Bike rack and hitch advice needed

Pulling isn’t going to be your problem, it’s stopping the car and the trailer with your load if you have to do it fast and the Prius brakes are not made to handle that kind of stress, that’s why Toyota puts a warning in all of their manuals NOT MADE FOR TOWING. Or something like that. Because if you cause the accident say goodbye to any money you ever had or will have, Insurance Company’s don’t like to payout if you go against the manufacturers warnings.

 

just bending of cantilever beams... Not going crazy here just a basic calc of a beam attached to a fixed point to see what kind of stress could be on that section of tube. Its not a perfect comparison but not far off. Just eyeballing anyway.

Beam Stress & Deflection | MechaniCalc

StressMax=BendingMoment * DistanceFromCenter / MomentInertia

Example: rack that extends 3 feet past vehicle with concentrated load at 2 feet. Max load for the rack I have is 2 bikes, 45 lbs each. Rack itself weights 37 lbs.

Now that I have the rack the square section is solid and measures 1.2x1.2. The Moment of Inertia for a solid is: [(1.2)^4 ]/ 12 = .1728 in^4

This is going to be your best case. I assumed it was going to be a square tube.
bending moment I used: 24" * (90+37) = 3048 in lbs

stress max = 3048*(1.2'/2)/.1728 = 3048*.6/.1728 = 10,583 psi

1018 Steel tensile strength is about 53,000 psi.
AISI 1018 Mild/Low Carbon Steel


This is about 1/5 the stress required to start to deform that section of the rack. Factor of safety of 5.
Its pretty good, but for shock loading and repetitive loads that change direction it should be designed with a factor of safety of 10x when lives are on the line. I know... a bit overkill. But this rack is prob one of the stronger 1.25" racks and it will fail after some amount of time under extreme use.

So anyway, this is why I purchased this rack. I though the design was good for the prius, and it seemed well built. It's ok - you can see the bugger welds in the picture. The size of the material is great thought. The hatch opens without tilting it back if your handle bars are short.

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I just found a calculator. Play around and eyeball the racks you find. Let me know if i'm crazy...
ENGINEERING.com | Beam Deflection Calculators



For fun I went to amazon and searched for 5 bike hitch mount and estimated the weights and dimensions. Scary stuff.

I am assuming the cheap rack bellow is made of 2" .089" wall tubing. Lets put 5 bikes, 30 lbs each on it. say the load is concentrated at 36" past the hitch connection. 13ksi - not bad for all that weight.

*** see what im saying about a 2" hitch. Even a cheap thin walled attachment point will be difficult to bust. (someone did it, look at 1 star reviews! )

Check out the round section that holds the bikes, the page says it is 28" long. Under round tube beams use 20" 1.25" .089 wall and 150 lbs. 34Ksi. This will fail on a road trip for sure. Its too close to 53Ksi. If you change the numbers to anything reasonable it wont be too far off. try 1.5, 1.0 etc. maybe thicker wall.. .125".

Ok, now check out the 1 star reviews and check out the pictures... Amazon.com: Customer reviews: Allen Sports Deluxe 5-Bike Hitch Mount Rack, Model 552RR-R

The material is so thing on that thing. Scary stuff. Perhaps I need a therapist.

 

The better built bike racks do not need it. I bought that on amazon but returned it because when the Kuat transfer showed up it has a locking pin with with threads. You screw the pin in and it pulls the square section against the hitch.

 

My point was, whether you use a hitch lock or built-in anti rattle mechanism, the torque on the 1.25" bar is too high. Let's say you have a total mass of 120lbs (2 bikes plus the rack), and the center of mass is 36" above the hitch. If the rack experiences a 1 "G" load that's 360 ft-lb on the bar. And I think 1G would be a low estimate because that's pretty gentle compared to hitting a big bump and hitting the bump stops on the suspension (which I have done many times on road trips with lots of bikes and cargo and people in the car).

If the hitch bar is 1.2" wide that's a force of about 3600 lbs (it's probably not that simple but close enough) on the edge of the bar as it starts to rotate. If your anti-rattle "hitch lock" uses a 1/4" U-bolt (grade 5 to be generous), the proof load is 2700 lbs. That is less than 3600 lbs, so the bolt will deform or break. Again it's probably not that simple, but close enough to show that it's to close for comfort.

And if there's any play that lets the rack rock side to side, that will increase the force and can eventually work the anti-rattle mechanism loose until it or the rack fails. I have had it happen on my 1up-usa rack on rough dirt roads (admittedly an extreme use case).

 

The hitch locks will help since it will hold it tight on one side of the square and the regular bolt on the other.

I didnt see a need for it on my install since it had a good fit and was super locked in, but it sure will help especially if your fits are bad.

Two mates surfaces are certainly better than one!

