I went riding with David Beardsley Sunday before last. We met at the Tanji River Bridge at 9:30; the sun sharing sky room with cumulus banks. The point of the exercise was 1. To have a good time riding, 2. Get to know each other a bit, and 3. For me to learn some off-road riding skills, especially how to get through deep sand.
David was leery of my relatively smooth “dual sport” tires (Metzeler Tourance), which are designed for relatively firm surfaces: asphalt, gravel roads, light sand. “Let’s,” he suggested, “follow a track down here that’s reasonably well-packed. There are sections of deep stuff, but they don’t last long. Are you up for it?” I certainly was. I was also reasonably nervous about the deep stuff. Same sort of anxiety I would feel standing at the top of a steep ski slope with waist-deep powder. Not something I’m used to and afraid of consequences.
David rides a H*nda XR-650; a dual-sport closer to a pure dirt machine than my BMW. It is old technology, with a bullet-proof, carbureted engine from the ‘80s, traditional tank layout, and a suspension he has tweaked a bit. He rode trials, enduro and moto-cross for many years as a member of the British Army racing team. He knows his machine. He knows how to ride it. And he is essentially fearless. Not my traits at all!
With a twist of the throttle and a scatter of dirt and pebbles from his knobbies, he skidded the back end around 180 degrees and was off with me in teeth-gritting pursuit. Onto the asphalt of the coastal road, down a kilometer, then abruptly off the pavement onto a dirt road into the bush. Mourning doves and ring-necked doves exploded from the road before David’s tires. I watched him cross a deep patch of rutted sand with a sudden blast of throttle. I tried to stay in the tire tracks left by automobiles while trying to follow David’s line and emulate his throttle technique. Nonetheless, my rear tire was fishtailing and occasionally the front tire would start weaving back and forth in the sand. My heart was in my nose by now and twice I almost lost control of the front wheel. I was sitting back on the “step” of the passenger seat, trying to keep my shoulders relaxed, wondering if my death-grip on the handlebars would leave dents. I was also wondering when the seemingly inevitable dump would happen and hoping I wouldn't have any dents of my own.
The deep sand was just a bitch. I was scared every moment I was in it and was right at the edge of control. After a few kilometers I finally told David it wasn’t working for me. He agreed. He’d noticed I’d almost lost it a few times and suggested we go back and ride some packed roads to a beach side restaurant he knew and have a nice cuppa tea. So we did. We made it back to the coast road without me making a fool or a cripple of myself. After that sand the dirt road was a piece of cake. Past ladies carrying buckets or baskets on their heads, sheep wandering the road, and men driving donkey carts.
We settled down to cups of good English tea at an al fresco bar overlooking the beach. One doesn’t converse with David. One attends. He told me stories about his days with the British Army Racing Team, racing in the UK and on the Continent. Well worth listening too they were, as well. He talked about arriving, for example, at a track in the mountains in Germany. The team would get there a couple of hours early for the race and spend their time running the bikes (BSA 350 and 500 singles) and experimenting with the jetting of the carburetors and suspension setup. He had a notebook in which he’d enter the parameters for that specific track with the particular atmospheric conditions and, armed with this notebook, he could return years later to the same track and know how to jet his carb, what tires to use, etc. Sort of like the rutters kept by ancient navigators.
We talked about tires and about suspension setup. I asked him if he’d take a ride on my bike to get an impression of what needed to be done. He declined. “With those tyres I won’t like it. Get your proper tyres first,” he said. “Then I’ll ride the bike and we can start setting up the suspension.”
“In the sand,” he said, “you’ve got to use the throttle and ride fast!” He got off his chair and squatted down on the sand. “Here’s what happens when you’re riding slow.” He took the knife-edge of his hand and pushed it through the sand. I watched a bow wave of sand form and flow away from the sides of his advancing hand. “See, you’re pushing the sand out of the way. Now, hit the sand with your fist.” He punched the sand. I punched the sand. It hurt! Its behavior was different when struck than when pushed.
“If you hit the sand hard with your tire you don’t give it a chance to flow away,” he said. “It acts like a solid.”
It hit me suddenly, the analogy with boating physics. Sand, like water, is a fluid when pushed slowly. It is solid when it’s hit fast.
The motorcycle tire moving slowly through sand is like a boat with a displacement hull. The sand parts and flows along the sides. Speed isn’t going to happen. Once you start moving faster a different physics goes into play. The tires become planing hulls. They ride up on top of the sand and, as in a boat, the captain has to keep the power on or the hull/tires settle back down, become displacement hulls and the sand becomes a fluid again. Ride fast enough to turn the sand to a solid and things become more predictable.
“It’s the transition that’s tricky,” he said. “It’s while you’re getting from riding slow to where you’re up to speed that’ll get you in trouble.” He was, of course, right. Giving it power, just like in street riding, settles the back end down (just like a boat "squatting"), lightens the front wheel, and gets you through.
So, how is changing direction accomplished? A boat, planing at high speed and trying to change direction will drift sideways unless prevented by some lateral force. In the case of the boat it is the keel or skeg or centerboard. In the case of my motorcycle it is the meager side ridges on the tire treads. On David’s bike it was the sides of the knobs on his tires. On mine, I had very little of this lateral resistance and I tended to drift out. Some of that drift could be countered.
There are various techniques for turning but the one David explained to me and the one I used was contrary to all my street training. You stay upright and lean the bike under you. Weight on the outside peg forces the tire straight down into the sand. With the bike leaned over and weight pushing down the centrifugal force of the tire pushes sand to the outside of the turn and, at speed, this wall of displaced sand becomes an instant berm, pushing back against the tire. Force and counterforce. Pure Third-Law Newton.
We finished our tea and David proposed we ride the beach back home. Fine with me. I’m up for a nice, leisurely ride to settle my still-frazzled nerves. We mushed through the deep stuff and got out to the firm sand at water’s edge. There were gulls and fairy terns overhead, broken shell and cuttlebone in the sand and David set off like he had a hot date waiting at home. (Perhaps he did!) 60, 70, 80, 90 kilometers per hour. I kept trying to relax my shoulders and grip and gradually it worked. I experimented with leaning the bike, sensing the counter-push of the sand against the tires.
David would get well ahead of me and then noodle off into the deep sand up the beach, wasting time until I caught up. He did this twice and then I figured, “Sod it!”, turned up the wick and raced him all the way to Brufut Village. I kept thinking of the book A Twist of the Wrist, II, and the constant admonishment to fight one’s normal survival instinct to back off the throttle. Crank it on! Apply power. Power is what settles the machine; what gets you through.
We parted in Brufut, both with many things to do with the rest of our day. It has been nine days since that ride and my confidence in the sand has leaped exponentially. Thanks for the great mentoring, David.

that's a great read in many ways. many thanks!

Hung on every word. Thanks!

Yep, I sure enjoyed that and could almost feel that sickening, smooth tire on deep sand feeling as you described it, then I started wondering about the quality of the hospitals in Gambia. Deep sand with full gear (hot!) might not be too bad but no wreck is a good wreck.

Lee

Lee, the quality of the hospitals is, well, .....not up to western standards. For a year (2006-7) I was the principal doctor in the emergency department in the main hospital in the capitol, so I know it well. Good part is that there are some competent Cuban docs here as well as British-trained Gambian docs. The government is working hard to improve things but the lack of trained personnel and reliable equipment is always a problem. Then, of course, there is the lack of minor items like electricity and running water in some clinics....

There's an interesting string on ADVrider about some Brits who rode across Mauritania. Talk about sand!!!! I think it is titled "Gold & Salt" or some such.

Great tale!