Which Hitch?

<div class='quotetop'>QUOTE(chitown guzzler @ Sep 2 2007, 10:04 AM) [snapback]505890[/snapback]</div>

The REAL reason to have a larger hitch ball is to allow larger loads, but the Prius isn't going to ever be in the position where you can tow larger loads. So, the 1 1/4" ball is fine for whatever you're going to put on the back of a Prius ... except for those manufacturer's who will put a 2" receiver on their bike carrier!

So check the ball size of those carriers you want to use first, and size the ball on your hitch to fit.

I haven't seen the hitch setup for the Prius, but if you can swap out the ball, its usually pretty easy to do. I have a receiver mounted under my truck and two ball assemblies that slide into it, the 2" and 1 1/4" ball.

 

I need a hitch too for my Prius to carry my bikes.. After reading the numerous threads on PriusChat, I think it boils down to this:

- The only 2" hitch available for the Prius is the one by Coastal Electronics, and it is not a very strong hitch. Some forum members have experienced failure in the weld joints with this hitch.

- The other hitch available for the Prius is the one by Curt Manufacturing (also available at U-Haul). It is better-designed and will carry heavier loads than the Coastal, but it only comes in 1.25".

I think I'm going to take my Prius to a U-Haul and have them install the Curt for me.

 

I think the same criteria applies to the hitch receiver. I'm trying to remember, but I had a Ford Taurus with a 1 1/4" receiver, and the shop that put it on told me something about the difference being that you needed 2" with heavier loads (but that the 1 1/4" hitch would handle more than the Taurus can tow anyway, and its more compact and looks nicer under the bumper.)

 

<div class='quotetop'>QUOTE(chitown guzzler @ Sep 2 2007, 01:04 PM) [snapback]505890[/snapback]</div>

I know you just want it for the small things you suggest but keep in mind the ability to brake safely with extra capicity added to the car. I would just go with the 1 1/4 inch as it is lighter and will do what you need it to do. The fact that the smaller one takes up less room before hitting the ground should ring some bells also. Yes, ground clearance is less with the 2 inch also. The 2 inch is only for larger loads which we know the Prius can not handle. Not that it can not handle the acceleration but the fact is that most towing systems are designed around the ability to safely brake the load. There is a very good reason that the Prius has a max weight design for total passenger/cargo weight.......braking. The regen braking and the friction braking can only handle the design of the car. Anything extra and you are pushing your luck. Hope this helps for your particular situiation.

 

I went with the 2" on 2 Prii. I would not trust a cargo basket with a 1.25" receiver without some sort of end bracing at the left & right corners. Downward forces on a smaller-diameter reciever translate into a great deal more stress on the receiver and, probably more to the point, on the receiver-centric connection points (welds/bolts/etc) on the basket.

Suppose you have an unloaded basket 30" wide on the back of your Prius. A 180-lb person sits or stands on it, to change shoes, get a better view, step around the car, etc. Their weight is centered, oh, 12" from the centerline of the basket. The back-side of a 2" hitch receiver experiences a twisting (upward) force of 990 lbs. The load-side of the receiver is worse off (downward force), since it's getting the 990 lbs plus it's carrying the 40 lb basket and the 180 lb person.

Now put the same load on a 1.25" receiver. A 1.25" receiver gets a twisting force of 1638 lbs! (Again, the load-side gets the extra 220 lbs load to boot.)

The wider baskets just make this worse. Suppose you go with a 48" basket... a 2" receiver is getting 1800 lbs of twisting force -- more than I'd be willing to risk. (Plus 220 lbs for the down-side, assuming the basket's the same weight.) But your poor little 1.25" receiver is getting 2934 lbs of twisting force. (Again, plus 220 lbs for the down-side.)

With a heavy-duty enough bike rack, the lateral forces are probably manageable. Nobody's likely to run into the side of the bike rack with 200 lbs of force, and if they do it won't be sustained. The only real risk is in turns, and you'll probably take it easier because you'll feel the car rolling. But the cargo baskets I've seen tend to use lighter-gauge metal, and even if they've been patched up with strap steel I don't think they'll take 2000 lbs of force without deforming or weakening.

 

<div class='quotetop'>QUOTE(bobdavisnpf @ Sep 4 2007, 01:22 PM) [snapback]506848[/snapback]</div>

Sounds like you've more than thought this through. Do you mind explaining your calculations (in a very basic way), so I have some idea of the basis of the evaluation? Perhaps I won't understand it, but I'd like to give it a shot.

One reason I ask is that I recall other threads discussing the problems with the 2" receiver made by one particular manufacturer. As I remember it, some welds didn't hold. Some of the problem had to do with the higher leverage certain loads were exerting on that receiver (versus the Curt-designed 1.25" receiver). The 2" receiver gives us more options in terms of bike carriers and baskets, but I was not going to buy one that had the failures I'd read about here in PC.

Also, since you've had success with the 2" receiver, would you be willing to describe what systems you've used and how they have been better than others you'd considered (in your opinion, of course)?

Also, would the use of a 1.25" to 2" converter (to allow a specific bike rack or carrier to be used with a 1.25" receiver) make the situation worse? Meaning: Would the extension (additional leverage) allow too much lateral stress?

