Calculating Design Weights
One aspect of the SVA test that can cause confusion is the calculation of Design Weights. A quick glance at any modern mass-produced car, will reveal a plate with a set of weights marked on it – usually part of the manufacturer’s VIN or 'chassis' plate. The four weights are listed in an internationally agreed order:
Maximum permitted vehicle weight (sometimes called 'Gross Vehicle Weight')
Maximum permitted vehicle plus trailer weight (sometimes called 'Gross Train Weight')
Maximum permitted front axle weight
Maximum permitted rear axle weight.
The first is the maximum that the vehicle is allowed to weigh when fully loaded. It is always (on modern cars) expressed in kilograms.
The second, is the maximum that the vehicle and any trailer that it is pulling can weigh. Sometimes, when this is checked against the maximum trailer weight in the handbook and the maximum vehicle weight, it will be found that the two added together are greater than the value shown on the vehicle’s VIN or chassis plate. This simply means that the fully-loaded car can’t tow as heavy a trailer as it could if it were more lightly loaded. For example, if the maximum permitted vehicle weight is 2500kg and the Gross Train Weight is 3000kg but the maximum permitted trailer weight is 1000kg, it just means that you can’t load the car up to 2500kg and then put a 1000kg trailer on the back – you’d need to take 500kg out of the car if you wanted to tow a 1000kg trailer – or you could keep the extra 500kg in the car and only tow a 500kg trailer.
The maximum permitted front axle weight is the most weight that can be put on the road by the front axle, and the maximum permitted rear axle weight is the most weight that can be put on the road by the rear axle. NOTE that when looking at the vehicle’s plate, it is quite common to see that the maximum permitted front and rear axle weights, when added together, come to MORE than the maximum permitted vehicle weight. This simply means that you can’t load the front and rear axles to their respective maximum weights simultaneously. For example, if you have a Gross Vehicle Weight of 2000kg and a maximum front axle weight of 1100kg and a maximum rear axle weight of 1200kg, you can’t load the vehicle up to 2300kg – you’d have to reduce the weight on one or both axles until the total was less than the Gross Vehicle Weight. You could, however, carry something very heavy in the back provided the weight shared by the rear tyres didn’t come to more than 1200kg but you’d then have to make sure that the weight carried by the front tyres at the same time didn’t exceed 800kg.
It is these weights which the police can prosecute you for exceeding!
The object of this part of the SVA test is to check that the car has sufficient load carrying capacity to cater for it's fully-loaded weight. The fully-loaded weight MUST be AT LEAST the empty weight of the car (with full tank of fuel) PLUS 75kg times the number of seats in it. This is because the authorities make the assumption that an adult weighs 75kg and you MUST be able to carry at least that much for every seat you have in the vehicle. (in fact, they make the assumption that each adult weighs 68kg and comes with 7kg of luggage).
When you fill in the SVA application form, you need to declare the 'Design Weights' for each axle and for the car. These will be the maximum permitted weights in each case. If at all possible, it is best to obtain these design weights from the kit manufacturer. VOSA mainly uses these to carry out the brake test and tyre inspection so your manufacturer ought to be able to provide data that will ensure a pass. If you have built the car substantially differently to the way stipulated in the build manual (e.g. an engine option not covered by the manufacturer) you will have to provide your own weights.
The starting point for calculating design weights is the weight of the empty car with a full tank of fuel, spare wheel (if fitted) jack, wheelbrace, screenwash, coolant engine oil etc. Basically, everything needed to drive the car. This, of course, is difficult to measure accurately unless you have access to corner weight scales. It is also difficult to 'guess' the weight accurately and an incorrect guess could result in an SVA fail once the passenger weights are added. VOSA will weigh the car in this condition during the test. If you want to get your own 'empty' weight values, it might be possible to trailer the car to a public weighbridge or a local scrap metal merchant and ask if the car can be put on the weighbridge. Often this only costs a few pounds. The weighbridge ought to be pretty accurate but as they are generally used for weighing lorries full of scrap, they are unlikely to weigh more finely than +/- 10kg.
