Selecting Wheels and Tires

[bootlegend]

I am trying to learn a little about selecting tires and rims for trailer loads. I just purchased the 2018 Tire and Rim Association Yearbook and plan to read through that but based on older versions it doesn't have all the info I need.

For example, if I have a tri-axle that needs to carry 60 kips, is there a reference document to select a rim? Or do I have to go to each rim manufacturer and look through their info? Further, what other factor should I consider when selecting rim size? The trailer I have in mind is actually a chassis for a portable equipment that can be hauled on the highway and then operated in a quarry where the wheels and tires aren't in because it is operated on hydraulic jacks.

Do you generally select the rim first or the tire?

For over-inflation load increases with speed reduction is there some standard on how the max speed should be posted?

Appreciate any info you can share or anywhere you can point me to learn about the topic.

 

[MikeHalloran]

One wheel dimension that's important is the offset.

Look at a sectional drawing of tire, wheel, and axle, as typically found in a shop manual.
Note that the centerplane of the tire typically crosses the axle centerline somewhere between the wheel bearings.

Offset betwen the rim centerplane and the faying surface for mounting the wheel is the last, easiest place to adjust the location of the tire vs. the bearings. (I forget which way is positive.)

For a while, it was fashionable to put giant tires on, e.g., Jeeps. For which purpose, you may even need spacers to make the wheels clear the body. I recall reading a magazine article where some ink stained wretches paid someone to install giant tires on a brand new Jeep, and drove it from England to Australia ... where an axle failed. Jeep should be proud that it went that far.

There was a short-lived fad where rice rockets got undersized wheels with offsets such that the tires were fully outside of the body, lowering the CG and increasing the track width for greatly improved cornering ability. ... and wheel bearing life measured in weeks, which is likely why the fad faded so quickly.


Mike Halloran
Pembroke Pines, FL, USA

 

[bootlegend]

Mike,

Thanks for the info and I have noticed the offset in the drawings. I think I'm asking for even more elementary stuff though.

What advantages does one rim size have over another? Or should I simply start with the type of tire I need and will carry the load and then select the rim that will fit it?

Skimming through the Tire and Rim Yearbook it appears that they intend you to pick a tire and then select a rim for it because they don't give load rating for rims that I can see.

 

[GregLocock]

I've only worked on light trucks and passenger cars.

Increasing the rim width for a given tire (up to a point) will increase the stiffness of the tire vertically and laterally and will also increase its cornering stiffness. Increasing its pressure will do much the same. Whether that is a good thing depends on a detailed model of the system.

The T&RA book quotes a measuring rim width for a given tire. It is a good place to start.

OEM passenger car rims are designed for the particular car, often, rather than the max load capacity of the tire, but for light trucks the max axle weight is usually defined by the tire, so in that case the wheel and tire are designed for the same loads. I suspect a rim from e reputable supplier for a truck will be designed for the tire in question.



Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

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[CapriRacer]

It appears you are designing a trailer from scratch.

First, the Tire and Rim Yearbook was ah ...... Mmmmm ...... a waste of time. It doesn't have the info you are going to need.

First, find a tire website that has trailer tires. Select a tire that has enough load carrying capacity. May I suggest you look at Tire Rack and the Goodyear Endurance line. For practical purposes, that line of tires is about as complete a line as you will find.

Load carrying capacity: Get an estimate of the load on each tire. Don't forget the front to rear and side to side weight variation - that is, even though the axles are more or less in the same location, the load doesn't evenly distribute. When I've estimated travel trailers, I've used a 15% weight variation, but I think the value is closer to 10%.

ST tires are restricted to 65 mph, but I think everyone runs them beyond that speed. So unless you are sure the 65 mph speed will be observed, I suggest you add 20% to get 85 mph speed restriction.

Further, my experience is that tires should be loaded to no more than 85% of their rated load.

I know that is a lot of additional load carrying capacity, but that is what I think is needed. Disclaimer: I am a retired tire engineer. I have not done any work in selecting tires for vehicle, but I have been involved in many situations where how the load on a tires is calculated from a vehicle point of view. I think Greg Locock above has done some of this and the principles are the same regardless of the vehicle.

Once you've done those calculations, you should have a tire size and Load Range. Find a trailer tire retailer and they ought to be able to supply you with a tire/wheel combination. If I remember correctly, trailer tires generally have zero offset. I suspect you already have a way to select axles. Make sure the bolt holes on the axle hub are compatible with the wheel - they should be, because I think they are based on the load carrying capacity.

If you need any more advise, you can find me at my website. I know the moderators of this site don't like to have email addresses posted because it attracts spam, so google my website using my name, Barry, and the words tire and tech. You'll find an email adress there.

 

[bootlegend]

I suspect a rim from e reputable supplier for a truck will be designed for the tire in question.

I thought this might be the case. Accuride does put load and inflation ratings in their catalog but I have yet to cross check and see if the load increase pressures from the tire manufacturers are covered by the wheel rating.

It appears you are designing a trailer from scratch.

Essentially yes. It is a piece of equipment that will serve as it's own trailer during transportation. We have a kingpin on one end and a axle group beneath the tail end. Wheels and tires are only needed for transportation not normal use.

First, the Tire and Rim Yearbook was ah ...... Mmmmm ...... a waste of time. It doesn't have the info you are going to need.

I am now aware of that, but the last edition I had here in a dusty notebook was the 1992 yearbook so I figured we were due for an update anyway. Also, the general tire ratings seem like a decent place to start instead of specifying a specific brand, no?

