Many people have a new year’s resolution to go on a diet to try to lose weight. Here at Motorsport Essentials we’re fans of a rather different form of weight saving, namely losing weight from your car to help you go faster! This article will be discussing the benefits of weight saving and where best to start to put your car on a diet.
Let’s start with the basics; the key word to all weight saving, which you’ll see crop up in multiple forms throughout this article, is inertia. The definition of inertia is how resistant an object is to a change in its state of motion, such as changing speed and direction. The heavier an object is, the more inertia it has and therefore the harder it is to accelerate and decelerate. It is easy to understand this – throw a punch with a clenched fist and you can move your hand very fast. Hold a 10kg dumbell and try and throw the punch and the movement of your hand will be far slower.
What does this mean in practice for a car? Simply put the lighter a car is, the faster it should accelerate for a given power output, this is of course assuming the vehicle can use all of its power in the first place i.e. it isn’t traction limited. In addition to accelerating faster, the reduction in inertia should also mean that the car can decelerate faster for a given amount of braking force. This is interesting as under braking a car is usually grip limited (you brake up to the limit of grip rather than the force that the brakes can actually exert). This means that for a lighter car, with lower inertia and therefore momentum, you actually just use less energy with the brakes to reach the same limit of grip. You cannot decelerate any faster as you are grip limited anyway however you will experience less brake wear and heat generated through the brakes in a given braking zone.
So far we have discussed accelerations in the car’s direction of travel. Now we can consider lateral accelerations (cornering). If we think back to the initial definition of inertia we know that by reducing the inertia (weight) we can make our car less resistant to changes in direction as well as speed. This means that our car should be keener to turn and change direction (through a chicane for example) as it gets lighter. This is clearly of benefit in terms of lap time as the car will be much more positive to drive and less likely to “push on” into a corner.
All weight in a car is not considered equal. It is usually split into two subcategories – sprung and unsprung.
The simple textbook definition of unsprung weight is any mass of the car that is not suspended by the car’s suspension. In practice this means the wheels, tyres, brake discs and calipers, hubs and control arms of a car. So why do we want to reduce it?
First unsprung weight should also be separated into two categories; rotating unsprung weight (wheels, brake discs etc.) and static unsprung weight (hubs, control arms, brake calipers) this is important as they both have separate effects on the car’s performance.
Let’s first consider rotating unsprung weight. The main benefit of reducing this is an improvement in acceleration. By reducing the mass that has to be rotated to accelerate the car (the wheel and brake disc) there is less rotational inertia, this means, assuming you aren’t grip limited, that a higher rate of acceleration can be achieved as the wheels are less resistant to being accelerated. It must be noted that this won’t produce a night and day difference in acceleration however every little helps, especially on track! There is a downside or tradeoff to this reduction however, by reducing the rotational inertia of the wheels you are essentially making them able to decelerate faster. This has the potential to make the wheels easier to lock and modulation of the brakes on the limit harder. This is not necessarily undesirable but the driver has to be aware of this trait as this can make wheel locking easier. Rotational unsprung weight should be the first area to be looked at to reduce. This is because the very edge of a wheel is actually rotating at a far higher velocity than the car itself is moving. This means that any weight in this region has far greater affect on the acceleration than that of the sprung car itself.
Now let’s think about static unsprung weight. The main benefit to reducing this is an increase in grip and traction. Essentially by lowering the unsprung mass, the amount of work that the springs and dampers of the car have to do to maintain good contact of the tyres with the road is reduced. This is because higher unsprung weight again means greater inertia, making the unsprung components harder to accelerate and decelerate. This increases the time that it takes for the suspension to react to vertical accelerations from the wheels by, for example, hitting a bump on the road or track. In a car with a very light unsprung assembly, when the wheel hits a bump it can move upwards over the bump very fast and back down, remaining in contact with the track surface. A heavy unsprung assembly will struggle to react as quickly, and may leave the surface momentarily.
So why don’t manufacturers reduce unsprung mass as much as possible from the factory? Well firstly they do. Manufacturers will aim to reduce unsprung weight wherever possible as it gives a significant performance benefit. However whenever a car is being designed, cost is a large factor, there’s no point making a car if it doesn’t make money. Manufacturers have to carefully consider the cost implications of any engineering decision that is made. This is why many suspension components are made of steel rather than aluminium or magnesium. Steel is simply cheaper and easier to manufacture in large volumes making it a “no brainer” for manufacturers to take this route. That being said in recent times more manufacturers, especially at the high end, are moving towards aluminium as manufacturing costs are reduced and weight is of greater consideration than the past.
So the next thing to consider is how can we actually lose weight from our car? The first place to consider on any car (road or track car) is the spare wheel and any toolkit that would come as standard. Simply removing these parts will give a good reduction in weight for minimal effort when venturing out on track especially. The next simplest weight reduction that can be achieved is the removal of rear seats and associated trim. This can generally give a weight saving of in excess of 10kg with minimal tools and a few hours of your time. This is also good as it is easily reversible should you want to put a car back to “standard” or simply use them in future.
After those couple of free solutions to reduce weight the next methods are slightly more involved/compromised, especially for road use. Changing one or both front seats for more sports oriented options not only gives you an improvement in driver environment and connection to the car, but it also will generally give a good weight saving. Dependent on the car as much as 30kg can be saved by changing from OEM to a good quality bucket seats in the front.
The next weight savings that can be gained easily are in the unsprung area although this of course still contributes to an overall weight saving. Wheels are a simple way to lose weight from a car that almost anyone can do themselves. Savings of 10kg or more can be made by changing to a good set of lightweight racing wheels on some cars. This is the best bang for buck weight reduction you can get, because as previously mentioned, the rotating weight has a larger impact of overall acceleration than weight in any other part of the car. This being said I’d advise to steer clear of magnesium wheels in general, especially if your car is a road car. This is because a magnesium wheel is designed for racing and for being as light as possible. This means they aren’t designed to cover any sort of miles and can be easily buckled or cracked during use. This isn’t such an issue for a race car where you may change the wheels every season anyway but not so good for your road car where you do 20,000 miles a year!
Another area where unsprung weight can be reduced is the braking system. A good reduction can be made by changing from OEM brakes to an aluminium monoblock caliper, although generally this means it’s necessary to switch to a floating brake disc to allow for the fact that the caliper is rigid. Another region of the braking system where a weight saving can be gained is the brake discs. This can be done using a floating rotor design with a central aluminium bell, reducing the amount of iron in the disc itself, thus giving a weight reduction. The most extreme example of reducing unsprung mass however is ceramic braking systems. Not only do ceramic brakes improve braking performance, especially in regards to heat resistance, they also give a drastic weight reduction of again over 10kg.
Once you don’t have to consider road worthiness, even more extreme approaches can be made for race cars. Replacing glass windows for plastic can give a good weight saving, although it’s generally inadvisable on a road car as the plastic scratches more easily than glass and can go cloudy after a period of time. Another method of weight saving we employ on racecars is the “skinning” of body panels. This involves cutting out the inner skins of body panels and doors to reduce their weight. This causes a reduction to structural integrity of these parts however this generally isn’t much of a concern in a racecar as there’s a rollcage to protect the driver in the case of impacts. The final similar method that can be used is replacing the actual body panels of a car. Replacing steel panels with fibreglass or carbon fibre is another good way to give a weight reduction, although it is generally expensive to do and even more expensive if you damage them!
So with all of that in mind go away and get thinking of ways to put your car on a diet! It’s generally the cheapest and easiest way to improve the all round performance of your car.
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