 

etrailer just told me that Curt no longer requires straps for their hitches when using a bike rack. I don't see any change to the design so sway might still be an issue. The Draw Tite's design might have some additional torsional rigidity but would have to do a FEA to determine that. I just bought my 2017 Prime Premium. I had a 2004 Prius for about 9 years and hated how it handled but the Prime is so much better. I'm riding a 45 lb emountain bike now. I've been hauling it with a rather flimsy Transite rack (Performance). I don't think it will fit without using an extension on either of these hitches and the extensions are 7". I think I could add some holes to shorten that a bit but I need at least another 3" or so. etrailer has not installed the Curt but it looks like it's closer to the end of the bumper. The Draw Tite is 7.25" away from the bumper. Does anyone have the Curt class 1 installed on a Prime that can measure this. I thought this decision would be much easier than this. I'm probably going to end up buying a new bike rack too. The 1-up is nice but I think it will hit the bumper on the Prime without an extension.

 

I was etrailer's first customer to purchase that hitch for Prime. The strap requirement never made any sense. It works fine without.

Below is a photo of mine. I couldn't imagine using a rack without an extension. Sure, it would fit, but then you couldn't open the hatch. We travel with the bikes on back. Being able to still access cargo is from the rear is important.

http://john1701a.com/prius/photos/prime/Prius-Prime_Receiver-Hitch_06.jpg

 

^ Looks like you'd have no trouble putting a 1-up on that.

The 1up doesn't require a fixed depth into the receiver. As long as the ball lock can pressurize the fitting it'll hold. And of course it swings down and to the rear to grant trunk/hatch access.

 

Thanks John. Looks like the extension might be rubbing on the bottom of the bumper. I see some scuffing in the pic. I’m not sure but I think I read somewhere that the draw title might be a bit lower. The extensions have a flange around the end that could be hitting and maybe it could be ground off to avoid that? What’s your thought John and thanks again,

 

Looking at the one up fitment diagram, when the one up rack is inserted to its minimum depth it is only 3.1” beyond the end of the receiver. Since the end of the receiver is 7.25” from the end of the prime bumper, you still need an extension or am I missing something? I do like the one up rack but I still need at least a 4” extension I think. The minimum extensions are 7” but I think that can be reduced some by adding a set of mounting holes. Not sure about this last point. I have to get my hands on an extension to see.

 

I think Brophy makes a 5" extender, couldn't tell you if there are any shorter ones out there.

Not sure I'd love the idea of trying to drill a new lockpin guide hole in what may be heat-treated steel.

Good luck!

 

Hitch balls are heat treated for good wear resistance. The hitch itself won't be so hard (250 max Brinell) that a carbide drill on a drill press could not drill thru it like butter. Hardening the entire hitch would not only be expensive but would leave it brittle and not very tough too. Using the right speed, a rigid set up and some cutting oil a carbide tipped drill only drilling two holes will not even be dulled a bit.

Brophy does make a 5 3/4" extender for about $51. Thanks for pointing that out. Pretty expensive. I ordered the Curt one for $23. so we'll see how that goes. The Brophy can be my backup choice just in case.

CR Brophy Hitch Extender for 1-1/4" Hitch Receivers - 5-3/4" Long Brophy Hitch Adapters hes6

 

I haven't ever had any trouble, even when loaded down. I make a point of having the bike rack on for our EV shows too, which draws attention to the practical nature of Prius. I never get any comments/concerns of that nature when they see it in person.

 

invest another $100 to get the swing-away rear bike carrier.
Hanging off the back on swing-away rig is better than roof-top. Less drag and much easier to mount and dismount the bikes, and fiddle with the bikes. Can't tell you how many times I've put a bike on roof, only to second guess myself and need to take down to check something.
IMO, should get roof racks for wide low-profile cargo "box", and that is where you keep all your mission-specific outdoorsy stuff, easy to find in big wide "tray" of box (as long as you aren't too short)

 

I have no problems with a trunk mount bike rack like this:

 

I bought the draw tite from etrailer and it worked out perfectly. My friend, that owns an auto repair shop, installed it ($85.) with some help from me (he didn't need much help). Getting to the wiring harness snap fits was the major issue. We ended up loosening the steel under bumper piece before removing the fascia to gain better access (his idea) and that worked perfectly. And I didn't need an extension at all! I'll post some pics with the bike on it the first time I use it.

 

Can you open the hatch while bikes are on the rack?

 

John, I have not tried yet. I'll never put more than one bike on the rack since it's an ebike. A lighter one, 45 lbs but still pretty heavy. I will take a pic on Sunday since I have to transport the bike the first time. It's been pretty soggy out and I've mostly ridden right from home lately so the car wasn't required. On another note, I'm getting over 32 miles for a full charge on the Prime, 5.3-5.4 miles/kw-hr. Pretty happy so far with the Prime. It's so much better than my 2004 Prius. And I had the higher end version but it was not very well built in my opinion. Lots of dash rattles and wandered all over the highway at times.

 

OK here are the pics and no I can't open the hatch with the bike on the rack. Not a big deal for me but an extension would be needed for that. I prefer to keep the cantilever load down since my bike is 45 lbs.

 

My used 2017 came with a 1.25 curt hitch, put on a Swagman bike rack, works fine for 2 bikes , each less than 35#