- What kind of brakes do I need on my horse trailer and why?
- What is a breakaway brake?
- Are safety chains really necessary?
- Why are people getting away with hauling with illegal equipment and a really scary looking trailer?
- Do I need a logbook, medical card, or commercial driver's license?
- What is the difference between GVW, GVWR, GCVWR, and GW?
- What is Unladen Weight?
- Why is it important to know the GVWR or the GW of my vehicle and trailer?
- Should I stop at weigh stations?
- What kind of hitch do I need for a tag-along trailer?
- Is a gooseneck trailer better than a tag-along?
- What are the most important things I should consider when looking for a new trailer?
- How do I know I am getting the right tow vehicle for my horse trailer?
- What are the best construction materials for horse trailers?
- What should I look for in a used trailer?
- Are sport utilities and cars adequate to pull trailers?
- What is the proper way to have weight distribution bars installed on your trailer tongue?
- How do I use a a weight-distributing hitch?
- How far should one travel before stopping for a rest and checking the horse?
- When hauling one horse in a 2-horse bumper pull straight load, which side of the trailer should he ride on (in the US)?
- What are the definition of terms like a coupler, undercarriage or chassis?
- When hauling a horse trailer with living quarters with a 1 ton truck what are the advantages of 4 vs. 2-rear wheels?
- Are fiberglass roofs common and are they recommended?
Thirty one states require brakes on at least one axle (2-wheel brakes) on trailers over 3000 lbs. GVW and 11 states require brakes on both axles (4-wheel brakes) on trailers over 3000 lbs. GVW. In addition, 3 of those states that require brakes on trailers over 3000 lbs., require 4 wheel brakes on trailers over 4,000 lbs. (No matter how light your trailer is, when it is loaded with horses it will weigh 3000 lbs. or more!) The remaining states that do not require brakes per se have regulations that require the ability to stop the combination without sway from a specified speed over a specified distance.
For those who are classified "commercial", 4 wheel brakes are mandatory for vehicles and combinations of vehicles over 10,001 lbs. These regulations have been put into effect because having brakes on the trailer reduces the possibility of accidents, and makes hauling not only safer for you and your horse, but for other people on the road. Without brakes, the trailer will push against the tow vehicle every time you stop, and if there is a sway, the trailer can jackknife because there is nothing to control it.
Trailers come equipped with several different types of brakes, but electric brakes are the most common type. When the brakes are adjusted properly, stepping on the tow vehicle brake pedal activates the trailer brakes just slightly after the tow vehicle brakes are activated. Because there is a control box located on the dashboard within reach of the driver, it is possible to activate the brakes from the driver's seat without stepping on the brake pedal in the tow vehicle. If the trailer starts to sway out of control while you are driving, you can work the trailer brakes by hand and gain control of the trailer without braking your vehicle.
There are other types of brakes such as surge or hydraulic brakes, which are activated when the trailer pushes up against the vehicle when it slows down. These types of brakes are usually legal, but may not be legal in the states that require the brake to be able to be activated from the driver's seat, especially if you are classified as "commercial". (This requirement may be open to interpretation by the law officer.) Those who believe that brakes are not necessary for a horse trailer are asking for trouble. Besides increasing the chance of an accident, there are fines for driving with illegal equipment. Even if you never get stopped and checked, if you have an accident, your liability will be increased and you may be held at fault for the accident. You, your horse, or other drivers on the road may be injured or killed. It's just like wearing a seatbelt or a riding helmet. You may think you don't need it until its too late!
A breakaway brake is an independent device located on the coupler of the trailer that activates the trailer brakes if the trailer should come off the tow vehicle. To be legal, it must be equipped with a fully charged battery that will engage the trailer brakes for 15 minutes. A removable pin on the battery box is attached to a cable that is hooked onto the tow vehicle. If the trailer pulls free, the pin pulls out and the breakaway device activates the trailer brake. The cable should be attached to the frame of the vehicle or to the permanent part of the hitch, not to the ball or ball mount since either of these parts may also pull free with the trailer and the brakes will not be activated.. At least seventeen states require a breakaway brake on recreational and privately owned horse trailers, although some states have a "grandfather" clause that exempts older trailers. All commercial trailers must have one.
