because the coefficient of friction has changed.

So do wider tyres give better grip?
If the contact patch remains the same size and the coefficient of friction and frictional force remain the same, then surely there is no difference in performance between narrow and wide tyres? Well there is but it has a lot to do with heat transfer. With a narrow tyre, the contact patch takes up more of the circumference of the tyre so for any given rotation, the sidewall has to compress more to get the contact patch on to the road. Deforming the tyre creates heat. With a longer contact patch and more sidewall deformation, the tyre spends proportionately less time cooling off than a wider tyre which has a shorter contact patch and less sidewall deformation. Why does this matter? Well because the narrower tyre has less capacity for cooling off, it needs to be made of a harder rubber compound in order to better resist heating in the first place. The harder compound has less mechanical keying and a lower coefficient of friction. The wider tyres are typically made of softer compounds with greater mechanical keying and a higher coefficient of friction. And voila - wider tyres = better grip. But not for the reasons we all thought.

What about lateral force in cornering?
In terms of the lateral force applied to a tyre during cornering, you eventually come to a point where slip angle becomes important. The plot below shows an example of normalised lateral force (in Kg) versus slip angle (in degrees). Slip angle is best described as the difference between the angle of the tyres that you've set by steering, and the direction in which the tyres actually want to travel. As you corner the lateral force increases on your tyres, and at some point, the lateral force is going to overcome the mechanical grip of the tyres and that point is defined by the peak slip angle, as shown in the graph. ie. there comes a point at which no matter how much vertical load is applied to the tyre (from the vehicle weight), it's going to be overcome by the lateral force and 'break away' and slip. So why do wider tyres perform better when cornering? Well apart from the softer rubber compound giving better mechanical keying and a higher coefficient of friction, they have lower profile sidewalls. This makes them more resistant to deforming under lateral load, resulting in a more predictable and stable contact patch. In other words, you can get to a higher lateral load before reaching the peak slip angle.
M
I'll admit I'm not a mathematician.

A bit of Googling has resulted in several places same the same sort of thing:


That while tyre pressures and tyre widths do indeed alter the contact patch shape. They also alter its size. Unless I'm reading this incorrectly, it is saying the wider the tyre, the bigger the contact patch. Maybe at higher pressures it would change or be less so?

camel_landy

4,863 posts

182 months

Monday 13th July 2020
quotequote all
300bhp/ton said:
But who knows, maybe they should have come to you for advice wink
rolleyes

...conveniently forgetting they choose different tyre/wheel combinations, depending on the rally.

...and conveniently forgetting you are driving on the road and not a rally stage.

M

camel_landy

4,863 posts

182 months

Monday 13th July 2020
quotequote all
300bhp/ton said:
A bit of Googling has resulted in several places same the same sort of thing:
^^^ That's for a bike tyre...

M

300bhp/ton

Original Poster:

41,030 posts

189 months

Monday 13th July 2020
quotequote all
camel_landy said:
300bhp/ton said:
A bit of Googling has resulted in several places same the same sort of thing:
^^^ That's for a bike tyre...

M
Does the physics work differently with bike tyres?

300bhp/ton

Original Poster:

41,030 posts

189 months

Monday 13th July 2020
quotequote all
camel_landy said:
rolleyes

...conveniently forgetting they choose different tyre/wheel combinations, depending on the rally.

...and conveniently forgetting you are driving on the road and not a rally stage.

M
Some of the events described in my op are a bit like tarmac stage rallies wink

And yes, they did run different tyres for different surfaces. I don't recall them running narrow 185's on any tarmac rallies however.

camel_landy

4,863 posts

182 months

Monday 13th July 2020
quotequote all
300bhp/ton said:
I don't recall them running narrow 185's on any tarmac rallies however.
Maybe not but I suggest you read and inwardly digest my post from earlier. I suspect they would also be generating a lot more heat in their tyres...

M

camel_landy

4,863 posts

182 months

Monday 13th July 2020
quotequote all
300bhp/ton said:
Does the physics work differently with bike tyres?
When you've finished trying to compare apples with bananas, give me a shout.

