[Rehaan]

Originally Posted by styx71

So if we are using the same pressure, the 215 wide tyre is not at the same inflation level as the 185 wide tyre,

Remember that air pressure (psi) is measured on a per unit area basis. (Pounds per Square Inch). So changing the volume doesn't change the "inflation level" if the pressure is filled to the same amount.

Originally Posted by styx71

wouldn't all the weight be sufficient enough to generate the required resistance against rolling?

Not sure what you mean by "resistance against rolling". Are you talking about traction? ("resistance against sliding")?

Or are you talking about "rolling resistance" -- which is something different?

Can you summarize your question in a simple manner?

cya
R

[Rehaan]

Hi Macfreak7,

I don't really agree with most of your points here. I'll explain why below:

Originally Posted by Macfreak7

However for the average joe who has never taken their car to track, the application of a wider tire set up would be noticeable primarily (and this is my point) during straight line take offs in the form of reduced wheel spin / increased traction.

On a track, i think the order in which an average joe would rate wider/better tyres and their usefulness is :
1st : Cornering (hold the line better and carry more speed through corners)
2nd : Braking (later braking points, more aggressive braking, more composed)
3rd : Acceleration / launching (less wheelspin / power loss)

Originally Posted by Macfreak7

Lots of power accompanied by wide tires for a killer set up. Would 255mm wide tires do any good for a car with 150HP? Most likely not.

Completely disagree here.
A 60 BHP car can benefit hugely from some better rubber. It doesn't require power to corner fast or to brake hard.

Take a look at last years diesel VW Polo cars (for an on-track example). They put out 129 BHP. Now, just between different brands of slick tyres, the lap times were 3-5 seconds apart!! Imagine how big the difference would be between the grippy slicks and an average road legal tyre!

Originally Posted by Macfreak7

Reduced braking distances are a by product of wider tires. You don't switch to wider tires for better braking. If you want better braking you get a bigger and more powerful brake set up, ABS and such.

Completely disagree here as well.

Remember that the strongest chain breaks as fast as the weakest link.

Virtually every STOCK braking system is capable of overpowering the stock tyres when in motion. Ever heard a Premiere Padmini screech its tyres? Yep, thats whats happening even with those drum brakes. They are out-doing the tyres' grip.

Bigger more powerful brakes aren't going to help at all if your link to the road (the tyres) is weak. Its like buying a 4G phone when your service provider is still offering only a 2G network.

Originally Posted by Macfreak7

Also, to generate enough lateral traction on a car you're going to need a powerful enough engine to push the car to those limits where wider tires become a necessity. Again, the engine power dictates that.

You can generate enough lateral g-forces to challenge the tyres at as low as 50 km/h - and it doesn't take much engine power to get there.

To sum it up, there's more to grippy tyres than just putting the power down. In fact, putting the power down is the least important part of it from a well-rounded point of view. (Obviously different if we are talking 1/4 mile etc).

cya
R


@ Harbir - Completely agree with your theory, but there are some examples in your post that on paper seem right, but in reality i wonder if the hundreds of other factors influencing them would change things around. I don't think the final results would be as linear as expected! Maybe i'll raise some questions point by point if i get the time. Cheers

[Harbir]

Originally Posted by styx71

^^ Huh ?! This is a bit confusing for me. Could someone please clarify:

Based on this site : http://www.club80-90syncro.co.uk/Syn...calculator.htm, the tyre volume of 185/65 R15 is 11.15 Liters and that of 215/55 R15 is 12.69 Liters. So if we are using the same pressure, the 215 wide tyre is not at the same inflation level as the 185 wide tyre, which would theoretically mean, more contact area with the road.

Also, If the Pressure (Force per unit Area) is less due to wider contact, if we consider a car weighing 1 tonne plus passenger(s), fuel and may be luggage,
wouldn't all the weight be sufficient enough to generate the required resistance against rolling?

