Tire Safety

[iolair]

After reading the contents of thread68-161928, I checked out the specs of a wheel/tire swap and am slightly concerned about the safety of the tire choice of a fellow forum member for street and occasional track day use.

The spec OEM front tire:

275/40ZR 17 - 98Y - Max Load 1653@44psi
_________________________________________

The tire that is on it now:

275/35ZR 18 - 95Y - Max Load 1521@51psi
_________________________________________

I called BFGoodrich and of course they will definitely
not sanction this particular swap, but OTOH as previously stated,the max load safety margin is for worst case scenarios/legal protection.

Is this a legitimate concern on my part or should I just mind my own business?

Thanks!

 

[NormPeterson]

You didn't give the car make/model, but if it makes any difference, 275/35-18 is listed as an OE front tire size on the Corvette Z06. I think it's also found on the front of the Viper. The front tire on the Callaway C16 is slightly smaller in terms of load index.

Perhaps this individual is partially compensating with a little higher inflation pressure.


Norm

 

[iolair]

Hi Norm,

The OEM spec that I quoted is for a 2004 Mustang SVT Cobra.

Thanks!

 

[NormPeterson]

That information helps make the case for running LI 95 tire much stronger, as tires of that capacity were fitted to the '04 Mustang GT (245/45-17's). 275/40's are essentially the same OD at 25.7-ish inches, which simplifies a number of other issues.

 

[iolair]

Hi Norm,

I would imagine that the increased front end weight of a SVT Cobra, as opposed to a GT.....DOHC heads, supercharger, intercoolers, pumps, hose assemblies etc. warrants the increased load capacity of the 275/40ZR 18.

So bottom line, I assume that you agree with his choice of front tires?

Thanks!

 

[Tmoose]

I think those tire sizes are chosen for sales appeal and speed rating appropriate for the car's top speed more than having the precise capacity to hold the car up.

Z rating means it has a speed rating of 149 mph plus.
here's a link that shows (1) the pressure increase above OEM spec necessary for speeds over 118 mph for Z-rated tires, and (2) the effect on load capacity if the pressure increase for speed would exceed the max pressure on the sidewall.

I don't see how a Mustang with driver could weigh much over 4200 lbs.

http://www.rsportscars.com/ford/2004-ford-svt-mustang-cobra/

Or have much less than 55% or so on the front wheels.

http://www.fast-autos.net/vehicles/...12_9504_ford_mustangs_r_trim/competition.html

>>Maybe<< it can go faster than 149 mph. I'd want to see it on a GPS rather than any factory speedo.

So, driving straight ahead the weight the pair of front tires will carry continuously is 2300 lbs or so, and each tire will carry less than 1200 lbs.

Just looking at the max load you provided it looks like either tire could easily handle the Mustang's weight at whatever the max speed rating is, but might need max pressure to go that fast for a long time.

I'm guessing it would take a banked superspeedway to allow 149 mph driving AND enough weight tranfer to require considering non-equal front tire loads of any duration.

 

[iolair]

Alright....that is interesting.........it also opens up a much larger selection for my GT500 if I choose to ignore the stock OEM tire spec of 1709@44psi (255/45ZR18).

Thanks!

John

 

[CapriRacer]

Just to add some more fuel to the fire:

I've heard that using tires that are larger capacity than needed helps handling. Specifically, that the carpet plots are much more linear in the lower load areas - and that translates into more predictable handling.

 

[GregLocock]

Oh, that is for sure. A very easy way to improve the handling in the model world is to rescale your Pacejka tire model by 120% in both axes (slip and force). The improved linearity is very helpful, and the extra grip won't hurt.

Cheers

Greg Locock

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

 

[iolair]

Ah, ha...so that may be one very important reason to specify a tire of extra load capacity! Cool!

Hmmm.......OTOH, if there is room/no rules for the largest load capacity tire possible......could there be a case of diminishing returns if too large a tire were fitted as it relates to turning corners?

Thanks!

John

 

[GregLocock]

Yes, there is an upper limit of sorts- aquaplaning.

In the dry I don't think you'd really see an upper limit, other than by practical considerations..



Cheers

Greg Locock

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

 

[patprimmer]

There are other trade offs, not in grip, but in weight which increases unsprung weight and increases polar moment which requires extra power for the same acceleration rate. It also might increase aero drag. It might change rolling resistance either way, depending on details.

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.

 

[iolair]

Decisions, decisions......thank you all very much!

John