I'm in the hunt for the right setup, and I had thought the Curt receiver (1.25") was the right way to go, so I'd greatly appreciate your additional insight and comment.

a priori

 

For reference, here's the thread on the failures suffered by a few Coastal 2" hitch owners: http://priuschat.com/index.php?showtopic=9326&st=20#

Looks like the Curt can handle more tongue weight than the Coastal even though the Curt is only 1.25".

 

<div class='quotetop'>QUOTE(JustLurkin @ Sep 4 2007, 02:16 PM) [snapback]506878[/snapback]</div>

Thanks for the reference.

Still, I wonder, is tongue weight the issue bobdavisnpf was describing? Truly, I wonder about this, I just don't understand it all. He was talking about the twisting stress of (what I assumed were) a different type of lateral load. Whether the sqare nature of the receiver opening has any bearing on this (no pun intended) I do not know.

I don't suppose I'm looking for the full engineering explanation on this, but I do want to know that if I had the Curt 1.25" receiver with a 30" basket on the back, that I could stand on it without fearing I would damage the carrier, the receiver, the wonderful Prius, or even my own backside. Likewise, I want to know whether the same activity on a 2" receiver (the Coastal one, for example) would produce any different result.

 

a,
Thanks for your interest in this: you're right, I put a lot of thought into it.

I use a 20x30x2 basket from Coastal Tech and it is great. I've got their 2" receiver on the '05 and I do not trust it with any but the lightest of loads. I took it to a hitch shop to get it beefed up and they declined, saying they would charge almost as much to make that one passable as they would charge to build and install a new custom assembly.

Your idea to use the Curtis 1.25" with a step-up adapter is pretty good, really: I think the Curtis is beefy enough that, in the twisting-force scenario I describe, the basket will give way before the Curtis receiver will. At that piont I'd worry more about the rocking from adding one extra semi-loose fit into the mix. I didn't mentioin in my little engineering-calculations piece that the rocking potential in a possible loose-fit scenario was also on my mind.

Well, let's start in on the calculations then, shall we? Techie rant begins here.

Basically we all know that, to exert more force on something, you use (1) more force or (2) a smaller something. The force (weight of man changing shoes) is not predicted to increase substantially, so it's a constant 180. In this case, the 1.25" hitch is the smaller something, relative to the 2" hitch.

Next we need to come up with the appropriate mathematical formula. As engineers go, I'm what is best described as "the fake kind", so the most accurate formula is found via search engine. I found one at, fittingly, Engineer's Edge:
http://www.engineersedge.com/calculators/l...ge_levers_1.htm
In this formula:
W = 180 (man changing shoes)
L = 2 (hitch size)
X = 11 (center of man to load-side edge of hitch)
Force = 990
Now, to interpret this force number, it's important to note this calculator is designed to tell you the downward force exerted on the short end of the lever (the "uphill" side of the 2" hitch). The fulcrum is under a greater load because it must bear the added weight of the man and the cargo basket, so I simply added this weight to the "load-side" edge of the hitch receiver. It might be more accurate to DOUBLE the 990 - after all, 990 is the force needed for equilibrium (a state where both sides are equal).

Anyway, now to plug in a 1.25" hitch. The load is leveraged out a little further, since the basket stayed the same but the hitch decreased, so both the L and the X change:
W = 180
L = 1.25
X = 11.375
Force = 1638
So, I can pretty confidently state that a 1.25" hitch is going to deliver at least 40% more lateral stress, metal fatigue, and wibble-wobble than a 2" hitch.

Techie rant mode off!

 

<div class='quotetop'>QUOTE(bobdavisnpf @ Sep 4 2007, 10:55 PM) [snapback]507140[/snapback]</div>

I knew joining up with this site would work out for everybody. You get to do all of this work, and I get to thank you for it and reap all of the benefits. What a country!

<div class='quotetop'>QUOTE(bobdavisnpf @ Sep 4 2007, 10:55 PM) [snapback]507140[/snapback]</div>

Tell me more! You don't think that the step-up adapter would add another stresser? Or do you believe the design of the Curt receiver would overcome much of the additional stress coming from the increased leverage?

<div class='quotetop'>QUOTE(bobdavisnpf @ Sep 4 2007, 10:55 PM) [snapback]507140[/snapback]</div>

Love it! Keep it coming!

 

The step-up adaptor is likely to be a solid cast piece. A 3000 lb twisting stress load is probably ok on it.

The main thing the extr adaptor introduces is another point of possible "play" in rocking or sided-to-side motion in the event of a loose connection. The scientific term for this motion potential is "wibble-wobble".

If you have an 0.5 mm play on a 2" hitch, then at the far corner of a 30" basket you have 15mm of vertical rocking motion (7.5mm up & 7.5mm down). On a 1.25" hitch, the same 0.5mm play gives you 24mm of vertical play: 1 full inch. A 48" basket on a 1.25" hitch would have 38mm play.

If you put in a step-up adaptor, your stress vulnerability is mainly moved into the 2" realm, which is great. But now you have a double connection. If each connection has 0.5mm play, then a 30" basket now has 2 full inches of play; a 48" basket has 76" play, or about 3 inches (1.5" up+1.5" down).

 

Oh, I do love this forum: let me see if I can wake up this thread:

Isn't the hitch size (1.25" or 2") measured as the width and height of its opening and not the "L" (="Length from Fulcrum") as explained on the link that bobdavisnpf gave:
Simple Lever Calculator Case #1 - Engineers Edge