In middle biking with my friend, he asked i we should switch bikes, we both agreed, the bike path was very dark,and i bumped his bike into somone coming in to me, we all didn’t have light in front of the bike, do i have to pay for the damage, is it considered “baalav imo”?
There are two reasons why you might be exempt from paying the damage:
1. As you note, this might be considered “baalav imo”. The Tur (Choshen Mishpat 305) mentions two opinions as to whether a case of “lend me and I will lend you” is considered to be “baalav imo” or not.
The Beis Yosef asks why this should be the case, because the general concept of “baalav imo” refers to a lender who is working for the borrower, and not to a lender who has borrowed something. Later authorities explain (see Taz; Sema 10; Nesivos 3) various reasons why this should be the case: A borrower (who is the lender in our case) has to look after the borrowed item for the lender, and ensure that it gets back to him; a borrower has to prepare the item for the lender to take; a borrower who also lends is not considered as having complete benefit from the borrowed item (see Rashi, Bava Metzia 81).
Since according to this opinion, which is mentioned in the Shulchan Aruch, the loan is considered “Baalav Imo,” there won’t be an obligation to pay the damage.
2. The damage can be considered “meisa machmas melachah.” There is room to argue that the damage was done in the course of normal use of the bicycle, and therefore the borrower will not be liable. It can be argued that cycling in extreme conditions (very dark) is not “normal use,” but the fact that both parties agreed to cycle in the dark together strengthens the argument that this is considered “machmas melachah” and the borrower is exempt.
All the above relates to the strict halachah; in practice, it is of course commendable to reach an amicable agreement, and perhaps to split the cost of the damages between you.