I've only had a few gravel tires over the years (mainly a roadie) and those I had I never wore down...ended up swapping tires or selling that bike.
So I'm wondering if, when I wear down a tire that has a tread (say the GKSK, Touareg, Terra Speed, etc.) is it equivalent to a slick, or is it garbage?
I've been wondering the same thing. I've got a Terra Speed on the back that's pretty worn down, but not riding offroad much until things dry out, so wondering if I can just continue using it on pavement.
Bicyclerollingresistance.com did a test a while back of older tires vs a new version of the same tire. I think it was a GP 4k. The older tire was a bit slower.
Not a direct comparison since you're removing the nubbins from the equation. I wish we could send them our old tires for testing or filter our new tires through them before we use the tires ourselves.
Do I remember correctly that knobbies have thinner rubber under the knobs than a slick would have? So using a worn knobby as a slick would tend to wear quicker than a proper slick?
Bicyclerollingresisitance measures the tread thickness on tires, including the knobs. If I remember correctly, a tire like the Conti RaceKing has the same tread thickness as a GP4000/5000 when the knobs are not included. I tried to test a worn tire vs a newer tire with tread but it was inconclusive. I did not have enough repeititions to draw any conclusions. However, I suspect the aging of the tire probably makes it a wash when you take away the extra resistance of the knobs. Correction: The comparison I gave above was not correct. A RaceKing is 1.8mm in the center with 2.5mm knobs. A GP5000TL is 2.8 in the center.
it depends on the tire. Some if my tires are consistent down to the cord, some tires get really really hard (i.e. no traction) once you wear off the outer cap. Usually you can see this though, when there is an evident change in rubber compound when you get close to the cord.
