Vintage Dirt Bike Q & A

What tire sizes equall a 3.00x21 and a 4.00x18?

80/21 front
100/18 rear

Just do the metric to english conversion i.e. 3.00" X 25.4mm/in -> 76.2 so 80 is close enough.

The aspect ratio when given is expressed just like car tires. Lower number means fatter like 70s or 60s radials. Forgot what no aspect ratio given means, defaults to something like 100% or 95%

Just me and I am not a performance rider: Getting tire size to within ~1/2" is good'nuff. A 110 rear or 90 front if you are in soft dirt would be fine.

Why in soft dirt?? I ask because my original Bridgestone rear tire was completely worn and the guy at cycle shop convinced me the 110 was the replacement. But now it slightly rubs the muffler. No big deal there, but it absolutely doesn't hold a tight corner like the old Bridgestone did even when completely worn out. I have to really watch it or I'll lose the rear end in a tight corner.

Just a little more ~floatation in soft stuff.

I'm well over 200lbs. On my older smaller bikes, like a KS125 that only has 4" of suspension, I notice a dramatic softening when I bottom the suspension. Difference between chipped teeth and just getting punched in the gut.

Wish I could help you on the traction but there are at least two reasons you lose traction in a turn.

There could be the obvious tread pattern mismatch to the surface or 'walking out.' What happens in the latter is the tall nobs on a nobby tire bend/flex as you are cornering and allow the rear end to kind of crab crawl sideways. You can really notice this when you push a bike with nobbies hard into a turn on pavement: Quite disconcerting to have your back tire pass your front w/o a skid.