In a message dated 1/27/98 12:58:19 AM Eastern Standard Time, BobMGT@aol.com
writes:
> So how does this apply to handling vs tire width? I have a running argument
> with some of my buddies about this. I maintain that wide tires don't
improve
> handling all that much, and I use the coefficient of friction reasoning.
But
> I
> notice that the current trend is toward wider tires, especially on the
> performance
> cars.
Bob,
Wide tires do indeed improve the handling ability of the cars, very much
indeed! It all has to do with the shape of the footprint. As someone else
noted on this thread, the number of square inches of tire in contact with the
road is the same on a given car with a wide tire as it is for a narrow tire,
if the tire pressure is the same (a generality, as the stiffness of the
sidewall and other factors will have an impact as well).
The term "pounds per square inch" used in the inflation value for a give tire
determines the number of square inches of tire in contact with the road. As
an example, consider a 2000 pound car with all four tires inflated to 30 PSI.
With 500 pounds on each tire, the area in contact will be 16.67 inches
For a very narrow tire, the contact patch could be something like 2 inches
wide by 8.33 inches long, whereas on a very wide tire, the patch could be
something like 8.33 inches wide by 2 inches long. Now consider what happens
when you try to turn a corner. The rubber on the tire, being somewhat soft,
will deform as the car tries to maintain a straight line, allowing the tire to
"walk" towards the outside of the turn. IOW, as the tire rotates, each
successive portion of the tire comes into contact with the road a little
further out than the preceding portion. There is more resistance to this
"walking" with a wide tire than with a narrow tire. For this same reasoning,
an old tire, with very little tread, will handle better than a new tire. The
shorter blocks of tread will deform less than the longer, newer blocks. In
SCCA showroom stock class, where the cars must use street legal tires, the
drivers have the tread shaved (you should see the look on a tire dealers face
when he's asked to shave the tread off a brand new, expensive, high
performance tire!).
Also, as pointed out above, the sidewall stiffness is a factor -- the
sidewalls on the "rubber band" type tires, ie the 35 - 50 series tires -- are
much stiffer than the sidewalls on a 70 series, further reducing the walking.
When you get into racing tires, there is a little more involved. The softness
of the tires have the same effect on cornering as it does on the acceleration
of a dragster. Friction is not the only thing at work here -- the tires
actually "grip" the minute irregularities in the road surface.
I hope all this makes sense. It would help a lot if you could see all the
handwaving I'm doing as I write this!
Dan Masters,
Alcoa, TN
'71 TR6---------3000mile/year driver, fully restored
'71 TR6---------undergoing full restoration and Ford 5.0 V8 insertion - see:
http://www.sky.net/~boballen/mg/Masters/
'74 MGBGT---3000mile/year driver, original condition, slated for a V8 soon!
'68 MGBGT---organ donor for the '74
|