Any disadvantages in running an 18in rear while riding MX?

I have an 18” on my Gas Gas. I can’t tell the difference.

I ride an 18 on my KTM and a 19 on my GasGas and also can’t tell the difference.  

I got a RMX (18") rear wheel for my RMZ, I prefer the extra cushion of the taller sidewall, yes there is more spinning mass, I'm sure it would be a major point for the pro's.

yeah, I'd make a terrible test rider. although I've only ever pinchflatted 19s

Just run it and smile. 

I’d rather run an 18” at the track, than a 19” in some rocky single track.  Mx rear wheels were 18” up until around 1990. 

From what I’ve heard, the main difference is less bounce through supercross style whoops. For 99% of us I don’t think it’s an issue, lol…

Sand is a different story, 19" is much better than 18". But as far as choppy hardpack goes, the 18" is a lot smoother.

That and more rear steering feel in hardpack SX turns was always my understanding back when the switch happened as well.

Everyone keeps saying weight, but has anyone ever weighed a 19 and 18 setup with the same tire?

I can't see it being a significant difference and if you factor in the rim being closer to the hub on the 18 I speculate the weight and especially the rotating inertia are almost a wash. 

I sure can't tell the difference on a track, but def can in roots/rocks.

Pretty much any popular tire you can get in 18” MX34 or AT82 or if you ride a ton the Tusk tires are great. 

I’d try the Dunlop AT82 front and rear. 

I have bikes with the 18” and the 19” rims. I switch back and forth regularly on the same tracks and woods courses. I think the 18” works better in all conditions and I don’t see any drawbacks. For reference, I’m a Vet intermediate skill level and have been riding for 35 years. 

The 18 will perform well on hard surfaces, as the carcass will absorb the chop better like rock gardens. 

There is an MX14 18" tire if you need a sand tire.

I'm not a fan of the extra rotating mass personally, but it's not the end of the world. 

How does this extra rotating mass affect your riding ? 

Just changes how the bike reacts off the faces of jumps and in the air. Mostly feel it while scrubbing and the difference in front/rear balance in the air. Like I said though, not the end of the world.  

When I was still riding tracks, I use to ride my KTM 500 once in awhile and there was too many differences to really feel the difference in the 18” vs 19” tires. 
 

Im an old vet. I run the 19 to keep my whoop speed at max. 11.456 lbs pressure. 102.25 mm sag. I also back off my motor mounts .25 ft/lbs

Back when they switched from 18 to 19 the difference was immediately apparent. The hook-up and feel was vastly superior especially on hard pack..

Now tires have changed so...

Also 2 strokes only.

Try backing off the motor mounts an extra .07 ft/lbs.  That made all the difference for me.

I've never ridden both back to back and that would obviously be the best test. I have ridden enduro bikes on the track and at speed I haven't had anything to complain about. I notice the soft suspension far more than anything else that changes between XC/MX bikes. The sound of my kickstand hitting the ground on impact is my biggest complaint😂 If it's cheaper/easier in your current circumstance to run 18", run it and have fun with it!

Don't forget the good old 17's.

When was that? early 70s?

My buddy had a 450 xcf and the 18 seemed to work really well in our local sand track. He preferred the 18 in those conditions. 

If you find the 18" wheel not to your liking, before you give up on it you might want to try adjusting the torque on the rim locks to vary the amount of flex in the tire sidewall. You can also try drilling out the sides of the rimlocks for the same reason. 

If after that you still aren't happy, we hope to have the titianium and composite versions of our new rimlocks available soon so discriminating riders can enjoy the huge improvement they offer and was previously available only to factory riders. And more good news - we're keeping them affordable, with the MSRP being in the $995.99 to $1295.99 range.

One of my bikes is set up with an 18" rear, I actually rode it on the track this weekend. Only thing I don't like is I feel the tire roll a bit when I'm pushing into jump faces while leaned. Makes the rear hard a little hard to trust in that circumstance. 

Other than that, I don't really notice. The extra sidewall is nice on square edge acceleration bumps. 

Husky's, Honda XR's and some Yam IT's ran them in the 80's, after Honda's 23" front, tall sidewall, big footprint, they were an experience :-)

Yes, but into the late seventies, very early 80's