Im my opinion, and I know others will disagree, any axle under 1.25" is not large enough. Personally, Ive seen three 1" axles just break off, entire wheel hub tire assembly just fall off on the track. My guess is you are planning on using pillow block type bearings, which not only dont roll freely...but are very heavy. I would use the 1.25" size....because its the std. kart size. They are inexpensive plus there is a world of very high quality lightweight componants available for it. And the afore mentioned free rolling bearings ARE free rolling. Additionally, I would set it up for #35 chain, plenty strong, and light weight. Do not use the bulk chain available at Tractor Supply and places like that. It WILL NOT hold up.