When measuring the car’s axle weights, you will need to park the car with only its front wheels on the weighbridge and note the weight (empty front axle weight). You will then have to do the same with only the rear wheels on the weighbridge to get the empty rear axle weight. BE SURE to have nobody sitting in the car or standing on the weighbridge during these measurements! As a quick check, you can always push the whole car on to the weighbridge and get a third weight – this should be the same as the total of the first two weights (give-or-take a bit for the accuracy and resolution of the weighbridge).
The next stage is to take the following measurements, as accurately as possible, using a tape measure.
Distance 'A' - is the distance from a convenient datum point to the centre of the front wheel. It is often easiest to park the car up against a wall and measure from the wall but this is not essential, as long as the same datum point is used for all measurements. It is also worth taking the same measurement on each side of the vehicle and averaging the two so that if the front wheels aren’t exactly straight ahead or the car isn’t exactly square to the wall, the discrepancy should be averaged-out.
Distance 'B' - is from the datum point (wall) to a point where VOSA considers the weight of the occupant would be concentrated. For an adjustable seat, it is 250mm FORWARD of the point where the backrest of the front seats meet the cushions of the front seats. If the seats are not adjustable, the measurement is to a point 200mm FORWARD of where the backrest meets the cushion.
NOTE: The seats need to be fully rearwards (if adjustable) for this measurement. In some cars, the driver and passenger seats might not have the same travel. If this is the case, you will need to do each seat separately and call one of them B and the other one C. Subsequent measurements will therefore need more letters!
Distance C - is from the datum point (wall) to a point where VOSA considers the weight of the occupant would be concentrated. For an adjustable seat, it is 250mm FORWARD of the point where the backrest of the rear seats meet the cushions of the rear seats. If the seats are not adjustable, the measurement is to a point 200mm FORWARD of where the backrest meets the cushion. As with the front seats, there may be more than one rear seat and they may be adjustable. In either of these cases, more measurements will have to be taken and more letters will have to be used.
Distance D - is from the datum point (wall) to the centre of the rear axle.
Distance E - is the distance from the wall to the centre of the luggage area. In a car with no luggage area, this may be left out but instead of putting 68kg in each seat and 7kg per occupant in the luggage area, it will be necessary to put 75kg in each seat during the calculations.
An example is featured below so that the basic calculations can be done automatically for the most common configurations of kitcar (one or two rows of seats and all the seats in any one row having the same amount of travel). For other configurations of car, the calculations will have to be carried out manually as shown in the examples below.
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
Distance (A) |
75 |
|
Distance (B) (Remember the 250/200mm correction factor!) |
180 |
|
Distance (C) (Remember the 250/200mm correction factor!) |
0 |
|
Distance (D) |
255 |
|
Distance (E) |
310 |
|
|
|
|
Weight on front axle with empty car and full tank of fuel (kg) |
550 |
|
Weight on rear axle with empty car and full tank of fuel (kg) |
450 |
|
|
|
|
Number of seating positions in the first row of seats |
2 |
|
Number of seating positions in the second row of seats |
0 |
|
|
|
|
Total additional load on rear axle |
97.6 |
|
Total additional load on front axle |
52.4 |
|
Total additional load |
150 |
|
|
|
|
MINIMUM front axle "design weight" that can be declared |
602.4 |
|
MINIMUM rear axle "design weight" that can be declared |
547.6 |
|
MINIMUM "Gross Vehicle Weight" that can be declared |
1150 |
EXAMPLE 1.
Two-seater car with a luggage area behind the seats. Both seats have the same rearmost position.
Distance A = 75cm
Distance B = 180cm (including the 25cm taken off for an adjustable seat)
Distance C doesn’t exist.
Distance D = 255cm
Distance E = 310cm
Front axle weight (empty vehicle) = 550kg
Rear axle weight (empty vehicle) = 450kg
Total additional weight on rear axle when loaded = (((B-A)x(68x2))+((E-A)x(7x2))) / (D-A)
= (((180-75)x(68x2))+((310-75)x(7x2))) / (255-75)
= ((105 x 136) + (235 x 14)) / 180
= (14280 + 3290) / 180
= 97.61kg
Total weight added = (68x2) + (7x2) = 150kg
So additional weight on FRONT axle when loaded = 150 – 97.61kg = 52.39kg
So the SMALLEST “Design Weight” that can be quoted for the front axle is 550 + 52.39 = 602.39kg.
