Frame material 6061 T6 Aluminum, fork material 4130 CroMo
Top tube diameter: 1.5"
Seat tube diameter: 1.5" (26.8mm post) - vs 1.25" on the 1988
Down tube diameter: 1.75"
Seat stays: 0.75 box section
Chain stays: 1.0" box section
Drop outs: 0.375" plate
BB Shell: 90mm - vs 68mm on the 1988
Rear axle spacing: 145mm
Front axle spacing: 115mm
Cantilever brake mounts front and rear
The main visible difference is the larger 1.5" seat tube as compared to the 1.25" on the 1988. The seat tube necks down at the top to accomodate a 26.8mm seatpost just like the 1988. A custom front derailleur band clamp (Shimano didn't make anything over 1 3/8" at the time) is used to mount the XT front derailleur to the massive seat tube. It seems Doug used Aluminum for the early clamps, but later switched to steel as the Al versions seemd to fatigue and break. I've seen photos in one of the MBA reviews (late 1988 I think) that show a seat tube starting at 1.5" near the BB and then necking down to the 1.25" at the middle and up. This would be a transitional design element going from the 1988 to 1989. Although linking a specific feature on a DB Manitou to any given year is silly as he changed things as they suited him, it seem that the increasingly larger seat tube was a consistent feature going from 88 to 89 and onto 90. Eventually in 91/92 he started using 31.8mm seat posts and removed the neck down feature at the top of the seat tube altogether.
As best as I can tell Manitou stems came on some 1988s but not all. My 88 is an early one and so it didn't get the stem, while some later ones I've seen did have them. It appears that by 1989 all of them came equipped with the custom stems and this bike is no different. The stem really completes the custom kit for any DB Manitou and although simple in design is rather aesthetically pleasing to look at.
Unlike the 1988 frame which featured horizontal dropouts this 90 Manitou uses vertical dropouts, a design feature found on all subsequent Manitous. The rear end is basically the same, with the main point of departure being the way the chainstays are mounted to the BB shell. I need to go and take another picture to better depict the change, but suffice it to say where the 88 frame used a custom formed, asymmetrical 1" box section to offset the drive side chainstay around the tire the 90 uses a 3/8" Aluminum plate on both sides with a cross member to join the now symmetrical chainstays to the larger 90mm (vs 68mm) bottom bracket shell.
Other differences include the way the dropouts are mounted onto the fork legs. On the early 88 forks Doug simply pinched the steels legs together and then welded in the dropouts. On this 90 he formed the lower legs into a taper which serves as a much cleaner transition to the dropouts (will add photos later).
Lastly, one of the coolest things about this bike and the thing most people notice first is the color. Doug painted very few of his bikes and according to the previous owner this bike was one of maybe 10 that were painted for an order to Japan and Doug added this bike (which was sold to Los Angeles) as a favor. I've seen pictures of three or four of the other blue Manitous and actually one black one. But aside from those I have not come across any other painted Manitous, making this a rare bird. I just like the color.