Statens väg- och trafikinstitut (VTI) • Fack • 58101 Linköping
National Road & Traffic Research Institute - Fack - 58101 Linköping - Sweden
Nr 108A 1976
108A
Tire Dimensions
Properties of Wide and Low versus Narrow
and High Tires
by Ulf Sandberg, Carl Formgren
and Evert Ohlsson
Statens väg- och trafikinstitut (VTI) • Fack • 58101 Linköping
National Road & Traffic Research Institute - Fack - 58101 Linköping - Sweden
Nr108A 1976
108 A
Tire Dimensions
Properties of Wide and Low versus Narrow
and High Tires
by Ulf Sandberg, Carl Formgren
and Evert Ohlsson
ACKNOWLEDGEMENT
This work was done as a part of the project "Tire Noise - Influence of Tire and Road Surface" ("Externt däck buller. Däck- och vägbaneparametrars inflytande")
financed by the Swedish Board for Technical Development (Styrelsen för teknisk utveckling) and run by IFM Bureau of Acoustics Co (IFM Akustikbyrån AB) in cooperation with the Institute.
Page
ABSTRACT I
REFERAT II
1 INTRODUCTION 1
2 PREMISES 2
3 INFLUENCE OF DIMENSION ON TIRE 4
CHARACTERISTICS
3.1 Load capacity 4
3.2 Vehicle handling 4
3.3 Ride characteristics 5
3.4 Friction 7
3.5 Rolling resistance and fuel consumption 8
3.6 Wear 10
3.7 Generation of splash and spray 10
3.8 Road clearance and center of gravity 11
of vehicles
3.9 Space requirements 11
3.10 Available space for brakes 12
4 WHY THIS TREND TOWARDS LOWER ASPECT RATIO 13
5 CONCLUSIONS 14
6 REFERENCES 15
The chapter about vehicle handling is written by Carl Formgren, the chapter about friction by Evert Ohlsson and the other parts by Ulf Sandberg.
I
ABSTRACT
In order to examine the characteristics of a tire of extremely small width and large overall diameter rela tive to recent tire constructions, a study of available literature dealing with dimensional influence on tire performance was made. Relevant experimental data was found to be surprisingly rare, and thus definite con clusions regarding the dimensional influence was difficult to make. The results show that the premised tire constructions have advantages and disadvantages in largely different respects; the perhaps most import ant being decreased cornering stability but increased hydroplaning limits for a high and narrow relative to a low and wide tire. On a total basis, none of the studied dimensional alternatives seems to be remarkably better or worse than the other, and the ultimate choice of dimensions depends on how their respective qualities are valued.The present trend towards lower and wider tires, undoubtedly influenced by "styling" considera tions, therefore should be discuss'ed further as it is not well motivated in every respect.
REFERAT
DÄCKDIMENSIONER
Egenskaper hos breda och låga gentemot smala och höga däck
Med avsikt att undersöka vilka egenskaper som fås för ett bildäck med extremt liten bredd och stor diameter relativt konventionella, moderna däckkonstruktioner, har en litteraturstudie inom ämnesområdet utförts. Användbara experimentella data befanns förekomma
ytterst sparsamt, och således blev det mycket svårt att dra säkra slutsatser angående storleksinflytandet. Resultaten visar att de förutsatta däckkonstruktionerna har för- och nackdelar i vitt skilda avseenden? de kanske viktigaste är försämrade sidkraftsegenskaper men ökade vattenplaningsgränser för ett högt och smalt relativt ett lågt och brett däck. Totalt sett synes inget av de studerade dimensionsalternativen vara klart sämre eller bättre än det andra, och vilket däck man föredrar beror på hur man värderar respektive företräden. Den nuvarande entydiga trenden mot allt lägre och bred are däck, som flera representanter för däcktillverkarna har erkänt vara påverkad av utseendeskäl, förefaller således inte i alla avseenden vara välmotiverad.
