Diamond Back Tires
Diamond Back Tires



No one has ever identified or explained how a radial tire can actually cause additional stress on a rim. I’ve read and listened to opinions and that’s all they are is opinions unsupported by scientific testing data. I believe this myth is backwards. I believe radials cause less stress on rims than the old rigid bias ply tires.

The Department of Transportation has created the new FMVSS 139 (Tread Act)* for the purpose of “establishing new and more stringent tire performance requirements that apply to all new tires for use on light vehicles.” Here is the DOT’s assessment of the differences in performance characteristics between bias and radial tires.“A bias passenger car tire carcass is typically made up of two or four plies of cord material that run from bead to bead at an angle of approximately 35 degrees to the centerline of the tire. This type of construction provides a very strong, durable carcass for the tire. Because the ply cords criss-cross and all the cords are anchored to the beads, the carcass is stiff and relatively inflexible. This type of construction prevents the different parts of the tire from acting independently of another when forces are applied to the tire. As a result, a bias construction is susceptible to impact breaks because it does not easily absorb road irregularities.” This causes more rim stress.

“By comparison, a radial passenger tire carcass is typically made up of one of more plies of cord material that run from bead at an angle of approximately 90 degrees to centerline of the tire. Because the cords do not criss-cross and because the opposite ends of each cord are anchored to the beads at points that are directly opposite each other, the radial tire carcass is flexible. The radial tire is reinforced and stabilized by a belt that runs circumferentially around the tire under the tread. This construction allows the sidewalls to act independently of the belt and the tread area when forces are applied to the tire. The “independent” action is what allows the sidewalls to readily absorb road irregularities without overstressing the cords. This ‘independent’ action allows two important things to happen (1) the tread of a radial tire remains fully in contact with the road over the entire tread width, and (2) the ply cords and sidewall are able to absorb forces without exerting the twisting force on the beads that are exerted by bias construction.”

Because a radial tire is so flexible, it must have some type of lower sidewall support. Remember the upper third of a radial’s sidewall does all the work. The lower two thirds of the sidewall is supported by the portion of the tire called the Apex. The Apex consists of two stiff, triangular shaped pieces of rubber that extend upward from the bead where the tire sits on the rim. The apex is thickest at the bottom and tapers upwards to a small point. The sole purpose at the apex is stability and support. The apex is a very strong, rigid piece of rubber – so rigid that it becomes the principal component of the run flat tire. How does the apex relate to rim stress? Simple - it transfers road force up to the flex area so that the flexible upper sidewall can do its job of absorbing road forces without “exerting the twisting force on the beads that are exerted by bias constructions.”

Don’t forget about inflation. Radials typically run about 10 psi more than the old bias tires. A poorly inflated radial causes more drag and will create more rim stress. A properly inflated radial with more rigid sidewalls will have less rolling resistance and less
stress on the rim.

The following paragraph is from Old Cars Weekly – 12-6-07 issue. Here’s a condensed version of what someone has written:

“Rims for bias-ply tires and radial tires are made out of different alloys. Rims for bias-ply tires cannot be used on radials. The forces exerted by bias-ply versus radial-ply are different and needs rims made specific for each application. Otherwise there is over flexing leading to rim metal fatigue and breakage. One can tell bias-ply, radial-ply and disc brake rims by their markings.”

I witnessed the transition from bias to radials and I don’t agree with this letter. The letter also says, “ Rims for bias-ply and radial tires are made of different alloys.” Again, I disagree. There were no industry standard markings to identify rims for radial use or rims for bias use, and no alloy changes just to accommodate radial tires.

In summary, all rims are subjected to stress (flex). The stress (flex) point can change its location on the rim (bead seat) depending on many factors – tire size, tire width, inflation, vehicle weight, bias or radial tires. If you have a 50’s or 60’s car, you would be safer with new rims simply because you don’t know the history of the older rims. In the last sentence of the letter someone wrote to Old Cars Weekly he states, “If you cannot find the correct rims you should continue to use bias-ply tires.” This is at best very bad advice, simply because of the safety advantage of radials versus the poor handling characteristics of bias tires. If you are using old rims, you could have a rim failure. The chances are extremely remote, and in my opinion, the cause won’t be whether you’re using radial or bias tires- it’s just an old rim.

Bill Chapman
Diamond Back Classic Tires

*CFR Part 571
Docket #NHTS-030-15400
RIN 2127-A154

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Diamond Back Tires

Diamond Back Classic Tires
Conway, South Carolina