Bike racks on the rear bumper?

[Bill]

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

[G-V_Driver]

FYI, there is a double mount bike rack for trailer bumpers (one wheel fits in each rack; some will accomodate two wheels in each rack therefore two bikes. So it's still attached to the bumper, but one on each end rather than a single mount in the middle. They are bolt-on-fall-down-simple units that even I can install.

Ther reason I know about these racks is that a nice guy named Ted Smith gave me one in Santa Fe this summer when he found it necessary to "lighten his load" because the bumper on the new trailer he bought (not a TM) was bending due to the weight of the bike racks and his storage rack on which he was carrying a big tool box and a portable generator. Ted and his wife were camped next to us and when we got ready to leave he pointed at the pile and told us to take anything we could use. So we did.

I was tickled to get them (but haven't installed them since we haven't gone anywhere since) but nice to know they exist. And they will be especially nice for us since we ride the long-wheelbase recumbent bikes.

Bill,

Our experience with TM sway has been much the same as others on the board. My wife and I transport our tandem in a rear bumper mounted rack. With the rack and bike mounted, we experience a slight amount of sway. No sway is experienced when the rack and bike are not mounted. We have installed a Reese Friction Sway control unit, and inspite of the negative experiences others have reported with such units, we have had good results. As I said, the sway was only slight, but the friction unit seems to put a stop to it.

Michael

[Bill]

Hmmm - That's good input, Michael. I'll have to go back and revisit my theory. If, as I theorized, the problem was bounce, then a friction sway control would not have cured it.

Still (he said, clutching at straws), I have seen more than one trailer go by with the bikes bouncing steadily up and down, up and down ...

Bill

Has anyone considered the changes in aerodynamics and not the weight distribution? Don't know if a bike would change the aerodynamics that much, but I know the faster I go over a certain speed, the more sway I have on my 3124ks.
Of course that certain speed is 75 mph -- as fast as I dare go.
I live in Colorado and constantly go up and down I-70 with my 6 cyl 2000 4-runner and can get speed downhill very easily.... 35-40 mph uphill on the 7% grades.
I have air shocks "100 psi when towing" and no sway bar or equalizer hitch. May consider buying the equilzer hitch.http://www.trailmanorowners.com/foru...ote=1&p=16323#
New Eyecrazy

[RockyMtnRay]

Originally Posted by ddnavar

Has anyone considered the changes in aerodynamics and not the weight distribution? Don't know if a bike would change the aerodynamics that much, but I know the faster I go over a certain speed, the more sway I have on my 3124ks.
Of course that certain speed is 75 mph -- as fast as I dare go.
I live in Colorado and constantly go up and down I-70 with my 6 cyl 2000 4-runner and can get speed downhill very easily.... 35-40 mph uphill on the 7% grades.
I have air shocks "100 psi when towing" and no sway bar or equalizer hitch. May consider buying the equilzer hitch.http://www.trailmanorowners.com/foru...ote=1&p=16323#
New Eyecrazy

The reason you have sway isn't aerodynamics, it's mostly because you're unwisely using air shocks to level your 4-Runner and partly because a 3124KS is just too long a trailer for your 4-Runner.

The air shocks are masking a fairly severe front/rear loading on your 4-Runner's suspension...and that's where most of your sway is coming from. When you hookup a trailer with a heavy hitch weight like the 3124KS (probably around 600 lbs), you're not only pushing the rear end down, you're also causing the front end to rise through a teeter-totter effect with the rear axle as the fulcrum. Because 200 to 300 lbs of weight was taken off the front end, your front tires have much less control over the vehicle and much less ability to fight transients coming from the trailer. The net effect is you get sway. The 2000 4-Runner is a short (105 inch) wheelbase vehicle that's barely long enough to control a 24 foot trailer; raising its front suspension is making a bad situation a lot worse.

