I haven't lowered mine yet and I went to a 21". The biggest difference is your going to feel the road a little bit more and low speed handling (under 15mph) is a little more challenging. Regular speeds you should notice no difference. In fact highway speeds feel better for me.
"A zest for living must include a willingness to die."
Are you talking about the front or rear? What brand tire are you going with? What are the specs on the tire? A lot of it depends on this as with the Metzeler me880 200/50/17 with a 17" rim the bike will sit at the same height in the rear but if you go with the 55mm height sidewall it will sit a little bit higher. As far as the front is concerned the 21" will sit the bike about 1.25" higher in the front. A lot fo people will lower the front 1" to compensate but at the same time you will be losing 1" of travel in your front forks so it is a double edged sword.
I rode my bike without lowering it for several years and the slower steering was what was affected. When I did lower it 1" it steered a little easier at slow speeds but the suspension in the front is a little firmer. I added Ricor intiminators to the front with the lowering springs I used.
The bottom line is that everyone responds and feels differently to changes on their bike. What I consider to be a subtle change may be consider a major change to others so all I can do is state the facts the best that I can.
If I were putting another 21" on I would lower the bike 1" with Ricor Intiminators.
I have a 21,17 set up on my Glide, have not lowered he front and like the ride very much, my ife likes it also, slow steering is a different experience, I call it "the bars get heavy " witch is understandable with the ncrease in size.
As with all mods ride it slow and easy as you get the feel.
If your like me you will love the ride with out lowering the front.
SE Heavy Breather,D&D 2in1 Fatcat, SE 255 cam's SE Tuner,21",17",Renegade Phantom Cut Whistler,HD Edge Cut Grips,Mirrors,Levers , Etc. 12"Yaffe Monkey Bars 13" LRS
Thanks for all the info. My tire in the rear is a 55mm so it will be a little higher, but not as high as the agitator rear. I will ride it for a season and see if it needs to be lowered. Went out today for a bit still with stock wheels on, it was in the 30s here, need those heated gloves now...