There are two main types of defense combos: ones that rely on weight and ones that rely on grip.
The purpose of defense types is to:
1. Withstand heavy hits (from attack types primarily)
2. Not be easily knocked out of the stadium
Types that rely on weight are customs like this:
AR: Master Dragoon/Tiger Defenser/Galeon
WD: Wide Defense
SG: Neo Right HMC
BB: SG Metal Ball with 4 metal balls
If you think this is too much weight you can either use two metal balls or switch to a lighter SG Core.
Typically I prefer these types of defense combos. The extra weight increases spin rate. The heavier something is the harder it is to knock around.
Types that rely on gripped tips will make use of Wolborg 2's tip most of the time.
AR: Tiger Defenser
WD: Wide Defense
SG: Wolborg 2
SP: Defense Ring
BB: Customize Grip
or
AR: Tiger Defenser
WD: Wide Defense
SG: Wolborg 2
BB: Wolborg 2
In the second generation of zombie type beyblades, players started using Wolborg 2's gripped tip more frequently because it started to become more common that people would try to take out zombies with upper attack type combos. Wolborg 2's rubbery grip tip creates more friction between the beyblade and the stadium floor making it harder to be knocked around.
It would be better if you posted all the beyblades you have so we can help you better.
Here is a link to a discussion on defense types:
http://forums.beywiki.com/showthread.php?tid=581&page=1
Here is a link to Composer's physics page. He talks about friction at one point. Maybe this will help you understand things more clearly:
http://forums.beywiki.com/showthread.php?tid=836
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