Tire Upgrades

Tires and shock absorbers play an important part in road grip, and it is road grip that ultimately allows the car to be accelerated and decelerated while also providing predictable handling and stability. As such road grip is an important element in car safety. Upgrading your brakes, for example, is a useless endeavor if your car has little grip due to worn tires or worn shock absorbers. This is obvious as worn tires with low grip may lock up and skid when you apply brakes, and the wheels need to be able to rotate to slow the car down. Similarly when the tires are bouncing through the air when you brake over bumps, road grip is lost and braking becomes ineffective.

A simple safety precaution would be to check your tires' thread depth and tire wear, but on modified cars with extra horse power, the grip provided by standard tires may not be sufficient even if the tires are in good condition. On modified cars you may want to fit larger or wider tires that provide a larger contact area with the road, or low profile tires that have shorter, less flexible sidewalls and provide more precise handling. You may also want to fit a tire that has a higher maximum speed rating and higher load rating. If you do opt for larger or wider tires, you will need to upgrade your shock absorbers (or dampers as they are also called) and possibly your springs to handle the increased unsprung weight.

But first you need to know what tires your car has, what tires are suitable for your car, and the effect of changing your tires.

Understanding Tire Codes

Ever wondered what the code on your tire wall meant? Wonder no more. All tires have a standardized alpha-numerical code that is molded on the sidewall. This tire code indicates the width of the thread area, the height of the sidewall relative to the width (also known as the aspect ratio), the construction type of tire, the size of the rim the tire will fit, the tire's load carrying capacity (or load index), and the tire's speed rating. A tire with the code 205/55 R 15 91V, for example, has a thread width of 205 mm and an aspect ratio of 55%. The aspect ratio indicated the height of the sidewall as a percentage of the thread width. In this case the sidewall height is 55% of 205 mm, or 112.75 mm. The 'R' in 205/55 R 15 91V indicates the construction type of tire, and can be 'R' for radial, 'B' for cross-ply with belted bias, and 'D' for cross-ply with diagonal bias, though B and D are becoming increasingly rare. Radial tires are better for traction and grip. The '15' in 205/55 R 15 91V indicates the size of the rim (in inches) that the tire is designed for, which is 15 in. in this case and the '91V' indicates the load carrying capacity and the speed rating as indicated below in Table 1 and Table 2.

TABLE 1: TIRE LOAD INDICES
IndexLoad
51195 Kg
52200 Kg
53206 Kg
54212 Kg
55218 Kg
56224 Kg
57230 Kg
58236 Kg
59243 Kg
60250 Kg
61257 Kg
62265 Kg
63272 Kg
64280 Kg
IndexLoad
65290 Kg
66300 Kg
67307 Kg
68315 Kg
69325 Kg
70335 Kg
71345 Kg
72355 Kg
73365 Kg
74375 Kg
75387 Kg
76400 Kg
77412 Kg
78425 Kg
IndexLoad
79437 Kg
80450 Kg
81462 Kg
82475 Kg
83487 Kg
84500 Kg
85515 Kg
86530 Kg
87545 Kg
88560 Kg
89580 Kg
90600 Kg
91615 Kg
92630 Kg
IndexLoad
93650 Kg
94670 Kg
95690 Kg
96710 Kg
97730 Kg
98750 Kg
99775 Kg
100800 Kg
101825 Kg
102850 Kg
103875 Kg
104900 Kg
105925 Kg
106950 Kg
IndexLoad
107975 Kg
1081000 Kg
1091030 Kg
1101060 Kg
1111090 Kg
1121120 Kg
1131150 Kg
1141180 Kg
1151215 Kg
1161250 Kg
1171285 Kg
1181320 Kg
1191360 Kg
1201400 Kg
TABLE 2: TIRE SPEED RATING
RatingSpeed
H130 mph (210 Km/h)
L75 mph (120 Km/h)
M81 mph (130 Km/h)
N87 mph (140 Km/h)
P95 mph (150 Km/h)
Q100 mph (160 Km/h)
R105 mph (170 Km/h)
S113 mph (180 Km/h)
T118 mph (190 Km/h)
U125 mph (200 Km/h)
V150 mph (240 Km/h)
W168 mph (270 Km/h)
Y186 mph (300 Km/h)
Z150+ mph (240+ Km/h)

The load index is the maximum weight each tire can carry. So, if your car weighs 2,000 Kg with 55% weight bias on the front wheels and 45% on the rear, then the front wheels should have a minimum load index of 88 as each front wheel would need to carry 550 Kg each (2,000 × 55% ÷ 2), assuming an even spread on the two front tires. Add another 500 Kg in weight and each wheel would need to carry an additional 125 Kg on average, which will require a tire with a load index of 95!

