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What are the risks of allowing a fork to bottom out, could this cause damage to or worse a catastrophic failure of the fork?

I'm wanting to increase my preset sag to around 30% and believe I should be fine with 120mm of travel on relatively tame XC single track. However the pressure for this is significantly under the recommended psi for my weight.

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It CAN cause a catastrophic failure of your teeth. –  Daniel R Hicks Oct 14 '13 at 21:12
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2 Answers

up vote 2 down vote accepted

Bottoming out once in a while is OK and will probably not damage your fork. The forks have bottom out bumpers which prevent any metal to metal impact.

If you find that you are bottoming out too frequently then you need to do one of the following:

  • increase the PSI (in case of air spring)
  • change coil to a harder one (in case of coil)
  • increase high speed compression damping
  • change fork and/or bicycle in case your riding style goes beyond of the capabilities of your current equipment (for example you are doing AM on an XC bike, or DH on an AM bike)

However the pressure for this is significantly under the recommended psi for my weight.

"Recommended psi" are just recommendations and your weight is just one parameter. Other parameters which play role in this are:

  • gear weight
  • fork condition (in regards of servicing, tuning, oil wts and oil condition)
  • riding style
  • stance on the bike

You should only care about sag % and the amount of travel that you use in your regular do-it-all rides (more is better; 80-90% of travel used is the usual).

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Bottoming out of a fork CAN cause a failure of a fork. I'm speaking from my experience where I ruined 2 DH forks in a space of a week. Both by bottoming out on harsh landings. However, this might be a fault of the fork construction (both of them were the same brand/model).

While you are talking about XC and tame trail, I can't see the same things happening to you. I've bottomed out forks many times without failures. Bottoming out is not nice, but the forks survived in most cases (apart from the 2 above).

So go and play with the air pressure. You'll feel that air is lacking before you bottom out. Just pump some air in. Also, air forks are working as a progressive springs, so you'll need to try hard to bottom out 30% sag on 120mm travel.

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Which forks did you destroy? –  cherouvim Oct 15 '13 at 2:48
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One additional recomendation: Carry your shock pump on the first few rides (if you don't already do) so that you can experiment on the trail. –  arne Oct 15 '13 at 6:11
    
@cherouvim these were Rock Shox Boxxer Race'09. Both of them. –  trailmax Oct 15 '13 at 9:14
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Interesting. Well done, and hope it was worth the wreckage ;) –  cherouvim Oct 15 '13 at 9:24
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