wider is better? or not? size of contact patch arguement 8

[steved33]

In a very heated debate on another forum, some associates and I have been debating whether or not wider is truly better. So I address this forum...(it's been forever since I posted last)

The basis is that for the same compound of tire, running in it's proper temperature zone, that wider is not always better.

On one side of the arguement, the insistence is that as the area of the contact patch increases (you go to a wider tire), that increase in area is what gives you more grip/traction.

On the other side, grip/traction is controlled by the coeff. of friction x the normal force, and width really doesn't matter.

really the question is...Does the size of the contact patch matter?

Apply this to an autocross or road race car that could be setup on anything from a 195 to a 275 width tire.

I can supply a pointer to the arguement if needed.

steve

 

[GregLocock]

Even boring tyres easily achieve mu greater than 1. For instance in braking the front tyre of a typical car will develop a longitudinal force of about 120% of the vertical load on it.

Cheers

Greg Locock

 

[ccw]

From

http://www.unexplained-mysteries.com:

In order to exceed 300 mph in 4.5 seconds dragsters must accelerate at an average of over 4 g’s.
* In order to reach 200 mph well before half track, the launch acceleration approaches 8 g’s.

Let's see, that's mu coef fric = thrust force / normal force = (m*a)/(m*g) = wow!
Try that on your incline plane sliding block tests.

 

[Warpspeed]

Are we talking ultimate grip here, or how far a competent driver dare push it ? Slip angles, and subjectively how it feels leading up to actual loss of adhesion can have rather a lot to do with it too.

Footprint area probably has more to do with tyre pressure than anything else, so tyre aspect will only really change the shape of the footprint, not actually plant more rubber on the road as many believe.

The racers that run slicks, and the F1 guys back in the old days used special qualifying tyres that were very soft and had a much higher frictional coefficient than the harder long distance tyres. I hardly think they would have bothered if frictional coefficient is irrelevant.

As previously stated a very wide flat tyre can have higher lateral stiffness right at the contact patch, and hence a reduced slip angle which gives the impression of more grip. It will also obviously have more water to shift in the wet. Keeping the whole enormous tread width flat on the road may be quite a challenge too for the suspension designer.

There are probably theoretical advantages of very wide tyres, but suspension geometry may limit what is achievable in practice, and beyond a certain practical width the problems begin to rapidly outweigh the advantages.

 

[awhittle]

I recently changed the 295-45-18 tires on 9.5" wheels to 255-65-16 tires on 7" wide wheels. The vehical is a 2000 Ford Lightning. The new wheels are about 20# per corner lighter than the 18's. To my supprise vertualy ever aspect of the performance of the truck is better now. I attribute most of the improvment to the unsprung weight ratio improvment and to the increased sidewall deflection allowing the tire to follow the road surface. Then tire noise in the interior is greatly improved. The idea that large fat wheels and tires sells overpriced vehicals is looking quite possible.

Andy

 

[evelrod]

Well, Andy...Under the heading "...better to light one candle than curse the darkness", it appears, IMO, you've hit upon the 'crux' of the matter....big wheels, wide and low profile tires make big 'bucks' for the OEM.
Take a 'fad', run with it, add great 'gobs' of $$$ spent in advertising and, voila,--- even bigger $$$ in profit for the OEM and, by default, the specialty equipment mfgrs. that started the whole idea in the first place. We seem to be so easily swayed by 'advertising' that we will buy into any thing we are told is 'hot', no matter what it's real worth. Do you really 'need' 22" wheels on your Mitsuhondayota?

Rod

 

[ItSnotjavi22]

Simply put
"On one side of the arguement, the insistence is that as the area of the contact patch increases (you go to a wider tire), that increase in area is what gives you more grip/traction."

which is true because your tire will now cover more ground and will now be harder to lose stability because of the coeff. of friction x the normal force that you mentioned...the 235 tire will now have more friction than a 195 tire in regards to autocross and such, it depends on the track a wider tire will not be as nimble at low speeds and for that reason not needed as the speed pick up you would want a wider tire for the high speed stabilty now if there is downforce involved this probably isnt' your local autocross were' talking about where lips and spoiler can't generated enough D.F. to do much when trying to slow down from 180 mph

 

[NormPeterson]

Huh? The only thing known for certain from a comparison of tires in which only the widths are given . . . is that one is wider.

