Originally Posted by
70sSanO
While bikeman715 may have given more of the attributes of the visible differences, it is not entirely wrong. Cage length is to accommodate for chain wrap based on chainring and cassette gearing range. To that end mountain bikes generally have wider range gearing that road bikes which drives longer cages.
What I don't know are the actual parallelogram or other design differences that are used to optimize a derailleur to specific road or mountain cassettes. There has to be some differences since there are limits in max cog regardless of capacity.
For the OP, you can run a wider 7 speed freewheel/cassette with a larger low gear cog with a mountain bike derailleur.
John
RE: parallelograms... i have three ders. in front of me... An ANCIENT Suntour 7 MTB, a 105 road long cage, and a Deore MTB... all use the exact same angle.... further comparisons will yield the same info.... interesting to note, the upper knuckles on the two Shimanos place the parallelogram further to the rear, compared to the Suntour... this also effects Wrap.... cable mount points are similar on the Shimanos, but very different on the old Suntour 7.