In Chapter 3 Bertie goes into great detail about the effects of corrugations in the road on their travels. Wikipedia defines corrugations as:- “Washboarding or corrugation of roads comprises a series of ripples, which occur with the passage of wheels rolling over unpaved roads at speeds sufficient to cause bouncing of the wheel on the initially unrippled surface. Most studies of washboarding pertain to granular materials, including sand and gravel.”
As you will see from Bertie’s description below, he got it spot on.
“The second reason why average speeds appear high is that there is nothing to slow down for, except the bad condition of the track itself. I mean that there are few corners; after all, the width of the road is considerable! So one has plenty of choice – there are no towns or even villages. There are no thirty mile limits, there is no traffic and there are no police. Therefore one’s average speed is much higher than it would be if one drove at the same speed in more civilized countries. We proceeded on our way and soon after passing Tibreut we came on the surface that is the bugbear of African motoring. One meets it here, in Northern Africa, in Kenya, in the Union of South Africa. This particular bugbear is known as “corrugations”. Presumably it begins as a small ridge in the road, over which the wheels of passing vehicles bump into the air: when they come down again they make another short jump and so on, each bump becoming a little less high until the vehicle again resumes its smooth travel. The next vehicle that comes along does the same thing, but as the preceding vehicle has by its impact deepened the hollows following the original ridge, the second vehicle’s second bump is rather higher than the first ones, so that its blow to the road beyond the second ridge is harder. Hence instead of making perhaps, five shortening jumps before it travels smoothly again, it will make six or seven. The next vehicle does the same but more so, until gradually the ridges become higher, the hollows become deeper and the series of ridges and hollows extends further and further along the road. These ridges and hollows are called “corrugations.”
On a really badly corrugated surface like the one we have just met with each ridge will be perhaps six inches high and the ridges will be about 18 inches apart. These corrugations are apt to occur wherever all traffic is compelled to travel on the same path, either, as on this stretch, because it is the only practicable one owing to deep sand or tussocks of coarse grass on each side of the track or, as in Kenya or South Africa, because the road is a road and it is not possible for traffic to follow any other course than the road. Travelling on corrugation is simply hell. It punishes a car as much, if not more than, any other surface one can meet with, anything that is liable to shake loose will certainly do so, and one cannot help the feeling that something must certainly break.
As for the crew of the car, it is indescribably awful, the jarring and jolting seem to loosen the very brains in one’s head, and the misery of this becomes intensified until it is almost unbearable. Once one has managed to obtain a certain critical speed, in our case about 40mph it is just endurable, apparently the wheels do not then have time to descend into the hollows and the car rides comparatively smoothly from ridge to ridge. But the real agony is when for any reason one has to slow down: perhaps for a patch of deeply rutted sand which it is highly inadvisable to cross at high speed as for the moment the car is more or less out of control: perhaps for some deep gully across the track. But then begins the awful business of accelerating from say 15mph up to the normal 40. At 20mph the vibration is appalling, at 25 worse, at 30 worse still, at 35 it begins to smooth out and at 40 the car will again be comparatively comfortable. Maybe one may get a mile of this, maybe only 200 yards, then onto the brakes for some obstacle and the ghastly business begins again. No, corrugated roads do not make pleasant travelling. For the tourist a speed of say 10 – 12mph is quite pleasant but that was of no use to us. We had to get on with the business and we and the car had to put up with what became sheer horror.”
These pictures are of the corrugated roads in the Northern Cape.