Durometer readings....when is it bad?

Steven Carter · **Posted:** Sun Aug 12, 2012 7:51 pm

So on a whim I ordered a tire (Shore A) durometer. I have three pairs of Star Specs at various levels of wear, and thought it would be interesting to see what hardness each pair was....

Pair A is my current front pair. Formerly on the WRX, it has approximately 125 runs and 8000 miles. They started at 6/32 this season and are now just above the wear bars. Durometer reads 72.

Pair B is the other 2 from the WRX, only 60 runs and 6000 miles. Durometer reads 63. About 6/32 tread remaining

Pair C is the new rears ordered for this year. 2000 miles, 60 runs, nearly full tread (double wishbone FTW!) durometer reads 60.

So this all seems about what I would expect....more wear, more heat cycles, tires getting harder with age and use. My question is at what durmoeter reading should I consider the tires to be beyond their useful range? I have been noticing more undertseer lately despite tir pressure, FSB and shock adjustment, but I expected the fronts to have a reading closer to 80. How bad is a 73 relative to a 60?

Any thoughts? Thanks!

Cash Davidson · **Posted:** Sun Aug 12, 2012 9:51 pm

Steven Carter wrote:

How bad is a 73 relative to a 60?

Knowing nothing about tire Durometers, I'd guess 73 is 21.6% worse that 60. :-)

Steven Carter · **Posted:** Sun Aug 12, 2012 10:27 pm

Ha!

My plan is to take readings at various temps of each tire as a baseline to see how the tire softens from ambient to 180ish. When one set begins to "go" I will recheck the temp/hardness curve and see how how much change there has been. We'll see if it turns out to be useful info.

Chuck Branscomb · **Posted:** Mon Aug 13, 2012 9:05 am

Steve,

First off, I have no clue on comparing the readings you have obtained. Random thoughts: (1) make sure you record the temperature of the rubber/tire you are measuring if you want to make comparisons with other similar tires -- all should be at the same temperature in order to begin to make any comparisons (the stiffness of the rubber compound will vary significantly with temperature); (2) sample the repeatability of your measurement methodology -- various locations on the same tire should yield readings very close to each other (note: unless this is a multi-compound tire like a Pilot Super Sport and various others); (3) it wouldn't surprise me if a brand new tire shaved to 2/32 yielded different (stiffer) readings than a 10/32 new tire although this is likely a small effect (?) given the small size of the durometer probe although I don't know its insertion depth (i.e. if the probe insertion depth becomes a significant percentage of the tread depth (tread face to tire carcass structure), then I would "assume" that stiffness might likely register higher even though that might not correlate to the tractive force capability).

All the above is just BS off the top of my head knowing next to nothing about this stuff. Oh, one other thought -- if you want to compare these tires in order to gain insight into how durometer readings might effect grip, I would wager you want to do that durometer comparison at as close to full operating temperature as you can (just off the track, just after an autox run, etc). Just a hunch.

It should be interesting to study this though -- keep your results posted.

JamesShort · **Posted:** Mon Aug 13, 2012 9:31 am

Nit completely applicable, but I've seen motor mounts that come in 60-90 durometer urethane. The 60 is street comfort and the 90 is race. My crx has 80 durometer motor mounts and they are pretty damn stiff (make your teeth rattle

).

MarcusMcRae · **Posted:** Mon Aug 13, 2012 9:39 am

James, from now on you must always use nit instead of not.

Steven Carter · **Posted:** Mon Aug 13, 2012 9:46 am

from what I gather, 60 is the typical reading for street tires. Since 80 is so harsh (like delrin hard, right?) I figure the relationship may not be as linear as I initially expect it to be...I do appreciate Cash's math, however :wink:

Chuck, those are great points. I am going to try some "in vitro" experiments first by heating the tire with a heat gun over a 15-20 sqin area then taking measurements with my pyrometer in 20 degree increments over ambient. It may or may not translate to the effects of whole-tire heating during an event, but I bet it will be close. As for the probe, it extends 2mm into the tread, so I figure as long as I am above the wear bars I should be away from the carcass. My graphing skills are pretty poor, but I will post what I get, at least in table form.

RodneyWright · **Posted:** Mon Aug 13, 2012 10:00 am

MarcusMcRae wrote:

James, from now on you must always use nit instead of not.

damn, that was funny Marcus....

MarcusMcRae · **Posted:** Mon Aug 13, 2012 12:16 pm

RodneyWright wrote:

MarcusMcRae wrote:

James, from now on you must always use nit instead of not.

damn, that was funny Marcus....

Made me chuckle.

Steven, did you see these?

http://www.intercomp-racing.com/tech/Me ... rdness.pdf

http://www.cab-racing.com/extras/UsingA ... ometer.pdf

Seems like the data will be relevent only if you want to switch between several different sets of tires and to your tires only.

JamesShort · **Posted:** Mon Aug 13, 2012 12:48 pm

Autocorrect sucks on my new Rezound...it's always odd words it messes up. Os instead of is. This instead of the.

Anyway, it is not nit, it is nee!

MarcusMcRae · **Posted:** Mon Aug 13, 2012 1:11 pm

JamesShort wrote:

Anyway, it is not nit, it is nee!

You will say Nit!

Steven Carter · **Posted:** Mon Aug 13, 2012 1:25 pm

MarcusMcRae wrote:

JamesShort wrote:

Anyway, it is not nit, it is nee!

You will say Nit!

We are no longer the knights who say Ni. We are now the Knights who say... "Ekki-Ekki-Ekki-Ekki-PTANG. Zoom-Boing. Z'nourrwringmm.

RodneyWright · **Posted:** Mon Aug 13, 2012 1:58 pm

Ah, so good to see you guys educated in the classics! Glad I'm not the only Python addict in the group.

shawnwhipple · **Joined:** Wed Aug 20, 2003 6:28 pm **Posts:** 603

And after you are done taking all your durometer readings you must seek the largest tree in the forest and cut it down with......(nit)......A Herring!! :thumbsup:

Steven Carter · **Posted:** Sun Nov 18, 2012 6:01 pm

As a loose follow-up I just checked the durometer readings on my tires. With significant changes in ambient temps, the comparison from August is not entirely valid, but the tires on the car (after another 20 runs and 1000 miles) are now reading 73 front and 68 rear. I chalk up the difference more to temperature (50s today vs 80s in August) than wear. Interesting datapoint however is with the 255 Z1* I got with Stephen Westerfield's TRM STR wheels. They measured 82 for the nearly bald ones (beyond the wear bars w/ only a trace of tread) and 78 (at the wear bars).

THSCC Forums

Durometer readings....when is it bad?

Who is online