M

GravelBen

15,655 posts

229 months

Tuesday 14th July 2020
quotequote all
camel_landy said:
300bhp/ton said:
A bit of Googling has resulted in several places same the same sort of thing:
^^^ That's for a bike tyre...
A fatbike in particular by the look of it, and appears to be more about wider rims allowing you to run the same width tyre at lower pressures for a larger contact patch (because the soft tyre and low pressure lets it distort).

Quite different proportions, construction and loads to car tyres.


Honeywell

1,368 posts

97 months

Tuesday 14th July 2020
quotequote all
I took the HP Primacy eco tyres off my tweaked GT86 and put same size Michelin PS4s on.

Whilst I loved the Primacy low grip in the dry they had two drawbacks. I was triggering the ABS sometimes in the dry when pressing on and that suggested to me that I often had little 'emergency braking' margin. Then I had two occasions on cornering on dry roads where I encountered water - a leaking water pipe and then another time just a blocked drain. The sudden loss of grip on both axles was a bum clenching experience.

The PS4's are less playful at low speed which is sad. But I drive with much more confidence at speed. Never ever trigger the ABS in the dry now and the dry2wet unexpected transitions are much less scary.

Just today on a sweeping B road at about 70 in third in the dry full throttle the back end started gentley drifting out and I held it with no drama and about 20 degrees of opposite lock. Lovely. Really progressive and communicative.

Stops in the Wet extremely well. Only cost £80 on 17 inch rims. Beautiful tyre that won't scare you in the wet or kill you.

On my other cars (L322 RR and BMW 535d) I run a proper off-road tyre - General Grabber AT - and Michelin CrossClimates because they suit the profile of use. I think prioritising dry grip on a car tyre is wrong in most cases unless you are running out of dry braking performance regularly.

Tyres are the most important aspect of a cars performance.

cerb4.5lee

30,189 posts

179 months

Tuesday 14th July 2020
quotequote all
Honeywell said:
Tyres are the most important aspect of a cars performance.
Tyres are important I agree, but I find that having really good brakes are more important. I never had any confidence under braking in my E92 M3 and I don't have any either in my Mini Cooper S.

If I swapped the Primacy to PS4 on the Mini I think that it would give me more overall grip/less wheelspin, but I'd still be left with the problem of poor brakes though sadly.

It is surprising how many cars I've actually had with decent brakes, and off the top of my head I'd only say that 3 cars that I've had-had decent stoppers as standard(Cerbera/TTS/370Z were/are the only ones that I rated as having decent brakes).

I need a Porsche I reckon, and their brakes come really highly regarded from what I read.

ATM

18,094 posts

218 months

Tuesday 14th July 2020
quotequote all
cerb4.5lee said:
Honeywell said:
Tyres are the most important aspect of a cars performance.
Tyres are important I agree, but I find that having really good brakes are more important. I never had any confidence under braking in my E92 M3 and I don't have any either in my Mini Cooper S.

If I swapped the Primacy to PS4 on the Mini I think that it would give me more overall grip/less wheelspin, but I'd still be left with the problem of poor brakes though sadly.

It is surprising how many cars I've actually had with decent brakes, and off the top of my head I'd only say that 3 cars that I've had-had decent stoppers as standard(Cerbera/TTS/370Z were/are the only ones that I rated as having decent brakes).

I need a Porsche I reckon, and their brakes come really highly regarded from what I read.
Depends on the Porsche. You might be more interested in the feel from the pedal than the performance of the system or its ability to actually stop the car.

Honeywell

1,368 posts

97 months

Tuesday 14th July 2020
quotequote all
I'm often impressed by the GT86 brakes. Once warm they have amazing stopping power I find (on PS4 rubber). The size and specification of them is rather hum drum but some combination of the low weight and ultra low centre of gravity makes them feel tremendous. More so than any other cars I've owned of considerably higher performance.

Plus a conventional handbrake lever means you can handbrake turn in the car park when nobody's about and you can't do that in in any new Ferrari!