I've been asked to inspect and fix someone's bike. Its a heavy full-suspension Kona "Stab Deluxe" downhill monster from 2006.

The main fault is that the chain gets stuck between the front chainring and the chainguide.

It is a single 36 tooth chainring branded as Saint, with Hussefelt cranks and a 4 bolt Truvativ crank. Not sure what the BB is other than external. Seems to be stock/original components.

There's also a trailing lifter wheel vaguely like a jockey wheel but without teeth, so seems to be a tensioner but has no tension, perhaps best described as a keeper?

Normal pos Normal position

Rotated forward Rotated forward by hand.

Shape of support system Outline showing support system in the back.

My gut feeling is that the whole mounting bracket should not rotate like this. From below I can see threads of the external cup (?) housing between the BB case and the chain keeper's support plate.

There is a lot of scoring and worn-off plastic inside the chain guide itself, suggests this has been loose or wrong for a long time.

Question What's wrong with this setup? And how do I fix it?

  • On further investigation I believe the root cause is a stuffed Bottom Bracket. Still want to know if the chainkeeper should rotate around the BB or be completely rigid to the frame.
    – Criggie
    Oct 21 '18 at 3:51
  • 1
    It seems odd to me that the curve of the keeper doesn't align better with the curve of the ring. Oct 21 '18 at 12:01

Looks like the chainguard is of the type that mounts with a number of bolts that screw into the frame. You can see the slots in the mounting plate the screws go through to allow setting the chainguard angle. Presumably those bolts just need tightening?

I'd pull the crank and guard off and clean everything up as there may be dirt behind the guard plate. Looks like you will be doing this anyway if the BB is toast.

GMBN has a video on fitting this type of chainguard:

  • You got it - the BB cup on the right hand side was backed out till it tried to push the crank off. I guess the chain guide got caught sometimes, and was pulled back by pressure from the rear derailleur. The chainguard is supposed to be clamped firmly between the frame and the drive side cup. As it wiggled back and forth the drive side cup was unwound off over time. The bearings are munched on that side (oddly, bearings are perfect on the left/non-drive side) The axle's keyways are also damaged, may be saveable.
    – Criggie
    Oct 22 '18 at 3:37

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