Even if it is not required in your state, it is a good idea to have your trailer so equipped. Most new trailers come with a breakaway brake as standard equipment. Some newer trailers are equipped with a rechargeable battery.
Forty-six states require safety chains on tag-along trailers and 29 of those states also require them on gooseneck trailers. The remaining states strongly recommend safety chains. The safety chains should be attached to the frame of the vehicle or to the frame-mounted hitch. They should be crossed underneath to catch the trailer in a sort of cradle if the trailer comes off the ball. The chains should not touch the ground and they should be long enough to allow the trailer to turn corners without pulling. Trailers do come off, especially if they have not been properly hitched or if equipment fails. Don't take a chance - use safety chains!
I see people hauling with illegal equipment and really scary looking trailers. Why are they allowed to get away with it?
The officers who enforce the laws are not always interested in horse trailers unless there has already been an accident, so sometimes it seems that they let some really dangerous vehicles go by. However, things are not always as they seem, because people do get stopped and when they do, the fines can be heavy. Unfortunately, if these people continue on the road, they will probably eventually cause someone to get hurt or killed, or their horses will suffer from their negligence.
When a horse gets into a trailer, he is completely dependent on the driver for his life. A good horseperson takes this responsibility seriously.
If your vehicle or combination of vehicles has a GVWR (Gross Vehicle Weight Rating) or GCVWR (Gross Combined Vehicle Weight Rating) of less than 10,001 lbs., you do not need to be concerned about these things. However, if your rig has a GVWR or GCVWR of 10,001 lbs. or more, you must make some decisions. (The GVWR is determined by the manufacturer and should be marked on your trailer and your tow vehicle. The GCVWR can be obtained by adding the two together. A two-horse trailer and a pickup truck can easily be rated over 10,001 lbs.).
Even if you don't haul horses for money, the US Department of Transportation (USDOT) can consider you "commercial". Operating a commercial stable, hauling horses for show (with intent to profit), rodeo, race, sale, training, or for compensation are examples of "commercial ventures" as defined by the DOT for those who are traveling interstate. Those who only travel intrastate need only be concerned with the regulations within their own state of residence. Most states have the same regulations and some have even more stringent laws.
If you fall into the commercial category, you must follow the Federal Motor Carrier Safety Regulations (FMCSR). This means that you must carry a logbook, medical card, certain safety equipment (such as flares, triangles, fire extinguisher etc.) and your vehicle must be equipped with certain features such as required lights, safety chains, brakes, and breakaway brake. You must also have a federal inspection sticker, ID number, and a sign on your truck.
For interstate travel, a commercial drivers license is only necessary for driving a vehicle or combination of vehicles over 26,001 lbs. Some states have additional classified licenses for those who drive vehicles or combinations under 26.001 lbs.
Farm exemptions are available, but if you have a farm plate, you may not travel more than 100 miles from your farm.
GVW (Gross Vehicle Weight) and GW (Gross Weight) are interchangeable terms meaning the actual weight of the vehicle (trailer) and its complete load. This weight can be determined by loading the horses, tack, feed, and hay etc. into the fully equipped trailer (mats, spare tire, etc.) and taking it to a truck scale to have it weighed. Most gravel yards or truck stops have truck scales.
GVWR (Gross Vehicle Weigh Rating) is the value specified by the manufacturer as the recommended maximum loaded weight of a single vehicle. For a trailer this value is determined by the axle capacity and the coupler capacity. For example, a trailer will be rated at 5000 lbs. GVWR by the manufacturer if it has two 2500 lb. axles and a 2 inch ball coupler that is rated 5000 lbs. Loading the trailer to excess of the GVWR is not only unsafe, but is illegal.
