Check the option parts that Junior World Champion and member of team XRAY, Daniel Kobbevik from Norway has been using at his XRAY XB4'21. Daniel explains also the benefit of each part, for which track conditions they should be used and what performance improvement you will experience
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Differential Bevel & Pin & Satellite Gears #335030 Bevel & Gears #335080 Pin
>>> The diff bevel with satellite gears and pin are as must have for me on high grip
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(click to enlarge)
Composite Gear Differential Cover & Case - Large Volume - Graphite #364912-G Case #364920-G Cover
>>> The composite gear diff cover and case keep temperature down and overall feeling of diffs is better
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(click to enlarge)
Composite Suspension Lower Arms - Graphite #362112-G Front #363112-G Rear Right #363122-G Rear Left
>>> Graphite arms make less traction roll feeling and pushes the job over to the shocks. Also provide better landings.