As a polite, general comment from an occasional standardized test evaluator, intended constructively: it is not at all clear just what point your second paragraph is trying to make.

Norm

 

[CSBM5]

When going to a wider tire and assuming that the contact patch is now wider (but shorter in the longitudinal direction -- assume same inflation pressure and load in each case hence we end up with same contact patch area), it would seem that the benefit could also be coming from the distribution of the cornering load across the contact patch as it relates to the distance from the leading edge of the patch?

I'm trying to build a mental image of how dramatically different the contact patch to road interface looks between two different width tires. For example, consider a 205mm tire versus a 275mm tire both with the same sidewall heights. At their respective limits of adhesion under cornering, there would seem to be a dramatic difference in that load (and slip) distribution on the contact patch. Is there any source for further information on this subject?

 

[GregLocock]

http://www.ftire.com

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[crysta1c1ear]

I'm just thinking out loud here ...., this isn't knowledge.

Wider tyre => more even 'radius' over the shorter contact patch => less tyre deformation => lower deformation component to the coefficient of friction
which from another thread results basically from adhesion, deformation and abrasion.

I'm not saying that's right. It is just food for thought.

 

[GregLocock]

"Wider tyre => more even 'radius' over the shorter contact patch => less tyre deformation => lower deformation component "

is dead right - that is why the Michelin solar car tyre was a 2.5 inches wide radial when every body else was running BMX tyres - the short wdie contact patch deformed less and gave less rolling resistance. The best tyre previously ran about .0055 rolling resistance, the Michelins were half that.



Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[CSBM5]

When the term "deformation" is used in describing the three components of grip, I always took it to mean local deformation of the tread due to asperities of the pavement surface (as opposed to a more global deformation of the tread face due to contact patch shape).

 

[GregLocock]

Good point, the global deformation would have something to do with grip on glass, the local deformations would give additional grip on rougher surfaces.

Just thinking about it, most tires have roughly the same peak grip in botht he longitudinal and lateral directions. But the shape of the contact patch is wildly different for those two cases, in the direction of the force. So I'm beginning to think that maybe the contact patch shape as such is not especially important.



Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[tireman9]

We seem to have a couple different thought trends working here.
One being the theoretical improvement in vehicle handling based on the assumption that wider tires have a larger foot print. Sorry but in general this is not the case. The total contact area is essentially the same for a wide range of sizes as long as corner load and infl remain the same.

I believe a strong case can be made that with a standard OEM suspension the handling will become worse as you go wider as the camber curve will drastically alter the contact patch.

The reality is that other changes in tire construction and tread compounding make the comparison impossible unless you work for a tire company and someone is building specific constructions for you.

37 yrs Tire Eng. Designed basic rain Firestone for CART. SCCA & IMSA Pro & Am. Set lap records at 6 different road courses in '89-91.

 

[Knap]

If looking in general at handling balance in the case of tyres, the cof drops off as you increase the weight on the wheel and you can tune a car's handling by adjusting the anti-roll bar.

The bigger the contact patch the more grip you can get. In a drag race, dropping the tyre pressure increases the contact patch area and increases grip.

When you look at lateral grip other factors start to matter. The tyre develops side force because of the slip angle between the tyre and the road. This slip angle means the tread is being pulled sideways by the road surface. At the front of the contact patch the deflection is relatively small. As you move back along the contact patch the deflection increases steadily. At some point, the sideways forces in the tyre exceed the friction between the tread and the road and the tread starts to slip relative to the road. When the tread is slipping like this it produces less grip on the road. As the slip angle increases the sideways deflection builds up quicker so the front of the contact patch works harder. But more and more of the back of the contact patch is sliding and losing grip. At some point you reach a maximum point where more slip angle means less side force because you are losing more grip at the rear of the contact patch that you are gaining at the front.

