[Santoshbhat]

Just read up an interesting article on Car & Driver about the effects of upsizing tyres. THe article gives a good perspective on what effect an upsized tyre has, given that the tyre compound and other variables are constant.

They ran a VW Golf on various sizes of tyres ranging for stock to about +4 upsize, keeping the overall wheel diameter the same. The tyres used were also identical, just the size and profiles varied. The results are quite interesting

- Wider tyres suffered poorer fuel economy
- Acceleration was poorer with wider tyres
- Braking was marginally better
- Lateral grip improved upto + 2 upsize, beyond that it diminished
- Bigger rim and low profile tyre combo adds unsprung weight as metal is heavier than rubber.

Read the article here http://www.caranddriver.com/features...d-tires-tested

[Harbir]

AS I was saying earlier, seemingly to no effect at all:

http://www.roadandtrack.com/motorhea...t-tire-patches

Originally Posted by Santoshbhat

Just read up an interesting article on Car & Driver about the effects of upsizing tyres. THe article gives a good perspective on what effect an upsized tyre has, given that the tyre compound and other variables are constant.

They ran a VW Golf on various sizes of tyres ranging for stock to about +4 upsize, keeping the overall wheel diameter the same. The tyres used were also identical, just the size and profiles varied. The results are quite interesting

- Wider tyres suffered poorer fuel economy
- Acceleration was poorer with wider tyres
- Braking was marginally better
- Lateral grip improved upto + 2 upsize, beyond that it diminished
- Bigger rim and low profile tyre combo adds unsprung weight as metal is heavier than rubber.

Read the article here http://www.caranddriver.com/features...d-tires-tested

The 18 and 19" are not comparable to the 15,16, and 17, because its a different compound. THe jump in grip from 17 to 18 is too large and then negligible again from 18 to 19 (which are the same compound), for increase to be due to size rather than compound.

Between 15, 16 and 17, on identical compound, virtually negligible changes in performance occur, within the boundaries of test error (if you conduct a test 10 times, you will not get 10 identical results. you will get varying results with a mean and a median. These results are close enough to fall within the variation from test to test that is liable to occur).

The car and driver test shows conclusively that if the compound is not changed, wider, lower profile tyres make negligible difference to performance.

My own experience has been to seek the smallest, lightest wheels that will fit over the brakes, mated to the lightest tires I can find in the appropriate compound, with reduced diameter. The increase in agility and responsiveness from ultra low unsprung weight and ultra low rotational inertia, are hugely more desirable to me. The reduced diameter also drops the gearing, which further increases throttle response and reaction, over and above the loss of inertia. Sure it looks really dorky, but I couldn't give a flying flip about how the car looks, if its feeling just right from behind the wheel.

[FlatOut]

Originally Posted by SS-Traveller

The question occurred to me some days ago, but I have been unable to convince myself either way, or find any relevant answers from books or the internet.

My question is:
With respect to temperature build-up in a tyre when travelling long distances at high speeds, is it the narrower tyre that will heat up more, or the wider tyre? Or is it that there can be an optimum width for the given weight (and engine power) of a vehicle, and anything narrower or wider generates more heat than the optimum?

Arguments for the narrower tyre heating up more:
At high speeds, especially when running over minor bumps and imperfections on the road, the narrower tyre is likely to lose some grip and 'skid' on the road surface while rotating. Also, the narrower tyre suffers heavier loading per unit area when braking or accelerating. Therefore, there is more heat buildup.

Arguments for the wider tyre heating up more:
Primarily, a wider tyre has more rolling resistance - more the resistance, more the heat generated. Can we say... More grip = more heat?

Look forward to your opinions.

Apologies for the delay in replying - hadn't seen your post, SS-Traveller.

A narrower tyre will heat up more if the sidewall is correspondingly taller (to keep the rolling circumference the same) since the tyre will be distorting slightly more where the tread contacts the road. This is one reason lower-profile tyres are fitted to faster cars. To cope with the extra heat generated in a taller tyre, the compound is slightly harder.

The wider tyre with a lower profile will distort less so generate less heat. There is less resistance to roll within the tyre itself, but more resistance through the air with the larger frontal area.

You are again assuming there is more grip with a wider tyre which is not necessarily the case (for all the reasons I postulated in my first post). The tyre will feel to turn into a corner more eagerly since there is less contact patch distorted with slip angles, but it will not necessarily corner harder - this depends on many factors, some of which I touched on in my post earlier. If it does generate more cornering grip, the the break-away will be more sudden and with less warning.

