Tire Sidewall - Tall vs Short

[johnc]

From Mark Ortiz's latest newletter:

 

TALL VS. SHORT SIDEWALLS

 

I am a Racecar Engineering subscriber and have enjoyed your later articles regarding tyre width and grip.

 

I seem to recall another one regarding wheel and tyre profile effects as well, but can't find it. Was it written by you? Do you remember in which issue it was published?

 

Basically, there have been some discussions about why the current trend to big wheels/small tyre profiles and why some racecars like F1 don't use a small-profile tyre. One of the opinions is that F1 is limited to 13" wheels (to limit brake size), but if it wasn't, bigger wheels would be of benefit. Personally I think the bigger wheels/smaller profiles are (beyond a certain point) just a styling and marketing exercise.

 

Any insight on this point?

I think I can recall having discussions about this, but I don't think I've addressed the subject in the newsletter or column before. Perhaps I have done so on a forum at some time, or in private correspondence. Checking my newsletter back issues list, I don't see any such article listed – only the one on why wide tires are better than narrow ones.

 

Are big wheels with low-profile tires just a styling fad or marketing gimmick? In some market sectors they have become that, but this is a styling fad with some basis in engineering.

 

Actually, the trend to very low tire aspect ratios began in racing. Low-profile tires are still used in racing, where they are allowed: Trans-Am cars, touring sedans and other production-based road racing cars, autocross cars, sports racing cars. Tall sidewalls are only seen where the rules limit wheel diameter, and on the driven wheels in drag racing.

 

 

 

Thirty-five years ago, there were no street tires below a 60% aspect ratio. If you wanted the low-profile look that the racing cars had, you had to buy racing tires. People sometimes ran racing tires on the street, although of course that was illegal. But that was the only way to get racy-looking tires with stiff sidewalls and sticky tread rubber. Car shows were full of display-only vehicles with race tires. The reason we now see low profiles on the street, and tall profiles in F1, is that street tire technology has belatedly caught up with discoveries originally made in racing, while racing rules have restricted F1 cars, and some other classes as well, to anachronistic wheel sizes.

 

Is this a bad thing? Not necessarily, although it is a bit odd. If you are running a racing series, it makes economic sense to restrict any technological progress that would require competitors to replace equipment or require manufacturers to retool. If you are trying to keep speeds down and keep fields full, why would you permit anything that raises both speeds and costs? As long as the cars go fast enough and make enough noise to put on a good show, that's enough, isn't it?

 

Certainly it is if you are openly promoting budget racing, with technologically restricted cars. But if you are billing your series as the premier class in motorsports; if you are charging an arm and a leg for tickets; if everybody knows the expense to compete is ridiculous but this is part of the draw – then it becomes harder to defend draconian restrictions on tires and wheels. And if you are still trying to justify the show as an exercise in "improving the breed", that really does get rather awkward. Wide, low-profile tires are the main street-applicable technological advancement that racing can claim to have originated during the last forty years.

 

Are they really an advancement? The questioner appears to have some doubt.

 

I say yes, they are an advancement, although they are something of a mixed blessing and have become a customizing fad.

 

Making the sidewall shorter and the wheel diameter bigger has two, or perhaps three, advantages. First, it makes the sidewall stiffer. Second, it makes room for bigger brakes. And, if we don't use all the extra room for bigger brakes, we can get more air through the wheel. If we shape the fenders properly, that lets us extract more air from under the car through the wheel wells. This not only helps brake cooling, but also aids lift reduction/downforce creation. This assumes, of course, that the car has fenders.

 

Are stiffer sidewalls always better? I think we can say that for racing and for high performance applications, we want as much lateral stiffness as we can get. There is some penalty in directional stability, because the car will have more tendency to "tramline", or follow edges in the road surface that nearly parallel the vehicle's direction of travel. But a performance-oriented driver will generally put up with this to get more responsiveness and greater cornering power. Greater lateral stiffness helps keep the tread flat to the road and prevent the tire from rolling under and concentrating load on the outside shoulder of the tread.

