Version 0.90 - April 6, 2009 - Added photo of fitting.
Version 0.10 - April 5, 2009 - Adding ready brake link and cleaned up some formatting.
Version 0.01 - April 3, 2009 - First release.

Introduction

This web page describes how I approached flat-towing my 2003, NHW11 Prius behind my wife's 1993, Coachman Class B RV that is built on the body of a GMC G30 van:

This is a risky effort and given the litigious nature of our society, I have no plans to offer a product. However, I have no problem with describing how I approached and solved the technical issues:

Mechanical Attachment

Modern bumpers are a plastic, aerodynamic shell covering a sacrificial, safety bumper:
Designed to absorb energy in the event of a crash, they provide no structure anchor points to the body. However, there are four tie-down points used to anchor the Prius to the ship.

My Prius has a 2" receiver hitch that bolts on to where the rear, tow anchors attach. I had already attached a standard 4-conductor, tow light plug to handle a light-weight trailer. This also provides access to rear brake and running lights of my Prius.

The front tow anchors are a permanent part of the frame. In the case of the 2003, NHW11 Prius, each consists of two stamped sheet metal, ~2 mm. thick with strength webs that are part of the front frame. These anchors are back-to-back and as long as the webs are not crushed, provide great strength, rigidity and a permanent part of frame.

Fittings

Although tempting to just put a huge u-bolt, any slack would rapidly deform these webs and weaken the tow anchors and beat themselves into useless scrap. On a ship this is not a problem as the anchor chains are hitched down tight, thus compressing the front and rear struts. Preloaded in tension, the ship motion does not deform them and the vehicle remains firmly anchored in transit.

The first step is to 'blueprint' these anchor webs in my case, using a photo and PowerPoint. I traced the inside of the anchor web and used this to make a template for the insert. I then shaped the external plates that will carry the stress and tested paper cutouts to make sure they would fit:

The following photos show one of the two fittings:

The U-bolt is held by plastic-insert, locking nuts. Since these are never removed, this should be good enough.

The side plates with inserts have enough slack to slide over the tie-down anchors. The mechanical strength comes from the 5/8" bolt being firmly tightened to clamp the anchors. It is important that the inserts do not crush the webs:

With a little looseness, finger tighten until the inserts fit in the anchor webs. Then slowly tighten and jossle until the bolt is firmly seated. The last step is to insert a safety clip to prevent the nut from backing out.

Things That Didn't Work

At one point, I tried to use spackle to make a mold of the web insert. However, the dried material crumbled when I tried to take it out of the web and some of the material can still be seen in the photo.

At one point, I considered making a plywood, disposable version out of marine plywood:

This allowed me to check form and fit but thankfully, were never tested:


This photo shows the problems of the plywood web:

To be large enough to handle the stress, it extends down too far and can easily scrape on speed bumps and potentially stressing the anchors and frame. The form has to extend from the tie-down anchors forward of the bumper to attach to the tow bar.

Tow Bar

The plywood forms were useful for testing the tow-bar attachment. I used a 1/2 inch stainless steel u-bolt to attach the tow bar fitting:

Tow Lights

Normally the towing vehicle powers a set of passive lights at the rear of the towed vehicle. In my case, I'm using the existing rear lights but to prevent any possible voltage feedback, I put blocking diodes. Although it drops about 1.2 V so the lights are not quite a bright, it prevents the Prius 12 VDC system from ever getting in a regulator fight with the GMC G30.

Break-away Braking

Ready Brake is a receiver hitch that has a spring shock absorber and lever that pulls a cable to activate the Prius brake when the RV shows.

The Ready Brake system is the one I've gone with but I haven't installed the braking cable assembly, yet.

The best approach includes a latch and mechanical fuse on the brake cable. If the tow bar falls off and the safety chains break, the last thing the Prius will do is pull the main brake cable, latch and then the mechanical fuse will release the latched cable with the Prius brake fully applied.

Towing Operation

The Prius comes with a two hook and instructions that it can be flat towed up to about 20 mph. But this is only for retrieving or moving a broken Prius about. To be practical, highway speeds up to 80 mph although many tow bars are placarded to just 55 mph. Even 55 mph poses a risk.

The Prius MG2 is always spinning with the front wheels are turning and work as a generator. As the speed increases, the voltage increases and could damage the inverter. There is also MG1, a critical part of the transaxle, that with the engine off spins and generate substantial voltages that could also destroy an unpowered inverter. Toyota papers and the control laws set 42 mph as the maximum speed before the engine has to turn. However, with the engine running, there is no risk to the car at ordinary highway speeds.

"N" with engine running

One approach is to start the Prius and with the engine running, slip it into "N." This will keep the Prius engine running and the inverter in an "idle" state. Unfortunately, the vehicle electrical system will be powered by the slow discharge of the traction battery.

One can 'force charge' the traction battery by holding the brake and pressing the accelerator until the engine shuts down. So before beginning the tow, force charge the battery and then use one of the diagnostic techniques to get the engine spinning and slip it into "N". Then begin the tow. Be sure to take regular breaks to check the state of charge and replenish the traction battery as needed.

"D"

The second option, the one I favor, is to put the Prius in "D" and drive. Initially the Prius will charge the traction battery from the energy of the RV but when it is fully charged, the control laws will block further charging. What we don't know, yet, is if the Prius goes into some form of "B" mode using the engine to provide drag. However, in this mode the vehicle can be towed any distance without harm. It is as if it were coasting down the largest hill in the world, the distance of the tow.

One variation is to combine the techniques. Tow the Prius for about 15-20 minutes in "D" to fully charge the traction battery. Then stop to inspect the tow setup and make sure no fittings are coming loose. Trick the engine into running and slip it into "N" for the next couple of hours. Then stop to check the traction battery state of charge.

Safety

Towing a 3,000 pound vehicle at speeds of 55 mph+ is inherently dangerous so it makes sense have have a checklist for how to connect and stop a tow. Stuff can be replaced but humans are not.

Connection Checklist

NEVER GET BETWEEN THE PRIUS AND THE TOW VEHICLE WHEN THE PRIUS OR TWO VEHICLES HAVE RUNNING ENGINES!! NEVER, NEVER, NEVER!!

CONCLUSION

I have used this system without the break-away receiver to take the RV to various repair shops and towing the Prius at speeds under 30 mph in the city. Once at the repair facility, I disconnect the Prius and do my other errands. But I keep the speeds low and distances short.

If you have questions about this, I hang around at: