I read again, on the forum, one group saying, "I need more tire surface so I can stop better", and another group saying, "area does not matter." Both are right, and wrong at the same time.
The "area doesn't matter" group is correct that friction = the coefficient of friction of the materials, multiplied by the force pushing the two objects together. This is absolutely true for 2 hard surfaces. One hundred pounds of steel, on concrete, has the same friction whether there is 1 square inch of contact area or 1,000 square inches of contact area. Areas does not matter.
However, a tire is not a hard surface and rubber has some special friction qualities. The above equation does not apply to soft, adhesive, rolling, or fluid materials. Tires are made of rubber to take advantage of some special qualities of rubber not represented by simple coefficients of friction for hard surfaces.
Obviously, tire area has a drastic effect on total friction. Racers use fat, soft, low pressure tires to get much more friction.
Rubber also generates friction from adhesion, deformation, and wear. The total effect of each of these methods are dependent on area.
Adhesion is the momentary molecular bonding between the rubber and the pavement. This is how adhesive tape works. This depends on contact area. On the microscopic level, neither the tire, nor the pavement, is flat. Instead, the surfaces are full of tiny peaks and valleys. A soft tire means the rubber can deform into these peaks and valleys and increase the surface area in contact. Another way of increasing the adhesive contact area is to apply more force to mash the rubber into the pavement's micro peaks and valleys. This is why putting your passengers in the back seat or putting your spares and tools in the trunk works: it increases the force acting on the tires and increases the adhesion area by mashing the rubber into the surface irregularities of the pavement.
This force causes deformation. That is physical keying of the tire and pavement. In the original steel on concrete example, the is no deformation which is one reason the simple friction equation is not applicable to tires. Rubber, on something smooth and hard, like glass, is generating friction mainly by adhesion. But, pavement is rough and deformation dominates. The rubber deforms and drapes over the micro peaks in the pavement. This isn't necessarily "micro" as a newly paved track has much more friction than after only a few month's wear because the sand size particles in the asphalt are still sharp and not yet worn down and the tiny valleys are not yet filled in by eroded rubber. A wet surface prevents contact between the rubber and the surface, blocking the formation of adhesive forces. Friction forces due to deformation, also called mechanical keying, provide most of the friction force between a tire and a wet, oily, or dusty surface since the film on the pavement prevents adhesion.
Rubber tires also have tearing and wear. When local forces exceed the tensile strength of the rubber, the rubber is stressed past its point of elastic recovery. Skid marks.
The friction equation for rubber tires now becomes the total of friction from adhesion, deformation, and wear. These three factors are a linear function of contact area. More rubber on the road means more friction from adhesion, deformation, and wear.
With rubber tires, there are also issues of viscoelasticity and hysteresis.
"This is why putting your passengers in the back seat or putting your spares and tools in the trunk works: it increases the force acting on the tires and increases the adhesion area by mashing the rubber into..."
Are you referring to a T with rear only brakes? The traction is simply proportional to the percentage of weight on the braking wheels. As the braking force increases, the weight on rear wheels decreases until there is a skid. With 4-wheel brakes, the forward shift of the weight equals out, so moving the load to the rear gains nothing as far as stopping distance.
You're welcome to improve these:
Now I understand why Iv always put the women in the back, It was all that coefficient friction of the materials, multiplied by the force pushing the two objects together! Mystery solved.
Thank You, Jim... that pretty much confirms what I wrote, but with better detail, in the 15th post of this topic: http://www.mtfca.com/discus/messages/331880/403102.html?1385151254
Ralph, your diagram above does not take into account the fact that Model T's have their wheels attached to the chassis via movable suspension.
SVSA (contact patch to Instant-Center) effects anti-dive (front brakes) and brake hop (rear brakes). However, the CoG I suspect is located way too high in your calculations, so the numbers may actually be somewhat accurate with multiple wrongs making it close to right.
Please explain SVSA. Upward thrust on the wishbone yoke counteracts nose dive, but not entirely. Why would there be brake hop?
I probably have the cg too high, but nobody has suggested what it might be.
Side View Swing Arm
It's the imaginary line from the center of the contact patch to the Instant Center. (wishbone socket on front, 4th main ballcap on rear)
On the front axle, I suspect that short distance and steep incline from contact patch to wishbone socket means that the anti-dive is quite a bit greater than 100%, and with front brakes, the nose of your car would lift rather than dive. If I knew anyone locally with front brakes on a T, I'd offer the use of my GoPro camera pointed at the front spring to verify...