INTRODUCTION
Results from the Swedish research project "Tire noise - influence of tire and road surface" indicate that it should be possible to construct tires with less noise generation by accepting a radical change in their
dimensions. A positive effect on noise should be gained if the tires were narrower and higher than normal. This dimensional change is unfortunately opposite to the present trend towards wider and lower tires.
A quiet tire will be worthless for practical use, if it cannot fulfil requirements set also by traffic safety and vehicle dynamics. Therefore it was decided to examine the effects of the presumed dimensional change on important performance characteristics other than noise. This report is a result of a study of available literature on the influence of tire dimen sions on tire performance made within the above men tioned research project.
The purpose was to examine if a quiet tire based on a dimensional change could be a realistic alternative to present, more noisy, tires. The subject has, despite its fundamental importance, been treated to a surpris ingly small extent in literature, and mostly as compari sons of characteristics of tires with different aspect ratios , covering a relatively small dimensional differ ence. Therefore, it is difficult to draw definite
conclusions on the characteristics of the compared tires, which have widely different dimensions, and generally the "conclusions" should be considered as personal opinions based on the studied literature.
s e c t i o n h e i g h t
m aspect ratio = — — — — r— -— s e c t i o n w i d t h
PREMISES
In order to get a concrete starting-point some premises regarding to tires were stated.
The study shall make it possible to predict the relative characteristics of two theoretical tires with the
following dimensions, but else of the same basic con struction and material:
1. "Reference tire". Approximately corresponding to a modern tire for passenger cars.
Aspect ratio 75% Overall radius R Rim radius r (~R/2) Section height h ( ^r) Section width bs Tread width bt Air volume V
The air volume and the section height are the same in both tires; the important differences are the alterna tive tire being 25% narrower and having an overall dia meter 25% higher. The given ratios implies a. simplifi cation to an approximate rectangular or quadratic cross- sectional shape. 2 2. "Alternative tire". Aspect ratio 100% Overall radius 1,25-R Rim radius 1,5-r Section height h Section width °'75*bs Tread width 0,75-bt Air volume V
3
Of course it was not expected that a relevant comparison between tires of that different and idealized dimensions could be found in literature, but notwithstanding it was desirable to try to project results to such a comparison (even as an uncertain estimation), because a smaller dimensional difference would have too little effect on noise generation to be justified.
3 INFLUENCE OF DIMENSIONS ON TIRE CHARACTERISTICS
3.1 Load capacity
The load capacity of a tire is determined mainly by the volume of the enclosed air and the air pressure (Carr /7/). Scandinavian Tire and Rim Organization
(STRO), /24/, states a so-called load formula
where X ^ 0 , 6 and K is a "variable depending on the use and operation of the tires, mainly the speed".
Vukan /29/ and Elliot et al /12/ present the corre
sponding load formula used in the USA (by TRA). It is
far more complicated but on application to the presumed "reference" and "alternative" tires one finds that the load capacity is the same (in a few percent).
Conclusion: The "reference" and "alternative" tires will have essentially the same load capacity provided that their air volume and pressure are the same, which was primarily assumed.
Most authors seem to agree on the advantages of the low profile tire compared to a tire of higher profile in the field of vehicle handling on dry surface, mainly due to a higher cornering stiffness for the tire of low profile. Brooker /6/ for instance writes about development of low section height passenger tires that improved tire stability, and better car handling charac teristics were invariably sensed.
Load = K . (air pressure • air volume)X
5
Roland, Rice and Kakaley /23/ have studied the effects of size, aspect ratio, and wheel dimensions independ ently of construction type and construction materials and vice versa. From the results of their tests can be found that radial ply tires of the 78 series (78 per cent aspect ratio) have 18 to 20 per cent less corner ing stiffness with only 6 to 8 per cent less section width and 0.5 per cent larger overall diameter than radial ply tires of 70 series on 15" rims. Even more so, bias ply tires of the 78 series have about 35 per cent less cornering stiffness with only 5 per cent less section width and 1 per cent larger overall diameter than bias ply tires of 70 series on 14" rims.