 

 

 

Greater vertical stiffness is more of a mixed blessing, and a more complex issue. The tire is to some extent a secondary suspension system, acting in series with the main suspension system. In stiffly sprung winged formula and sports racing cars, tire compliance may be as much as half of the total: the suspension may be as stiff as the tire sidewalls.

 

Considered as a suspension system, a set of tires is very good in some respects, and horrible in others. For unsprung weight, it's unbeatable. The only unsprung components are the contact patch and some material near it. It has no camber change in ride. On the other hand, it has no camber recovery in roll, and it is seriously underdamped.

 

We might be tempted to decide that we could accept a very high vertical stiffness from the tire, i.e. a very high tire spring rate, and get our compliance from the suspension proper, where we can get camber recovery in roll and control the damping properties.

 

However, there is one other factor: the tire's vertical spring rate is inextricably related to the contact patch size, and the contact patch size is related to the amount of grip we have. As the tire spring rate approaches infinity, the contact patch length and area approach zero. If the spring rate were truly infinitely large, the contact patch would be a line of zero width front to back, and the contact area would be zero.

 

The original objective of radial tires was to have greater vertical compliance and a longer contact patch, while still keeping the contact patch flat to the road in cornering.

 

Theoretical considerations aside, for most purposes practical considerations limit our sidewall height. We have to have enough distance between the rim and the ground so that we don't damage the rim on pavement slab edges and potholes. One of the features that has made today's short sidewalls possible has been the development of sidewall designs with a meaty region near the rim flange that protects the rim aginst both curb scuffs and pothole damage. This is combined with a flexible zone closer to the tread shoulder that provides vertical compliance, albeit over a smaller range than with traditional construction.

 

It is important to note that sidewall height is not the sole determinant of sidewall properties. It is quite possible to put a big, strong bead stiffener in a tall sidewall, and make it act like a short sidewall. It will weigh a bit more, and the brake size will suffer, but the car will corner about the same as would with a short sidewall. I am told that most "radial" racing tires are not actually radials at all anyway. The main plies are actually not as close to 90 degrees as physically possible. They are more like 70 or 80 degrees – closer to radial than a true bias-ply or the bias-belted street tires we saw in the '70's, but not truly radial. In other words, relatively stiff construction is still favored where cornering performance is paramount, and therefore the trend toward short sidewalls for performance tires is fundamentally sound.

[Armand]

Good stuff! :thumbs up:

[blueovalz]

Of all that we know about automotive science, the tires still seem like a "black art" (no pun intended).

[A. G. Olphart]

It is interesting that he makes no mention of rotational inertial or polar moment or whatever it should properly be called. The article is interesting, but seems simplistic/dumbed down in that there must be optimal sizes; bigger is not always better, due to at least the following considerations.

 

• Rotational Inertia
  
• Chassis and bodywork clearances (turn and bump)
  
• Aerodynamics
  
• tire and wheel weight, and weight of larger braking components
  

 

Huge tires dictate a huge vehicle, which in turn would demand a huge roadway.

 

What is the optimally sized tire for a Z?

 

 

 

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[johnc]

Hoosier is coming out this March with a 275/35-15" A6/R6 tire that is probably the perfect size tire for a racing 240Z. Should be about 22.5" tall and 10" wide.

[JMortensen]

That does seem like a pretty kick ass tire size. I like the 22.5" tall part, keeps the car nice and low.

[RTz]

Hoosier is coming out this March with a 275/35-15" A6/R6 tire that is probably the perfect size tire for a racing 240Z. Should be about 22.5" tall and 10" wide.

 

Cool!

[A. G. Olphart]

Hoosier is coming out this March with a 275/35-15" A6/R6 tire that is probably the perfect size tire for a racing 240Z. Should be about 22.5" tall and 10" wide.

 

Thanks John, and for ignoring my stating the obvious (which somehow at the time seemed to involve actual thought).

 

 

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