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Wheels And Tires For Your Trailer

There are a million tires out there with a ton of classifications and designations.  What tires do I choose for my trailer?  That is not a silly question at all.

Truly, the tire industry can feel like black magic with all the various designations to keep straight.  Then, at the tire store, they rattle off the numbers and letters so fast your head spins.  Do I really want that?  Will I look silly if I ask too many questions?  While it may feel intimidating, NO, you don’t look silly asking questions.  You’re learning, and that’s important.

The real problem with tire stores is the people you interface with a sales folks.  Their job is to get you out the door with new tires.  Best if they can sell you what they have in stock, add a lengthy warranty and they’re done.  That is a little at odds with your goals — of getting the tires that will serve you best down the road.  So, to do that, knowledge is your friend.

Anyway, an informed customer is better to work with.  And, if you come prepared, it’s much more likely that you’ll walk away with the right tires.  So, let’s dive into in just a little.

First, Tire Designations

Tires have a bunch of different types of classifications, and not all tires are consistent.  For the sake of space, we’ll talk about the most common, and refer you to Google to find the odd-balls.

The most common way of defining a tire is with this formula that also appears on the outside of your tires.  It’s normally raised rubber lettering on the sidewall in a form like this:

(Service Type) (Width)/(Aspect Ratio) (Construction)(Rim Size) (Load Index)(Speed Rating) (Load Range) (Extras)

Appearing something like:

LT 215 / 75 R15 106Q D1 BSW

OK, what can I do with that?  Don’t worry too much about the details, because even the tire experts have to look up some of these things.  And to keep you on your toes, sometimes they mix things up a little or leave things out.  Anyway, here’s how to decipher it.

Deciphering:
Trailer Tires vs. Light Truck Tires
Tire Examples:  Light Truck Tires (LT) and Trailer Specific Tires (T or ST).  They don’t all look like this.

Service Type — Passenger Car (P), Light Truck (LT), Trailer (T or ST), etc..  There are more, but we won’t worry about that right now.  For trailers, we recommend LT or ST for most.  See Trailer Tires or Automotive Tires for more information.  For small tires, there are some different designations, and we’ll leave that for another time too.

Width — Nominal tire width in millimeters from bulging side to bulging side.  It is NOT the width of the tire tread, nor is it the width of the bulge from the weight of the vehicle resting on the road.  It’s the “theoretical” widest point of the tire when properly inflated and sitting by itself.  (If you start measuring, they don’t always come out exact.)

Aspect Ratio — This is a percentage number of the tire section height to it’s width.  This is the tire section, NOT including the wheel.  See the image.  If you have a 75 aspect ratio, that means the distance from the rim to the tread is 75% of the width.

Construction — The make-up inside.  Radial (R) is all you really need.  While there are more, Bias ply tires are not available as much any more for consumer applications.  For heavy trailer applications there are some, however.

Rim Size — This is the tire seat diameter in inches, and it must match the rim.  It is NOT the total outside diameter of the rim.  Interestingly, rim width is not part of the tire designation, but it is important when buying your rims and tires.  Since the tire is flexible, it will have a range to work with, like 6.5″ to 8″ width or something.  The rim is obviously rigid, so make sure your tires match the rims.

Load Index — This is a specification for load the tire will carry, but it’s actually a code.  106, for instance, means it will carry at least 2094 lbs.  You just have to look up the index number based on the load capacity you want.  Here’s a page with charts for both Load Index and Load Range from Discount Tire.

Speed Rating — This tells the max speed for the tire.  Again, it’s a code to look it up.  From our example above, Q means up to 100 mph.  Since many trailer tires are more like:  L – Up to 75 mph;  M – Up to 81 mph;  N – Up to 87 mph, make sure your tires are made for the speed you travel.  Here’s a chart of Speed Ratings.

Load Range — Again, a coded number to show the ply’s and max pressure rating of the tire.  Standard Load (SL), Extra Load (XL), C, D, E, F, G, H and maybe with a number like a “1” – E1 for instance.  This is less important to me, because the load index is really a better way to choose a tire.