The overall diameter of a tire will probably not be of any great significance for vehicle handling in itself, but a large overall diameter will impose higher bending stresses on the rim which for that reason will be heavier, and together with the larger tire it will give a higher unsprung mass which on uneven roads can give worse vehicle handling.
Conclusion: From what has been stated in various reports about the vehicle handling characteristics of tires of different aspect ratio there seems to be little doubt about that low aspect ratio tires give better vehicle handling than tires of high aspect ratio. The evolution of racing car tires will also reflect this opinion.
Ride characteristics
Reports in literature concerning ride characteristics (comfort) of low profile (low aspect ratio) contra high profile tires of bias ply construction seems to be relatively uniform in the opinion that the former give a harsher ride than the latter. One exception is
Strigle /27/ who has compared "wide base" tires of 74% aspect ratio to "conventional" tires of 96% aspect ratio, and found that the 74% tires have a lower spring rate (approx 10-15%) which, according to Strigle, gives a better ride. This, however, must not necessarily be true; the combined effect of the tire and the vehicle suspension should be considered.
Others have noticed a decrease in ride comfort for low
profile tires, as in /2/ and by Bowler / 4 / . Clapson /9/
has compared wheel acceleration in the range 10-500 Hz for bias tires of 95% resp 77% aspect ratio, and found:
1. Vertical transmission is 5-10 dB higher for 77% tires. 2. Longitudinal transmission is approximately the same
for both tire dimensions.
This is due to the higher vertical stiffness for low profile bias tires which is due to lower cord angle. This explanation is also given by Hulswit /17/, Setright /26/ and Hutchinson /18/. The lower cord angle is advantageous for the construction of the wide tires. Brooker /6/ gives another explanation to the decreased comfort; it could be partly due to the smaller overall diameter for low profile tires which can give increased vertical motions on the road.
No relevent data concerning ride comfort for radial tires of different dimensions have been found, but
general discussions by authors reveal that most of them have a similar opinion about the dimensional influence on ride comfort of radial tires. They agree that the increased transmission for low profile tires must be cured by a different design of the vehicles* suspension which has so far been a successful solution.
7
From the above mentioned reports it can be summarized: For bias tires increased width has a significant nega tive influence on ride comfort. An increase in overall diameter may give decreased vertical motions and
therefore better ride comfort.
Conclusion: The "alternative" tire is likely to have the best ride comfort if it is of the bias construction because it is both narrower and has a larger diameter. For radial tires there is no reason to suspect some thing else, but relevant data is missing.
Friction
The effect of the tire dimensions upon friction seems not to be very commonly dealt with in literature. Naturally, t h e r e might be various reasons, but it is probably connected to the difficulties in performing the experiments in such a way that the effect of the dimensions is obtained separately.
It is, however, quite obvious that an increased tire size gives rise to an increased dynamic and viscous hydroplaning speed, i.e., the risk of loosing the grip by hydroplaning is displaced towards a higher speed area. It is assumed that the tire proportions are altered so that the contact area becomes narrower but longer. The influence of the dimensions at hydroplaning is emphasized in among others ref. /I, 22/. However, the influence is not quantified according to what has been found.
In reference /23/ a direct experimental comparison between the frictional qualities for tires of various dimensions (less than 10 per cent variations of the dimensions) has been carried out, showing an improvement 3.4
in friction of about one per cent per each per cent of increase in the diameter, but no attempt has been made to explain the result. It is known, however, - for example according to ref. /16/ and ref. /21/ - that sliding motions at the places of contact, where the tire is transformed from a toroid into a plane surface at the entering of and leaving from the contact surface, is of great importance to friction. Consequently, it might be possible that the relatively less important
flattening of the large tire and the lower sliding motions give rise to the effect shown by the experiment. Less heating of the large tyre (ref. /22/) might have a similar effect (ref. /21/).
In ref. /22/ is also stated, however without being theoretically or experimentally proved, that the friction is maintained when the tire dimensions are increased.
Conclusively there are no indications of any unfavour able effects on friction properties of an increase in diameter and a decrease in width of a tire.