Your suggestion that more heat=more grip is correct, but manufacturers make the tread of lower profile wider tyres from a slightly different compound to compenate.

The downsides of tyres which are wider than necessary, with a lower-profile than necessary are multiple. There will be more drag through the air, more shocks through the suspension, more tramlining (adverse steering effects), more tendency to lose grip in the wet, less stability, less warning of breakaway and less ability to put down power in less-than-perfect conditions. All for an eagerness to turn in to corners. As with all things, there is a balance in these matters.



This is such a complicated area of discussion, requiring a good understanding of tyre behaviour, suspension layout, suspension behaviour, wheel geometries, kinematics and driving towards tyres' and vehicles' limits. I was led to considering the common misconception of 'a wider tyre means more grip because there is more rubber on the road' when I outpaced a modern car (perhaps 180bhp) with wide tyres (probably 215-55-16) in a car with narrow, high-profile tyres (165-80-400) and only 65bhp. The road was winding with many small and a few larger bumps (including drain grids) and changing camber - like many.

One of the most interesting results of my consideration is that a car with wider, lower profile tyres should corner more easily to nearer their limits and so should be capable of driving more quickly on a winding road. But it is their lack of steering feedback (resulting from the much smaller part of the contact patch with slip) as to how close these limits are which often seems to prevent this sort of really quick driving on real roads. Once you introduce surface changes, ripples, bumps, camber changes and other normalities, the reason for my ability to go so fast on what most would consider to be the very opposite of a 'performance tyre' becomes clearer.

I think these massive misconceptions are born from watching motor racing on a (relatively) smooth, flat track where racing cars have quite wide tyres to prevent the heat build up and give a twitchy-handling machine. If we were all to watch racing on normal roads and tracks, then we wouldn't necessarily have these odd ideas about tyre width.

[saket77]

My question will be a little off-topic from the current discussion, but since its a relatively smaller query and related to the wheels, I felt that creating a new thread is not necessary.

I would like to ask the wheel gurus that what are the advantages and disadvantages of smaller (say upto 12-13 inch) wheels over bigger ones (mostly 14 & above) and vice-versa?
I have read that smaller wheels generally mean a smoother ride, but would like to know the physics behind. I am aware that taller sidewalls would mean more cushioning hence better comfort )at the cost of handling, but would like to know about the differences due to circumference of different sizes.

Thanks,
Saket

[amit_purohit20]

Originally Posted by Rehaan

This can be put down to several other factors like suspension tune, kerb weight, steering responsiveness, etc.
.....
cya
R

Originally Posted by Harbir

This is a very imporant point and points to why .....design and set up or differences in tyre design and construction.

Keeping aside the suspension things and the increase in overall tyre diameter some points:

1) Wider tyre does not necessarily mean more grip.
Frictional force = Coefficient of friction X Weight. Nowhere area is mentioned in the formula.
Amontons law of friction states that asperities (hacksaw shaped teeth profile found on the microscopic surfaces) get engaged with mating asperities and their shearing during sliding creates friction. So more the area more the friction. But experiments have found out that its not true because increased area also reduces the contact pressure between the two bodies. Also the friction caused by such asperities shearing is negligible for all practical purposes. Same is for rubber adhesion. The more the contact pressure more the asperities dig deep in the mating asperities but still it doesnot contribute significantly to the available friction.

So ways available for increasing friction-
a) Increase the Mu (Coefficient of friction) by opting for a softer compound. In real world the Mu is also not linear for rubber and it varies based on the contact pressure.
b) Increase the weight. Adding weight increases the frictional force available but it also increases the momentum of the vehicle inturn demanding more traction from tyres to brake. So what amount of weight is good?

2) Wider tyres allows the manufacturer to have a more softer rubber compound because of more rubber available for wear and still maintain useful life of tyre.

3) In Indian context it does not necessarily mean that a wider tyre will have a softer compound. It all depends on manufacturers wish. But its experienced by one of the team-bhpian that for motorcycle tyre MRF Zapper C 120/80 x 17 performed better than 100/80 x 17.

4) Tyres are also made wide for their own construction purpose. For eg more load carrying ability, more no. of plies etc.

5) Wider tyres should run cooler because of increased surface area to dissipiate heat and should not get affected easily by localized heat generation.

6) For snow and wet grip narrow tyres are preferred over wide ones.

7) If area doesnot come into picture why are tyre manufacturers so bothered about the contact patch of the tyre?

8) Wide off-road tyres should help increase traction because they are shearing more soft/loose soil between their treads. For eg. Tractor tyres.
Also tyres are made wide in agricultural tractors to avoid soil compaction which inhibits seeds germination.