GCVWR (Gross Combined Vehicle Weight Rating) is the value specified by the manufacturer as the recommended maximum loaded weight of a combination of vehicles. (The GVWR of the tow vehicle plus the GVWR of the trailer.) In the absence of a value specified by the manufacturer, GCVWR will be determined by adding the GVWR of the power unit and the total weight of the towed unit and any load thereon.
Unladen Weight is the actual weight of the trailer as it is equipped with mats, spare, etc. but empty of its load. This weight may be stated on the Certificate of Origin or the title of the trailer.
Most states require trailers to be registered by weight. Some states require registration by GVWR or GVW whichever is greater and some states require unladen weight. Other states have a dividing weight that determines license plate classification. Whichever your state requires, you must know the weight of your trailer. In most cases, if the weight of your rig exceeds the weight on the registration, you can be cited.
The weight of your trailer is also important in choosing the proper tow vehicle.
I have been hauling horses for years and I have never stopped at a weigh station. I have been told I should pull in when I have a horse trailer. Is this true?
Each state has a different weigh station policy. All commercial vehicles must stop, but sometimes even non-commercial vehicles must pull in. As a general rule, if the sign says "All Trucks" must pull in, it probably means pickup trucks too. Some states want to see vehicles that have "commercial" plates even if they are not classified "commercial" by use. They may want to check vehicle registration, driver's license, weight, or safety equipment and often they will want to see the health papers of the horses or do a brand inspection. Most of the time, the weigh station personnel will be too busy with big trucks to bother with you and they will probably wave you on. Some people have even been told they should not have stopped. Horse trailers fall into the cracks, and any enforcement official has the option to enforce the regulations as he sees fit. Any sign that says "Vehicles with Trailers" or with "Livestock" means you must pull in. In this case, they will want to inspect the horses. If you do not stop, they may pursue you and bring you back. The fines can be very steep and you can be held for a very inconvenient period of time.
A frame mounted Class III or Class IV receiver hitch is mandatory for safe hauling, and is the only hitch that is legal in all states. This type of hitch is bolted or welded to the frame of the tow vehicle and has a square receiver for a slide in ball mount. The ball mount itself can be purchased with the proper drop so the trailer travels absolutely level. (A trailer that is not level will not travel properly and may sway or wear the tires unevenly because the tongue weight of the trailer will be compromised. There is also a likelihood that the trailer can pop off the ball. The horses inside the trailer will find it hard to balance which can cause trailer control problems and extra stress on the horses.)
Each part of the hitch should be rated to match the GVWR of the trailer. For example, if the trailer has a GVWR of 5000 lbs., the hitch, ball mount, and ball must also be rated at least 5000 lbs. The entire rig is only rated as much as its weakest link! A ball that is mounted on a "step bumper" of the truck does not distribute the weight throughout the tow vehicle and it is not adjustable. Federal Motor Carrier Safety Regulations do not allow a trailer weighing 5000 lbs. or more to be towed by a ball on a step bumper.
The rating of the hitch will be imprinted on the hitch itself. There are two classifications of ratings: "Weight Carrying" determines the maximum weight the hitch can support with the weight of the trailer carried only on the hitch without weight distribution bars. "Weight Distribution" rating determines the amount that can be safely accepted by the hitch with weight distribution bars. Both ratings will be stated on the hitch. The ball and ball mount will also each have the weight rating imprinted in a visible place.
Weight distribution bars, sometimes called "stabilizer bars" or "equalizer bars" distribute the weight of the trailer and tow vehicle throughout the entire combination. When the tongue weight of the trailer rests completely on the rear of the tow vehicle, it can lift the front end of the tow vehicle. The weight distribution bars prevent this from happening. They also increase the GVWR of the hitch. They are very important when towing with a downsized vehicle, a vehicle with a short wheelbase, or a long trailer.
