Can i use 6 speed on my 8 speed

[cockney]

After some wheels 26'' for my mtb so i can swap my wheels over if i want to go offroad,have found some but they are 6 cog where mine are 8 what would happen if i put them on?Would it work?

 

[mr Mag00]

non​

 

[staffy]

If your bike was made for an 8 speed rear wheel ie the gear shifters having 8 speeds then the trouble you will most likely have is the rear derailleur throwing the chain off the sprockets when you select high and low gears and generally not indexing properly , also there is a good chance your chain is an 8speed chain which can cause shifting problems and the 6 speed wheels you mention are probably an oldstyle freewheel which you can't swap with your 8speed which is probably a cassette http://sheldonbrown.com/free-k7.html

 

[andrew_s]

Also, a 6 speed rear wheel will be 126mm wide, whilst your existing wheel and frame will be 135mm wide.
It may be possible to screw in the QR to squeeze a steel frame (though a right nuisance), but an aluminium frame would break in due course.