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This article was first published in 1998.
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While we often think of how adding weight to a car slows down acceleration and increases braking distances, it's pretty seldom that any consideration is given to the mass of the wheels and tyres. These weights are important in two regards - unsprung mass and rotating inertia.
The unsprung mass refers to the mass of the tyres, wheels, brakes, part of the suspension linkages and the other components that move vertically with the wheels. Unsprung mass in a typical car can comprise as much as 15 per cent of the overall vehicle mass. Leaving aside the springiness of the tyres, these masses move in direct accordance with the road surface.
The ratio between unsprung and sprung mass is important because the vertical accelerative forces imparted to the unsprung mass by road bumps must be controlled by the sprung mass. When a tyre encounters a bump, the wheel is accelerated vertically at a speed dependent on the characteristics of the tyre, the size of the bump, and the forward speed of the car.
The greater the accelerated mass (the unsprung weight), The greater the kinetic energy that must be absorbed by the suspension if the sprung mass (the body of the car) isn't also to be vertically accelerated. Keeping unsprung mass as low as possible therefore benefits ride. If the ratio of unsprung and sprung mass is poor, the tyre will also not be as firmly pushed down against the road over bumpy surfaces. Thus not only will ride suffer, but also roadholding will be decreased.
Inertia is the property of an object to resist change. So the inertia of a car needs to be overcome to accelerate it. Once it's moving, its inertia will also need to be overcome to slow it! More precisely, inertia is "the property of matter by which it remains at rest or in uniform motion unless acted upon by an external force." So, what's that got to do with the wheels and tyres, except in the fact that they're part of the weight of the car?
Unlike most parts of the car, the wheels and tyres are rotationally accelerated when you put your foot down. In other words, they need the input of power to spin them up to speed. Like miniature flywheels, the heavier that they are, The greater the power required to change their speed. In addition to the wheels and tyres, the brake discs or drums, and driveshafts also rotate at wheel speed. Spinning even quicker are assorted gearbox bits and (in RWD and 4WD cars) the tailshaft. Finally, the crankshaft, harmonic balancer, flywheel and clutch all spin at engine speed - the very quickest rpm. In lower gears the rotating components are required to accelerate in speed much faster, meaning that that their inertial effects are more pronounced.
So what are the practical implications of all of this? US magazine Sport Compact Car found some of these effects when their late model Honda Civic was fitted with an upgraded wheel/tyre package. The starting point was a set of 185/65 Dunlops on 14 inch steel wheels. Each wheel/tyre weighed 15.5kg. They then went to 205/40 Nitto tyres on 17 inch alloys, which increased each corner's mass to 19.5kg. This 4kg (26 per cent) per wheel increase in mass was enough to drop the power measured at the wheels on a Dynojet chassis dyno by nearly 5 per cent! This means that even a kilogram added to the rotating assembly is important - each kilogram that was added to the Civic's individual wheel/tyre mass decreased power at the wheels by over 1 per cent.
Upsizing Wheels & Tyres
Let's say that you're running 15 inch steel rims with 205/65 tyres. (For Australian readers - a 'police' wheel and tyre set on a VL Commodore.) The 15 x 6 steel rim weighs 9.5kg and a typical 205/65 tyre just under 10kg. That means that each corner weighs in at 19.5kg.
You decide to keep the tyres but swap to factory 15 inch alloys (from the Calais). These weigh 8kg each (total = 18kg), so the tyre/wheel combo has dropped in mass by 1.5kg or 8 per cent.
You're happy with that for a while then you decide to upgrade to 225/50 16s. You go for a ROH Reflex rim which is a light 8kg, matching it with a Yokohama S1-Z that weighs in at 10.7kg. That means that your new wheels/tyres have a combined mass each corner of 18.7kg - you've gained only a little bit of weight (0.7kg) as well as getting a good tyre upgrade.
Always greedy, you then fit 235/45s on 17 x 8.5 ROH Z5s, each with a mass of 10.1kg. With Yokohama A520s fitted (each weighing 11.9kg) the mass each corner is 22kg - up by over 20 per cent from the alloy wheel starting point. This will quite definitely reduce the power available to give linear acceleration to the car....
For further thought, we measured the masses of 19 wheels and 12 tyres, and found major variations between both different sizes and different models of wheels and tyres. It might just be worth taking along a cheap and simple spring balance when thinking about buying wheels and tyres....
Wheel Name |
Wheel Size |
Wheel Mass
(kg) |
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Viper Legarno |
15x6 |
8 |
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VL Police Rims |
15x6 |
9.5 |
|
VL Calais (alloy) |
15x6 |
8 |
|
Subaru Liberty (steel wheel) |
15x6 |
8.5 |
|
Subaru Liberty (alloy wheel) |
15x6 |
7 |
|
ROH Gambler |
15x7 |
5.5 |
|
Simmons F15 |
15x7 |
6.1 |
|
Cheviot Armalite |
15x7 |
6.5 |
|
Polished Magnum |
15x6 |
6 |
|
Subaru WRX (alloy) |
16x7 |
7 |
|
ROH Reflex |
16x7 |
8 |
|
Viper A2 |
16x7 |
8 |
|
Simmons F16 |
16x7 |
8 |
|
BBS |
16x6.5 |
9.5 |
|
Speedy Cluster |
16x7 |
9.2 |
|
Simmons F17 |
17x7 |
9.2 |
|
ROH Z5 |
17x8.5 |
10.1 |
|
ROH Chicane |
17x12 |
10 |
|
Azev |
20x10 |
14.2 |
Tyre Name |
Tyre Size |
Tyre Mass
(kg) |
|
Yokohama S306 |
205/60 14 |
8 |
|
Yokohama A509 |
205/65 15 |
9.9 |
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Yokohama A509 |
225/50 16 |
10.7 |
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Yokohama S1-Z |
225/50 16 |
10.7 |
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Kelly Charger HP Plus |
225/50 16 |
11.2 |
|
Yokohama A032 (competition) |
245/45 16 |
10 |
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Goodyear Eagle F1 |
205/45 16 |
8.9 |
|
Pirelli P700 |
205/50 16 |
9.1 |
|
Kelly Charger HP Plus |
215/45 17 |
9.2 |
|
Yokohama A520 |
235/45 17 |
11.9 |
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Falken GRB |
235/45 17 |
11.2 |
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Dunlop Sport 9000 |
235/45 17 |
11.1 |
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