The "pulse" actueally is pre trek, has pretty much all the features of the more expensive frames but a 1 1/8 standard headset. It has the pressed in bottom bracket bearings, internal cable guids, boxed chainstays, chain control device, microdropouts, smooth welds, very very durable and tough paint.
The "pulse 2" is "post trek", but the frame ist still the same. No Changes that I know of.
The "pulse comp" then lost pretty much all klein features. No more internal cables, no fancy tubing (except the rear triangle), no pressfit bb, no more nice welds. Still good frames, but nothing special any more. Just another trek frame with a klein badge.
I dont know why the pulse frames go that cheap, as for me those are the best Klein frames at that time. But thats just me liking the suspension fork choice. One of those boxed forks would kill me, I guess. They're pure race machines
And now that I think about it, the pulse frames are sold for 250 Euros, thats not bad for a frame that old. The pulse 2 sells for 200, the comp models for 150. They just dont cost those silly prices of a attitude garage queen. Those are considered art. So no sane rules apply...
