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Static Loadad Wheel/Tyre Radius 2

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brentonschnaars

brentonschnaars

Automotive
Jun 1, 2008
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AU
Hey guys just seeking some input for a formula to work out the Static Loaded Radius of a wheel/tyre. The variables that come to mind are Tyre pressure, Unladen mass of vehicle, Strength of side wall of tyre, Stiffness of suspension. When wheel manufacturers calculate for test loads what data do they use for their tests? eg Static loaded radius of largest tyre to be used on wheel will depend on what tyre and which vehicle the data is derived from. I understand i can ascertain the STL using a FEA but am lost as to how to define the variables to a point that will be sufficient for standards testing. Any input you could give would be appreciated

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Could you give us an idea what your ultimate goal is here? I suspect the SLR of a tire is an intermediate step, since you mentioned standards testing and vehicle suspension.(Vehicle suspension would play no role in SLR - and I'll bet the deflection of the rim under load would be so small compared to the tire that it can be ignored.)
 
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CapriRacer,

What I'm refering to is a figure that ill need for an equation to come up with the required test load for a Dynamic Cornering Fatigue Test. The maximum STL of the largest tyre to be used on wheel is needed to complete this equation. Also May i ask why you say the springs/ suspension do not affect the STL? I had the thought pattern that those parts play a role in force/weight absorption that would effect the downforce on a wheel and tyre. Please point me in the right direction if I'm on the wrong path.


Thanks

Never be afraid to try something new. Remember, amateurs built the ark. Professionals built the Titanic
 
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Incidentally the usual approach short of measuring it would be to look up the SLR in the T&RA book, or work it out from the tire size (ie diameter width and aspect ratio). That is close enough for fatigue work.



Cheers

Greg Locock

SIG:please see FAQ731-376 for tips on how to make the best use of Eng-Tips.
 
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Sorry Greg, was a typo, meant to be SLR. I have copy of the T&RA manual and it doesn't have any data on SLR. The other variable for SLR would be tyre brand hence why there is nothing on this in T&RA.Also even if i did go and physically measure the SLR, it would still depend on what tyre i was using so for the sake of testing for a standard how do they take this variable out of the equation? Do they say for arguments sake that the difference of the SLR throughout the different tyre Brands/designs for that respective size of tyre have a difference <10mm and work on a worst case scenario?

Does the suspension not support some of the weight of the car even though it is static at this point? I used the thought pattern of if i sat in my car while it was stationary my suspension would absorb some of the new added weight.

Thanks

Never be afraid to try something new. Remember, amateurs built the ark. Professionals built the Titanic
 
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The suspension transmits the entire sprung weight of the car through the axle, wheel bearings, hub, wheel and finally the tyre. In a static situation it obviously has no effect on the load transferred to the tyre.

Regards

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Give me a tire size and I will show you where the OD is in the T&RA(A).

For instance a 295/40R18 has an OD of 693 mm

Yes, the SLR varies little between brands for a given tire size. If you ask the manufacturer they should be able to give you what is known as the grown profile for each of their tires.

Admittedly that only gives you the SLR at zero load (I think) and nominal pressure. IF it is important to account for the deflection at a given load then you need the pressure/load/deflection graph for that tire - the manufacturer has that. For a passenger car tire use 200-250 N/mm at 32 psi, rate is proportional to psi, near enough.

In most cases it is a trivial exercise to measure the SLR for your particular load and inflation pressure.



Cheers

Greg Locock

SIG:please see FAQ731-376 for tips on how to make the best use of Eng-Tips.
 
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The reason I'm seeking the SLR of a wheel/tyre is so i can define that variable in an equation that will equal the required amount of force the wheel will have to meet in testing to conform to Australian standards, so its not such a trivial value. Obviously one particular wheel could have any number of differing tyres installed on it, how can i make sure i have the SLR of the largest tyre to be used on the wheel? Do tyre manufacturers meet specifications for tyre deflection and so forth? If they do are they published standards or in house specs?


Thanks for taking the time to reply

Never be afraid to try something new. Remember, amateurs built the ark. Professionals built the Titanic
 
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For crying out loud.

Take the published radius of the largest tyre possibly used, add 10mm then do your sums.

If you are designing the thing so close to the wire that brand to brand variation and tyre pressure and temperature and wear put you over the safety limit, you are flirting with danger to save almost no metal in the wheel.

Even if you accurately factor all the most minute variables, once dynamic it will all go out the window.

Also someone might make a slightly different tyre soon after you do your meticulous calculations.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.
 
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Thanks Pat thats what i wanted to know, i just want to make sure i have the correct test forces to satisfy my OCD, not because iv designed the wheel so close to the point of failure.





Thanks guys for your help

Never be afraid to try something new. Remember, amateurs built the ark. Professionals built the Titanic
 
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Brentonschnaars,

I am still unclear on what your ultimate goal is.

From the conversation, I'd guess you are designing a rim. If that is the case, I'd do a literature research on testing methods of rims. That ought to point you towards methodologies that work.

BTW, I am surprised that folks didn't know that the TRA yearbook doesn't have SLR - and I'm also surprised that anyone would think that it would vary from brand to brand and not from tire type to tire type (meaning all season / high performance / etc.)

And..... if I'm correct you are designing a rim, then I understand now why you were concerned about the suspension. That would cause asymetrical loading on the rim flange area.
 
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CapriRacer,

You seem very switched on i must say. I have done the reasearch on testing methods and have the the relevent AS {Australian Standard} that confirms what tests are required and how to work out the test force loads to meet TYPE approval. What it doesnt state is how to work out the SLR ( which is part of an equation to work out test force for test which i stated above). I do understand that the SLR changes over the range of tyres available to go on to a rim. so my question was how to work out the largest possible SLR of any tyre combination on a wheel.


Thanks

Never be afraid to try something new. Remember, amateurs built the ark. Professionals built the Titanic
 
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By the way. The SAE fatigue tests are NOT the limiting load case for wheel design, unless you are designing a steel wheel.

The limiting load case for the rim of an alloy wheel tends to be semi-abuse cases like potholes and kerbs. Your options there are rather limited if you are using conventional wheelweights.





Cheers

Greg Locock

SIG:please see FAQ731-376 for tips on how to make the best use of Eng-Tips.
 
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Carnage,


So your suggestion is to go and obtain every possible tyre that would fit a wheel, inflate each tyre to every specified pressure for each vehicle that the wheel and tyre will fit on then measure it..... Go figure


Greg,


I do appreciate the help you guys have tried to give but unfortunately you seem to have misunderstood, Australian standards requires the Static loaded radius to be known for test loads so maybe you should let all the members that played a role in the development of this standard that they can remove this part of the equation because it is such a trivial part


Never be afraid to try something new. Remember, amateurs built the ark. Professionals built the Titanic
 
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Ah, AS have to be bought, so I can't look it up.

I think the cornering fatigue test is based on the SAE test. If so it is up to the manufacturer to recommend a max SLR and weight. Then you just test at that combination.



Cheers

Greg Locock

SIG:please see FAQ731-376 for tips on how to make the best use of Eng-Tips.
 
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