I have an older Kona 26" cruiser that has what appears to be a BMX style frame that I installed a Sachs 3 speed hub on several years ago. It has worked fine.
Small wheel bike service is actually easier on a gear hub as the primary gearing needs to be higher to give reasonable overall gearing. This decreases the input torque the hub is subjected to.
The Kona model is a Humuhumu-nukunuku-a-pua'a according to the top tube label. From about 1995 or so as I recall.
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Gear Hubs Owned: Rohloff disc brake, SRAM iM9 disc brake, SRAM P5 freewheel, Sachs Torpedo 3 speed freewheel, NuVinci CVT, Shimano Alfine SG S-501, Sturmey Archer S5-2 Alloy. Other: 83 Colnago Super Record, Univega Via De Oro
Visit and join the Yahoo Geared Hub Bikes group for support and links.
http://groups.yahoo.com/group/Geared_hub_bikes/