text: how do i finish my bmx? ive built a bmx and finished it all, but then my chain kept falling off ,so a freind told me i need to replace the axle because the rear wheel wobled,  so i took the axle out. would a mountain bike axle fit on to my bmx its a 20" frame? and what would you sudgest i do? cheers pete pete,  first, you need to make sure the chain tension is appropriate. you should be able to move the chain a half an inch up or down.  if there's too much slack, the chain will of course fall off if you hit any bumps. try loosening up your rear wheel axle nuts and pulling back on the wheel. don't forget to make sure the wheel is straight in the dropouts before you tighten the nuts again. also, check to make sure the wheel has equal spacing up at the joints of the chain and seat stays.   this is the other issue. if your wheel is crooked, that means your cassette is crooked too, which would create sort of a twisting effect, causing the chain to pop off. just make sure your wheel is as straight has possible, and the chain is relatively tight (half an inch of slack maximum), and you should be good.   i wouldn't suggest putting a mountain bike axle in your hub. it's not going to be very cost-effective, and if your bmx bike is high-quality brand (not next, huffy, magna, vertical, etc...), chances are it has a 14 mm axle. the mountain bike axle won't fit in your dropouts correctly, which is a safety hazard, plus you could potentially damage your bike.  if none of this works, take a look at the teeth on both your rear cassette and your front chainring; if even one tooth is bent, that could cause your chain to fall off. sometimes you can get lucky and bend the teeth back with a small pipe wrench, but in many cases the parts will need to be replaced.  hope that helps some, pete!