Scooterzilla

Dave Broyles 2003

 

In the OZ report, someone made mention of my scooter-tow system and only because it looks ugly and crude, called it the Frankenstein scooter-tow system. Well, I coined the phase "scooter-tow" in 1996, and my first system was based on a bright and shiny Yamaha Riva 125, but I found that it did not have enough power to tow really big pilots or tandems. I checked a local motorcycle junkyard and found a wrecked Honda Elite that had been sitting there for at least 5 years, buried to the axles in the mud. With little hope of making it into anything useful, I bought it cheap. The plastic body had been ripped off by the accident that put it there, but surprisingly, when I took it home, cleaned up the carb, aired the tires and put a new battery on it, it started right up. I built a winch wheel drum and tested its towing capacity. I rapidly found that it was so powerful that the rope wrapping on the wheel drum crushed it and distorted the sides of the drum. I quickly built a stronger wheel with a reinforced center and found that it was a Godzilla of a tow system. Thus, Scooterzilla.

On the day mentioned in the OZ Report, the wind was blowing about 10 mph and Scooterzilla was towing a 200 lb. hang glider pilot on a Vision Mk 4-19 to over 700 ft.

It has towed a tandem hang glider loaded to 400 lb to over 500 ft and a tandem paraglider loaded to 450 lb to 600 ft in light winds. It has done 15 tows of the same pilot in one hour. I am sure that there are few stationary winches uglier, but few more reliable or better working.

 

While this system looks crude, it is actually sophisticated in design. The scooter itself provides one of the major features built in. The scooter, a Honda Elite 250, has a constantly variable transmission (CVT) and a centrifugal clutch. These features make operation relatively easy since clutch operation and shifting are unnecessary. The CVT is designed to keep the engine running at an RPM consistent with maximum power output. The centrifugal clutch allows the rope to pay out with no load when the engine is idling or off. The size of the rear wheel makes the overall gearing appropriate for this application without alteration. The CVT also compensates for the change in diameter of the rope wrap as the rope winds up and it provides a changing gear ratio to compensate for changing loads due to winds.

Scooterzilla has a built-in line tension gauge. It consists of a surplus aircraft hydraulic cylinder that luckily had an inner cylinder area of approximately 1 sq. inch. (Don't ask me where to get another. I bought it in 1975 and have never seen another like it.) Air actuator cylinders filled with mineral oil actually work pretty well, but require that the face of the pressure gauge be recalibrated or use a conversion table since it is probably impossible to find an actuator with a 1" internal area (on the shaft side).

 

The cylinder is connected to a 0-500 lb pressure gauge on the handlebar of the scooter.

 

The scooter is mounted on a frame with rollers that is restrained by the hydraulic cylinder one end, and the rope comes out through a fixed set of rollers at the other. Due to Newton's 3rd law, the force on the cylinder is equal to the tow rope tension. The trailer it is on has a trailer tongue jack to level the trailer

 

The picture of the fixed guide rollers shows no tow rope. The blue strap anchors the frame for travel. It must move freely back and forth in operation and the strap is removed for towing.The guide rollers on the front of the scooter are to make the rope wind evenly on the wheel drum, and are centered on the centerline of the scooter as is the rear wheel. They also keep the frame lined up evenly with the fixed guide rollers.

 

An important feature of this system is the rear wheel brake of the scooter. If the rope is pulled out and stopped rapidly the wheel drum doesn't stop, the rope will backlash or tangle. An operating rear wheel foot brake is necessary to stop the wheel when turning at inappropriate moments.

This system uses 4000 ft or more of 1/8" Spectron 12 rope. Spectron 12 is a trade name of Sampson rope company. It is also sold as Amsteel 12 by Sampson.

 See http://www.samsonrope.com/home/recmarine/de_12_am.cfm and http://www.samsonrope.com/products/Arborist/ar_spectron_12.htm It is a HMWPE fiber also known as Spectra and Dyneema. It isn't cheap, usually about 25 cents per foot.

Because the amount of rope needed is much greater than the capacity of the wheel drum, The rope is stored on a separate drum with a motorcycle starter motor powering it. (Imagine hand cranking 4000 ft.of rope).

