PCA SDR

I recently bought a durometer to help me assess the status of my tires. All it’s done is thoroughly confuse me. Measuring my Toyo RA1’s (1 year old, AX and track) which have about 25-30 heat cycles on them, the durometer reads 56 to 59 (out of 100 – it’s an InterComp Type A). My Khumo V710’s (only track), which I have about 15 heat cycles on, read 70. Unfortunately, I don’t have readings on either of these when they were new. Brand new Hankooks Ventus Z21, not yet mounted, have a reading of 80. Finally, I have a set of Toyo R888 which I have a weekend at Chuckwalla and an AX on. When new these had a duro reading of 63 – now they are at 60. All of the tires ‘look’ in good shape with no balding and tread grooves are still obvious. The readings are when the durometer is initially pressed into the tire ( it goes down with time). The tires were clean and I also checked different tread positions on each tire and the results were consistent. All the tires were at the same temperature (room temp).
I expected as tires got more use they got harder. It’s not clear to me how the RA1’s can read what they do when they should be getting towards the end of their life. Also, the R888 started at 63 and are now at 60 – going the ‘wrong way’? The new, unused Hankooks are reading the highest of all (80).
I’m curious what experience others have had with these durometers (and tires) and what typical readings are when the tires have ‘gone south’. I recognize different tires will have different initial readings, however, what increment to expect between new and ‘used up’ readings would be helpful. I can’t find any data on the internet of what to expect with different tires and Tire Rack and other suppliers don’t list the hardness for their tires on durometer scales, only the UTQG rating. Thanks for any help.

Bill

Bill,

Longacre has an article on the proper use of Durometers which may help.

https://www.longacreracing.com/articles ... p?ARTID=18

Thanks, Mark. That link was helpful. It appears I'm taking the measurements correctly. I know others in the club use these durometers - I was hoping to see some numbers that others have read from their tires.
Maybe these readings are not as common as I thought.

-Bill

Bill -
Brand new tires are coated with a substance which helps the rubber (or silca) retain good polymeric properties while they are sitting in storage (sometimes for years). This applies to R-compound tires as well. For lack of a better term, this coating is often referred to as "wax".This is why all tires will seem slippery for the first couple of miles (or feet, if you have a big enough motor). Because of this, durometer readings will often be wrong (harder) until you scuff off the wax (this is where the term "scuff tires" or "scuffs" come from).

Tire aging (sitting in storage) can also contribute to the outer surface of a tire getting harder, until you scuff off that layer.

Durometers are best used to measure the relative hardness of a tire from after it is scuffed, eventually hardening through successive heat cycles, so measure your tires periodically, preferably when they're warm. The measurements should correlate to slower lap times and even handling issues when the tires "go hard", sometimes with an unexpected suddenness.

I remember one time Chuck and I were going over his car at Buttonwillow because it was suddenly undrivable. It turned out that his rear tires had heat-cycled out way before he expected them to (e.g., he was expecting to run them all the way to the chord like his other tires . )

.... and those tires were Toyo R888's. Never again!
If you can't run them until they cord - they are either real race tires or lousy R888's.

ChuckS wrote:If you can't run them until they cord - they are either real race tires or lousy R888's.

Ahem,
I can personally confirm that Chuck does indeed practice what he preaches.