9) What is better a longer narrow contact patch as in narrow high profile tyres or a shorter but wider contact patch as in with low profile tyres.( Considering contact patch area remains same either way)?

10) I still do not get how the P=F/A is applied in explaining that a wide tyre and a narrow tyre both have equal contact patch. Because in that equation only the F is constant rest other things can vary depending on the tyre width. If you calculate area by using A=F/P and consider pressure remaining constant its wrong because contact pressure can change depending on the F (which is constant here) or Area which is dependent on the actual contact patch shape determined by the construction and geometric dimensions of the tyre.
So I still do not get fully how a wider tyre has always the contact patch area equal to narrow tyre?

[Harbir]

Originally Posted by amit_purohit20

10) I still do not get how the P=F/A is applied in explaining that a wide tyre and a narrow tyre both have equal contact patch. Because in that equation only the F is constant rest other things can vary depending on the tyre width. If you calculate area by using A=F/P and consider pressure remaining constant its wrong because contact pressure can change depending on the F (which is constant here) or Area which is dependent on the actual contact patch shape determined by the construction and geometric dimensions of the tyre.
So I still do not get fully how a wider tyre has always the contact patch area equal to narrow tyre?

First, please note that A=F/P does not apply strictly because other factors such as mechanisms of deformation introduce non-linearity. So there will be some variation. But not enough to matter.

If A=F/P, and pressure remains constant (as it should), then A has to remain constant because F is constant (applied by the weight of the car sitting on that tire).

You remember geometry from school? A tangent to a circle touches the circle only at a point which has no area. THe circle is the tire, the tangent is the road surface. In order to get an area to the contact point, the circle has to deform from the perfectly circular shape, which it does due to the vehicle's weight (F). THe greater the weight, the more the tire flattens out, increasing the area. If you increase the width of the tire, it will flatten out less. Due to the width, less deformation is required to achieve the same area. and the area has to remain the same by A =F/P.

You can decrease the pressure to cause the tire to flatten out more, but then you've regressed because the tire will have to deform more than the narrower tire (a greater amount of rubber has to be deformed to form the larger contact patch) and that increases power loss, causes the tire to run hotter, makes the sidewalls less stable, etc etc.

[venkyhere]

Originally Posted by amit_purohit20

10) I still do not get how the P=F/A is applied in explaining that a wide tyre and a narrow tyre both have equal contact patch. Because in that equation only the F is constant rest other things can vary depending on the tyre width. If you calculate area by using A=F/P and consider pressure remaining constant its wrong because contact pressure can change depending on the F (which is constant here) or Area which is dependent on the actual contact patch shape determined by the construction and geometric dimensions of the tyre.
So I still do not get fully how a wider tyre has always the contact patch area equal to narrow tyre?

Originally Posted by Harbir

First, please note that A=F/P does not apply strictly because other factors such as mechanisms of deformation introduce non-linearity. So there will be some variation. But not enough to matter.

If A=F/P, and pressure remains constant (as it should), then A has to remain constant because F is constant (applied by the weight of the car sitting on that tire).
.

@amit_purohit20,
'pressure remains constant (as it should)' => this refers to the filled up air pressure in the tyre , say 32psi.
Then, car weight = mg = 32 psi * area of a section of the wheel rim touching the air = 32 psi * area of the tyre patch in contact with the ground (of course, neglecting sidewall bulging)

So if both the thin tyre with higher profile and fat tyre with narrow profile are both filled to 32psi, the area patch on the ground has to remain same. (of course this assumes that the unsprung weight offered by the wheel+tyre remains constant in both cases).

Hope this clears it.

[amit_purohit20]

Originally Posted by Harbir

First, please note that A=F/P does not apply strictly because other factors such as mechanisms of deformation introduce non-linearity. So there will be some variation. But not enough to matter.

If A=F/P, and pressure remains constant (as it should), then A has to remain constant because F is constant (applied by the weight of the car sitting on that tire).
....run hotter, makes the sidewalls less stable, etc etc.

Thanks Harbir, I was already clear upto this point. (Blame it to my writing skills that I was not able to explain the point clearly. I will try now...)

Originally Posted by venkyhere

@amit_purohit20,
'pressure remains constant (as it should)' => this refers to the filled up air pressure in the tyre , say 32psi.
Then, car weight = mg = 32 psi * area of a section of the wheel rim touching the air = 32 psi * area of the tyre patch in contact with the ground (of course, neglecting sidewall bulging)

Venky has made it more clear.
Assuming Tyre Internal Pressure remains constant.

