By Bill Dobson
As arranged on the ICEBIKE mailing list, I volunteered to perform hardness tests on a few studs from winter bicycling tires. Several others agreed to send samples of studs and shortly I received studs from 3 different studded tires: Nokian, IRC, and Nashbar, to measure their hardness.
At last I have the stud hardness test results! The following is my report.
A little background first...
In a hardness test, a small indenter (in this case made of diamond) is pressed into the surface of a metal sample with a known force. The result is a small impression, and the size of the impression is an indication of hardness. The smaller the impression, the harder the material. The term "Hardness" is really a measure of the ability of a sample to resist indentation, and in itself it is meaningless, and only becomes useful when compared to the hardness of something else. There is a definite relation between hardness and strength and wear resistence. Harder materials are stonger and more wear resistant.
To measure the hardness, the sample must be properly prepared. Standard metallographic technique is to embed the sample in a plastic "mount" which allows the technician to hold the sample in proper alignment for preparation. In this case, the studs were laid on their side in the mount. The face of the mount was ground down, and in the process the studs themselves were partially ground away, In this case, we ground the mount until about half the studs were ground away, revealing a cross-section through the stud centerline. This surface is then polished to a mirror smooth finish, since the hardness impressions are microscopic and surface imperfections could affect the result, thus the need for a flat smooth surface.
Here is an image of the the prepared mount with the ground and polished studs showing their cross sections. Click on the image for a closeup view.
It can be seen that the studs have a "T" shape cross section. There is a head at one end (like a nail head), a body, and a tapered end, which is the "business" end of the stud that makes contact with the ground.
Now the results:
It is immediately obvious that the Nokian and IRC studs are made from two pieces: the stud body which is steel, and an insert brazed into the center of the stud. It is the brazed insert that is in contact with the ground, and the body is simply a holder. The Nashbar stud is a single piece of steel.
The Nokian insert had a hardness of 1700 KN (Knoop hardness scale). The IRC insert had a hardness of 1400 KN The Nashbar had a hardness of 350 KN (corresponding to about 57 HRC for those who care about such things).
The Nashbar stud is by visual examination and hardness testing, simply hardened steel. It was harder on the surface than in the center, which implies a low alloy and/or low carbon steel (ie: less expensive steel), as something this small should harden throughout its thickness when heat treated - there should be no hardness gradient. I made no attempt to identify the composition of the IRC or Nokian inserts (that would have been a bit expensive), but my best guess is they are "carbide", a very hard, wear resistant material commonly used for high wear applications like metal working tools, saw blade edges, and so on.
A word of caution: Don't try to attach a "value" to the relative hardness of the studs. The Nokian is not "5 times better than the Nashbar" nor will it "last 5 times longer". All that can be said is that the Nokian and IRC are "substantially harder and will be much more wear resistant" than the Nashbar.
As a metallurgist, and knowing the environment these tires run in, I would be very surprised to see significant wear on a Nokian or IRC stud, whereas on road surfaces, I would expect wear on the Nashbar. Running on dirt or ice the Nashbar may hold up much longer.
On a personal note, I am impressed with the degree of technology exhibited by the Nokian and IRC tires. There is considerably more cost involved in preparing the stud to receive the insert, and then going through the brazing process, compared to the Nashbar stud which is simply machined and then heat treated. I can understand why the Nokian tire is so expensive! .