Fork travel won't be affected unless you lower the front end as well. Stock FLH fenders fit over 21's with only enlarging the holes to 3/8" and tilting them forward. Ride will be different as it does sit around an inch higher so it's going to throw off rake/trail of the bike.

Just my personal opinion, take it for what it's worth, aggressive on a Harley isn't even warming up the tires on a sport bike, I don't think the difference in rake and trail is going to make it handle any more like a slug than it already does. Not saying they aren't fun to take down back roads and drag your lower fairings, just that they aren't fast enough at it that a 21 is suddenly going to turn an F1 car into a yugo.

 

A 21" wheel and tyre is around 2" larger in diameter than a stock 19". By contrast the 16", 17", 18" and 19" stock front wheel/tyre combos the MoCo has used in recent years are all pretty well the same OD. So there is likely to be a difference in ride and lean angle with a 21".

 

I would say 9 out of 10 guys would get use to the ride of a 21" over the 19" and never think about it. Maybe 1 out of 10 would notice the ride quality difference and go back to a 19" so really depends on how sensite you are to the ride of your bike. If you rather have a little better performance and sacrafice a little of the looks then stick with the 19" but it is a minimal difference IMHO. Both use a low profile tire so on the highway they feel pretty similar and in the canyons the 21" handles good.

 

Old post, but funny and direct to the point.