Is your HRmax when biking different then for running, or can you train to raise the HR you can achieve when biking to comparable levels? I think your absolute HRmax - the point when you faint - is different than the HRmax achievable by just running, so maybe its different for different activities?
In Faster Road Racing, by Pfitzinger and Latter, there is a section about cross training. They make a point whereby your MaxHR for different activities is in fact different. If I recall correctly, they also noted that the MaxHR achieved by cycling was not as high as that for running - you you can expect to "max out" at a lower HR.