Upsides and Downsides of wheel sizes

I read in these magazines about professional riders using 20inch front tires as oppose to using 21inch. And I read about them using 18inch rear tires as oppose to using 19inch. Standard sizes being 21inch front and 19inch rear.

What would be best for strictly motocross track riding?

What would be best for strictly woods-trail riding?

What would be best for someone who does both?

Any thoughts on this? Please be as informative as you can be in your answer.

Thank you :thumbsup::bonk:

The front wheel thing you could discuss for weeks and get nowhere. There was a 20" movement some years ago (my 1971 BSA B50MX had one) that started mostly as a way to use wider tires than the 80/100x21. Now, you can get several tires in 90/100x21, so you can experiment with those without building a custom wheel. My son likes the Bridgestone M601 in 90/100 on the front of his bike, although the 80/100 D756 still works better in the sand. I tried it on mine, and the tire bites well, but the wider tire makes the front end too squirrelly with my offset clamps.

The rears are simpler. The off road models use an 18 because a 110/100x18 is a taller tire from the rim up, and offers the rim more protection from rock damage. MX'ers use a 110/90x19. That size tire is roughly the same overall diameter, but the shorter sidewalls make the tire considerably lighter and also stiffen it so it holds its shape better in corners when running low air pressure.

I put an 18" rear on my new YZ because I am a lard ass and I very rarely ride track, and when I do, I suck. I feel like the 18 not only provides more rim protection, especially considering my size(270lbs), but gives me a better ride offroad. I have also been told is greatly reduces the chances of 'pinch flats'. I have one ride with the stock 19 and one ride with the 18, not a huge difference, but when I get it in the mountains is where I think I will notice it the most. Also, I believe most 18 tires are slightly cheaper.

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