I asked someone how is very well informed about this subject . Here the reply he gave mabye you will find
it usefull " Here is what I know. STEEMBackedDollar is like a gold backed dollar or paper gold. It's a note that say you can exchange this for X amount of Y. It was designed to be converted (not exchanged) to 1 USD worth STEEM. I also remember reading that SBD was designed in a way that the amount of SBD doesn't exceed 10% of the STEEM supply. So SBD supply should be 10% the USD value of STEEM .

STEEM marketcap 500 million means 50 million SBD coin supply. I don't know if any of these changed because of recent developments. Anyway the point is as long as the value of STEEM isn't zero the lowest value of 1 SBD is 1 USD (assuming dollar doesn't hit zero)

It's a safety net mechanism that was assumed to be stable but the free market pumped it up. SBD now has $12 price tag and $1 intrinsic Value"
Check his content out Hive account@vimukthi. Here is a post of his that goes little bit deeper about the subject steem dollars https://steemit.com/steemit/@vimukthi/my-take-on-what-to-do-about-sbd-free-market-all-the-way Hopefully I helped you engouh to get by !