I have magnetic walls at my track as well
You can either use a modified BRP TC3 bumper or the pdub bumper. I have now tried both. Either will save you arms and other suspension parts over the the standard bumper. I do not believe you need to use both.
However, depending on your driving, you will still break parts. If you get tagged, you will also break parts.
Schumacher has two sets of front arms. The plastic set and the graphite/nylon set. The plastic (S1 - U2716) set go for about $9 depending on who sells them. The graphite/nylon (CF - U2765)go for about $13. The plastic ones seem to be a bit more flexible under a crash and are cheaper. You might want those to start with.
If you use the CF ones, check for hair line cracks if you hit a wall during the race. Sometimes they crack before they break. I have been able to extend their life until at least the end of the race day by using some very thin CA or JB Weld. They will however eventually break but it has worked for me in a pinch without spares (which are hard to come by in my area)
I keep at least two spare fronts and one rear. I have to says that even with the hardest hit, I have never broken a rear arm and I have smacked and have been smacked pretty good on some of the tighter tracks I run.
Also, most of my breaking has been indoor on foams, outdoor the barriers we use seem to be a bit more forgiving so I never broke a front arm until I ran indoors.
I recommend you have spare:
- Front A-arms
- Front hub carriers (2, 4 & 6 degree depending on your setup)
- and at least 1 spare steering bell crank
I am an intermediate level racer and not a pro and have been racing the Mi2 since about September on a regular basis and that is all I have broken. To date, I have broken 5 pairs of fronts, 3 sets of hub carriers and painfully 3 sets of bell cranks.
Sorry for the long winded answer, hope that helps