Wheel Design

[CapriRacer]

I've fielded a question in another forum concerning the use of high pressures on Pickup truck wheels. Specifically, if an LT type tire using its max pressure of 80 psi were to be put on a wheel designed for P metric type tires (35 psi!), would the wheel be able to stand up to the increased pressure.

First, I know that the real problem here is fatigue - that inflating the tire to 80 psi isn't going to cause the wheel to fail IMMEDIATELY! That the problem may be thousands of miles down the road.

I'm hoping that Tim Flater, Senior Designer, Enkei America, Inc.,

http://www.enkei.com

is still around and can help shed some light on this

- BUT -

I'd welcome any others who have experience in wheel design.

 

[Xwheelguy]

I am considerably flattered that you specifically sought my advice on this subject - thank you. A very good question, indeed.

Unfortunately, I am unable to give you a yes or no answer, for a few reasons.

Most of my experience comes from OEM passenger car wheels, which are sold directly to the auto makers versus OES or aftermarket wheels which can be designed to fit many different vehicles and tire applications. We test our wheels to specific criteria, given to us by our customers. Our wheels are made for specific vehicles with specific sized tires and tests performed at max. inflation pressure for the specified tires. We have never been asked to test a passenger car rim with a truck tire installed and inflated to maximum recommended inflation pressure of the truck tire. If it makes you feel any better, the fatigue simulation test typically uses 65 psi for passenger car tire inflation pressure.

If you are putting an LT tire on a passenger car wheel, then I cannot say what may or may not happen in either case because that wheel was probably not designed to have that tire installed on it and therefore testing was more than likely NOT performed with truck (LT) tires.

However, I can say this: I feel that you are correct in stating that fatigue is the big concern here - increasing air pressure should increase the loading on the wheel. (Note this is my opinion and NOT an official statement or fact from Enkei as a company).

I know you were seeking more of a yes or no answer, but I just cannot give you that type of answer and feel comfortable in doing so with my experiences (or lack of) so far in the wheel industry. I also must stress that any response I may give on such matters can reflect directly on my employer's reputation and I would rather avoid being put in a situation like that - can't look bad in the public eye.

I wish I were able to give you a better answer, but my experiences just haven't covered those bases in my years at Enkei - there just hasn't been a need for it so far, given the route we take at this facility in regards to wheel design and testing.

Tim Flater
Senior Designer

 

[CapriRacer]

Tim,

Thanks for the response.

Knowing that the flanges absorb all the force pushing the beads apart - and knowing that this force is proportional to the inflation pressure - I think that an increase from 35 psi to 80 psi means the force at the flange is over twice as high.

If we further assume that the load on the vehicle doesn't change, then ....... and this is where I start to lose my expertise. I am pretty confident that the load is carried by the beads - and therefore to the rim - in a somewhat spreadout fashion around the circumference of the rim. Is there a way to confirm this?

Put another way, do you have access to Finite Element Models of rims - and is it easy to run a simulation where you compare the max stresses for 2 different inflation pressures? What would be the result?

 

[Xwheelguy]

When we perform FEA simulation for radial fatigue (basically a load to the beads), we omit the tire and all pressures associated to it. With that being said, please keep in mind what I said about specific tires - we aren't considering an LT tire on a passenger car rim, hence the increased air pressure (beyond 65 psi) isn't a concern for us. But like I said yesterday, we're working with wheels that are designed for a specific set of criteria (certain tires, etc.)

I don't believe it would be very wise of me to get into FEA specifics on a public forum, as that sort of information is kept under tight wraps in the wheel industry (as well as tire industry), and we aren't in business to help our competition (sorry). However, I can say that you are correct about the location of the loading - it's the distribution that isn't what one might expect at first thought.

Yes, I do have access to FEA, however no access to tire models nor the method of setting up the simulation for air pressures as well as lacking any info on the tire materials for the FEA to be anywhere near accurate. It's not just a matter of modeling the geometric shapes and throwing them into the FEA software - the tire would have to be modeled quite accurately (belts, bands, tread and proper wall thicknesses, etc.) and that information just isn't out there for us to use. We get very little information on tires and the tire makers aren't very willing to share that info, even with us (a non-competitor).

Tim Flater
Senior Designer

 

[CapriRacer]

Tim,

Again, thanks for the response.

It is quite understandable that FEA of both tires and wheels is closely guarded. There's been a lot of work done in this area and it represents some real competitive advantage to have that experience.

I guess for now I will just continue to respond to queries about the use of high pressures in tires by pointing out that wheels were designed for low pressures!

Thanks again.

 

[pennswoodsed]

Capri racer ,why use lt tire at lt pressure on pass wheel ? Unless maybe we are talking trailer tires ?
Regards,Ed Flanagan

 

[CapriRacer]

Ed,

There are 2 different situations that I have encountered:

1) Replacing a LT metric with a P metric - and in that case, of course, the inflation pressure goes up by about 15 psi. This is a somewhat common change that folks make bnecause they feel that the increased "strength" of the LT is better for their road (or lack of road) conditions.

