The lost art of tire truing aka (tire shaving)

JCR · Oct 10, 2021

The lost art of tire truing | ClassicCars.com Journal

Article is on tire truing aka "shaving" on a XWX equipped Bora. This was mostly done in the bias ply tire days and many garages had the equipment to perform it.

The "why" is in the article but here is some more info from Barry's Tire Tech (barrystiretech.com) He posts as CarpriRacer on Tires & Wheels | Bob Is The Oil Guy where the following came from.

Allow me to explain about Out of Round tires.
The technical term is Uniformity, where both the physical dimensions (Runout) and the tire stiffness variation are combined.
The measurement is force and sometimes the term “Force Variation” is used to describe the phenomenon.
If you measure a loaded tire at the axle, you can plot the amount of force variation In pounds) around the tire’s circumference.
Interestingly, you can break down the force variation (mathematically) around a tire into a series of Sine waves – once per revolution, twice per revolution, etc.
We refer to these as 1st Harmonic, second Harmonic, etc..
It turns out that a vehicle’s suspension is a Spring-Mass-Damper system and those have resonant frequencies where the mass vibrates as though the damper (shock) is not there.
In cars, that usually occurs in the 50 to 70 mph range.
So if the 1st Harmonic is high enough, it can be detected by a human in spite of all the other vibrations that a vehicle makes.
This is particularly true when the road is very smooth.
One way to deal with this is to match the high point of 1st harmonic of the tire to the low point of the 1st harmonic of the wheel.
The net effect is an assembly that is overall “Rounder”. That’s what the Hunter Engineering RoadForce machines do.
In the case of wheels the 1st harmonic peak generally occurs very close to the max runout point.
This is not as true for tires, so the old idea of “Trueing” a tire didn’t work as well as it should have.
So why can’t tire manufacturers make tires round? Because of the way they are manufactured.
First, the tire molds are heated and cooled – and the metal warps. Second, tires are made in a series of layers where each layer has an overlap (called a splice)
Each splice adds a bit of variation to the uniformity.
Further, over the years, cars have been made stiffer and stiffer and that makes the vehicle more sensitive to vibrations.
Ergo, it is a lot easier to make a vehicle twice as stiff than it is to improve the tire making process to reduce the force variation by half.
Yes, tire uniformity has always been there as has vehicle sensitivity, but vehicle sensitivity has progressed much more rapidly than improvement in tire uniformity.
CapriRacer
Click to expand...

More here:
www.barrystiretech.com/unifandbal.html Vibration - Balance, Runout, and Uniformity:
K&T Vintage Sports Cars | Tire Truing Video (ktvintagecars.com) Good video on wire wheels
Technical - Shaving Bias Tires | The H.A.M.B. (jalopyjournal.com)

energy88 · Oct 10, 2021

I can attest to tire truing.

Back in the late 1960s, I had a 4-4-2 that would develop a vibration starting at about 75 mph. Balancing the wheels would not cancel out the vibration. Somehow, someone told me about the art of tire truing and I found a tire & front end shop that did this type of work. Results were outstanding! As a buddy of mine stated, before the truing it was like the car was working against itself as the speed increased; afterwards, the car was smooth and felt like it was accelerating better with increasing speed.

Highly recommended.

fatbillybob · Oct 10, 2021

That lost art is no longer needed because of modern manufacture. The only people shaving tires are racers to cut tire blocks smaller to reduce tire squirm and thus lower laptime

JCR · Oct 10, 2021

fatbillybob said: ↑

That lost art is no longer needed because of modern manufacture. The only people shaving tires are racers to cut tire blocks smaller to reduce tire squirm and thus lower laptime
Click to expand...

Still needed on bias ply and older radial designs used on classic cars.

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