Does anyone tow with their Prius? Yes,I know that it might void the warranty.I'm not talking about towing 2500 lbs.I have in mind maybe 250-300 lbs maximum.We take lonf\g driving trips(3300 miles round trip),and currently use a Pilot.It gets decent-20.5mpg at 70-80 mph,and 23-24 mpg at 68 mph), but we will be selling it soon.The Prius will have to be our trip vehicle,and we travel heavy-2 adults, one leggy greyhound, one MTB, 2 small ice chests, several plastic bins-20x13x13".We also sleep at rest stops in the back of the vehicle.The Prius will allow about 68 inches front to back(or 90 inches depending on how we configure the ft seats),and almost 37 inches side to side.This is enough-barely-for 2 (140lb and 170 lb) adults,and a 75 lb dog(ft seat).It leaves no room for cargo.I don't think we can carry it all on/in our aluminum(26 lbs) 49x22" hitch carrier.I would be reluctant to put more than 70 lbs of cargo in it(100 lb tongue wt).
A light trailer(maybe 70 lbs) might be better.Maybe I could adapt MTB wheels/tires and frames to cobble together a trailer?I have grave doubts about bicycle wheels/tires taking the 70 mph speeds.MC wheels/tires could, but they are about 23lbs per wheel/tire.They would probably be the way to go.
Anyone towing with a Prius? BobW??
Thanks,
Charlie

 

Hi Charlie,

I have the Coastal Electronics receiver hitch on my 2003 NHW11 Prius and I use it for a 1,400 lb rated, folding trailer. I use it for plywood sheets and other oversized loads around town. But I also have the 'oversided load' platform that I used to get the trailer kit home.

I'd recommend looking at the small trailers at Harbor Freight and don't think there would be a big problem. You'll be happier with something that easily fits in the draft behind the Prius, say a 12 ft. Jon boat. However, I don't really see it as a problem.

The usual problem with a trailer is overloading the transmission and getting excess heat. I suspect that moderate speeds, ~60-65 mph, would not be a problem. However, I would recommend you think about instrumenting the transaxle to read out the oil temperature. If it starts heading warmer than 100C, slow down and make a new plan.

Bob Wilson

 

So Bob, you wouldn't have any qualms(assuming transaxle temps stay under 210 or so) about towing 500 lbs or so??We would have less than 500 lbs of humans/dog/junk in the actual interior,so the vehicle would only be propelling~4000 lbs .Now I'm guessing that the aero drag might double at 60 mph.The Prius is so clean that any trailer is going to add lots of drag-keeping it in the shadow will sure help. If I get the time/energy I'll see if I can cobble together some sort of fairing.Maybe a couple of junkyard hoods would be a good place to start.They are pretty light,and about the right size-One for the front, one for the back.
Thanks,
Charlie

 

Hi,

No problem at all as long as the temperatures are OK. Heat is the enemy.

If you can keep the trailer profile at or a little less than the Prius, you'll be OK. The rear spoiler lip should mark the boundary of a trailing vortex.

One of the experiments I'd like to see performed would be to attach ribbons to the rear and take photos from a trailing car. This would map the vortex flow and if the trailer body can fit in that vortex, that would be perfect.

I've been a fan of foam and fiberglass construction. It is light weight and can be shaped into curved surfaces very easily. The trick is to map the trailing vortex and fit everything into that area.

Bob Wilson

 

There is a Yahoo! group called priustrailers where you may find kindred spirits. Except for MS Dickerson, who has been towing a trailer for years with an 01 or 02 (forget which he has).

DAS

 

Bob, Tochatihu-thanks.
-Tochatihu- I didn't know there was such a yahoo group-thanks.
Bob, when you refer to the vortex behind the Prius,are you referring to the tumbling dirty air that is behind it? Is the idea to keep the trailer in this tumbling air,since it is "bad" already, it can't be made worse?
If I trailed ribbons, what would I be looking for-clean straight ribbons above and to the sides of this tumbling vortex,and flipping, flopping ribbons "in it"?
There are some nice aero looking trailers built for motorcycles to tow, but no way would I be interested in a $1500+ trailer.
A trailer weighing~125lbs with a 300 lb load would suit me fine. The Harbor freight ones are 4x5' and 4x8' with total width about 60" with 12" wheels.They come without a floor;I could use some very thin Luan plywood(about 1/8") for the floor-save some wt. I would get the 12" wheels rather than the 8 or 10" stock wheels.I don't know what they weigh-maybe 125lbs without the floor?
Making a cast lid/fairing by laying fiberglass over foam would certainly give a smoother, better fitting result than teepeeing a couple of car hoods.
Thanks,
Charlie

 

You've got the idea. Laminar flow (Google it up) is air that flows around an object smoothly. This air takes energy at the start but delivers it back at the end. In the case of our Prius, there is a 'burble' of chaotic, non-laminar air that follows behind the car. This air has an average velocity equal to the car that decreases as the distance increases. If you can get the main profile of the trailer in that burble, you'll minimize its drag at very little cost to the towing vehicle. The trailer is 'drafting' the tow vehicle.

What you want to avoid is having the trailer parts extend out into the laminar flow around the Prius. This is where the ribbons come in.

