I own a 2014 Kia K2700 and have noticed a distressing characteristic when it is not loaded: there’s no braking on the rear axle. I have confirmed this anomaly on similar K2700s owned by friends and even a new one from the showroom floor. I feel it’s putting lives at risk, but according to the Automobile Association, it’s road legal in the mentioned state – something I don’t agree with. This problem is especially prevalent on dirt roads, where the front wheels lock too easily and the rear axle does not assist braking. Can you please comment?
DAWIE COETZEE
Reddersburg
Answer: After contacting a roadworthy centre, it became clear there are many commercial vehicles of this ilk that fail the braking-force test on the rear axle when not loaded. However, they are classed as roadworthy.
They’re equipped with a load sensor that adjusts the brake bias (braking force) between the front and rear axle. There’s a good reason for this: a commercial vehicle such as the K2700 has the cabin and engine at the front, which means that the bulk of the vehicle mass is focused on the front axle when the load bay is empty. Remember that the normal force between the tyre and the road (given a fixed friction coefficient) limits the amount of braking force available on a wheel.
Therefore, there is little braking force available on the rear axle of the vehicle when it’s not loaded. Added to this is weight transfer during braking that increases the normal force between the front wheels and tar, and decreases the normal force on the rear axle by the same amount. In this scenario, it is less safe to “over brake” the rear wheels, as they tend to lock easily and render the vehicle unstable (which is akin to pulling the handbrake). This is especially prevalent on vehicles without ABS, such as the K2700.
It is, however, possible that the calibration of the load sensor/braking bias is not suited to dirt-road use and we therefore ask readers with similar braking problems on their Kia K2700s to please write in. The best and safest solution is for manufacturers to fit ABS to all vehicles.
