Bunk vs Scissors Trailers
Posted: Wed Feb 04, 2009 8:58 pm
yahoo, i finally found the stability discussion i posted over at OPDB back in July 2005. So I am reposting it here so we have something else to argue about. Let the games begin!
07/20/2005 02:13:29 PM
I don't think a scissors trailer initiates a rollover much before a bunk trailer, but I think the problem is that the scissors trailer allows it to continue to a full rollover more easily.
The severity of turn goes as speed squared divided by turn radius. This is termed the centripetal acceleration. Ignoring tire tuck, for the inside tire to lift the condition is b/h equals twice the centripetal acceleration divided by the acceleration of gravity, where b is the track (side-to-side distance between tires) and h is the height of the boat+trailer center of gravity above the ground. In other words, a trailer's tendency to start tipping depends on the ratio of track to CG height. Considering the motor, deck, toons and gear, I would estimate that a pontoon boat's CG is pretty close to the deck height. The trailer's mass will lower the CG of the total package somewhat. My scissors trailer puts the deck at about 3 feet high or so, so the package CG is probably near 2.5 feet. The track is something like 4.5 feet. So the ratio is 4.5/2.5 = 1.8. A friend of mine has a bunk trailer, and by memory I will estimate the deck at 5 feet, so the CG around 4.5 feet, and the track around 9 feet. So the ratio is 9/4.5 = 2. These are similar ratios. We could measure the track widths more accurately, but it's hard to measure the package CG.
The point is this: the scissors trailers have a much reduced track width, but that is somewhat offset by the lowered center of gravity.
I think the real issue is what happens once the inside tire leaves the pavement. For the rollover to complete, ignoring the hitch loads, the CG of the boat+trailer package needs to pass over the contact of the outside tire. This requires lifting the CG. The bunk trailer has a much wider track, so the CG must be lifted considerably higher than with a scissors trailer. You can get a bunk trailer up on one side and have it come back down at higher speeds than a scissors trailer.
I think this is an issue mainly for high speed evasive maneuvers, which are when the centripetal acceleration is most likely to get large. Like everyone else, I take it very easy through turns and have never had a problem. I deal with the high speed maneuver issue by leaving a lot of space between me and the car ahead of me. I pull the trailer with the mindset that any deer that runs out in front of me will see in-line braking only and may be history.
So anyway, I don't think the concern is rolling the boat over in a turn on the road (unless it is driven by a maniac). I think the concern is rolling it during an evasive maneuver.
But there may be a price for having that boat up higher--that head-on collision thing, where a boat on a scissors trailer may hit the vehicle frame and stop, but the boat on a bunk trailer may cruise through the vehicle and chop your whole family in half. We've beat that discussion to death in the past, and it's all just conjecture, because we have no data.
I suppose the bottom line is "be careful out there".
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07/20/2005 02:13:29 PM
I don't think a scissors trailer initiates a rollover much before a bunk trailer, but I think the problem is that the scissors trailer allows it to continue to a full rollover more easily.
The severity of turn goes as speed squared divided by turn radius. This is termed the centripetal acceleration. Ignoring tire tuck, for the inside tire to lift the condition is b/h equals twice the centripetal acceleration divided by the acceleration of gravity, where b is the track (side-to-side distance between tires) and h is the height of the boat+trailer center of gravity above the ground. In other words, a trailer's tendency to start tipping depends on the ratio of track to CG height. Considering the motor, deck, toons and gear, I would estimate that a pontoon boat's CG is pretty close to the deck height. The trailer's mass will lower the CG of the total package somewhat. My scissors trailer puts the deck at about 3 feet high or so, so the package CG is probably near 2.5 feet. The track is something like 4.5 feet. So the ratio is 4.5/2.5 = 1.8. A friend of mine has a bunk trailer, and by memory I will estimate the deck at 5 feet, so the CG around 4.5 feet, and the track around 9 feet. So the ratio is 9/4.5 = 2. These are similar ratios. We could measure the track widths more accurately, but it's hard to measure the package CG.
The point is this: the scissors trailers have a much reduced track width, but that is somewhat offset by the lowered center of gravity.
I think the real issue is what happens once the inside tire leaves the pavement. For the rollover to complete, ignoring the hitch loads, the CG of the boat+trailer package needs to pass over the contact of the outside tire. This requires lifting the CG. The bunk trailer has a much wider track, so the CG must be lifted considerably higher than with a scissors trailer. You can get a bunk trailer up on one side and have it come back down at higher speeds than a scissors trailer.
I think this is an issue mainly for high speed evasive maneuvers, which are when the centripetal acceleration is most likely to get large. Like everyone else, I take it very easy through turns and have never had a problem. I deal with the high speed maneuver issue by leaving a lot of space between me and the car ahead of me. I pull the trailer with the mindset that any deer that runs out in front of me will see in-line braking only and may be history.
So anyway, I don't think the concern is rolling the boat over in a turn on the road (unless it is driven by a maniac). I think the concern is rolling it during an evasive maneuver.
But there may be a price for having that boat up higher--that head-on collision thing, where a boat on a scissors trailer may hit the vehicle frame and stop, but the boat on a bunk trailer may cruise through the vehicle and chop your whole family in half. We've beat that discussion to death in the past, and it's all just conjecture, because we have no data.
I suppose the bottom line is "be careful out there".
fb