 

 

For training and for use with the retrieval winch, the rope is run through a turn-around pulley attached to a 3 ft long earth anchor or a car bumper stationed 2000' away from the winch. It is important to protect the rope and the pulley from rope recoil by wrapping duct tape around the top of the pulley so that there is nothing for the rope to catch on. It is also possible to replace the snatch block, swivel and hook with a triangular aluminum 3/8" plate with a hole for a carabiner to reduce the chance of the rope snagging on the pulley. The real value of the snatch block though is that it allows the pulley to be put on the middle of the 4000 ft rope rather than at one end.

 

The retrieval winch has a number of special features. It is based on a 12V Ford long shaft starter, which is rare since most Ford long shaft starters were 6V. The stock endplates had plain bearings which wore out the armature shaft and themselves at a high rate of speed. They were replaced by "Real Balls" aluminum endplates with ball bearings which will last much longer in this winch application. The winch drum is adapted from a radio control sailplane winch, but is much stronger since wrapping the nylon retrieval string on the drum at 50 lb or more of tension blew the stock side plates off.

 

Normally the drum would mount directly on the starter shaft, but at the speed the starter turned, it would instantly break the retrieval line, so a jack  shaft was mounted with a chain reduction of about 4 to 1. In addition, a low RPM centrifugal clutch was added to keep the motor from turning when the line was being paid out.

 

A brake was added to stop the drum from spinning wildly when the payout stopped.

 

The only thing cheap about this winch is the string, which is the best #18 nylon twine available. (Avoid braided string. Twisted string is much stronger. Braided #18 string lasts for hours and twisted #18 string lasts for months. Also avoid cheap imported string. The best made string lasts much longer.) This winch works very well if the string used is high quality and is replaced when it starts wearing out.

 

Various people have suggested a more elegant lever for the anti-backlash brake than a piece of 1/4" steel with a hole in the bent end. They worry that the steel will wear out the string. We should point out that the string goes through a hole  in the end that started out 1/8" in diameter, but is now about 3/8". The string wore the steel out... so we don't worry about about the string. The original battery fit in the box with the winch motor. It was an Optima deep cycle. It was always dead before the end and required recharging the battery with my truck. Now I use a really serious battery. This Interstate Batteries 4DM is a deep cycle marine battery and the biggest battery of this type I could find. It usually lasts for a full day of instruction if it is fully charged before going out. This battery is not your grandmother's battery and needs more than your grandmother's charger.

 

Here is a picture of a tow with the retrieval winch in action. What the retrieval winch does is provide a really short turn-around between tows.

 

After saying how wonderful Scooterzilla is, we still must mention a few operational issues. In strong winds, the scooter will put all it's power into holding the rope still. That means that all its power is dissipated into the centrifugal clutch. Twenty horsepower can get a clutch really hot in a very short time. Surprisingly, a very small amount of power will hold the rope still and still give the glider a good climb rate so the wise thing to do is to throttle down nearly to an idle. It may not give as good a climb rate, but a new clutch is about $200 even if you put it in yourself. Scooterzilla also requires care in operation because of the possibility of backlash. The towline needs a parachute and the operator must hit the foot brake the instant the pilot releases the tow rope. When retrieving the rope either with the retrieval winch or manually, it is essential to hit the brake the instant the retrieval stops. The operator must always insure that the rope is not tangled or backlashed before starting to pull rope in.

If using a pulley, the operator should realize that an elastic rope such as Dacron or nylon has an enormous recoil at the pulley when the pilot releases and there is even some recoil with Spectra. This means that if there is anything that the rope can hang on at the pulley, it will, including the top of the turnaround pulley. It is essential to wrap the top of the turnaround pulley with duct tape so that the rope won't hang on it. Hanging the rope on something at the turn around pulley will usually ruin a length of rope. On the other hand, Spectra will usually rip up the pulley or what ever it hangs on. So don't forget the duct tape.

Scooterzilla is still under development. I have some ideas for improvement. I don't think the changes will necessarily improve its looks but form follows function. My goal is easier operation, better training classes and higher tows. If, however, someone wants to give me a nice scooter that hasn't been wrecked...

For those who want plans, I have drawn up a set of plans for $25 plus postage. I know that there now many scooter-tow systems out there with different designs and I think that it is fine to roll your own. Your goal should be improve safety, get better performance, more ease of operation and lastly to look the way you want it. Whatever you build, send me a picture and I will put it on my web site.

 

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For more information, call 972-390-9090 or email to d.broyles@comcast.net Dave Broyles.