Back to Technical Information Directional Accuracy and Stability Static Stability Uphill Slopes Downhill Slopes The WmegaTracŪ, BetaTracŪ and AlphaTracŪ are steered and driven through TEFTEC's patented steering transmission. This is a total departure from all other wheelchairs of the world and solves the directional instability problem that plagued the industry for decades. When you want to go straight, it will go straight, even with the driving surface leaning to the side, up to the point that the tires break loose. Directional Accuracy and Stability The importance of going straight, especially when the driving surface is not flat or has interruptions, increases as the user's ability to steer is reduced. If the chair can only be steered by sip and puff or head array, or foot switches, driving unerringly straight has the utmost importance because without it, the user must stop and correct their course each time the chair deviates from its intend path. If this is important, do not take anyone's word for what you should get. Directional Stability, or the lack of it, is where this all got started. In the early part of Jim Finch's power chair use, he was always in peril when spasms would momentarily keep him from controlling his powerchair. Since his popular brand power chairs had front casters and two drive motors, and in those days no brakes, the chairs had no resolve to go straight; they only wanted to go down hill. That may be exactly where a Peterbuilt or Kenworth, or Greyhound was coming. To this day, that is still a scary thought. It was discovered through the search for a better powered wheelchair, that the industry had, for decades, tried to solve this directional stability problem. They simply could not get their powerchairs to go straight on warped and slanting surfaces, much less to go precisely where their users wanted them to go. That is not surprising, because propelling each drive wheel separately is like two people pushing a shopping cart with one hand each, and pushing with that one hand on opposite sides. If the shopping basket encounters a side slope, one person has to hold back while the other has to push harder. TEFTEC developed and patented a steering transmission that has this age-old problem solved decisively. It has a drive motor for propelling and a steering motor for steering. If the steering motor is not steering, the chair goes only straight up to the point that the tires skid. That means that on a surface that variably slopes to one side or the other, the WmegaTracŪ goes essentially straight. That is what allowed us to quit worrying about Jim and the Greyhound bus. If you have access to an WmegaTracŪ have someone drive it straight ahead with the speed turned down so you can keep up with it, and try to force it to turn by grabbing its push handle and pulling sideways. It seems to be on an invisible railroad track. Unlike others who only claim to have true tracking, the WmegaTracŪ and its siblings and only them, actually. It takes 21 gears clustered in 15 individual parts to do this job, so even if it was not patented, other manufacturers would probably never try to do this. Top of the Document Static Stability If the likelihood to upset your, or your client's, wheelchair by turning it over or breaking off a caster creates a hazard in your or their type of operation, then it is important to look deeply into this aspect. What may have been seen as safe operation may not be at all as it seems in the light of a really stable powered wheelchair. A powered wheelchair standing still on a flat seems very safe, no matter what its configuration is. Consider, however, cases where the center of gravity changes because of reclining or tilting or both and the casters are swiveled to their least conservative position, towards the other wheels, shortening the wheelbase. This may still not be a problem unless the user is obese. In this case, powered seating can change the center of gravity beyond a safe limit. When the surface slopes at a severe angle, static stability becomes very important. If the center of gravity of the powerchair is not very low, or if the ratio of the weight of the chair versus its occupant is unfavorable, then the chair may tip over easily. These factors are aggravated as the length between the wheel axles decreases or the width of the drive wheels decreases, depending on the direction of the slope. On side slopes, the closer the center of gravity is to the drive axle the more likely the person can get the chair to an unsafe side sloping angle. Since powerchairs without TransmissionSteering™ are more difficult to steer, the center of gravity is moved closer to the drive axle to reduce the steering difficulty and therefore makes the chair more capable of getting to the side tipping angle before loosing traction. In front-wheel-drive wheelchairs, the center of gravity is usually more displaced from the drive wheels to prevent forward tipping. As the side slope angle increases, the traction on the uphill drive wheel is lost and the chair turns uphill, reducing the likelihood to tip sideways. A similar event occurs in long wheelbase rear-wheel-drive wheelchairs except the chair turns down hill. Short wheelbase rear-wheel-drive wheelchairs are more likely to reach the side tipping point. Mid-wheel-drive wheelchairs are the most likely to reach the side tipping point because they have the drive wheels closest to the center of gravity. With user and seating total weights less than 