Also a lot of members recommend dropping the tyre pressure when upsizing. A clasic example being the old swift : 165/80 R14 to 185/70 R14.
Even though the volume of 185/70 R14 is more than 165/80 R14, lot of members have reported using reduced tyre pressure as compared to the stock size.

I didn't bother to go into this in my previous post for the sake of simplicity. At the correct tyre pressure for each tire, everything I said is true. For a certain given weight on it, a wider section tyre will reach its correct inflation pressure before its contact patch expands front to back to the same extent as a narrower tires under the same load and at IT'S correct pressure, and will end up with the approximately the same area contact patch.

I said "same pressure" because I didn't want to get into a conversation of "correct pressure". But lets deal with that now. I define correct pressure as "evenly flat contact patch." Most people don't know how to determine the correct tyre pressure for optimal performance of a tire, so there is a lot of guessing, conjecture and misinformation.

The correct pressure for a tyre is the one that produces a flat contact patch. As everyone knows, if a tyre is underinflated, its contact patch is concave and puts more weight on the edges rather than the middle, and if its overinflated, its the other way around.

But how do you determine the correct pressure?

Very simple. Get some thick chalk of the sort used to right on a classroom blackboard and take your car to a place where the surface is absolutely flat and level, such as a large parking lot. Use a good gauge to measure the tyre pressure. Draw a straight line on a tire, making sure its not at an angle, and drive the car forward and backward in an ABSOLUTELY straight line, say 50 meters forward and 50 back from the original point an few times. Get out and check the line. lf it is evenly worn out, the pressure is bang on. If its worn in the middle of the tyre more than on the edges, the pressure is too much. If its worn more on the sides than the middle, its under inflated.

Of course this assumes you don't have a radical wheel alignment setting and are choosing the right pressure for street use , but in general this works.

Originally Posted by Rehaan

@ Harbir - Completely agree with your theory, but there are some examples in your post that on paper seem right, but in reality i wonder if the hundreds of other factors influencing them would change things around. I don't think the final results would be as linear as expected! Maybe i'll raise some questions point by point if i get the time. Cheers

Indeed, it is generally correct but not perfectly linear, as I alluded to. Given the huge number of variables that interact, its quite possible to get a result not consistent with what I have said. The question is what general rule of thumb should guide people. In that, what I have said is correct. Generally speaking, wider tyres of the same model and overall diameter will not increase maximum grip. But I'd be happy to discuss this more. I am sure I have as much to learn as you do!

[Rehaan]

Originally Posted by Harbir

I define correct pressure as "evenly flat contact patch."

Just to clarify for the sake of others, Harbir means "flat" (ie even pressure) across the width of the contact patch (from inner sidewall to outer sidewall). Not flat from front to back, as there would obviously be lesser flatness/pressure on the leading and trailing edges of the contact patch.

Depending on the pressure, the excessive wear (shown in red) differs. A correctly inflated tyre will have even wear in most cases:

Source

cya
R

[AbhiJ]

When you compare 2 tyres, solely based on width, its like comparing Applies and Oranges.

Grip is HUGELY dependent on tyre compound and tyre design.

I wont be surprised if a 175 mm Bridgestone Potenza GIII outperforms the 185 mm Bridgestone OEM (S/B series) tyre!

Originally Posted by Chewbacca

The hardening effect applies to even Michellin and Yokos but to a lesser extent since they have more of the Silica content.

Sorry, Silica content of a tyre has no connection to aging.

Aging resistance is dependent on a whole lot of other factors.

[styx71]

Originally Posted by Rehaan

Remember that air pressure (psi) is measured on a per unit area basis. (Pounds per Square Inch). So changing the volume doesn't change the "inflation level" if the pressure is filled to the same amount.



Not sure what you mean by "resistance against rolling". Are you talking about traction? ("resistance against sliding")?

Or are you talking about "rolling resistance" -- which is something different?

Can you summarize your question in a simple manner?

cya
R

I was referring to "Rolling Resistance" - essentially the resistance by the tyre + road against the forward motion of the vehicle.

What i am trying to say is if the upsize results in increase in overall diameter and width of the tyre,
doesn't the contact area increase with use of same pressure as that of the stock ones?

Another question is, why is there is a drop in Fuel Efficiency of 1-2 km
when we upsize - isn't that because of more Friction?

Thanks

[Harbir]

Originally Posted by styx71

What i am trying to say is if the upsize results in increase in overall diameter and width of the tyre,
doesn't the contact area increase with use of same pressure as that of the stock ones?

I have said repeatedly qualified "for the same outer diameter"

In most cases, when increasing width and/or wheel size, you reduce the aspect ratio to keep the overall diameter the same. This is done not just to make sure that the gearing and speedometer are unaffected, but also to make sure that sensors such as for ABS and stability control do not get thrown off. Changing the tyre diameter also alters the suspension geometery. The angle from the inboard mounting point of the control arm or wishbown to center point of the contact patch changes.

[styx71]

Originally Posted by Harbir

I have said repeatedly qualified "for the same outer diameter"

In most cases, when increasing width and/or wheel size, you reduce the aspect ratio to keep the overall diameter the same. This is done not just to make sure that the gearing and speedometer are unaffected, but also to make sure that sensors such as for ABS and stability control do not get thrown off. Changing the tyre diameter also alters the suspension geometery. The angle from the inboard mounting point of the control arm or wishbown to center point of the contact patch changes.

Thanks for the clarification Harbir. I missed the "same overall diameter" point, my bad.
I am actaully in the process of changing from 175/70 R14 to 195/65 R14 forn my Verna (older model).
And i ended up being confused with the discussion on this thread.

[Rehaan]

Originally Posted by styx71

What i am trying to say is if the upsize results in increase in overall diameter and width of the tyre, doesn't the contact area increase with use of same pressure as that of the stock ones?

Nope, because of the f=p/a equation. See this para once again:

Originally Posted by Harbir

[/b]This is because the size of contact patch of the 185/65 tyre will be the same size as that of the 215/55 tyre. Only the shape will change. As the contact patch becomes wider, it will shrink in length from front to back. So where the 185/65 has a contact patch closer to a square, the 215/55 has a patch shape closer to a rectangle. This is because the size of the contact patch is largely determined by the pressure in the tyre and load bearing down on the tyre. If you maintain pressure and weight the same, a wider tyre of the same overall diamater will maintain about the same contact area and provide no more grip!!

Originally Posted by styx71

Another question is, why is there is a drop in Fuel Efficiency of 1-2 km when we upsize - isn't that because of more Friction?

There's a couple of reasons:
1) Possible increase in rolling resistance
2) Increase in mass that the engine has to rotate (ie heavier wheel+tyre combo)

cya
R

[Harbir]

Rolling resistance is the result of how the tyre deforms as it rolls (the part of the tyre reaching the contact patch flattens out, the part leaving resumes its shape). That deformation takes energy, which is dissipated as heat. But how much energy a tyre dissipates due to its rolling deformation is a complex problem that involves not just the size but also the construction and materials of the tyre, the load its under, the speed at which its rolling.

Typically, the decrease in fuel consumption of larger tyres is due not just to their greater rolling resistance, but also due to their greater rotational inertia. They absorb a greater amount of energy to spin up to speed, and then that energy is dissipated as heat during braking. This additional energy dissipated due to the greater rotational inertia of the tyres shows up as increased fuel consumption.

increase in rolling resistance is harder to figure out. If you maintain exactly the same model of tyre and exactly the same diameter, only increasing width and reducing aspect ratio, the rolling resistance change will still not be a straightforward one because the tyre's construction has changed. The ratio of sidewall height to section width has changed and their respective effect on rolling resistance has changed.

But in general, a wider tyre will have greater rolling resistance because at any given instant in time (think back to your differential calculus class) the slice of the tyre that is undergoing deformation has more material being deformed at that instant, thus absorbing more mechanical energy and dissipating it as heat.