Extras — While there are additional things to appear at the end, most are not so important for function.   Things like BSW (Black Side Wall), or the manufacturer or manufacturer’s code, or other things may appear.

Tire Specifications from Les Schwab
An excellent tire specification diagram from Les Schwab to illustrating the information above.

Then, Wheel Designations

Wheels (also called rims) are much easier.  They come with a load capacity, a diameter, a width, a bolt pattern, and an offset.  Some also include a pressure designation, but mostly that is part of the load capacity.  This image from Katana Wheels is a nice diagram of the rim an some of the specifications.

Wheel SpecificationsRim Diameter — The tire seat diameter.  This is not the actual full diameter of the wheel.  It’s a matching diameter that must match the tire size designation.  Rims come in many sizes that, in short hand, define the wheel — like a 14″ or 15″ wheel.  There’s more to it, but this is the primary measure.

Bolt Pattern — A designation of how many bolts there are in what size pattern.  The bolt pattern must match the axle pattern.  It’s often given as 5 on 5″ or something like that — which is deciphers as 5 holes on a 5″ (bolt circle) diameter, equally spaced.  Higher capacity wheels usually have more holes on larger a ‘Bolt Circle Diameter’.  Examples are 6 on 5.5″ and 8 on 9″ diameter.

Width — Rim width is the space between the flanges where the tire goes.  This is a linear dimension like 7″ and it must match within an acceptable range of the tire.  A wider rim allows more tire “bulge”.  Trailer specific rims tend to be on the narrower side for clearance.  Just make sure the tire specifications match the wheel width, diameter, and load capacity.

Offset — This is usually zero for trailer wheels.  The offset is a distance from the center of the wheel to the mounting face.  We sometimes call it “Dish”.  On Front Wheel Drive cars, they have an offset “out” that allows the axle shafts more space inside.  Dually applications have a large offset so wheels can bolt together with space still between the tires.  Choosing wheels with an offset other than zero can allow the tires to “stick out” or “pull in” a little.  Generally we don’t do this with trailers because it loads the axle bearings a little different which de-rates the axle load capacity.  We recommend sticking to Zero Offset to avoid trouble.

Choosing Your Trailer Tires

Now we’ve got the needling details out of the way, let’s talk more specific about tires for your trailer.  Not every tire is appropriate for a trailer.  Of course you can put the wide, low aspect ratio tires on a trailer if you want, maybe to match the tow vehicle for show.  Those are always fun to see.  However, in a more practical sense, there are many we can easily rule out.

Size Things to Consider

In general, smaller diameter wheel sizes carry smaller diameter tires — and carry less total load.  On the other hand, they offer lower trailers and less vertical intrusion above (smaller wheel wells, fenders, etc.), albeit, they also give less ground clearance.

In general, wider tires carry more load.  While a wider footprint is nice in some ways, they do require a little more space side to side.  If you drive a lot over soft ground — like the lawn, deep gravel or sand — wider tires are helpful.

Tire pressure can be a big deal, especially if you pull your trailer around empty.  Trailer ‘bounce’ is a real thing, and pressure is one way of dealing with it as mentioned in this article about trailer bounce.  Small tires with high pressure are particularly susceptible.  Choosing tires with a larger Aspect Ratio (like 75 or 80) can help help that because there is more sidewall to add some “suspension” effect.

Speed and Load

The Speed Rating and Load Index numbers mean a lot more than just what’s safe.  Don’t skimp on either of these.  If you think you’ll never pull your trailer on the highway — so you buy tires rated below 75 mph.  What if sometime you need to be on the highway and find traffic is moving all around you at 80+ mph?  Don’t flirt with it.  My personal opinion is there is no reason to even consider a tire with a speed rating lower than N.

I approach Load Index the same way.  While it’s easy to think about axle capacity translating directly to tire capacity, it’s again ignoring the way things actually tend to occur.  Tires are often overloaded, accidently or by dynamics (going around a corner).  Since tire failures with trailers are not uncommon, it just doesn’t make sense (to me) to skirt by with the minimum.  I argue to use tires that have 10% to 15% extra capacity.  See the discussion below for an example with numbers.

