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May 10, 1998 |
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Q: Rúnar Sigurjónsson wrote:Nice article you write there on Rockcrawler, one of the nicest ones I've seen so far. A: Thanks for the kind words. It's sometimes difficult to try to put all the stuff you do without thinking about anymore into words. Q: Rúnar Sigurjónsson wrote: However I didn't agree with one part of it, the one that deals with high current rivers. "....In fast flowing water, a slight angle downstream will put the front of the rig in a sheltered position, and may drop the subjective water level some. But it will put you out in the water for a longer distance, and increase the likelihood of stumbling across a deep spot, or other underwater surface irregularity. Climbing the far bank without straightening the rig out will create an off camber situation. ......" I'm from Iceland, a driver in a SAR team. The trucks we have are two 80-series Land-Cruiser Turbo Diesels (44" tires, and 38" tires, built for > snow driving), and one MAN 4x4 truck, 16 seater (Unimog kind of truck). A: Thanks for the kind words. It's sometimes difficult to try to put all the stuff you do without thinking about anymore into words. Q: Rúnar Sigurjónsson wrote: The rivers we are crossing in the summertime are glacial runoffs, that often change on daily basis (hourly sometimes). They flow fast, at human running speeds, 15-20 km/h (I guess). A: These type rivers makes up a large part of our crossing concerns as well. Q: Rúnar Sigurjónsson wrote: The general rule up here is to cross slowly at an downward angle of some 20 - 45 degrees. This means that the current breaks on the trucks rear corner, causing less sideways force on the truck. If you go just straight the truck kind of works like a dam, and the current just grabs it. Specially true in narrow rivers. Other benefits of going downward is that the current "assists" you by pushing on the rear of the truck, and you have a easier way of controlling the truck in case the current starts to play with it. Its always the the rear end of the truck that first floats up, and starts going sideways. If you are going over straight, the current turns your front end upstream in just a second, and there is nothing you can do about it. If going downstream at an angle, you can always turn your front tires further down, and floor the truck, to make the front end go downstream at the same speed as the rear, preventing the truck to turn face up.... A: Crossing downstream does take a bit less power from the engine, due to the fact that you do not have to work as hard to push the water out of your way. But while you lessen the side aspect of the rig exposed to current when you turn downstream, you increase the rear aspect that is subject to the current. This largely offsets any reduction in force against the side of the rig, until you exceed about 45 degrees in relation to the flow. Additionally, when you turn the front of the rig downstream, the angular force of the water pushing against the side of the rig will create a vector which will work against your efforts to reach the bank. The water will actually be trying to push you back toward the bank that you started on. This is not a tremendouse force, but it is a very real one. So long as your tires retain their grip on the bottom, it is not a factor, but if they start bouncing, slipping and clawing for traction, it will definitely come into play. In white water rafting this same effect is a primary means of controlling the raft in swift water. If the raft or vehicle is moving with the current and providing no resistance to the flow, then this cross channel "push" does not exist. As soon as resistance is offered to the flow (by back oaring in the raft, or by the grip that the vehicle's tires have on the bottom) the it appears. Again, so long as the tires can resist with no slippage, they can negate the push, but as soon as they enter a partial traction state (slipping and sliding without completely loosing all grip) In the mean time, while the current against the rear of the rig will supply some push while your tires have traction, as soon as the traction is lost, any assistance the the curent is providing in the direction that the vehicle is pointed will disappear. You do lessen the resistance to the passage of the water that your rig creates when you angle up or down stream instead of turning at a right angle to the flow. The fact that you are traveling downstream, also lessens the speed at which the water impacts against the vehicle, which therefore lessesn the effect that is has. And the force of the current against the full side of the rig on a ninety degree crossing is larger than the force against both the side and the rear of rig when it is turned at an angle. In my experience, I have not noticed a tremendous difference in the amount of time it takes for the tail end to come around when the vehicle is turned at an angle, as opposed to when it is perpendicular to the flow. Of course it has further to swing before you find yourself pointed upstream if you begin the situation pointed downstream some. We are in complete agreement, that pointed up stream in fast water is usually not a good position to be in. In the smaller rigs, we usually first experience lateral displacement with little turning of the rig as it approaches it's depth-current limitation. There are definitely times when I agree with the approach that you advocate. I feel that the largest advantage offered by this method is the reduction of force against the side of the vehicle which can use up most or even all of the traction capability of the tires. In the situations where I fell that a downstream angle is advantageous, I usually feel that a faster crossing speed will multiply the benefits. With your speed close to or the same as that of the current, there will be no lateral traction demands on your tires. The current will not be exerting any force to wash you downstream or to spin either end of the vehicle around. Since you will essentially be motionless in relation to the surrounding water (at least so far as the downstream flow is concerned), there will be little bow wave or splashing to contend with, and less power required to push the smaller amounts of water to the side. You will be able to carry much momentum in you arppoach to the opposite bank if that is needed. Of course as you know, a higher speed crossing will increase the concerns about and the hazards of underwater obstacles, or irregularities in the bottom. When everything is said and done, While I agree that there are times when a downstream angle is the way to approach a crossing, in general, for most crossings, I prefer to head straight across if possible. But in actuality, there are few serious crossing where the banks, bottom, water depths or other factors will not complicate the equation anyway. And to be perfectly honest, I'm a bit of a chicken. I always look for the shallowest and widest crossing I can find, so that I can avoid those situations which demand all of the capability of both the driver and the vehicle. Sometimes even the easiest crossing available can be pretty daunting though... I guess that's why we build more capable rigs as time goes by, and spend the time is pondering the best choices in technique... Q: Rúnar Sigurjónsson wrote: To get a real feeling on the forces that are playing with the truck, I've found playing on a mountain bike in knee deep streams to be most educating... A: As part of the swift water rescue training required for the whitewater rafting, we spend a few days every year training in some pretty fast (and very cold) water. Swimming through class III rapids ( know a couple crazies who have swum class IV stuff on purpose!!!), wading flows that are trying to tear your feet out from under you, and trying to figure out how to make it that last impossible 4 feet across a rushing channel will REALLY teach you respect for the power of a fast flowing river. Send your questions to Mark at cruiser@rockcrawler.com |
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