To limit the travel of the suspension, there is a rubber bumper stop on both sides of the rear axle. Without this stop, the rear axle can have too much spring, with all the consequences that this implies. If this rubber shows cracks or crumbles, replace it in time.
They are pressed onto a ball that is integral with the rear axle support. Use only glycerin (see options tab) to press them onto the ball. If you use any other lubricant, chances are it will damage the rubber and you won't enjoy your new stop rubbers for long.
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