Tire choices
 

Just before I get ready to go to bed and sleep a couple hours before having to drive 2 hours away and tow my wife's car home AGAIN, I thought about something.

The 2 bolt patterns for hi-load hi-speed trailer tires are 5x4.5" and 4x4", which also happen to be two very common vehicle bolt patterns.

The tires on my trailer are 4.80x12 hi-speed, load C. They can handle a passenger car or very light truck at highway speed, in other words. I wonder how unsafe it would be to use these at 60 PSI (the sidewall rating) for a city vehicle? (Never exceeding 45MPH).

Thoughts?

Trailer tires are designed for trailers. There is no consideration given to handling - braking, cornering, etc. Plus there is indeed a speed limitation on these tires - 65 mph.

Consider for the moment: A 4.80-12 Load Range C has a load carrying capacity of 785 pounds at 60 psi. A 175/70R13 (a pretty common but pretty small tire) has a load carrying capacity at 1036 # at 35 psi. That's quite a difference.

Why do you suppose that car manufacturers put such large capacity tires on their cars? Surely the cost alone would be a disincentive. They must know something more about tires than what is written on the sidewall.

And indeed they do. The more capability the tire has, the more safe it is. This applies to almost anything.

Let's face it - a tire failure on a trailer is just not as dramatic as a tire failure on a car.

I've never seen much drama in low-speed tire failures on cars, either. :rolleyes:

Looking at the tire sizes you suggest:

175mm = 6.88976378 inches tread width. (6.9" for ease)

The overall circumference of a 175/70R13 tires is 71.12 inches, figuring total area which will make road contact during the rotation of the tire:

71.12*6.9 = ~490.75 square inches.

The trailer tire has 4.8 inches of tread width, and close to 20" diameter.

Pi*20 = 62.8318531

62.8 * 4.8 = 301.44 square inches.

By your admission, a trailer tire at nominal inflation pressure can handle 785#, and a 175/70R13 tire can handle 1036#

Dividing out:

785 / 301.4 = 2.60451228, or 2.6# of capacity per sq. in. of tread face.

1036 / 490 = 2.11428571, or 2.1# per square inch...

Hm. seems like the trailer tire is actually stronger? We can't really assume that, because I estimated the diameter of a trailer tire. It's been awhile since I've measured one.

As far as handling, braking, etc... yeah, those things actually are designed into trailer tires... trailers have brakes, my friend. They also have to maintain a course in a straight line behind the towing vehicle, which means that they have grip while cornering. It may not be the same handling characteristics as a car tire might have, since they're not designed to be car tires, but for a very small city vehicle, I still don't see a safety issue.

not to endores either side of this.... but 4.80x12's check that they'll actually bolt on, i wouldnt be surprised if the wheels hit the brakes.

I wouldn't be surprised either, by any means.

Economically, it's almost worthwhile to do this, since trailer tires come pre-mounted for like $60, vs. even smaller passenger tires which are $60+ usually, without being mounted and balanced. Balancing isn't really an issue at under 45 MPH, though.

Yeah, I had a friend try that and that was the case.

The only way I know they fit is b/c I used to use them on the back of my Civic when I went to track day for timed runs... less weight, rotating mass, width, etc.

EDIT: Drum brakes, they fit over. I never tried them on the front, because, well, that would defeat the purpose.

Ah ........ Mmmmmm ............. Where to start?

The load carrying capacity of a tire is determined by quite a few things - directly: the amount of allowable deflection - which is a function of the size of the air chamber, inflation pressure, and some factors to account for the type of service. Indirectly: the size of the air chamber is a function of the section width and the diameter.

The factors used to account for the type of service can be boiled down to:

The higher the speed, the less the load capacity.

The rougher the road, the less the load capacity.

The more sensitive the service, the less the load capacity. (Meaning if people are involved or a tire failure causes a major problem - like on an airplane.)

In the case of trailer tires, they aren't stronger. They are just allowed a higher load capacity because of the lower speed limitiation and the fact that they don't carry people, and a tire failure on a trailer wouldn't have as large an impact as one on a car.

BTW, a P175/70R13 Standard Load has a rated inflation pressure of 35 psi, while a 4.80-12 Load Range B has a rated inflation pressure of 60 psi. That's quite a difference!

One of the issues with higher inflation pressures is impact resistance. A tire's ability to absorb the energy (Force X distance) of a penetrating object is a function of its spring rate. Lower inflation pressures allow a tire to absorb more energy. While impact type failures are fairly rare, because they completely destroy the integrity of the tire, they need to be avoided if possible.

BTW 4.80-12's are mostly used in light trailer applications - boat and utility trailers - and those usually don't have brakes. Cornering? Well, not so much.

Overall, I could see these type of tires being used on an city type electric vehicle where the top speed is limited by the size of the electric motor. Sort like those golf cart type vehicles - which in fact use these types of tires.

Thanks - That's actually what I was pointing the question toward, something that won't go over 45 even if you try, but something more-so designed to commute through cities or even in suburbia or some of the less sparsely populated country areas... like the 3 miles to the local store around here, on a highway that is frequented by farm equipment. (PA SR14N)

A golf cart then. Those tires look like trailer tires. Are they?