What is the minimum trail on a Dyna below which the bike would experience a wobble?

Has anyone out there mounted a sport bike 17-inch wheel on their Dyna? If so, what offset was in the triple trees you used, and how did the change affect the handling?

I see many different comments and online guidance regarding how sensitive handling is to changes in the trail figure. Some articles state that experienced racers can detect even one millimeter of difference. The Chopper Builder's Handbook states that as long as the trail is between 2 and 8 inches, the bike is rideable. Both may be true. Most sport bikes seem to be between 3.8 to 4.1 inches of trail.

I have a 2004 Dyna FXDX that I bought new. It has a stock 28 degree rake and a stock narrow-glide triple tree with an offset of 2.5 inches that support a stock 19-inch cast wheel with dual discs. Rear shocks are 13-inch Ohlins units from Mr. Messner. My bike's steering has always seemed heavy compared to how I like to ride, more so now that I installed clip-on hand controls.

I have decided to get a lightweight front wheel to reduce the rotating mass, and have a smaller wheel diameter at the same time, both with the goal of making the bike more nimble. The issue is the trail figure. The stock Dyna has a trail of 4.1 inches. A 17-inch wheel with a 120/70 profile has an overall diameter that will reduce my trail to only 3.45 inches. A taller tire would increase the trail, but at some point an 18-inch wheel makes more sense. An 18-inch front wheel with a 120/70 tire would provide a trail figure of 3.8 inches using my stock triple trees.

Any comments would be most welcome.

 

I suspect that you find there is no fixed trail that makes all dynas wobble. It depends on a number of things. Wheel weight is only part of the equation. Tire selection, rider position, suspension, phases of the moon, etc all effect handling.

I ride an 2000 FXDX and the steering is pretty light compared to other HDs but I'm still running the stock bars. I do run lighter alloy spooked wheels and a fatter tire in back.. I suspect that going to clip-ons will make the bike feel heavy but then it is heavy.

Not sure what you can do except try the wheels and see how they work for you. You can try to extend the forks. There was someone that posted a link to place that makes tubes to fit. You'd have to add a spacer to the cartridge tho.

One of the things you could try, is simply sliding the tubes up some to see how the reduced tail effects the handling. It's not exactly the same as a smaller wheel but it should give you some idea as what you experience with the smaller wheel.

One thing to note is lowering the front end will reduce cornering clearance..

 

I reckon you're over-thinking this, just get stuck in! Harleys seem to work almost regardless of what indignity we subject them to. My Firebolt has very little trail by 'normal' standards and is fine. By contrast your Dyna has very modest geometry numbers and losing a little trail is hardly likely to make much change IMHO.