The saga continues.
There are a few snags resulting from having decided to do the rear disc conversion on the GT750 I’m working on – although the 1975 2 stroke, and the 1978 4 stroke models have about the same overall length (87.2 inches or 221.5 cm for the GT750 versus 87.6 inches or 222.5 cm for the GS750) the GS750 swing arm is roughly 3 inches (or about 8 cm) longer than the swing arm on the GT750, and has just over an inch (3 cm) of additional wheelbase. I suspect this is a good thing, as it should improve the tracking and overall handling, and also there is room now to perhaps install a larger rear tire. The downside is that the arc that the rear wheel travels, doesn’t look right with the stock GT750 rear fender. As I had planned to use a rear deck and fender off a 1978 GS750, I did not initially think this would be a major problem, however it did result in a number of frame modifications being required for the rear fender mount, the seat and the tank, as well as of the tank itself.
If you compare the photo of a GT750 (left) with a GS750 (right), you can see that the GS tank is longer than the GT tank by about 3 inches, as it extends to just over the side cover, whereas on the GT750, the tank stops roughly even with the front of the air filter box (red line in both photos). Also note that although the top shock mount is still in roughly the same location on both frames relative to the steering head, you can see the effect of the longer swing arm in the increased angle of the shock itself (green line).
The problem then, is having started from the rear of the bike to try and get the rear fender to look right with the new swing arm, and extending the frame to allow the rear deck to be mounted in the same manner as the GS750 stock frame, once I have the seat mounted there is a 3 to 4 inch gap to be filled in some way, either between the front edge of the seat and the rear of the tank, or between the rear of the seat and the rear deck being mounted on the fender. You can actually see it clearly in this photo as being the gap (yellow lines) between the scrap rear fender off a GS750 that I’m using for clearance checks as I fab up the frame mount extensions, and the original inner fender that come stock with the GT750.
I’m tending toward modifying the tank and have mocked up what it might look like in the photo below. Lengthening the GT750 tank actually makes it about the same length as a GS750 tank, and together with the rear deck makes it appear a bit more modern.. At the moment, I’m waiting for a few parts to arrive before finalising the changes, but I expect to make a decision in about two weeks. Nothing is ever easy !
Also – I continue to document what I’ve been doing with this project here, just to try and keep everything in one place.