2) Where the wheel from a 1/2 ton pickup is used in a 3/4 ton application. I know there are bolt circle differences that are supposed to prevent this, but I recall one situation when this was possible - not to mention that some folks think that increasing the tire load carrying capacity is the only thing you need to do to increase the capacity of the truck.

 

[Xwheelguy]

If people are taking wheels designed for lighter vehicles and putting them on vehicles designed for heavier loads (1/2 wheels/tires being put on 3/4 ton vehicles) - well, that's pretty risky, IMO, from a wheel standpoint UNLESS the axle ratings are exactly the same for both vehicles.

The loads for wheel durability tests are derived from axle load ratings (in a non-aftermarket scenario). In other cases, wheel design loads are taken from the highest load rated tire that will fit the rim (aftermarket). Either way, I see it as a gamble, but I tend to lean on the conservative side when it comes to that sort of issue (rather be safe than sorry).

Many things about a wheel's design can affect the vehicle in surprising ways. For example, a larger pilot/hub diameter can decrease the ride quality causing vibrations while traveling (according the to the vehicle manufacturers). As a result, the tolerances on wheel pilots/hubs are some of the tightest tolerances on the whole wheel.

Tim Flater
Senior Designer

 

[bradleyelwood]

Make sure that your life insurance is paid up and that your beneficiary is someone you like before inflating the tire.

Overinflating a truck tire (or car tire) is like playing Russian Roulette.

I investigated an accident where first one, then the second tire blew, causing catastropic damage to a wheeled vehicle.

In short, dont even think about overloading or overpressuring a tire or wheel on a vehicle. The proper designed tire/wheel is way cheaper than an accident.

 

[CapriRacer]

Bradeyelwood,

First, Tim and I are both experts on our fields - Tim in wheel design, me in tires.

For my side, I have examined over 10,000 tires that have been involved in accidents, so I am well aware of the problems associated with loading and inflation pressure of tires.

The scenario I posed involved a properly sized tire, but an improperly spec'd wheel - in the form of a question asked of me in my role as an expert in tires. Most of the time I am dealing with folks who - oh, how to best say this - have a little bit of knowledge and are therefore dangerous. I was looking for something to counter these folk's natural tendency to ignore the expert's advice and do what they have already decided to do. Unfortunately, I didn't get it.

Nevertheless, I ALWAYS advise people to properly size and inflate their tires. But surprisingly, many tires and wheels will endure overloading and overinflating without failure - hence the problem of convincing non-experts that these are very bad things to do.

BTW, the next time you get a case involving 2 tire failures in the same accident, you can be sure that the tires didn't cause the accident. The tire failures were the result of the accident.

 

[Xwheelguy]

I had a typo in my last post, first paragraph should have been:

"...putting them on vehicles designed for heavier loads (1/2 TON wheels/tires being put on 3/4 ton vehicles)..."

In summary, non-aftermarket wheels are designed for maximum loads and you should not ever assume they can handle more of a load than the vehicle for which they are manufactured.

Aftermarket wheels generally have a maximum vehicle load rating cast or stamped into the backside of the wheel (look in the spoke or mounting area pockets or any as-cast surface) - or at the very least in some literature that comes with the wheel when purchased. I'm sure if someone looked hard enough they might be able to find wheels that lack this information/marking, but from my personal experience, it's preferred practice to have that info ON the wheel somewhere, similar to the tire's sidewall having the max. inflation pressure molded into the tire.


Tim Flater
Senior Designer

 

[CapriRacer]

Tim,

I'm going to disagree with you on a couple of points. My experience is that load information is rarely written on the inside of the wheel. I can always find the size of the wheel and the part number, and usually some sort of logo for the wheel manufacturer, but that seems to be the extent of the info.

But I agree that putting a wheel designed for a 1/2 ton and putting it on a 3/4 ton is pretty foolish.

 

[Xwheelguy]

Capri,

I assure you there are plenty of wheels out there with load ratings cast into the backside of the spokes. It might not be as common a practice today, as I haven't dealt with aftermarket wheels in quite some time - but they ARE out there.

Tim Flater
Senior Designer

 

[bradleyelwood]

Capri,

To put my example in perspective, the first tire separated at the bead bundle, and the resulting fire compromised the second wheel which separated the flange from the rim base. When this happened, the wheel separated from the hub, breaking all of the bolts. Then the wheel went out thru the side of the vehicle, shearing a 10" wide flange beam.

If you "have examined over 10,000 tires that have been involved in accidents", you have way more experience than me.

 

[Xwheelguy]

Not to make this the Lazarus thread, nor to boast about my past experience, I just would like to point out that I have changed jobs recently and will no longer be in the wheel industry. I appreciate all who sought my advice, opinion as well as those who shared their knowledge of wheels as well as corrected me when I was was mistaken.

I will still keep my eye out on this area, but current practices, standards, etc. may change in the future, so the only info I could offer would be limited up to 2011.

Thanks to all, it's been a fun RIDE! (Get it? - HA!)

Tim Flater
NX Designer