Join Date: Oct 2005
Location: Southwestern Ontario
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Ummm ... (overall) wheel diameter would be contributing pretty much all of the difference (what isn't contributed by possible differences in tire quality/construction).
As you've specced it above, your wheel diameter on your mtb is about 25.5 inches; on your hybrid about 27.8 inches. That results in a very large -- and very noticeable -- reduction in 'angle of attack' in favour of the hybrid. When each wheel hits a bump (especially a sharpish one), or encounters a dip, in the road surface, the larger wheel will go up and over more easily, or not fall in as far (and again, roll out more easily once it has fallen in), as the significantly smaller wheel. Lower rolling resistance (in this sense) and a smoother ride are the result.
My 700c Sirrus w/32mm Paselas is smoother riding, in this way, than my 26er mtb w/1.6" (40mm) road slicks, even with the higher pressure required in the former.
It's the whole principle behind "29er" (and now 650B) mtbs.
The front suspension on the mtb isn't really intended to compensate for this kind of difference, nor (as you've found) does it do so.