View Full Version : Tire Size

11-29-2004, 05:29 PM
For those of you with the old style MC trailers (1991 and older) that ride a little lower than the newer trailers, what size tire are you running? I currently have a pair of 215/70/14s that are for a car application and they really need to be replaced ( they are overloaded and now also starting to check on the sidewalls). My ski partner with a newer (96) trailer runs 215/75/14 Marathons and had a nearly new set that he gave me for free but they are to big for my trailer (bummer:( ). They fit barely but there is only a 1/4" clearance from the outside lip of the fender. Far to close if I were to hit a bump on the road. From all I hear I want to stay with the Marathons but the next size down is a 205/75/14 and am wondering if that is going to be to short. The clearance at the prop gurad is small as it is.

What brand and size are you running with this trailer? Also has anyone flipped the axel over on this trailer? Doing so looks like it would raise the trailer about 3" which might be a good thing.

11-29-2004, 07:04 PM
Just buy some longer shackles for the rear of the leaf spring. Mine will adjust up or down about 3".My trailer came with extra long ones already installed on the trailer.It will rotate your axle a few degrees,but I don't think it would hurt your springs.Don't know if this will work on your trailer just something to look into.

11-29-2004, 08:42 PM
I put new Marathons on my trailer (1984) this summer and they were 205/75R14. I also put a new axle on since the old one was bent. The ride has been fine. I don't have a prop guard so I drive carefully, but haven't had a problem in the 4 years I've had the boat. The 215 refer to the width (in mm I believe) and I'd expect the 205s to give you about the same ride height.

Do a search on the board. I know Doug put lift blocks under his springs to lift up the trailer a couple of inches. This might allow you to run 15 inch tires as well if you wanted to go all out and get new wheels.

11-29-2004, 08:50 PM
I know that the 215 or 205 refers to the width but the 75 is the height of the tire reffered to in a percentage of the width. A 215/75 would have a 161.25mm sidewall the 205/15 would have a 153.75 mm sidewall. A difference of 8mm. The current tires I have are 215/70 and would have a 150.5mm sidewall. I guess that answers my question. Although the 205 marathons would be 10mm narrower than my current tire they would actually be 3 mm taller. Darn. I guess I am going to have to spend the bucks on new tires. I am going to check into the shackels though. Thanks for the help.

11-30-2004, 06:49 AM
Tire Dim's....

Width x Aspect Ratio = Section Height x 2 = Combined Section Height + Wheel Diameter = Tire Diameter
Example...185/60R14 85H or 185/60HR14

185mm x .60=111mm x 2=222mm + 355.6mm(14")= 577.6mm or 22.74"

The first number is the width of the tire in millimeters, measured from sidewall to sidewall. To convert to inches, divide by 25.4 In the example above, the width is 185mm or 7.28".

The second number is the aspect ratio. This is a ratio of sidewall height to width. In the example above, the tire is 7.28" wide, multiply that by the aspect ratio to find the height of one sidewall. In this case, 185x0.60=111mm or 7.28"x0.60=4.36".

The last number is the diameter of the wheel in inches.

To figure the outside diameter of a tire, take the sidewall height and multiply by 2,(remember that the diameter is made up of 2 sidewalls, the one above the wheel, and the one below the wheel) and add the diameter of the wheel to get your answer.

11-30-2004, 02:21 PM
BrianM, I'll buy the 215's if they're in good shape and you want to sell them cheap. You have 2, correct?
205's will be slightly shorter and narrower, but only about 1/4" each way. My trlr has a combo of both (tandem) sizes and you can't hardly see the difference in size unless you're really looking for it. Car 205s are theoretically the same size. Ride height is prob a tad shorter because the tires are more flexible than trlr tires, but s/b the same for clearance around the fender (+/- manufacturer's differences).

12-01-2004, 11:20 AM
Brianm....If you have a drop axle on your MC trailer (like I do) and flip it over to increase lift height I think your going to get a lot more than 3 inches. I have been thinking about just getting a new straight axle and bolting it on to get a couple inches. Shackles, Springs, Blocks and changing axles seem to be the options. r

12-01-2004, 12:35 PM
I can't say for sure, but the engineer in me seems to think that flipping a drop axle over would result in a lot of stress and axle/leaf twist that would result in the axle wobbling quite a bit and potentially being unsafe. I doubt you could put a strong enough spring perch and u-bolts on the axle to hold it in place, especially if you hit a pothole...

12-01-2004, 01:14 PM
On the single axle trlrs, the springs are pretty stout. 2 springs holding up 3000# +. As heavy as rear leafs in a 1/2 ton pickup, and you can put 4" lift blocks on the back of a p/u w/o any issue unles you're doing extreme off roading or have alot of hp.. The engineer in me, thinks it wouldn't be much of an issue. You're not going to get any axle wrap, as the wheels are not driven, just coasting. It will raise the cg up a few inches, but Prostars are pretty low profile anyway. Alot of i/o trlrs have a much higher cg than MC.

12-01-2004, 05:57 PM
Hmmm.... While the leafs on my trailer do hold up 3000lbs, they are nowhere near as beefy as the leafs on my truck. The U-bolts on my truck are a good 3"-4" across which helps control the moment arm of the axle. On the trailer it's like 2". That puts a lot of stress, which the springs weren't designed to take (because they shouldn't get much twisting with a drop axle... the natural position is neutral.) Flip the axle around and you now have a 4" lever arm (and the natural position of the axle wants to be flipped around) concentrated in a small location that wants to twist. Flipping the axle results in an almost 8" difference, assuming 4" drop and that you relocate the spring perches to the other side. I guess you could keep the same perches and convert the mounting to the other side of the springs (top if it was bottom and vice versa) and then you'd only be lifting 6" or so. Still makes for a pretty big lever arm. It might work, but I'd rather put 4" blocks on the axle. At least the axle will be somewhat neutral with respect to twist instead of unstable. Did that make sense? I had to draw it out, but it's hard to explain in words. My other concern would be that the drop is designed for tension (wheel force is up, axle force is down) and not compression and might not be sized properly for compressive forces. No clue if this is true or not since I don't have the axle to look at.

12-03-2004, 01:41 PM
Good points. I am trying to remember if my tandem axle is a drop axle or not. On a drop axle, that makes perfect sense. I may have spoken out of line there.