To trace the laminar flow, you'll want a temporary 'stalk' on the spoiler, both sides and one or two on the bottom, rear bumper. The ribbons need to be about a foot or so long so they are clearly visible. Then have someone drive the car while following it with another. If possible, have a camera in the trailing car. Get both cars up to say 50-55 mph where aerodynamic effects predominate.

Now if the ribbons pretty well flow straight back or slightly curve, they are in the laminar flow region. Nothing in the trailer should intrude into that space. Then you need to map the turbulent region.

Using the same ribbons, attach them to the underside of the spoiler and to the left and right sides and the bottom of the bumper. Again reach 50-55 mph and take photos. The ribbons should be doing 'a dance' and the extreme outer range should be the limits of turbulent flow. That is the profile you want to achieve with the trailer.

Now to build a nicely flaired trailer, I would get the wheels and frame that best fits. Narrower will be better than wider because you don't want even the wheels to extend into the laminar flow area. I would then put some 2x4s in the front and side with bracing to be an 'open' frame. If you have time, repeat the 'ribbon' test to verify they are in the laminar flow area and cut down as needed.

I would then go to a hardware or marine supply store and buy the thickest foam blocks they have. You'll also need some marine epoxy and fiberglass.

Cut foam squares to fit on the front and side. Then using epoxy and toothpicks, glue the foam blocks together so they will be thick enough to shape. Using large knives, cut to rough shape and finish with a sanding disk and coarse sandpaper.

Lay the fiberglass over the foam core using straight pins as needed to make it cover the foam. Then starting from one side, mix a little epoxy, brush it on the foam and fit the fiberglass over the foam. Use pins or toothpicks to hold it as needed. Be sure to squeegie the glass so it is fully 'wet' but not dripping. Take your time and a second pair of hands are very useful. (Wear gloves, disposable clothing, paper towels and a mask. Assume the epoxy will find a way to stick to everything and never go away.)

The goal is to have no air gaps between the epoxy and the glass. You also want to glue the foam and epoxy to the 2x4s and have wax paper protecting the trailer. This let's you take off the aerodynamic front and sides when you have oversized loads to move.

Let it cure but check from time to time to make sure you don't have any puddling or other defects. If the upper parts appear to get too dry, don't worry about it because you can always apply an external coat later.

You may or may not choose to sand everything smooth. At our speeds, the surface texture is not that important but a smooth surface looks much nicer. When the surface is good, paint it and you are almost done.

Decide what sort of top cover you want and build it out of thin plywood with re-enforcing. You might want it hinged, my preference, or pegged so it can come completely off. Either prime and paint it or my preference, fiberglass it.

As for the rear, you may want to 'boat tail' it but I suspect a cargo-net would work just as well. There isn't going to be a whole lot of air there to worry about. I would want something easy to remove for quick access and possibly water resistant.

Hopefully this gives you some ideas.

Bob Wilson

 

Bob,
Thanks for the specific info. Looks like I will be in the Nascar business with this trailer drafting me.Shorter front to back is obviously better also,I guess.The 5 ft long harbor freight would be plenty big enough; I 'm not sure how long the tongue is, but probably not too important.
Thanks,
Charlie
PS I got a lesson on practical aerodynamics when I used a roof basket on out Pilot.It had about 3 sq ft frontal area-the Pilot has about 30 sq ft with a .35 or so coe.-so it was effectively 12 sq ft.
Our mpg was 15.9 over 2 3000 mile round trips with the basket.I got smart,and built a carrier that I could suspend from our swing out bike rack.It was completely in the shadow of the Pilot.On the next two trips-same route, same speed we got 22.25 and 22.4 mpg over the 6000 miles!!! Just 3 square feet of very dirty frontal area costs us over 6 mpg,and increased our fuel use by almost 40%!!!!! I wouldn't have believed it if some else had told me that the roof basket would cause that much drag!!I kept careful records-it did!!Our average speed was about 67 mph with some stretches of 75+ in Az.
Thanks,
Charlie

 

When I carry a roof rack load on 2001 Prius at highway speeds, fuel economy is reduced by about 20%.

Slowing down *really helps* when you have a weird load.

DAS

 

Tochatihu,
Sooo right!!Speed really kills mpg and it is accentuated when you have an odd shaped or flat shaped load that has a aero coe. of 1 (or maybe more than one).It not only adds a high aero load because of its frontal area, but it ruins the aero profile of the vehicle to a greater extent that its frontal area would indicate.The 15.9mpg vs 22.4 mpg is by far the biggest mpg improvement any mod ever got me(or ever will get me).Roof racks and baskets are poison mpg wise.A hitch carrier or even a trailer are much better solutions(easier to get to also).The baskets tend to produce a lot of noise also.
You and Bob are correct; if you keep the load in the shadow of the vehicle, it is almost free in respect to mpg. Put it up in the wind,and it is poison.
Thanks,
Charlie
PS I went to Priustrailers-the folks there feel pretty confident with 900 lb trailer loads.One optimist wanted to tow a 1500 lb boat in a 500 lb trailer, but he was dissuaded from it. I feel better about my trailered load-should be 400lbs or less-trailer included.