Here is the calculation for % anti-dive:
On the rear, the axle is trying to do the opposite. Brake hop is the opposite effect of "anti-squat", which refers to acceleration. Cars with greater than 50% anti-squat are likely to brake-hop with too much rear brake bias (a stock T is ALL rear brake)
Braking torque pulls downward on the front of the axle, which forces are all transferred to the chassis at the ballcap. Because the mass of the body is greater than that of the axle, and because bodies at rest tend to stay at rest, these forces tend to compress the spring and lift the axle off the ground. As soon as the tires are lifted off the ground, all torque applied to the axle is gone, and the spring forces the axle back to the pavement, which then grips the pavement and kicks off a cyclical hopping action/reaction.
I suspect a T anti-squat is very close to 100%.
100% anti-squat is an imaginary line from the rear tire contact patch to the intersection vertical of the front axle centerline and horizontal of the CoG. An Instant Center above that line is greater than 100%, and below the line is less than 100%. Without knowing the exact location of the CoG (which varies by body style anyway) it appears that the IC is very close to the line, if not slightly below it.
Not sure if anyone will read all that... I bored myself right out of proofreading the whole thing.
All this BS and you still don't have any more than what you started with.
Way too complicated for me! Guess that's why I keep it under 35 mph, and leave lots of room behind the other guy, and start coasting way ahead of time.
The fore-aft length of the front wishbone is mebbe 30". I measured it once, but forget. Assuming the center of rotation force of braking is 15", that means the upward push on the wishbone socket is only half the downward force of weight plus weight shift. Will that result in nosedive, or lift?
Rear braking pulls down on the rear wishbone, causing the rear wheels to get lighter, especially if there is front braking.
Rear drum brakes have a single leading shoe in each direction, so to have braking when rolling backward. Front drum brakes have both shoes leading when rolling forward, so you automatically have front brakes doing 2/3 of the job.
The next time Gene stays in town long enough, we'll make a video. I don't know how to load to utube, but I'm semi-teachable. I bolted a video camera under the runningboard back in '97, and Gus Stangeland put it on this site. It didn't include braking, however.
I have ridden with Ralph and his car stops very nice. He helped to inspire me to work on front brakes for my own car and to continue the development of a bolt on kit for the ordinary T. And that project is continuing and is being tested. Perhaps in another year I will offer it to a few people for further trials. I do get discouraged at times and leave it by all the "nay-sayers" on this forum. Then I get inspired and come back to it. It will happen God willing.
One of the things I struggle with is the handing issues associated with front brakes and slippery roads. Ralph has solved this by using a "better" steering box (non-reversible). On my '27 I totally reworked the front axle steering geometry (gave it "Ackerman"). Both solutions work. I have embarked on reproducing 10 Ross "Cam and Lever" steering boxes for the T. I am also considering making a small run of new tubular front axles that will have "Ackerman" steering geometry and will potentially incorporated 12" drum front brakes.
The front suspension has been proven to handle the forces if you add a good radius rod "doubler"
I have focused on drum brakes. My first effort was disc brakes, but I just don't like the look.
That modification is unnecessary, troublesome, gaudy and ruins the whole character of the Model T!
12", really? I like the 8" Metros, because they match the size of the small drum rear.
I understand what you are saying. The thing is the 12" backing plate assemblies are cheap and readily available in either hydraulic or mechanical. The shoes are 2" wide. The 10" version is 2 1/2" wide and as such the geometry gets more complicated. I am thinking of making the drums in a "spoke" version rather than solid to get something resembling a Buggati feel. I have the wood for the hub/drum pattern all glued up and just need to finish it and get it to the foundry for casting. Part of this project is also that these parts can be adapted to a really workable front wheel drive/four wheel drive T speedster/truck
You could get a 12" backing plate to fit behind the spindle, huh? That would help. Metro shoes are only 1 1/8" wide. Some of the Triumphs, etc., used the same brakes.
Yes. Realize what I am doing involves a new spindle and all that. NO welding!!! And all the parts are current production (and should be for many years to come)
Where do you get new backing plates?