Weight distribution bars can only benefit your situation. Weight distribution bars differ from a "sway bar". If there is a need for a sway bar, there is something wrong with the trailer that shouldn't be happening such as uneven tire pressure, suspension problems, or an unlevel trailer.
For more than two horses, yes. For a two-horse trailer, a tag-along trailer is just a safe as a gooseneck trailer if it is properly hitched to the proper tow vehicle. The advantages of a tag-along include cost, ability to be towed by something other than a pick-up truck, and a smaller size for storing and hauling. If you do not need the extra space for sleeping or tack/feed storage, the extra expense for a gooseneck trailer may not be necessary.
For anything more than a two-horse trailer, a gooseneck trailer has more stability, and a smaller turning radius.
There is some difference between the two when it comes to driving. A tag-along trailer will follow the wheels of the tow vehicle when turning a corner, but a gooseneck will cut in on the turn. Sometimes new gooseneck drivers will catch the fenders while learning to make the turns.
(Read the full interview by clicking here.)
The first thing you should consider is your horse(s). Horses are claustrophobic by nature so the more room, light and ventilation in the trailer, the less stress on the horse. In practical terms, this means that the horse will travel more comfortably, will load more easily, will be less likely to injure himself, and will be less likely to suffer from stress related health problems like shipping fever, dehydration, and colic.
No matter which style of trailer you prefer, the size of the horse is an important factor. The horse should have enough room to move its legs forward and sideways to keep its balance while moving. Narrow trailers and trailers with full center dividers can cause the horse to scramble because he can not spread his legs enough to keep his balance. Slant load trailers are not styled for horses that are over 15.3h because the length of the stall is not long enough to allow the horse freedom of movement. Horses over 15.3h should have 7 feet of stall length and 3 feet of head area - 10 feet overall. It used to be that a 7 foot tall trailer was extra tall, but nowadays 7'4" to 7'6" are common and should be considered for any horse over 15.3h. The horse should also be allowed to lower its head so he may remove debris (hay dust and other contaminants in the trailer) by coughing, therefore keeping his respiratory tract clear.
Rubber torsion suspension is available on almost all newer trailers. This type of suspension greatly reduces the amount of shock the horse absorbs through the floor of the trailer, also reducing stress. There is also a safety advantage to this type of suspension. If you have a flat tire, the remaining three wheels will maintain the trailer until you can get to a safe place to change the tire.
There should be no sharp edges or protrusions anywhere on the trailer, inside or out. Floor boards should run vertically (the length of the trailer), not horizontally (across the trailer) and there should be good support underneath. Horse trailer mats should not be slippery. Ramps should be non-slip and not steep. (Step-up trailers can be dangerous when backing the horse out because it can slip under the trailer causing major injury. The horse should be allowed to walk out headfirst when unloading from a step-up trailer.
All tie rings, center dividers, chest bars, and butt bars should be easily worked by quick release. All parts should also be strong enough to hold up to the largest, strongest horse you will be hauling. When considering construction material, think about how well it will hold up to a panicky horse, or a traffic accident. If you have large horses, strength, not weight should be your first priority.
When you have made your decision about your horses' needs, consider yours. Will you be hauling long distances, or will you only need your trailer for short trips around town? Do you need a dressing room for showing, or a gooseneck for sleeping. Are you a timid driver, or do you have the confidence to haul a large rig across the country?
The final decision is the tow vehicle. Now that you know what trailer you need, buy the vehicle to haul it.
The most dependable information you can get on tow vehicles is a manufacturer's Trailer Towing Guide. Each manufacturer publishes such a guide that easily explains how each vehicle is rated to tow. Unfortunately, many dealers do not have this towing guide at the dealership. You should insist that your dealer consult this guide before you buy. Many a car salesman has sold an unsuitable vehicle simply because he is not fully informed on the specifics of hauling a horse trailer. The second most reliable source of information is a reputable horse trailer dealer.