300 pounds, the WmegaTracŪ demonstrated tip angles above 20 degrees (36%) in forward, and side tip tests for the FDA static demonstration. And backward tipping tests were abandoned at 103% (46 degrees) tip angle with no tipping. Users should stop at half those angles to account for dynamics. That means that grades of twice the ADA maximum of 8% can be navigated safely with normal configurations of the WmegaTracŪ . The WmegaTracŪ cannot tip backward while climbing a hill because it will lose climbing traction at around 45% grade and will not tip at grades less than 100%. Rapid stopping, high user and seating weights, elevated seating, and abuses of seat location will reduce the angles at which tipping will occur. Rapid deceleration while driving down hill will reduce the angles at which tipping will occur, and should be avoided. As the downhill grade steepens, speed must be reduced. This is analogous to driving a car in small radius turns. As the turn radius decreases, the speed must be reduced. It is safe to back an WmegaTracŪ off a curb up to the height of the foot rests (minus 2 inches to keep from damaging the foot rests), up to a height of 8 inches for passenger and seat weights of less than 300 pounds and using non elevated seats. Top of the Document Uphill Slopes On uphill slopes, the rear-wheel-drive chairs get increasing traction and if they have enough power, will climb a steepening grade until they tip over backward. To reduce the chance of aft tipping, wheelie bars or anti tippers are installed to catch the chair as it tips. This may be adequate if the rate of tipping is low and the anti tippers are far enough behind the center of gravity, and they are strong enough to catch the weight. However, if the wheelie bars or anti tippers are back far enough and low enough to prevent the chair from rolling over backwards, the chair can not drop off a driveway lip of any height without raising the drive wheels hopelessly in the air. Longer wheelbase rear-wheel-drive powerchairs have less likely hood of tipping on a given slope. Front-wheel-drive wheelchairs have the weight transfer off the drive wheels as the uphill grade steepens until the chair can no longer climb because of loss of traction. This is an inherently safe condition. Mid-wheel drive chairs also will loose traction as the grade steepens if the chair has enough power but, because of its mid wheel layout which moves the casters nearer the center of gravity, the casters may swing under the center of gravity as the chair slides back and care must be exercised. If the mid wheel drive is sliding back on uneven ground, because now the weight on the casters is far higher than it was while climbing, it may make the casters dig into the surface if it is soft, or be reluctant to rise over an obstacle that is rolled over easy with the reduced weight when the casters were trailing as the chair moved forward. If the casters dig in or fail to roll over an obstacle while the chair is rolling backward out of control, the chair will likely tip backward. Top of the Document Downhill Slopes Downhill Slopes can be the most dangerous condition if the chair is operated carelessly. Rear-wheel-drive powerchairs have the weight transfer forward, making the casters even more reluctant to roll over any kind of an obstacle and a sudden stop by an obstacle can pitch the chair over forward. When the downhill grade steepens enough, there is no weight on the drive wheels and no way to stop the chair. This is a catastrophic case unless the grade gradually decreases. If it suddenly flattens and the chair is sliding out of control, it can pitch the chair over forward. Obviously, long wheelbase rear-wheel-drive chairs are less likely to have this problem on a given slope than short wheelbase rear-wheel-drive chairs. Front-wheel-drive powerchairs will not run out of drive wheel traction on steepening downhill grades. For that reason, careless operation on very steep downhill grades can put the user in a position of forward tipping. The downhill grade on which forward tipping can occur, varies with the center of gravity of the user, chair, and the rate of deceleration. Because the driver of the front-wheel-drive powerchair is not out of control, careful operation can prevent a problem. In chairs with adjustable seat location, it is better to locate the seat more rearward on front-wheel-drive chairs because it makes the descending grade capability steeper than the ascending grade that can be climbed, and therefore keeps the user out of trouble. He or she would not be able to climb what they will not be able to descend. Front wheel drive powerchairs should always be operated more slowly going downhill than uphill. When descending curbs or steep grades, turn the speed knob or switch to its lowest setting, which allows the joystick to be advanced fully and with better control. Do not stop while dropping off a curb or going down a particularly steep grade. Wait until the grade levels out of the curb drop is completed before stopping the chair. Mid-wheel-drive powerchairs behave similarly to rear-wheel-drive chairs on downhill slopes and will lose the ability to stop when the grade steepens enough to take the weight off the drive wheels and put it on the anti tip forward wheels. Mid-wheel-drive units with sprung front anti tip wheels that are raised for curb clearance may be unstable when rocking over on the anti tippers. This condition must be individually evaluated. Top of the Document