So there are now two cases:
1) Pressure (Tyre pressure which remains constant)= Weight of the car on the tyre/Area
And the area discussed here is area of a section of the wheel rim touching the air.
Agreed upto this point.

2) Pressure (Contact pressure between the ground and the tyre)= Weight of the car on the tyre/Area (Contact Patch Area of the tyre with the ground).

Now in Case 2 please note the Pressure is not the tyre pressure but the contact pressure between the tyre and the ground.


Although Venky did brilliantly equate the both cases saying as Weight of the car is common in both cases so :

Tyre pressure X Area(area of a section of the wheel rim touching the air) = Contact pressure between ground and tyre X Area (Contact patch area between ground and the tyre).

If we can say Tyre Internal pressure= Contact pressure between ground and tyre then I very well understand the whole concept.

But my question still remains how do we say that:
Tyre Internal pressure= Contact pressure between ground and tyre

And are they really equal or are we just assuming that both are equal?

[Harbir]

For this discussion, they are equal. the pressure on the contact patch pushing the tyre to deform has to be equal to the pressure on the inside of the pressure pushing it out. otherwise the tyre would not be in static shape. It would be collapsing or expanding.

This is why when the internal air pressure is reduced, the tyre flattens out. As it flattens out, the weight spreads over a wider area, reducing the pressure between tire and road till stasis between inside and outside pressure is reached.

Note, its not EXACTLY equal due to voids in the tread, larger external vs internal area, and side wall deformation, but when all the forces are taken into consideration, the total force on the outside of the ttire and the inside of the tire is in balance.

[amit_purohit20]

@Harbir Can you throw some light on different compounds used in rubber and how they affect the dry and wet grip.

Is there any way for the common man to judge a tyre before buying the compound of the tyre?

Am I demanding too much

Also while contact patch area is of not much importance then why are even the tyre manufacturers after that?

[Harbir]

Compounds are a very wide subject. Broadly speaking it comes down to the softness of the rubber. Softer rubber conforms to the texture of the surface better, increasing the actual contact area, and therefore increasing friction. However, softer rubber is not as resistant to abrasion. THe greater abrasion due to increased contact, plus reduced resistance to abrasion means that softer compound tires don't last as long.

So it is a balance of tread life vs grip.

There are other factors at play as well. Soft compound tires will not grip well on colder surfaces because the rubber hardens to a level harder than otherwise harder tires, so their grip can fall below hard compound tires, simply because they are more sensitive to temperature. Likewise, hard compound tires driven hard will overheat and start losing chunks of rubber from the tread because the rubber compound is not designed for high temperature use.

These temperature factors are mostly irrelevant in India because we don't have cold temperatures in the vast majority of the country, and we don't have cars fitted with tires that have trouble in below freezing temperatures, and we don't have autobahns and racetracks where high temperature stresses can become a problem.

Dry and wet grip a function of tread design and operation temperatures. rain reduces the tyre's temperature, so the best rain tyres will remain pliant at low operating temperatures. Water is a very effective coolant so a tire running in 20 degree C rain weather can loose more heat than one running in the dry at 5 degrees (illustrative). THen you also need good tread pattern design to evacuate water. THis is a balancing act. wider voids in the tread patter improve water evacuation, but reduce contact patch area and increase flexibility of the tread blocks, which increases their squirm, reducing feel and accuracy.

Also very important for rain are sipes. these are tiny cuts in the tread. Water is sticky. it sticks to itself. These sipes fill with water, which creates an attraction to the wet surface, increasing the stickiness of the tire to the surface. Well siped tyres are amazingly grippy on wet surfaces. High performance tyres will have minimal or no siping at all.

There are a lot of factors at play in tyre behaviour so endless combinations of compounds and tread designs are possible.

Tyres are complex business. More grip is the simplest and least sophisticated way to look at them. THere will be tyres in any category that will be very sticky, but not have good breakaway characteristics at the limit. There will be others that give up grip and steering accuracy for very high levels of predictability and superior rain performance. Others still will balance all year performance, noise, ride comfort, longevity, fuel economy, and grip.

The combinations are endless.

How to select? educate yourself and do lots of research. Ask people who know. use google. etc.

To your last question, contact patch area IS important. The question was whether increasing tyre section width increases contact patch area. it doesn't. if more contact patch area is desired, a larger diameter tire has to be fitted, which is not a straight forward decision.

WHen people do plus sizing, the greatest difference in steering and handling comes from the shorter sidewalls and different compounds, not from the contact path width increase.

