Is there any problems with  16 rear an a 21 front   On a street bob 06 dyna 

No good for....?

I don't know much, but it might inspire someone else to post.

I guess a 21 inch front wheel might get you through a pot hole better, but might change the geometry of your bike.

I think a smaller front wheel will turn a little more nimbly, and a larger wheel might cause a slightly larger turning circle, and a less agile cornering.

It will mean that you'll need to change brackets to run a front fender from the same mounting holes on your front forks. I run very high profile tires, which caused me to have to fabricate brackets to raise the front fender, so I could use the stock mounting point, but stop it rubbing on the tire.

Also will need to check 21 inch front wheel hub width and ensure that it'll fit width wise without having to use spacers on your axle, either side, as then you'd deal with disc rotor size and fitment issues.
Also, make sure the wheel can fit your axle size and bearing size.

Just thinking aloud.

Good luck

Cheers yer no major issues with it more of a cosmetic thing on my behalf

considering that was the main combo 16 on the rear 21 on the front with Harleys for along time might help if there was more info on what bike

It's on a 06 street bob

so you have a 17 and 19? I would keep the 17 , 21 wide glide might fit or relace your hub or new spacers ,might have to raise guard too
but as said some people prefer a 19 and have gone from 21 o 19 I am sure it would have been done though

I've got a 21/16 on my Evo Softy that works well.
Only difference if you go up would be under steer I suppose.
My other bike has is 16/15 and it tends to over steer a bit.

Check the tyre diameters. From what I found the 16 to 19 inch Harley wheels have the same rolling diameter, so a 19 can be swapped in easily. Swapped back easily too. Changing to the 21 means the guard will not fit as the rolling diameter is larger, if using a standard Softail guard from a wide rear wheel model it is also offset to one side (as is the wheel and guard mounts on the fork lowers) so you will have to use spacers, buy/make a guard, or buy/swap a wideglide front end onto your bike. Most 21 inch Harley wheels only have one brake disc, which means it might be easier to have a new rim put onto your wheel if you want to have twin discs. If going to a single disc then you will need to find a fork lower without the brake lugs on it for one side. I went the other way and took a 21 off and fitted a Fartbob wheel to improve the handling/braking and go to twin disc's, love it. I found it was better than I hoped it would be and won't be swapping back, parts already sold so can't help you out with bits. Some of the single disc 19 wheels have the mounting for the second disc already there, just covered with a bit of trim. About 2006 the axle diameter increased but think this can be fixed by swapping the bearings if using a later model wheel.