Common Sense Problem Avoidance

If you can keep a problem from happening, that’s way better than solving a problem later.  Trailer tires certainly fit that category.  Tires and tire issues are the #1 cause of trailer towing problems on the road.  How many times have you seen a trailer on the side of the road with a wheel missing?  Seriously, this is something we can avoid with a few common sense steps.

First:

Above all, make sure the tires are are in good shape.  If there is dry rot, or sidewall damage, or extra bulges, or uneven wear — fix things BEFORE pulling the trailer.  If axles are not straight, for instance, it can cause extra tire wear.  The wear itself is not the issue — it’s the heat generation with the wear that denigrates the tire core (which you can’t see).  That weakens the tire and at some inopportune moment, it will give way.

The avoidance is easy.  Just inspect your tires before going.  Most of the time the signs are there early telling you when to replace worn trailer tires.  Another good indicator is heat.  Put a hand on the tires when you stop for gas.  If the tires or bearings are hot, figure it out, and fix it before continuing.

Second:

Make sure your trailer is ready for the service you want.  I see lots of trailers that are “probably” overloaded.  I see even more with an axle situation that causes tire overload — read about proper axle load sharing.  It’s not very easy to change these things in the moment, but just know you might need to adjust your trip if what you’re trying to do doesn’t fit the available equipment.

The same can be said for equipment that’s no longer ready for the load.  See this obnoxious tire situation that changed their plans.  The signs were there . . . .

Third:

Choose trailer tires with more capacity than you need.  I recommend 10% to 15% or so over.  Here’s an example with numbers.

If you have a 3500# axle, each wheel and tire must handle at lease half of that load (because there is a tire at each end of the axle).  That would say 1750# capacity for each.  Looking at the charts, a Load Index of 100 for 1764# fits the bill.  Right?  Not in my book.  This example assumes both ends of the axle carry the same amount, yet rarely are trailer loads perfectly even.  Secondly, when you go around a corner, the outside tire carries more just do to centrifugal force.  If tires are near the limit standing, then one overloads at every corner.

Of course, manufacturers make the tires to handle some amount of overload for situations like cornering and bumps in the road.  Yet, when you’re pushing the limits, that’s flurting with disaster.  And, it’s only a small price difference for tires with 15% extra capacity.

Using the example above for the 3500# axle, I’d add 15% to the 1750# making it 2012# as the minimum tire capacity.  That’s a Load Index of 105 for 2039#.  It’s a simple way to avoid problems.

Fourth:

Though not specifically about tires, make sure your axle spindles operate properly and have appropriate grease.  Yes, it takes a few minutes, but take the time to inspect once every few years — especially if the trailer is stored outdoors.

It’s really pretty easy.  Jack up one wheel and spin it.  Listen to it.  If it spins well and has no scraping or grinding noise, go on to the next.  Every few years, you should also pull a wheel and hub to check the bearings and grease.  If you have a boat with a trailer that goes in the water, you should check the bearings every season — maybe more often.  If you travel long distances or on frequent trips, check the bearings more often.

Is this ridiculous?  Sometimes the grease lasts for years.  However, when it fails, it’s usually catastrophic, and it’s NEVER convenient.

Finally:

Check function of brakes once in a while.

There was a car pulling a trailer in Idaho years ago with a locked brake.  It heated up the hub and wheel enough that the tire caught fire.  It actually started a series of fires all along the road.  Horrible.

On another occasion, someone pulled one to me with the “park brake” still engaged, and it took a few hours till it cooled enough that we could take the wheel off.  Completely cooked the bearings and grease.

Just do it as part of the normal checks and regular maintenance — or take it in for the service.

Good Luck with Tires For Your Trailer

It doesn’t need to rely on luck.  As mentioned, the most common problems are mostly avoidable.  Do regular inspection and maintenance, and go a little overrated with the specifications.  Now you know one more secret to trailer towing success.

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