Rolling resistance and fuel consumption
A lot of authors present data on, and discuss the influ ence of tire dimensions (mainly different aspect ratios) on rolling resistance (Hulswit /17/, Davisson /ll/, Carson /8/, Bajer /3/, Curtiss /10/, Floyd /13/, Strigle /27/, Walter and Conant /30/, Glemming and Bowers /15/). They show that rolling resistance is considerably lower for a low profile tire relative to a standard tire. Results range from 30% decrease in rolling resistance for 80% increase in tire width, to 2% for a tire with aspect ratio of 72% relative to one of 84%. The tires compared approximately has the same overall diameter but different widths. A 20% change in rolling resistance generally corresponds to approximately 5% change in total fuel consumption.
9
The general opinion is that it is the smaller deflec tion of the wider tires due to stiffer sidewalls that gives the improvement in rolling resistance. This can be achieved also by increasing the diameter of the rim
(tire section height and profile held constant), which according to Curtiss /10/ will give a significant improvement if for example rim diameter is increased from 12 to 16".
Kummer and Meyer /20/ writes that rolling resistance (hysteresis) can be decreased by altering the contact area to a longer and more narrow form. This is supported by Moore /22/.
Schuring et al /25/ have presented a simple mathematical model for the tire with respect to rolling resistance, but certain important tire constants are assumed to be measured, making it impossible to apply the model for computation of the rolling resistance of the theoretical tires we are interested in. Elliott et al /12/ also have presented a mathematical formula describing rolling resistance as a function of, among other things, tire dimensions. By using this formula on the "reference" and "alternative" tires, the latter (which is narrow and high) was found to have 9% larger rolling resistance. The validity of the formula seems, however, to be un certain .
Pacejka /28/ relates a formula for load deflection as a function of width and height of the tire and rim diameter, telling that deflection is inversely propor
tional to width •\frinTTadius. The formula is claimed
to correlate well with a great variety of tires. Accord ing to it, the "reference" tire should have about 8% smaller deflection than the "alternative" tire. As rolling resistance is strongly related to deflection this supports the prediction by the formula of Elliott et a l .
Conclusion: It seems reasonable to (from the above mentioned literature) believe that the increase in rolling resistance for the narrow relative to the wide tire is, at least partly, compensated by a decrease due to the greater rim and tire diameter. However, a confident quantitative estimation is impossible to make, as no real experimental data is available for the
influence of increased rim or tire diameter.
Wear
For wide tires a special moulding technique, "concave11
or "contour moulding", is applied,giving a flatter tread (increased crown radius). This is favourable because it makes the contact area more independent of inflation. Another advantage concerning wear for low profile tires, is the increased tread area (wider) giving more material to wear off.
According to Moore /22/ a tire with a long and narrow contact area (corresponding to the "alternative tire") will produce increased transverse slip motions towards the rear of the contact so that there is an increase in tread wear.
Brooker /6/ states that low profile tires (bias) wears 15-50% less than "standard" tires. Strigle /27/ reports that a tire 8.00-16.5 wears 10% less than 7.00-17.5. In the former tire the tread area is approximately 10% greater.
Generation of splash and spray
The amount of water that can be picked up by the tire from the road is depending on the width of the track, that is the width of the tire. A wid^r tire can collect more water than a narrow tire, and this will increase
splash and spray. See, for instance, work by Braun /5/ and Koessler et al /19/.
3.6
11
The influence of tire diameter can be discussed as follows: Decreasing tire diameter will increase the
2
radial acceleration of the water drops, v /rr where v = vehicle velocity and r = tire radius. If the adhesion forces are the same, then these will be over come by drops with a smaller mass, thus water spray will consist of smaller but more numerous drops if the tire has a small radius. This is a drawback, because smaller drops will more easily be caught by air turbu lences at the side and back of the vehicle.
Conclusion: The "alternative" tire will probably gener ate less splash and spray than the "reference" tire.
