Weight vs performance - The basics

Interestingly many people who want to increase the performance of their car overlook the importance of the weight issue.

One point of consideration is adding weight. Whenever we purchase accessories they do literally weigh in. Throughout this topic the effect on 2 examples will illustrate the impact on weight adding to the car as well as reducing weight.

Additionally the options with different engines in some models and the comparison between them shall be looked at.

The different aspect to optimize the performance of a given car are:

1. Over all weight
2. Weight balance
3. Center of gravity
4. Unsprung weight
5. Rotating mass

Over all weight

A car with 910kg a few bits of clutter and a driver tips the scale with 1,000kg, At 60bhp this results in 60bhp per ton. I have met quite a few people who wanted to get more performance out of such a car, but at the same time fit some ICE. After all the work done the performance was not any better or only marginal better than having done nothing to the car. The last system I removed from a smaller car was over 70kg.

The 60bhp car now weighs with driver 1,070kg, which means that the power to weight ratio decreases to 56.075bhp. To recoup the power to weight loss, which would allow the same straight line acceleration as the standard car the heavier the power output needs to be increased by 7% or over 4bhp. This means the car will need for 0-60mph/0-100kph a good second longer.

In a 1,200kg car (with driver) with a 150bhp power output 70kg ICE have a lesser effect in % on the straight line acceleration. However the absolute figure is quite sobering. The power to weight ratio decreases from the 125bhp per ton to 118.11bhp per ton. This is close to 7bhp per ton! To regain the performance loss 8.89bhp have to be found.

Let's take both cars and remove 40kg and then another 60kg, which can be achieved without losing any driving comfort.

The 60bhp car will have a power to weight ratio of 62.5 and 66.67bhp per ton respectively and the 150bhp car 129.31 and 136.36bhp per ton respectively.

In other words this means that the 60bhp car would need at standard weight to have a power increase to 62.5 and 66.67bhp and the 150bhp car 155.17 and 163.63bhp respectively.

Removing weight does not only aid straight line speed,but also allows to take more speed through bends improving lap time and B-road blast times considerably.

And an added effect is a lower fuel consumption.

Some people are convinced it is not worth tuning a 60bhp car and advise to go to the same model with a more powerful engine. Well, let us then take the top end version of this model. It tips the scale at1150 with driver and power output is 136bhp. This makes it 118.26bhp per ton. If the 60bhp is a Fiat Punto Mk2 with a 1.2 8v engine then it is easy to extract 80 and with some more effort 95bhp before it gets expensive. Removing 150kg (partly replacing by better material) is not something out of this world without losing driving comfort. However, we first remove 50kg from the small engine powered version.

At 80bhp removing 50kg results in over 84bhp per ton and at 95bhp it is as much as 100bhp per ton.

Removing 150kg at 80bhp would result in over 94bhp per ton and at 95 in over 111bhp per ton.

The straight line acceleration of the latter modification is slightly slower than the 136bhp car. However on a B-road blast it will be quicker due to the much lower weight.

Whether one goes for one or the other solution to enjoy b-road blasts or traffic light starts is a matter of preference rather than a discussion on whether it is worth it.

Weight balance

Balancing the weight is of a lesser importance when it come to pure straight line acceleration. However, this changes dramatically when cornering comes into play.

Unfortunately there is no universal set-up. The occupants of the vehicle have a big impact on the balance. Even if the driver (and front seat passenger) sit in the middle (between front and rear axle), there is the balance between driver's side and passenger's side. This means optimization can only be done for the preferred use.

Because most of the front wheel driven cars are rather front heavy due to the fact that everything needed is for simplicity there in the front naturally they are front heavy. To balance this out a lot of weight is added to the rear. Some more luxurious cars have the battery in the boot for this reason.

In practical terms it means if the best balance is 50:50 and the car has got a battery with a weight of 13kg then the weight that can be removed from the rear after moving the battery to the rear - is 26kg.

covering the basics in this thread does not allow to go to deep into the issue. However it should be mentioned in this context that the weight that needs balancing should be within the four corner points of the wheels. Any weight over the the axle (or wheel base) is as unwanted as the weight over the track.

Additionally the higher the weight is located the more effort is made to remove as much as possible of it.

Center of gravity

The perfect balance brings the center of gravity into the right line. However, it is necessary to aim for a low center of gravity, which means that preferably no weight is above the ground. Naturally this is not possible, which calls for a compromise. The compromise is to remove as much weight as possible from as high as possible as well as moving whatever is needed to as low as possible. The obvious result is increased cornering speed and less body roll.
Obviously the center of gravity effect can artificially be enhanced by aerodynamic measures creating a lot of down force enabling higher cornering speeds. But this is a separate subject.

Unsprung weight

If vibration, movement and uneven surfaces weren't an issue when driving unsprung mass would consist only of 2 problems for the performance on the road, which is because of the weight of the unsprung mass a reduction obviously would decrease over all weight aiding acceleration and cornering.

Unfortunately in the real world are other factors influencing too. Unsprung weight does not aid traction. To improve this side of the performance gain any reduction in unsprung mass will help. Due to the fact that suspension arrangements are rather complicated the weight savings won't be substantial except the rims ans tires. On some cars 15kg per corner can be saved.

Assuming alloy rims are lighter than steel rims is not always justified. In many cases steel rims are lighter. Pressed aluminium rims seem to be the lightest in these days.

Rotating mass

Rotating mass is not too good for acceleration. Most people who have fitted a lightened flywheel will tell you. It definitively has got this advantage, but has got some disadvantages too (not part of the discussion in this thread).

However, the flywheel is not the only point to look at. The discs, drums and wheels have their effect on acceleration too.

The more rotating mass can be found the further away from the center of the wheel. the more force needs to be applied to rotate this mass. Ultra wide rims with ultra wide tires will not only lower top speed and acceleration because of the added drag, but will lengthen the acceleration time because of the extra mass to rotate. This is the reason why a lot of small cars have smaller rims and are much quicker off the line then we would have thought. Bigger brake discs will also have their effect as will have some other rotating components.

To get the best out of any car just on the weight issue means a lot of effort and in many instances a lot of skill, which is the reason why racing can be very expensive. However, it is worth the effort.