There are variables of course. the lower sidewalls will deform less, so that will be a benefit. And the part of the tyre's circumference that is deformed is smaller because the contact patch is not as big (back to front).

A CORRECTLY inflated (as described above) wider tyre will have a slightly greater rolling resistance than a correctly inflated narrow tyre. Certainly not 2km/L worth more.

Even if you add the effect of greater rotational inertia, you will still not get a loss of 15% in the fuel economy. To get that much of a loss, in addition to the loss caused by increased rolling resistance due to greater width and due to greater rotational inertia, you have some combination of under inflation and rolling resistance increase that is caused by the wider tyre's design rather than its size.

For example, on my pajero SFX I am using 31x10.5R15 mud terrain tyres in place of the 265/70R15 OE tyres. I have an increase in diameter of about 1.4 inches. The width is approximately the same. But my fuel economy is lower. its caused entirely by the fact that mud terrain tyres have much greater rolling resistance than street tyres because of all the flexing of the treadblocks and the tyre carcass. Add in the larger diameter and thus greater rotation inertia, and you can see that I have a fuel economy drop.

[Harbir]

One more thing I wanted to add, though its been 5 days since my last post.

Rotational inertia is related both to the weight and diameter. If you fit a larger tyre that has the same overall diameter, you will still get a big increase in rotational inertia because not only is the tyre heavier, all the weight gain is on the outside of the rotating element (wheel plus tyre). The same weight increase but distributed between wheel and tyre would have produced a smaller increase in rotational inertia because the moment of inertia would have been smaller.

Increasing the overall diameter only exaggerates this effect.

Greater rotational inertia not only increases fuel consumption (extra energy needed to spin the tyre then bled away as heat under braking), it also impacts steering and handling negatively because it has greater gyroscopic stability. The more rotational inertia that the car is carrying in its wheels, crankshaft, transmission, etc, the greater is the tendency of the car to keep travelling in a straight line (conservation of momentum. A body in motion tends to stay in motion unless acted upon by an external force). With greater rotational inertia in the tyres, a car becomes less agile because its more reluctant to change direction.

Also, keep in mind that added weight is unsprung weight so it negatively impacts the ride and handling of a car. I see that the indian market has filled up with cheap cast wheels that are ridiculously heavy, which mated with larger tyres, inevitably produce a poorer handling, poorer riding, poorer steering car. But since people don't know any better, this sense of added heft and smaller slip angles makes them think they've gotten something awesome. Unfortunately, the manufacturers of quality wheels have not found distribution in India because there seems to be little appreciation in the aftermarket for the need for lightness (balanced with strength and cost).

People who know better know obsess about wheel and tyre weights. My 4th MX-5 had 9lb BBS wheels on it fitted with 15lb tyres, for a total weight of 24lbs per corner. But typical aftermarket wheels for the car are around 15lbs to as much as 18-20lbs, plus 20 lbs for the tyre, for 35-40lbs per corner, which has a devastating effect on the car's exquisite steering and handling.

[Chewbacca]

Originally Posted by Harbir

People who know better obsess about wheel and tyre weights.

I am one of those obsessed types

I was recently forced to upgrade the 13" OE alloys on my Lancer to 14" alloys because the stock tyre size 175/80/R13 is longer available. It took me 3 months to find everything to my liking. My ultimate goal was to NOT deviate too much from OE specs.

old worn-out tubed tyre Bridgestone S248 175/80/R13 = 9230 gms
butyl rubber tube = 980 gms
Mitsubishi OE 13*5J alloy wheel = 5500 gms
OE steel lug nuts (4) = 140 gms
Total weight per wheel = 15.85 kg

all above replaced with ....

