Bill
Senior Member
Site Team
RV LIFE Pro
A perennial topic on this board is the addition of a bike rack on the rear bumper. Several people have experienced sway when they do this, and the question is, WHY does it cause sway? The usual answer is that the weight of the bikes, mounted so far behind the wheels, reduces the hitch weight, which in turn causes sway. I admit I’ve had some trouble with this explanation.
First let me say that I thoroughly believe in the importance of proper weight distribution. A properly-loaded TM does not sway, and I support the idea that this is because the wheels are located aft of the center of gravity. And there is no question that moving the CG backward substantially would increase the tendency to sway.
But I’m not convinced that putting 70 pounds of bikes on the back bumper changes the CG enough to make the TM sway. As Denny pointed out, for each pound you add to the back bumper, you remove about 2/3 of a pound of hitch weight, so 70 pounds of bikes removes 45 pounds from the hitch. When the starting hitch weight is in the neighborhood of 500 pounds, though, 45 pounds just doesn’t seem like enough to have a real effect. (Note that since hitch weight is an indicator of the CG’s location, and is easier to measure, we talk about hitch weight rather than actual CG location.)
Why don’t I think it is enough? There are many common ways to change the hitch weight by 45 (or more!) pounds. For example, are your propane tanks full or empty? That’s 40 pounds right there. Do you carry the microwave on the floor aft of the refrigerator, or in front on the couch? And in the SL models, where the battery is located near the bumper, did you replace the factory battery (46 pounds) with a pair of golf-cart batteries (122 pounds) for a 76 pound weight gain at the rear bumper? That step alone is remarkably similar to adding bikes on the rear bumper. But none of these things seems to cause sway.
So if it isn’t the weight on the bumper, what accounts for the tendency to sway when bikes are mounted back there?
I first thought about aerodynamics. Could the slipstream coming over the top of the trailer be catching the bikes, and causing wind buffeting? Maybe, but since the bikes are largely shielded from the wind, I couldn’t convince myself. I gave up on that idea.
After a lot of thought, I’ve concluded that the problem may be bounce, not sway. These single-point-mount bike racks are VERY flexible. If you put your bikes on the rack, and then put your hand on the top of the rack and push hard, it will flex and the bikes will move up-and-forward 2 or 3 inches. And if you pull back, the bikes will move down-and-back 2 or 3 inches. At highway speed (especially on one of those cursed concrete roads in the south and midwest), the whole package - 70 pounds of bikes and carrier - begins to bounce repetitively, up and down, up and down. The bouncing motion could be converted to sway if the rack is mounted off center, but in most cases I would bet that the driver is simply feeling the bounce of the 70 pound package, which could easily be mistaken for sway.
By the way, this continuous bouncing is what breaks the welds where the bumper is attached to the frame, as reported by a couple people.
At this point, this is pure hypothesis. But if it is right, the answer is to mount the bikes on a carrier that doesn’t flex, or is restrained by straps that go forward over the top of the TM to a solid anchor point. Anyone own such a carrier? Are there any of these on the market? I don’t know – but I’ll look.
Any thoughts?
Bill
First let me say that I thoroughly believe in the importance of proper weight distribution. A properly-loaded TM does not sway, and I support the idea that this is because the wheels are located aft of the center of gravity. And there is no question that moving the CG backward substantially would increase the tendency to sway.
But I’m not convinced that putting 70 pounds of bikes on the back bumper changes the CG enough to make the TM sway. As Denny pointed out, for each pound you add to the back bumper, you remove about 2/3 of a pound of hitch weight, so 70 pounds of bikes removes 45 pounds from the hitch. When the starting hitch weight is in the neighborhood of 500 pounds, though, 45 pounds just doesn’t seem like enough to have a real effect. (Note that since hitch weight is an indicator of the CG’s location, and is easier to measure, we talk about hitch weight rather than actual CG location.)
Why don’t I think it is enough? There are many common ways to change the hitch weight by 45 (or more!) pounds. For example, are your propane tanks full or empty? That’s 40 pounds right there. Do you carry the microwave on the floor aft of the refrigerator, or in front on the couch? And in the SL models, where the battery is located near the bumper, did you replace the factory battery (46 pounds) with a pair of golf-cart batteries (122 pounds) for a 76 pound weight gain at the rear bumper? That step alone is remarkably similar to adding bikes on the rear bumper. But none of these things seems to cause sway.
So if it isn’t the weight on the bumper, what accounts for the tendency to sway when bikes are mounted back there?
I first thought about aerodynamics. Could the slipstream coming over the top of the trailer be catching the bikes, and causing wind buffeting? Maybe, but since the bikes are largely shielded from the wind, I couldn’t convince myself. I gave up on that idea.
After a lot of thought, I’ve concluded that the problem may be bounce, not sway. These single-point-mount bike racks are VERY flexible. If you put your bikes on the rack, and then put your hand on the top of the rack and push hard, it will flex and the bikes will move up-and-forward 2 or 3 inches. And if you pull back, the bikes will move down-and-back 2 or 3 inches. At highway speed (especially on one of those cursed concrete roads in the south and midwest), the whole package - 70 pounds of bikes and carrier - begins to bounce repetitively, up and down, up and down. The bouncing motion could be converted to sway if the rack is mounted off center, but in most cases I would bet that the driver is simply feeling the bounce of the 70 pound package, which could easily be mistaken for sway.
By the way, this continuous bouncing is what breaks the welds where the bumper is attached to the frame, as reported by a couple people.
At this point, this is pure hypothesis. But if it is right, the answer is to mount the bikes on a carrier that doesn’t flex, or is restrained by straps that go forward over the top of the TM to a solid anchor point. Anyone own such a carrier? Are there any of these on the market? I don’t know – but I’ll look.
Any thoughts?
Bill
