I was thinking about using a crf150r as a trials and trails bike. I like to sit and ride sometimes on trails and I am a newbie (relatively) to trials riding, so I don't think I am going to be doing any outrageous rock climbing anytime soon. I was looking to try and find trials type tires that would work on the bike. The crf150r expert has 90/100-16 in the back and I believe 90/100-19 in front if I am not mistaken. Any help or advice would be greatly appreciated. Also, any thoughts on changing the spocket a tooth or 2 to increase torque a little?

Thanks!

I think that bike would work well as a trials/trail bike. I ride a RM100 for that and while it is light like a trials bike at 150lbs, it doesn't have the low end power that a 4 stroke has. Here are the mods that I did: 13/60 gearing, homemade skid plate, Rox risers, cut the seat foam down and moved the footpegs back. I bought the huge rear sprocket from PMP. It is good for tight singletrack but on open trails it's too slow. You can't find a good 16 inch rear trials tire so I use a Kenda Trackmaster 2 which is softer than most. Shinko makes a good 19 inch front trials tire but I use a knobby. Tires are not as important as gearing and getting a good standing riding position.

Here's a video of me doing trials on my RM100.

The fact you said tires instead of tyres makes me think you may be ok, I don't think it would be allowed if you did say tyres though.

What about getting a whole new back wheel from a bike like gasgas or a rear 18 wheel from the montesa and putting it on the bike?

You would have to check carefully that several things fit correctly such as:

axel bolt and snail cams,

spacers on each side

alignment of sprockets - front to back.

alignment od brake hardware, left to right and braking surface on disk to caliper location

Also check clearance of tire to frame, fender, chain, etc. when the shock is fully compressed

It might be easier to have someone lace a trials rim onto a CRF hub. That would ensure all the hub and axel related alignment issues. Still need to check clearances when the shock is fully compressed.

Many club rules do say "Trails tires only" but it depends how strict the club is.

How is the turning radius on the CRF? There are lots of tight turns in trials. Lower grearing would be a big help for the trials riding.

An 18 inch wheel won't fit on a super mini MX bike. What might work is a 17 inch rim and Vee Rubber makes a real radial 17 inch trials tire. If you want a hybrid bike, just use knobbies since trials tires isn't such a huge advantage. Knobbies are better in the soft stuff. You don't want to spend too much time riding slowly because of overheating with a 4T, but on my 2T super mini I haven't overheated yet.