[Harbir]

I have been waiting for someone to ask why a larger diameter tire will yield a bigger contact patch. The bigger tire isn't going to squash down more than the smaller tire under the same weight so it should produce the same contact area, right?

wrong. A bigger tyre can be run at a lower pressure. BEcause it is larger, it can support the same weight at the same level of physical stress at a lower pressure. That will cause it to have a larger contact patch without compromising its thermal and mechanical stability and performance. BUt it does come with a cost. the larger patch means higher fuel consumption (because more rubber is transitioning from arc to flat to arc as it cycles past the contact patch), and there is more energy wasted in braking the bigger diameter tire with its greater inertia.

[Chewbacca]

Originally Posted by SS-Traveller

My question is:
With respect to temperature build-up in a tyre when travelling long distances at high speeds, is it the narrower tyre that will heat up more, or the wider tyre?

Based on my experience, tyre width has negligible influence on more/less temperature build-up. This applies to normal passenger cars driven on highways and NOT for cars on the racing circuits.

Points to ponder ....
>> The biggest heat contributor is the "less than recommended" tire pressure. With less pressure, the air molecules have more room to wander and collide with other molecules, thus generating heat. More so with flexing sidewall (bumps) and twisty roads (squirms)

>> With less pressure, more the flex & squirm of the sidewall, thus more collision of molecules. This is a vicious cycle that aggravates the heat built-up. Agreed that fatter tyres have lower profiles, thus less flex/squirm of sidewall = less molecule collision = less heat BUT the larger footprint = greater traction = more heat. This nullify the gains.

>> Tyres with single-ply sidewall (polyester / polyamide = comfort) will flex and squirm more (again more heat) than 2-ply sidewall tyres. Correct tyre pressure for single-ply sidewall tyres is of prime importance. Else they are prone to sidewall cuts.

>> The 2nd largest heat contributor is concrete roads. Expect 50% higher temperatures and 4x wear when driving highspeed over concrete surfaces. Again, the pros and cons nullify each other.

>> The 3rd largest heat contributor is excessive braking. The brake disks dissipate the heat to the rim and ultimately to the air & tyre. Those who rely entirely on brakes always run hotter compared to those who combine "brakes + engine brake". People who use wheel trims (wheel caps) are effectively reducing the free flow of air to the rims = inefficient cooling = heat built-up.

[Harbir]

Originally Posted by Chewbacca

>> The biggest heat contributor is the "less than recommended" tire pressure. With less pressure, the air molecules have more room to wander and collide with other molecules, thus generating heat. More so with flexing sidewall (bumps) and twisty roads (squirms)

This is not correct. 1. Lower pressure at the same temperature means fewer encounters happen. 2. The ones that happen are of the same average energy. Just because molecules have more room to travel does not mean they are constantly accelerating in that time. So long as their average speed, represented by the temperature, remains the same, their average energy will remain the same, no matter what the pressure.

That is why two containers on a lab table with different pressure air in them maintain the same temperature as each other.

in other words, low pressure does not cause heat because air molecules have space to wander around and smash harder.

It happens for the reason you actually rejected. Lower pressure means there is greater deformation and reformation of rubber as it passes the contact patch. greater deformation and greater material being deformed causes enormous friction in the rubber material, internally. that causes the running temperature to shoot up on an underinflated tyre.

>> With less pressure, more the flex & squirm of the sidewall, thus more collision of molecules. This is a vicious cycle that aggravates the heat built-up

There is no vicious cycle (defined as one that grows by giving itself positive) here. Lower pressure increases friction in the rubber material, that has to be overcome by applying greater torque which comes from greater expenditure of energy from the fuel combustion. the excess heat energy being generated by the engine, after all wastage correction factor for losses in the driveline, will be the exact amount of excess heat the tires will be radiating by running at a higher temperature.

There is no vicious cycle.

>> The 2nd largest heat contributor is concrete roads. Expect 50% higher temperatures and 4x wear when driving highspeed over concrete surfaces. Again, the pros and cons nullify each other.

I am not sure what the pros and cons are that nullify each other in case of concrete roads.

Regards

[Rehaan]

Originally Posted by Chewbacca

>> The 2nd largest heat contributor is concrete roads. Expect 50% higher temperatures and 4x wear when driving highspeed over concrete surfaces. Again, the pros and cons nullify each other.

Do you have a source for this information? (50% more heat / 4x the wear rate)

Originally Posted by Chewbacca

Those who rely entirely on brakes always run hotter compared to those who combine "brakes + engine brake".

Yet another good point in favour of using engine braking.

cya
R