New tubeless Turanza ER60 185/70/R14 = weight 8640 gms
Dzire OE 14*5.5J alloy wheel = 5900 gms
Forged Aluminium Lug Nuts (4) = 55 gms
Total weight per wheel = 14.59 kg

Thus weight reduction per corner = 1255 gms
Total weight reduction of car = 5020 gms

This is unsprung weight and they say it multiplies 7 times and adds to the sprung weight when the car is in motion. So theoretically the car is now lighter by 35 kg !!

Now sir, can I say with authority that 0 -100 and 100 - 0 is sooner than before?
I haven't checked yet.

[condor]

The video was intresting. Looking at the behaviour between the stock & the experimental tyres, I was wondering if the video actually made a case for 1-size up over the stock tyres ?

[FlatOut]

A wider tyre means more rubber on the road. It's obvious, surely? It's why a car feels to corner better with wider tyres. If someone tried to tell you that a narrower tyre has just as much tread on the road, you'd think they were mad.


Well, this subject cropped up in the ABS thread, and I didn't want to hijack it too much, so here we are. One day I was driving a car with narrow tyres and it was cornering just as quickly on a winding road as a wide-tyred car in front which was being driven quickly. In fact, he tried to pull out a lead, and didn't apart from on a straight section. I caught him again in the corners. I was able to travel more quickly through the corners, that was obvious.

But why? My 165 width, high profile (80) tyres look like they could be good for low rolling resistance or cutting through standing water, but surely not excellent at generating cornering grip? Or was it the superiority of my car's suspension and chassis design? Maybe both. If I'm honest with you, the road suited my car very well, from a point of view of corner radius, speed, surface and foundations. I would also love to say I was the better driver, but I would never disrespect a Mercedes-Benz driver, for I am one too!

May I mention the design of car I was driving was first sold in 1934, and that the one in front was a Mercedes SLK, model year about 2002? You will now, of course, know I am quite mad!


I began thinking about tyres and their contact patches, and having applied a little paint to a few car tyres when jacked up, then let them down onto some card, I realised that narrower tyres have a contact patch which is inline with the car, whereas wider tyres' contact is wider and shorter. I began to wonder how much bigger a wide tyre's contact with the road was, everything else being the same (ie same car, same make of tyre, same pressure.)

I did a little more crude experimentation as above and began to wonder if a wide tyre's contact was actually bigger. It looked as if it may not be. Must be my crude expermentation, I thought.
So I remembered my school and university physics equations. Pressure=Force/Area or rearranged to Area=Pressure/Force. Well the downwards force on the tyres is the vehicle's weight, the pressure in the tyres only alters with temperature since it's a trapped gas in a fixed volume.

From this it becomes clear that the amount of area of tyre touching the road will be constant if the car's weight and tyre pressures remain the same. Oh.

I then tried to disprove my theory and looked online. It seemed I was right. Here are a couple of links which I read.

http://www.pistonheads.com/gassing/t...=48&h=&t=78848 - see GreenV8S's reply, about 3/4 the way down the page, if you don't want to read it all.
http://www.carbibles.com/tyre_bible_pg3.html

It even turns out that area of tyre on the road doesn't in theory affect grip. I'll leave it at that for now. We can discuss cheap front suspension which relies on anti-roll bars to work well and their need for wider tyres later. We could even digress into a discussion about anti-roll bars, and why they're largely an admission of cheap engineering.


to be continued...

Mod Note: Post Edited. Smiley usage is restricted to two per post on Team-BHP. Please do NOT use more than 2 smileys in a post.

We advise you to read the Forum Rules before proceeding any further.

[condor]

Width of tyres does affect steer (under-over steer). If you said that the width does not affect contact area, then why would a different contact area result in different amounts of under/over-steer ?

The effects may not have been apparent in the situation that you experienced - at least to human feel. But to get a better idea, I think you should try witha narrower & wider tyres on the two cars there.

There was this brilliant video (BBC ?) that showed the effects of width of tyre, and how much the car would under-steer / over-steer. Trying to search for that link. If any one remembers that, please share !