By Ricks - Surf City on Thursday, November 21, 2013 - 02:00 pm:
" Folks, our braking capability is very limited because even with the latest hydraulic, disc brakes, the footprint of each rear tire is only about the same size as that of a shot-glass."
You just showed us you're like most here, who have never studied, or have failed physics, Bob. You're also making two arguments: rear only, and size tire.
The size of the contact matters not. It is the coefficient of friction that counts. Clinchers of the same compound as wide tires will stop in the same distance. Tires harden with age and lose traction. I need to replace mine at ten years old.
Rear only brakes are really limited, as the harder the braking, the more the load shifts to the front.
I run 4-wheel drum brakes. Somebody at the last last local T meeting reminded me how I almost vaulted him over the windshield at 10 mph one time many years ago. I now require seat belts on my pax before the car moves.
By Derek Kiefer - Mantorville, MN on Thursday, November 21, 2013 - 03:37 pm:
"The size of the contact matters not. It is the coefficient of friction that counts. Clinchers of the same compound as wide tires will stop in the same distance."
This is absolutely false. Friction coefficients are entirely dependent on the size of the contact patch. All else being equal, a larger contact patch has more grip every time.
Smaller patch = more vertical load per square inch, which also gives more grip per square inch, but load and grip are not directly proportional. Increasing load gives diminishing returns on grip.
A simple example would be changing the camber angle on a racecar. When the car leans and camber goes positive, you lose grip because the contact patch is small. Increasing static negative camber to keep the contact patch larger in a turn increases the grip at that wheel, and changes the entire balance of the car. I do this frequently with my racecars...
By Bob Coiro on Thursday, November 21, 2013 - 04:32 pm:
Ricks, of Surf City,
Yes, you're quite right in that I was talking about the vast majority of Model T Fords that only brake the rear wheels. You and I are very much in agreement that such a setup is very limited as to stopping ability.
I never took college physics, so you have an advantage on me. I have absolutely no idea as to the meaning of "coefficient of friction." But hey, I'll look that up later on Wiki and learn something new.
I think, however, you and I will maintain an honest, gentleman's disagreement over point of contact size, and the reason for my belief is the same as the reason why the tires on modern, Formula-1 race cars look the way they do. I really do think I'd be on safe ground to assume that wide tires have a better grip on the road than skinny tires. But hey, I could be wrong—happens all the time—just ask my wife.
By Hal Davis-SE Georgia on Thursday, November 21, 2013 - 05:00 pm:
I've posted this before, but here goes again. Friction (Traction) is equal to the coefficient of friction multiplied by the normal force (Weight a scale would see if the tire were sitting on it). Surface area doesn't matter. It's high school physics.
HOWEVER.....the coefficient of friction changes with temperature and the smaller contact area doesn't dissipate heat as well.
I have no doubt that if you took two cars that were identical except for the tire width and set them on a variable inclined surface with their brake set and began to increase the angle of the incline until one began to skid downhill, they would both begin to skid at the same point. The narrow tired car would not begin sliding before the wide tire car. However, if they were both skidding, the heat would build up more in the narrow one and it would likely skid further than the wide one.
Here we go again!!!
Here's what a couple of physicists have to say:
I'm not sure they qualify as "physicists". One at least has a MS. The other a BA
I eagerly await your definition of a physicist, thanks!
Is a physicist one that uses Ex-lax as a candy treat then studies the results ?
But what if the tires are on a doctor's coupe using NOS oil?
As long as they had the same amount of NOS air in the tires, they would have the same stopping distance.
Watch "Big Bang Theory"
According to Wikipedia:
A physicist is a scientist who does research in physics. Physicists study a wide range of physical phenomena in many branches of physics spanning all length scales: from sub-atomic particles of which all ordinary matter is made (particle physics) to the behavior of the material Universe as a whole (cosmology).
The key words here are scientist and research.
A person can teach physics and not be considered a physicist.
I don't care what size tires you put on a Model T you still better drive it like it has no brakes!
To me Ricks is the closest answer.
From Wikipedia, "The coefficient of traction is defined as the usable force for traction divided by the weight on the running gear (wheels, tracks etc.) That is:
Usable Traction = Coefficient of Traction x Normal Force "
As you can see, contact patch has little to do with this. The biggest variable is the normal force. That is the weight being applied to the contact patch. As braking force is applied, weight will shift forward, decreasing the available traction at the rear wheels. I taught Motorcycle Safety Foundation (MSF) courses for many rears. This is why we stress front wheel braking on motorcycles. The technique being a progressive squeeze of the front brake, and light to lighter application of the rear. As the weight of the motorcycle shifts forward (increased force/weight at the front wheel), the available traction increases allowing for more braking force to be applied. You lighten braking force at the rear since available traction decreases at the weight shifts forward.
With a typical T setup of rear wheel brakes only, initially you can apply quite a bit of braking force, but as weigh shift forward, less traction is available at the rear. This is why so many quick stops using rear wheel braking result in skids. This is very dangerous on two wheels and significantly increases stopping distance in both two and four wheel vehicles. On many US made cars, early ABS systems were rear wheel only as the rear is most likely to break loose. So, as Val said, best to drive a T like you don't have brakes, since the system of rear wheel only is very inefficient.
I never used to agree with RD on this subject, but now I BELIEVE HE IS RIGHT.
there IS ONE THING THOUGH, A WIDER TIRE WILL HAVE MORE SURFACE ON THE ROAD when the wheel goes over stones and other loose things in the road.