The SMALLEST “Design Weight” that can be quoted for the rear axle is 450 + 97.61 = 547.61kg.
The SMALLEST “Maximum Gross Weight that can be quoted for the car is:
547.61kg + 602.39kg = 1150kg.
In reality, it would be silly to quote the bare minimum on the SVA form. It leaves no room for errors in measurement, but, more importantly, it leaves you no scope to add items to the vehicle during or after the build without risking breaking the law. It is surprising how small additions (CD changer, amplifier, a few tools, etc add to the weight of the car. It is even more important if the car was weighed before the build was complete and even small items like hoods can sneak the weight up! Finally, 68kg is not an especially heavy person to be carrying!
Is there a downside to declaring a MUCH higher set of design weights?
These weights are used in the braking test. The car needs to be able to stop its fully-loaded weight safely. The higher the weights you declare, the tougher you make the braking test for yourself. Remember also that the tyres load ratings will need to be adequate for the maximum weight that you quote. Lastly, the VOSA inspector will check that the vehicle structure looks like it’s up to carrying the weight that you quote. In reality, these are seldom a problem for a well-designed kitcar. Nearly all of them use brakes from much heavier donor vehicles and there shouldn’t be any trouble finding tyres that can cope with the weight of a typical kitcar.
Two final, and very important points are also worth making. Firstly, the 'Maximum Gross Weight (Gross Vehicle Weight) of the car (i.e. the most it can weigh as declared by you on the application form) must NEVER be MORE than the sum of the two maximum permitted axle weights. (although having it the other way round isn’t a problem).
In the example above, if we declare a Maximum Gross Weight of 1300kg, a maximum front axle weight of 700kg and a maximum rear axle weight of 550kg, the car would fail, because by loading it up to 1300kg, you would automatically be overloading at least one of its axles. If, we declare a Maximum Gross Weight of 1300kg, a maximum front axle weight of 700kg and a maximum rear axle weight of 600kg, the car would pass. It would ALSO pass if we declared a Maximum Gross Weight of 1300kg, a maximum front axle weight of 700kg and a maximum rear axle weight of 650kg. All it would mean is that you wouldn’t be able to load it up to its maximum permitted front and rear axle weights at the same time.
Secondly, the tester will check that when the vehicle is loaded up to the Maximum Gross Weight that you declare AND the maximum rear axle weight that you declare, at least 30% of the maximum Gross Weight is still left on the front axle. This is well worth checking for – especially on rear or mid-engined cars that might be inherently a bit tail-heavy!
To summarise:
1. You will need accurate weights for the front and rear axles separately on a COMPLETED car when empty of people but with a full tank of fuel.
2. Do NOT be tempted to 'guess' at the weight of the last few bits and pieces that you haven’t yet fitted!
3. Take accurate measurements from a convenient datum point (e.g. front bumper) to:
(a) The centre of the front wheels
(b) The point where the backrest of the front seat(s) meet the cushion(s) of the front seat(s) when the seat(s) are in the rearmost position.
(c) In the case of adjustable seats subtract 250mm from the measurement.
(d) Take the same measurement for the rear seat(s), if there are any and subtract the 250mm if they are adjustable or 200mm if fixed.
(e) Measure to the centre of the rear wheels.
(f) Measure to the centre of the luggage compartment (if there is one).
4. Carry out the calculation adding 68kg to each seating point and 7kg of luggage for each seating position, in the centre of the luggage area. NOTE that if the car doesn’t have a luggage area, the tester will put the additional 7kg per occupant when he or she sees fit.
5. Arrive at the MINIMUM calculated weights that can be declared for Maximum front axle, maximum rear axle and maximum gross weight of the vehicle.
6. Add something to allow for heavier loads and any errors that may have been introduced in the weighing process.
8. Check that the declared maximum front and rear axle weights added together do NOT come to LESS than the declared Gross Vehicle Weight.
9. Check that when the vehicle is laden to maximum gross weight and maximum rear axle weight, there is AT LEAST 30% of the maximum Gross Weight on the front axle.
10. Check that the tyres have a load rating suitable for the declared maximum.
By Ian Hopley
