Trailer Road Clearance

[DarrylR]

I have inherited from a good friend a 1981 Montauk 17. Its trailer needed an overhaul: new bunks, brackets, rollers, and axle.

The mechanic who did the trailer work set the axle above the springs. As a result the trailer frame is close to the ground. The mechanic explained he did that because the trailer would have been too high above the road if the axle were below the springs.

When parking the trailer in my driveway I had to remove the license plate to keep it from scraping the ground as I backed it up over the gutter.

I am concerned that the trailer road clearance is too low.

Q1: what should be the minimum clearance between the road and the boat frame?

When level to the ground, the rear end of the trailer has only a 7-inch clearance.

Q2: what should be the clearance when towing on the highway?

[Jefecinco]

I recommend checking the attitude of your trailer before concerning yourself with the road clearance. Ensure it is approximately level when on level terrain. The height of the hitch ball on your towing vehicle determines the attitude of the trailer under tow.

Seven inches of clearance seems low if that is the clearance while hitched to the tow vehicle on level terrain. What part of the trailer are you using for the measuring point.

The lowest part of my trailer is about 12 and 3/4-inches above the ground measured at the bottom of the fenders. The trailer is a torsion axle trailer with 15-inch wheels. It is under a 1981 Sport 13. This allows enough clearance for the boat engine to be in the fully down position while stored. When towing I tilt the engine enough to engage the towing support mechanism.

[Phil T]

Do you have before/after photos?

Please post.

You need to have the mechanic correct his mistake unless he got your okay to do the change in axle location.

[Tom Hemphill]

My trailer (under a classic 16-foot Whaler) has 12-inch wheels. The axle clamps and bunk brackets--the lowest components--have 9.5-inches of ground clearance.

The license plate is loosely fastened at the top with nylon wire ties, so it can pivot freely.

I have never had a problem with ground clearance, and my outboard engine can be stored in an vertical position.

[DarrylR]

Fig. 1. Clearance at exit of driveway.

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Fig. 2. View of underside of trailer.

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Fig. 3. Clearance between rear cross member of trailer and ground.

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[Phil T]

Have the mechanic move the axle to below the springs--or do it yourself.

Also replace the five-inch keel rollers with 12-inch non-marring keel roller.

Raise the engine mounting height by two holes--when you have a chance.

[undercover]

A ground clearance of 10-inches isn't necessarily bad. But, if you're towing level (hitch ball height mentioned above) and scraping, raise it up.

I'd be more concerned with a straight axle that high when you have V-crossmembers. To deduce the clearance from the images is hard.

Make sure the trailer axle cannot come in contact the boat keel when hitting a big bump at speed.

[jimh]

It is difficult to assess the ground clearance shown in Figure 1 because the camera is shooting downward. The clearance does look rather close.

When parking the trailer in my driveway I had to remove the license plate to keep it from scraping the ground as I backed it up over the gutter.

When towing a boat trailer you will soon learn to be alert for road elevation changes in which there is a sharp descent followed by a sharp ascent. When the trailer transits such a change in elevation there is a very great chance the trailer will bottom out--or even worse, that the skeg of the outboard engine will bottom out. If you want to be certain the trailer or engine skeg will never bottom out, you need to increase the clearance above the roadway, and always tow with the engine in a tilted-up and locked position.

As a general rule, having the trailer frame set low to the ground will put the boat low to the ground. This will be useful when at a launching ramp that has little water depth at the end of the submerged roadway portion, or just in general at any ramp, as the faster the boat immerses into the water, the less distance you have to back in the trailer. The less distance you have to back in the trailer, the farther from the water your towing vehicle will be.

I have seen situations on some launch ramps with some boats and trailer where the towing vehicle rear tires are in the water, and even on occasion where the exhaust pipe of the towing vehicle is underwater and bubbling out exhaust. The lower the boat is on the trailer and the lower the trailer to the ground, the less likely these situations will occur.

Of course, there is a limit to how low the trailer can be to the ground and not become a nuisance with bottoming out in normal driving.