Towing with a vehicle that is underrated will cause undue strain on the vehicle. Acceleration and deceleration will produce wear and tear that will decrease the life of the vehicle. Also, maneuverability will also be affected, resulting in unsafe operation.
There are many choices for construction materials today - all have advantages and disadvantages. You must make your choice based upon your own priorities.
Steel - Steel is the strongest material and it yields to stress, making it less brittle than some other metals. This is why automobiles are constructed of steel. Steel is also the least expensive material. One of the disadvantages of steel is the fact that it rusts, but some new trailers are constructed of "galvanealed" steel which is very rust resistant. Even good quality, non-galvanealed steel trailers can be protected by a good coat of paint or undercoating and these trailers can have a long life if maintained properly. The steel industry has made many improvements to steel in the last few years and rust is no longer much of a problem.
The main disadvantage of steel is weight, but since less steel is needed for strength than aluminum, a steel trailer can compare very favorably to aluminum in respect to weight. Learn more about aluminum vs. steel horse trailers.
Aluminum - Aluminum is lighter than steel. Aluminum does not rust, but it can corrode. Aluminum is not as strong as steel, and it is more brittle because it does not "give". Repairs to aluminum trailers can be more difficult because aluminum welding is more difficult that steel welding. People with smaller horses may be satisfied with an aluminum trailer, but larger horses may cause more wear and tear, shortening the life of the trailer. Aluminum is a good heat conductor, which is why so much cookware is made of aluminum - but do you really want to cook your horse? Also, contrary to popular opinion, aluminum is not maintenance free.
Aluminum must be thicker than steel to be as strong, therefore, an all aluminum trailer is not always lighter than a trailer made of another material. Aluminum is also the most expensive material.
Picture this: You have two pieces of metal pipe. One pipe is steel and the other is the same type of aluminum alloy that is used in most horse trailers. They are the same length and the aluminum pipe is twice the thickness of the steel pipe. The aluminum pipe is only slightly lighter than the steel pipe. If both pipes were struck with the same amount of force that would break the aluminum pipe, the steel pipe would only bend. In order for the aluminum pipe to be as strong as the steel pipe, it must be THREE times as thick as the steel pipe, which would make the aluminum pipe slightly heavier than the steel pipe.
Hybrids - Hybrids can combine the best of all the materials. Steel is used for the frame and chassis and another non-rusting material such as aluminum, fiberglass, or FRP (Fiberglass Reinforced Product) is used for the exterior and other parts that do not receive so much stress. Steel parts made of the newer galvanealed steel are a superior choice since it is so rust resistant. By combining these materials in this way, the weight can be taken out of the trailer without sacrificing the strength of the trailer. Hybrid trailers are more expensive than all steel trailers, but less expensive than all aluminum trailers.
A used horse trailer should have all the same criteria that we have listed for a new trailer, but in this case, the condition is an added consideration.
- Make sure the floor and undercarriage are in good condition. This goes for both wood and aluminum floors, and structural beams under the floor.
- Check the suspension and tires.
- Know if the brakes work, and find out how much it will cost to fix them if they don't. Uneven tire wear can signify some problem in the axle alignment or balance of the trailer. Horse trailer tires seldom wear out, but dry rot is a common problem.
- Sometimes the coupler can be worn inside, causing the coupler to be too large for the ball.
- Check for rust or cracks in places where there is stress. Stress fractures are a special consideration for all aluminum trailers. Make sure the frame and welds are structurally sound.
- Sometimes surface rust on steel may be unsightly, but if it is not in a place where it is supporting the integrity of the trailer it could be all right.
- If repairs need to be made, ask yourself if you will be putting more money into it than the trailer is worth. Spending too much money for restoration may make the trailer suitable for your own use, but do not expect to add that much value to the trailer when you sell it.
I don't want to buy a full size truck to tow my horse trailer. I see people using sport utility vehicles and cars to pull their trailers. Is this OK?