The longer the contact patch is, the more gradually break away occurs. If you shorten the contact patch, the break away occurs more abruptly but you get more absolute grip at the peak as there is less variation in sideways distortion between the front and back of the contact patch, more of the contact patch reaches maximum grip and starts to slide at the same point.

When you fit wider tyres the contact patch wider and shorter for the same tyre pressure. This means you get a more abrupt breakaway but more grip right on the limit.

If this logic is correct then increasing pressure in the tyre further improves grip, since more pressure = less contact patch area = shorter contact patch = better grip?
However more rubber on the road does help grip due to the hysteresis properties of rubber. As rubber expands to fill a depression in the road, it takes some time to do so. When a tyre is sliding (and due to the slip angle, the rear most portion of the contact patch slides at even low cornering forces), this means that the upward rise of the depression to which the tyre is moving has more rubber acting on it that does the upwards rise on the other side. This allows a pressure differential in the lateral plane, providing frictional resistance over and above that offered by simple friction. As the tyre vertical load increases, the rubber is forced more fully, and more quickly into the depressions, overcoming the hysteresis and reacting on both sides of the upward rise from the depression more evenly – giving less pressure differential and less grip.
Low tyre pressure is better for grip from deformation and hysteresis.
Tuning the pressure is about balancing the contact patch length (which is better as pressure goes up), and the contact patch pressure (which is better as tyre pressure goes down). Even though the optimum grip may be achieved at low pressures higher slip (because the hysteresis element is significant),low pressure increases tyre deflection, which increases heat (less even radius over longer contact patch). It therefore appears that the best way to increase grip is a wide tyre as this gives a shorter contact patch for the same inflation pressure.

However tuning the handling balance using tyre pressures, appears to be a combination of trying to match front and rear slip angles, ultimate grip at the limit, and effect of heat which may effect inflation pressure and tyre compound?

 

[ubrben]

Great post Knap - are you a tyre engineer?

Everything I've seen that models actual tread and road interaction suggests that grip has no dependency on load assuming isothermal conditions and ignoring carcasss effects.

Two hypotheses for load sensitivity assuming area change isn't one are: thermal effects reducing grip of the compound and carcass deflection causing the tread slip angle to be different to the wheel's slip angle (Trevorrow 2006 Monash University Phd thesis)

Wider tyres are better in both these cases, they have higher in plane belt stiffness (less carcass twist) and the tyre is not being heated as much because it is in contact with the road for less time and cooling for longer due to the shorter patch.

The effect of pressure (and therefore area) on compound bulk temperature is a big effect - particularly with softer race compounds. Indeed it is perhaps easier to see these effects if you are dealing with race compounds rather than hard road ones.

Ben Michell
Dunlop Motorsport - MotoGP

Senior Design Analyst
Dunlop Tyres
Motorsport Division

 

[Knap]

Would you say with same size tyres that the effect of changing pressures to achieve ultimate grip is therefore not related to contact patch area change, but relates to temperature change and carcass deflection changes?

The use of lower profile, wider rim, stiffer sidewall and higher pressures would all reduce carcass twist (tread slip angle to be different to the wheel's slip angle)

 

[tireman9]

I think it important to separate the effects of ride & handeling into at least 2 different areas.
One is for vehicles with completely adjustable suspension such as race cars. For production based, even with some mods to suspension there are some other items at work which can make what is theoretically good for a race car not good for the car without completely adjustable suspension.

