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By Design News Staff -- Design News, March 11, 2002
MIXED TIRES: TO IT?
While I do not disagree with your tire analysis and the different loaded radius being a factor in this accident, to me this is fairly minor issue. Isn't the largest contributor to this accident the overall static weight difference of a pickup? (The front wheels probably carried over 65% of the total vehicle weight statically.) The drawing shows W, vehicle weight centered between the wheels. Unless this pickup had a payload in the box, W is not centered between the wheels.
Secondly, while decelerating, the additional "weight transfer" to the front wheels further aggravates the weight, and therefore braking effort, available at the front wheels versus the rear wheels. This is true of all vehicles when braking, and a great reason for ABS brakes.
The smaller loaded radius of the rear tires would cause them to skid sooner than the proper tire. Once skidding occurs, the ride will probably be pretty much the same, and that's where weight balance and transfer take over.
Dave Majkrzak, Design News reader
I read your article on the improperly sized tires. If I had not driven pickups for the past 40 years I would have been lead to believe that the accident occurred because the driver had installed tires on the rear axle that were one inch smaller in diameter. Knowing from first-hand experience that pickups are designed to carry a load and one that is not loaded is a safety hazard I feel the driver was the problem. I would say that 90% of the blame belongs with the driver going too fast trying to do evasive tactics. It's a wonder that more accidents don't happen
Wally Skyrman, Central Point, OR
You state: "If braking system design is such that the friction force is the same on the front and rear axles..." This assumption is naive. On trucks, the rear brakes are sized for the rated payload, resulting in grossly oversized rear brakes on the typical, sparsely loaded pickup. Many pickups have rear brake-proportioning valves, which detect the ride height and then attempt to vary hydraulic pressures in the rear brakes to compensate for the variable weight on the rear tires. Such systems suffer from inordinate sensitivity to vehicle sway and forward tilt during emergency stops.
The most important parameter controlling braking is the distribution of vehicle weight, a situation that is dynamic, since inertia shifts effective weight towards the front wheels during hard braking. Your T100 pickup probably started out with a 60/40 front/rear weight distribution. During the emergency stop it shifted to perhaps 80/20. It's no wonder that the back wheels started to skid, given virtually no weight pressing them onto the pavement.
As far as the effects of tire size, the most important para-meter would be the width of tread on the road, not tire diameter. The 5% difference in width (205 versus 215) in your example would not have a significant effect on traction. More significant effects would come from differences in tire pressure or tread design.
James C. McLane III, P.E., Houston, TX
L. Zirkle's article about tires is interesting, but superficial. He apparently has not driven many pickup trucks, and did not input into the equations the fact that any car, except a few sports cars, has the center of gravity (CG) off center. Pickups are the worst and to boot, he does not refer to any load being carried. Assuming a centered CG for any car is a sure way to miscalculate braking forces.
The fact is that, even if you are driving a balanced car, if you lock the rear wheels the car will swing around, whether or not on a curve. Uneven grip of the dry road on the right or left, unbalanced tire air pressure, uneven wear of the threads, and other factors will start the rotation. The CG being behind the wheels with the better grip (the front ones, always) will be the force that spins the car around.
Assume a initial driving velocity about 60 mph and the car will do a 360, or more, before all the rotational energy is dissipated by the tire friction. Add to the road surface anything such as snow, ice, water, sand, gravel, or just enough dust, and who knows how long the car will keep spinning. The solution is to take the foot off the brakes before the car swings more than a few degrees. And even if you do have an antilock system, do not assume that you "know the car and the road" enough to drive faster than is safe.
Victor Rossi, Sr. Engr., the ROSSI design group
Zirkle's response: My basic objective is to provide an entertaining article with interesting and useful information about vehicle accidents—not to present an exhaustive treatment of the theory and practice of accident reconstruction. Since my articles have an obvious length constraint, it is impossible to present all aspects of any case. Focusing on a single issue allows me to explain the particular topic in more (but still limited) detail. I believe that this makes for a more interesting article, even though other significant aspects may not be discussed. (Names and some details also may be changed to preserve anonymity.) Nonetheless, I appreciate readers' feedback and comments about any and all aspects of a case. Some of you have specialized knowledge on a particular topic, and I find many of your comments to be very informative. So, keep those e-mails coming, realizing that I may not always be able to respond individually to each one. —Larry
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