I have a used 19" wheel & tire coming in tomorrow. The wheel has provisions for dual disks. I currently have a 21" with a single disk on my Custom. I ordered the hub cap & bolts for the missing disk, but they probably won't be here for a while.
Being the impatient chap that I am, does the hub cap act as a spacer or is it just for esthetics?

 

That cap doesn't perform any function except cover up where the rotor goes. You can see it on this parts website.

 

Yeah, I looked at it in my parts book before I ordered it. It appeared that it was purely cosmetic. I just wanted to be sure.
Thanks.

 

I'm actually looking for a 21" dual disk. Can a single disk be switched to dual?

 

Lets look at the whole dual disk thing.
It will give you superior braking power. It may save your life if some jerk does his/her thing in front of you. But it is worthless if your front tire is not up to it. I have 19". I'm using Continental Road Attack 110/80R currently and I am very careful with pressure (read: slightly underinflating) to keep the contact patch maximized under heavy braking, it also helps when you are forced to brake while leaning over. Please note, radial tires have better grip in these conditions, too, that's why I'm using R.
What is the best grip tire you can put on your 21" rim?

 

I can say the stock 21x2.15 spoke wheel has no provisions for a second disk. Down the road I may upgrade to a dual disk setup on the 19. But this would require new fork lowers for me. I'll be going the caliper upgrade route before I go dual.
I don't know what's the stickiest tire you can put on the 21". I was surprised to find the Michellin Commander IIs I put on last were bias ply. I'll be putting another Michellin on the 19 so it matches the rear. Next tire replacement I'll be looking for radials.

 

There is no need to match radial/bias ply front/rear on a motorcycle. It will make you feel uncomfortable if you do it on a car, yes, thus it is not recommended. But cars do not lean over in turns.

 

Chaps, there are two quite different 21" rims available, the traditional old thin ones used often on off-road bikes, but a wider one that can take a 120/70 tyre which is offered by some brands. I personally wouldn't use either, but the wide option seems to be a more wise one over the narrow one!

As for mixing bias and radials there is a convention (I don't know it's origin) for bikes which is that a radial rear with bias front is fine (which I run on my Glide), however radial front with bias rear is not wise. Obviously both bias or both radial is also fine!

 

Internet conventions should be taken with a big grain of salt. Somebody somewhere says something that seems to make sense and soon enough it becomes "truth". And of course, there are no references to original source.
I have bias ply rear right now, this is not first time I ride this configuration.
I ride fast, sometimes very fast. Bike handling is paramount for me. There are absolutely no ill effects having radial front and bias ply rear.

 

Here ya go, from Avon Tyres' website:

"Can I mix tyre construction front to rear?

Yes, but only to the combinations below:-

Front - Rear
Radial - Radial
Bias - Radial
X-ply - Radial
Bias - Bias
X-ply - Bias
X-ply - X-ply"