In general:
Contact area is primarily load/inflation pressure BUT the sidewall stiffness of the tire does carry a portion of the load and that portion can also vary with inflation pressure so there is no one simple answer which is what I think many seem to want or expect. As UBERBEN pointed out tires have a load sensitivity curve and this curve is based on a combination of construction, aspect ratio and sidewall curvature both as molded and as mounted on a rim and the curve is not linear so again there is no simple answer.
It is entirely possible to have tire "A" provide better "grip" up to a certain point of speed around a curve but tire "B" to be better at the upper ranges of cornering. You also have to decide if it is better to have a flatter response curve with a more predictable response or one with higher max grip but only over a very narrow cornering angle that can break away with little warning at it's max level.

If you have a very wide tire it becomes more important to have the suspension ensure there are minimal camber changes or you will loose the possible advantage of the wider tire.

The first post starts out stating the two different opinions and then asks which is correct. In my experience neither is completely correct and neither is completely wrong. This is because you haven't stated the boundary conditions.

EVELROD in an early made statements about his comparison of a "radial" from one mfg vs a "bias" from another. It appears he based his opinion of a faulty assumption that a tire marked "radial" had a 0° body ply angle.
Using the angle convention of 0° running from bead to bead. I have seen tires marked "radial" having 0°, 5° and 37°. I have not looked at bias lately but know than many have body angles at 45° to 60°.

The various arguments also have ignored the other variables such as tire weight and rotational inertia.

Tread width is also a variable that few seem to take into consideration. You can have 2 tires, both marked 225/50R16 but one can have a static contact width 8 to 20 mm wider than the other. Which is "wider".

There is no simple answer to tire design be it for ride, handling, noise, fuel economy etc, etc.


37 yrs Tire Eng. Designed basic rain Firestone for CART. SCCA & IMSA Pro & Am. Set lap records at 6 different road courses in '89-91.

 

[evelrod]

Actually I never considered the specific angles of the two types of tires I was evaluating for use on my race car.
My comments were more anectdotal and in the line of a general discussion of 'tread width' vs 'overall handling' (cornering ability in this case) in one specific case, my 1963 Austin Cooper race car at one specific track, Willow Springs. More in the line of comparing 30+ year old technology that "everyone" uses (Hoosier will no doubt love that, but there is just not enough demand in 10" wheel size to commit resources to newer technology) to the realitively new Yokohama Advan A032R tire, thankfully available in 10".

I am not a tire engineer, but I did "Sleep in a..." ---Oh, sorry, I got a bit off track...I did speak with several engineers at the SCCA Run Offs at Mid Ohio in '03. Kumho and Toyo folks were VERY helpful fielding most of my questions relating to my particular setup. Unfortunately, I have yet to hear from Yokohama tire reps...have not seen their support unit at any of the races I have attended lately.

My point, if I haven't lost track of it, as it pertains to this thread is that tire width alone is not the best way of determining overall performance as that performance relates to cornering speed. In my opinion, wider is NOT necessarily better.

As to some of the other stuff in my post, the Yokos finally did 'heat cycle out'--- at around 25 cycles they could not be brought up to temp and their 'bite' was just 'gone'. Probably will use 20 or fewer heat cycles as a referance point in the future, just to stay on the safe side. I still have not tried the chemical 'soak'...perhaps I will do it "next year"?

Thanks for your input, I find it invaluable.

Rod

 

[Knap]

Rod

With regard to Yoko Advan, it would appear that high performance tyres better suit the mid-engined roadsters. This observation was borne out by comments made by Vehicle Handling Solutions' (VHS) Alan Phillips, who was involved in the development of the MGF and more latterly TF suspension design. His comment was that all mid-engined cars appear to require a far stiffer side wall (than front engined cars) to promote predictable and reliable handling characteristics. Indeed, Toyota also recognised this - and with Yokohama, developed the Advan tyre range specifically for the MR2, with specially reinforced sidewalls.

Tireman9

Any inside information of the (development) of slicks and tyre width?

A quote "it's ultimate grip at maximum slip angles that matters- a decrease in front tyre pressure increases grip on that tyre by increasing contact patch area and could help cure an understeer problem ... A larger portion of contact patch gripping means more total grip".