The speed rating indicates the maximum speed that the tire can safely sustain for a ten minute period. Thus an H rated tire is good for up to 130 mph but not for more than 10 minutes at 130 mph! In other words, if your modified race car has a top speed of 130 mph, you'd be better off with a V rated tire capable of up to 150 mph than an H rated tire. Z rated tires are rated for over 150 mph but are being replaced by W rated tires (for up to 168 mph) and Y rated tires (for up to 186 mph).

Wider and/or Larger Tires?

One method of improving tire grip is to increase the contact area between the tire and the road. This can be accomplished by fitting a tire with a larger circumference (i.e. a larger or oversized tire) or fitting a tire with a wider thread area. A tire with a larger circumference has a larger longitudinal contact area, which will provide better straight line stability but will affect overall gearing and will increase the ride height and the car's center of gravity, and will also increase the rolling mass the bakes will have to overcome. There is nothing that can be done to rectify the increased ride height and center of gravity; but the drivetrain gearing can be rectified by fitting a differential with a higher diff ratio. The increased rolling mass that the brakes need to overcome can be rectified by upgrading the brakes. However, even though a larger tire may be able to fit under the wheel arches when the car is static, it may knock against the body or suspension components when suspension travel occurs and during cornering. Stiffer suspension springs with less travel and stiff shock absorbers may compensate for the tire knocking against the body during suspension travel but will result in a harder ride.

Wider tires, on the other hand, have a wider contact area but will need to have a smaller aspect ratio to retain the overall circumference of the wheel. Thus going from a 185/60 R 14 91H tire to a tire that is 215 mm wide will require a tire with an aspect ratio of 50% as a 185/60 R 14 91H tire has a sidewall height of 111 mm (60% of 185 mm) while a 215/50 R 14 91H will have a sidewall height of 107.5 mm. A tire with a similar overall circumference to the standard tire will not interfere with the body or suspension parts during suspension travel but will be slightly heavier, and will increase the rolling mass that the brakes need to overcome, but not to the same extent that larger tires do. However, wider tires are also prone to knocking against the body or suspension parts during cornering. This can be overcome by fitting a wider wheel arch and fitting a spacer on the wheel hubs to allow the wheels to be mounted further away from the suspension parts. This will, however, place greater stress on the front wheel bearings and on the steering mechanism.

Low Profile Tires

A tire with a low sidewall is referred to as a low profile tire. These tires provide more precise cornering as the lower sidewall has stiffer sidewalls with less inherent elasticity. This means that the tire will suffer less deflection during cornering and will provide better lateral stability. However, reduced flexibility is not ideal for handling in the snow where flexibility is actually desirable. The stiffer sidewall will also result in a harder ride, especially over bumps, and will transmit more road noise. There is thus a bit of compromise involved when deciding on low profile tires but, under normal road conditions, the lateral stability provided by low profile tires makes them a safer option while improving handling characteristics. A low profile tire can easily be achieved by fitting a larger diameter wheel rim. Thus, by going from a 14 in. wheel to a 16 in. wheel you can fit a tire with a sidewall that is approximately 1 in. (24.5 mm) lower. This means that if your standard tire is a 185/60 R 14 91H tire, which has a sidewall height of 111 mm, you can fit a 185/45 R 1 91H tire with a 83 mm sidewall (28 mm lower). Contrary to popular belief, fitting a wider tire does not allow you to fit low profile tires without affecting your car's gearing. Yes, the aspect ratio on a wider tire would be less but your sidewall will still have the same approximate height otherwise your car's gearing will be out. Increasing the wheel size, in what is sometimes called inching up, in thus the only way to fit low profile tires without affecting your car's gearing.

Regardless of what size tires you fit, you check your tire pressure regularly to ensure optimal grip. With underinflated tires, the cars weight is concentrated on the tread near the sidewalls rather than being spread out across the full width of the tread. This causes the sidewalls to flex when the tire rolls and results in heat buildup in the sidewall. Excessive heat buildup and the constant flexing of the sidewall may lead to tire bursts! Overinflated tires are also prone to tire bursts as the air in the tire does heat up under normal conditions. If the tire is overinflated the normal heating of the air in the tire will increase the tire pressure further and may result in a tire burst. Overinflated tires also do not have the optimum tread contact and will give a stiff ride. You should also ensure that your wheels and tires are balanced and aligned correctly.