Soapbox Brake Force and Gradients
The vast majority of soapbox rules contain a requirement that all carties have effective brakes, and many specify a minimum force that they must be able to hold. This is commonly 50 KgF. The wording is typically "The brakes must be capable of holding the vehicle on a dry horizontal road surface, when laden with driver and any ballast, against a horizontal load of 50KgF exerted on the front towing eye.".
This has been adequate for most of the races up until now, but with racers and race organisers now increasingly looking for steeper, twistier and more challenging courses, the "50Kg static brake test" needs to be reviewed.
50KgF should bring a 200Kg cartie travelling at 40mph to a stop in about 65m. However this only applies when the cartie is on a horizontal road so it is not very realistic for gravity races which by their very nature tend not to be held on a horizontal road surface.
When travelling down a hill the weight of the cartie is working against the brakes, so the effective braking force is reduced. The steeper the course, the less effective the brakes will become, and there is a gradient beyond which it will be impossible to stop. For a 200 Kg racer with 50KgF brakes, that is gradient is 1 in 4 (25%). Even at 1 in 8 (12.5%), the effectiveness of 50KgF brakes is reduced by 50%.
It follows that, while 50KgF brakes may well be perfectly acceptable on relatively shallow courses, there may well be more extreme courses where far stronger brakes will be required.
For safety reasons, it should be self evident that all carties should be able to bring themselves to a controlled stop in a reasonable distance at any point on the course, so it follows that there is, depending on which way you look at it, either;
1) A maximum gradient at which any given cartie can safely run
- or -
2) A minimum required brake force for any given course
The table on the right (click to enlarge) shows the static horizontal brake force required to be able to exert 50KgF at any gradient. Race organisers need to consider a "worst case" scenario, and a typical maximum all up weight for a cartie is around 200Kg.
Brake forces in excess of 100KgF are difficult to achieve for all but the most dedicated soapbox racer. Typical carties at most community/fun events often struggle to get even half this.
There are two ways to approach this. If you want to continue to set a relatively low and achievable brake limit of 50KgF to maximise participation in local fun races, then the course gradient should not exceed 1 in 10 and speeds should be relatively slow.
Alternatively, for more challenging events, the following is suggested;
|Maximum Gradient||Minimum Brake Force (KgF)|
|Up to 1 in 10 ||70 |
| Up to 1 in 7||80|
| Up to 1 in 5||90 |
|Up to 1 in 4 ||100 |
| Steeper than 1 in 4|| not advised|
A simpler way of specifying and testing this would be to re-word the rule currently in use to say;
"The brakes must be capable of holding the vehicle on the steepest part of the course, when laden with driver and any ballast, against a horizontal load of 50KgF exerted on the front towing eye."