V85TT Subframe Question / Strengthening the Small Block Swing-arm—Transmission

[Dirk_S]

TL;DR: Does this brace on the V85TT provide any structural bracing with the gearbox and/or swingarm, or is its purpose solely in supporting the foot controls? Pic attached:





Background
It’s always in the back of my mind, since I occasionally throw a sidecar onto my V7–what can I do to structurally improve the gearbox-swingarm / driveshaft region of my small block V7 in order to handle the torsion forces. Connecting the swing-arm to the gearbox rather than more-structurally sound frame, in my opinion, is the small block’s biggest setback as a tug, but I imagine also proves to be a concern for the off-road jaunts. Yes yes—“the bike isn’t meant for that”. Well, to those who are afraid to modify a bike and rely solely on factories to produce stock machines for them, I say…OK. I’m sure you’re using that creative problem-solving brain of yours more productively than I.

I’ve thought about creating a support plate on each side that connects from the main frame down to the lower frame rail (or that engine bolt hole), with the support plate going over the pivot area, to somehow assist in bracing and/or restricting the torsion. Still trying to figure out how this could actually be done, considering the swingarm pivot screws thread into the swingarm and thus turn with the swingarm. If a brace came into contact with the pivot bolts and remained rigid, you would have a restrictive wear point. A hole cut in the brace, with custom-machined pivot bolts that still thread into the swingarm but also jut out through the swingarm, perhaps resting in a bearing that’s seated in the swingarm brace could work, but I’m curious if there might be a simpler solution. Hence my aforementioned question about the V85TT brace.

(I can already hear the cynical jokes developing in the ether that a simpler idea would be to get a different bike)

[Grabcon]

Take a look at. the parts diagram for the frame.


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[Frenchfrog]

The Baja TT 650's that were hand built in the factory competition dept back in the 80's addressed this problem by grafting a big block swinging arm onto the frame, also solving the single jointed UJ problem too which is associated and possibly the real problem here. The newer small blocs change the single UJ to a double too. You still get the aluminum cast swinging arm that's not ideal for sidecar use and then not very suitable gear ratio's  though

[Dirk_S]

Take a look at. the parts diagram for the frame.


img host

Thanks for the pic. I should’ve done that in my original post. Yep, I’ve looked at this before multiple times—I see where it connects to the frame and transmission case, but I still have trouble discerning precisely if there’s any bracing to the swingarm pivot.

The Baja TT 650's that were hand built in the factory competition dept back in the 80's addressed this problem by grafting a big block swinging arm onto the frame, also solving the single jointed UJ problem too which is associated and possibly the real problem here. The newer small blocs change the single UJ to a double too. You still get the aluminum cast swinging arm that's not ideal for sidecar use and then not very suitable gear ratio's  though

I’m aware of those bikes, but I didn’t think those factory mods were done to address lateral torsion on the swingarm pivot. I know they addressed the small block driveshafts’ lacking travel capability, which led to loud noises and smaller pieces. Looking back on some pics of those bikes, it doesn’t appear like any of those changes really tackle that sideways force.

Edit: I do understand the small block swings arm is aluminum, while the big block’s is steel, which provides more rigidity, but I’m not fully sure how well that aids in protection against lateral forces…I suppose the swingarm’s pivot extensions wouldn’t twist as much.

(I want to keep the post on track, but yes—I know that the small block gearing ain’t great for sidecars, although the hemi’s lower torque delivery—especially the V9–are much better than the older Herons. Would LOVE to have one of those older V9 motors in my V7 III chassis for this reason…or one day cracking the motor apart and replacing all the bearings, cylinders, crank, and cam myself—yes, there are more parts than just those, but again, staying focused on the topic at hand)