ST tires are restricted to 65 mph, but I think everyone runs them beyond that speed. So unless you are sure the 65 mph speed will be observed, I suggest you add 20% to get 85 mph speed restriction.
Further, my experience is that tires should be loaded to no more than 85% of their rated load.

I admit that I have pushed the tires close to the rated load assuming they have a decent factor of safety built into the rating. Sounds like that may not be the case. I'll keep that in mind. A few more dollars on tires to know I'm not going to have a failure on the highway is an easy decision for me.

Once you've done those calculations, you should have a tire size and Load Range. Find a trailer tire retailer and they ought to be able to supply you with a tire/wheel combination. If I remember correctly, trailer tires generally have zero offset. I suspect you already have a way to select axles. Make sure the bolt holes on the axle hub are compatible with the wheel - they should be, because I think they are based on the load carrying capacity.

Right now I need a low profile/small diameter tire for overall equipment height and clearance during operation. That is limiting my options. I haven't gotten into researching the axles other than what "we've always used." That will be next.

I appreciate the help and will check out your site.

 

[GregLocock]

"I admit that I have pushed the tires close to the rated load assuming they have a decent factor of safety built into the rating. Sounds like that may not be the case. I'll keep that in mind. A few more dollars on tires to know I'm not going to have a failure on the highway is an easy decision for me."

I heartily agree, always overspec your tires for a one-off, and it sounds as if you are going off road with them, so you need all the plies you can afford in the sidewalls.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[CapriRacer]

A couple of thoughts:

The TRA Yearbook has EVERY possible tire size, not what is available on the market, particularly what is easily obtained from convenient sources.

Low aspect ratio? You might want to consider using an LT or a regular medium duty truck tire. ST tires generally come in fairly high aspect ratios - 75 series.

Plus, this sounds like where Low Platform Trailer tires were developed for. What individual tire loads are we talking about?

What part of the world are you designing the trailer for? Tire availablility varies quite a bit depending on where one is located.

 

[mark512]

The TRA Yearbook has EVERY possible tire size, not what is available on the market, particularly what is easily obtained from convenient sources.

I definitely second what CapriRacer is saying. If you're a huge OEM you can choose anything from a catalog you want or even get a custom size made for your product. But if you're a small manufacturer with low volumes, you'd best specify something popular if you're able to - it will be less expensive, easier to get, and easier on the end user's schedule if/when the tires need to be replaced on short order.

 

[bootlegend]

I apologize for the delayed response. I'm just now getting a few minutes to look into this again.

I was looking at a load on the axle group of 54,000 pounds. Previous design had used 215/75R17.5 tires which are rated for 4,540 lbs each in the dual configuration at 125 psi, or 54,480# total.

I am suggesting going to the 235/75R17.5 tires, rated at 5,675 lbs each or 68,100 for the group. The rim we have used (and I am sticking with for now) is a heavy construction Accuride 17.5x6.75 rated at 5,070 lbs at 125 psi, so they will control over the tire rating at that point. This may still be closer to capacity than you all would suggest.

Questions I'd still like to be able to answer at some point in the future, for my own understanding:

1. How much equalizing does an "equalizing" suspension do? At some point some old timer told me the equalizing leaf-spring suspensions share the load among all axle groups and I thought no more about it. Haven't sat down and tried to work out a free body diagram with the little rockers accounted for yet though. Right now I am basing my axle loads off of reactions at the center of the group.

2. How did the 17.5" rim get selected originally? Could I not select a 16", 17", or 18" rim and a corresponding tire that would carry the load and is equivalent diameter when matched with another tire? Although, with a quick look I can't seem to find a combination that results in about a 31" diameter like the current setup so I may have my answer.

I'm sure I'll have more questions later as I revisit this. Thanks for the helpful comments so far.

 

[Compositepro]

There is a freely pivoting link between the front and rear axles. Until the link hits its end of travel, the loads on the two axles will be the same.

 

[CapriRacer]

bootlegend,

Thanks for posting back.

I see we are in a totally different place with regard to loading than I suspected.

I can't answer the question about load distribution without a description of the suspension. A photo would be better! What I quoted earlier was based on my experience with travel trailers - and we are in a whole different ballpark!

17.5" tires are the tubeless equivalent of 15" LPT's (Low Platform Trailer) - which is where I would suspect they originally came from.

My yearbooks are a bit dated, but I don't see a 215/75R17.5 in TRA, but I do see it in ETRTO. And ETRTO only lists the tires as capable of about 3400# at about 100 psi dualled. So maybe I'm a bit behind the curve.

But to answer your question, No, there is no reason why you can't use a 16", 17", or 18" - BUT - I don't think you will find a tire in those rim diameters with enough load carrying capacity!

So I think you need to talk to your tire supplier. Find out what might be available with enough load carrying capacity with the dimensions required. This is starting to sound like a one-off type of situation, so care has to be taken to stay away from boxing yourself in with something totally off the wall and unavailable in the future.

 

[CapriRacer]

Followup:

So it turns out I am a bit behind the curve. Goodyear lists on their US commercial tire website a 215/75R17.5 Load Range H that matches Bootlegend's description - and they also list a 235/75R17.5 Load Range H with a load carrying capacity of 4,410# (200 kg) @ 112 psi (775 kPa) The even metric units probably means it's designed to ETRTO standards.