You may consider downsizing to a smaller vehicle as long as the vehicle is rated to tow the weight of your trailer with its full load. The weight of the horse(s) you will be hauling is the factor you cannot change in this combination. If you are hauling one 900 lb. horse, you can purchase a lighter trailer and keep your weight down considerably. If you have two 1500 lb. warmbloods, you cannot change the fact that you are hauling 3000 lbs. of horse no matter how much your trailer weighs. In addition, if you want a trailer that is strong enough to hold up to the extra wear and tear of such large horses, you should not have an extra light trailer. This is like carrying bricks in a plastic bag - sooner or later the bricks will break the bag.
The wheelbase is usually shorter in a downsized vehicle, which makes it more important to install the proper hitch with weight distribution bars.
Because a lighter weight vehicle can be pushed by the trailer when stopping, or pulled out of control by a trailer sway, your trailer brakes must be in working order. See our full line of horse trailers.
What is the proper way to have weight distribution bars installed on your trailer tongue and how do you determine how to adjust them for your load?
For example, I may leave with one horse but return with two. Do I change the links when adding another horse? How do I determine which link to use for the weight I'm hauling? We just eyeball the levelness of the vehicle and trailer. Is this the only way to decide if they're being used correctly?
Once you determine the link you need for your trailer, you shouldn't need to readjust for different load weights. If the trailer is properly balanced, the percentage of tongue weight to load should not change. To make the initial determination, you must have a perfectly flat area to hitch your vehicle and trailer. Measure the vehicle height from ground at a place in the front of the vehicle and a place in the rear of the vehicle and write it down. Do the same for the trailer. Adjust the height of the ball so that it is slightly higher than the coupler of the trailer. The height of the ball will vary according to how 'soft' the suspension is on the tow vehicle and how much it will pull the vehicle down. When the trailer is hitched, the combination should look level. If there is a difference of level, the trailer tongue should be slightly on the lower side, not the higher side.
Attach the spring bars to the hitch head. Approximate a link that looks like it will lift the hitch and pull up the brackets. Now go back to the tow vehicle and measure the front and back in the same place you measured before. Measure the trailer again. If the measurements are the same as before, the link is the correct one. If the front or back of either vehicle has changed, then adjust the hitch until you have a level combination. You can check the combination again when the trailer is loaded. If there is a difference, you can make adjustments and then you should not have to change it again.
I have a truck and trailer that I bought used that have a weight-distributing hitch. It has bars and chains. I know this is good but I don't know exactly why or what it is supposed to do exactly. Can you tell me?
A weight distribution system works like a wheelbarrow. A wheelbarrow carries its loaded weight on three points - the front wheel and the two legs. By lifting the handles of the wheelbarrow the weight is distributed through the front wheel and the handler's legs. The hitch weight distribution system lifts the tongue weight and transfers it to the front wheels of the tow vehicle and the wheels of the trailer, keeping the front end of the tow vehicle on the ground. This makes the combination work like one unit and greatly increases the stability of your rig.
Weight distribution bars also increase the capacity of the hitch as explained in an earlier question. The hitch has two ratings. The higher rating is the weight distributing rating and is only in effect when the bars are installed.
Load up, drive down the driveway, and get out to check everything over before you pull out onto the road. Check over your trailer hitch, make sure the ramps are up, the doors are closed, and the horses are tied. Make this part of your routine every time you take you trailer out.
After that, my co author of Hawkins Guide: Equine Emergencies, veterinarian Dr. Jim Hamilton suggests that as a general rule, 10 hours is considered a long trip for a horse. There are many variables to this rule, however. The health, temperament, and experience of the horse make a difference, and weather conditions should always be considered. It is most important that the horse does not become dehydrated, since dehydration can be the cause of several other conditions such as heat exhaustion, colic and shipping fever. If your horse drinks while in the trailer, stopping to check and water your horses when you need to stop for your own breaks should be sufficient. If, like most horses, yours doesn't like to drink when he is in the trailer, it would be a good idea to plan a rest stop after 5 hours or so depending on the weather conditions and take the horse off the trailer so he will drink.