So a clincher on a marble has almost or no grip on the road whereas a real wide tire can be on a marble and still have surface on the road.
Dragsters have big, wide, low pressure, gummy 'Slicks' on them for a reason.
Tires have both mechanical and chemical grip, and you have to consider BOTH types in order to understand how tire grip is effected by unit-loads.
Mechanical grip is the deformation and interlocking of the tread with the irregularities of the surface. This is pretty much a linear correlation between unit-loading and grip. (higher unit-load = equally higher grip)
Chemical grip is the adhesion of the tire to the tire to the road surface. It takes more effort to peel rubber from the surface than it does to stick it to it. Higher unit-loads do NOT have a linear correlation to chemical-grip.
The compound in your tires determines the traction. Race car compounds don't count, as they're like duct tape on glass, and have a coefficient of friction >1.
From the Bosch Automotive Handbook, 1976:
I asked Universal when the guy was on here, but he did not provide anything about the compounds used in the clinchers vs. the balloon tires for Ts. I can only assume it's the same compound if from the same factory. Do clinchers wear out a lot faster than balloon tires? If not, maybe they are a harder compound.
I get inspired to do some testing of things like this, then get distracted. My crude, limited testing came up with a coefficient of friction of close to 1 .
All tires have both mechanical and chemical grip, not just racing tires.
There’s no denying that passenger car tires have changed quite a bit since the days of wooden spoke wheels. No doubt, a lot of that transformation came about not only with the intention of increasing braking (and acceleration) performance but also to enhance the way a car handles going a round a curve. So far, a lot of what I’ve read here in this thread, and what I’ve been reading on the internet, leads me to believe that race car tires look the way they do in great part because of the increased cornering performance they impart.
But in this thread, we’ve been discussing whether there’s something about clincher tires that have a detrimental effect on braking performance. Now, so far, I’m hearing that in the sterile and perfect world of physics and math, the width of the tire makes no difference when it comes to braking traction. But something in my gut keeps insisting that there’s more to the picture than that. I wish I could put my finger on it. Maybe this: One of the things we haven’t talked a lot about is inflation and I think that issue necessarily goes hand in hand with the discussion. Why? Because all the brands and models of the clincher tires equipping Model T Fords are high-pressure tires requiring between 55 and 65 PSI of pressure as opposed to the 28 to 32 PSI of pressure necessarily associated with today’s much wider passenger tires.
Is this the objection of my inarticulate gut? Could it be that inflation is a game-changer that would throw a monkey-wrench into the cold, objective physics of this braking-action debate? Is the issue more a matter of the hardness of a skinny, clincher-type tire vs. the softness of a wide, low-pressure tire?
(Aside from this, I know the subject of rubber chemistry will come up, but there isn’t much data available for us to wield in this regard. If there’s a difference, it’s probably subtle because the rubber of a clincher tire feels pretty much the same to my hand as the rubber of a modern tire.)
Now let me get this straight. If the size of the footprint makes no difference, then a block of rubber 5feet wide and 10 feet long vs. our 4 skinny tires would slide at the same time on our proverbial incline?
Chemical grip is the reason why a larger contact patch has more grip.
We're not talking about something like rubbing 2 pieces of steel together, where those who have "passed physics" may in fact be right.
We have the factor of adhesion that needs to be included if we want to talk about real world application rather than theorizing what things should do on paper.
I guess the wide and the softest tires that would last the race we put on our race car were a waste of time and money.
A comment from the real world, back in the day of mud and caliche roads, the old narrow tires that I grew up with would cut down through the mush, be it mud, snow, or cow manure, and get a grip on the harder soil down below, and keep on pulling. I still remember when the bigger, fatter tires came out, a car or pickup would be almost helpless on mud or snow or ice, they just floated on top. Now I know this is not scientific, but like I said at the start, this was my real world experiences with skinny and/or fat tires. I also know that T's have the most marginal of a brake system, and like someone said, "Always drive like you don't have brakes". I did this back when I drove a truck in the oilfield moving huge loads with no trailer brakes, and pitiful hydraulics on the tractor portion.
You're leaving rubber on the road, Fred, like duct tape pulled off of glass. Street tires don't do that so much.
Bob, the tire pressure determines the size of patch. 60 psi, pounds per square inch, is going to take 7.5 square inches to support the typical 450 lb front wheel of a T. That's the road contact you will have. Drop the pressure, and contact area increases.
I will say this about cornering: there's nothing like narrow tires to let you get into a 4 wheel drift. I've done it a number of times in the Speedster and the ol' brass picup - sometimes when I shouldn't. If your T is too topheavy, then borrow an MGTC or other stock tired early sportscar. You get the drift.
No need here to borrow anything. I have an SCCA Solo II E-Stock '94 R-package Miata, and a C-Prepared '69 Chevelle... been sideways in each many times
Have also raced an XP Miata, SSM Miata, CS Miata, DS BMW, and league race indoor rental karts on Monday nights during the winter.
Wider tires = more grip on all of them. ;)
Here is a pretty good discussion from a college physics class:
This is about half art and half science -- the problem is: which half is which?
I live in the frozen north
As Grady says if you are in snow or muddy roads narrow tires work better. Snow and ice, soft compound with LOTS of "edges" work best. Trade off is they don't last long. It the snow is deep you want narrow tires and soft compound
I "Ice raced" a Datsun 510. In the straight "rubber" class (no studs) we ran really narrow SOFT grip tires and were very competitive. Especially if the track had much snow on it