One size fits both

[Ray Ride4life]

Sorry if it's alteady out there but what is the best tube size to take as a spare that will fit in the 120 21" front and the 150 18" rear?

[AZJW]

I carry a 80/90-21 regular duty tube in my swingarm for trailside repairs. 

 

Hopefully this will help. 

 

Motorcycle Inner Tube Sizing and Cross Reference Guide

 

[Landshark]

In a pinch, the front tube can be used in the rear tire as well.  Just carry a front tube and you're covered.  Unless you get two flats. 

[Ray Ride4life]

10 minutes ago, Landshark said:

Unless you get two flats. 

It's Just to have a backup in case it's not possible to patch.

When you have 2 unrepairable tubes at the same time you probably have other things to worry about.

I hope i never get so unlucky to have 2 slashed tyres before i have the chance to get a new spare.