In Figure 3 there appears to be about 10-inches clearance between the road and the bottom of the rear cross member of the trailer. I assume that when this was measured the trailer frame was aligned to be level with the ground.

The trailer should be towed with the trailer frame level to the ground. I frequently see trailers being towed that are not level, but those instances are usually yahoos towing jet-ski boats or U-Haul trailers. The hitch height on the towing vehicle should be adjusted so the trailer tows level to the ground.

Q3: If you transposed the position of the axle and spring, how much increase in height above the pavement would the trailer frame rise?

By the way, I have never measured any clearance on my trailer. Using the metric of the license plate scraping on the pavement, I know that was occurring with my trailer set up. I discovered this when I noticed that the license plate was gone. There was just a bit of one corner of the plate left, firmly bolted to the trailer, but all the rest of the plate was gone. I was concerned I was going to have to pay $75 for a new trailer license plate, but the Secretary of State office kindly gave me a replacement plate for $4.

Fig. 4. Trailer clearance. The lowest object on my boat trailer is the bottom edge of the keel roller brackets. In this view I would guess there is about 10-inches clearance, at most.

trailerClearance.jpg (40.02 KiB) Viewed 2256 times

[jimh]

If you happen to decide to move the axle and springs to the alternate arrangement, take a careful measurement of the distance from the axle at the frame on each side to the ball hitch. The axle should be square to the frame and the distance to the ball hitch equal for both sides. This will allow the trailer to be towed in-line. If the axle is not square to the hitch, the trailer will tend to hunt back and forth, and the tire wear will increase.

[DarrylR]

Fig. 5. There is 7-inches of clearance at the lowest part of the trailer when level with pavement.

fig4a.jpg (6.05 KiB) Viewed 2194 times

The mechanics still are uncertain about raising the springs above the axle.

I am not sure how to proceed.

[jimh]

I am not sure how to proceed.

Take the boat and trailer to a launch ramp. See how it works out.

You did not answer my question, so let's try again:

Q3: If you transposed the position of the axle and spring, how much increase in height above the pavement would the trailer frame rise?

[DarrylR]

Sorry about that. If I flipped the springs above the axle it would raise the frame about 4 inches. I think I decided that I'm just going to go try it out and hope I don't hit some bumpy road and the frame bottoms out.

[Phil T]

The concern for the trailer height is for it's break-over-angle.

With a small break-over-angle, the boat transom may make contact with the ground when traveling over a:

- speed bump or lump (reverse bump)
- curb cut, driveway apron (gas station)
- ramp head (area of transition from flat grade to ramp grade)

Been there, done that.

[Jefecinco]

If a mechanic is unsure about doing what you ask him to do, find another mechanic.

If I have read the posts correctly you now have seven inches of road clearance with your trailer. If you installed the springs above the axle it would increase the clearance by four inches. That would provide eleven inches of ground clearance.

Eleven inches of ground clearance does not seem excessive. The higher ground clearance would necessitate immersing the trailer four inches deeper in the water to launch your boat. It would also eliminate concerns about "grounding" the trailer under usual towing conditions.

Perform a test launch, as recommended, before proceeding with any changes.

Do you know the trailer's ground clearance before the repairs were made?

[jimh]

With a small break-over-angle, the boat transom may make contact with the ground when traveling over a...ramp head (area of transition from flat grade to ramp grade)

I am sure that is how I lost the license plate on my trailer. The ramp I use now has a very sharp break over from flat to down-slope. Also, with my towing vehicle, the distance between the rear ties and the hitch ball is several feet. That long distance exacerbates the effective rise of the trailer hitch when transiting the break-over.

Fig. 6. A an exaggerated sketch showing how the effective height of the towing car hitch increases when transiting a sharp break-over angle from incline to flat pavement.

breakOverHitchHeight.jpg (15.92 KiB) Viewed 2135 times

Note in Figure 6 above how the distance from the tow vehicle rear axle to the hitch ball increases the effect of the break-over angle on increase in effective hitch height.