PLEASE READ...Some Clarifications (archive)

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In Reply to: Re: Need Wheel/Tire Help posted by Christopher Graff on January 21, 2001 at 16:58:59:

I was wrong in saying that weight transfer has nothing to do with the tire width. It does, because the more tire you have, the more grip you get and so therefore the more body roll, aka weight transfer. I do not agree with RA/BOB's statement about the wider the tire the worse the handling. This is not true. If you have a wider tire, you are getting more grip no matter what, so even if this causes more body roll, it is only because you are able to go through the corner at a faster speed. The only time you can say that wider is not better is when you start creating too much aero drag, so that the motor cannot efficiently push the car down the straight.

What I was trying to explain with the brush, is tire role over. Generally, the larger the sidewall on the tire, the more tire roll over you have. The more roll over, the less speed you can carry through the corner, because the sidewall does not provide the grip that the tread does.

As for the ratings, I have one more thing to clarify. While the ratings are based on speed, there is build quality also considered in the rating. To build a tire that will withstand 155mph, you have to make sure that it is very stiff, and that it disapates heat very well. Here is why it has to be stiff. When you are driving the car at 155mph, the tire actually wants to compress. The way it would do this is for the center of the tire to bulge out and the sidewalls to flex in. A tire that can withstand 155 with have a lot stiffer sidewalls to be able to withstand this force, over a tire that can only withstand 120. So if you keep the 14" wheels, you might want to think about putting on Z rated tires and you will get some more feel. Not nearly as much as going to a larger size but difference. As for the heat disapation, the tire that can do 155 will have to diapate more heat than a tire that is limited to 120. The reason is that at 155 the tire generates a lot more heat, and so the compounds in the tire have to be made to withstand the heat. This is why you will not find good snow tires that are Z rated. This is purely because even though the carcas of the tire may be able to resist the forces of going 155, it cannot diapate the heat, because it uses that heat to its advantage in snowy conditions.

You cannot use 205/60 R14 tires. The rolling diameter is smaller and will screw up your odometer and speedo. 215/65 R14 tires are the ones that fit CORRECTLY.

Also, weight transfer has nothing to do with the width of the tires. What you have to consider is that the taller the sidewall, the more movement it will have. For example, take a brush with long bristles, versus a brush with shorter bristles. If you press down and to the side, you will see the brush with the longer bristles flex and move more than the shorter bristles of the other brush. This is what happens with the sidewall of a tire in cornering. And this is what you feel when the tire is scrubbing along in a corner. You "feel" the wallowy-ness of the tire sidewall flexing. A bigger tire, and thus smaller sidewall, reduces this flex and gives better handling and feel. But, because there is less sidewall to compress, road irregularities and harshness gets transmitted more easily to the suspension, the car, and you (i.e., a bigger sidewall has more room to flex if you hit a pothole, whereas a smaller sidewall has less room to flex).

Weight transfer is a much more complex topic that needs discussing mostly in extreme handling conditions. You will not need to worry about this, but a simplified explanation for the layman's terms, weight transfer is the movement of downward the force that the car puts on the suspension, wheels, and tires from front to back or side to side. Let's assume that a car is perfectly balanced at rest, and it's putting 800 lbs on each tire at rest (for argument's sake). In a right hand corner, what happens is that the car will lean over to the left, and the left-side (or ouside) tires will have MORE weight on them, than the right side (or inside) tires will have Less weight on them. (Let's say 1000 lbs on each outside tire, and 600 lbs on each inside tire). The same applies for braking and acceleration. You only need to worry about weight transfer when you start talking about handling conditions, suspension travel/movement, and other suspension topics. For tires, all you need to make sure is that the tire can handle the maximum load it will be required to handle. Nowadays, tires are made well enough that you don't have to worry about load ratings. But for example, if you add weight to the car (you, your 3 other friends, and a trunk full of stuff), say 150 lbs each, you're adding 750 lbs of weight to the car that say weights 3250 lbs. This means it now weighs 4000lbs total, and if the car has perfect distribution (which in reality it doesn't but for argument's sake let's assume it does), then each tire will need to handle a 1000lb load. This load rating will be written on the tire, and often time it is high enough that you will not need to worry about it.

H-rated tires are rated for speed. Any letter rating is a rating for the maximum speed those tires can go without failing. This rating means nothing if you're looking at all-season H-rated tires versus summer only H-rated tires. What counts there is the compound. Snow tires, all-seaons tires, performance tires, high performance summer tires, all have different compounds made to work in certain conditions (such as low heat, low grip for snow, or high heat, high grip for summer tires). The speed rating is more a by product of what these compounds can handle. It all matters on how the tires are made. Usually a snow tire won't be able to travel as fast as a high-performance tire because of the compound and the "job" it has to do. Of course you can find V and maybe even Z-rated snow tires, but they will cost a LOT more than the "regular" snows because more development was needed to make that tire work well in the snow AND go fast.

What you need to do is consider the conditions you drive in, and figure out what you want your tires and wheels to do. Then you can start looking for the set-up that you will need. These requirements can range from snow/winter driving, to track and driver schools. And more often than not, you will need certain tires to do certain jobs (i.e, snow tires for winter, and performance tires that are good in the wet for spring, summer, and fall driving).

Now that we're done with all the math,in practical terms a 205/60/14 will act better than the 195's you have now.Using a 225 size tire on a 14" rim,with the weight of your car, will cause too much weight transfer in corners,and be worse,not better.
If you plan to keep the car,invest in 15" or 16" wheels,and use a 50 or 55 series tire.Far better handling,safety,and stability.Otherwise use a GOOD "H" rated or better 205/60/14 tire.
I specialize in setting up BMWs for street/track,mostly stock and/or extreme mods.,and will be glad to discuss it if you can catch me here.

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