Road clearance and center of gravity of vehicles
Some authors, e.g. Carr /7/, point out that making lower tires (smaller diameter) is avoided because of the road clearance. Instead the rim diameter often is increased for low profile tires. The high "alternative" tire will have no such problems, but instead it will increase the center of gravity of the vehicle to a point higher above the road (for unchanged vehicle design). In order to have the same risk for overturning, conventional trucks and trailers cannot be loaded as high as else. For the "alternative" tire this will
decrease load capacity in the order of 5-10%. Moore /22/ writes that this undoubtedly is one reason for the
success of the lower tires.
Space requirements
For the "alternative" tire the wheel track width can be increased, however the space requirements around the wheels will increase somewhat. The spare tire will require more space. These aspects seems to be rather important for the automobile industry, for example French from Dunlop /14/ writes: "The need to minimize the space occupied by the tyres has been largely respons ible for changes in tyre crossectional shape".
3.8
Available space for brakes
Due to its larger diameter and identical section height compared to the reference tire, the "alterna tive" tire will admit a lot more space for brakes, making brake design much easier. For a given outer diameter on the other hand the trend towards lower aspect ratio has given more space for the brake designers because the rim diameter has usually been
increased (Bajer /3/ and Setright /26/). Brooker /6/
says this is one of the reasons for the popularity of the low profile tires.
Frictional heat from brake drums or discs is more readily dissipated for a tire with a greater diameter
(Moore /22/). 3.10
13
WHY THIS TREND TOWARDS LOWER ASPECT RATIO?
During the past 15 years the aspect ratio of automobile tires has decreased from about 95 % to 50 - 80 % for "modern" tires. Why this? As we see it, these so called low profile tires certainly has important advantages over conventional tires, such as giving better vehicle handling and lower power consumption, but also disadvan tages such as worse riding comfort and greater risk for hydroplaning. Better vehicle handling together with a lower center of gravity and higher speed ratings have been the main reasons for the popularity of the wide low profile tire in racing. This association with racing seems to have influenced our apprehension of these tires as being "high performance". Probably (according to
designers) most of us also think they are more beautiful (or less ugly) than the old tires with a high aspect ratio. The visual appearance has nothing to do with the quality or performance of the tires, still it seems like these aspects have influenced the development very much. Many of the authors presenting these new tires with low aspect ratio also admit that styling has played an important role in the evolution (/2/, Clapson /9/,
Setright /26/, Strigle /27/). Bajer of Ford /3/ writes
it is impossible to decrease the tire diameter because of styling, and Carr /7/ of Goodrich writes "design of an ultra low profile tire, aspect of 55, was motivated primarily by styling".
It would be interesting to know how our automobile tires would have looked like today;if styling had no influence on the development.
CONCLUSIONS
The relative characteristics of the tires of different dimensions can, according to our opinion, be summarized as follows:
Reference tire Alternative tire
(Aspect ratio 75% (Aspect ratio 100%
"Normal" radius Increased radius,
and width) decreased width)
o Improved cornering o Increased dynamic and
stability viscous hydroplaning
limits
o Less tread wear o Improved security on roads
covered with snow slush
o Lower center of o Improved ride (comfort)
gravity performance
o Decreased space o Less splash and spray
requirements generation
o Somewhat lower un- o Easier brake design
sprung mass
This makes it difficult to judge which tire has the best overall characteristics. As each of them is advantageous in largely different respects, an appraisement of which tire should be recommended, should be made according to its expected use. Therefore a tire of small width and large overall diameter undoubtedly might be motivated under certain circumstances, for example requiring minimum noise generation. Further it is not excluded that such a tire could, despite a major drawback such as decreased cornering stability, be generally accepted or even preferred. Here, a discussion without styling considerations, as well as a thorough experimental investigation is recommended.
15
REFERENCES
References with assigned numbers are referred to in the text. The others were found to be interesting for this work, but are not specifically referred.
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/3/ BAJER, J: Low Aspect Ratio Passenger Car Tire from a
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983B. Soc of Automotive Eng., New York, 1965.
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Properties and their Effect on Braking Performance. SAE paper 741137. Soc of Automotive Eng., New York, 1974 .
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17
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