Then when you raised the rear suspension with the air shocks to make the 4-Runner level, both ends are now actually higher than stock. That makes a dangerous situation even worse...much worse. Reason: on the rear axle of your vehicle is a Brake Proportioning Valve. Its purpose is to limit the amount of brake pressure on the rear brakes when the vehicle is lightly loaded (to prevent lockup) and to enable more rear braking when the vehicle is loaded (rear suspension compressed). By raising the rear of the 4-Runner to level it with the too-high front, you've basically closed this valve and your rear brakes are doing virtually no braking...your front brakes are doing almost the braking. And worse, those front brakes are trying to brake tires that only have about 80% of their normal friction because the front end has been unweighted.

The bottom line is air shocks are downright dangerous and should be banned from the market. AIR SHOCKS SHOULD NEVER BE USED TO LEVEL A TOW VEHICLE.

You can solve all this by simply getting a weight distributing hitch. Then, instead of the front suspension being unweighted, it will be additonally weighted with about a third of the hitch weight. So instead of removing about 200 lbs from the front, you'll be adding 200 lbs. That additional 400 lbs on the front will make a huge improvement in your ability to steer and your sway problems will very likely mostly disappear. The sway may not entirely disappear because your trailer is simply too long for your 4-Runner to fully control.

And even better, because both the front and rear suspension on the 4-Runner are being equally compressed, the Brake Proportioning Valve will be opened more, thus enabling your rear brakes to actually contribute to the overall braking effort.

As is, your rig is so marginally controlled that I shudder at the thought of having to share a Colorado highway with you.

[fcatwo]

Ray's mention of the possibility of air shocks affecting the Brake Proportioning Valve rang my bell. Like many Honda Odyssey owners who frequent the Odyssey owners site, we have installed Airlift bags in our rear coil springs to inprove handling while both towing and solo. The bags only lift the unloaded rear of the van about 1/4" when pumped to 20 psi (which is where we normally run them) but keep it from squatting and wallowing while running loaded. I've found them to be an overall handling improvement but the possibility they are compromising our ABS brake system concerns me.

Hitched to our TM2619 with a 400/4000 one-armed WDH (no sway bar) we are level and weigh out at 2660 front axle, 2640 rear and 2890 trailer axle - all of which are within spec but we are uncomfortably close to GCWR. The Odyssey weighs 4300 dry, has an 118 wheelbase and we have experienced absolutely no sway (as in zip) in 14k miles of towing in the 55-70 mph range under a variety of condition and up to about 5000 ft. We've had no braking problems but have habitually avoided panic braking situations so our brakes have never been tested under severe conditions.

Anyone see a problem with our setup? yes I know a more capable TV would be nice, and I'm looking, but finding something as versatile and even close to as fuel efficient as the Odyssey has alluded me so far.

Frank

Originally Posted by Bill

Hmmm - That's good input, Michael. I'll have to go back and revisit my theory. If, as I theorized, the problem was bounce, then a friction sway control would not have cured it.

Still (he said, clutching at straws), I have seen more than one trailer go by with the bikes bouncing steadily up and down, up and down ...

Bill

Bill,

The bike rack we use is a solid mount type. By that I mean the front wheel of the bike is removed, the fork is secured to the rack with a quick disconnect, and the rear tire is strapped to the rack. I also use ratcheting tie-down straps from each end of the rack to the rear bumper of the TM. The hitch is a single point receiver type, but the straps prevent any bouncing action by the rack.

Michael

Thanks Ray -- I have panic stopped at red lights on off the I-70 ramps that are quite steep... > 10% grade with the 4-runner and the 3124ks. I have had to apply more brake pressure than I would like to -- so I will be going to the hitch store and buying an equalizer hitch.. You have convinced me to buy one. I use a Protogy brake controller and have had to increase the braking on the trailer for it's weight.

After realizing that I had created a brake and steering problem with my air shocks on my 4-runner and the 3124ks TM, I purchased the 1000 lb, equal-i-zer hitch.

Went to Las Vegas from Denver over I-70 this last week and it did improve the bounce, steering and brakeing considerably. It was very stable with no sway even in the strong winds I encountered.

"Good purchase".

Only problem I have now is the noise that the swing arms and L brackets have when turning.
Does anyone know of a fiber spacer one can put on the swing arms or L brackets to reduce the metal to metal noise???