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Okay, so I'm really scratching my head on this one. Before I took the front wheel off everything was okay. I could get the bike up on the center stand just fine and the cable had enough slack in it. I have pictures to prove this.

Tonight I put the forks back on and reassembled the front wheel. With the bike on the center stand, I could not reinstall the speedometer cable. It was too tight to allow me to thread both ferrules on. I then put the bike on the side stand, and was able install it since there was enough slack.

However, when I try and put it on the center stand, the speedometer cable pulls tight and acts as limiting strap, supporting the weight of the front end.

I'm afraid if I put it on the center stand it would break the speedometer or something else.

Anybody have any ideas?

Well, I've been watching this for a while and you did the front shocks, right?
I assume all the parts went in as they should and forks went back in triple tree correct. If oil level was set not right this will keep the forks extended and interfere with the cable. It might be a forks problem - oil level in forks. It also could be air pressure in forks. How much did you put in there? Within safe margin this should not make adverse effects. Either way, wrong oil level plus wrong pressure would extend the forks length beyond what the cable can take especially without load. I'd think you have shocks problem.

Is your speedo drive unit correct installed? In the video it seems to be not in the right groove.

Yes it's a good point to overall installation, but he mentioned to install it in the blocking groove correct (on facebook). Looking at the images it seems the cable to follow the correct curve.. Though the still image looks a bit weird I agree. Funny how you can't take people for granted. Yet there is one spot only to keep the housing in place.Otherwise it would turn one way or the other. If all is solid, it would mean it's in place if done correctly.

Regardless of quantity of oil or air, when the bike goes up on the center stand, the forks will extend fully to its mechanical stop.Oil level and air quantity will effect the compression of the shock and limit the travel from total extend to fully compressed.

So it would appear that the forks are now too long. Fortunately, about 2 weeks ago McBoney measured the length of his forks in order to determine if a set of Ducati USD forks would fit. He measured his forks at 830 mm in length which should be from the center of the front wheel bolt hole to the bottom edge of the cap on the upper tube.
So the first thing I would do is take that measurement with the bike on the center stand and see what you have. Possibly something is not right with the assembly allowing the fork to extend beyond its normal limit.

I'll ask Paul (McBoney) to confirm that the length as he measured it was 830 mm bolt center to bottom of the cap please.

I know I am going a bit silly here, but if the air pressure was negative, the vacuum would not even let the fork extend. So, there is some, at least theoretical, correlation there. I personally wouldn't sign up for any definite conclusion. From what I gather, fork are hyper extended. If all mechanical points are met, there has to be something else. SO, perhaps the OP should backtrack the assembly then move on onto adjustment and tuning.If nothing was changed, then the only two left are the fluid variables - oil and air pressure. OP discussed the oil in length on facebook, loving certain hydraulic oils and playing with ideas incorporating engine oils and using them as fork oil replacement. Those are two extremely different environments, but I can't stop anyone using engine oil for forks and vise versa. Overall, my impression was - oil. I will leav it at this. If re-assembly was done following reference photos, I can't see much alternative than oil vs. pressure. And gain, off the ground forks still extend as much as they are allowed by all involved. Including oil type, air and mechanical wear. It is not a equal default regardless of all, so I disagree with that assessment. That's would be a good general assumption, but not academic. More or less pressure will let the weight of the internals go lower or keep them higher, depending on what type of back pressure it creates. More pressure will help forks extend, less will keep them compact - a vacuum effect. Especially in the unforgiving rigid oil hydraulics environment. SO, unless a big mechanical screw up during the assembly my bet is on the oil level, oil type and air pressure.