Do not take any horse off the trailer at a rest stop or anywhere else where he may be in danger of getting away or eating grass that may be treated with toxic chemicals. You can plan ahead by contacting a stable or vet clinic along the way before you leave. If you don't have a safe place to unload, leave the horse on the trailer.
Also, know how to monitor your horse's condition by doing the "pinch test" for dehydration. Determine your horse's normal temperature, pulse, and respiration while he is at home, so you have something to compare when you take these vital signs on the road. Remember that horses need as much water in cold weather as in hot, but in hot weather your horse may be losing water through perspiration more rapidly, and you may not realize it if the breeze from traveling is drying him off before you see the sweat.
After 10 hours, it would be advisable to stop and stable the horses for a rest of several hours or more.
Dr. Hamilton suggests you feed your horse electrolytes for a few days before the trip, and consider plenty of water - about 20 gallons - part of your first aid kit. You may need water not only for drinking, but also for cooling a horse off. I like to soak the hay before putting it in the trailer so the horse can get more moisture though his feed.
When hauling just one horse in a 2-horse bumper pull straight load, which side of the trailer should he ride on (in the US)?
Some have said on the left because the road is higher in the middle, others say on the right b/c if something does happen, the horse isn't on the side of oncoming traffic. Which side is best?
It's best to put the horse on the roadside (left) because most roads are crowned in the center and the trailer will be more stable. Also, when there are two horses in the trailer, the heavier one should be on the roadside.
I don't understand what a coupler or undercarriage or chassis is as well as some other parts stated. Definitions for beginners would be helpful, or definitions with pictures.
A coupler is the part of the trailer that "couples" the trailer to the hitch ball whether the trailer is a tag-along or gooseneck trailer.
An undercarriage is the part of the trailer that supports the trailer from underneath. It supports the entire trailer and the axles are attached to the undercarriage.
A chassis is the frame of the trailer that the body of the trailer is built upon. The undercarriage is part of the chassis.
Our book, The Complete Guide to Buying, Maintaining, and Servicing a Horse Trailer has more complete explanations of trailer construction.
I will be purchasing a 1 ton truck to haul my horse trailer with living quarters. What are the advantages of 4 vs. 2-rear wheels.
You are referring to a "dually" as opposed to a single rear wheel on your truck. First of all, determine the weight of your loaded trailer and know the tongue weight. When you purchase your truck, ask the dealer for the manufacturer's Tow Vehicle Guide. This guide will tell you how much your truck is rated to pull and how much tongue weight it can carry. If the towing guide tells you that you can haul your trailer with the single wheel axle, then you should not have to have the dual wheel. If you are close to the load limit, you would want the dual wheel option because it will make the combination more stable from side to side. There may or may not be a difference in carrying capacity.
Some people feel more comfortable with the dual wheel, and if it gives you more piece of mind to have the dually, then you should choose that option. There are a few disadvantages of the dually. The extra width on the truck will make it a bit more difficult to find parking spaces if you drive around town in your truck, and the extra tires will be more expensive.
My used trailer had one, and in an accident it was a literal lifesaver for my mare. With the trailer on its side, and me alone, I could break away the fiberglass to get her out. I am having trouble finding another one like it.
I like fiberglass roof construction, too, which is why we have it on our EquiSpirit Trailer. Besides the advantage you mentioned, fiberglass does not get sharp if it tears in an accident like metal does, and cause more injury to the horse. Also, because it's not much of a heat conductor, it makes for cooler temperatures inside the trailer.
All fiberglass roofs are not created equal, however. I believe there should be a strong frame molded into the fiberglass to act as a roll bar effect if the trailer should turn over. Fiberglass itself is not strong enough to hold up to a rollover.