Hitch Mount Bike Rack Suspenders

In a prior topic there was a discussion about the viability of carrying more than 2 bicycles on a hitch mount carrier. It seems the general conclusion was that it was problematic with the Coastal hitch bending a bit too much for people's comfort. It doesn't seem to be related to receiver size either (1.25" vs 2.0"). One person had the idea to mount the carrier closer to the car by making a new hole in the receiver section of the carrier to mount the rack closer to the car. I've been thinking about this problem and have taken some pictures which mock-up an alternate solution. I'll call them bicycle carrier suspenders. There are two variations of this idea for different types of racks. I have not decided to go ahead and make one of these for myself as I first wanted critical feedback on what engineers out there think of the idea.

Here it goes...

The basic component of the suspenders are glass hatch clips made for fitting trunk mount bike racks on certain cars. Yakima, Thule and others make these. Here is an example of the Yakima version...
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In their intended usage they are placed inside the car at the top of the hatch with the strap going outside...
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Instead of holding on a trunk rack, I see them being used on a typical hanging style hitch bike carrier. The picture below shows a mockup of how they would work with a Yakima Kingpin 4, but the same idea should work with any rack of this style. In a "final" version I'd make some custom straps with cam buckles rather than the tied-on straps used in the mockup. The idea is that the straps add some support so the hitch isn't carrying the entire load. The rack shown is a borrowed one, and this isn't a solution that really works well for me as I need to carry two adult bikes and two 20" kids bikes. Getting two 20" kids bikes to fit on this style rack is a nightmare and isn't user friendly. Nevertheless, this may be a great solution for others. The big negative is that you need to undo the straps to get in the hatch.
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Since I use a tray style carrier from 1upusa, I've thought of an alternative using these same straps. It involves placing the straps at the bottom of the hatch like this...
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From the inside of the car with the hatch closed it would look like this...
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The suspenders would then lend support to the rack (which can be expanded to carry up to 4 bikes) like this (again with a custom strap and buckle arrangement, but for the mock-up you get the idea)...
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I can see the need to protect the bumper in some way. I also wonder if the force of the strap on the hatch might tend to do something bad over time. The hinge point of the hatch (where the solution above is mounted) is no doubt stronger than the latch point on the bottom.

It goes without saying that you'd never get Yakima, Thule, Saris or any retailer to "approve" of either of these setups. The only piece of supporting data is that Yakima (and probably Thule & Saris) rate their trunk style rack for the Prius to hold up to 3 bikes or 90 lbs. It doesn't seem like a huge stretch to think that straps and a hitch mount should be able to handle 4 bikes plus the weight of the rack. With that said, anyone who tries either of these ideas are still on our own.

What do people think of this idea?

 

For what it's worth as this seemed to be of little interest...

After playing around with the various options I decided that making the hanging style hitch mount bicycle rack work for me was the best option for carrying 4 bicycles with the greatest sense of safety. Compared to the tray style I've used for two bikes, this system takes longer to setup and load and offers no easy access to the hatch once the bikes are loaded and suspenders in place (although the hatch does clear the rack without the straps in place). The bikes also sit quite a bit higher than the tray style and as such completely block the rear view and affect wind resistance to a greater extent.

On the plus side, after a couple of trips including short highway sections, speed bumps and country roads it appears very stable and I'm not worried about littering the roads with fallen bikes or bent/overstressed hitch receivers. With that limited experience I decided to buy the Yakima Kingpin during the recent sale REI had on Yakima.

To make the "final" suspenders I got a pair of 2' straps from NRS and sewed the free end into a loop. To keep things tidy I also sewed on some Velcro (male/female back-to-back) to secure the loose end of the suspenders so they don't flap in the wind. Total cost of the suspenders was $16.00 for the Yakima glass hatch hooks plus $5.70 for the straps and $0.50 for the Velcro.

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The following goes into the ultra-nerd category...

The notion about carrying more than two bikes on a hitch receiver bike rack has gotten the better of me. Yes, I believe the suspenders work just fine, but I've been curious about whether they are really needed or if I was just being overly conservative.

Yesterday I decided to play around with some physics modeling software (i.e. interactive physics 2005) to try and understand the forces being placed on the hitch receiver. The following diagram shows the forces at play on the hitch for 1, 2, 3 and 4 bikes on a hanging style rack. The rack is modeled after the Yakima Kingpin in a simplifed way. In the model the rack is assumed to weigh 35 lbs and each bike (represented by the yellow disks) is 25 lbs. The model simulates what forces would be present if you just placed the rack in the receiver without being screwed in place. In reality this isn't what happens, but this method kind of relates to tongue weight limits.

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It's interesting that 1 and 2 bikes stay within (or very close to) the 200 lb tongue weight limit of the Prius hitches, and 3 and 4 bikes appear not OK. This also approximates people's observations.

The next diagram models the alternate technique where you mount the rack deeper in the receiver so it's closer to the car. This model was only done for 4 bikes. Moving it in 3" has a significant impact, but it seems like you need to move it in more to get under that 200 lb figure.

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Anyways, it was a fun exercise to do. The software makes physics fun.

 

<div class='quotetop'>QUOTE(mtsarpilot @ Oct 31 2006, 11:11 AM) [snapback]341199[/snapback]</div>

The SAE defines the specification for hitch classes which includes tongue weight. I believe tongue weight is nothing more than the force pushing vertically on the ball when towing. If you placed the load where it hooks to the receiver on a bathroom scale it would give you this measurement. As the simulation above shows, you can't just add up the weight of the bikes and rack and relate that directly to tongue weight. It's more complex and relates to the distribution of the load on the rack and the nature of the more complex lever arm holding the bikes (i.e. the rack). The simulation does jive with the general wisdom that to carry 4 bikes you need a Class III hitch (500 lb tongue weight). Given how Coastal seems to operate, I believe the 200 lb tongue weight limit is a somewhat casual specification that typically relates to Class I hitches. I'd like to believe Coastal did an actual design analysis and field test to prove that the hitch meets Class I specs (atleast with regard to tongue weight), but I have doubts. I'd have more trust that Curt did actual engineering work on their design, but they don't offer a 2" option. To your direct question, given that Toyota doesn't approve of any of this (towing or hitch mount bike racks), I'd guess the 200 lb limit is more what the hitch can withstand with acceptable bending rather than what the car can take. Clearly Toyota would wash their hands of all of this if any problem happened either during an accident or structural issues from everyday use. For that matter I don't remember seeing any written information saying that Coastal hitch meets any "Class" spec. The 200 lb limit appears to be more anecdotal or based on conversations with Coastal.

Thanks for the reference to your solution. That looks wonderful! Very cleaver solution! $120 seems to be a fair price for that amount of custom work. I do share your concern about what hitches and what your modification in particular may do to the structure during an accident. Where do the HV batteries sit relative to the central rear jack point?